Aftershock risks such as those demonstrated by the recent events in New Zealand and Japan

USGS Publications Warehouse

Shome, Nilesh; Luco, Nicolas; Gerstenberger, Matt; Boyd, Oliver; Field, Edward; Liel, Abbie; van de Lindt, John W.

2014-01-01

Recent earthquakes in New Zealand and Japan show that it is important to consider the spatial and temporal distribution of aftershocks following large magnitude events since the probability of high intensity ground motions from aftershocks, which are capable of causing significant societal impact, can be considerable. This is due to the fact that a mainshock will have many aftershocks, some of which may occur closer to populated areas and may be large enough to cause damage. When a large magnitude event strikes a region, the chance that aftershocks will cause damage can be significant as was observed after the 2011 Tohoku and 2010 Canterbury earthquakes (e.g., damage caused by Mw6.6 April 11, 2011 Fukushima-Hamadori earthquake following Tohoku earthquake or by Mw6.3 February 22, 2011 Christchurch earthquake following Canterbury earthquake). Aftershock events may further damage already damaged buildings, thereby further complicating assessments of risk to the built environment. In this paper, the issue of aftershock risk is addressed by summarizing current research regarding: (1) aftershock hazard, (2) structural fragility/vulnerability before and after the mainshock, and (3) change in risk due to aftershocks.

Temporal variation of aftershocks by means of multifractal characterization of their inter-event time and cluster analysis

NASA Astrophysics Data System (ADS)

Figueroa-Soto, A.; Zuñiga, R.; Marquez-Ramirez, V.; Monterrubio-Velasco, M.

2017-12-01

. The inter-event time characteristics of seismic aftershock sequences can provide important information to discern stages in the aftershock generation process. In order to investigate whether separate dynamic stages can be identified, (1) aftershock series after selected earthquake mainshocks, which took place at similar tectonic regimes were analyzed. To this end we selected two well-defined aftershock sequences from New Zealand and one aftershock sequence for Mexico, we (2) analyzed the fractal behavior of the logarithm of inter-event times (also called waiting times) of aftershocks by means of Holdeŕs exponent, and (3) their magnitude and spatial location based on a methodology proposed by Zaliapin and Ben Zion [2011] which accounts for the clustering properties of the sequence. In general, more than two coherent process stages can be identified following the main rupture, evidencing a type of "cascade" process which precludes implying a single generalized power law even though the temporal rate and average fractal character appear to be unique (as in a single Omorís p value). We found that aftershock processes indeed show multi-fractal characteristics, which may be related to different stages in the process of diffusion, as seen in the temporary-spatial distribution of aftershocks. Our method provides a way of defining the onset of the return to seismic background activity and the end of the main aftershock sequence.

Power-law rheology controls aftershock triggering and decay

PubMed Central

Zhang, Xiaoming; Shcherbakov, Robert

2016-01-01

The occurrence of aftershocks is a signature of physical systems exhibiting relaxation phenomena. They are observed in various natural or experimental systems and usually obey several non-trivial empirical laws. Here we consider a cellular automaton realization of a nonlinear viscoelastic slider-block model in order to infer the physical mechanisms of triggering responsible for the occurrence of aftershocks. We show that nonlinear viscoelasticity plays a critical role in the occurrence of aftershocks. The model reproduces several empirical laws describing the statistics of aftershocks. In case of earthquakes, the proposed model suggests that the power-law rheology of the fault gauge, underlying lower crust, and upper mantle controls the decay rate of aftershocks. This is verified by analysing several prominent aftershock sequences for which the rheological properties of the underlying crust and upper mantle were established. PMID:27819355

Intermediate-term forecasting of aftershocks from an early aftershock sequence: Bayesian and ensemble forecasting approaches

NASA Astrophysics Data System (ADS)

Omi, Takahiro; Ogata, Yosihiko; Hirata, Yoshito; Aihara, Kazuyuki

2015-04-01

Because aftershock occurrences can cause significant seismic risks for a considerable time after the main shock, prospective forecasting of the intermediate-term aftershock activity as soon as possible is important. The epidemic-type aftershock sequence (ETAS) model with the maximum likelihood estimate effectively reproduces general aftershock activity including secondary or higher-order aftershocks and can be employed for the forecasting. However, because we cannot always expect the accurate parameter estimation from incomplete early aftershock data where many events are missing, such forecasting using only a single estimated parameter set (plug-in forecasting) can frequently perform poorly. Therefore, we here propose Bayesian forecasting that combines the forecasts by the ETAS model with various probable parameter sets given the data. By conducting forecasting tests of 1 month period aftershocks based on the first 1 day data after the main shock as an example of the early intermediate-term forecasting, we show that the Bayesian forecasting performs better than the plug-in forecasting on average in terms of the log-likelihood score. Furthermore, to improve forecasting of large aftershocks, we apply a nonparametric (NP) model using magnitude data during the learning period and compare its forecasting performance with that of the Gutenberg-Richter (G-R) formula. We show that the NP forecast performs better than the G-R formula in some cases but worse in other cases. Therefore, robust forecasting can be obtained by employing an ensemble forecast that combines the two complementary forecasts. Our proposed method is useful for a stable unbiased intermediate-term assessment of aftershock probabilities.

Spatio-temporal evolution of the 2011 Prague, Oklahoma aftershock sequence revealed using subspace detection and relocation

USGS Publications Warehouse

McMahon, Nicole D; Aster, Richard C.; Yeck, William; McNamara, Daniel E.; Benz, Harley M.

2017-01-01

The 6 November 2011 Mw 5.7 earthquake near Prague, Oklahoma is the second largest earthquake ever recorded in the state. A Mw 4.8 foreshock and the Mw 5.7 mainshock triggered a prolific aftershock sequence. Utilizing a subspace detection method, we increase by fivefold the number of precisely located events between 4 November and 5 December 2011. We find that while most aftershock energy is released in the crystalline basement, a significant number of the events occur in the overlying Arbuckle Group, indicating that active Meeker-Prague faulting extends into the sedimentary zone of wastewater disposal. Although the number of aftershocks in the Arbuckle Group is large, comprising ~40% of the aftershock catalog, the moment contribution of Arbuckle Group earthquakes is much less than 1% of the total aftershock moment budget. Aftershock locations are sparse in patches that experienced large slip during the mainshock.

Three ingredients for Improved global aftershock forecasts: Tectonic region, time-dependent catalog incompleteness, and inter-sequence variability

USGS Publications Warehouse

Page, Morgan T.; Van Der Elst, Nicholas; Hardebeck, Jeanne L.; Felzer, Karen; Michael, Andrew J.

2016-01-01

Following a large earthquake, seismic hazard can be orders of magnitude higher than the long‐term average as a result of aftershock triggering. Because of this heightened hazard, emergency managers and the public demand rapid, authoritative, and reliable aftershock forecasts. In the past, U.S. Geological Survey (USGS) aftershock forecasts following large global earthquakes have been released on an ad hoc basis with inconsistent methods, and in some cases aftershock parameters adapted from California. To remedy this, the USGS is currently developing an automated aftershock product based on the Reasenberg and Jones (1989) method that will generate more accurate forecasts. To better capture spatial variations in aftershock productivity and decay, we estimate regional aftershock parameters for sequences within the García et al. (2012) tectonic regions. We find that regional variations for mean aftershock productivity reach almost a factor of 10. We also develop a method to account for the time‐dependent magnitude of completeness following large events in the catalog. In addition to estimating average sequence parameters within regions, we develop an inverse method to estimate the intersequence parameter variability. This allows for a more complete quantification of the forecast uncertainties and Bayesian updating of the forecast as sequence‐specific information becomes available.

Some facts about aftershocks to large earthquakes in California

USGS Publications Warehouse

Jones, Lucile M.; Reasenberg, Paul A.

1996-01-01

Earthquakes occur in clusters. After one earthquake happens, we usually see others at nearby (or identical) locations. To talk about this phenomenon, seismologists coined three terms foreshock , mainshock , and aftershock. In any cluster of earthquakes, the one with the largest magnitude is called the mainshock; earthquakes that occur before the mainshock are called foreshocks while those that occur after the mainshock are called aftershocks. A mainshock will be redefined as a foreshock if a subsequent event in the cluster has a larger magnitude. Aftershock sequences follow predictable patterns. That is, a sequence of aftershocks follows certain global patterns as a group, but the individual earthquakes comprising the group are random and unpredictable. This relationship between the pattern of a group and the randomness (stochastic nature) of the individuals has a close parallel in actuarial statistics. We can describe the pattern that aftershock sequences tend to follow with well-constrained equations. However, we must keep in mind that the actual aftershocks are only probabilistically described by these equations. Once the parameters in these equations have been estimated, we can determine the probability of aftershocks occurring in various space, time and magnitude ranges as described below. Clustering of earthquakes usually occurs near the location of the mainshock. The stress on the mainshock's fault changes drastically during the mainshock and that fault produces most of the aftershocks. This causes a change in the regional stress, the size of which decreases rapidly with distance from the mainshock. Sometimes the change in stress caused by the mainshock is great enough to trigger aftershocks on other, nearby faults. While there is no hard "cutoff" distance beyond which an earthquake is totally incapable of triggering an aftershock, the vast majority of aftershocks are located close to the mainshock. As a rule of thumb, we consider earthquakes to be aftershocks if they are located within a characteristic distance from the mainshock. This distance is usually taken to be one or two times the length of the fault rupture associated with the mainshock. For example, if the mainshock ruptured a 100 km length of a fault, subsequent earthquakes up to 100-200 km away from the mainshock rupture would be considered aftershocks. The fault rupture length was approximately 15 km in the 1994 Northridge earthquake, and 430 km in the great 1906 earthquake.

Aftershocks illuminate the 2011 Mineral, Virginia, earthquake causative fault zone and nearby active faults

USGS Publications Warehouse

Horton, J. Wright; Shah, Anjana K.; McNamara, Daniel E.; Snyder, Stephen L.; Carter, Aina M

2015-01-01

Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.

Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake

USGS Publications Warehouse

Herman, Matthew W.; Nealy, Jennifer; Yeck, William; Barnhart, William; Hayes, Gavin; Furlong, Kevin P.; Benz, Harley M.

2017-01-01

On 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double source W phase moment tensor inversion consists of a Mw 7.2 subevent and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west-northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple-event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20–45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust-faulting events, except for normal-faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along-strike events on the megathrust and intraplate extensional events.

Direct test of static stress versus dynamic stress triggering of aftershocks

USGS Publications Warehouse

Pollitz, F.F.; Johnston, M.J.S.

2006-01-01

Aftershocks observed over time scales of minutes to months following a main shock are plausibly triggered by the static stress change imparted by the main shock, dynamic shaking effects associated with passage of seismic waves from the main shock, or a combination of the two. We design a direct test of static versus dynamic triggering of aftershocks by comparing the near-field temporal aftershock patterns generated by aseismic and impulsive events occurring in the same source area. The San Juan Bautista, California, area is ideally suited for this purpose because several events of both types of M???5 have occurred since 1974. We find that aftershock rates observed after impulsive events are much higher than those observed after aseismic events, and this pattern persists for several weeks after the event. This suggests that, at least in the near field, dynamic triggering is the dominant cause of aftershocks, and that it generates both immediate and delayed aftershock activity.

Imaging 2015 Mw 7.8 Gorkha Earthquake and Its Aftershock Sequence Combining Multiple Calibrated Global Seismic Arrays

NASA Astrophysics Data System (ADS)

LI, B.; Ghosh, A.

2016-12-01

The 2015 Mw 7.8 Gorkha earthquake provides a good opportunity to study the tectonics and earthquake hazards in the Himalayas, one of the most seismically active plate boundaries. Details of the seismicity patterns and associated structures in the Himalayas are poorly understood mainly due to limited instrumentation. Here, we apply a back-projection method to study the mainshock rupture and the following aftershock sequence using four large aperture global seismic arrays. All the arrays show eastward rupture propagation of about 130 km and reveal similar evolution of seismic energy radiation, with strong high-frequency energy burst about 50 km north of Kathmandu. Each single array, however, is typically limited by large azimuthal gap, low resolution, and artifacts due to unmodeled velocity structures. Therefore, we use a self-consistent empirical calibration method to combine four different arrays to image the Gorkha event. It greatly improves the resolution, can better track rupture and reveal details that cannot be resolved by any individual array. In addition, we also use the same arrays at teleseismic distances and apply a back-projection technique to detect and locate the aftershocks immediately following the Gorkha earthquake. We detect about 2.5 times the aftershocks recorded by the Advance National Seismic System comprehensive earthquake catalog during the 19 days following the mainshock. The aftershocks detected by the arrays show an east-west trend in general, with majority of the aftershocks located at the eastern part of the rupture patch and surrounding the rupture zone of the largest Mw 7.3 aftershock. Overall spatiotemporal aftershock pattern agrees well with global catalog, with our catalog showing more details relative to the standard global catalog. The improved aftershock catalog enables us to better study the aftershock dynamics, stress evolution in this region. Moreover, rapid and better imaging of aftershock distribution may aid rapid response and hazard assessment after destructive large earthquakes. Existing multiple global seismic arrays, when properly calibrated and used in combinations, provide a high resolution image of rupture of large earthquakes and spatiotemporal distribution of aftershocks.

Modeling Explosion Induced Aftershocks

NASA Astrophysics Data System (ADS)

Kroll, K.; Ford, S. R.; Pitarka, A.; Walter, W. R.; Richards-Dinger, K. B.

2017-12-01

Many traditional earthquake-explosion discrimination tools are based on properties of the seismic waveform or their spectral components. Common discrimination methods include estimates of body wave amplitude ratios, surface wave magnitude scaling, moment tensor characteristics, and depth. Such methods are limited by station coverage and noise. Ford and Walter (2010) proposed an alternate discrimination method based on using properties of aftershock sequences as a means of earthquakeexplosion differentiation. Previous studies have shown that explosion sources produce fewer aftershocks that are generally smaller in magnitude compared to aftershocks of similarly sized earthquake sources (Jarpe et al., 1994, Ford and Walter, 2010). It has also been suggested that the explosion-induced aftershocks have smaller Gutenberg- Richter b-values (Ryall and Savage, 1969) and that their rates decay faster than a typical Omori-like sequence (Gross, 1996). To discern whether these observations are generally true of explosions or are related to specific site conditions (e.g. explosion proximity to active faults, tectonic setting, crustal stress magnitudes) would require a thorough global analysis. Such a study, however, is hindered both by lack of evenly distributed explosion-sources and the availability of global seismicity data. Here, we employ two methods to test the efficacy of explosions at triggering aftershocks under a variety of physical conditions. First, we use the earthquake rate equations from Dieterich (1994) to compute the rate of aftershocks related to an explosion source assuming a simple spring-slider model. We compare seismicity rates computed with these analytical solutions to those produced by the 3D, multi-cycle earthquake simulator, RSQSim. We explore the relationship between geological conditions and the characteristics of the resulting explosion-induced aftershock sequence. We also test hypothesis that aftershock generation is dependent upon the frequency content of the passing dynamic seismic waves as suggested by Parsons and Velasco (2009). Lastly, we compare all results of explosion-induced aftershocks with aftershocks generated by similarly sized earthquake sources. Prepared by LLNL under Contract DE-AC52-07NA27344.

Aftershock Comparisons of the Tehuantepec and Puebla Earthquakes: Implications for the Transition between Aseismic and Seismic Behavior?

NASA Astrophysics Data System (ADS)

Richardson, E.

2017-12-01

Reduced aftershock productivity has been observed in subduction zones where slow slip events and aseismic transients have also been observed. A comparison of the aftershock productivity of the recent Tehuantepec and Puebla earthquakes corroborates such observations. The Tehuantepec earthquake of 8 September 2017 produced hundreds of aftershocks and arguably still continues to produce them as of late October 2017, whereas the Puebla earthquake of 19 September 2017 notably lacks aftershocks. This difference in productivity cannot simply be ascribed to differences in mainshock magnitude or detection thresholds. The Puebla earthquake occurred downdip from and just adjacent to the eastern edges of previously observed slow slip events in the Guerrero Gap, whereas the Tehuantepec event is quite removed along strike from the Guerrero Gap and ruptured a patch of fault adjacent to other previous ruptures that also produced standard aftershock sequences. In order to compare aftershock productivity of earthquakes near the Guerrero Gap slow slip region with adjacent regions I used the Advanced National Seismic System catalog and counted aftershocks within a 14-day 100-km window of 42 M>=6.0 slab earthquakes that occurred since 2001 in a box bounded by 13°N and 20°N, and between 91°W and 103°W. This box includes the Guerrero Gap and significant portions of the plate boundary on either side. Preliminary results indicate that ordinary fast-rupturing earthquake productivity in general is much reduced near the location of known SSEs and aftershock productivity of those events that do occur is low compared to earthquakes outside that zone. Earthquakes with low aftershock productivity may represent transitional behavior from aseismic to seismic and in terms of frictional rheology, may represent the transition from velocity weakening to velocity strengthening.

Mainshock-Aftershocks Clustering Detection in Volcanic Regions

NASA Astrophysics Data System (ADS)

Garza Giron, R.; Brodsky, E. E.; Prejean, S. G.

2017-12-01

Crustal earthquakes tend to break their general Poissonean process behavior by gathering into two main kinds of seismic bursts: swarms and mainshock-aftershocks sequences. The former is commonly related to volcanic or geothermal processes whereas the latter is a characteristic feature of tectonically driven seismicity. We explore the mainshock-aftershock clustering behavior of different active volcanic regions in Japan and its comparison to non-volcanic regions. We find that aftershock production in volcanoes shows mainshock-aftershocks clustering similar to what is observed in non-volcanic areas. The ratio of volanic areas that cluster in mainshock-aftershocks sequences vs the areas that do not is comparable to the ratio of non-volcanic regions that show clustering vs the ones that do not. Furthermore, the level of production of aftershocks for most volcanic areas where clustering is present seems to be of the same order of magnitude, or slightly higher, as the median of the non-volcanic regions. An interesting example of highly aftershock-productive volcanoes emerges from the 2000 Miyakejima dike intrusion. A big seismic cluster started to build up rapidly in the south-west flank of Miyakejima to later propagate to the north-west towards the Kozushima and Niijima volcanoes. In Miyakejima the seismicity showed a swarm-like signature with a constant earthquake rate, whereas Kozushima and Niijima both had expressions of highly productive mainshock-aftershocks sequences. These findings are surprising given the alternative mechanisms available in volcanic systems for releasing deviatoric strain. We speculate that aftershock behavior might hold a relationship with the rheological properties of the rocks of each system and with the capacity of a system to accumulate or release the internal pressures caused by magmatic or hydrothermal systems.

Growth of a Structure Connecting the 2010 M 7.2 El Mayor - Cucapah Rupture with the Elsinore Faul

NASA Astrophysics Data System (ADS)

Donnellan, A.; Parker, J. W.

2015-12-01

The M 7.2 El Mayor - Cucapah earthquake occurred on 4 April 2010 in the northern part of Baja, Mexico. The rupture extended about 120 km from near the northern tip of the Gulf of California to the US - Mexican border south of the Elsinore fault zone. Most of the aftershocks occurred within days of the main event. On 14 June 2010 a M 5.7 late aftershock occurred 8 km southeast of Ocotillo, CA and is the largest aftershock in the sequence. The right-lateral event occurred in a cluster of aftershocks and was followed by its own aftershock sequence. UAVSAR data were collected for a swath covering the aftershock on 13 April, 2010 just after the El Mayor - Cucapah earthquake and before the earthquake on 21 October 2009. The line was reflown 1 July 2010 after the M 5.7 14 June 2010 aftershock. Data have been continued to be collected semi yearly to yearly since then. Repeat Pass Interferomety (RPI) products spanning the aftershock show the growth of a lineament that with an azimuth of 121.5° or a strike of -58.5°. The interferograms suggest that a stepover develops following the earthquake. The epicenter of the M 5.7 aftershock is proximal to the linear discontinuity in the postseismic interferogram and the mechanism of the event is consistent with slip on this stepover. Inversions for slip on the northeast linear structure that steps west of the mainshock rupture yield a moment magnitude ranging from 5.5 - 5.8, which is consistent with the magnitude of the aftershock. Slip occurs at a depth of 2-10 km on a steeply dipping fault.

DETERMINATION OF ELASTIC WAVE VELOCITY AND RELATIVE HYPOCENTER LOCATIONS USING REFRACTED WAVES. II. APPLICATION TO THE HAICHENG, CHINA, AFTERSHOCK SEQUENCE.

USGS Publications Warehouse

Shedlock, Kaye M.; Jones, Lucile M.; Ma, Xiufang

1985-01-01

The authors located the aftershocks of the February 4, 1975 Haicheng, China, aftershock sequence using an arrival time difference (ATD) simultaneous inversion method for determining the near-source (in situ) velocity and the location of the aftershocks with respect to a master event. The aftershocks define a diffuse zone, 70 km multiplied by 25 km, trending west-northwest, perpendicular to the major structural trend of the region. The main shock and most of the large aftershocks have strike-slip fault plane solutions. The preferred fault plane strikes west-northwest, and the inferred sense of motion is left-lateral. The entire Haicheng earthauake sequence appears to have been the response of an intensely faulted range boundary to a primarily east-west crustal compression and/or north-south extension.

Magnitude estimates of two large aftershocks of the 16 December 1811 New Madrid earthquake

USGS Publications Warehouse

Hough, S.E.; Martin, S.

2002-01-01

The three principal New Madrid mainshocks of 1811-1812 were followed by extensive aftershock sequences that included numerous felt events. Although no instrumental data are available for either the mainshocks or the aftershocks, available historical accounts do provide information that can be used to estimate magnitudes and locations for the large events. In this article we investigate two of the largest aftershocks: one near dawn following the first mainshock on 16 December 1811, and one near midday on 17 December 1811. We reinterpret original felt reports to obtain a set of 48 and 20 modified Mercalli intensity values of the two aftershocks, respectively. For the dawn aftershock, we infer a Mw of approximately 7.0 based on a comparison of its intensities with those of the smallest New Madrid mainshock. Based on a detailed account that appears to describe near-field ground motions, we further propose a new fault rupture scenario for the dawn aftershock. We suggest that the aftershock had a thrust mechanism and occurred on a southeastern limb of the Reelfoot fault. For the 17 December 1811 aftershock, we infer a Mw of approximately 6.1 ?? 0.2. This value is determined using the method of Bakun et al. (2002), which is based on a new calibration of intensity versus distance for earthquakes in central and eastern North America. The location of this event is not well constrained, but the available accounts suggest an epicenter beyond the southern end of the New Madrid Seismic Zone.

A catalog of aftershock sequences in Greece (1971 1997): Their spatial and temporal characteristics

NASA Astrophysics Data System (ADS)

Drakatos, George; Latoussakis, John

A complete catalog of aftershock sequences is provided for main earthquakes with ML 5.0, which occurred in the area of Greece and surrounding regions the last twenty-seven years. The Monthly Bulletins of the Institute of Geodynamics (National Observatory of Athens) have been used as data source. In order to get a homogeneous catalog, several selection criteria have been applied and hence a catalog of 44 aftershock sequences is compiled. The relations between the duration of the sequence, the number of aftershocks, the magnitude of the largest aftershock and its delay time from the main shock as well as the subsurface rupture length versus the magnitude of the main shock are calculated. The results show that linearity exists between the subsurface rupture length and the magnitude of the main shock independent of the slip type, as well as between the magnitude of the main shock (M) and its largest aftershock (Ma). The mean difference M-Ma is almost one unit. In the 40% of the analyzed sequences, the largest aftershock occurred within one day after the main shock.The fact that the aftershock sequences show the same behavior for earthquakes that occur in the same region supports the theory that the spatial and temporal characteristics are strongly related to the stress distribution of the fault area.

What Is Better Than Coulomb Failure Stress? A Ranking of Scalar Static Stress Triggering Mechanisms from 105 Mainshock-Aftershock Pairs

NASA Astrophysics Data System (ADS)

Meade, Brendan J.; DeVries, Phoebe M. R.; Faller, Jeremy; Viegas, Fernanda; Wattenberg, Martin

2017-11-01

Aftershocks may be triggered by the stresses generated by preceding mainshocks. The temporal frequency and maximum size of aftershocks are well described by the empirical Omori and Bath laws, but spatial patterns are more difficult to forecast. Coulomb failure stress is perhaps the most common criterion invoked to explain spatial distributions of aftershocks. Here we consider the spatial relationship between patterns of aftershocks and a comprehensive list of 38 static elastic scalar metrics of stress (including stress tensor invariants, maximum shear stress, and Coulomb failure stress) from 213 coseismic slip distributions worldwide. The rates of true-positive and false-positive classification of regions with and without aftershocks are assessed with receiver operating characteristic analysis. We infer that the stress metrics that are most consistent with observed aftershock locations are maximum shear stress and the magnitude of the second and third invariants of the stress tensor. These metrics are significantly better than random assignment at a significance level of 0.005 in over 80% of the slip distributions. In contrast, the widely used Coulomb failure stress criterion is distinguishable from random assignment in only 51-64% of the slip distributions. These results suggest that a number of alternative scalar metrics are better predictors of aftershock locations than classic Coulomb failure stress change.

Repeating aftershocks of the great 2004 Sumatra and 2005 Nias earthquakes

NASA Astrophysics Data System (ADS)

Yu, Wen-che; Song, Teh-Ru Alex; Silver, Paul G.

2013-05-01

We investigate repeating aftershocks associated with the great 2004 Sumatra-Andaman (Mw 9.2) and 2005 Nias-Simeulue (Mw 8.6) earthquakes by cross-correlating waveforms recorded by the regional seismographic station PSI and teleseismic stations. We identify 10 and 18 correlated aftershock sequences associated with the great 2004 Sumatra and 2005 Nias earthquakes, respectively. The majority of the correlated aftershock sequences are located near the down-dip end of a large afterslip patch. We determine the precise relative locations of event pairs among these sequences and estimate the source rupture areas. The correlated event pairs identified are appropriately referred to as repeating aftershocks, in that the source rupture areas are comparable and significantly overlap within a sequence. We use the repeating aftershocks to estimate afterslip based on the slip-seismic moment scaling relationship and to infer the temporal decay rate of the recurrence interval. The estimated afterslip resembles that measured from the near-field geodetic data to the first order. The decay rate of repeating aftershocks as a function of lapse time t follows a power-law decay 1/tp with the exponent p in the range 0.8-1.1. Both types of observations indicate that repeating aftershocks are governed by post-seismic afterslip.

Some statistical features of the aftershock temporal behavior after the M7.4 Izmit earthquake of august 17, 1999 in Turkey

NASA Astrophysics Data System (ADS)

Gospodinov, D.; Fajtin, H.; Rangelov, B.; Marekova, E.

2009-04-01

An earthquake of magnitude Mw=7.4 struck 8 km. southeast of Izmit, Turkey at 3:02 AM local time on August 17, 1999. The earthquake occurred on one of the world's longest and best studied strike-slip (horizontal motion) faults - the east-west trending North Anatolian fault. Seismologists are not able to predict the timing and sizes of individual aftershocks but stochastic modeling allows determinationof probabilities for aftershocks and larger mainshocks duringintervals following the mainshock. The most widely applied stochastic model to depict aftershocks temporal distribution is the non- homogenous Poisson process with a decaying intensity, which follows the Modified Omori Formula (MOF) (Utsu, 1961). A more complex model, considering the triggering potential of each aftershock was developed by Ogata (1988) and it was named Epidemic Type Aftershock Sequence (ETAS) model. Gospodinov and Rotondi (2006) elaborated a Restricted Epidemic Type Aftershock Sequence (RETAS) model. The latter follows the general idea that only aftershocks stronger than some cut-off magnitude possess the capability to induce secondary aftershock activity. In this work we shall consider the Restricted Epidemic Type Aftershock Sequence (RETAS) model, for which the conditional intensity function turns out to be ‘ K0eα(Mi-M0)- λ (t|Ht) = + (t- ti + c)p ti < t Mi ≥ Mth (1) Here the summation occurs for all aftershocks with magnitude bigger than or equal to Mth, which took place before time. Leaving Mth to take all possible values, one can examine all RETAS model versions between the MOF and the ETAS model on the basis of the Akaike Information Criterion AIC (Akaike, 1974) AIC = - 2max log L+ 2k (2) where k is the number of parameters used in the model and logL is the logarithm of the likelihood function. Then for the model providing the best fit, we choose the one with the smallest AIC value. The purpose of this paper is to verify versions of the RETAS model (including the MOF and the ETAS model) for the analysis of the aftershock sequence after the Mw=7.4 Izmit earthquake. The obtained results revealed that the best fit model is ETAS, for which the triggering magnitude coincides with the lower cut-off. In this case each event in the sequence can generate its own secondary events. The analysis points out that about 12 days before the Duzce earthquake a relative quiescence begins which then turns to a relative activization about 5 days before the strong shock. This can be explained by a possible realization of several stronger aftershocks before the Duzce event, which are considered as its foreshocks. Another result which can be outlined is that the MOF model is not good for a sequence in which we have secondary activity (strong aftershock or several main events). For a complex aftershock sequence as is the case for the Izmit-Duzce zone, it is adequate to apply the RETAS model which gives more possibilities to model the aftershock activity in time. This allows to perform a better geotectonic interpretation later. The analysis of the aftershock activity after the strong Izmit earthquake, executed through the RETAS model makes it possible to draw some conclusions when interpreting the results: - The RETAS model supplies a methodology to identify the best fit model of the relaxation temporal development and in our case this is the ETAS model (each aftershock is capable to trigger secondary activity). - The recognized best fit model for the sequence provides an opportunity to identify separate periods of relative quiescence and activization. A quiescence period of several days was identified before the strongest aftershock - the Duzce earthquake. - For standard aftershock sequences the different versions of the RETAS model (ETAS, MOF and the intermediate ones) are closer to each other. For a more complex sequence with strong aftershock activity the RETAS model provides better possibilities to model the temporal aspect of the relaxation process and to perform the most adequate geotectonic interpretation. References Akaike, H., 1974, A new look at the statistical model identification. IEEE Trans. Automat. Contr. AC-19, 716-723. Gospodinov, D., R. Rotondi, 2006, Statistical Analysis of Triggered Seismicity in the Kresna Region of SW Bulgaria (1904) and the Umbria-Marche Region of Central Italy (1997). Pure Appl. Geophys. 163, 1597-1615. Ogata, Y., 1988, Statistical models for earthquake occurrences and residual analysis for point processes. J. Am. Stat. Assoc. 83, 9-27. Utsu, T., 1961, A statistical study on the occurrence of aftershocks, Geophys. Mag., 30, 521-605

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal-faulting earthquake related to a fracture zone in the Pacific plate

NASA Astrophysics Data System (ADS)

Obana, Koichiro; Takahashi, Tsutomu; No, Tetsuo; Kaiho, Yuka; Kodaira, Shuichi; Yamashita, Mikiya; Sato, Takeshi; Nakamura, Takeshi

2014-04-01

describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

The 2010 M w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin

NASA Astrophysics Data System (ADS)

Hauksson, Egill; Stock, Joann; Hutton, Kate; Yang, Wenzheng; Vidal-Villegas, J. Antonio; Kanamori, Hiroo

2011-08-01

The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M > 2 (up to M4.4) that occurred during the 24 h preceding the mainshock. The foreshocks occurred along a north-south trend near the mainshock epicenter. The M w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15 s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The aftershock zone extends for 120 km from the south end of the Elsinore fault zone north of the US-Mexico border almost to the northern tip of the Gulf of California. The waveform-relocated aftershocks form two abutting clusters, each about 50 km long, as well as a 10 km north-south aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large aftershocks is asymmetric with five M5+ aftershocks located to the south of the mainshock, and only one M5.7 aftershock, but numerous smaller aftershocks to the north. Further, the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the aftershocks is similar to the rate of decay of a generic California aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific-North America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.

Detailed fault structure of the 2000 Western Tottori, Japan, earthquake sequence

USGS Publications Warehouse

Fukuyama, E.; Ellsworth, W.L.; Waldhauser, F.; Kubo, A.

2003-01-01

We investigate the faulting process of the aftershock region of the 2000 western Tottori earthquake (Mw 6.6) by combining aftershock hypocenters and moment tensor solutions. Aftershock locations were precisely determined by the double difference method using P- and S-phase arrival data of the Japan Meteorological Agency unified catalog. By combining the relocated hypocenters and moment tensor solutions of aftershocks by broadband waveform inversion of FREESIA (F-net), we successfully resolved very detailed fault structures activated by the mainshock. The estimated fault model resolves 15 individual fault segments that are consistent with both aftershock distribution and focal mechanism solutions. Rupture in the mainshock was principally confined to the three fault elements in the southern half of the zone, which is also where the earliest aftershocks concentrate. With time, the northern part of the zone becomes activated, which is also reflected in the postseismic deformation field. From the stress tensor analysis of aftershock focal mechanisms, we found a rather uniform stress field in the aftershock region, although fault strikes were scattered. The maximum stress direction is N107??E, which is consistent with the tectonic stress field in this region. In the northern part of the fault, where no slip occurred during the mainshock but postseismic slip was observed, the maximum stress direction of N130??E was possible as an alternative solution of stress tensor inversion.

Aftershock collapse vulnerability assessment of reinforced concrete frame structures

USGS Publications Warehouse

Raghunandan, Meera; Liel, Abbie B.; Luco, Nicolas

2015-01-01

In a seismically active region, structures may be subjected to multiple earthquakes, due to mainshock–aftershock phenomena or other sequences, leaving no time for repair or retrofit between the events. This study quantifies the aftershock vulnerability of four modern ductile reinforced concrete (RC) framed buildings in California by conducting incremental dynamic analysis of nonlinear MDOF analytical models. Based on the nonlinear dynamic analysis results, collapse and damage fragility curves are generated for intact and damaged buildings. If the building is not severely damaged in the mainshock, its collapse capacity is unaffected in the aftershock. However, if the building is extensively damaged in the mainshock, there is a significant reduction in its collapse capacity in the aftershock. For example, if an RC frame experiences 4% or more interstory drift in the mainshock, the median capacity to resist aftershock shaking is reduced by about 40%. The study also evaluates the effectiveness of different measures of physical damage observed in the mainshock-damaged buildings for predicting the reduction in collapse capacity of the damaged building in subsequent aftershocks. These physical damage indicators for the building are chosen such that they quantify the qualitative red tagging (unsafe for occupation) criteria employed in post-earthquake evaluation of RC frames. The results indicated that damage indicators related to the drift experienced by the damaged building best predicted the reduced aftershock collapse capacities for these ductile structures.

Comparison of aftershock sequences between 1975 Haicheng earthquake and 1976 Tangshan earthquake

NASA Astrophysics Data System (ADS)

Liu, B.

2017-12-01

The 1975 ML 7.3 Haicheng earthquake and the 1976 ML 7.8 Tangshan earthquake occurred in the same tectonic unit. There are significant differences in spatial-temporal distribution, number of aftershocks and time duration for the aftershock sequence followed by these two main shocks. As we all know, aftershocks could be triggered by the regional seismicity change derived from the main shock, which was caused by the Coulomb stress perturbation. Based on the rate- and state- dependent friction law, we quantitative estimated the possible aftershock time duration with a combination of seismicity data, and compared the results from different approaches. The results indicate that, aftershock time durations from the Tangshan main shock is several times of that form the Haicheng main shock. This can be explained by the significant relationship between aftershock time duration and earthquake nucleation history, normal stressand shear stress loading rateon the fault. In fact the obvious difference of earthquake nucleation history from these two main shocks is the foreshocks. 1975 Haicheng earthquake has clear and long foreshocks, while 1976 Tangshan earthquake did not have clear foreshocks. In that case, abundant foreshocks may mean a long and active nucleation process that may have changed (weakened) the rocks in the source regions, so they should have a shorter aftershock sequences for the reason that stress in weak rocks decay faster.

Stress history controls the spatial pattern of aftershocks: case studies from strike-slip earthquakes

NASA Astrophysics Data System (ADS)

Utkucu, Murat; Durmuş, Hatice; Nalbant, Süleyman

2017-09-01

Earthquake ruptures perturb stress within the surrounding crustal volume and as it is widely accepted now these stress perturbations strongly correlates with the following seismicity. Here we have documented five cases of the mainshock-aftershock sequences generated by the strike-slip faults from different tectonic environments of world in order to demonstrate that the stress changes resulting from large preceding earthquakes decades before effect spatial distribution of the aftershocks of the current mainshocks. The studied mainshock-aftershock sequences are the 15 October 1979 Imperial Valley earthquake ( Mw = 6.4) in southern California, the 27 November 1979 Khuli-Boniabad ( Mw = 7.1), the 10 May 1997 Qa'enat ( Mw = 7.2) and the 31 March 2006 Silakhor ( Mw = 6.1) earthquakes in Iran and the 13 March 1992 Erzincan earthquake ( Mw = 6.7) in Turkey. In the literature, we have been able to find only these mainshocks that are mainly characterized by dense and strong aftershock activities along and beyond the one end of their ruptures while rare aftershock occurrences with relatively lower magnitude reported for the other end of their ruptures. It is shown that the stress changes resulted from earlier mainshock(s) that are close in both time and space might be the reason behind the observed aftershock patterns. The largest aftershocks of the mainshocks studied tend to occur inside the stress-increased lobes that were also stressed by the background earthquakes and not to occur inside the stress-increased lobes that fall into the stress shadow of the background earthquakes. We suggest that the stress shadows of the previous mainshocks may persist in the crust for decades to suppress aftershock distribution of the current mainshocks. Considering active researches about use of the Coulomb stress change maps as a practical tool to forecast spatial distribution of the upcoming aftershocks for earthquake risk mitigation purposes in near-real time, it is further suggested that the background mainshocks along the neighbouring faults should be taken into account in producing the stress change maps for commenting on aftershock occurrences.

The 1992 Landers earthquake sequence; seismological observations

USGS Publications Warehouse

Egill Hauksson,; Jones, Lucile M.; Hutton, Kate; Eberhart-Phillips, Donna

1993-01-01

The (MW6.1, 7.3, 6.2) 1992 Landers earthquakes began on April 23 with the MW6.1 1992 Joshua Tree preshock and form the most substantial earthquake sequence to occur in California in the last 40 years. This sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long. The faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the San Andreas fault. The MW6.1 Joshua Tree preshock at 33°N58′ and 116°W19′ on 0451 UT April 23 was preceded by a tightly clustered foreshock sequence (M≤4.6) beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks. The aftershocks extended along a northerly trend from about 10 km north of the San Andreas fault, northwest of Indio, to the east-striking Pinto Mountain fault. The Mw7.3 Landers mainshock occurred at 34°N13′ and 116°W26′ at 1158 UT, June 28, 1992, and was preceded for 12 hours by 25 small M≤3 earthquakes at the mainshock epicenter. The distribution of more than 20,000 aftershocks, analyzed in this study, and short-period focal mechanisms illuminate a complex sequence of faulting. The aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults, and 40 km to the south, crossing the Pinto Mountain fault through the Joshua Tree aftershock zone towards the San Andreas fault near Indio. The rupture initiated in the depth range of 3–6 km, similar to previous M∼5 earthquakes in the region, although the maximum depth of aftershocks is about 15 km. The mainshock focal mechanism showed right-lateral strike-slip faulting with a strike of N10°W on an almost vertical fault. The rupture formed an arclike zone well defined by both surficial faulting and aftershocks, with more westerly faulting to the north. This change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west. A 20-km-long linear cluster of aftershocks occurred 10–20 km north of Barstow, or 30–40 km north of the end of the mainshock rupture. The most prominent off-fault aftershock cluster occurred 30 km to the west of the Landers mainshock. The largest aftershock was within this cluster, the Mw6.2 Big Bear aftershock occurring at 34°N10′ and 116°W49′ at 1505 UT June 28. It exhibited left-lateral strike-slip faulting on a northeast striking and steeply dipping plane. The Big Bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north. The Landers mainshock occurred near the southernmost extent of the Eastern California Shear Zone, an 80-km-wide, more than 400-km-long zone of deformation. This zone extends into the Death Valley region and accommodates about 10 to 20% of the plate motion between the Pacific and North American plates. The Joshua Tree preshock, its aftershocks, and Landers aftershocks form a previously missing link that connects the Eastern California Shear Zone to the southern San Andreas fault.

An Explosion Aftershock Model with Application to On-Site Inspection

NASA Astrophysics Data System (ADS)

Ford, Sean R.; Labak, Peter

2016-01-01

An estimate of aftershock activity due to a theoretical underground nuclear explosion is produced using an aftershock rate model. The model is developed with data from the Nevada National Security Site, formerly known as the Nevada Test Site, and the Semipalatinsk Test Site, which we take to represent soft-rock and hard-rock testing environments, respectively. Estimates of expected magnitude and number of aftershocks are calculated using the models for different testing and inspection scenarios. These estimates can help inform the Seismic Aftershock Monitoring System (SAMS) deployment in a potential Comprehensive Test Ban Treaty On-Site Inspection (OSI), by giving the OSI team a probabilistic assessment of potential aftershocks in the Inspection Area (IA). The aftershock assessment, combined with an estimate of the background seismicity in the IA and an empirically derived map of threshold magnitude for the SAMS network, could aid the OSI team in reporting. We apply the hard-rock model to a M5 event and combine it with the very sensitive detection threshold for OSI sensors to show that tens of events per day are expected up to a month after an explosion measured several kilometers away.

An explosion aftershock model with application to on-site inspection

DOE PAGES

Ford, Sean R.; Labak, Peter

2015-02-14

An estimate of aftershock activity due to a theoretical underground nuclear explosion is produced using an aftershock rate model. The model is developed with data from the Nevada National Security Site, formerly known as the Nevada Test Site, and the Semipalatinsk Test Site, which we take to represent soft-rock and hard-rock testing environments, respectively. Estimates of expected magnitude and number of aftershocks are calculated using the models for different testing and inspection scenarios. These estimates can help inform the Seismic Aftershock Monitoring System (SAMS) deployment in a potential Comprehensive Test Ban Treaty On-Site Inspection (OSI), by giving the OSI teammore » a probabilistic assessment of potential aftershocks in the Inspection Area (IA). The aftershock assessment, combined with an estimate of the background seismicity in the IA and an empirically derived map of threshold magnitude for the SAMS network, could aid the OSI team in reporting. Here, we apply the hard-rock model to a M5 event and combine it with the very sensitive detection threshold for OSI sensors to show that tens of events per day are expected up to a month after an explosion measured several kilometers away.« less

Forecasting the (un)productivity of the 2014 M 6.0 South Napa aftershock sequence

USGS Publications Warehouse

Llenos, Andrea L.; Michael, Andrew J.

2017-01-01

The 24 August 2014 Mw 6.0 South Napa mainshock produced fewer aftershocks than expected for a California earthquake of its magnitude. In the first 4.5 days, only 59 M≥1.8 aftershocks occurred, the largest of which was an M 3.9 that happened a little over two days after the mainshock. We investigate the aftershock productivity of the South Napa sequence and compare it with other M≥5.5 California strike‐slip mainshock–aftershock sequences. While the productivity of the South Napa sequence is among the lowest, northern California mainshocks generally have fewer aftershocks than mainshocks further south, although the productivities vary widely in both regions. An epidemic‐type aftershock sequence (ETAS) model (Ogata, 1988) fit to Napa seismicity from 1980 to 23 August 2014 fits the sequence well and suggests that low‐productivity sequences are typical of this area. Utilizing regional variations in productivity could improve operational earthquake forecasting (OEF) by improving the model used immediately after the mainshock. We show this by comparing the daily rate of M≥2 aftershocks to forecasts made with the generic California model (Reasenberg and Jones, 1989; hereafter, RJ89), RJ89 models with productivity updated daily, a generic California ETAS model, an ETAS model based on premainshock seismicity, and ETAS models updated daily following the mainshock. RJ89 models for which only the productivity is updated provide better forecasts than the generic RJ89 California model, and the Napa‐specific ETAS models forecast the aftershock rates more accurately than either generic model. Therefore, forecasts that use localized initial parameters and that rapidly update the productivity may be better for OEF than using a generic model and/or updating all parameters.

Aftershock decay, productivity, and stress rates in Hawaii: Indicators of temperature and stress from magma sources

USGS Publications Warehouse

Klein, Fred W.; Wright, Tom; Nakata, Jennifer

2006-01-01

We examined dozens of aftershock sequences in Hawaii in terms of Gutenberg-Richter and modified Omori law parameters. We studied p, the rate of aftershock decay; Ap, the aftershock productivity, defined as the observed divided by the expected number of aftershocks; and c, the time delay when aftershock rates begin to fall. We found that for earthquakes shallower than 20 km, p values >1.2 are near active magma centers. We associate this high decay rate with higher temperatures and faster stress relaxation near magma reservoirs. Deep earthquakes near Kilauea's inferred magma transport path show a range of p values, suggesting the absence of a large, deep magma reservoir. Aftershock productivity is >4.0 for flank earthquakes known to be triggered by intrusions but is normal (0.25 to 4.0) for isolated main shocks. We infer that continuing, post-main shock stress from the intrusion adds to the main shock's stress step and causes higher Ap. High Ap in other zones suggests less obvious intrusions and pulsing magma pressure near Kilauea's feeding conduit. We calculate stress rates and stress rate changes from pre-main shock and aftershock rates. Stress rate increased after many intrusions but decreased after large M7–8 earthquakes. Stress rates are highest in the seismically active volcano flanks and lowest in areas far from volcanic centers. We found sequences triggered by intrusions tend to have high Ap, high (>0.10 day) c values, a stress rate increase, and sometimes a peak in aftershock rate hours after the main shock. We interpret these values as indicating continuing intrusive stress after the main shock.

A case study of two M~5 mainshocks in Anza, California: Is the footprint of an aftershock sequence larger than we think?

USGS Publications Warehouse

Fritts, Karen R.; Kilb, Debi

2009-01-01

It has been traditionally held that aftershocks occur within one to two fault lengths of the mainshock. Here we demonstrate that this perception has been shaped by the sensitivity of seismic networks. The 31 October 2001 Mw 5.0 and 12 June 2005 Mw 5.2 Anza mainshocks in southern California occurred in the middle of the densely instrumented ANZA seismic network and thus were unusually well recorded. For the June 2005 event, aftershocks as small as M 0.0 could be observed stretching for at least 50 km along the San Jacinto fault even though the mainshock fault was only ∼4.5 km long. It was hypothesized that an observed aseismic slipping patch produced a spatially extended aftershock-triggering source, presumably slowing the decay of aftershock density with distance and leading to a broader aftershock zone. We find, however, the decay of aftershock density with distance for both Anza sequences to be similar to that observed elsewhere in California. This indicates there is no need for an additional triggering mechanism and suggests that given widespread dense instrumentation, aftershock sequences would routinely have footprints much larger than currently expected. Despite the large 2005 aftershock zone, we find that the probability that the 2005 Anza mainshock triggered the M 4.9 Yucaipa mainshock, which occurred 4.2 days later and 72 km away, to be only 14%±1%. This probability is a strong function of the time delay; had the earthquakes been separated by only an hour, the probability of triggering would have been 89%.

Aftershock Energy Distribution by Statistical Mechanics Approach

NASA Astrophysics Data System (ADS)

Daminelli, R.; Marcellini, A.

2015-12-01

The aim of our work is to research the most probable distribution of the energy of aftershocks. We started by applying one of the fundamental principles of statistical mechanics that, in case of aftershock sequences, it could be expressed as: the greater the number of different ways in which the energy of aftershocks can be arranged among the energy cells in phase space the more probable the distribution. We assume that each cell in phase space has the same possibility to be occupied, and that more than one cell in the phase space can have the same energy. Seeing that seismic energy is proportional to products of different parameters, a number of different combinations of parameters can produce different energies (e.g., different combination of stress drop and fault area can release the same seismic energy). Let us assume that there are gi cells in the aftershock phase space characterised by the same energy released ɛi. Therefore we can assume that the Maxwell-Boltzmann statistics can be applied to aftershock sequences with the proviso that the judgment on the validity of this hypothesis is the agreement with the data. The aftershock energy distribution can therefore be written as follow: n(ɛ)=Ag(ɛ)exp(-βɛ)where n(ɛ) is the number of aftershocks with energy, ɛ, A and β are constants. Considering the above hypothesis, we can assume g(ɛ) is proportional to ɛ. We selected and analysed different aftershock sequences (data extracted from Earthquake Catalogs of SCEC, of INGV-CNT and other institutions) with a minimum magnitude retained ML=2 (in some cases ML=2.6) and a time window of 35 days. The results of our model are in agreement with the data, except in the very low energy band, where our model resulted in a moderate overestimation.

On the origin of diverse aftershock mechanisms following the 1989 Loma Prieta earthquake

USGS Publications Warehouse

Kilb, Debi; Ellis, M.; Gomberg, J.; Davis, S.

1997-01-01

We test the hypothesis that the origin of the diverse suite of aftershock mechanisms following the 1989 M 7.1 Loma Prieta, California, earthquake is related to the post-main-shock static stress field. We use a 3-D boundary-element algorithm to calculate static stresses, combined with a Coulomb failure criterion to calculate conjugate failure planes at aftershock locations. The post-main-shock static stress field is taken as the sum of a pre-existing stress field and changes in stress due to the heterogeneous slip across the Loma Prieta rupture plane. The background stress field is assumed to be either a simple shear parallel to the regional trend of the San Andreas fault or approximately fault-normal compression. A suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared (allowing for uncertainties in both mechanism parameters and earthquake locations) to well-constrained mechanisms reported in the US Geological Survey Northern California Seismic Network catalogue. We also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes, assuming either plane to be a likely rupture plane. Various permutations of the assumed background stress field, frictional coefficients of aftershock fault planes, methods of comparisons, etc. explain between 52 and 92 per cent of the aftershock mechanisms. We can explain a similar proportion of mechanisms however by comparing a randomly reordered catalogue with the various suites of synthetic aftershocks. The inability to duplicate aftershock mechanisms reliably on a one-to-one basis is probably a function of the combined uncertainties in models of main-shock slip distribution, the background stress field, and aftershock locations. In particular we show theoretically that any specific main-shock slip distribution and a reasonable background stress field are able to generate a highly variable suite of failure planes such that quite different aftershock mechanisms may be expected to occur within a kilometre or less of each other. This scale of variability is less than the probable location error of aftershock earthquakes in the Loma Prieta region. We successfully duplicate a measure of the variability in the mechanisms of the entire suite of aftershocks. If static stress changes are responsible for the generation of aftershock mechanisms, we are able to place quantitative constraints on the level of stress that must have existed in the upper crust prior to the Loma Prieta rupture. This stress level appears to be too low to generate the average slip across the main-shock rupture plane. Possible reasons for this result range from incorrect initial assumptions of homogeneity in the background stress field, friction and fault geometry to driving stresses that arise from deeper in the crust or upper mantle. Alternatively, aftershock focal mechanisms may be determined by processes other than, or in addition to, static stress changes, such as pore-pressure changes or dynamic stresses.

Aftershock patterns and main shock faulting

USGS Publications Warehouse

Mendoza, C.; Hartzell, S.H.

1988-01-01

We have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms. Our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault. Aftershocks followng earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip. The spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture. -from Authors

Cataloging the 1811-1812 New Madrid, central U.S., earthquake sequence

USGS Publications Warehouse

Hough, S.E.

2009-01-01

The three principal New Madrid, central U.S., mainshocks of 1811-1812 were followed by extensive aftershock sequences that included numerous felt events. Although no instrumental data are available for the sequence, historical accounts provide information that can be used to estimate magnitudes and locations for the large aftershocks as well as the mainshocks. Several detailed eyewitness accounts of the sequence provide sufficient information to identify times and rough magnitude estimates for a number of aftershocks that have not been analyzed previously. I also use three extended compilations of felt events to explore the overall sequence productivity. Although one generally cannot estimate magnitudes or locations for individual events, the intensity distributions of recent, instrumentally recorded earthquakes in the region provide a basis for estimation of the magnitude distribution of 1811-1812 aftershocks. The distribution is consistent with a b-value distribution. I estimate Mw 6-6.3 for the three largest identifiable aftershocks, apart from the so-called dawn aftershock on 16 December 1811.

Aftershocks and triggered events of the Great 1906 California earthquake

USGS Publications Warehouse

Meltzner, A.J.; Wald, D.J.

2003-01-01

The San Andreas fault is the longest fault in California and one of the longest strike-slip faults in the world, yet little is known about the aftershocks following the most recent great event on the San Andreas, the Mw 7.8 San Francisco earthquake on 18 April 1906. We conducted a study to locate and to estimate magnitudes for the largest aftershocks and triggered events of this earthquake. We examined existing catalogs and historical documents for the period April 1906 to December 1907, compiling data on the first 20 months of the aftershock sequence. We grouped felt reports temporally and assigned modified Mercalli intensities for the larger events based on the descriptions judged to be the most reliable. For onshore and near-shore events, a grid-search algorithm (derived from empirical analysis of modern earthquakes) was used to find the epicentral location and magnitude most consistent with the assigned intensities. For one event identified as far offshore, the event's intensity distribution was compared with those of modern events, in order to contrain the event's location and magnitude. The largest aftershock within the study period, an M ???6.7 event, occurred ???100 km west of Eureka on 23 April 1906. Although not within our study period, another M ???6.7 aftershock occurred near Cape Mendocino on 28 October 1909. Other significant aftershocks included an M ???5.6 event near San Juan Bautista on 17 May 1906 and an M ???6.3 event near Shelter Cove on 11 August 1907. An M ???4.9 aftershock occurred on the creeping segment of the San Andreas fault (southeast of the mainshock rupture) on 6 July 1906. The 1906 San Francisco earthquake also triggered events in southern California (including separate events in or near the Imperial Valley, the Pomona Valley, and Santa Monica Bay), in western Nevada, in southern central Oregon, and in western Arizona, all within 2 days of the mainshock. Of these trigerred events, the largest were an M ???6.1 earthquake near Brawley and an M ???5.0 event under or near Santa Monica Bay, 11.3 and 31.3 hr after the San Francisco mainshock, respectively. The western Arizona event is inferred to have been triggered dynamically. In general, the largest aftershocks occurred at the ends of the 1906 rupture or away from the rupture entirely; very few significant aftershocks occurred along the mainshock rupture itself. The total number of large aftershocks was less than predicted by a generic model based on typical California mainshock-aftershock statistics, and the 1906 sequence appears to have decayed more slowly than average California sequences. Similarities can be drawn between the 1906 aftershock sequence and that of the 1857 (Mw 7.9) San Andreas fault earthquake.

Rapid Estimates of Rupture Extent for Large Earthquakes Using Aftershocks

NASA Astrophysics Data System (ADS)

Polet, J.; Thio, H. K.; Kremer, M.

2009-12-01

The spatial distribution of aftershocks is closely linked to the rupture extent of the mainshock that preceded them and a rapid analysis of aftershock patterns therefore has potential for use in near real-time estimates of earthquake impact. The correlation between aftershocks and slip distribution has frequently been used to estimate the fault dimensions of large historic earthquakes for which no, or insufficient, waveform data is available. With the advent of earthquake inversions that use seismic waveforms and geodetic data to constrain the slip distribution, the study of aftershocks has recently been largely focused on enhancing our understanding of the underlying mechanisms in a broader earthquake mechanics/dynamics framework. However, in a near real-time earthquake monitoring environment, in which aftershocks of large earthquakes are routinely detected and located, these data may also be effective in determining a fast estimate of the mainshock rupture area, which would aid in the rapid assessment of the impact of the earthquake. We have analyzed a considerable number of large recent earthquakes and their aftershock sequences and have developed an effective algorithm that determines the rupture extent of a mainshock from its aftershock distribution, in a fully automatic manner. The algorithm automatically removes outliers by spatial binning, and subsequently determines the best fitting “strike” of the rupture and its length by projecting the aftershock epicenters onto a set of lines that cross the mainshock epicenter with incremental azimuths. For strike-slip or large dip-slip events, for which the surface projection of the rupture is recti-linear, the calculated strike correlates well with the strike of the fault and the corresponding length, determined from the distribution of aftershocks projected onto the line, agrees well with the rupture length. In the case of a smaller dip-slip rupture with an aspect ratio closer to 1, the procedure gives a measure of the rupture extent and dimensions, but not necessarily the strike. We found that using standard earthquake catalogs, such as the National Earthquake Information Center catalog, we can constrain the rupture extent, rupture direction, and in many cases the type of faulting, of the mainshock with the aftershocks that occur within the first hour after the mainshock. However, this data may not be currently available in near real-time. Since our results show that these early aftershock locations may be used to estimate first order rupture parameters for large global earthquakes, the near real-time availability of these data would be useful for fast earthquake damage assessment.

Seismotectonic framework of the 2010 February 27 Mw 8.8 Maule, Chile earthquake sequence

USGS Publications Warehouse

Hayes, Gavin P.; Bergman, Eric; Johnson, Kendra J.; Benz, Harley M.; Brown, Lucy; Meltzer, Anne S.

2013-01-01

After the 2010 Mw 8.8 Maule earthquake, an international collaboration involving teams and instruments from Chile, the US, the UK, France and Germany established the International Maule Aftershock Deployment temporary network over the source region of the event to facilitate detailed, open-access studies of the aftershock sequence. Using data from the first 9-months of this deployment, we have analyzed the detailed spatial distribution of over 2500 well-recorded aftershocks. All earthquakes have been relocated using a hypocentral decomposition algorithm to study the details of and uncertainties in both their relative and absolute locations. We have computed regional moment tensor solutions for the largest of these events to produce a catalogue of 465 mechanisms, and have used all of these data to study the spatial distribution of the aftershock sequence with respect to the Chilean megathrust. We refine models of co-seismic slip distribution of the Maule earthquake, and show how small changes in fault geometries assumed in teleseismic finite fault modelling significantly improve fits to regional GPS data, implying that the accuracy of rapid teleseismic fault models can be substantially improved by consideration of existing fault geometry model databases. We interpret all of these data in an integrated seismotectonic framework for the Maule earthquake rupture and its aftershock sequence, and discuss the relationships between co-seismic rupture and aftershock distributions. While the majority of aftershocks are interplate thrust events located away from regions of maximum co-seismic slip, interesting clusters of aftershocks are identified in the lower plate at both ends of the main shock rupture, implying internal deformation of the slab in response to large slip on the plate boundary interface. We also perform Coulomb stress transfer calculations to compare aftershock locations and mechanisms to static stress changes following the Maule rupture. Without the incorporation of uncertainties in earthquake locations, just 55 per cent of aftershock nodal planes align with faults promoted towards failure by co-seismic slip. When epicentral uncertainties are considered (on the order of just ±2–3 km), 90 per cent of aftershocks are consistent with occurring along faults demonstrating positive stress transfer. These results imply large sensitivities of Coulomb stress transfer calculations to uncertainties in both earthquake locations and models of slip distributions, particularly when applied to aftershocks close to a heterogeneous fault rupture; such uncertainties should therefore be considered in similar studies used to argue for or against models of static stress triggering.

Foreshocks, aftershocks, and earthquake probabilities: Accounting for the landers earthquake

USGS Publications Warehouse

Jones, Lucile M.

1994-01-01

The equation to determine the probability that an earthquake occurring near a major fault will be a foreshock to a mainshock on that fault is modified to include the case of aftershocks to a previous earthquake occurring near the fault. The addition of aftershocks to the background seismicity makes its less probable that an earthquake will be a foreshock, because nonforeshocks have become more common. As the aftershocks decay with time, the probability that an earthquake will be a foreshock increases. However, fault interactions between the first mainshock and the major fault can increase the long-term probability of a characteristic earthquake on that fault, which will, in turn, increase the probability that an event is a foreshock, compensating for the decrease caused by the aftershocks.

Early warning system for aftershocks

USGS Publications Warehouse

Bakun, W.H.; Fischer, F.G.; Jensen, E.G.; VanSchaack, J.

1994-01-01

A prototype early warning system to provide San Francisco and Oakland, California a few tens-of-seconds warning of incoming strong ground shaking from already-occurred M ≧ 3.7 aftershocks of the magnitude 7.1 17 October 1989 Loma Prieta earthquake was operational on 28 October 1989. The prototype system consisted of four components: ground motion sensors in the epicentral area, a central receiver, a radio repeater, and radio receivers. One of the radio receivers was deployed at the California Department of Transportation (CALTRANS) headquarters at the damaged Cypress Street section of the I-880 freeway in Oakland, California on 28 October 1989 and provided about 20 sec of warning before shaking from the M 4.5 Loma Prieta aftershock that occurred on 2 November 1989 at 0550 UTC. In its first 6 months of operation, the system generated triggers for all 12 M > 3.7 aftershocks for which trigger documentation is preserved, did not trigger on any M ≦ 3.6 aftershocks, and produced one false trigger as a result of a now-corrected single point of failure design flaw. Because the prototype system demonstrated that potentially useful warnings of strong shaking from aftershocks are feasible, the USGS has completed a portable early warning system for aftershocks that can be deployed anywhere.

Characterization of Aftershock Sequences from Large Strike-Slip Earthquakes Along Geometrically Complex Faults

NASA Astrophysics Data System (ADS)

Sexton, E.; Thomas, A.; Delbridge, B. G.

2017-12-01

Large earthquakes often exhibit complex slip distributions and occur along non-planar fault geometries, resulting in variable stress changes throughout the region of the fault hosting aftershocks. To better discern the role of geometric discontinuities on aftershock sequences, we compare areas of enhanced and reduced Coulomb failure stress and mean stress for systematic differences in the time dependence and productivity of these aftershock sequences. In strike-slip faults, releasing structures, including stepovers and bends, experience an increase in both Coulomb failure stress and mean stress during an earthquake, promoting fluid diffusion into the region and further failure. Conversely, Coulomb failure stress and mean stress decrease in restraining bends and stepovers in strike-slip faults, and fluids diffuse away from these areas, discouraging failure. We examine spatial differences in seismicity patterns along structurally complex strike-slip faults which have hosted large earthquakes, such as the 1992 Mw 7.3 Landers, the 2010 Mw 7.2 El-Mayor Cucapah, the 2014 Mw 6.0 South Napa, and the 2016 Mw 7.0 Kumamoto events. We characterize the behavior of these aftershock sequences with the Epidemic Type Aftershock-Sequence Model (ETAS). In this statistical model, the total occurrence rate of aftershocks induced by an earthquake is λ(t) = λ_0 + \\sum_{i:t_i

Comment on "Localized water reverberation phases and its impact on back-projection images" by Yue et al. [2017

NASA Astrophysics Data System (ADS)

Fan, W.; Shearer, P. M.

2017-12-01

Fan and Shearer [2016] analyzed the 2012 Mw 7.2 Sumatra earthquake and reported that the earthquake dynamically triggered early aftershock/aftershocks 150 km away from the mainshock and 50 s later. The early aftershock/aftershocks were detected with teleseismic P-wave back-projection, coincided with passing surface waves, and showed observable seismic waveforms in a wide frequency range (0.02—5 Hz). Recently, however, Yue et al. [2017] interpreted these coda arrivals as water reverberations from the mainshock, based mostly on EGF analysis of a nearby M6 earthquake and a water-phase synthetic test. Here, we show detailed back-projection and waveform analysis of three M6 earthquakes within 100km of the Mw 7.2 earthquake, including the EGF event analyzed in Yue et al. [2017]. In addition, we examine the waveforms of three M5.5 reverse faulting earthquakes close to our detected early aftershock landward of the trench. Our results show that the coda energy in question is more likely caused by a separate earthquake near the trench than by a mainshock water reverberation phase, thus supporting our earlier conclusion that the detected coherent radiators are likely to be dynamically triggered early aftershock/aftershocks.

Tests of remote aftershock triggering by small mainshocks using Taiwan's earthquake catalog

NASA Astrophysics Data System (ADS)

Peng, W.; Toda, S.

2014-12-01

To understand earthquake interaction and forecast time-dependent seismic hazard, it is essential to evaluate which stress transfer, static or dynamic, plays a major role to trigger aftershocks and subsequent mainshocks. Felzer and Brodsky focused on small mainshocks (2≤M<3) and their aftershocks, and then argued that only dynamic stress change brings earthquake-to-earthquake triggering, whereas Richards-Dingers et al. (2010) claimed that those selected small mainshock-aftershock pairs were not earthquake-to-earthquake triggering but simultaneous occurrence of independent aftershocks following a larger earthquake or during a significant swarm sequence. We test those hypotheses using Taiwan's earthquake catalog by taking the advantage of lacking any larger event and the absence of significant seismic swarm typically seen with active volcano. Using Felzer and Brodsky's method and their standard parameters, we only found 14 mainshock-aftershock pairs occurred within 20 km distance in Taiwan's catalog from 1994 to 2010. Although Taiwan's catalog has similar number of earthquakes as California's, the number of pairs is about 10% of the California catalog. It may indicate the effect of no large earthquakes and no significant seismic swarm in the catalog. To fully understand the properties in the Taiwan's catalog, we loosened the screening parameters to earn more pairs and then found a linear aftershock density with a power law decay of -1.12±0.38 that is very similar to the one in Felzer and Brodsky. However, none of those mainshock-aftershock pairs were associated with a M7 rupture event or M6 events. To find what mechanism controlled the aftershock density triggered by small mainshocks in Taiwan, we randomized earthquake magnitude and location. We then found that those density decay in a short time period is more like a randomized behavior than mainshock-aftershock triggering. Moreover, 5 out of 6 pairs were found in a swarm-like temporal seismicity rate increase. They locate mostly in high geothermal gradient areas, which are probably triggered by a small-scale aseismic process. Thus it rather supports the argument of Richards-Dingers et al. in which dynamic triggering by small mainshock is untenable.

Spectral scaling of the aftershocks of the Tocopilla 2007 earthquake in northern Chile

NASA Astrophysics Data System (ADS)

Lancieri, M.; Madariaga, R.; Bonilla, F.

2012-04-01

We study the scaling of spectral properties of a set of 68 aftershocks of the 2007 November 14 Tocopilla (M 7.8) earthquake in northern Chile. These are all subduction events with similar reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments. The seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega-square spectral decay; radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect. Using a graphical approach, we test the scaling of seismic spectrum, and the scale invariance of the apparent stress drop with the earthquake size. To test whether the Tocopilla aftershocks scale with a single parameter, we introduce a non-dimensional number, ?, that should be constant if earthquakes are self-similar. For the Tocopilla aftershocks, Cr varies by a factor of 2. More interestingly, Cr for the aftershocks is close to 2, the value that is expected for events that are approximately modelled by a circular crack. Thus, in spite of obvious differences in waveforms, the aftershocks of the Tocopilla earthquake are self-similar. The main shock is different because its records contain large near-field waves. Finally, we investigate the scaling of energy release rate, Gc, with the slip. We estimated Gc from our previous estimates of the source parameters, assuming a simple circular crack model. We find that Gc values scale with the slip, and are in good agreement with those found by Abercrombie and Rice for the Northridge aftershocks.

Mapping the rheology of the Central Chile subduction zone with aftershocks

NASA Astrophysics Data System (ADS)

Frank, William B.; Poli, Piero; Perfettini, Hugo

2017-06-01

The postseismic deformation following a large (Mw >7) earthquake is expressed both seismically and aseismically. Recent studies have appealed to a model that suggests that the aseismic slip on the plate interface following the mainshock can be the driving factor in aftershock sequences, reproducing both the geodetic (afterslip) and seismic (aftershocks) observables of postseismic deformation. Exploiting this model, we demonstrate how a dense catalog of aftershocks following the 2015 Mw 8.3 Illapel earthquake in Central Chile can constrain the frictional and rheological properties of the creeping regions of the subduction interface. We first expand the aftershock catalog via a 19 month continuous matched-filter search and highlight the log-time expansion of seismicity following the mainshock, suggestive of afterslip as the main driver of aftershock activity. We then show how the time history of aftershocks can constrain the temporal evolution of afterslip. Finally, we use our dense aftershock catalog to estimate the rate and state rheological parameter (a - b)σ as a function of depth and demonstrate that this low value is compatible either with a nearly velocity-neutral friction (a≈b) in the regions of the megathrust that host afterslip, or an elevated pore fluid pressure (low effective normal stress σ) along the plate interface. Our results present the first snapshot of rheology in depth together with the evolution of the tectonic stressing rate along a plate boundary. The framework described here can be generalized to any tectonic context and provides a novel way to constrain the frictional properties and loading conditions of active faults.

Heterogeneity of direct aftershock productivity of the main shock rupture

NASA Astrophysics Data System (ADS)

Guo, Yicun; Zhuang, Jiancang; Hirata, Naoshi; Zhou, Shiyong

2017-07-01

The epidemic type aftershock sequence (ETAS) model is widely used to describe and analyze the clustering behavior of seismicity. Instead of regarding large earthquakes as point sources, the finite-source ETAS model treats them as ruptures that extend in space. Each earthquake rupture consists of many patches, and each patch triggers its own aftershocks isotropically. We design an iterative algorithm to invert the unobserved fault geometry based on the stochastic reconstruction method. This model is applied to analyze the Japan Meteorological Agency (JMA) catalog during 1964-2014. We take six great earthquakes with magnitudes >7.5 after 1980 as finite sources and reconstruct the aftershock productivity patterns on each rupture surface. Comparing results from the point-source ETAS model, we find the following: (1) the finite-source model improves the data fitting; (2) direct aftershock productivity is heterogeneous on the rupture plane; (3) the triggering abilities of M5.4+ events are enhanced; (4) the background rate is higher in the off-fault region and lower in the on-fault region for the Tohoku earthquake, while high probabilities of direct aftershocks distribute all over the source region in the modified model; (5) the triggering abilities of five main shocks become 2-6 times higher after taking the rupture geometries into consideration; and (6) the trends of the cumulative background rate are similar in both models, indicating the same levels of detection ability for seismicity anomalies. Moreover, correlations between aftershock productivity and slip distributions imply that aftershocks within rupture faults are adjustments to coseismic stress changes due to slip heterogeneity.

The southeastern Illinois earthquake of 10 June 1987: the later aftershocks

USGS Publications Warehouse

Langer, C.J.; Bollinger, G.A.

1991-01-01

The 10 June 1987 southeastern Illinois earthquake (mbLg=5.2) was located about 200 km east of St Louis, Missouri, caused minor damage in the epicentral area, had a contiguous felt area of about 433 000 km2, and had a total felt area over 1 million km2. Within 47 hours after the main shock, a 15-station aftershock monitoring network (later expanded to 21 instruments) was installed that recorded more than 100 aftershocks in the folllowing 4-day period. Results from the 56 aftershocks that were well located indicate a compact, cylindrically shaped aftershock volume about 1.7 km long, 0.8 km wide, and with a vertical distribution between about 9 and 12 km in depth. Composite focal mechanism solutions of the aftershocks suggest that the predominant mode of faulting is reverse slip, but some strike-slip type motion occurred similar to the mechanism for the main shock as determined from teleseismic data. The maximum principal compressive stress (P axes) is oriented E-ESE and is subhorizontal in plunge. -from Authors

Aftershocks of the 13 May 1993 Shumagin Alaska earthquake

USGS Publications Warehouse

Lu, Zhong; Wyss, Max; Tytgat, Guy; McNutt, Steve; Stihler, Scott

1994-01-01

The 13 May 1993 Ms 6.9 Shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1.5 by 1 June 1993. Of these aftershocks, 79 were located by using S-P travel times at the only two stations within 570 km of the mainshock epicenter. The rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1.0 m and a 28 bar stress drop. The magnitude-frequency plots give a b-value for the aftershock sequence of about 0.4, which is low compared to the background value of approximately 0.8. The decay of the aftershock sequence followed the modified Omori law with a p-value of 0.79, which is also lower than the typical values of about 1.1 observed in Alaska. Both of these facts can be interpreted as indicating relatively high ambient stress in the Shumagin seismic gap and the possibility that the 13 May earthquake was a foreshock to a larger gap-filling event to occur within the next few years.

The Use of Explosion Aftershock Probabilities for Planning and Deployment of Seismic Aftershock Monitoring System for an On-site Inspection

NASA Astrophysics Data System (ADS)

Labak, P.; Ford, S. R.; Sweeney, J. J.; Smith, A. T.; Spivak, A.

2011-12-01

One of four elements of CTBT verification regime is On-site inspection (OSI). Since the sole purpose of an OSI shall be to clarify whether a nuclear weapon test explosion or any other nuclear explosion has been carried out, inspection activities can be conducted and techniques used in order to collect facts to support findings provided in inspection reports. Passive seismological monitoring, realized by the seismic aftershock monitoring (SAMS) is one of the treaty allowed techniques during an OSI. Effective planning and deployment of SAMS during the early stages of an OSI is required due to the nature of possible events recorded and due to the treaty related constrains on size of inspection area, size of inspection team and length of an inspection. A method, which may help in planning the SAMS deployment is presented. An estimate of aftershock activity due to a theoretical underground nuclear explosion is produced using a simple aftershock rate model (Ford and Walter, 2010). The model is developed with data from the Nevada Test Site and Semipalatinsk Test Site, which we take to represent soft- and hard-rock testing environments, respectively. Estimates of expected magnitude and number of aftershocks are calculated using the models for different testing and inspection scenarios. These estimates can help to plan the SAMS deployment for an OSI by giving a probabilistic assessment of potential aftershocks in the Inspection Area (IA). The aftershock assessment combined with an estimate of the background seismicity in the IA and an empirically-derived map of threshold magnitude for the SAMS network could aid the OSI team in reporting. We tested the hard-rock model to a scenario similar to the 2008 Integrated Field Exercise 2008 deployment in Kazakhstan and produce an estimate of possible recorded aftershock activity.

A detailed study of the Pernik (Bulgaria) seismic sequence of 2012

NASA Astrophysics Data System (ADS)

Raykova, Plamena; Solakov, Dimcho; Simeonova, Stela; Dimitrova, Liliya

2014-05-01

A detailed study of the Pernik (Bulgaria) seismic sequence of 2012 D.Solakov, S.Simeonova ,I. Georgiev, P.Raykova, L.Dimitrova and V.Protopopova National Institute of Geophysics, Geodesy and Geography-BAS, Sofia, Bulgaria The spatial and temporal clustering of aftershocks is the dominant non-random element of seismicity, so that when aftershocks are removed, the remaining activity can be modelled (as first approximation) as a Poisson process. The properties of aftershock sequences (distinct cluster, for example; even aftershocks can have aftershocks) allow time-dependent prediction of aftershock probabilities. Consideration of recent earthquake sequences suggests that aftershocks to large earthquakes although they are still, by definition, smaller events, can be very damaging and should be addressed in emergence planning scenarios. Because of the factors such as location and radiation pattern and the cumulative nature of building damage, aftershocks can cause more damage than the main shock. An earthquake of moment magnitude 5.6 hit Sofia seismic zone, on May 22nd, 2012. The earthquake occurred in the vicinity of Pernik city, at about 25 km south west of the city of Sofia (the capital of Bulgaria). The event was followed by intensive activity. The active area is situated in the central part of western Bulgaria. That is the most populated (more than 1.2 mil. inhabitants), industrial and cultural region of Bulgaria. Seismicity in the zone is related to the marginal neotectonic faults of Sofia graben. The boundaries of the graben are represented by SE-NW fault system with expressive neotectonic activity. This zone is characterized by shallow earthquakes. The strongest known event in the region is the 1858 quake with intensity I0=9-10 MSK. The 1858 earthquake caused heavy destruction in the city of Sofia and the appearance of thermal spring. It is worth mentioning that the seismic sequence of May 2912 occurred in an area characterized by a long quiescence (of 95 years) for moderate events. Moreover, a reduced number of small earthquakes has also been registered in the recent past. The manifold purpose of this study is first to study spatial and temporal distribution of aftershocks than to analyze wave forms and to determine individual focal mechanisms of the largest shocks. Additionally, a joint hypocenter determination and composite focal mechanism of a large number of small aftershocks were carried out. Finally, the current state of stress in the considered region, obtained on the base of aftershock focal mechanisms, was compared with horizontal crustal movement inferred from GPS measurement.

Comparing the stress change characteristics and aftershock decay rate of the 2011 Mineral, VA, earthquake with similar earthquakes from a variety of tectonic settings

NASA Astrophysics Data System (ADS)

Walsh, L. S.; Montesi, L. G.; Sauber, J. M.; Watters, T. R.; Kim, W.; Martin, A. J.; Anderson, R.

2011-12-01

On August 23, 2011, the magnitude 5.8 Mineral, VA, earthquake rocked the U.S. national capital region (Washington, DC) drawing worldwide attention to the occurrence of intraplate earthquakes. Using regional Coulomb stress change, we evaluate to what extent slip on faults during the Mineral, VA, earthquake and its aftershocks may have increased stress on notable Cenozoic fault systems in the DC metropolitan area: the central Virginia seismic zone, the DC fault zone, and the Stafford fault system. Our Coulomb stress maps indicate that the transfer of stress from the Mineral, VA, mainshock was at least 500 times greater than that produced from the magnitude 3.4 Germantown, MD, earthquake that occurred northwest of DC on July 16, 2010. Overall, the Mineral, VA, earthquake appears to have loaded faults of optimum orientation in the DC metropolitan region, bringing them closer to failure. The distribution of aftershocks of the Mineral, VA, earthquake will be compared with Coulomb stress change maps. We further characterize the Mineral, VA, earthquake by comparing its aftershock decay rate with that of blind thrust earthquakes with similar magnitude, focal mechanism, and depth from a variety of tectonic settings. In particular, we compare aftershock decay relations of the Mineral, VA, earthquake with two well studied California reverse faulting events, the August 4, 1985 Kettleman Hills (Mw = 6.1) and October 1, 1987 Whittier Narrow (Mw = 5.9) earthquakes. Through these relations we test the hypothesis that aftershock duration is inversely proportional to fault stressing rate, suggesting that aftershocks in active tectonic margins may last only a few years while aftershocks in intraplate regions could endure for decades to a century.

Foreshocks and aftershocks of the Great 1857 California earthquake

USGS Publications Warehouse

Meltzner, A.J.; Wald, D.J.

1999-01-01

The San Andreas fault is the longest fault in California and one of the longest strike-slip faults anywhere in the world, yet we know little about many aspects of its behavior before, during, and after large earthquakes. We conducted a study to locate and to estimate magnitudes for the largest foreshocks and aftershocks of the 1857 M 7.9 Fort Tejon earthquake on the central and southern segments of the fault. We began by searching archived first-hand accounts from 1857 through 1862, by grouping felt reports temporally, and by assigning modified Mercalli intensities to each site. We then used a modified form of the grid-search algorithm of Bakum and Wentworth, derived from empirical analysis of modern earthquakes, to find the location and magnitude most consistent with the assigned intensities for each of the largest events. The result confirms a conclusion of Sieh that at least two foreshocks ('dawn' and 'sunrise') located on or near the Parkfield segment of the San Andreas fault preceded the mainshock. We estimate their magnitudes to be M ~ 6.1 and M ~ 5.6, respectively. The aftershock rate was below average but within one standard deviation of the number of aftershocks expected based on statistics of modern southern California mainshock-aftershock sequences. The aftershocks included two significant events during the first eight days of the sequence, with magnitudes M ~ 6.25 and M ~ 6.7, near the southern half of the rupture; later aftershocks included a M ~ 6 event near San Bernardino in December 1858 and a M ~ 6.3 event near the Parkfield segment in April 1860. From earthquake logs at Fort Tejon, we conclude that the aftershock sequence lasted a minimum of 3.75 years.

Detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 Mw 7.6 Nicoya earthquake

NASA Astrophysics Data System (ADS)

Yao, Dongdong; Walter, Jacob I.; Meng, Xiaofeng; Hobbs, Tiegan E.; Peng, Zhigang; Newman, Andrew V.; Schwartz, Susan Y.; Protti, Marino

2017-01-01

We apply a waveform matching technique to obtain a detailed earthquake catalog around the rupture zone of the 5 September 2012 moment magnitude 7.6 Nicoya earthquake, with emphasis on its aftershock sequence. Starting from a preliminary catalog, we relocate 7900 events using TomoDD to better quantify their spatiotemporal behavior. Relocated aftershocks are mostly clustered in two groups. The first is immediately above the major coseismic slip patch, partially overlapping with shallow afterslip. The second one is 50 km SE to the main shock nucleation point and near the terminus of coseismic rupture, in a zone that exhibited little resolvable afterslip. Using the relocated events as templates, we scan through the continuous recording from 29 June 2012 to 30 December 2012, detecting approximately 17 times more than template events. We find 190 aftershocks in the first half hour following the main shock, mostly along the plate interface. Later events become more scattered in location, showing moderate expansion in both along-trench and downdip directions. From the detected catalog we identify 53 repeating aftershock clusters with mean cross-correlation values larger than 0.9, and indistinguishably intracluster event locations, suggesting slip on the same fault patch. Most repeating clusters occurred within the first major aftershock group. Very few repeating clusters were found in the aftershock grouping along the southern edge of the Peninsula, which is not associated with substantial afterslip. Our observations suggest that loading from nearby afterslip along the plate interface drives spatiotemporal evolution of aftershocks just above the main shock rupture patch, while aftershocks in the SE group are to the SE of the observed updip afterslip and poorly constrained.

Systematic deficiency of aftershocks in areas of high coseismic slip for large subduction zone earthquakes

PubMed Central

Wetzler, Nadav; Lay, Thorne; Brodsky, Emily E.; Kanamori, Hiroo

2018-01-01

Fault slip during plate boundary earthquakes releases a portion of the shear stress accumulated due to frictional resistance to relative plate motions. Investigation of 101 large [moment magnitude (Mw) ≥ 7] subduction zone plate boundary mainshocks with consistently determined coseismic slip distributions establishes that 15 to 55% of all master event–relocated aftershocks with Mw ≥ 5.2 are located within the slip regions of the mainshock ruptures and few are located in peak slip regions, allowing for uncertainty in the slip models. For the preferred models, cumulative deficiency of aftershocks within the central three-quarters of the scaled slip regions ranges from 15 to 45%, increasing with the total number of observed aftershocks. The spatial gradients of the mainshock coseismic slip concentrate residual shear stress near the slip zone margins and increase stress outside the slip zone, driving both interplate and intraplate aftershock occurrence near the periphery of the mainshock slip. The shear stress reduction in large-slip regions during the mainshock is generally sufficient to preclude further significant rupture during the aftershock sequence, consistent with large-slip areas relocking and not rupturing again for a substantial time. PMID:29487902

Longer aftershocks duration in extensional tectonic settings.

PubMed

Valerio, E; Tizzani, P; Carminati, E; Doglioni, C

2017-11-27

Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes.

On the adaptive daily forecasting of seismic aftershock hazard

NASA Astrophysics Data System (ADS)

Ebrahimian, Hossein; Jalayer, Fatemeh; Asprone, Domenico; Lombardi, Anna Maria; Marzocchi, Warner; Prota, Andrea; Manfredi, Gaetano

2013-04-01

Post-earthquake ground motion hazard assessment is a fundamental initial step towards time-dependent seismic risk assessment for buildings in a post main-shock environment. Therefore, operative forecasting of seismic aftershock hazard forms a viable support basis for decision-making regarding search and rescue, inspection, repair, and re-occupation in a post main-shock environment. Arguably, an adaptive procedure for integrating the aftershock occurrence rate together with suitable ground motion prediction relations is key to Probabilistic Seismic Aftershock Hazard Assessment (PSAHA). In the short-term, the seismic hazard may vary significantly (Jordan et al., 2011), particularly after the occurrence of a high magnitude earthquake. Hence, PSAHA requires a reliable model that is able to track the time evolution of the earthquake occurrence rates together with suitable ground motion prediction relations. This work focuses on providing adaptive daily forecasts of the mean daily rate of exceeding various spectral acceleration values (the aftershock hazard). Two well-established earthquake occurrence models suitable for daily seismicity forecasts associated with the evolution of an aftershock sequence, namely, the modified Omori's aftershock model and the Epidemic Type Aftershock Sequence (ETAS) are adopted. The parameters of the modified Omori model are updated on a daily basis using Bayesian updating and based on the data provided by the ongoing aftershock sequence based on the methodology originally proposed by Jalayer et al. (2011). The Bayesian updating is used also to provide sequence-based parameter estimates for a given ground motion prediction model, i.e. the aftershock events in an ongoing sequence are exploited in order to update in an adaptive manner the parameters of an existing ground motion prediction model. As a numerical example, the mean daily rates of exceeding specific spectral acceleration values are estimated adaptively for the L'Aquila 2009 aftershock catalog. The parameters of the modified Omori model are estimated in an adaptive manner using the Bayesian updating based on the aftershock events that had already taken place at each day elapsed and using the Italian generic sequence (Lolli and Gasperini 2003) as prior information. For the ETAS model, the real-time daily forecast of the spatio-temporal evolution of the L'Aquila sequence provided for the Italian Civil Protection for managing the emergency (Marzocchi and Lombardi, 2009) is utilized. Moreover, the parameters of the ground motion prediction relation proposed by Sabetta and Pugliese (1996) are updated adaptively and on a daily basis using Bayesian updating based on the ongoing aftershock sequence. Finally, the forecasted daily rates of exceeding (first-mode) spectral acceleration values are compared with observed rates of exceedance calculated based on the wave-forms that have actually taken place. References Jalayer, F., Asprone, D., Prota, A., Manfredi, G. (2011). A decision support system for post-earthquake reliability assessment of structures subjected to after-shocks: an application to L'Aquila earthquake, 2009. Bull. Earthquake Eng. 9(4) 997-1014. Jordan, T.H., Chen Y-T., Gasparini P., Madariaga R., Main I., Marzocchi W., Papadopoulos G., Sobolev G., Yamaoka K., and J. Zschau (2011). Operational earthquake forecasting: State of knowledge and guidelines for implementation, Ann. Geophys. 54(4) 315-391, doi 10.4401/ag-5350. Lolli, B., and P. Gasperini (2003). Aftershocks hazard in Italy part I: estimation of time-magnitude distribution model parameters and computation of probabilities of occurrence. Journal of Seismology 7(2) 235-257. Marzocchi, W., and A.M. Lombardi (2009). Real-time forecasting following a damaging earthquake, Geophys. Res. Lett. 36, L21302, doi: 10.1029/2009GL040233. Sabetta F., A. Pugliese (1996) Estimation of response spectra and simulation of nonstationary earthquake ground motions. Bull Seismol Soc Am 86(2) 337-352.

Aftershock seismicity of the 2010 Maule Mw=8.8 Chile, earthquake: Correlation between co-seismic slip models and aftershock distribution?

USGS Publications Warehouse

Rietbrock, A.; Ryder, I.; Hayes, G.; Haberland, C.; Comte, D.; Roecker, S.

2012-01-01

The 27 February 2010 Maule, Chile (Mw=8.8) earthquake is one of the best instrumentally observed subduction zone megathrust events. Here we present locations, magnitudes and cumulative equivalent moment of the first -2 months of aftershocks, recorded on a temporary network deployed within 2 weeks of the occurrence of the mainshock. Using automatically-determined onset times and a back projection approach for event association, we are able to detect over 30,000 events in the time period analyzed. To further increase the location accuracy, we systematically searched for potential S-wave arrivals and events were located in a regional 2D velocity model. Additionally, we calculated regional moment tensors to gain insight into the deformation history of the aftershock sequence. We find that the aftershock seismicity is concentrated between 40 and 140 km distance from the trench over a depth range of 10 to 35 km. Focal mechanisms indicate a predominance of thrust faulting, with occasional normal faulting events. Increased activity is seen in the outer-rise region of the Nazca plate, predominantly in the northern part of the rupture area. Further down-dip, a second band of clustered seismicity, showing mainly thrust motion, is located at depths of 40–45 km. By comparing recent published mainshock source inversions with our aftershock distribution, we discriminate slip models based on the assumption that aftershocks occur in areas of rapid transition between high and low slip, surrounding high-slip regions of the mainshock.

The Mechanisms and Spatiotemporal Behavior of the 2011 Mw7.1 Van, Eastern Turkey Earthquake Aftershocks

NASA Astrophysics Data System (ADS)

Ezgi Guvercin Isik, Sezim; Ozgun Konca, A.; Karabulut, Hayrullah

2016-04-01

We studied the mechanisms and spatiotemporal distribution of the aftershocks of the Mw7.1 Van Earthquake, in Eastern Turkey. The 2011 Van Earthquake occurred on a E-W trending blind thrust fault in Eastern Turkey which is under N-S compression due to convergence of the Arabian plate toward the Eurasia. In this study, we relocated and studied the mechanisms of the M3.5-5.5 aftershocks from regional Pnl and surface waves using the "Cut and Paste" algorithm of Zhu and Helmberger (1996). Our results reveal that the aftershocks in the first day following the mainshock are in the vicinity of the co-seismic slip and have mostly thrust mechanism consistent with the mainshock. In the following day, a second cluster of activity at the northeast termination of the fault ( North of Lake Erçek) has started. These aftershocks have approximately N-S lineation and left lateral source mechanisms. The aftershocks surrounding the mainshock rupture are deeper (>20 km) than the aftershocks triggered on the north (<15km). We also observe strike slip earthquakes on the south of the mainshock. Both of delayed activities (north of the mainshock and south of the mainshock) are consistent with the Coulomb stress increase due to slip on the mainshock. We propose that the Van Fault is truncated by two strike-slip faults at each end, which has determined the along-strike rupture extent of the 2011 mainshock.

Internal tectonic structure of the Central American Wadati-Benioff zone based on analysis of aftershock sequences

NASA Astrophysics Data System (ADS)

Å PičáK, Aleš; Hanuš, VáClav; VaněK, JiřÃ.­; BěHounková, Marie

2007-09-01

Relocated Engdahl et al. (1998) global seismological data for 10 aftershock sequences were used to analyze the internal tectonic structure of the Central American subduction zone; the main shocks of several of these were the most destructive and often referenced earthquakes in the region (e.g., the 1970 Chiapas, 1983 Osa, 1992 Nicaragua, 1999 Quepos, 2001 El Salvador earthquakes). The spatial analysis of aftershock foci distribution was performed in a rotated Cartesian coordinate system (x, y, z) related to the Wadati-Benioff zone, and not in a standard coordinate system (ϕ, λ, h are latitude, longitude, focal depth, respectively). Available fault plane solutions were also transformed into the plane approximating the Wadati-Benioff zone. The spatial distribution of earthquakes in each aftershock sequence was modeled as either a plane fit using a least squares approximation or a volume fit with a minimum thickness rectangular box. The analysis points to a quasi-planar distribution of earthquake foci in all aftershock sequences, manifesting the appurtenance of aftershocks to fracture zones. Geometrical parameters of fracture zones (strike, dip, and dimensions) hosting individual sequences were calculated and compared with the seafloor morphology of the Cocos Plate. The smooth character of the seafloor correlates with the aftershock fracture zones oriented parallel to the trench and commonly subparallel to the subducting slab, whereas subduction of the Cocos Ridge and seamounts around the Quepos Plateau coincides with steeply dipping fracture zones. Transformed focal mechanisms are almost exclusively (>90%) of normal character.

Internal tectonic structure of the Central American Wadati-Benioff zone based on analysis of aftershock sequences

NASA Astrophysics Data System (ADS)

Špičák, Aleš; Hanuš, Václav; Vaněk, Jiří; Běhounková, Marie

2007-09-01

Relocated Engdahl et al. (1998) global seismological data for 10 aftershock sequences were used to analyze the internal tectonic structure of the Central American subduction zone; the main shocks of several of these were the most destructive and often referenced earthquakes in the region (e.g., the 1970 Chiapas, 1983 Osa, 1992 Nicaragua, 1999 Quepos, 2001 El Salvador earthquakes). The spatial analysis of aftershock foci distribution was performed in a rotated Cartesian coordinate system (x, y, z) related to the Wadati-Benioff zone, and not in a standard coordinate system ($\\varphi$, λ, h are latitude, longitude, focal depth, respectively). Available fault plane solutions were also transformed into the plane approximating the Wadati-Benioff zone. The spatial distribution of earthquakes in each aftershock sequence was modeled as either a plane fit using a least squares approximation or a volume fit with a minimum thickness rectangular box. The analysis points to a quasi-planar distribution of earthquake foci in all aftershock sequences, manifesting the appurtenance of aftershocks to fracture zones. Geometrical parameters of fracture zones (strike, dip, and dimensions) hosting individual sequences were calculated and compared with the seafloor morphology of the Cocos Plate. The smooth character of the seafloor correlates with the aftershock fracture zones oriented parallel to the trench and commonly subparallel to the subducting slab, whereas subduction of the Cocos Ridge and seamounts around the Quepos Plateau coincides with steeply dipping fracture zones. Transformed focal mechanisms are almost exclusively (>90%) of normal character.

Fault structure in the Nepal Himalaya as illuminated by aftershocks of the 2015 Mw 7.8 Gorkha earthquake recorded by the local NAMASTE network

NASA Astrophysics Data System (ADS)

Ghosh, A.; Mendoza, M.; LI, B.; Karplus, M. S.; Nabelek, J.; Sapkota, S. N.; Adhikari, L. B.; Klemperer, S. L.; Velasco, A. A.

2017-12-01

Geometry of the Main Himalayan Thrust (MHT), that accommodates majority of the plate motion between Indian and Eurasian plate, is being debated for a long time. Different models have been proposed; some of them are significantly different from others. Obtaining a well constrained geometry of the MHT is challenging mainly because of the lack of high quality data, inherent low resolution and non-uniqueness of the models. We used a dense local seismic network - NAMASTE - to record and analyze a prolific aftershock sequence following the 2015 Mw 7.8 Gorkha earthquake, and determine geometry of the MHT constrained by precisely located well-constrained aftershocks. We detected and located more than 15,000 aftershocks of the Gorkha earthquake using Hypoinverse and then relatively relocated using HypoDD algorithm. We selected about 7,000 earthquakes that are particularly well constrained to analyze the geometry of the megathrust. They illuminate fault structure in this part of the Himalaya with unprecedented detail. The MHT shows two subhorizontal planes connected by a duplex structure. The duplex structure is characterized by multiple steeply dipping planes. In addition, we used four large-aperture continental-scale seismic arrays at teleseismic distances to backproject high-frequency seismic radiation. Moreover, we combined all arrays to significantly increase the resolution and detectability. We imaged rupture propagation of the mainshock showing complexity near the end of the rupture that might help arresting of the rupture to the east. Furthermore, we continuously scanned teleseismic data for two weeks starting from immediately after the mainshock to detect and locate aftershock activity only using the arrays. Spatial pattern of the aftershocks was similar to the existing global catalog using conventional seismic network and technique. However, we detected more than twice as many aftershocks using the array technique compared to the global catalog including many aftershocks that were undetected by the global network. This method might provide new tool to rapidly detect aftershock activity immediately after a large damaging earthquake to guide fast and more effective disaster response.

Dynamic Aftershock Triggering Correlated with Cyclic Loading in the Slip Direction

NASA Astrophysics Data System (ADS)

Hardebeck, J.

2014-12-01

Dynamic stress changes have been shown to contribute to aftershock triggering, but the physical triggering mechanisms are not fully understood. Some proposed mechanisms are based on dynamic stress loading of the target fault in a direction that encourages earthquake slip (e.g. dynamic Coulomb stress triggering), while other mechanisms are based on fault weakening due to shaking. If dynamic stress loading in the fault slip direction plays a role in aftershock triggering, we would expect to see a relationship between the dynamic stress orientations and the aftershock focal mechanisms. Alternatively, if dynamic stress change triggering functions only through a fault weakening mechanism that is independent of the slip direction of the target fault, no such relationship is expected. I study aftershock sequences of 4 M≥6.7 mainshocks in southern California, and find a small but significant relationship between modeled dynamic stress direction and aftershock focal mechanisms. The mainshock dynamic stress changes have two observed impacts: changing the focal mechanisms in a given location to favor those aligned with the dynamic stress change, and changing the spatial distribution of seismicity to favor locations where the dynamic stress change aligns with the background stress. The aftershock focal mechanisms are significantly more aligned with the dynamic stress changes than the preshock mechanisms for only the first 0.5-1 year following most mainshocks, although for at least 10 years following Hector Mine. Dynamic stress effects on focal mechanisms are best observed at long periods (30-60 sec). Dynamic stress effects are only observed when using metrics based on repeated stress cycling in the same direction, for example considering the dominant stress orientation over the full time series, and not for the peak dynamic stress. These results imply that dynamic aftershock triggering operates at least in part through cyclic loading in the direction of fault slip, although non-directional fault weakening may be important as well. This suggests that the orientation of the dynamic stresses, as well as their amplitude, should be considered in the development of physics-based aftershock forecasting models.

Evaluating spatial and temporal relationships between an earthquake cluster near Entiat, central Washington, and the large December 1872 Entiat earthquake

USGS Publications Warehouse

Brocher, Thomas M.; Blakely, Richard J.; Sherrod, Brian

2017-01-01

We investigate spatial and temporal relations between an ongoing and prolific seismicity cluster in central Washington, near Entiat, and the 14 December 1872 Entiat earthquake, the largest historic crustal earthquake in Washington. A fault scarp produced by the 1872 earthquake lies within the Entiat cluster; the locations and areas of both the cluster and the estimated 1872 rupture surface are comparable. Seismic intensities and the 1–2 m of coseismic displacement suggest a magnitude range between 6.5 and 7.0 for the 1872 earthquake. Aftershock forecast models for (1) the first several hours following the 1872 earthquake, (2) the largest felt earthquakes from 1900 to 1974, and (3) the seismicity within the Entiat cluster from 1976 through 2016 are also consistent with this magnitude range. Based on this aftershock modeling, most of the current seismicity in the Entiat cluster could represent aftershocks of the 1872 earthquake. Other earthquakes, especially those with long recurrence intervals, have long‐lived aftershock sequences, including the Mw">MwMw 7.5 1891 Nobi earthquake in Japan, with aftershocks continuing 100 yrs after the mainshock. Although we do not rule out ongoing tectonic deformation in this region, a long‐lived aftershock sequence can account for these observations.

Abundant aftershock sequence of the 2015 Mw7.5 Hindu Kush intermediate-depth earthquake

NASA Astrophysics Data System (ADS)

Li, Chenyu; Peng, Zhigang; Yao, Dongdong; Guo, Hao; Zhan, Zhongwen; Zhang, Haijiang

2018-05-01

The 2015 Mw7.5 Hindu Kush earthquake occurred at a depth of 213 km beneath the Hindu Kush region of Afghanistan. While many early aftershocks were missing from the global earthquake catalogues, this sequence was recorded continuously by eight broad-band stations within 500 km. Here we use a waveform matching technique to systematically detect earthquakes around the main shock. More than 3000 events are detected within 35 d after the main shock, as compared with 42 listed in the Advanced National Seismic System catalogue (or 196 in the International Seismological Centre catalogue). The aftershock sequence generally follows the Omori's law with a decay constant p = 0.92. We also apply the recently developed double-pair double-difference technique to relocate all detected aftershocks. Most of them are located to the west of the hypocentre of the main shock, consistent with the westward propagation of the main-shock rupture. The aftershocks outline a nearly vertical southward dipping plane, which matches well with one of the nodal planes of the main shock. We conclude that the aftershock sequence of this intermediate-depth earthquake shares many similarities with those for shallow earthquakes and infer that there are some common mechanisms responsible for shallow and intermediate-depth earthquakes.

Did stress triggering cause the large off-fault aftershocks of the 25 March 1998 MW=8.1 Antarctic plate earthquake?

USGS Publications Warehouse

Toda, S.; Stein, R.S.

2000-01-01

The 1998 Antarctic plate earthquake produced clusters of aftershocks (MW ??? 6.4) up to 80 km from the fault rupture and up to 100 km beyond the end of the rupture. Because the mainshock occurred far from the nearest plate boundary and the nearest recorded earthquake, it is unusually isolated from the stress perturbations caused by other earthquakes, making it a good candidate for stress transfer analysis despite the absence of near-field observations. We tested whether the off-fault aftershocks lie in regions brought closer to Coulomb failure by the main rupture. We evaluated four published source models for the main rupture. In fourteen tests using different aftershocks sets and allowing the rupture sources to be shifted within their uncertainties, 6 were significant at ??? 99% confidence, 3 at > 95% confidence, and 5 were not significant (< 95% level). For the 9 successful tests, the stress at the site of the aftershocks was typically increased by 1-2 bars (0.1-0.2 MPa). Thus the Antarctic plate event, together with the 1992 MW=7.3 Landers and its MW=6.5 Big Bear aftershock 40 km from the main fault, supply evidence that small stress changes might indeed trigger large earthquakes far from the main fault rupture.

Aftershocks of the 2010 Mw 7.2 El Mayor-Cucapah earthquake revealcomplex faulting in the Yuha Desert, California

USGS Publications Warehouse

Kroll, K.; Cochran, Elizabeth S.; Richards-Dinger, K.; Sumy, Danielle

2013-01-01

We detect and precisely locate over 9500 aftershocks that occurred in the Yuha Desert region during a 2 month period following the 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake. Events are relocated using a series of absolute and relative relocation procedures that include Hypoinverse, Velest, and hypoDD. Location errors are reduced to ~40 m horizontally and ~120 m vertically.Aftershock locations reveal a complex pattern of faulting with en echelon fault segments trending toward the northwest, approximately parallel to the North American-Pacific plate boundary and en echelon, conjugate features trending to the northeast. The relocated seismicity is highly correlated with published surface mapping of faults that experienced triggered surface slip in response to the EMC main shock. Aftershocks occurred between 2 km and 11 km depths, consistent with previous studies of seismogenic thickness in the region. Three-dimensional analysis reveals individual and intersecting fault planes that are limited in their along-strike length. These fault planes remain distinct structures at depth, indicative of conjugate faulting, and do not appear to coalesce onto a throughgoing fault segment. We observe a complex spatiotemporal migration of aftershocks, with seismicity that jumps between individual fault segments that are active for only a few days to weeks. Aftershock rates are roughly consistent with the expected earthquake production rates of Dieterich (1994). The conjugate pattern of faulting and nonuniform aftershock migration patterns suggest that strain in the Yuha Desert is being accommodated in a complex manner.

Utsu aftershock productivity law explained from geometric operations on the permanent static stress field of mainshocks

NASA Astrophysics Data System (ADS)

Mignan, Arnaud

2018-03-01

The aftershock productivity law is an exponential function of the form K ∝ exp(αM), with K being the number of aftershocks triggered by a given mainshock of magnitude M and α ≈ ln(10) being the productivity parameter. This law remains empirical in nature although it has also been retrieved in static stress simulations. Here, we parameterize this law using the solid seismicity postulate (SSP), the basis of a geometrical theory of seismicity where seismicity patterns are described by mathematical expressions obtained from geometric operations on a permanent static stress field. We first test the SSP that relates seismicity density to a static stress step function. We show that it yields a power exponent q = 1.96 ± 0.01 for the power-law spatial linear density distribution of aftershocks, once uniform noise is added to the static stress field, in agreement with observations. We then recover the exponential function of the productivity law with a break in scaling obtained between small and large M, with α = 1.5ln(10) and ln(10), respectively, in agreement with results from previous static stress simulations. Possible biases of aftershock selection, proven to exist in epidemic-type aftershock sequence (ETAS) simulations, may explain the lack of break in scaling observed in seismicity catalogues. The existence of the theoretical kink, however, remains to be proven. Finally, we describe how to estimate the solid seismicity parameters (activation density δ+, aftershock solid envelope r∗ and background stress amplitude range Δo∗) for large M values.

The Ms4.4 2016 Yuncheng, Shanxi, China, Seismic Sequence: Source Characterizations and Tectonic Implications

NASA Astrophysics Data System (ADS)

Xie, Z.; Riaz, M. S.; Zheng, Y.; Xiong, X.

2017-12-01

On 12 March 2016 at 11:14 (Beijing time) , a moderate earthquake with magnitude Ms4.4 struck Yuncheng County, Shanxi Province, Central China. Seismic activity continued with numerous aftershocks, and another event with little smaller magnitude of Ms4.0 occurred on 27 March 2016 at 3:58 as well as a lot of aftershocks followed. Seismic waves from the earthquakes were recorded by a dense local broadband seismic network in Shanxi and its surrounding provinces, enabling detailed the source characterizations of this earthquake sequence. The hypocenters of the Yuncheng earthquake sequence determined by the TomoDD method displayed that the events occurred in the middle of Yuncheng Basin which locates between Weihe Basin and Fenhe Basin, one of renowned active seismic belts in mainland China. The relocation of the aftershocks presented a NNW-SSE orientated distribution, and the earlier aftershocks tended to fault at shallow depths, concentrating at 5 km. After the Ms4.0 event, the aftershocks extended to the SSE direction with deeper focal depths. The focal mechanisms of the mainshock and the biggest aftershock obtained by the CAP (Cut And Paste) method indicated consistent right-lateral strike-slip faults. Since the magnitude was relatively small, no surface rupture associated with this shock sequence had been observed, and no known faults exited around the epicenter, indicating that the causative fault was ambiguous just based on the focal mechanism. According to the aftershocks distribution and the recent GPS results, the ruptured fault was apt to the nodal plane striking at 194°, dipping 79° and rake 151° for Ms4.4 mainshock and 199°/73°/141°, representing strike/dip/rake, for Ms4.0 aftershock. This earthquake sequence was regarded as an adjustment of stress accumulation in the pull-apart Yuncheng Basin, implying that the energy around the boundary faults of the basin has accumulated continuously.

Nonlinear Viscoelastic Rheology and the Occurrence of Aftershocks

NASA Astrophysics Data System (ADS)

Shcherbakov, R.; Zhang, X.

2017-12-01

Aftershocks are ubiquitous in nature. They are the manifestation of relaxation phenomena observed in various physical systems. In one prominent example, they typically occur after large earthquakes. The observed aftershock sequences usually obey several well defined non-trivial empirical laws in magnitude, temporal, and spatial domains. In many cases their characteristics follow scale-invariant distributions. The occurrence of aftershocks displays a prominent temporal behavior due to time-dependent mechanisms of stress and/or energy transfer. There are compelling evidences that the lower continental crust and upper mantle are governed by various solid state creep mechanisms. Among those mechanisms a power-law viscous flow was suggested to explain the postseismic surface deformation after large earthquakes. In this work, we consider a slider-block model to mimic the behavior of a seismogenic fault. In the model, we introduce a nonlinear viscoelastic coupling mechanism to capture the essential characteristics of crustal rheology and stress interaction between the blocks and the medium. For this purpose we employ nonlinear Kelvin-Voigt elements consisting of an elastic spring and a dashpot assembled in parallel to introduce viscoelastic coupling between the blocks and the driving plate. By mapping the model into a cellular automaton we derive the functional form of the stress transfer mechanism in the model. We show that the nonlinear viscoelasticity plays a critical role in triggering of aftershocks. It explains the functional form of the Omori-Utsu law and gives physical interpretation of its parameters. The proposed model also suggests that the power-law rheology of the fault gauge and underlying lower crust and upper mantle controls the decay rate of aftershocks. To verify this, we analyze several prominent aftershock sequences to estimate their decay rates and correlate with the rheological properties of the underlying lower crust and mantle, which were estimated from the postseismic surface deformation. Our modelling suggests that the power-law rheology exponent n controls the decay rate of aftershocks and is related to the parameter p of the Omori-Utsu law.

Modelling the 2013 North Aegean (Greece) seismic sequence: geometrical and frictional constraints, and aftershock probabilities

NASA Astrophysics Data System (ADS)

Karakostas, Vassilis; Papadimitriou, Eleftheria; Gospodinov, Dragomir

2014-04-01

The 2013 January 8 Mw 5.8 North Aegean earthquake sequence took place on one of the ENE-WSW trending parallel dextral strike slip fault branches in this area, in the continuation of 1968 large (M = 7.5) rupture. The source mechanism of the main event indicates predominantly strike slip faulting in agreement with what is expected from regional seismotectonics. It was the largest event to have occurred in the area since the establishment of the Hellenic Unified Seismological Network (HUSN), with an adequate number of stations in close distances and full azimuthal coverage, thus providing the chance of an exhaustive analysis of its aftershock sequence. The main shock was followed by a handful of aftershocks with M ≥ 4.0 and tens with M ≥ 3.0. Relocation was performed by using the recordings from HUSN and a proper crustal model for the area, along with time corrections in each station relative to the model used. Investigation of the spatial and temporal behaviour of seismicity revealed possible triggering of adjacent fault segments. Theoretical static stress changes from the main shock give a preliminary explanation for the aftershock distribution aside from the main rupture. The off-fault seismicity is perfectly explained if μ > 0.5 and B = 0.0, evidencing high fault friction. In an attempt to forecast occurrence probabilities of the strong events (Mw ≥ 5.0), estimations were performed following the Restricted Epidemic Type Aftershock Sequence (RETAS) model. The identified best-fitting MOF model was used to execute 1-d forecasts for such aftershocks and follow the probability evolution in time during the sequence. Forecasting was also implemented on the base of a temporal model of aftershock occurrence, different from the modified Omori formula (the ETAS model), which resulted in probability gain (though small) in strong aftershock forecasting for the beginning of the sequence.

The enigma of the Arthur's Pass, New Zealand, earthquake 1. Reconciling a variety of data for an unusual earthquake sequence

USGS Publications Warehouse

Abercrombie, R.E.; Webb, T.H.; Robinson, R.; McGinty, P.J.; Mori, J.J.; Beavan, R.J.

2000-01-01

The 1994 Arthur's Pass earthquake (Mw6.7) is the largest in a recent sequence of earthquakes in the central South Island, New Zealand. No surface rupture was observed the aftershock distribution was complex, and routine methods of obtaining the faulting orientation of this earthquake proved contradictory. We use a range of data and techniques to obtain our preferred solution, which has a centroid depth of 5 km, Mo=1.3??1019 N m, and a strike, dip, and rake of 221??, 47??, 112??, respectively. Discrepancies between this solution and the Harvard centroid moment tensor, together with the Global Positioning System (GPS) observations and unusual aftershock distribution, suggest that the rupture may not have occurred on a planar fault. A second, strike slip, subevent on a more northerly striking plane is suggested by these data but neither the body wave modeling nor regional broadband recordings show any complexity or late subevents. We relocate the aftershocks using both one-dimensional and three-dimensional velocity inversions. The depth range of the aftershocks (1-10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the mainshock fault plane. This structure and the Harper fault, ???15 km to the south appear to have acted as boundaries to the extensive aftershock zone trending NNW-SSE Most of the ML???5 aftershocks, including the two largest (ML6.1 and ML5.7), clustered near the Harper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in the south, is able to match the observed displacements. The occurrence of this earthquake sequence implies that the level of seismic hazard in the central South Island is greater than previous estimates. Copyright 2000 by the American Geophysical Union.

Aftershock sequence of ML6.1 earthquake in Sakhalin: recovery with waveform cross correlation

NASA Astrophysics Data System (ADS)

Kitov, Ivan; Konovalov, Alexey; Stepnov, Andrey; Turuntaev, Sergey

2017-04-01

The Sakhalin Island is characterized by relatively high seismic activity. The largest measured earthquake of Mw=7.0 occurred in 1995 near the town of Neftegorsk. It was followed by a long-lasting aftershock sequence. Based on the results of our previous analysis of this aftershock sequence with the method of waveform cross correlation (WCC), we have recovered an aftershock sequence of the ML 6.1 earthquake occurred on August 14, 2016 at 11:15:13.1 (UTC). The epicentre of this earthquake estimated by near-regional data has geographic coordinates 50.351N i 142.395E, with the focal depth of 9 km. The aftershock catalogue compiled by the eqaler.ru resource includes 133 events within 20 days from the main shock. We used P- and S-wave signals from the main shock and a few largest aftershocks from the catalogue as waveform templates. Cross correlation of continuous waveforms with these templates was carried out at six closest seismic stations of the regional network, with four stations to northeast and two stations to southwest of the epicentre. For detection, we used standard STA/LTA method with thresholds depending on seismic phase and station. The accuracy of onset time estimation by the STA/LTA detector based on the obtained CC-traces is close to a few samples, with the sampling rate of 40 Hz at all stations. Arrival times of all detected signals were reduced to origin times using the observed travel times from the master-events to six stations. For a given master event, clusters of origin times are considered as event hypotheses in a local association procedure. When several master events find the same physical signal, we resolve conflict using the number of associated stations and then the RMS origin time residual. In total, more than 190 aftershocks were found with three and more associated stations and five and more associated phases. This is by 40% more than the number of aftershocks in the original catalogue. Their magnitudes vary between 1.5 and 4.5. We also applied the relative location procedure to all found aftershocks, which were moved closer to the main shock. The epicentres of relocated aftershocks tend to cluster in the narrow zone corresponding to the western (hanging) wing of the Central Sakhalin upthrow-thrust fault zone, which defines the boundary between the Okhotsk and Eurasian (or Amur) plates.

Aftershocks to Philippine quake found within nearby megathrust fault

NASA Astrophysics Data System (ADS)

Schultz, Colin

2013-02-01

On 31 August 2012 a magnitude 7.6 earthquake ruptured deep beneath the sea floor of the Philippine Trench, a powerful intraplate earthquake centered seaward of the plate boundary. In the wake of the main shock, sensors detected a flurry of aftershocks, counting 110 in total. Drawing on seismic wave observations and rupture mechanisms calculated for the aftershocks, Ye et al. found that many were located near the epicenter of the main intraplate quake but at shallower depth; all involved normal faulting. Some shallow thrusting aftershocks were located farther to the west, centered within the potentially dangerous megathrust fault formed by the subduction of the Philippine Sea plate beneath the Philippine microplate, the piece of crust housing the Philippine Islands.

Seismicity of the Pahute Mesa area, Nevada Test Site: 8 October 1975 to 30 June 1976

USGS Publications Warehouse

Rogers, A.M.; Wuollet, Geraldine M.; Covington, P.A.

1977-01-01

A total of 1,075 earthquakes occurred in the Pahute Mesa area with 2.5≤ML≤4.9 during the period October 28, 1975, to June 28, 1976. The majority of these earthquakes are aftershocks of the nuclear events, Kasseri, Inlet, Muenster, Fontina, Cheshire, Estuary, Colby, and Pool (5.8≤ML≤6.3). Smaller nuclear events (ML≤5.5) on Rainier Mesa and Yucca Flat detonated in the same time period did not trigger aftershock sequences. The aftershock series were displaced laterally from ground zero and occurred deeper (at 4–6 km) than the nuclear even depth of burial (~1 km). The aftershocks appear to occur on vertical faults with approximately north-south strike.

Complex faulting associated with the 22 December 2003 Mw 6.5 San Simeon California, earthquake, aftershocks and postseismic surface deformation

USGS Publications Warehouse

McLaren, Marcia K.; Hardebeck, Jeanne L.; Van Der Elst, Nicholas; Unruh, Jeffrey R.; Bawden, Gerald W.; Blair, James Luke

2008-01-01

We use data from two seismic networks and satellite interferometric synthetic aperture radar (InSAR) imagery to characterize the 22 December 2003 Mw 6.5 San Simeon earthquake sequence. Absolute locations for the mainshock and nearly 10,000 aftershocks were determined using a new three-dimensional (3D) seismic velocity model; relative locations were obtained using double difference. The mainshock location found using the 3D velocity model is 35.704° N, 121.096° W at a depth of 9.7±0.7 km. The aftershocks concentrate at the northwest and southeast parts of the aftershock zone, between the mapped traces of the Oceanic and Nacimiento fault zones. The northwest end of the mainshock rupture, as defined by the aftershocks, projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the Oceanic fault, near the Santa Lucia Range front and the >5 mm postseismic InSAR imagery contour. The Oceanic fault in this area, as mapped by Hall (1991), is therefore probably a second-order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth. The southeast end of the rupture projects closer to the mapped Oceanic fault trace, suggesting much of the slip was along this fault, or at a minimum is accommodating much of the postseismic deformation. InSAR imagery shows ∼72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture, and ∼48 and ∼45 mm at the northwest and southeast end of the aftershock zone, respectively. From these observations, we model a ∼30-km-long northwest-trending northeast-dipping mainshock rupture surface—called the mainthrust—which is likely the Oceanic fault at depth, a ∼10-km-long southwest-dipping backthrust parallel to the mainthrust near the hypocenter, several smaller southwest-dipping structures in the southeast, and perhaps additional northeast-dipping or subvertical structures southeast of the mainshock plane. Discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic Nacimiento fault zone at depth. The InSAR data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone. The well-defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the Santa Lucia Range as a popup block; aftershocks in the southeast part of the zone also indicate a popup block, but it is less well defined. The absence of backthrust features in the central part of the zone suggests range-front uplift by fault-propagation folding, or backthrusts in the central part were not activated during the mainshock.

Using waveform cross correlation for automatic recovery of aftershock sequences

NASA Astrophysics Data System (ADS)

Bobrov, Dmitry; Kitov, Ivan; Rozhkov, Mikhail

2017-04-01

Aftershock sequences of the largest earthquakes are difficult to recover. There can be several hundred mid-sized aftershocks per hour within a few hundred km from each other recorded by the same stations. Moreover, these events generate thousands of reflected/refracted phases having azimuth and slowness close to those from the P-waves. Therefore, aftershock sequences with thousands of events represent a major challenge for automatic and interactive processing at the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Organization (CTBTO). Standard methods of detection and phase association do not use all information contained in signals. As a result, wrong association of the first and later phases, both regular and site specific, produces enormous number of wrong event hypotheses and destroys valid event hypotheses in automatic IDC processing. In turn, the IDC analysts have to reject false and recreate valid hypotheses wasting precious human resources. At the current level of the IDC catalogue completeness, the method of waveform cross correlation (WCC) can resolve most of detection and association problems fully utilizing the similarity of waveforms generated by aftershocks. Array seismic stations of the International monitoring system (IMS) can enhance the performance of the WCC method: reduce station-specific detection thresholds, allow accurate estimate of signal attributes, including relative magnitude, and effectively suppress irrelevant arrivals. We have developed and tested a prototype of an aftershock tool matching all IDC processing requirements and merged it with the current IDC pipeline. This tool includes creation of master events consisting of real or synthetic waveform templates at ten and more IMS stations; cross correlation (CC) of real-time waveforms with these templates, association of arrivals detected at CC-traces in event hypotheses; building events matching the IDC quality criteria; and resolution of conflicts between events hypotheses created by neighboring master-events. The final cross correlation standard event lists (XSEL) is a start point for interactive analysis with standard tools. We present select results for the biggest earthquakes, like Sumatra 2004 and Tohoku 2011, as well as for several smaller events with hundreds of aftershocks. The sensitivity and resolution of the aftershock tool is demonstrated on the example of mb=2.2 aftershock found after the September 9, 2016 DPRK test.

Some characteristics of the complex El Mayor-Cucapah, MW7.2, April 4, 2010, Baja California, Mexico, earthquake, from well-located aftershock data from local and regional networks.

NASA Astrophysics Data System (ADS)

Frez, J.; Nava Pichardo, F. A.; Acosta, J.; Munguia, L.; Carlos, J.; García, R.

2015-12-01

Aftershocks from the El Mayor-Cucapah (EMC), MW7.2, April 4, 2010, Baja California, Mexico, earthquake, were recorded over two months by a 31 station local array (Reftek RT130 seismographs loaned from IRIS-PASSCAL), complemented by regional data from SCSN, and CICESE. The resulting data base includes 518 aftershocks with ML ≥ 3.0, plus 181 smaller events. Reliable hypocenters were determined using HYPODD and a velocity structure determined from refraction data for a mesa located to the west of the Mexicali-Imperial Valley. Aftershock hypocenters show that the El Mayor-Cucapah earthquake was a multiple event comprising two or three different ruptures of which the last one constituted the main event. The main event rupture, which extends in a roughly N45°W direction, is complex with well-defined segments having different characteristics. The main event central segment, located close to the first event epicenter is roughly vertical, the northwest segment dips ~68°NE, while the two southeast segments dip ~60°SW and ~52°SW, respectively, which agrees with results of previous studies based on teleseismic long periods and on GPS-INSAR. All main rupture aftershock hypocenters have depths above 10-11km and, except for the central segment, they delineate the edges of zones with largest coseismic displacement. The two southern segments show seismicity concentrated below 5km and 3.5km, respectively; the paucity of shallow seismicity may be caused by the thick layer of non-consolidated sediments in this region. The ruptures delineated by aftershocks in the southern regions correspond to the Indiviso fault, unidentified until the occurrence of the EMC earthquake. The first event was relocated together with the aftershocks; the epicenter lies slightly westwards of published locations, but it definitely does not lie on, or close to, the main rupture. The focal mechanism of the first event, based on first arrival polarities, is predominantly strike-slip; the focal plane coinciding with neighboring aftershocks has N13°E and left-lateral slip. From the above, we conclude that the EMC earthquake consisted of a first event that triggered the main right-lateral strike-slip rupture and probably another rupture indicated by a NS trending aftershock alignment.

Variation of b and p values from aftershocks sequences along the Mexican subduction zone and their relation to plate characteristics

NASA Astrophysics Data System (ADS)

Ávila-Barrientos, L.; Zúñiga, F. R.; Rodríguez-Pérez, Q.; Guzmán-Speziale, M.

2015-11-01

Aftershock sequences along the Mexican subduction margin (between coordinates 110ºW and 91ºW) were analyzed by means of the p value from the Omori-Utsu relation and the b value from the Gutenberg-Richter relation. We focused on recent medium to large (Mw > 5.6) events considered susceptible of generating aftershock sequences suitable for analysis. The main goal was to try to find a possible correlation between aftershock parameters and plate characteristics, such as displacement rate, age and segmentation. The subduction regime of Mexico is one of the most active regions of the world with a high frequency of occurrence of medium to large events and plate characteristics change along the subduction margin. Previous studies have observed differences in seismic source characteristics at the subduction regime, which may indicate a difference in rheology and possible segmentation. The results of the analysis of the aftershock sequences indicate a slight tendency for p values to decrease from west to east with increasing of plate age although a statistical significance is undermined by the small number of aftershocks in the sequences, a particular feature distinctive of the region as compared to other world subduction regimes. The b values show an opposite, increasing trend towards the east even though the statistical significance is not enough to warrant the validation of such a trend. A linear regression between both parameters provides additional support for the inverse relation. Moreover, we calculated the seismic coupling coefficient, showing a direct relation with the p and b values. While we cannot undoubtedly confirm the hypothesis that aftershock generation depends on certain tectonic characteristics (age, thickness, temperature), our results do not reject it thus encouraging further study into this question.

Geophysical data reveal the crustal structure of the Alaska Range orogen within the aftershock zone of the Mw 7.9 Denali fault earthquake

USGS Publications Warehouse

Fisher, M.A.; Ratchkovski, N.A.; Nokleberg, W.J.; Pellerin, L.; Glen, J.M.G.

2004-01-01

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 Mw 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25?? north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (>10 ??-m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after-shock zone.

Tectonic Setting and Aftershocks of the Mw 6.7 Feburary 14, 2013 Earthquake in Yakutia, Northeast Russia

NASA Astrophysics Data System (ADS)

Rappolee, E.; Burk, D. R.; Mackey, K. G.; Fujita, K.; Shibaev, S. V.; Koz'min, B. M.

2016-12-01

The details of the seismotectonics along the boundary between the Eurasian, North American, and Okhotsk plates are poorly understood. Infrequent earthquakes of moderate size (Mw > 4) in this remote region make it difficult to characterize its tectonic activity. On February 14, 2013, an Mw 6.7 earthquake along this boundary in Northern Yakutia, Russia, resulted in a long sequence of aftershocks that provide an opportunity to better understand the region's geology. A temporary deployment of four seismic stations was installed around the main shock to supplement regional station coverage. During the ten day deployment, several thousand aftershocks were recorded. We have located 112 events using both first-arriving Pn and Sn and secondary arriving Pg and Sg phase time picks. The located aftershocks define a SSE striking zone approximately 30 km long and 10 km wide, east of the Illin'-Tas fault and northwest of the Indigirka River. Location depths range from 0 to 20 km. In conjunction with locating aftershocks, a local three-layer best-fit velocity was determined consisting of an upper crust (14 km thick, VPg = 6.06 km/s and VSg = 3.53 km/s), a lower crust (21 km thick, VP* = 6.45 km/s and VS* = 3.65 km/s), and a Moho (35 km deep, VPn = 7.98 km/s and VSn = 4.53 km/s). The mainshock epicenter falls in the northwestern corner of the aftershock zone, however its focal depth is not well established. Aftershock analysis is ongoing and will possibly provide a better understanding of the earthquake rupture zone. Nonetheless, results of this study support active thrusting and mountain building as a mechanism to accommodate compression along the North America-Eurasia boundary.

Stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 Northridge, California, earthquake

USGS Publications Warehouse

Deng, J.; Hudnut, K.; Gurnis, M.; Hauksson, E.

1999-01-01

Following the M(w) 6.7 Northridge earthquake, significant postseismic displacements were resolved with GPS. Using a three-dimensional viscoelastic model, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several weeks of the main shock, may have been triggered by continuous stress loading from viscous flow. The long-term decay time of aftershocks (about 2 years) approximately matches the decay of viscoelastic loading, and thus is controlled by the viscosity of the lower crust. Our model provides a physical interpretation of the observed correlation between aftershock decay rate and surface heat flow.Following the Mw 6.7 Northridge earthquake, significant postseismic displacements were resolved with GPS. Using a three-dimensional viscoelastic model, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several weeks of the main shock, may have been triggered by continuous stress loading from viscous flow. The long-term decay time of aftershocks (about 2 years) approximately matches the decay of viscoelastic loading, and thus is controlled by the viscosity of the lower crust. Our model provides a physical interpretation of the observed correlation between aftershock decay rate and surface heat flow.

Nonlinear Viscoelastic Mechanism for Aftershock Triggering and Decay

NASA Astrophysics Data System (ADS)

Shcherbakov, R.; Zhang, X.

2016-12-01

Aftershocks are ubiquitous in nature. They are the manifestation of relaxation phenomena observed in various physical systems. In one prominent example, they typically occur after large earthquakes. They also occur in other natural or experimental systems, for example, in solar flares, in fracture experiments on porous materials and acoustic emissions, after stock market crashes, in the volatility of stock prices returns, in internet traffic variability and e-mail spamming, to mention a few. The observed aftershock sequences usually obey several well defined non-trivial empirical laws in magnitude, temporal, and spatial domains. In many cases their characteristics follow scale-invariant distributions. The occurrence of aftershocks displays a prominent temporal behavior due to time-dependent mechanisms of stress and/or energy transfer. In this work, we consider a slider-block model to mimic the behavior of a seismogenic fault. In the model, we introduce a nonlinear viscoelastic coupling mechanism to capture the essential characteristics of crustal rheology and stress interaction between the blocks and the medium. For this purpose we employ nonlinear Kelvin-Voigt elements consisting of an elastic spring and a dashpot assembled in parallel to introduce viscoelastic coupling between the blocks and the driving plate. By mapping the model into a cellular automaton we derive the functional form of the stress transfer mechanism in the model. We show that the nonlinear viscoelasticity plays a critical role in triggering of aftershocks. It explains the functional form of the Omori-Utsu law and gives physical interpretation of its parameters. The proposed model also suggests that the power-law rheology of the fault gauge and underlying lower crust and upper mantle control the decay rate of aftershocks. To verify this, we analyze several prominent aftershock sequences to estimate their decay rates and correlate with the rheological properties of the underlying lower crust and mantle.

The 26 May 2006 Yogyakarta earthquake fault observed by seismic data and satellite data based surface features

NASA Astrophysics Data System (ADS)

Anggraini, Ade; Sobiesiak, Monika; Walter, Thomas R.

2010-05-01

The Mw 6.3 May 26, 2006 Yogyakarta Earthquake caused severe damage and claimed thousands lives in the Yogyakarta Special Province and Klaten District of Central Java Province. The nearby Opak River fault was thought to be the source of this earthquake disaster. However, no significant surface movement was observed along the fault which could confirm that this fault was really the source of the earthquake. To investigate the earthquake source and to understand the earthquake mechanism, a rapid response team of the German Task Force for Earthquake, together with the Seismological Division of Badan Meteorologi Klimatologi dan Geofisika and Gadjah Mada University in Yogyakarta, had installed a temporary seismic network of 12 short period seismometers. More than 3000 aftershocks were recorded during the 3-month campaign. Here we present the result of several hundred processed aftershocks. We used integrated software package GIANTPitsa to pick P and S phases manually and HYPO71 to determine the hypocenters. HypoDD software was used for hypocenters relocation to obtain high precision aftershock locations. Our aftershock distribution shows a system of lineaments in southwest-northeast direction, about 10 km east to Opak River fault, at 5-18 km depth. The b-value map from the aftershocks shows that the main lineaments have relatively low b-value at the middle part which suggests this part is still under stress. We also observe several aftershock clusters cutting these lineaments in nearly perpendicular direction. To verify the interpretation of our aftershocks analysis, we will overlay it on surface feature we delineate from satellite data. Hopefully our result will give significant contribution to understand the near surface fault systems around Yogyakarta Area in order to mitigate similar earthquake hazard in the future.

Characteristics of Gyeongju earthquake, moment magnitude 5.5 and relative relocations of aftershocks

NASA Astrophysics Data System (ADS)

Cho, ChangSoo; Son, Minkyung

2017-04-01

There is low seismicity in the korea peninsula. According historical record in the historic book, There were several strong earthquake in the korea peninsula. Especially in Gyeongju of capital city of the Silla dynasty, few strong earthquakes caused the fatalities of several hundreds people 1,300 years ago and damaged the houses and make the wall of castles collapsed. Moderate strong earthquake of moment magnitude 5.5 hit the city in September 12, 2016. Over 1000 aftershocks were detected. The numbers of occurrences of aftershock over time follows omori's law well. The distribution of relative locations of 561 events using clustering aftershocks by cross-correlation between P and S waveform of the events showed the strike NNE 25 30 o and dip 68 74o of fault plane to cause the earthquake matched with the fault plane solution of moment tensor inversion well. The depth of range of the events is from 11km to 16km. The width of distribution of event locations is about 5km length. The direction of maximum horizontal stress by inversion of stress for the moment solutions of main event and large aftershocks is similar to the known maximum horizontal stress direction of the korea peninsula. The relation curves between moment magnitude and local magnitude of aftershocks shows that the moment magnitude increases slightly more for events of size less than 2.0

Spatial-temporal evolutions of early aftershocks following the 2013 Mw 6.6 Lushan earthquake in Sichuan, China

NASA Astrophysics Data System (ADS)

Wu, Jing; Yao, Dongdong; Meng, Xiaofeng; Peng, Zhigang; Su, Jinrong; Long, Feng

2017-04-01

We perform a comprehensive detection of early aftershocks following the 2013 Mw 6.6 Lushan earthquake, which occurred in the southern Longmenshan Fault Zone in Sichuan Province, China, about 5 years after the 2008 Mw 7.9 Wenchuan earthquake. We use events in both standard and relocated catalogs as templates to scan through continuous waveforms 2 days before and 3 days after the main shock. We successfully reduce the magnitude of completeness Mc by more than 1 order and obtain up to 6 times more events than listed in both catalogs. Aftershocks in the first hour mostly occur around the main shock slip region, and aftershocks at later times show systematic expansions in the along-strike, perpendicular-strike, and updip directions. Although postseismic deformation following the Lushan main shock has not been clearly identified, we suggest that early aftershock expansions are likely driven by afterslip of the Lushan main shock. This is consistent with the observations that most aftershocks were in the stress shadow of the Lushan main shock and that there was significant slip deficit in the top 10 km of the crust. We also find that seismicity on the back thrust fault was activated as soon as 20 min after the main shock, earlier than previously reported. We are unable to detect any clear foreshocks in the last 2 days before the Lushan main shock.

A stochastic estimate of ground motion at Oceano, California, for the M 6.5 22 December 2003 San Simeon earthquake, derived from aftershock recordings

USGS Publications Warehouse

Di, Alessandro C.; Boatwright, J.

2006-01-01

The U.S. Geological Survey deployed a digital seismic station in Oceano, California, in February 2004, to investigate the cause of damage and liquefaction from the 22 December 2003 M 6.5 San Simeon earthquake. This station recorded 11 M > 2.8 aftershocks in almost 8 weeks. We analyze these recordings, together with recordings of the mainshock and the same aftershocks obtained from nearby stations in Park Hill and San Luis Obispo, to estimate the mainshock ground motion in Oceano. We estimate the Fourier amplitude spectrum using generalized spectral ratio analysis. We test a set of aftershocks as Green's functions by comparing simulated and recorded acceleration amplitude spectra for the mainshock at San Luis Obispo and Park Hill. We convolve the aftershock accelerograms with a stochastic operator to simulate the duration and phase of the mainshock accelerograms. This approximation allows us to extend the range of aftershocks that can be used as Green's functions to events nearly three magnitude units smaller than the mainshock. Our realizations for the mainshock accelerogram at Oceano yield peak ground accelerations distributed as 28% ?? 4%g. We interpret these realizations as upper bounds for the actual ground motion, because our analysis assumes a linear response, whereas the presence of liquefaction indicates that the ground behaved nonlinearly in Oceano.

Using the 2011 Mw9.0 Tohoku earthquake to test the Coulomb stress triggering hypothesis and to calculate faults brought closer to failure

USGS Publications Warehouse

Toda, Shinji; Lin, Jian; Stein, Ross S.

2011-01-01

The 11 March 2011 Tohoku Earthquake provides an unprecedented test of the extent to which Coulomb stress transfer governs the triggering of aftershocks. During 11-31 March, there were 177 aftershocks with focal mechanisms, and so the Coulomb stress change imparted by the rupture can be resolved on the aftershock nodal planes to learn whether they were brought closer to failure. Numerous source models for the mainshock have been inverted from seismic, geodetic, and tsunami observations. Here, we show that, among six tested source models, there is a mean 47% gain in positively-stressed aftershock mechanisms over that for the background (1997-10 March 2011) earthquakes, which serve as the control group. An aftershock fault friction of 0.4 is found to fit the data better than 0.0 or 0.8, and among all the tested models, Wei and Sladen (2011) produced the largest gain, 63%. We also calculate that at least 5 of the seven large, exotic, or remote aftershocks were brought ≥0.3 bars closer to failure. With these tests as confirmation, we calculate that large sections of the Japan trench megathrust, the outer trench slope normal faults, the Kanto fragment beneath Tokyo, and the Itoigawa-Shizuoka Tectonic Line, were also brought ≥0.3 bars closer to failure.

Are triggering rates of labquakes universal? Inferring triggering rates from incomplete information

NASA Astrophysics Data System (ADS)

Baró, Jordi; Davidsen, Jörn

2017-12-01

The acoustic emission activity associated with recent rock fracture experiments under different conditions has indicated that some features of event-event triggering are independent of the details of the experiment and the materials used and are often even indistinguishable from tectonic earthquakes. While the event-event triggering rates or aftershock rates behave pretty much identical for all rock fracture experiments at short times, this is not the case for later times. Here, we discuss how these differences can be a consequence of the aftershock identification method used and show that the true aftershock rates might have two distinct regimes. Specifically, tests on a modified Epidemic-Type Aftershock Sequence model show that the model rates cannot be correctly inferred at late times based on temporal information only if the activity rates or the branching ratio are high. We also discuss both the effect of the two distinct regimes in the aftershock rates and the effect of the background rate on the inter-event time distribution. Our findings should be applicable for inferring event-event triggering rates for many other types of triggering and branching processes as well.

A change in fault-plane orientation between foreshocks and aftershocks of the Galway Lake earthquake, ML = 5.2, 1975, Mojave desert, California

USGS Publications Warehouse

Fuis, G.S.; Lindh, A.G.

1979-01-01

A marked change is observed in P/SV amplitude ratios, measured at station TPC, from foreshocks to aftershocks of the Galway Lake earthquake. This change is interpreted to be the result of a change in fault-plane orientation occurring between foreshocks and aftershocks. The Galway Lake earthquake, ML= 5.2, occurred on June 1, 1975. The first-motion fault-plane solutions for the main shock and most foreshocks and aftershocks indicate chiefly right-lateral strike-slip on NNW-striking planes that dip steeply, 70-90??, to the WSW. The main event was preceded by nine located foreshocks, ranging in magnitude from 1.9 to 3.4, over a period of 12 weeks, starting on March 9, 1975. All of the foreshocks form a tight cluster approximately 1 km in diameter. This cluster includes the main shock. Aftershocks are distributed over a 6-km-long fault zone, but only those that occurred inside the foreshock cluster are used in this study. Seismograms recorded at TPC (?? = 61 km), PEC (?? = 93 km), and CSP (?? = 83 km) are the data used here. The seismograms recorded at TPC show very consistent P/SV amplitude ratios for foreshocks. For aftershocks the P/SV ratios are scattered, but generally quite different from foreshock ratios. Most of the scatter for the aftershocks is confined to the two days following the main shock. Thereafter, however, the P/SV ratios are consistently half as large as for foreshocks. More subtle (and questionable) changes in the P/SV ratios are observed at PEC and CSP. Using theoretical P/SV amplitude ratios, one can reproduce the observations at TPC, PEC and CSP by invoking a 5-12?? counterclockwise change in fault strike between foreshocks and aftershocks. This interpretation is not unique, but it fits the data better than invoking, for example, changes in dip or slip angle. First-motion data cannot resolve this small change, but they permit it. Attenuation changes would appear to be ruled out by the fact that changes in the amplitude ratios, PTPC/PPEC and ptpc/pcsp, are observed, and these changes accompany the changes in P/SV. Observations for the Galway Lake earthquake are similar to observations for the Oroville, California, earthquake (ML = 5.7) of August 1, 1975, and the Brianes Hills, California, earthquake (ML = 4.3) of January 8, 1977 (Lindh et al., Science Vol. 201, pp. 56-59). A change in fault-plane orientation between foreshocks and aftershocks may be understandable in terms of early en-echelon cracking (foreshocks) giving way to shear on the main fault plane (main shock plus aftershocks). Recent laboratory data (Byerlee et al., Tectonophysics, Vol. 44, pp. 161-171) tend to support this view. ?? 1979.

Earthquakes, July-August 1992

USGS Publications Warehouse

Person, W.J.

1992-01-01

There were two major earthquakes (7.0≤M<8.0) during this reporting period. A magnitude 7.5 earthquake occurred in Kyrgyzstan on August 19 and a magnitude 7.0 quake struck the Ascension Island region on August 28. In southern California, aftershocks of the magnitude 7.6 earthquake on June 28, 1992, continued. One of these aftershocks caused damage and injuries, and at least one other aftershock caused additional damage. Earthquake-related fatalities were reportred in Kyrgzstan and Pakistan. 

Omori's law in the Internet traffic

NASA Astrophysics Data System (ADS)

Abe, S.; Suzuki, N.

2003-03-01

The Internet is a complex system, whose temporal behavior is highly nonstationary and exhibits sudden drastic changes regarded as main shocks or catastrophes. Here, analyzing a set of time series data of round-trip time measured in echo experiment with the Ping Command, the property of "aftershocks" (i.e., catastrophes of smaller scales) after a main shock is studied. It is found that the aftershocks obey Omori's law. Thus, the Internet shares with earthquakes and financial-market crashes a common scale-invariant feature in the temporal patterns of aftershocks.

Aftershock locations and rupture characteristics of the 2006 May 27, Yogyakarta-Indonesia earthquake

NASA Astrophysics Data System (ADS)

Irwan, M.; Ando, M.; Kimata, F.; Tadokoro, K.; Nakamichi, H.; Muto, D.; Okuda, T.; Hasanuddin, A.; Mipi A., K.; Setyadji, B.; Andreas, H.; Gamal, M.; Arif, R.

2006-12-01

A strong earthquake (M6.3) rocked the Bantul district, south of Yogyakarta Special Province (DIY) on the morningof May 27, 2006. We installed a temporary array of 6 seismographs to record aftershocks of the earthquake. The area of aftershocks, which may be interpreted as mainshock ruptured area has dimensions of about 25 km length and 20 km width, in the N48E direction. At depth the seismicity mainly concentrated between 5 to 15 km. The distribution of aftershock does not appear to come very close to the surface. There is no obvious surface evidence of causative fault in this area, though we find many crack and fissures that seem to have produced by the strong ground motion. We used the orientation and size of the fault determined from our aftershock results to carry out an inversion of teleseismic data for the slip distribution. We used broad- band seismograms of the IRIS network with epicentral distances between 30 and 90 degrees. We assume a single fault plane, strike 48 degree and dip 80 degree, which is inferred from the aftershock distribution. The total seismic moment is 0.369 x 10(19) Nm with maximum slip 0.4 meters. The asperity is located about 5 km away southwest of USGS estimated epicenter. Although the distances from the seismic source to heavily damaged areas Bantul and Klaten are 10 to 50 km, soft sedimentary soil likely to have generated very damaging motions within the area.

Structural context of the 2015 pair of Nepal earthquakes (Mw 7.8 and Mw 7.3): an analysis based on slip distribution, aftershock growth, and static stress changes

NASA Astrophysics Data System (ADS)

Parameswaran, Revathy M.; Rajendran, Kusala

2017-04-01

The Great Himalayan earthquakes are believed to originate on the Main Himalayan Thrust, and their ruptures lead to deformation along the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8), earthquake was east-directed, with no part relayed to the MFT. The aftershock distribution, coseismic elevation change of 1 m inferred from the InSAR image, and the spatial correspondence of the subtle surface deformations with PT2, a previously mapped out-of-sequence thrust, lead us to explore the role of structural heterogeneities in constraining the rupture progression. We used teleseismic moment inversion of P- and SH-waves, and Coulomb static stress changes to map the slip distribution, and growth of aftershock area, to understand their relation to the thrust systems. Most of the aftershocks were sourced outside the stress shadows (slip >1.65 m) of the April 25 earthquake. The May 12 (Mw 7.3) earthquake that sourced on a contiguous patch coincides with regions of increased stress change and therefore is the first known post-instrumentation example of a late, distant, and large triggered aftershock associated with any large earthquake in the Nepal Himalaya. The present study relates the slip, aftershock productivity, and triggering of unbroken stress barriers, to potential out-of-sequence thrusts, and suggests the role of stress transfer in generating large/great earthquakes.

Analysing the 1811-1812 New Madrid earthquakes with recent instrumentally recorded aftershocks

USGS Publications Warehouse

Mueller, K.; Hough, S.E.; Bilham, R.

2004-01-01

Although dynamic stress changes associated with the passage of seismic waves are thought to trigger earthquakes at great distances, more than 60 per cent of all aftershocks appear to be triggered by static stress changes within two rupture lengths of a mainshock. The observed distribution of aftershocks may thus be used to infer details of mainshock rupture geometry. Aftershocks following large mid-continental earthquakes, where background stressing rates are low, are known to persist for centuries, and models based on rate-and-state friction laws provide theoretical support for this inference. Most past studies of the New Madrid earthquake sequence have indeed assumed ongoing microseismicity to be a continuing aftershock sequence. Here we use instrumentally recorded aftershock locations and models of elastic stress change to develop a kinematically consistent rupture scenario for three of the four largest earthquakes of the 1811-1812 New Madrid sequence. Our results suggest that these three events occurred on two contiguous faults, producing lobes of increased stress near fault intersections and end points, in areas where present-day microearthquakes have been hitherto interpreted as evidence of primary mainshock rupture. We infer that the remaining New Madrid mainshock may have occurred more than 200 km north of this region in the Wabash Valley of southern Indiana and Illinois-an area that contains abundant modern microseismicity, and where substantial liquefaction was documented by historic accounts. Our results suggest that future large midplate earthquake sequences may extend over a much broader region than previously suspected.

Decay of aftershock density with distance does not indicate triggering by dynamic stress

USGS Publications Warehouse

Richards-Dinger, K.; Stein, R.S.; Toda, S.

2010-01-01

Resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard. Felzer and Brodsky examined the distance distribution of earthquakes occurring in the first five minutes after 2 ≤ M  M  M ≥ 2 aftershocks showed a uniform power-law decay with slope −1.35 out to 50 km from the mainshocks. From this they argued that the distance decay could be explained only by dynamic triggering. Here we propose an alternative explanation for the decay, and subject their hypothesis to a series of tests, none of which it passes. At distances more than 300 m from the 2 ≤  M< 3 mainshocks, the seismicity decay 5 min before the mainshocks is indistinguishable from the decay five minutes afterwards, indicating that the mainshocks have no effect at distances outside their static triggering range. Omori temporal decay, the fundamental signature of aftershocks, is absent at distances exceeding 10 km from the mainshocks. Finally, the distance decay is found among aftershocks that occur before the arrival of the seismic wave front from the mainshock, which violates causality. We argue that Felzer and Brodsky implicitly assume that the first of two independent aftershocks along a fault rupture triggers the second, and that the first of two shocks in a creep- or intrusion-driven swarm triggers the second, when this need not be the case.

The Mw 5.8 Mineral, Virginia, earthquake of August 2011 and aftershock sequence: constraints on earthquake source parameters and fault geometry

USGS Publications Warehouse

McNamara, Daniel E.; Benz, H.M.; Herrmann, Robert B.; Bergman, Eric A.; Earle, Paul; Meltzer, Anne; Withers, Mitch; Chapman, Martin

2014-01-01

The Mw 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M0 5.7×1017  N·m) occurred near Mineral, Virginia, within the central Virginia seismic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast‐striking reverse fault that nucleated at a depth of approximately 7±2  km. The distribution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Near‐source and far‐field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and academic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best‐recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence magnitude–frequency and temporal distribution. The observed slope of the magnitude–frequency curve or b‐value for the aftershock sequence is consistent with previous EUS studies (b=0.75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approximately 2–8 km in depth and 8–10 km along the strike of the fault plane. Best‐fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east‐southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast–southwest striking southeast‐dipping fault plane.

Seismotectonics of the May 19, 2011 Simav- Kutahya Earthquake Activity

NASA Astrophysics Data System (ADS)

Komec Mutlu, Ahu

2014-05-01

Aftershock sequence of May 19, 2011 Simav earthquake (Mw = 5.8) is relocated with a new 1-D seismic velocity model and focal mechanisms of largest aftershocks are determined. The May 19, 2011 Simav-Kutahya earthquake is occured in the most seismically active region of western Turkey. During six months after the mainshock, more than 5000 earthquakes are recorded and aftershocks followed over a period of almost two years. In this study, more than 7600 aftershocks occured between years 2011 and 2012 with magnitudes greater than 1.8 relocated. Waveform data is collected by 13 three component seismic stations from three different networks (Kandilli Observatory and Earthquake Research Institute (NEMC-National Earthquake Monitoring Center), Prime Ministry Disaster and Emergency Management Presidency, Department of Earthquake and Canakkale Onsekiz Mart University Geophysics Department). These seismic stations are deployed closer than 80 km epicentral distance in the Simav-Kutahya. Average crustal velocity and average crustal thickness for the region are computed as 5.68 km/sn and 37.6 km, respectively. The source mechanism of fifty aftershocks with magnitudes greater than 4.0 are derived from first motion P phases. Analysis of focal mechanisms indicate mainly normal fault motions with oblique slip.

Seismic constraints and coulomb stress changes of a blind thrust fault system, 2: Northridge, California

USGS Publications Warehouse

Stein, Ross S.; Lin, Jian

2006-01-01

We review seismicity, surface faulting, and Coulomb stress changes associated with the 1994 Northridge, California, earthquake. All of the observed surface faulting is shallow, extending meters to tens of meters below the surface. Relocated aftershocks reveal no seismicity shallower than 2 km depth. Although many of the aftershocks lie along the thrust fault and its up-dip extension, there are also a significant number of aftershocks in the core of the gentle anticline above the thrust, and elsewhere on the up-thrown block. These aftershocks may be associated with secondary ramp thrusts or flexural slip faults at a depth of 2-4 km. The geological structures typically associated with a blind thrust fault, such as anticlinal uplift and an associated syncline, are obscured and complicated by surface thrust faults associated with the San Fernando fault that overly the Northridge structures. Thus the relationship of the geological structure and topography to the underlying thrust fault is much more complex for Northridge than it is for the 1983 Coalinga, California, earthquake. We show from a Coulomb stress analysis that secondary surface faulting, diffuse aftershocks, and triggered sequences of moderate-sized mainshocks, are expected features of moderate-sized blind thrust earthquakes.

Relocation of the 2012 Ms 7.0 Lushan Earthquake Aftershock Sequences and Its Implications

NASA Astrophysics Data System (ADS)

Fang, L.; Wu, J.; Sun, Z.; Su, J.; Du, W.

2013-12-01

At 08:02 am on 20 April 2013 (Beijing time), an Ms 7.0 earthquake occurred in Lushan County, Sichuan Province. Lushan earthquake is another devastating earthquake occurred in Sichuan Province after 12 May 2008 Ms 8.0 Wenchuan earthquake. 193 people were killed, 25 people were missing and more than ten thousand people were injured in the earthquake. Direct economic losses were estimated to be more than 80 billion yuan (RMB). Lushan earthquake occurred in the southern part of the Longmenshan fault zone. The distance between the epicenters of Lushan earthquake and Wenchuan earthquake is about 87 km. In an effort to maximize observations of the aftershock sequence and study the seismotetonic model, we deployed 35 temporal seismic stations around the source area. The earthquake was followed by a productive aftershock sequence. By the end of 20 July more than 10,254 aftershocks were recorded by the temporal seismic network. The magnitude of the aftershock ranges from ML-0.5 to ML5.6. We first located the aftershocks using Hypo2000 (Kevin, 2000) and refined the location results with HYPODD (Waldhauser & Ellsworth, 2000). The 1-D velocity model used in relocation is modified from a deep seismic sounding profile near Lushan earthquake (Wang et al., 2007). The Vp/Vs ratio is set to 1.83 according to receiver function h-k study. A total of 8,129 events were relocated. The average location error in N-S, E-W and U-D direction is 0.30, 0.29 and 0.59 km, respectively. The relocation results show that the aftershocks spread approximately 35 km in length and 16 km in width. The dominant distribution of the focal depth ranges from 10 to 20 km. A few earthquakes occurred in the shallow crust. Focal depth sections crossing the source area show that the seismogenic fault dips to the northwest, manifested itself as a listric thrust fault. The dip angle of the seismogenic fault is approximately 63° in the shallow crust, about 41° near the source of the mainshock, and about 17° at the bottom of the fault. The focal depths of 28 aftershocks with ML≥4.0 were determined using Himalaya Seismic Array and sPn phase. The focal depths obtained from sPn phase are consistent with HYPODD, which also reveals a northwest-dipping fault. Since the earthquake did not cause significant surface rupture, the seismogenic structure of Lushan earthquake remains controversial. On the basis of aftershock relocation results, we speculate that the seismogenic fault of Lushan earthquake may be a blind thrust fault on the eastern side of the Shuangshi-Dachuan fault. The relocation results also reveal that there is a southeastward tilt aftershock belt intersecting with the seismogenic fault with y-shape. We infer that it is a back thrust fault that often appears in a thrust fault system. Lushan earthquake triggered the seismic activity of the back thrust fault.

Coseismic Stress Changes of the 2016 Mw 7.8 Kaikoura, New Zealand, Earthquake and Its Implication for Seismic Hazard Assessment

NASA Astrophysics Data System (ADS)

Shan, B.; LIU, C.; Xiong, X.

2017-12-01

On 13 November 2016, an earthquake with moment magnitude Mw 7.8 stroke North Canterbury, New Zealand as result of shallow oblique-reverse faulting close to boundary between the Pacific and Australian plates in the South Island, collapsing buildings and resulting in significant economic losses. The distribution of early aftershocks extended about 150 km to the north-northeast of the mainshock, suggesting the potential of earthquake triggering in this complex fault system. Strong aftershocks following major earthquakes present significant challenges for locals' reconstruction and rehabilitation. The regions around the mainshock may also suffer from earthquakes triggered by the Kaikoura earthquake. Therefore, it is significantly important to outline the regions with potential aftershocks and high seismic hazard to mitigate future disasters. Moreover, this earthquake ruptured at least 13 separate faults, and provided an opportunity to test the theory of earthquake stress triggering for a complex fault system. In this study, we calculated the coseismic Coulomb Failure Stress changes (ΔCFS) caused by the Kaikoura earthquake on the hypocenters of both historical earthquakes and aftershocks of this event with focal mechanisms. Our results show that the percentage of earthquake with positive ΔCFS within the aftershocks is higher than that of historical earthquakes. It means that the Kaikoura earthquake effectively influence the seismicity in this region. The aftershocks of Mw 7.8 Kaikoura earthquake are mainly located in the regions with positive ΔCFS. The aftershock distributions can be well explained by the coseismic ΔCFS. Furthermore, earthquake-induced ΔCFS on the surrounding active faults was further discussed. The northeastern Alpine fault, the southwest part of North Canterbury Fault, parts of the Marlborough fault system and the southwest ends of the Kapiti-Manawatu faults are significantly stressed by the Kaikoura earthquake. The earthquake-induced stress increments would raise the probability of earthquake occurrence on these faults.

Improved detection and relocation of micro-earthquakes applied to the Sea of Marmara

NASA Astrophysics Data System (ADS)

Tary, J. B.; Evangelia, B.; Géli, L.; Lomax, A.

2016-12-01

The Sea of Marmara is located at the western end of the North Anatolian Fault (NAF). This part of the NAF is considered as a seismic gap, being between the Izmit and Duzce earthquakes to the East and the Ganos earthquake to the West. Improved detection and location of seismicity in the Sea of Marmara is important for defining the seismic hazard in this area.On July 25, 2011, a Mw 5 earthquake occurred below the Western High in the western part of the Sea of Marmara. This earthquake as well as its aftershock sequence were recorded by a network of 10 ocean bottom seismometers (Ifremer) as well as seafloor observatories (KOERI). The OBSs were deployed from mid-April, 2011, to the end of July, 2011.The aftershock sequence is characterized by deep seismicity ( 10-15 km) around the main shock and shallow seismicity. Some of the shallow seismicity could be located at a similar depth as gas prone sediment layers below the Western High. The exact causes of these shallow aftershocks are still unclear. To better define this aftershock sequence, we use the match filter technique with a selection of aftershocks as templates to dig out child events from the continuous data streams. The templates are cross-correlated with the continuous data for stations with absolute time picks. The cross-correlation coefficients are then summed over all stations and components, and we then compute its median absolute deviation (MAD). Signals are detected when the summed cross-correlation time series exceeds a given number of times the MAD. Using a conservative detection threshold, we obtain a 10-fold increase in the number of events. The newly detected events are then relocated using the double-difference technique. With these newly detected events, we investigate the nucleation phase of the main shock and the aftershock sequence, as well as the possible triggering of the shallow aftershocks by the deeper seismicity.

Spatio-temporal analysis of aftershock sequences in terms of Non Extensive Statistical Physics.

NASA Astrophysics Data System (ADS)

Chochlaki, Kalliopi; Vallianatos, Filippos

2017-04-01

Earth's seismicity is considered as an extremely complicated process where long-range interactions and fracturing exist (Vallianatos et al., 2016). For this reason, in order to analyze it, we use an innovative methodological approach, introduced by Tsallis (Tsallis, 1988; 2009), named Non Extensive Statistical Physics. This approach introduce a generalization of the Boltzmann-Gibbs statistical mechanics and it is based on the definition of Tsallis entropy Sq, which maximized leads the the so-called q-exponential function that expresses the probability distribution function that maximizes the Sq. In the present work, we utilize the concept of Non Extensive Statistical Physics in order to analyze the spatiotemporal properties of several aftershock series. Marekova (Marekova, 2014) suggested that the probability densities of the inter-event distances between successive aftershocks follow a beta distribution. Using the same data set we analyze the inter-event distance distribution of several aftershocks sequences in different geographic regions by calculating non extensive parameters that determine the behavior of the system and by fitting the q-exponential function, which expresses the degree of non-extentivity of the investigated system. Furthermore, the inter-event times distribution of the aftershocks as well as the frequency-magnitude distribution has been analyzed. The results supports the applicability of Non Extensive Statistical Physics ideas in aftershock sequences where a strong correlation exists along with memory effects. References C. Tsallis, Possible generalization of Boltzmann-Gibbs statistics, J. Stat. Phys. 52 (1988) 479-487. doi:10.1007/BF01016429 C. Tsallis, Introduction to nonextensive statistical mechanics: Approaching a complex world, 2009. doi:10.1007/978-0-387-85359-8. E. Marekova, Analysis of the spatial distribution between successive earthquakes in aftershocks series, Annals of Geophysics, 57, 5, doi:10.4401/ag-6556, 2014 F. Vallianatos, G. Papadakis, G. Michas, Generalized statistical mechanics approaches to earthquakes and tectonics. Proc. R. Soc. A, 472, 20160497, 2016.

Full Aftershock Sequence of the M w 6.9 2003 Boumerdes Earthquake, Algeria: Space-Time Distribution, Local Tomography and Seismotectonic Implications

NASA Astrophysics Data System (ADS)

Kherroubi, Abdelaziz; Yelles-Chaouche, Abdelkrim; Koulakov, Ivan; Déverchère, Jacques; Beldjoudi, Hamoud; Haned, Abderrahmane; Semmane, Fethi; Aidi, Chafik

2017-07-01

We present a detailed analysis of the aftershocks of the May 21, 2003 Boumerdes earthquake ( M w = 6.9) recorded by 35 seismological stations and 2 OBS deployed in the epicentral area. This network recorded the aftershock activity for about 1 year and resulted in locating about 2500 events. The five main aftershocks (4.7 < M <5.8) display thrust faulting consistent with the main shock, except for the second event (M5.8, 29/05/2003) which depicts a strike-slip focal solution at the western tip of the rupture zone. Most aftershocks are clustered near the main rupture plane, in the footwall or at the westernmost tip of the 2003 Boumerdes rupture area. Many aftershocks last over the whole seismic crisis ahead (north) of the main rupture zone, forming a diffuse, low-angle surface within the footwall where the coseismic static stress change is predicted to increase. At the SW tip of the rupture, short-lived clusters locate at intersections of faults near the contact between the inner (Kabylia) and outer (Tell) zones. The tomographic inversion depicts high-velocity P- and S-wave anomalies coinciding with Miocene magmatic intrusive bodies in the upper crust, partially hidden by surrounding basins. The area of the main shock is associated with a large low-velocity body subdivided into sub-domains, including Neogene basins on land and offshore. Our results support a rupture model strongly controlled by geological inhomogeneities and extending as ramp-flat-ramp systems upward, favoring heterogeneous slip and segmentation in the fault plane with strong afterslip toward the surface. The diffuse aftershock activity in the footwall evidences an inherited discontinuity at mid-crustal depth that we interpret as the contact of Kabylian and African (Tethyan) continental crusts that were stacked during the Upper Miocene collision.

International Aftershock Forecasting: Lessons from the Gorkha Earthquake

NASA Astrophysics Data System (ADS)

Michael, A. J.; Blanpied, M. L.; Brady, S. R.; van der Elst, N.; Hardebeck, J.; Mayberry, G. C.; Page, M. T.; Smoczyk, G. M.; Wein, A. M.

2015-12-01

Following the M7.8 Gorhka, Nepal, earthquake of April 25, 2015 the USGS issued a series of aftershock forecasts. The initial impetus for these forecasts was a request from the USAID Office of US Foreign Disaster Assistance to support their Disaster Assistance Response Team (DART) which coordinated US Government disaster response, including search and rescue, with the Government of Nepal. Because of the possible utility of the forecasts to people in the region and other response teams, the USGS released these forecasts publicly through the USGS Earthquake Program web site. The initial forecast used the Reasenberg and Jones (Science, 1989) model with generic parameters developed for active deep continental regions based on the Garcia et al. (BSSA, 2012) tectonic regionalization. These were then updated to reflect a lower productivity and higher decay rate based on the observed aftershocks, although relying on teleseismic observations, with a high magnitude-of-completeness, limited the amount of data. After the 12 May M7.3 aftershock, the forecasts used an Epidemic Type Aftershock Sequence model to better characterize the multiple sources of earthquake clustering. This model provided better estimates of aftershock uncertainty. These forecast messages were crafted based on lessons learned from the Christchurch earthquake along with input from the U.S. Embassy staff in Kathmandu. Challenges included how to balance simple messaging with forecasts over a variety of time periods (week, month, and year), whether to characterize probabilities with words such as those suggested by the IPCC (IPCC, 2010), how to word the messages in a way that would translate accurately into Nepali and not alarm the public, and how to present the probabilities of unlikely but possible large and potentially damaging aftershocks, such as the M7.3 event, which had an estimated probability of only 1-in-200 for the week in which it occurred.

Stress evolution following the 1999 Chi-Chi, Taiwan, earthquake: Consequences for afterslip, relaxation, aftershocks and departures from Omori decay

USGS Publications Warehouse

Chan, C.-H.; Stein, R.S.

2009-01-01

We explore how Coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system. The 1999 September 21 Mw = 7.6 Chi-Chi shock is typical of continental ramp-d??collement systems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable. First, we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated Coulomb stress changes imparted by the coseismic rupture. Some 61 per cent of the M ??? 2 aftershocks and 83 per cent of the M ??? 4 aftershocks lie in regions for which the Coulomb stress increased by ???0.1 bars, and there is a 11-12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre-Chi Chi control period. Second, we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and d??collement, subject to the condition that friction is high on the ramp and low on the d??collement. Third, viscoelastic relaxation is evident from the fit of the post-seismic GPS data on the footwall. Fourth, we find that the rate of seismicity began to increase during the post-seismic period in an annulus extending east of the main rupture. The spatial extent of the seismicity annulus resembles the calculated ???0.05-bar Coulomb stress increase caused by viscoelastic relaxation and afterslip, and we find a 9-12 per cent gain in the percentage of focal mechanisms with >0.01-bar shear stress increases imparted by the post-seismic afterslip and relaxation in comparison to the control period. Thus, we argue that post-seismic stress changes can for the first time be shown to alter the production of aftershocks, as judged by their rate, spatial distribution, and focal mechanisms. ?? Journal compilation ?? 2009 RAS.

Aftershock distribution and heterogeneous structure in and around the source area of the 2014 northern Nagano Prefecture earthquake (Mw 6.2) , central Japan, revealed by dense seismic array observation

NASA Astrophysics Data System (ADS)

Kurashimo, E.; Hirata, N.; Iwasaki, T.; Sakai, S.; Obara, K.; Ishiyama, T.; Sato, H.

2015-12-01

A shallow earthquake (Mw 6.2) occurred on November 22 in the northern Nagano Prefecture, central Japan. Aftershock area is located near the Kamishiro fault, which is a part of the Itoigawa-Shizuoka Tectonic Line (ISTL). ISTL is one of the major tectonic boundaries in Japan. Precise aftershock distribution and heterogeneous structure in and around the source region of this earthquake is important to constrain the process of earthquake occurrence. We conducted a high-density seismic array observation in and around source area to investigate aftershock distribution and crustal structure. One hundred sixty-three seismic stations, approximately 1 km apart, were deployed during the period from December 3, 2014 to December 21, 2014. Each seismograph consisted of a 4.5 Hz 3-component seismometer and a digital data recorder (GSX-3). Furthermore, the seismic data at 40 permanent stations were incorporated in our analysis. During the seismic array observation, the Japan Meteorological Agency located 977 earthquakes in a latitude range of 35.5°-37.1°N and a longitude range of 136.7°-139.0°E, from which we selected 500 local events distributed uniformly in the study area. To investigate the aftershock distribution and the crustal structure, the double-difference tomography method [Zhang and Thurber, 2003] was applied to the P- and S-wave arrival time data obtained from 500 local earthquakes. The relocated aftershock distribution shows a concentration on a plane dipping eastward in the vicinity of the mainshock hypocenter. The large slip region (asperity) estimated from InSAR analysis [GSI, 2014] corresponds to the low-activity region of the aftershocks. The depth section of Vp structure shows that the high Vp zone corresponds to the large slip region. These results suggest that structural heterogeneities in and around the fault plane may have controlled the rupture process of the 2014 northern Nagano Prefecture earthquake.

Aftershocks of the june 20, 1978, Greece earthquake: A multimode faulting sequence

USGS Publications Warehouse

Carver, D.; Bollinger, G.A.

1981-01-01

A 10-station portable seismograph network was deployed in northern Greece to study aftershocks of the magnitude (mb) 6.4 earthquake of June 20, 1978. The main shock occurred (in a graben) about 25 km northeast of the city of Thessaloniki and caused an east-west zone of surface rupturing 14 km long that splayed to 7 km wide at the west end. The hypocenters for 116 aftershocks in the magnitude range from 2.5 to 4.5 were determined. The epicenters for these events cover an area 30 km (east-west) by 18 km (north-south), and focal depths ranges from 4 to 12 km. Most of the aftershocks in the east half of the aftershock zone are north of the surface rupture and north of the graben. Those in the west half are located within the boundaries of the graben. Composite focalmechanism solutions for selected aftershocks indicate reactivation of geologically mapped normal faults in the area. Also, strike-slip and dip-slip faults that splay off the western end of the zone of surface ruptures may have been activated. The epicenters for four large (M ??? 4.8) foreshocks and the main shock were relocated using the method of joint epicenter determination. Collectively, those five epicenters form an arcuate pattern convex southward, that is north of and 5 km distant from the surface rupturing. The 5-km separation, along with a focal depth of 8 km (average aftershock depth) or 16 km (NEIS main-shock depth), implies that the fault plane dips northward 58?? or 73??, respectively. A preferred nodal-plane dip of 36?? was determined by B.C. Papazachos and his colleagues in 1979 from a focal-mechanism solution for the main shock. If this dip is valid for the causal fault and that fault projects to the zone of surface rupturing, a decrease of dip with depth is required. ?? 1981.

Source parameters and effects of bandwidth and local geology on high- frequency ground motions observed for aftershocks of the northeastern Ohio earthquake of 31 January 1986

USGS Publications Warehouse

Glassmoyer, G.; Borcherdt, R.D.

1990-01-01

A 10-station array (GEOS) yielded recordings of exceptional bandwidth (400 sps) and resolution (up to 96 dB) for the aftershocks of the moderate (mb???4.9) earthquake that occurred on 31 January 1986 near Painesville, Ohio. Nine aftershocks were recorded with seismic moments ranging between 9 ?? 1016 and 3 ?? 1019 dyne-cm (MW: 0.6 to 2.3). The aftershock recordings at a site underlain by ???8m of lakeshore sediments show significant levels of high-frequency soil amplification of vertical motion at frequencies near 8, 20 and 70 Hz. Viscoelastic models for P and SV waves incident at the base of the sediments yield estimates of vertical P-wave response consistent with the observed high-frequency site resonances, but suggest additional detailed shear-wave logs are needed to account for observed S-wave response. -from Authors

The Constantine (Algeria) seismic sequence of 27 October 1985: a new rupture model from aftershock relocation, focal mechanisms, and stress tensors

NASA Astrophysics Data System (ADS)

Ousadou, F.; Dorbath, L.; Dorbath, C.; Bounif, M. A.; Benhallou, H.

2013-04-01

The October 27, 1985 Constantine earthquake of magnitude MS 5.9 (NEIC) although moderate is the strongest earthquake recorded in the eastern Tellian Atlas (northeast Algeria) since the beginning of instrumental seismology. The main shock locations given by different institutions are scattered and up to 10 km away northwest from the NE-SW 30 km long elongated aftershocks cloud localized by a dedicated temporary portable network. The focal mechanism indicates left-lateral strike-slip on an almost vertical fault with a small reverse component on the northwest dipping plane. This paper presents relocations of the main shock and aftershocks using TomoDD. One hundred thirty-eight individual focal mechanisms have been built allowing the determination of the stress tensor at different scales. A rupture model has been suggested, which explains the different observations of aftershock distribution and stress tensor rotation.

Depth dependent stress revealed by aftershocks

NASA Astrophysics Data System (ADS)

Narteau, C.; Shebalin, P.

2017-12-01

Aftershocks occur in response to perturbations of the state of stress induced either by earthquakes or human activities. Along major strike-slip fault segments of the San Andreas fault system, the time-delay before the onset of the power-law aftershock decay rate (the c-value) varies by three orders of magnitude in the first twenty kilometers below the surface. Despite the influence of the lithostatic stress, there is no continuous change in c-value with respect to depth. Instead, two decay phases are separated by an abrupt increase at an intermediate depth range of 2 to 5 km. This transitional regime is the only one observed in fluid-injection-induced seismic areas. This provides strong evidence for the role of fluid and a porosity reduction mechanism at depth of few kilometers in active fault zones. Aftershock statistics can then be used to predict the evolution the differential shear stress with depth until the brittle-ductile transition is reached.

Shallow megathrust earthquake ruptures betrayed by their outer-trench aftershocks signature

NASA Astrophysics Data System (ADS)

Sladen, Anthony; Trevisan, Jenny

2018-02-01

For some megathrust earthquakes, the rupture extends to the solid Earth's surface, at the ocean floor. This unexpected behaviour holds strong implications for the tsunami potential of subduction zones and for the physical conditions governing earthquakes, but such ruptures occur in underwater areas which are hard to observe, even with current instrumentation and imaging techniques. Here, we evidence that aftershocks occurring ocean-ward from the trench are conditioned by near-surface rupture of the megathrust fault. Comparison to well constrained earthquake slip models further reveals that for each event the number of aftershocks is proportional to the amount of shallow slip, a link likely related to static stress transfer. Hence, the spatial distribution of these specific aftershock sequences could provide independent constrains on the coseismic shallow slip of future events. It also offers the prospect to be able to reassess the rupture of many large subduction earthquakes back to the beginning of the instrumental era.

2010 Chile Earthquake Aftershock Response

NASA Astrophysics Data System (ADS)

Barientos, Sergio

2010-05-01

The Mw=8.8 earthquake off the coast of Chile on 27 February 2010 is the 5th largest megathrust earthquake ever to be recorded and provides an unprecedented opportunity to advance our understanding of megathrust earthquakes and associated phenomena. The 2010 Chile earthquake ruptured the Concepcion-Constitucion segment of the Nazca/South America plate boundary, south of the Central Chile region and triggered a tsunami along the coast. Following the 2010 earthquake, a very energetic aftershock sequence is being observed in an area that is 600 km along strike from Valparaiso to 150 km south of Concepcion. Within the first three weeks there were over 260 aftershocks with magnitude 5.0 or greater and 18 with magnitude 6.0 or greater (NEIC, USGS). The Concepcion-Constitucion segment lies immediately north of the rupture zone associated with the great magnitude 9.5 Chile earthquake, and south of the 1906 and the 1985 Valparaiso earthquakes. The last great subduction earthquake in the region dates back to the February 1835 event described by Darwin (1871). Since 1835, part of the region was affected in the north by the Talca earthquake in December 1928, interpreted as a shallow dipping thrust event, and by the Chillan earthquake (Mw 7.9, January 1939), a slab-pull intermediate depth earthquake. For the last 30 years, geodetic studies in this area were consistent with a fully coupled elastic loading of the subduction interface at depth; this led to identify the area as a mature seismic gap with potential for an earthquake of magnitude of the order 8.5 or several earthquakes of lesser magnitude. What was less expected was the partial rupturing of the 1985 segment toward north. Today, the 2010 earthquake raises some disturbing questions: Why and how the rupture terminated where it did at the northern end? How did the 2010 earthquake load the adjacent segment to the north and did the 1985 earthquake only partially ruptured the plate interface leaving loaded asperities since 1906? Since the number of M>7.0 aftershocks has been low, does the distribution of large-magnitude aftershocks differ from previous events of this size? What is the origin of the extensional-type aftershocks at shallow depths within the upper plate? The international seismological community (France, Germany, U.K., U.S.A.) in collaboration with the Chilean seismological community responded with a total of 140 portable seismic stations to deploy in order to record aftershocks. This combined with the Chilean permanent seismic network, in the area results in 180 stations now in operation recording continuous at 100 cps. The seismic equipment is a mix of accelerometers, short -period and broadband seismic sensors deployed along the entire length of the aftershock zone that will record the aftershock sequence for three to six months. The collected seismic data will be merged and archived to produce an international data set open to the entire seismological community immediately after archiving. Each international group will submit their data as soon as possible in standard (mini seed) format with accompanying meta data to the IRIS DMC where the data will be merged into a combined data set and available to individuals and other data centers. This will be by far the best-recorded aftershock sequence of a large megathrust earthquake. This outstanding international collaboration will provide an open data set for this important earthquake as well as provide a model for future aftershock deployments around the world.

Identical Aftershocks from the Main Rupture Zone 10 Months After the Mw=7.6 September 5, 2012, Nicoya, Costa Rica, Earthquake

NASA Astrophysics Data System (ADS)

Protti, M.; Alfaro-Diaz, R.; Brenn, G. R.; Fasola, S.; Murillo, A.; Marshall, J. S.; Gardner, T. W.

2013-12-01

Over a two weeks period and as part of a Keck Geology Consortium summer research project, we installed a dense broad band seismic array directly over the rupture zone of the Nicoya, September 5th, 2012, Mw=7.6 earthquake. The network consisted of 5 Trillium compact seismometers and Taurus digitizers from Nanometrics, defining a triangular area of ~20 km per side. Also located within this area are 3 stations of the Nicoya permanent broadband network. One side of the triangular area, along the west coast of the Nicoya peninsula, is parallel to the trench and the apex lies 15 km landward. The plate interface and rupture zone of the Nicoya 2012 earthquake are located 16 km below the trench-parallel side and 25 km below the apex of this triangular footprint. Station spacing ranged from 3 to 14 km. This dense array operated from July 2nd to July 17th, 2013. On June 23rd, eight days before we installed this array, an Mw=5.4 aftershock (one of the only 5 aftershocks of the Nicoya Mw=7.6 earthquake with magnitudes above 5.0) occurred directly beneath the area of our temporary network. Preliminary analysis of the data shows that we recorded several identical aftershocks with magnitudes below 1.0 that locate some 18 km below our network. We will present detailed locations of these small aftershocks and their relationship with the June 23rd, 2013 aftershock and the September 5th, 2012, mainshock.

Statistical Features of the 2010 Beni-Ilmane, Algeria, Aftershock Sequence

NASA Astrophysics Data System (ADS)

Hamdache, M.; Peláez, J. A.; Gospodinov, D.; Henares, J.

2018-03-01

The aftershock sequence of the 2010 Beni-Ilmane ( M W 5.5) earthquake is studied in depth to analyze the spatial and temporal variability of seismicity parameters of the relationships modeling the sequence. The b value of the frequency-magnitude distribution is examined rigorously. A threshold magnitude of completeness equal to 2.1, using the maximum curvature procedure or the changing point algorithm, and a b value equal to 0.96 ± 0.03 have been obtained for the entire sequence. Two clusters have been identified and characterized by their faulting type, exhibiting b values equal to 0.99 ± 0.05 and 1.04 ± 0.05. Additionally, the temporal decay of the aftershock sequence was examined using a stochastic point process. The analysis was done through the restricted epidemic-type aftershock sequence (RETAS) stochastic model, which allows the possibility to recognize the prevailing clustering pattern of the relaxation process in the examined area. The analysis selected the epidemic-type aftershock sequence (ETAS) model to offer the most appropriate description of the temporal distribution, which presumes that all events in the sequence can cause secondary aftershocks. Finally, the fractal dimensions are estimated using the integral correlation. The obtained D 2 values are 2.15 ± 0.01, 2.23 ± 0.01 and 2.17 ± 0.02 for the entire sequence, and for the first and second cluster, respectively. An analysis of the temporal evolution of the fractal dimensions D -2, D 0, D 2 and the spectral slope has been also performed to derive and characterize the different clusters included in the sequence.

Aftershocks of microearthquakes as probes of the mechanics of rupture

NASA Astrophysics Data System (ADS)

Rubin, Allan M.

2002-07-01

Using a waveform cross-correlation technique, Rubin and Gillard [2000] obtained precise relative locations for 4300 0.5 < M < 3.5 earthquakes occurring along 50 km of the San Andreas fault. This study adds to that another 5000 earthquakes distributed along 10 km of the San Andreas fault and 20 km of the Calaveras fault. Errors in relative location are typically tens of meters for earthquakes separated by hundreds of meters and, after correcting for time-dependent station delays, meters for earthquakes separated by tens of meters. Along both faults, the minimum separation between consecutive earthquakes scales with magnitude in a manner consistent with a magnitude-independent stress drop. By treating each earthquake on the San Andreas as if it were a main shock, scaling the distances to all subsequent earthquakes by main shock size, and stacking the results, a ``composite'' aftershock sequence is produced that has many of the characteristics predicted by rate-and-state friction models. Projected onto the fault surface, these aftershocks outline a quasi-elliptical, roughly 4-MPa stress drop main shock elongate in the slip-parallel direction by ~40%. At the ends of the major axes of this ellipse over twice as many aftershocks occur to the NW than to the SE, an asymmetry attributed to the contrast in material properties across the fault. Unlike the San Andreas, the Calaveras fault exhibits little P wave velocity contrast and no discernible aftershock asymmetry; however, the earliest part of the aftershock sequence on the Calaveras might be truncated by the ~30-s ``blind'' time of the network following a triggering event.

Seismicity and stress transfer studies in eastern California and Nevada: Implications for earthquake sources and tectonics

NASA Astrophysics Data System (ADS)

Ichinose, Gene Aaron

The source parameters for eastern California and western Nevada earthquakes are estimated from regionally recorded seismograms using a moment tensor inversion. We use the point source approximation and fit the seismograms, at long periods. We generated a moment tensor catalog for Mw > 4.0 since 1997 and Mw > 5.0 since 1990. The catalog includes centroid depths, seismic moments, and focal mechanisms. The regions with the most moderate sized earthquakes in the last decade were in aftershock zones located in Eureka Valley, Double Spring Flat, Coso, Ridgecrest, Fish Lake Valley, and Scotty's Junction. The remaining moderate size earthquakes were distributed across the region. The 1993 (Mw 6.0) Eureka Valley earthquake occurred in the Eastern California Shear Zone. Careful aftershock relocations were used to resolve structure from aftershock clusters. The mainshock appears to rupture along the western side of the Last Change Range along a 30° to 60° west dipping fault plane, consistent with previous geodetic modeling. We estimate the source parameters for aftershocks at source-receiver distances less than 20 km using waveform modeling. The relocated aftershocks and waveform modeling results do not indicate any significant evidence of low angle faulting (dips > 30°. The results did reveal deformation along vertical faults within the hanging-wall block, consistent with observed surface rupture along the Saline Range above the dipping fault plane. The 1994 (Mw 5.8) Double Spring Flat earthquake occurred along the eastern Sierra Nevada between overlapping normal faults. Aftershock migration and cross fault triggering occurred in the following two years, producing seventeen Mw > 4 aftershocks The source parameters for the largest aftershocks were estimated from regionally recorded seismograms using moment tensor inversion. We estimate the source parameters for two moderate sized earthquakes which occurred near Reno, Nevada, the 1995 (Mw 4.4) Border Town, and the 1998 (Mw 4.7) Incline Village Earthquakes. We test to see how such stress interactions affected a cluster of six large earthquakes (Mw 6.6 to 7.5) between 1915 to 1954 within the Central Nevada Seismic Belt. We compute the static stress changes for these earthquake using dislocation models based on the location and amount of surface rupture. (Abstract shortened by UMI.)

Aftershock Seismicity of the 27 February 2010 Maule Earthquake and its Relation to Postseismic Displacements from GPS

NASA Astrophysics Data System (ADS)

Lange, D.; Moreno, M. S.; Tilmann, F. J.; Baez, J.; Barrientos, S. E.; Beck, S. L.; Bernard, P.; Bevis, M. G.; Brooks, B. A.; Contreras Reyes, E.; Heit, B.; Methe, P.; Tassara, A.; Vilotte, J.; Vigny, C.

2011-12-01

On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic landstations was installed along the whole rupture zone in order to capture the aftershock activity. We present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. Processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, Caltech and GFZ, we determined 19,908~hypocentres with magnitudes Mw between 1 and 6.2. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25-35 km depth and 50-120 km distance to the trench. Along strike, the aftershocks occur largely within the zone of co-seismic slip but extend ~50 km further north. Along dip, the events are either within the zone of co-seismic slip, or downdip from it, depending on the slip model used. 3.) A third band of seismicity is observed further downdip at 40-50 km depth and further inland at 150-160 km trench perpendicular distance, with mostly shallow dipping thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the co-seismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 to 120 km depth are present north of 36°S. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the mainshock 6.) Pronounced crustal aftershock activity is found in the region of Pichilemu (~34.5°S). The time-series of postseismic deformation analyzed here show rapid transient deformation immediately following the Maule earthquake. We examine the relation between the spatial-temporal properties of the aftershock distribution and postseismic displacements from GPS. First results show a linear relationship between cumulative displacement and cumulative number of aftershocks at large times (>25d, when the local aftershock catalog is available). This relationship may be use to infer rheological properties. Similar relations have been observed in other large subduction zone earthquakes.

Making Initial Earthquake Catalogs from a Temporary Seismic Network for Monitoring Aftershocks

NASA Astrophysics Data System (ADS)

Park, J.; Kang, T. S.; Kim, K. H.; Rhie, J.; Kim, Y.

2017-12-01

The ML 5.1 foreshock and the ML 5.8 mainshock earthquakes occurred consecutively in Gyeongju, the southeastern part of the Korean Peninsula, on September 12, 2016. A temporary seismic network was installed quickly to observe aftershocks followed this mainshock event in the vicinity of the epicenter. The network was consisting of 27 stations equipped with broadband sensors initially and it has been operated in off-line system which required a periodic manual backup of the recorded data. We detected P-triggers and associated events by using SeisComP3 to make an initial catalogue of aftershock events rapidly. If necessary, manual picking was performed to obtain precise P- and S-arrival times from a module, scolv, included in SeisComP3. For cross-checking of reliable identification of seismic phases, a seismic python package, PhasePApy, was applied in parallel with SeisComP3. Then we get the precise relocated coordinates and depth of the aftershock events using the velellipse algorithm. The resulting dataset comprises of an initial aftershock catalog. The catalog will provide the means to address some important questions and issues on seismogenesis in this intraplate seismicity region including the 2016 Gyeongju earthquake sequence and to improve seismic hazard estimation of the region.

Aftershock seismicity and Tectonic Setting of the 16 September 2015 Mw 8.3 Illapel earthquake

NASA Astrophysics Data System (ADS)

Lange, Dietrich; Geersen, Jacob; Barrientos, Sergio; Moreno, Marcos; Grevemeyer, Ingo; Contreras-Reyes, Eduardo; Kopp, Heidrun

2016-04-01

Powerful subduction zone earthquakes rupture thousands of square kilometers along continental margins but at certain locations earthquake rupture terminates. On 16 September 2015 the Mw. 8.3 Illapel earthquake ruptured a 200 km long stretch of the Central Chilean subduction zone, triggering a tsunami and causing significant damage. Here we analyze the spatial pattern of coseismic rupture and the temporal and spatial pattern of local seismicity for aftershocks and foreshocks in relation to the tectonic setting in the earthquake area. Aftershock seismicity surrounds the rupture area in lateral and downdip direction. For the first 24 hours following the mainshock we observe aftershock migration to both lateral directions with velocities of approximately 2.5 and 5 km/h. At the southern earthquake boundary aftershocks cluster around individual subducted seamounts located on the prolongation of the downthrusting Juan Fernández Ridge indicating stress transfer from the main rupture area. In the northern part of the rupture area a deeper band of local seismicity is observed indicating an alternation of seismic to aseismic behavior of the plate interface in downdip direction. This aseismic region at ~30 km depth that is also observed before the Illapel 2015 earthquake is likely controlled by the intersection of the continental Moho with the subducting slab.

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October-November 2002, M = 7.9 Denali fault earthquake

USGS Publications Warehouse

Fisher, M.A.; Nokleberg, W.J.; Ratchkovski, N.A.; Pellerin, L.; Glen, J.M.; Brocher, T.M.; Booker, J.

2004-01-01

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

The 20 April 2013 Lushan, Sichuan, mainshock, and its aftershock sequence: tectonic implications

NASA Astrophysics Data System (ADS)

Lei, Jianshe; Zhang, Guangwei; Xie, Furen

2014-02-01

Using the double-difference relocation algorithm, we relocated the 20 April 2013 Lushan, Sichuan, earthquake ( M S 7.0), and its 4,567 aftershocks recorded during the period between 20 April and May 3, 2013. Our results showed that most aftershocks are relocated between 10 and 20 km depths, but some large aftershocks were relocated around 30 km depth and small events extended upward near the surface. Vertical cross sections illustrate a shovel-shaped fault plane with a variable dip angle from the southwest to northeast along the fault. Furthermore, the dip angle of the fault plane is smaller around the mainshock than that in the surrounding areas along the fault. These results suggest that it may be easy to generate the strong earthquake in the place having a small dip angle of the fault, which is somewhat similar to the genesis of the 2008 Wenchuan earthquake. The Lushan mainshock is underlain by the seismically anomalous layers with low-VP, low-VS, and high-Poisson's ratio anomalies, possibly suggesting that the fluid-filled fractured rock matrices might significantly reduce the effective normal stress on the fault plane to bring the brittle failure. The seismic gap between Lushan and Wenchuan aftershocks is suspected to be vulnerable to future seismic risks at greater depths, if any.

Optimal Scaling of Aftershock Zones using Ground Motion Forecasts

NASA Astrophysics Data System (ADS)

Wilson, John Max; Yoder, Mark R.; Rundle, John B.

2018-02-01

The spatial distribution of aftershocks following major earthquakes has received significant attention due to the shaking hazard these events pose for structures and populations in the affected region. Forecasting the spatial distribution of aftershock events is an important part of the estimation of future seismic hazard. A simple spatial shape for the zone of activity has often been assumed in the form of an ellipse having semimajor axis to semiminor axis ratio of 2.0. However, since an important application of these calculations is the estimation of ground shaking hazard, an effective criterion for forecasting future aftershock impacts is to use ground motion prediction equations (GMPEs) in addition to the more usual approach of using epicentral or hypocentral locations. Based on these ideas, we present an aftershock model that uses self-similarity and scaling relations to constrain parameters as an option for such hazard assessment. We fit the spatial aspect ratio to previous earthquake sequences in the studied regions, and demonstrate the effect of the fitting on the likelihood of post-disaster ground motion forecasts for eighteen recent large earthquakes. We find that the forecasts in most geographic regions studied benefit from this optimization technique, while some are better suited to the use of the a priori aspect ratio.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Diansari, Angga Vertika, E-mail: anggav.bmkg@gmail.com; Purwana, Ibnu; Subakti, Hendri

The 11 April 2012 earthquakes off-shore Aceh-North Sumatra are unique events for the history of Indonesian earthquake. It is unique because that they have similar magnitude, 8.5 Mw and 8.1 Mw; close to epicenter distance, similar strike-slip focal mechanism, and occuring in outer rise area. The purposes of this research are: (1) comparing area of earthquakes base on models and that of calculation, (2) fitting the shape and the area of earthquake rupture zones, (3) analyzing the relationship between rupture area and magnitude of the earthquakes. Rupture area of the earthquake fault are determined by using 4 different formulas, i.e.more » Utsu and Seki (1954), Wells and Coppersmith (1994), Ellsworth (2003), and Christophersen and Smith (2000). The earthquakes aftershock parameters are taken from PGN (PusatGempabumiNasional or National Earthquake Information Center) of BMKG (Indonesia Agency Meteorology Climatology and Geophysics). The aftershock epicenters are plotted by GMT’s software. After that, ellipse and rectangular models of aftershock spreading are made. The results show that: (1) rupture areas were calculated using magnitude relationship which are larger than the the aftershock distributions model, (2) the best fitting model for that earthquake aftershock distribution is rectangular associated with Utsu and Seki (1954) formula, (3) the larger the magnitude of the earthquake, the larger area of the fault.« less

A high-resolution aftershock seismicity image of the 2002 Sultandaği-Çay earthquake (Mw = 6.2), Turkey

NASA Astrophysics Data System (ADS)

Ergin, Mehmet; Aktar, Mustafa; Özalaybey, Serdar; Tapirdamaz, Mustafa C.; Selvi, Oguz; Tarancioglu, Adil

2009-10-01

A moderate-size earthquake (Mw = 6.2) occurred on 3 February 2002 (07:11:28 GMT) in the Sultandağı-Çay region of southwest Turkey. The mainshock was followed by a strong aftershock of Mw = 6.0 just 2 h after the mainshock, at 09:26:49 GMT. A temporary seismic network of 27 vertical component seismometers was installed to monitor aftershock activity. One thousand sixty nine aftershocks (0.2 < ML < 3.3) were recorded during the period from 5 to 10 February 2002. We analyzed the P and S arrival times and P wave first motion data to obtain high-quality hypocenters and focal mechanisms, which revealed fine details of the fault zone. We infer that the mainshock has ruptured a segment of the Sultandağ Fault Zone that is approximately 37 km long and 7 km wide at depth. The average slip over the rupture plane during the mainshock is estimated to be 32 cm. The linear distribution of the aftershocks and the location of the mainshock epicenter suggest that rupture has initiated in the eastern bending of the fault and propagated unilaterally to the west. The majority of fault plane solutions indicate E-W to ESE-WNW striking oblique-normal faulting mechanisms with an average dip angle of 62° N ± 10° . The high-resolution aftershock seismicity image also shows that faulting involved a complex array of synthetic and possibly antithetic structures during the evolution of the aftershock sequence. The steady increase of the b value towards the west implies that the highest moment release of the mainshock occurred to the west of the epicenter. The study clearly shows the activation of the WNW-ESE-trending Sultandağ Fault Zone along the southern margin of the Akşehir-Afyon Graben (AAG). The westernmost end of the aftershock activity corresponds to a structurally complex zone distinct from the main rupture. It is characterized by both ENE-WSW- and NNE-SSW-trending oblique-slip normal faulting mechanisms, the latter being associated with the NNE-SSW-trending Karamık Graben. The intersection of these two grabens, AAG and Karamık Graben, provides abundant faults available for failure in this region. The occurrence pattern of large events in recent years indicates a possible migration of earthquakes from east to west. Thus, we conclude that this has an important implication for earthquake hazard for the city of Afyon, which lies along the same fault line and only 20 km west of the termination point of the aftershock zone.

Gravity and magnetic anomalies used to delineate geologic features associated with earthquakes and aftershocks in the central Virginia seismic zone

NASA Astrophysics Data System (ADS)

Shah, A. K.; Horton, J.; McNamara, D. E.; Spears, D.; Burton, W. C.

2013-12-01

Estimating seismic hazard in intraplate environments can be challenging partly because events are relatively rare and associated data thus limited. Additionally, in areas such as the central Virginia seismic zone, numerous pre-existing faults may or may not be candidates for modern tectonic activity, and other faults may not have been mapped. It is thus important to determine whether or not specific geologic features are associated with seismic events. Geophysical and geologic data collected in response to the Mw5.8 August 23, 2011 central Virginia earthquake provide excellent tools for this purpose. Portable seismographs deployed within days of the main shock showed a series of aftershocks mostly occurring at depths of 3-8 km along a southeast-dipping tabular zone ~10 km long, interpreted as the causative fault or fault zone. These instruments also recorded shallow (< 4 km) aftershocks clustered in several areas at distances of ~2-15 km from the main fault zone. We use new airborne geophysical surveys (gravity, magnetics, radiometrics, and LiDAR) to delineate the distribution of various surface and subsurface geologic features of interest in areas where the earthquake and aftershocks took place. The main (causative fault) aftershock cluster coincides with a linear, NE-trending gravity gradient (~ 2 mgal/km) that extends over 20 km in either direction from the Mw5.8 epicenter. Gravity modeling incorporating seismic estimates of Moho variations suggests the presence of a shallow low-density body overlying the main aftershock cluster, placing it within the upper 2-4 km of the main-fault hanging wall. The gravity, magnetic, and radiometric data also show a bend in generally NE-SW orientation of anomalies close to the Mw5.8 epicenter. Most shallow aftershock clusters occur near weaker short-wavelength gravity gradients of one to several km length. In several cases these gradients correspond to geologic contacts mapped at the surface. Along the gravity gradients, the aftershocks appear to cluster near areas with cross-cutting geologic features such as Jurassic diabase dikes. These associations suggest that local variations in rock density and/or rheology may have contributed to modifications of local stress regimes in a manner encouraging localized seismicity associated with the Mw5.8 event and its aftershocks. Such associations are comparable to results of previous studies recognizing correspondences between seismicity and features such as intrusive bodies and failed rifts in the New Madrid seismic zone and elsewhere. To explore whether similar correspondences may have occurred in the past, we use regional gravity and magnetic data to consider possible relations between historical earthquakes and comparable geologic features elsewhere in the central Virginia seismic zone.

Effects of Aftershock Declustering in Risk Modeling: Case Study of a Subduction Sequence in Mexico

NASA Astrophysics Data System (ADS)

Kane, D. L.; Nyst, M.

2014-12-01

Earthquake hazard and risk models often assume that earthquake rates can be represented by a stationary Poisson process, and that aftershocks observed in historical seismicity catalogs represent a deviation from stationarity that must be corrected before earthquake rates are estimated. Algorithms for classifying individual earthquakes as independent mainshocks or as aftershocks vary widely, and analysis of a single catalog can produce considerably different earthquake rates depending on the declustering method implemented. As these rates are propagated through hazard and risk models, the modeled results will vary due to the assumptions implied by these choices. In particular, the removal of large aftershocks following a mainshock may lead to an underestimation of the rate of damaging earthquakes and potential damage due to a large aftershock may be excluded from the model. We present a case study based on the 1907 - 1911 sequence of nine 6.9 <= Mw <= 7.9 earthquakes along the Cocos - North American plate subduction boundary in Mexico in order to illustrate the variability in risk under various declustering approaches. Previous studies have suggested that subduction zone earthquakes in Mexico tend to occur in clusters, and this particular sequence includes events that would be labeled as aftershocks in some declustering approaches yet are large enough to produce significant damage. We model the ground motion for each event, determine damage ratios using modern exposure data, and then compare the variability in the modeled damage from using the full catalog or one of several declustered catalogs containing only "independent" events. We also consider the effects of progressive damage caused by each subsequent event and how this might increase or decrease the total losses expected from this sequence.

Determination of the fault plane and rupture size of the 2013 Santa Cruz earthquake, Bolivia, 5.2 Mw, by relative location of the aftershocks

NASA Astrophysics Data System (ADS)

Rivadeneyra-Vera, C.; Assumpção, M.; Minaya, E.; Aliaga, P.; Avila, G.

2016-11-01

The Central Andes of southern Bolivia is a highly seismic region with many active faults, that could generate earthquakes up to 8.9 Mw. In 2013, an earthquake of 5.2 Mw occurred in Santa Cruz de la Sierra, in the sub-Andean belt, close to the Mandeyapecua fault, one of the most important reverse faults in Bolivia. Five larger aftershocks were reported by the International Seismological Centre (ISC) and 33 smaller aftershocks were recorded by the San Calixto Observatory (OSC) in the two months after the mainshock. Distances between epicenters of the events were up to 36 km, which is larger than expected for an earthquake of this magnitude. Using data from South American regional stations and the relative location technique with Rayleigh waves, the epicenters of the five larger aftershocks of the Santa Cruz series were determined in relation to the mainshock. This method enabled to achieve epicentral locations with uncertainties smaller than 1 km. Additionally, using data of three Bolivian stations (MOC, SIV and LPAZ) eight smaller aftershocks, recorded by the OSC, were relocated through correlation of P and S waves. The results show a NNW-SSE trend of epicenters and suggest an E dipping plane. The maximum distance between the aftershocks is 14 km, which is not consistent with the expected subsurface rupture length, in accordance with the magnitude of the mainshock. The events are located away from the Mandeyapecua fault and show an opposite dip, demonstrating that these events were generated by another fault in the area, that had not been well studied yet.

Foreshocks and delayed triggering of the 2016 MW7.1 Te Araroa earthquake and dynamic reinvigoration of its aftershock sequence by the MW7.8 Kaikōura earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Warren-Smith, Emily; Fry, Bill; Kaneko, Yoshihiro; Chamberlain, Calum J.

2018-01-01

We analyze the preparatory period of the September 2016 MW7.1 Te Araroa foreshock-mainshock sequence in the Northern Hikurangi margin, New Zealand, and subsequent reinvigoration of Te Araroa aftershocks driven by a large distant earthquake (the November 2016 MW7.8 Kaikōura earthquake). By adopting a matched-filter detection workflow using 582 well-defined template events, we generate an improved foreshock and aftershock catalog for the Te Araroa sequence (>8,000 earthquakes over 66 d). Templates characteristic of the MW7.1 sequence (including the mainshock template) detect several highly correlating events (ML2.5-3.5) starting 12 min after a MW5.7 foreshock. These pre-cursory events occurred within ∼1 km of the mainshock and migrate bilaterally, suggesting precursory slip was triggered by the foreshock on the MW7.1 fault patch prior to mainshock failure. We extend our matched-filter routine to examine the interactions between high dynamic stresses resulting from passing surface waves of the November 2016 MW7.8 Kaikōura earthquake, and the evolution of the Te Araroa aftershock sequence. We observe a sudden spike in moment release of the aftershock sequence immediately following peak dynamic Coulomb stresses of 50-150 kPa on the MW7.1 fault plane. The triggered increase in moment release culminated in a MW5.1 event, immediately followed by a ∼3 h temporal stress shadow. Our observations document the preparatory period of a major subduction margin earthquake following a significant foreshock, and quantify dynamic reinvigoration of a distant on-going major aftershock sequence amid a period of temporal clustering of seismic activity in New Zealand.

Simulation of spatial and temporal properties of aftershocks by means of the fiber bundle model

NASA Astrophysics Data System (ADS)

Monterrubio-Velasco, Marisol; Zúñiga, F. R.; Márquez-Ramírez, Victor Hugo; Figueroa-Soto, Angel

2017-11-01

The rupture processes of any heterogeneous material constitute a complex physical problem. Earthquake aftershocks show temporal and spatial behaviors which are consequence of the heterogeneous stress distribution and multiple rupturing following the main shock. This process is difficult to model deterministically due to the number of parameters and physical conditions, which are largely unknown. In order to shed light on the minimum requirements for the generation of aftershock clusters, in this study, we perform a simulation of the main features of such a complex process by means of a fiber bundle (FB) type model. The FB model has been widely used to analyze the fracture process in heterogeneous materials. It is a simple but powerful tool that allows modeling the main characteristics of a medium such as the brittle shallow crust of the earth. In this work, we incorporate spatial properties, such as the Coulomb stress change pattern, which help simulate observed characteristics of aftershock sequences. In particular, we introduce a parameter ( P) that controls the probability of spatial distribution of initial loads. Also, we use a "conservation" parameter ( π), which accounts for the load dissipation of the system, and demonstrate its influence on the simulated spatio-temporal patterns. Based on numerical results, we find that P has to be in the range 0.06 < P < 0.30, whilst π needs to be limited by a very narrow range ( 0.60 < π < 0.66) in order to reproduce aftershocks pattern characteristics which resemble those of observed sequences. This means that the system requires a small difference in the spatial distribution of initial stress, and a very particular fraction of load transfer in order to generate realistic aftershocks.

Seismic constraints and coulomb stress changes of a blind thrust fault system, 1: Coalinga and Kettleman hills, California

USGS Publications Warehouse

Lin, Jian; Stein, Ross S.

2006-01-01

This report reviews the seismicity and surface ruptures associated with the 1982-1985 earthquake sequence in the Coalinga region in California, and the role of Coulomb stress in triggering the mainshock sequence and aftershocks. The 1982-1985 New Idria, Coalinga, and Kettleman Hills earthquakes struck on a series of west-dipping, en echelon blind thrust faults. Each earthquake was accompanied by uplift of a Quaternary anticline atop the fault, and each was accompanied by a vigorous aftershock sequence. Aftershocks were widely dispersed, and are seen above and below the thrust fault, as well as along the up-dip and down-dip projection of the main thrust fault. For the Coalinga and Kettleman Hills earthquakes, high-angle reverse faults in the core of the anticlines are evident in seismic reflection profiles, and many of these faults are associated with small aftershocks. The shallowest aftershocks extended to within 3-4 km of the ground surface. There is no compelling evidence for aftershocks associated with flexural slip faulting. No secondary surface rupture was found on any of the anticlines. In contrast, the 1983 Nu?ez rupture struck on a high-angle reverse fault 10 km west of the Coalinga epicenter, and over a 40-80-day period, up to 1 m of oblique surface slip occurred. The slip on this Holocene fault likely extended from the ground surface to a depth of 8-10 km. We argue that both the Nu?ez and Kettleman earthquakes were triggered by stresses imparted by the Coalinga mainshock, which was the largest of the four events in the sequence.

Teleseismic Body Wave Analysis for the 27 September 2003 Altai, Earthquake (Mw7.4) and Large Aftershocks

NASA Astrophysics Data System (ADS)

Gomez-Gonzalez, J. M.; Mellors, R.

2007-05-01

We investigate the kinematics of the rupture process for the September 27, 2003, Mw7.3, Altai earthquake and its associated large aftershocks. This is the largest earthquake striking the Altai mountains within the last 50 years, which provides important constraints on the ongoing tectonics. The fault plane solution obtained by teleseismic body waveform modeling indicated a predominantly strike-slip event (strike=130, dip=75, rake 170), Scalar moment for the main shock ranges from 0.688 to 1.196E+20 N m, a source duration of about 20 to 42 s, and an average centroid depth of 10 km. Source duration would indicate a fault length of about 130 - 270 km. The main shock was followed closely by two aftershocks (Mw5.7, Mw6.4) occurred the same day, another aftershock (Mw6.7) occurred on 1 October , 2003. We also modeled the second aftershock (Mw6.4) to asses geometric similarities during their respective rupture process. This aftershock occurred spatially very close to the mainshock and possesses a similar fault plane solution (strike=128, dip=71, rake=154), and centroid depth (13 km). Several local conditions, such as the crustal model and fault geometry, affect the correct estimation of some source parameters. We perfume a sensitivity evaluation of several parameters, including centroid depth, scalar moment and source duration, based on a point and finite source modeling. The point source approximation results are the departure parameters for the finite source exploration. We evaluate the different reported parameters to discard poor constrained models. In addition, deformation data acquired by InSAR are also included in the analysis.

The Stress Transfer and Seismic Interaction Revealed by the Aftershocks of the 2011 Van Earthquake

NASA Astrophysics Data System (ADS)

Konca, A. O.; Işık, S. E.; Karabulut, H.

2016-12-01

We studied the aftershocks of the 2011 Mw7.2 Van, Eastern Turkey, earthquake. This earthquake ruptured an E-W striking blind thrust fault in a region where N-S convergence of the Arabian and Anatolian Plates dominate the tectonic regime. The double-difference relocation of the aftershocks reveal a Z pattern, where in addition to the E-W lineated aftershocks, perpendicular N-S lineated acitivities at each end of the co-seismic rupture are observed. The depths of the aftershocks associated with these two clusters get shallower as their location gets further away from the main fault. Both of the clusters inititated during the first 6 hours following the mainshock and spread away from the mainshock zone in the following days. The focal mechanisms of these aftershocks show that these two clusters are associated with left lateral faults with N-S strikes. These two left-lateral faults seem to cut the Van Fault and possibly determined the co-seismic rupture extent during the 2011 earthquake. This suggested geometry where two off-set left-lateral faults which are connected by a thrust fault is consistent with N-S convergence in the region and also helps explain the post-seismic GPS motion which is not consistent with a single thrust fault. In addition, a third strike-slip cluster to the south of the mainshock has initiated 17 days following the mainshock. This third cluster is associated with an E-W trending right-lateral fault. All of the three activated clusters are on faults which experienced Coulomb stress increase due to the co-seismic slip. Moreover, most seismic activity in the vicinity of the mainshock is on regions where there is Coulomb stress increase.

Stress-based aftershock forecasts made within 24h post mainshock: Expected north San Francisco Bay area seismicity changes after the 2014M=6.0 West Napa earthquake

USGS Publications Warehouse

Parsons, Thomas E.; Segou, Margaret; Sevilgen, Volkan; Milner, Kevin; Field, Edward; Toda, Shinji; Stein, Ross S.

2014-01-01

We calculate stress changes resulting from the M= 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress-based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 h after the main shock and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults.

A model of return intervals between earthquake events

NASA Astrophysics Data System (ADS)

Zhou, Yu; Chechkin, Aleksei; Sokolov, Igor M.; Kantz, Holger

2016-06-01

Application of the diffusion entropy analysis and the standard deviation analysis to the time sequence of the southern California earthquake events from 1976 to 2002 uncovered scaling behavior typical for anomalous diffusion. However, the origin of such behavior is still under debate. Some studies attribute the scaling behavior to the correlations in the return intervals, or waiting times, between aftershocks or mainshocks. To elucidate a nature of the scaling, we applied specific reshulffling techniques to eliminate correlations between different types of events and then examined how it affects the scaling behavior. We demonstrate that the origin of the scaling behavior observed is the interplay between mainshock waiting time distribution and the structure of clusters of aftershocks, but not correlations in waiting times between the mainshocks and aftershocks themselves. Our findings are corroborated by numerical simulations of a simple model showing a very similar behavior. The mainshocks are modeled by a renewal process with a power-law waiting time distribution between events, and aftershocks follow a nonhomogeneous Poisson process with the rate governed by Omori's law.

Stress-based aftershock forecasts made within 24 h postmain shock: Expected north San Francisco Bay area seismicity changes after the 2014 M = 6.0 West Napa earthquake

NASA Astrophysics Data System (ADS)

Parsons, Tom; Segou, Margaret; Sevilgen, Volkan; Milner, Kevin; Field, Edward; Toda, Shinji; Stein, Ross S.

2014-12-01

We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress-based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 h after the main shock and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults.

Aftershocks of the India Republic Day Earthquake: the MAEC/ISTAR Temporary Seismograph Network

NASA Astrophysics Data System (ADS)

Bodin, P.; Horton, S.; Johnston, A.; Patterson, G.; Bollwerk, J.; Rydelek, P.; Steiner, G.; McGoldrick, C.; Budhbhatti, K. P.; Shah, R.; Macwan, N.

2001-05-01

The MW=7.7 Republic Day (26 January, 2001) earthquake on the Kachchh in western India initiated a strong sequence of small aftershocks. Seventeen days following the mainshock, we deployed a network of portable digital event recorders as a cooperative project of the Mid America Earthquake Center in the US and the Institute for Scientific and Technological Advanced Research. Our network consisted of 8 event-triggered Kinemetrics K2 seismographs with 6 data channels (3 accelerometer, 3 Mark L-28/3d seismometer) sampled at 200 Hz, and one continuously-recording Guralp CMG40TD broad-band seismometer sampled at 220 Hz. This network was in place for 18 days. Underlying our network deployment was the notion that because of its tectonic and geologic setting the Republic Day earthquake and its aftershocks might have source and/or propagation characteristics common to earthquakes in stable continental plate-interiors rather than those on plate boundaries or within continental mobile belts. Thus, our goals were to provide data that could be used to compare the Republic Day earthquake with other earthquakes. In particular, the objectives of our network deployment were: (1) to characterize the spatial distribution and occurrence rates of aftershocks, (2) to examine source characteristics of the aftershocks (stress-drops, focal mechanisms), (3) to study the effect of deep unconsolidated sediment on wave propagation, and (4) to determine if other faults (notably the Allah Bundh) were simultaneously active. Most of our sites were on Jurassic bedrock, and all were either free-field, or on the floor of light structures built on rock or with a thin soil cover. However, one of our stations was on a section of unconsolidated sediments hundreds of meters thick adjacent to a site that was subjected to shaking-induced sediment liquefaction during the mainshock. The largest aftershock reported by global networks was an MW=5.9 event on January 28, prior to our deployment. The largest aftershock we recorded was MW=5.3. Our network recorded signals from nearly 2000 earthquakes, all on-scale.

High Frequency Recordings of the Parkfield M=6 and its Aftershocks in the 1.1 km Deep SAFOD Pilot Hole

NASA Astrophysics Data System (ADS)

Malin, P.; Shalev, E.; Chavarria, A.

2004-12-01

Seismic waves from the September 28th Parkfield event and its aftershocks were recorded by the SAFOD Pilot Hole seismic array. This array currently consists of seven levels of 3-component 15 Hz seismometers within the Salinian granite. Its sensors are spaced at 40 m intervals between depths of 856 to 1156 meters below ground. Our deep borehole recordings with high signal-to-noise ratios has allowed us to explore the high frequency content and distribution of both the main event and a large number of aftershocks not detected by the local surface network. We have determined the spectral characteristics for events of different sizes and have related them to their source characteristics. Events close to the PH array contain surprisingly similar distributions of high frequency energy irrespective of their seismic moment. For example, the seismic waves of nearly co-located M~2 and M~5 aftershocks have instrument-corrected corner frequencies that are different by only a few Hz: ~58 Hz versus ~50 Hz. The M~5 can thus be thought of as having broken numerous small but strong fault patches - a model previously suggested by others based on both theoretical and observational grounds. The M~6, which was much further away than these aftershocks, also contains high frequency signals, not quite, but almost, to the same degree. Our results suggest that strong attenuation of high frequency waves in the fault zone area, as well as in shallow weathering layers, prevents more distantly located instruments from recording a complete picture of the actual radiation. Further, in keeping with this suggestion, we have found that, at least for the first nine minutes after the main event, the number of aftershocks observed at the PH is almost ten times higher than that reported in the NCEDC catalog. The rate and size of these events does not fit previous notions of aftershock activity, but may fit with our suggested heterogeneous fault patch and near-source attenuation models.

Application of Subspace Detection to the 6 November 2011 M5.6 Prague, Oklahoma Aftershock Sequence

NASA Astrophysics Data System (ADS)

McMahon, N. D.; Benz, H.; Johnson, C. E.; Aster, R. C.; McNamara, D. E.

2015-12-01

Subspace detection is a powerful tool for the identification of small seismic events. Subspace detectors improve upon single-event matched filtering techniques by using multiple orthogonal waveform templates whose linear combinations characterize a range of observed signals from previously identified earthquakes. Subspace detectors running on multiple stations can significantly increasing the number of locatable events, lowering the catalog's magnitude of completeness and thus providing extraordinary detail on the kinematics of the aftershock process. The 6 November 2011 M5.6 earthquake near Prague, Oklahoma is the largest earthquake instrumentally recorded in Oklahoma history and the largest earthquake resultant from deep wastewater injection. A M4.8 foreshock on 5 November 2011 and the M5.6 mainshock triggered tens of thousands of detectable aftershocks along a 20 km splay of the Wilzetta Fault Zone known as the Meeker-Prague fault. In response to this unprecedented earthquake, 21 temporary seismic stations were deployed surrounding the seismic activity. We utilized a catalog of 767 previously located aftershocks to construct subspace detectors for the 21 temporary and 10 closest permanent seismic stations. Subspace detection identified more than 500,000 new arrival-time observations, which associated into more than 20,000 locatable earthquakes. The associated earthquakes were relocated using the Bayesloc multiple-event locator, resulting in ~7,000 earthquakes with hypocentral uncertainties of less than 500 m. The relocated seismicity provides unique insight into the spatio-temporal evolution of the aftershock sequence along the Wilzetta Fault Zone and its associated structures. We find that the crystalline basement and overlying sedimentary Arbuckle formation accommodate the majority of aftershocks. While we observe aftershocks along the entire 20 km length of the Meeker-Prague fault, the vast majority of earthquakes were confined to a 9 km wide by 9 km deep surface striking N54°E and dipping 83° to the northwest near the junction of the splay with the main Wilzetta fault structure. Relocated seismicity shows off-fault stress-related interaction to distances of 10 km or more from the mainshock, including clustered seismicity to the northwest and southeast of the mainshock.

Transient poroelastic stress coupling between the 2015 M7.8 Gorkha, Nepal earthquake and its M7.3 aftershock

NASA Astrophysics Data System (ADS)

Tung, S.; Masterlark, T.; Dovovan, T.

2018-05-01

The large M7.3 aftershock occurred 17 days after the 2015 M7.8 Gorkha earthquake. We investigate if this sequence is mechanically favored by the mainshock via time-dependent fluid migration and pore pressure recovery. This study uses finite element models of fully-coupled poroelastic coseismic and postseismic behavior to simulate the evolving stress and pore-pressure fields. Using simulations of a reasonable permeability, the hypocenter was destabilized by an additional 0.15 MPa of Coulomb failure stress change (ΔCFS) and 0.17 MPa of pore pressure (Δp), the latter of which induced lateral and upward diffusive fluid flow (up to 2.76 mm/day) in the aftershock region. The M7.3 location is predicted next to a local maximum of Δp and a zone of positive ΔCFS northeast of Kathmandu. About 60% of the aftershocks occurred within zones having either Δp > 0 or ΔCFS > 0. Particularly in the eastern flank of the epicentral area, 83% of the aftershocks experienced postseismic fluid pressurization and 88% of them broke out with positive pore pressure, which are discernibly more than those with positive ΔCFS (71%). The transient scalar field of fluid pressurization provides a good proxy to predict aftershock-prone areas in space and time, because it does not require extraction of an assumed vector field from transient stress tensor fields as is the case for ΔCFS calculations. A bulk permeability of 8.32 × 10-18 m2 is resolved to match the transient response and the timing of the M7.3 rupture which occurred at the peak of the ΔCFS time-series. This estimate is consistent with the existing power-law permeability-versus-depth models, suggesting an intermediately-fractured upper crust coherent with the local geology of the central Himalayas. The contribution of poroelastic triggering is verified against different poroelastic moduli and surface flow-pressure boundaries, suggesting that a poroelastic component is essential to account for the time interval separating the mainshock and the M7.3 aftershock.

Analysis of the 2012 Ahar-Varzeghan (Iran) seismic sequence: Insights from statistical and stress transfer modeling

NASA Astrophysics Data System (ADS)

Yazdi, Pouye; Santoyo, Miguel Angel; Gaspar-Escribano, Jorge M.

2018-02-01

The 2012 Ahar-Varzeghan (Northwestern Iran) earthquake doublet and its following seismic sequence are analyzed in this paper. First, it is examined the time-varying statistical characteristics of seismic activity since the occurrence of the doublet (two large events with Mw = 6.4 and 6.2) that initiated the sequence on 11 August 2012. A power law magnitude-frequency distribution (1.9 ≤ M ≤ 6.4) is obtained, with relatively low b-values for the complete series indicating the existence of relatively large magnitudes and high-stress level in the area. The Omori-Utsu model of the aftershock population decay with time shows a moderate decrease in activity rate. An epidemic-type aftershock sequence model that separates background seismicity from triggered aftershocks is then used to describe the temporal evolution of the seismicity during the period following the occurrence of the doublet. Results for the entire series (above cutoff magnitude Mc = 1.9) indicate a relatively low productivity related to the earthquake-earthquake triggering. Indeed, the majority of these events seems to be generated by underlying transient or aseismic processes, which might be added to the tectonic loading stress. The proportion of aftershock events significantly increases when the analysis is limited to larger events (M ≥ 3.0) suggesting that the triggered large aftershocks entail a substantial portion of the energy released. In order to analyze the spatial distribution of the sequence, new source models are proposed for the two main shocks. For the first shock, the coseismic slip distribution is constrained by the available data on surface ruptures. A Coulomb failure stress transfer model produced by the first event along optimally-oriented planes allows identifying the areas with positive stress loads where the rupture of the subsequent aftershocks may have occurred. The positive Δ CFS areas are compared for two depth intervals: 3-10 km and 15-22 km overlapping over 350 relocated hypocenters, giving arguments supporting the interpretation of Δ CFS as a main mechanism for aftershock triggering in deeper zones of the upper crust.

The 3-D aftershock distribution of three recent M5~5.5 earthquakes in the Anza region,California

NASA Astrophysics Data System (ADS)

Zhang, Q.; Wdowinski, S.; Lin, G.

2011-12-01

The San Jacinto fault zone (SJFZ) exhibits the highest level of seismicity compared to other regions in southern California. On average, it produces four earthquakes per day, most of them at depth of 10-17 km. Over the past decade, an increasing seismic activity occurred in the Anza region, which included three M5~5.5 events and their aftershock sequences. These events occurred in 2001, 2005, and 2010. In this research we map the 3-D distribution of these three events to evaluate their rupture geometry and better understand the unusual deep seismic pattern along the SJFZ, which was termed "deep creep" (Wdowinski, 2009). We relocated 97,562 events from 1981 to 2011 in Anza region by applying the Source-Specific Station Term (SSST) method (Lin et al., 2006) and used an accurate 1-D velocity model derived from 3-D model of Lin et al (2007) and used In order to separate the aftershock sequence from background seismicity, we characterized each of the three aftershock sequences using Omori's law. Preliminary results show that all three sequences had a similar geometry of deep elongated aftershock distribution. Most aftershocks occurred at depth of 10-17 km and extended over a 70 km long segments of the SJFZ, centered at the mainshock hypocenters. A comparative study of other M5~5.5 mainshocks and their aftershock sequences in southern California reveals very different geometrical pattern, suggesting that the three Anza M5~5.5 events are unique and can be indicative of "deep creep" deformation processes. Reference 1.Lin, G.and Shearer,P.M.,2006, The COMPLOC earthquake location package,Seism. Res. Lett.77, pp.440-444. 2.Lin, G. and Shearer, P.M., Hauksson, E., and Thurber C.H.,2007, A three-dimensional crustal seismic velocity model for southern California from a composite event method,J. Geophys.Res.112, B12306, doi: 10.1029/ 2007JB004977. 3.Wdowinski, S. ,2009, Deep creep as a cause for the excess seismicity along the San Jacinto fault, Nat. Geosci.,doi:10.1038/NGEO684.

Statistical Analysis of Seismicity in the Sumatra Region

NASA Astrophysics Data System (ADS)

Bansal, A.; Main, I.

2007-12-01

We examine the effect of the great M=9.0 Boxing day 2004 earthquake on the statistics of seismicity in the Sumatra region by dividing data from the NEIC catalogue into two time windows before and after the earthquake. First we determine a completeness threshold of magnitude 4.5 for the whole dataset from the stability of the maximum likelihood b-value with respect to changes in the threshold. The split data sets have similar statistical sampling, with 2563 events before and 3701 after the event. Temporal clustering is first quantified broadly by the fractal dimension of the time series to be respectively 0.137, 0.259 and 0.222 before, after and for the whole dataset, compared to a Poisson null hypothesis of 0, indicating a significant increase in temporal clustering after the event associated with aftershocks. To quantify this further we apply the Epidemic Type Aftershock Sequence (ETAS) model. The background random seismicity rate £g and the coefficient Ñ, a measure of an efficiency of a magnitude of an earthquake in generating its aftershocks, do not change significantly when averaged over the two time periods. In contrast the amplitude A of aftershock generation changes by a factor 4 or so, and there is a small but statistically significant increase in the Omori decay exponent p, indicating a faster decay rate of the aftershocks after the Sumatra earthquake. The ETAS model parameters are calculated for different magnitude threshold (i.e. 4.5, 5.0, 5.5) with similar results for the different magnitude thresholds. The Ñ values increases from near 1 to near 1.5, possibly reflecting known changes in the scaling exponent between scalar moment and magnitude with increasing magnitude. A simple relation of magnitude and span of aftershock activity indicates that detectable aftershock activity of the Sumatra earthquake may last up to 8.7 years. Earthquakes are predominantly in the depth range 30-40 km before 20-30 km after the mainshock, compared to a CMT centroid depth of the earthquake of 28.6 km.

A Fluid-driven Earthquake Cycle, Omori's Law, and Fluid-driven Aftershocks

NASA Astrophysics Data System (ADS)

Miller, S. A.

2015-12-01

Few models exist that predict the Omori's Law of aftershock rate decay, with rate-state friction the only physically-based model. ETAS is a probabilistic model of cascading failures, and is sometimes used to infer rate-state frictional properties. However, the (perhaps dominant) role of fluids in the earthquake process is being increasingly realised, so a fluid-based physical model for Omori's Law should be available. In this talk, I present an hypothesis for a fluid-driven earthquake cycle where dehydration and decarbonization at depth provides continuous sources of buoyant high pressure fluids that must eventually make their way back to the surface. The natural pathway for fluid escape is along plate boundaries, where in the ductile regime high pressure fluids likely play an integral role in episodic tremor and slow slip earthquakes. At shallower levels, high pressure fluids pool at the base of seismogenic zones, with the reservoir expanding in scale through the earthquake cycle. Late in the cycle, these fluids can invade and degrade the strength of the brittle crust and contribute to earthquake nucleation. The mainshock opens permeable networks that provide escape pathways for high pressure fluids and generate aftershocks along these flow paths, while creating new pathways by the aftershocks themselves. Thermally activated precipitation then seals up these pathways, returning the system to a low-permeability environment and effective seal during the subsequent tectonic stress buildup. I find that the multiplicative effect of an exponential dependence of permeability on the effective normal stress coupled with an Arrhenius-type, thermally activated exponential reduction in permeability results in Omori's Law. I simulate this scenario using a very simple model that combines non-linear diffusion and a step-wise increase in permeability when a Mohr Coulomb failure condition is met, and allow permeability to decrease as an exponential function in time. I show very strong spatial correlations of the simulated evolved permeability and fluid pressure field with aftershock hypocenters from this 1992 Landers and 1994 Northridge aftershock sequences, and reproduce the observed aftershock decay rates. Controls on the decay rates (p-value) will also be discussed.

Extending earthquakes' reach through cascading.

PubMed

Marsan, David; Lengliné, Olivier

2008-02-22

Earthquakes, whatever their size, can trigger other earthquakes. Mainshocks cause aftershocks to occur, which in turn activate their own local aftershock sequences, resulting in a cascade of triggering that extends the reach of the initial mainshock. A long-lasting difficulty is to determine which earthquakes are connected, either directly or indirectly. Here we show that this causal structure can be found probabilistically, with no a priori model nor parameterization. Large regional earthquakes are found to have a short direct influence in comparison to the overall aftershock sequence duration. Relative to these large mainshocks, small earthquakes collectively have a greater effect on triggering. Hence, cascade triggering is a key component in earthquake interactions.

The 2011 Mineral, Virginia, earthquake and its significance for seismic hazards in eastern North America: overview and synthesis

USGS Publications Warehouse

Horton, J. Wright; Chapman, Martin C.; Green, Russell A.

2015-01-01

The earthquake and aftershocks occurred in crystalline rocks within Paleozoic thrust sheets of the Chopawamsic terrane. The main shock and majority of aftershocks delineated the newly named Quail fault zone in the subsurface, and shallow aftershocks defined outlying faults. The earthquake induced minor liquefaction sand boils, but notably there was no evidence of a surface fault rupture. Recurrence intervals, and evidence for larger earthquakes in the Quaternary in this area, remain important unknowns. This event, along with similar events during historical time, is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.

The 2008 Mw 6.0 Wells, Nevada Earthquake Sequence

NASA Astrophysics Data System (ADS)

Smith, K.; Depolo, D.; Torrisi, J.; Edwards, N.; Biasi, G.; Slater, D.

2008-12-01

The Mw 6.0 February 21, 2008 (06:16 AM PDT) Wells, Nevada normal faulting earthquake occurred in Town Creek Flat about 8 km northeast of the small community of Wells. A preliminary set of about 1000 aftershock relocations clearly defines a 55-60 degree southeast dipping fault plane. The structure projects to the surface along the southern end of the Snake Range, although no surface offsets have been identified. The earthquake occurred east of the Ruby Mountains and Snake Range west dipping range front faults, possibly on a northern extension of an east dipping normal fault system on the eastern side of the East Humbolt Range. The depth of the mainshock is estimated to be 10.5 km with the aftershock sequence extending to about 15 km. Typical of moderate sized Basin and Range earthquakes, the early aftershock period included several earthquakes of M > 4 and these were felt strongly by the residents of Wells. From the preliminary relocations, the source radius of the mainshock is estimated to be about 4 km, resulting in an estimated displacement of 55 to 83 cm and static stress drop of 72 to 86 bars, depending on the seismic moment estimate used. Aftershock relocations suggest a radial rupture mechanism. Fortunately, the EarthScope USArray network was operating in Nevada at the time of the event and provided unique controls on the mainshock and early aftershock locations. The earthquake occurred in an area of relatively low seismic hazard and the only permanent seismograph in the region was the U.S. National Network broadband station east of the Ruby Mountains south of Wells. The University of Utah and University of Nevada deployed locally recorded strong motion instruments in the Wells area. Also, an 8 station IP telemetered strong motion network, jointly deployed by the U.S. Geological Survey and University of Nevada Reno, provided real-time data for quick high-quality aftershock relocations and ground motion estimates. In addition, the University of Utah established several telemetered analog stations for improved aftershock locations. IP data communications was routed through the Nevada Department of Information Technology microwave communications site north of Wells. The aftershock deployment was not possible without the considerable support of a number of public and private agencies in the Wells area and the Wells community itself. Many unreinforced masonry structures in old-town Wells, dating to the early 1900's, experienced significant damage. There was also damage to homes and businesses within the community, including the local High School, but fortunately there were no serious injuries associated with the earthquake.

Rupture Speed and Dynamic Frictional Processes for the 1995 ML4.1 Shacheng, Hebei, China, Earthquake Sequence

NASA Astrophysics Data System (ADS)

Liu, B.; Shi, B.

2010-12-01

An earthquake with ML4.1 occurred at Shacheng, Hebei, China, on July 20, 1995, followed by 28 aftershocks with 0.9≤ML≤4.0 (Chen et al, 2005). According to ZÚÑIGA (1993), for the 1995 ML4.1 Shacheng earthquake sequence, the main shock is corresponding to undershoot, while aftershocks should match overshoot. With the suggestion that the dynamic rupture processes of the overshoot aftershocks could be related to the crack (sub-fault) extension inside the main fault. After main shock, the local stresses concentration inside the fault may play a dominant role in sustain the crack extending. Therefore, the main energy dissipation mechanism should be the aftershocks fracturing process associated with the crack extending. We derived minimum radiation energy criterion (MREC) following variational principle (Kanamori and Rivera, 2004)(ES/M0')min≧[3M0/(ɛπμR3)](v/β)3, where ES and M0' are radiated energy and seismic moment gained from observation, μ is the modulus of fault rigidity, ɛ is the parameter of ɛ=M0'/M0,M0 is seismic moment and R is rupture size on the fault, v and β are rupture speed and S-wave speed. From II and III crack extending model, we attempt to reconcile a uniform expression for calculate seismic radiation efficiency ηG, which can be used to restrict the upper limit efficiency and avoid the non-physics phenomenon that radiation efficiency is larger than 1. In ML 4.1 Shacheng earthquake sequence, the rupture speed of the main shock was about 0.86 of S-wave speed β according to MREC, closing to the Rayleigh wave speed, while the rupture speeds of the remained 28 aftershocks ranged from 0.05β to 0.55β. The rupture speed was 0.9β, and most of the aftershocks are no more than 0.35β using II and III crack extending model. In addition, the seismic radiation efficiencies for this earthquake sequence were: for the most aftershocks, the radiation efficiencies were less than 10%, inferring a low seismic efficiency, whereas the radiation efficiency was 78% for the main shock. The essential difference in the earthquake energy partition for the aftershock source dynamics indicated that the fracture energy dissipation could not be ignored in the source parameter estimation for the earthquake faulting, especially for small earthquakes. Otherwise, the radiated seismic energy could be overestimated or underestimated.

Spatial and Temporal Variations in the Moment Tensor Solutions of the 2008 Wenchuan Earthquake Aftershocks and Their Tectonic Implications

NASA Astrophysics Data System (ADS)

Lin, X.; Dreger, D.; Ge, H.; Xu, P.; Wu, M.; Chiang, A.; Zhao, G.; Yuan, H.

2018-03-01

Following the mainshock of the 2008 M8 Wenchuan Earthquake, there were more than 300 ML ≥ 4.0 aftershocks that occurred between 12 May 2008 and 8 September 2010. We analyzed the broadband waveforms for these events and found 160 events with sufficient signal-to-noise levels to invert for seismic moment tensors. Considering the length of the activated fault and the distances to the recording stations, four velocity models were employed to account for variability in crustal structure. The moment tensor solutions show considerable variations with a mixture of mainly reverse and strike-slip mechanisms and a small number of normal events and ambiguous events. We analyzed the spatial and temporal distribution of the aftershocks and their mechanism types to characterize the structure and the deformation occurring in the Longmen Shan fold and thrust belt. Our results suggest that the stress is very complex at the Longmen Shan fault zone. The moment tensors have both a spatial segmentation with two major categories of the moment tensor of thrust and strike slip; and a temporal pattern that the majority of the aftershocks gradually migrated to thrust-type events. The variability of aftershock mechanisms is a strong indication of significant tectonic release and stress reorganization that activated numerous small faults in the system.

Triggered aseismic slip adjacent to the 6 February 2013 Mw 8.0 Santa Cruz Islands megathrust earthquake

USGS Publications Warehouse

Hayes, Gavin P.; Furlong, Kevin P.; Benz, Harley M.; Herman, Matthew W.

2014-01-01

Aseismic or slow slip events have been observed in many subduction zones, but whether they affect the occurrence of earthquakes or result from stress changes caused by nearby events is unclear. In an area lacking direct geodetic observations, inferences can be made from seismological studies of co-seismic slip, associated stress changes and the spatiotemporal nature of aftershocks. These observations indicate that the February 2013 Mw 8.0 Santa Cruz Islands earthquake may have triggered slow or aseismic slip on an adjacent section of the subduction thrust over the following hours to days. This aseismic event was equivalent to Mw 7.6, significantly larger than any earthquakes in the aftershock sequence. The aseismic slip was situated within the seismogenic portion of the subduction interface, and must have occurred to the south of the main seismic slip and most aftershocks in order to promote right-lateral faulting in the upper plate, the dominant deformation style of the aftershock sequence. This plate boundary segment can support either stable sliding (aseismic) or stick-slip (seismic) deformation in response to different driving conditions. The complete lack of aftershocks on the thrust interface implies this pair of megathrust slip episodes (seismic and aseismic) released a substantial portion of the stored strain on the northernmost section of the Vanuatu subduction zone.

Seismo-Tectonics of the 2014 Chiang Rai, Thailand, Earthquake Sequence

NASA Astrophysics Data System (ADS)

Furlong, K. P.; Pananont, P.; Herman, M. W.; Waldhauser, F.; Pornsopin, P.; Warnitchai, P.; Kosuwan, S.

2016-12-01

On 5 May 2014, a Mw 6.2 strike-slip earthquake struck in the Mae Lao region of Chiang Rai province in Thailand. This earthquake occurred in a region of known faults, but identified as relatively low earthquake hazard, and caused substantial damage and injuries. Detailed field reconnaissance and deployment of a dense, temporary seismometer network allowed details of the damage and its relationship to seismicity to be analyzed. The aftershock sequence associated with this mainshock occurs on two well-defined trends, reflecting the two potential fault planes in earthquake focal mechanisms for the mainshock and the majority of the aftershocks. The damage area was relatively large for an event of this magnitude, but primarily occurs within the primary rupture (aftershock) region or along regional rivers with soils susceptible to liquefaction of other ground failure. Stress modeling combined with the time-series and pattern of aftershock activity lead us to propose that the initial mainshock rupture continued slightly onto its conjugate faults near its northern termination, helping to trigger the distinct pattern of two discrete, conjugate trends of aftershock activity that mirror the kinematics of the mainshock fault mechanism. Although this earthquake occurred in a region of known faults, it cannot be directly linked to a previously mapped structure. This coupled with the substantial damage from the event indicates that there is potentially a higher earthquake hazard in northern and central Thailand than previously recognized.

Relationship between Near-Field and Teleseismic Observations of Seismic Source Parameters

DTIC Science & Technology

1978-07-01

For example, foreshocks and aftershocks of the Utah-Idaho border and Oroville, California sequences of 1975, as recorded at the Albuquerque SRO station...have been analyzed and compared; in both cases the principal foreshock exhibited the same mechanisms as the main shock, while the aftershocks are

Anomalous stress diffusion, Omori's law and Continuous Time Random Walk in the 2010 Efpalion aftershock sequence (Corinth rift, Greece)

NASA Astrophysics Data System (ADS)

Michas, Georgios; Vallianatos, Filippos; Karakostas, Vassilios; Papadimitriou, Eleftheria; Sammonds, Peter

2014-05-01

Efpalion aftershock sequence occurred in January 2010, when an M=5.5 earthquake was followed four days later by another strong event (M=5.4) and numerous aftershocks (Karakostas et al., 2012). This activity interrupted a 15 years period of low to moderate earthquake occurrence in Corinth rift, where the last major event was the 1995 Aigion earthquake (M=6.2). Coulomb stress analysis performed in previous studies (Karakostas et al., 2012; Sokos et al., 2012; Ganas et al., 2013) indicated that the second major event and most of the aftershocks were triggered due to stress transfer. The aftershocks production rate decays as a power-law with time according to the modified Omori law (Utsu et al., 1995) with an exponent larger than one for the first four days, while after the occurrence of the second strong event the exponent turns to unity. We consider the earthquake sequence as a point process in time and space and study its spatiotemporal evolution considering a Continuous Time Random Walk (CTRW) model with a joint probability density function of inter-event times and jumps between the successive earthquakes (Metzler and Klafter, 2000). Jump length distribution exhibits finite variance, whereas inter-event times scale as a q-generalized gamma distribution (Michas et al., 2013) with a long power-law tail. These properties are indicative of a subdiffusive process in terms of CTRW. Additionally, the mean square displacement of aftershocks is constant with time after the occurrence of the first event, while it changes to a power-law with exponent close to 0.15 after the second major event, illustrating a slow diffusive process. During the first four days aftershocks cluster around the epicentral area of the second major event, while after that and taking as a reference the second event, the aftershock zone is migrating slowly with time to the west near the epicentral area of the first event. This process is much slower from what would be expected from normal diffusion, a result that is in accordance to earthquake triggering in global scale (Huc and Main, 2003) and aftershocks diffusion in California (Helmstetter et al., 2003). While other mechanisms may be plausible, the results indicate that anomalous stress transfer due to the occurrence of the two major events control the migration of the aftershock activity, activating different fault segments and having strong implications for the seismic hazard of the area. Acknowledgments. G. Michas wishes to acknowledge the partial financial support from the Greek State Scholarships Foundation (IKY). This work has been accomplished in the framework of the postgraduate program and co-funded through the action "Program for scholarships provision I.K.Y. through the procedure of personal evaluation for the 2011-2012 academic year" from resources of the educational program "Education and Life Learning" of the European Social Register and NSRF 2007- 2013. References Ganas, A., Chousianitis, K., Batsi, E., Kolligri, M., Agalos, A., Chouliaras, G., Makropoulos, K. (2013). The January 2010 Efpalion earthquakes (Gulf of Corinth, central Greece): Earthquake interactions and blind normal faulting. J. of Seism., 17(2), 465-484. Helmstetter, A., Ouillon, G., Sornette, D. (2003). Are aftershocks of large California earthquakes diffusing? J. of Geophys. Res. B, 108(10), 2483. Huc, M., Main, I. G. (2003). Anomalous stress diffusion in earthquake triggering: Correlation length, time dependence, and directionality. J. of Geophys. Res. B, 108(7), 2324. Karakostas, V., Karagianni, E., Paradisopoulou, P. (2012). Space-time analysis, faulting and triggering of the 2010 earthquake doublet in western Corinth gulf. Nat.Haz., 63(2), 1181-1202. Metzler, R., Klafter, J. (2000). The random walk's guide to anomalous diffusion: a fractional dynamics approach. Physics Reports, 339, 1-77. Michas, G., Vallianatos, F., Sammonds, P. (2013). Non-extensivity and long-range correlations in the earthquake activity at the West Corinth rift (Greece). Nonlin. Processes Geophys., 20, 713-724. Sokos, E., Zahradník, J., Kiratzi, A., Janský, J., Gallovič, F., Novotny, O., Kostelecký, J., Serpetsidaki, A., Tselentis, G.-A. (2012). The January 2010 Efpalio earthquake sequence in the western Corinth gulf (Greece). Tectonophysics, 530-531, 299-309. Utsu, T., Y. Ogata, Matsu'ura R. S. (1995). The centenary of the Omori formula for a decay law of aftershock activity. J. Phys. Earth, 43, 1- 33.

AFTERSHOCK SEQUENCES AND CRUSTAL STRUCTURE IN THE REGION OF GREECE.

DTIC Science & Technology

the strain release characteristics and other properties of the aftershock and foreshock sequences (1) of all shocks of M 5.9 which have occurred in...relation between the water loading of two artificial lakes in the region of Greece and the earthquake activity in foreshocks or swarm of shocks triggered

Main shock and aftershock records of the 1999 Izmit and Duzce, Turkey earthquakes

USGS Publications Warehouse

Celebi, M.; Akkar, Sinan; Gulerce, U.; Sanli, A.; Bundock, H.; Salkin, A.

2001-01-01

The August 17, 1999 Izmit (Turkey) earthquake (Mw=7.4) will be remembered as one of the largest earthquakes of recent times that affected a large urban environment (U.S. Geological Survey, 1999). This significant event was followed by many significant aftershocks and another main event (Mw=7.2) that occurred on November 12, 1999 near Duzce (Turkey). The shaking that caused the widespread damage and destruction was recorded by a handful of accelerographs (~30) in the earthquake area operated by different networks. The characteristics of these records show that the recorded peak accelerations, shown in Figure 1, even those from near field stations, are smaller than expected (Çelebi, 1999, 2000). Following this main event, several organizations from Turkey, Japan, France and the USA deployed temporary accelerographs and other aftershock recording hardware. Thus, the number of recording stations in the earthquake affected area was quadrupled (~130). As a result, as seen in Figure 2, smaller magnitude aftershocks yielded larger peak accelerations, indicating that because of the sparse networks, recording of larger motions during the main shock of August 17, 1999 were possibly missed.

More major earthquakes at the Nepal Himalaya? - Study on Coulomb stress perspective

NASA Astrophysics Data System (ADS)

Som, S. K.; Sarkar, Subhrasuchi; Dasgupta, Soumitra

2018-07-01

On April 2015 a major earthquake of 7.9 Mw occurred in the Nepal Himalaya, followed by 553 earthquakes of local magnitude greater than 4.0 within the first 43 days including another major event of 7.3 Mw. We resolve the static coulomb failure stress (CFS) change onto the finite fault models of 7.9 Mw after Elliott et al. (2016) and Galezka et al. (2015) and its effect on associated receiver faults. Correlation of aftershocks with the enhanced CFS condition shows that the Elliott et al. (2016) model explains 60.4% and the Galezka et al. (2015) model explains about 47.7% of the aftershocks in high stress regions. Aftershocks were poorly spatially correlated with the enhanced CFS condition after the 7.9 Mw main shock and can be explained by correlation with release of seismic energy from the associated secondarily stressed prominent thrust planes and transverse faults. Stress resolved on the associated receiver faults show increased stress on both transverse and thrust fault systems with the potential of triggering significant aftershocks or subsequent main shocks.

Ground-motion variability resulting from the January 17, 1994, M = 6.6 Northridge earthquake at the interchange between highways 14 and I-5 in the northern San Fernando Valley

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hutchings, L.; Jarpe, S.; Kasameyer, P.

1994-02-04

Lawrence Livermore National Laboratory responded to the 17 January 1994, Northridge earthquake by sending an information gathering team to observe and study the collapse of the interchange between highways 14 and I-5 in the northern San Fernando Valley. This field team examined the structural failure at the interchange and the surface soil conditions, and they installed high-grain RefTek seismic recorders to record aftershocks. We recorded aftershocks for two weeks. Analyses of aftershock recordings in this report illustrate the degree of differential support motion for this site, and the higher than expected ground motion from an earthquake of this size andmore » distance. We used the aftershock recordings of small earthquakes as empirical Green`s functions and synthesized strong ground motion at three sites in the interchange area. Results presented here are based on an assumption that the geology of the interchange area remained linear in its response to the main event.« less

Detection of the Wenchuan aftershock sequence using waveform correlation with a composite regional network

DOE PAGES

Slinkard, Megan; Heck, Stephen; Schaff, David; ...

2016-06-28

Using template waveforms from aftershocks of the Wenchuan earthquake (12 May 2008, M s 8.0) listed in a global bulletin and continuous data from eight regional stations, we detected more than 6000 additional events in the mainshock source region from 1 May to 12 August 2008. These new detections obey Omori’s law, extend the magnitude of completeness downward by 1.1 magnitude units, and lead to a more than fivefold increase in number of known aftershocks compared with the global bulletins published by the International Data Centre and the Inter national Seismological Centre. Moreover, we detected more M > 2 eventsmore » than were listed by the Sichuan Seismograph Network. Several clusters of these detections were then relocated using the double-difference method, yielding locations that reduced travel-time residuals by a factor of 32 compared with the initial bulletin locations. Finally, our results suggest that using waveform correlation on a few regional stations can find aftershock events very effectively and locate them with precision.« less

Strategies for automatic processing of large aftershock sequences

NASA Astrophysics Data System (ADS)

Kvaerna, T.; Gibbons, S. J.

2017-12-01

Aftershock sequences following major earthquakes present great challenges to seismic bulletin generation. The analyst resources needed to locate events increase with increased event numbers as the quality of underlying, fully automatic, event lists deteriorates. While current pipelines, designed a generation ago, are usually limited to single passes over the raw data, modern systems also allow multiple passes. Processing the raw data from each station currently generates parametric data streams that are later subject to phase-association algorithms which form event hypotheses. We consider a major earthquake scenario and propose to define a region of likely aftershock activity in which we will detect and accurately locate events using a separate, specially targeted, semi-automatic process. This effort may use either pattern detectors or more general algorithms that cover wider source regions without requiring waveform similarity. An iterative procedure to generate automatic bulletins would incorporate all the aftershock event hypotheses generated by the auxiliary process, and filter all phases from these events from the original detection lists prior to a new iteration of the global phase-association algorithm.

The 1954 and 1980 Algerian earthquakes: implications for the characteristic-displacement model of fault behavior

USGS Publications Warehouse

Dewey, J.W.

1991-01-01

Joint epicenter determination of earthquakes that occurred in northern Algeria near Ech Cheliff (named Orleansville in 1954 and El Asnam in 1980) shows that the earthquake of 9 September 1954 (M=6.5) occurred at nearly the same location as the earthquake of 10 October 1980 (M=7.3). The 1954 main shock and earliest aftershocks were concentrated close to the boundaries of segment B (nomenclature of Deschamps et al., 1982; King and Yielding, 1984) of the 1980 fault system, which was to experience approximately 8 m of slip in the 1980 earthquake. Later aftershocks of the 1954 earthquake were spread over a broad area, notably in a region north of the 1980 fault system that also experienced many aftershocks to the 1980 earthquake. The closeness of the 1954 main shock and earliest aftershocks to the 1980 segment B implies that the 1954 earthquake involved either 1) rupture of segment B proper, or 2) rupture of a distinct fault in the hanging wall of footwall block of segment B. -from Author

Aftershock stress analysis of the April 2015 Mw 7.8 Gorkha earthquake from the NAMASTE project

NASA Astrophysics Data System (ADS)

Pant, M.; Velasco, A. A.; Karplus, M. S.; Patlan, E.; Ghosh, A.; Nabelek, J.; Kuna, V. M.; Sapkota, S. N.; Adhikari, L. B.; Klemperer, S. L.

2016-12-01

Continental collision between the Indian plate and the Eurasian plate, converging at 45 mm/yr, has uplifted the northern part of Nepal forming the Himalaya. Because of this convergence, the region has experienced large, devastating earthquakes, including the 1934 Mw 8.4 Nepal-Bihar earthquake and two recent earthquakes on April 25, 2015 Mw 7.8 (Gorkha earthquake) and May 12, 2015 Mw 7.2. These quakes killed thousands of people and caused billion dollars of property loss. Despite some recent geologic and geophysical studies of this area, many tectonic questions remain unanswered. Shortly after the Gorkha earthquake, we deployed a seismic network, NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake), to study the aftershocks of these two large events. Our network included 45 different seismic stations (16 short period, 25 broadband, and 4 strong motion sensors) that spanned the Gorkha rupture area. The deployment extends from south of the Main Frontal Thrust (MFT) to the Main Central Thrust region (MCT), and it to recorded aftershocks for more than ten months from June 2015 to May 2016. We are leveraging high-precision earthquake locations by measuring and picking P-wave first-motion arrival polarity to develop a catalog of focal mechanisms for the larger aftershocks. We will use this catalog to correlate the seismicity and stress related of the Indo-Eurasian plate margin, hoping to address questions regarding the complex fault geometries and future earthquake hazards at this plate margin.

Coherent Seismic Arrivals in the P Wave Coda of the 2012 Mw 7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock?

NASA Astrophysics Data System (ADS)

Fan, Wenyuan; Shearer, Peter M.

2018-04-01

Teleseismic records of the 2012 Mw 7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100 s after the direct P arrival. Azimuthal variations in these arrivals, together with back-projection analysis, led Fan and Shearer (https://doi.org/10.1002/2016GL067785) to conclude that they originated from early aftershock(s), located ˜150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (https://doi.org/10.1002/2017GL073254) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back-projection and waveform analyses of three M6 earthquakes within 100 km of the Mw 7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (https://doi.org/10.1002/2017GL073254). In addition, we examine the waveforms of three M5.5 reverse-faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (https://doi.org/10.1002/2016GL067785). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

Aftershock distribution as a constraint on the geodetic model of coseismic slip for the 2004 Parkfield earthquake

USGS Publications Warehouse

Bennington, Ninfa; Thurber, Clifford; Feigl, Kurt; ,

2011-01-01

Several studies of the 2004 Parkfield earthquake have linked the spatial distribution of the event’s aftershocks to the mainshock slip distribution on the fault. Using geodetic data, we find a model of coseismic slip for the 2004 Parkfield earthquake with the constraint that the edges of coseismic slip patches align with aftershocks. The constraint is applied by encouraging the curvature of coseismic slip in each model cell to be equal to the negative of the curvature of seismicity density. The large patch of peak slip about 15 km northwest of the 2004 hypocenter found in the curvature-constrained model is in good agreement in location and amplitude with previous geodetic studies and the majority of strong motion studies. The curvature-constrained solution shows slip primarily between aftershock “streaks” with the continuation of moderate levels of slip to the southeast. These observations are in good agreement with strong motion studies, but inconsistent with the majority of published geodetic slip models. Southeast of the 2004 hypocenter, a patch of peak slip observed in strong motion studies is absent from our curvature-constrained model, but the available GPS data do not resolve slip in this region. We conclude that the geodetic slip model constrained by the aftershock distribution fits the geodetic data quite well and that inconsistencies between models derived from seismic and geodetic data can be attributed largely to resolution issues.

Are Earthquakes Predictable? A Study on Magnitude Correlations in Earthquake Catalog and Experimental Data

NASA Astrophysics Data System (ADS)

Stavrianaki, K.; Ross, G.; Sammonds, P. R.

2015-12-01

The clustering of earthquakes in time and space is widely accepted, however the existence of correlations in earthquake magnitudes is more questionable. In standard models of seismic activity, it is usually assumed that magnitudes are independent and therefore in principle unpredictable. Our work seeks to test this assumption by analysing magnitude correlation between earthquakes and their aftershocks. To separate mainshocks from aftershocks, we perform stochastic declustering based on the widely used Epidemic Type Aftershock Sequence (ETAS) model, which allows us to then compare the average magnitudes of aftershock sequences to that of their mainshock. The results of earthquake magnitude correlations were compared with acoustic emissions (AE) from laboratory analog experiments, as fracturing generates both AE at the laboratory scale and earthquakes on a crustal scale. Constant stress and constant strain rate experiments were done on Darley Dale sandstone under confining pressure to simulate depth of burial. Microcracking activity inside the rock volume was analyzed by the AE technique as a proxy for earthquakes. Applying the ETAS model to experimental data allowed us to validate our results and provide for the first time a holistic view on the correlation of earthquake magnitudes. Additionally we search the relationship between the conditional intensity estimates of the ETAS model and the earthquake magnitudes. A positive relation would suggest the existence of magnitude correlations. The aim of this study is to observe any trends of dependency between the magnitudes of aftershock earthquakes and the earthquakes that trigger them.

Analysis of strong ground motions and site effects at Kantipath, Kathmandu, from 2015 Mw 7.8 Gorkha, Nepal, earthquake and its aftershocks

NASA Astrophysics Data System (ADS)

Dhakal, Yadab P.; Kubo, Hisahiko; Suzuki, Wataru; Kunugi, Takashi; Aoi, Shin; Fujiwara, Hiroyuki

2016-04-01

Strong ground motions from the 2015 Mw 7.8 Gorkha, Nepal, earthquake and its eight aftershocks recorded by a strong-motion seismograph at Kantipath (KATNP), Kathmandu, were analyzed to assess the ground-motion characteristics and site effects at this location. Remarkably large elastic pseudo-velocity responses exceeding 300 cm/s at 5 % critical damping were calculated for the horizontal components of the mainshock recordings at peak periods of 4-5 s. Conversely, the short-period ground motions of the mainshock were relatively weak despite the proximity of the site to the source fault. The horizontal components of all large-magnitude (Mw ≥ 6.3) aftershock recordings showed peak pseudo-velocity responses at periods of 3-4 s. Ground-motion prediction equations (GMPEs) describing the Nepal Himalaya region have not yet been developed. A comparison of the observational data with GMPEs for Japan showed that with the exception of the peak ground acceleration (PGA) of the mainshock, the observed PGAs and peak ground velocities at the KATNP site are generally well described by the GMPEs for crustal and plate interface events. A comparison of the horizontal-to-vertical ( H/ V) spectral ratios for the S-waves of the mainshock and aftershock recordings suggested that the KATNP site experienced a considerable nonlinear site response, which resulted in the reduced amplitudes of short-period ground motions. The GMPEs were found to underestimate the response values at the peak periods (approximately 4-5 s) of the large-magnitude events. The deep subsurface velocity model of the Kathmandu basin has not been well investigated. Therefore, a one-dimensional velocity model was constructed for the deep sediments beneath the recording station based on an analysis of the H/ V spectral ratios for S-wave coda from aftershock recordings, and it was revealed that the basin sediments strongly amplified the long-period components of the ground motions of the mainshock and large-magnitude aftershocks.

Remote detection of weak aftershocks of the DPRK underground explosions using waveform cross correlation

NASA Astrophysics Data System (ADS)

Le Bras, R.; Rozhkov, M.; Bobrov, D.; Kitov, I. O.; Sanina, I.

2017-12-01

Association of weak seismic signals generated by low-magnitude aftershocks of the DPRK underground tests into event hypotheses represent a challenge for routine automatic and interactive processing at the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization, due to the relatively low station density of the International Monitoring System (IMS) seismic network. Since 2011, as an alternative, the IDC has been testing various prototype techniques of signal detection and event creation based on waveform cross correlation. Using signals measured by seismic stations of the IMS from DPRK explosions as waveform templates, the IDC detected several small (estimated mb between 2.2 and 3.6) seismic events after two DPRK tests conducted on September 9, 2016 and September 3, 2017. The obtained detections were associated with reliable event hypothesis and then used to locate these events relative to the epicenters of the DPRK explosions. We observe high similarity of the detected signals with the corresponding waveform templates. The newly found signals also correlate well between themselves. In addition, the values of the signal-to-noise ratios (SNR) estimated using the traces of cross correlation coefficients, increase with template length (from 5 s to 150 s), providing strong evidence in favour of their spatial closeness, which allows interpreting them as explosion aftershocks. We estimated the relative magnitudes of all aftershocks using the ratio of RMS amplitudes of the master and slave signal in the cross correlation windows characterized by the highest SNR. Additional waveform data from regional non-IMS stations MDJ and SEHB provide independent validation of these aftershock hypotheses. Since waveform templates from any single master event may be sub-efficient at some stations, we have also developed a method of joint usage of the DPRK and the biggest aftershocks templates to build more robust event hypotheses.

Characterizing spatial heterogeneity based on the b-value and fractal analyses of the 2015 Nepal earthquake sequence

NASA Astrophysics Data System (ADS)

Nampally, Subhadra; Padhy, Simanchal; Dimri, Vijay P.

2018-01-01

The nature of spatial distribution of heterogeneities in the source area of the 2015 Nepal earthquake is characterized based on the seismic b-value and fractal analysis of its aftershocks. The earthquake size distribution of aftershocks gives a b-value of 1.11 ± 0.08, possibly representing the highly heterogeneous and low stress state of the region. The aftershocks exhibit a fractal structure characterized by a spectrum of generalized dimensions, Dq varying from D2 = 1.66 to D22 = 0.11. The existence of a fractal structure suggests that the spatial distribution of aftershocks is not a random phenomenon, but it self-organizes into a critical state, exhibiting a scale-independent structure governed by a power-law scaling, where a small perturbation in stress is sufficient enough to trigger aftershocks. In order to obtain the bias in fractal dimensions resulting from finite data size, we compared the multifractal spectrum for the real data and random simulations. On comparison, we found that the lower limit of bias in D2 is 0.44. The similarity in their multifractal spectra suggests the lack of long-range correlation in the data, with an only weakly multifractal or a monofractal with a single correlation dimension D2 characterizing the data. The minimum number of events required for a multifractal process with an acceptable error is discussed. We also tested for a possible correlation between changes in D2 and energy released during the earthquakes. The values of D2 rise during the two largest earthquakes (M > 7.0) in the sequence. The b- and D2 values are related by D2 = 1.45 b that corresponds to the intermediate to large earthquakes. Our results provide useful constraints on the spatial distribution of b- and D2-values, which are useful for seismic hazard assessment in the aftershock area of a large earthquake.

The January 2014 Northern Cuba Earthquake Sequence - Unusual Location and Unexpected Source Mechanism Variability

NASA Astrophysics Data System (ADS)

Braunmiller, J.; Thompson, G.; McNutt, S. R.

2017-12-01

On 9 January 2014, a magnitude Mw=5.1 earthquake occurred along the Bahamas-Cuba suture at the northern coast of Cuba revealing a surprising seismic hazard source for both Cuba and southern Florida where it was widely felt. Due to its location, the event and its aftershocks (M>3.5) were recorded only at far distances (300+ km) resulting in high-detection thresholds, low location accuracy, and limited source parameter resolution. We use three-component regional seismic data to study the sequence. High-pass filtered seismograms at the closest site in southern Florida are similar in character suggesting a relatively tight event cluster and revealing additional, smaller aftershocks not included in the ANSS or ISC catalogs. Aligning on the P arrival and low-pass filtering (T>10 s) uncovers a surprise polarity flip of the large amplitude surface waves on vertical seismograms for some aftershocks relative to the main shock. We performed regional moment tensor inversions of the main shock and its largest aftershocks using complete three-component seismograms from stations distributed throughout the region to confirm the mechanism changes. Consistent with the GCMT solution, we find an E-W trending normal faulting mechanism for the main event and for one immediate aftershock. Two aftershocks indicate E-W trending reverse faulting with essentially flipped P- and T-axes relative to the normal faulting events (and the same B-axes). Within uncertainties, depths of the two event families are indistinguishable and indicate shallow faulting (<10 km). One intriguing possible interpretation is that both families ruptured the same fault with reverse mechanisms compensating for overshooting. However, activity could also be spatially separated either vertically (with reverse mechanisms possibly below extension) or laterally. The shallow source depth and the 200-km long uplifted chain of islands indicate that larger, shallow and thus potentially tsunamigenic earthquakes could occur just offshore of northern Cuba posing a potential hazard to Florida and the Bahamas.

New insights on co- and post-seismic deformation and slip behavior associated with the Mw7.8 2016 Pedernales, Ecuador earthquake and its aftershock sequence

NASA Astrophysics Data System (ADS)

Soto-Cordero, L.; Nealy, J. L.; Meltzer, A.; Agurto-Detzel, H.; Alvarado, A. P.; Beck, S. L.; Benz, H.; Bergman, E. A.; Charvis, P.; Font, Y.; Hayes, G. P.; Hernandez, S.; Hoskins, M.; Leon Rios, S.; Lynner, C.; Regnier, M. M.; Rietbrock, A.; Stachnik, J. C.; Yeck, W. L.

2017-12-01

On April 16, 2016, a Mw7.8 earthquake, associated with oblique subduction of the Nazca Plate under South America, ruptured a segment approximately 130x100km in the region north of the intersection of the Carnegie ridge with the Ecuador subduction zone. The rupture coincides with the rupture area of the Mw7.8 1942 earthquake. To characterize the aftershock sequence, we analyze seismic data recorded by 30 stations from April 17, 2016 to May 8, 2017; 11 stations belong to Ecuador's national network and 19 are part of a PASSCAL temporary deployment. We apply a kurtosis detector to obtain automatic P- and S-wave picks. Earthquake locations, magnitudes, and regional moment tensors are obtained using the U.S. Geological Survey National Earthquake Information Center (NEIC) processing system. We also determine calibrated relocations using the Hypocentroidal Decomposition approach for a subset of events for which we combine phase readings from local and temporary PASSCAL stations with regional and teleseismic phase readings from the NEIC. In contrast with other earthquake relocation approaches, this method evaluates absolute location uncertainties for each event in the cluster, which allows us to more confidently assess the relationships between mainshock slip and aftershock activity. We find the aftershock sequence is characterized by a series of event clusters that predominantly surround the main rupture patches. However, the aftershocks extend beyond the mainshock rupture area, covering a region approximately 250x100km. Aftershocks north of the 2016 rupture fall in the rupture area of the Mw7.7 1958 earthquake. The southernmost region of elevated seismicity occurs south of a region of low coupling where the Carnegie ridge meets the subduction zone. The characterization of this sequence allows a detailed spatial and temporal analysis of the rupture processes, stress patterns and slip behavior during this earthquake sequence in Ecuador subduction zone.

Båth's law and its relation to the tectonic environment: A case study for earthquakes in Mexico

NASA Astrophysics Data System (ADS)

Rodríguez-Pérez, Q.; Zúñiga, F. R.

2016-09-01

We studied 66 mainshocks and their largest aftershocks in the Mexican subduction zone and in the Gulf of California with magnitudes in the range of 5.2 < Mw < 8.0 from 1932 to 2015. Three different types of earthquakes were analyzed: shallow thrust interplate, intermediate-depth inslab and transform strike-slip earthquakes (26, 19 and 21 events, respectively). We focus on observational aspects of the Båth's law. By studying the magnitude difference, energy ratios and energy partitioning of the mainshock-largest aftershock sequences, we analyze the physics of the mainshock-largest aftershock relationship (Båth's law). The partitioning of energy during a mainshock-aftershock sequence shows that about 96-97% of the energy dissipated in a sequence is associated with the mainshock and the rest is due to aftershocks. Our results for radiated seismic energy and energy-to-moment ratio are partially in agreement with worldwide studies supporting the observation of mechanism dependence of radiated seismic energy. The statistical tests indicate that the only significant difference is for shallow thrust and strike-slip events for these parameters. The statistical comparison of stress drop of shallow thrust versus that of inslab events shows a strongly significant difference with a confidence better than 99%. The comparison of stress drop of shallow thrust events with that of strike-slip events, also indicates a strongly significant difference. We see no dependence of stress drop with magnitude, which is strong evidence of earthquake self-similarity. We do not observe a systematic depth dependence of stress drop. The results also reveal differences in the earthquake rupture among the events. The magnitude difference between the mainshock and the largest aftershock for inslab events is larger than interplate and strike-slip events suggesting focal mechanism dependence of Båth's law. For the case of this parameter, only that for inslab and strike-slip events present a significant difference with 95% confidence.

High-Resolution Uitra Low Power, Intergrated Aftershock and Microzonation System

NASA Astrophysics Data System (ADS)

Passmore, P.; Zimakov, L. G.

2012-12-01

Rapid Aftershock Mobilization plays an essential role in the understanding of both focal mechanism and rupture propagation caused by strong earthquakes. A quick assessment of the data provides a unique opportunity to study the dynamics of the entire earthquake process in-situ. Aftershock study also provides practical information for local authorities regarding the post earthquake activity, which is very important in order to conduct the necessary actions for public safety in the area affected by the strong earthquake. Refraction Technology, Inc. has developed a self-contained, fully integrated Aftershock System, model 160-03, providing the customer simple and quick deployment during aftershock emergency mobilization and microzonation studies. The 160-03 has no external cables or peripheral equipment for command/control and operation in the field. The 160-03 contains three major components integrated in one case: a) 24-bit resolution state-of-the art low power ADC with CPU and Lid interconnect boards; b) power source; and c) three component 2 Hz sensors (two horizontals and one vertical), and built-in ±4g accelerometer. Optionally, the 1 Hz sensors can be built-in the 160-03 system at the customer's request. The self-contained rechargeable battery pack provides power autonomy up to 7 days during data acquisition at 200 sps on continuous three weak motion and triggered three strong motion recording channels. For longer power autonomy, the 160-03 Aftershock System battery pack can be charged from an external source (solar power system). The data in the field is recorded to a built-in swappable USB flash drive. The 160-03 configuration is fixed based on a configuration file stored on the system, so no external command/control interface is required for parameter setup in the field. For visual control of the system performance in the field, the 160-03 has a built-in LED display which indicates the systems recording status as well as a hot swappable USB drive and battery status. The detailed specifications and performance are presented and discussed.;

Use of microearthquakes in the study of the mechanics of earthquake generation along the San Andreas fault in central California

USGS Publications Warehouse

Eaton, J.P.; Lee, W.H.K.; Pakiser, L.C.

1970-01-01

A small, dense network of independently recording portable seismograph stations was used to delineate the slip surface associated with the 1966 Parkfield-Cholame earthquake by precise three dimensional mapping of the hypocenters of its aftershocks. The aftershocks were concentrated in a very narrow vertical zone beneath or immediately adjacent to the zone of surf ace fracturing that accompanied the main shock. Focal depths ranged from less than 1 km to a maximum of 15 km. The same type of portable network was used to study microearthquakes associated with an actively creeping section of the San Andreas fault south of Hollister during the summer of 1967. Microearthquake activity during the 6-week operation of this network was dominated by aftershocks of a magnitude-4 earthquake that occurred within the network near Bear Valley on July 23. Most of the aftershocks were concentrated in an equidimensional region about 2 1 2km across that contained the hypocenter of the main shock. The zone of the concentrated aftershocks was centered near the middle of the rift zone at a depth of about 3 1 2km. Hypocenters of other aftershocks outlined a 25 km long zone of activity beneath the actively creeping strand of the fault and extending from the surface to a depth of about 13 km. A continuing study of microearthquakes along the San Andreas, Hayward, and Calaveras faults between Hollister and San Francisco has been under way for about 2 years. The permanent telemetered network constructed for this purpose has grown from about 30 stations in early 1968 to about 45 stations in late 1969. Microearthquakes between Hollister and San Francisco are heavily concentrated in narrow, nearly vertical zones along sections of the Sargent, San Andreas, and Calaveras faults. Focal depths range from less than 1 km to about 14 km. ?? 1970.

Spatiotemporal Analysis of the Foreshock-Mainshock-Aftershock Sequence of the 6 July 2017 M5.8 Lincoln, Montana Earthquake

NASA Astrophysics Data System (ADS)

McMahon, N. D.; Stickney, M.; Aster, R. C.; Yeck, W.; Martens, H. R.; Benz, H.

2017-12-01

On 6 July 2017, a Mw 5.8 earthquake occurred 11 km southeast of Lincoln, Montana. The event was widely-felt from Edmonton, Alberta, Canada (750 km north), Seattle, Washington (800 km west), the Idaho/Utah and Idaho/Nevada borders (550 km south), and Rapid City, South Dakota (750 km east). This is the largest earthquake to occur in the state since the 1959 M 7.3 Hebgen Lake event 250 km to the southeast. In the three weeks following the 6 July 2017 Mw 5.8 main shock, the U.S. Geological Survey and Montana Bureau of Mines and Geology located more than 300 aftershocks. Preliminary observations show most of these aftershocks form a short NNE zone that suggests that the main shock may have slipped on a NNE left-lateral fault. A smaller number of aftershocks extend along a longer WNW-trending zone. These faults are part of the Lewis and Clark line, a prominent zone of Middle Proterozoic to Holocene age strike-slip, dip slip, and oblique slip faulting trending 400 km east-southeast from northern Idaho to east of Helena, Montana, and terminating southeast of this earthquake. We use identified aftershock waveforms as templates to examine the data from 1 June 2017 through 27 July 2017 with cross-correlation techniques on nearby permanent and temporary seismic stations deployed shortly after the mainshock to identify foreshocks and additional small aftershocks. Locating these events allows us to study subsurface geology, map fault structures, and provide insight on the spatial and temporal evolution of the earthquake sequence, which may continue to produce aftershocks for years. Other notable earthquakes in the region include a damaging M 6.6 earthquake 100 km to the south in June 1925, M 6.2 and M 6.0 earthquakes near Helena, Montana in October 1935 that caused significant damage and four fatalities, and a M 5.6 earthquake 170 km to the south in July 2005 that caused minor damage in Dillon and the surrounding region. We hope this work not only allows us to map the involved faults and detail hazards associated with them, but helps to raise awareness in general about the underrepresented hazard associated with intraplate seismicity and the reactivation of older preexisting faults that have few, if any, Quaternary expressions.

Rapid Seismic Deployment for Capturing Aftershocks of the September 2017 Tehuantepec, Mexico (M=8.1) and Morelos-Puebla (M=7.1), Mexico Earthquakes

NASA Astrophysics Data System (ADS)

Velasco, A. A.; Karplus, M. S.; Dena, O.; Gonzalez-Huizar, H.; Husker, A. L.; Perez-Campos, X.; Calo, M.; Valdes, C. M.

2017-12-01

The September 7 Tehuantepec, Mexico (M=8.1) and the September 19 Morelos-Puebla, Mexico (M=7.1) earthquakes ruptured with extensional faulting within the Cocos Plate at 70-km and 50-km depth, as it subducts beneath the continental North American Plate. Both earthquakes caused significant damage and loss of life. These events were followed by a M=6.1 extensional earthquake at only 10-km depth in Oaxaca on September 23, 2017. While the Morelos-Puebla earthquake was likely too far away to be statically triggered by the Tehuantepec earthquake, initial Coulomb stress analyses show that the M=6.1 event may have been an aftershock of the Tehuantepec earthquake. Many questions remain about these earthquakes, including: Did the Cocos Plate earthquakes load the upper plate, and could they possibly trigger an equal or larger earthquake on the plate interface? Are these the result of plate bending? Do the aftershocks migrate to the locked zone in the subduction zone? Why did the intermediate depth earthquakes create so much damage? Are these earthquakes linked by dynamic stresses? Is it possible that a potential slow-slip event triggered both events? To address some of these questions, we deployed 10 broadband seismometers near the epicenter of the Tehuantepec, Mexico earthquake and 51 UTEP-owned nodes (5-Hz, 3-component geophones) to record aftershocks and augment networks deployed by the Universidad Nacional Autónoma de México (UNAM). The 10 broadband instruments will be deployed for 6 months, while the nodes were deployed 25 days. The relative ease-of-deployment and larger numbers of the nodes allowed us to deploy them quickly in the area near the M=6.1 Oaxaca earthquake, just a few days after that earthquake struck. We deployed them near the heavily-damaged cities of Juchitan, Ixtaltepec, and Ixtepec as well as in Tehuantepec and Salina Cruz, Oaxaca in order to test their capabilities for site characterization and aftershock studies. This is the first test of these instruments in a region heavily affected by aftershocks, outside of Oklahoma. Analysis of the nodal and broadband data will allow us to investigate fault geometries from aftershock locations, stress release and orientation from the determination of fault plane solutions, and site effects and characteristics in regions of extensive damage.

Tsunami waves generated by dynamically triggered aftershocks of the 2010 Haiti earthquake

NASA Astrophysics Data System (ADS)

Ten Brink, U. S.; Wei, Y.; Fan, W.; Miller, N. C.; Granja, J. L.

2017-12-01

Dynamically-triggered aftershocks, thought to be set off by the passage of surface waves, are currently not considered in tsunami warnings, yet may produce enough seafloor deformation to generate tsunamis on their own, as judged from new findings about the January 12, 2010 Haiti earthquake tsunami in the Caribbean Sea. This tsunami followed the Mw7.0 Haiti mainshock, which resulted from a complex rupture along the north shore of Tiburon Peninsula, not beneath the Caribbean Sea. The mainshock, moreover, had a mixed strike-slip and thrust focal mechanism. There were no recorded aftershocks in the Caribbean Sea, only small coastal landslides and rock falls on the south shore of Tiburon Peninsula. Nevertheless, a tsunami was recorded on deep-sea DART buoy 42407 south of the Dominican Republic and on the Santo Domingo tide gauge, and run-ups of ≤3 m were observed along a 90-km-long stretch of the SE Haiti coast. Three dynamically-triggered aftershocks south of Haiti have been recently identified within the coda of the mainshock (<200 s) by analyzing P wave arrivals recorded by dense seismic arrays, parsing the arrivals into 20-s-long stacks, and back-projecting the arrivals to the vicinity of the main shock (50-300 km). Two of the aftershocks, coming 20-40 s and 40-60 s after the mainshock, plot along NW-SE-trending submarine ridges in the Caribbean Sea south of Haiti. The third event, 120-140 s was located along the steep eastern slope of Bahoruco Peninsula, which is delineated by a normal fault. Forward tsunami models show that the arrival times of the DART buoy and tide gauge times are best fit by the earliest of the three aftershocks, with a Caribbean source 60 km SW of the mainshock rupture zone. Preliminary inversion of the DART buoy time series for fault locations and orientations confirms the location of the first source, but requires an additional unidentified source closer to shore 40 km SW of the mainshock rupture zone. This overall agreement between earthquake and tsunami analyses suggests that land-based earthquake ruptures and/or non-thrust main shocks can generate tsunamis by means of dynamically-triggered aftershocks. It also provides an independent verification to the back-projection seismic method, and it indicates that the active NE-SW shortening of Hispaniola extends southward into the Caribbean Sea.

Hurricane Irene's Impacts on the Aftershock Sequence of the 2011 Mw5.8 Virginia Earthquake

NASA Astrophysics Data System (ADS)

Meng, X.; Peng, Z.; Yang, H.; Allman, S.

2013-12-01

Recent studies have shown that typhoon could trigger shallow slow-slip events in Taiwan. However, it is unclear whether such extreme weather events could affect the occurrence of regular earthquakes as well. A good opportunity to test this hypothesis occurred in 2011 when an Mw 5.8 earthquake struck Louisa County, Virginia. This event ruptured a shallow, reverse fault. Roughly 5 days later, hurricane Irene struck the coast of Norfolk, Virginia, which is near the epicentral region of the Virginia mainshock. Because aftershocks listed in the ANSS catalog were incomplete immediately after the main shock, it is very difficult to find the genuine correlation between the seismicity rate changes and hurricane Irene. Hence, we use a recently developed waveform matched filter technique to scan through the continuous seismic data to detect small aftershocks that are previously unidentified. A mixture of 7 temporary stations from the IRIS Ramp deployment and 8 temporary stations deployed by Virginia Tech is used. The temporary stations were set up between 24 to 72 hours following the main shock around its immediate vicinity, which provides us a unique dataset recording the majority aftershock sequence of an intraplate earthquake. We us 80 aftershocks identified by Chapman [2013] as template events and scan through the continuous data from 23 August 2011 through 10 September 2011. So far, we have detected 704 events using a threshold of 12 times the median absolute deviation (MAD), which is ~25 times more than listed in the ANSS catalog. The aftershock rate generally decayed with time as predicted by the Omori's law. A statistically significant increase of seismicity rate is found when hurricane Irene passed by the epicentral region. A possible explanation is that the atmosphere pressure drop unloaded the surface, which brought the reverse faults closer to failure. However, we also identified similar fluctuations of seismicity rate changes at other times. Hence, it is still possible that the seismicity rate increase during the hurricane Irene's passing by could be by coincidence. We are currently in the process of relocating all the newly detected events and investigate if any aftershock migration pattern might relate to the path of hurricane Irene. If true, it would suggest that extreme weather events such as hurricanes could trigger not only slow-slip events, but also regular earthquakes.

The aftershock signature of supershear earthquakes.

PubMed

Bouchon, Michel; Karabulut, Hayrullah

2008-06-06

Recent studies show that earthquake faults may rupture at speeds exceeding the shear wave velocity of rocks. This supershear rupture produces in the ground a seismic shock wave similar to the sonic boom produced by a supersonic airplane. This shock wave may increase the destruction caused by the earthquake. We report that supershear earthquakes are characterized by a specific pattern of aftershocks: The fault plane itself is remarkably quiet whereas aftershocks cluster off the fault, on secondary structures that are activated by the supershear rupture. The post-earthquake quiescence of the fault shows that friction is relatively uniform over supershear segments, whereas the activation of off-fault structures is explained by the shock wave radiation, which produces high stresses over a wide zone surrounding the fault.

Artificial seismic acceleration

USGS Publications Warehouse

Felzer, Karen R.; Page, Morgan T.; Michael, Andrew J.

2015-01-01

In their 2013 paper, Bouchon, Durand, Marsan, Karabulut, 3 and Schmittbuhl (BDMKS) claim to see significant accelerating seismicity before M 6.5 interplate mainshocks, but not before intraplate mainshocks, reflecting a preparatory process before large events. We concur with the finding of BDMKS that their interplate dataset has significantly more fore- shocks than their intraplate dataset; however, we disagree that the foreshocks are predictive of large events in particular. Acceleration in stacked foreshock sequences has been seen before and has been explained by the cascade model, in which earthquakes occasionally trigger aftershocks larger than themselves4. In this model, the time lags between the smaller mainshocks and larger aftershocks follow the inverse power law common to all aftershock sequences, creating an apparent acceleration when stacked (see Supplementary Information).

Aftershocks following crash of currency exchange rate: The case of RUB/USD in 2014

NASA Astrophysics Data System (ADS)

Usmanova, Vasilya; Lysogorskiy, Yury V.; Abe, Sumiyoshi

2018-02-01

The dynamical behavior of the currency exchange rate after its large-scale catastrophe is discussed through a case study of the rate of Russian rubles to US dollars after its crash in 2014. It is shown that, similarly to the case of the stock market crash, the relaxation is characterized by a power law, which is in analogy with the Omori-Utsu law for earthquake aftershocks. The waiting-time distribution is found to also obey a power law. Furthermore, the event-event correlation is discussed, and the aging phenomenon and scaling property are observed. Comments are made on (non-)Markovianity of the aftershock process and on a possible relevance of glassy dynamics to the market system after the crash.

Urban seismology - Northridge aftershocks recorded by multi-scale arrays of portable digital seismographs

USGS Publications Warehouse

Meremonte, M.; Frankel, A.; Cranswick, E.; Carver, D.; Worley, D.

1996-01-01

We deployed portable digital seismographs in the San Fernando Valley (SFV), the Los Angeles basin (LAB), and surrounding hills to record aftershocks of the 17 January 1994 Northridge California earthquake. The purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas, such as site amplification, sedimentary basin effects, and the variability of ground motion over short baselines. We placed seismographs at 47 sites (not all concurrently) and recorded about 290 earthquakes with magnitudes up to 5.1 at five stations or more. We deployed widely spaced stations for profiles across the San Fernando Valley, as well as five dense arrays (apertures of 200 to 500 m) in areas of high damage, such as the collapsed Interstate 10 overpass, Sherman Oaks, and the collapsed parking garage at CalState Northridge. Aftershock data analysis indicates a correlation of site amplification with mainshock damage. We found several cases where the site amplification depended on the azimuth of the aftershock, possibly indicating focusing from basin structures. For the parking garage array, we found large ground-motion variabilities (a factor of 2) over 200-m distances for sites on the same mapped soil unit. Array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct 5 waves consist of surface waves traveling from the same azimuth as that of the epicenter. These surface waves increase the duration of motions and can have frequencies as high as about 4 Hz. For the events studied here, we do not observe large arrivals reflected from the southern edge of the San Fernando Valley.

The 2014 Mihoub earthquake (Mw4.3), northern Algeria: empirical Green's function analysis of the mainshock and the largest aftershock

NASA Astrophysics Data System (ADS)

Semmane, F.; Benabdeloued, B. Y. N.; Heddar, A.; Khelif, M. F.

2017-11-01

On November 15, 2014, an Mw4.3 earthquake occurred 2 km west of Mihoub village, 60 km SE of Algiers. In this study, we retrieve the relative source-time functions of the mainshock and largest aftershock (Mw3.9) for rupture analysis using the empirical Green's function method. The two events are nearly colocated with a smaller aftershock (Mw3.5), which is treated as the empirical Green's function. Moreover, these three events have similar focal mechanisms, suggesting that deconvolution is well posed in this case. The three events were recorded by nine stations of the Algerian permanent network. We use mainly P-wave data. The focal mechanism solution shows dominant reverse faulting with a strong strike-slip component. The two nodal planes align almost E-W, dipping to the south, and NNE-SSW, dipping to the NW, respectively; the fault and auxiliary planes cannot be resolved from hypocenter locations alone because too few aftershocks were recorded by the permanent network. The results show unilateral rupture propagation to the ENE and complex rupture with multiple episodes for the mainshock. The largest aftershock shows similar behavior with slightly less pronounced directivity at some sites. The rupture directivity for the mainshock is estimated at about N66° E, and the rupture velocity is Vr = 0.66 β. The E-W nodal plane of the best-fit focal mechanism is the preferred fault plane because it best agrees with the directivity direction and is consistent with the E-W faulting that dominates in the region.

Seismicity rate changes along the central California coast due to stress changes from the 2003 M 6.5 San Simeon and 2004 M 6.0 Parkfield earthquakes

USGS Publications Warehouse

Aron, A.; Hardebeck, J.L.

2009-01-01

We investigated the relationship between seismicity rate changes and modeled Coulomb static stress changes from the 2003 M 6.5 San Simeon and the 2004 M 6.0 Parkfield earthquakes in central California. Coulomb stress modeling indicates that the San Simeon mainshock loaded parts of the Rinconada, Hosgri, and San Andreas strike-slip faults, along with the reverse faults of the southern Los Osos domain. All of these loaded faults, except for the San Andreas, experienced a seismicity rate increase at the time of the San Simeon mainshock. The Parkfield earthquake occurred 9 months later on the loaded portion of the San Andreas fault. The Parkfield earthquake unloaded the Hosgri fault and the reverse faults of the southern Los Osos domain, which both experienced seismicity rate decreases at the time of the Parkfield event, although the decreases may be related to the decay of San Simeon-triggered seismicity. Coulomb stress unloading from the Parkfield earthquake appears to have altered the aftershock decay rate of the southern cluster of San Simeon after-shocks, which is deficient compared to the expected number of aftershocks from the Omori decay parameters based on the pre-Parkfield aftershocks. Dynamic stress changes cannot explain the deficiency of aftershocks, providing evidence that static stress changes affect earthquake occurrence. However, a burst of seismicity following the Parkfield earthquake at Ragged Point, where the static stress was decreased, provides evidence for dynamic stress triggering. It therefore appears that both Coulomb static stress changes and dynamic stress changes affect the seismicity rate.

The impact of static stress change, dynamic stress change, and the background stress on aftershock focal mechanisms

USGS Publications Warehouse

Hardebeck, Jeanne L.

2014-01-01

The focal mechanisms of earthquakes in Southern California before and after four M ≥ 6.7 main shocks provide insight into how fault systems respond to stress and changes in stress. The main shock static stress changes have two observed impacts on the seismicity: changing the focal mechanisms in a given location to favor those aligned with the static stress change and changing the spatial distribution of seismicity to favor locations where the static stress change aligns with the background stress. The aftershock focal mechanisms are significantly aligned with the static stress changes for absolute stress changes of ≥ 0.02 MPa, for up to ~20 years following the main shock. The dynamic stress changes have similar, although smaller, effects on the local focal mechanisms and the spatial seismicity distribution. Dynamic stress effects are best observed at long periods (30–60 s) and for metrics based on repeated stress cycling in the same direction. This implies that dynamic triggering operates, at least in part, through cyclic shear stress loading in the direction of fault slip. The background stress also strongly controls both the preshock and aftershock mechanisms. While most aftershock mechanisms are well oriented in the background stress field, 10% of aftershocks are identified as poorly oriented outliers, which may indicate limited heterogeneity in the postmain shock stress field. The fault plane orientations of the outliers are well oriented in the background stress, while their slip directions are not, implying that the background stress restricts the distribution of available fault planes.

Do aftershock probabilities decay with time?

USGS Publications Warehouse

Michael, Andrew J.

2012-01-01

So, do aftershock probabilities decay with time? Consider a thought experiment in which we are at the time of the mainshock and ask how many aftershocks will occur a day, week, month, year, or even a century from now. First we must decide how large a window to use around each point in time. Let's assume that, as we go further into the future, we are asking a less precise question. Perhaps a day from now means 1 day 10% of a day, a week from now means 1 week 10% of a week, and so on. If we ignore c because it is a small fraction of a day (e.g., Reasenberg and Jones, 1989, hereafter RJ89), and set p = 1 because it is usually close to 1 (its value in the original Omori law), then the rate of earthquakes (K=t) decays at 1=t. If the length of the windows being considered increases proportionally to t, then the number of earthquakes at any time from now is the same because the rate decrease is canceled by the increase in the window duration. Under these conditions we should never think "It's a bit late for this to be an aftershock."

Long aftershock sequences within continents and implications for earthquake hazard assessment.

PubMed

Stein, Seth; Liu, Mian

2009-11-05

One of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries. Plate tectonics gives no insight, however, into where and when earthquakes will occur within plates, because the interiors of ideal plates should not deform. As a result, within plate interiors, assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes. Here, however, we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago. We present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded. Aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries. These predictions are in accord with observations. So the common practice of treating continental earthquakes as steady-state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere.

The 2008 Wells, Nevada earthquake sequence: Source constraints using calibrated multiple event relocation and InSAR

USGS Publications Warehouse

Nealy, Jennifer; Benz, Harley M.; Hayes, Gavin; Berman, Eric; Barnhart, William

2017-01-01

The 2008 Wells, NV earthquake represents the largest domestic event in the conterminous U.S. outside of California since the October 1983 Borah Peak earthquake in southern Idaho. We present an improved catalog, magnitude complete to 1.6, of the foreshock-aftershock sequence, supplementing the current U.S. Geological Survey (USGS) Preliminary Determination of Epicenters (PDE) catalog with 1,928 well-located events. In order to create this catalog, both subspace and kurtosis detectors are used to obtain an initial set of earthquakes and associated locations. The latter are then calibrated through the implementation of the hypocentroidal decomposition method and relocated using the BayesLoc relocation technique. We additionally perform a finite fault slip analysis of the mainshock using InSAR observations. By combining the relocated sequence with the finite fault analysis, we show that the aftershocks occur primarily updip and along the southwestern edge of the zone of maximum slip. The aftershock locations illuminate areas of post-mainshock strain increase; aftershock depths, ranging from 5 to 16 km, are consistent with InSAR imaging, which shows that the Wells earthquake was a buried source with no observable near-surface offset.

Distribution of the largest aftershocks in branching models of triggered seismicity: theory of the universal Båth law.

PubMed

Saichev, A; Sornette, D

2005-05-01

Using the epidemic-type aftershock sequence (ETAS) branching model of triggered seismicity, we apply the formalism of generating probability functions to calculate exactly the average difference between the magnitude of a mainshock and the magnitude of its largest aftershock over all generations. This average magnitude difference is found empirically to be independent of the mainshock magnitude and equal to 1.2, a universal behavior known as Båth's law. Our theory shows that Båth's law holds only sufficiently close to the critical regime of the ETAS branching process. Allowing for error bars +/- 0.1 for Båth's constant value around 1.2, our exact analytical treatment of Båth's law provides new constraints on the productivity exponent alpha and the branching ratio n: 0.9 approximately < alpha < or =1. We propose a method for measuring alpha based on the predicted renormalization of the Gutenberg-Richter distribution of the magnitudes of the largest aftershock. We also introduce the "second Båth law for foreshocks:" the probability that a main earthquake turns out to be the foreshock does not depend on its magnitude rho.

Random variability explains apparent global clustering of large earthquakes

USGS Publications Warehouse

Michael, A.J.

2011-01-01

The occurrence of 5 Mw ≥ 8.5 earthquakes since 2004 has created a debate over whether or not we are in a global cluster of large earthquakes, temporarily raising risks above long-term levels. I use three classes of statistical tests to determine if the record of M ≥ 7 earthquakes since 1900 can reject a null hypothesis of independent random events with a constant rate plus localized aftershock sequences. The data cannot reject this null hypothesis. Thus, the temporal distribution of large global earthquakes is well-described by a random process, plus localized aftershocks, and apparent clustering is due to random variability. Therefore the risk of future events has not increased, except within ongoing aftershock sequences, and should be estimated from the longest possible record of events.

Nonlinear and linear site response and basin effects in Seattle for the M 6.8 Nisqually, Washington, earthquake

USGS Publications Warehouse

Frankel, A.D.; Carver, D.L.; Williams, R.A.

2002-01-01

We used recordings of the M 6.8 Nisqually earthquake and its ML 3.4 aftershock to study site response and basin effects for 35 locations in Seattle, Washington. We determined site amplification from Fourier spectral ratios of the recorded horizontal ground motions, referenced to a soft-rock site. Soft-soil sites (generally National Earthquake Hazard Reduction Program [NEHRP] class E) on artificial fill and young alluvium have the largest 1-Hz amplifications (factors of 3-7) for both the mainshock and aftershock. These amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction. There are several indications of nonlinear response at the soft-soil sites for the mainshock ground motions, despite relatively modest peak accelerations in the S waves of 15%-22%g. First, the mainshock spectral ratios do not show amplification at 2-8 Hz as do the aftershock spectral ratios. Spectral peaks at frequencies below 2 Hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios. At one soft-soil site, there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock. The frequency of this resonance increases in the coda of the mainshock record, indicating that the site response during the weaker motions of the coda is more linear than that of the initial S wave. Three of the soft-soil sites display cusped, one-sided mainshock accelerograms after the S wave. These soft-soil sites also show amplification at 10-20 Hz in the S wave, relative to the rock site, that is not observed for the aftershock. The cusped waveforms and 10-20-Hz amplification are symptomatic of nonlinear response at the soft-soil sites. These sites had nearby liquefaction. The largest amplifications for 0.5 Hz occur at soft-soil sites on the southern portion of the Seattle Basin. Stiff-soil sites (NEHRP classes D and C) on Pleistocene-age glacial deposits display similar spectral amplification for the mainshock and aftershock, indicating approximately linear response. The stiff-soil sites generally have moderate amplification (factors of 1.1-2.4) at 0.5 and 1 Hz. Amplifications at 1 and 5 Hz for all sites generally increase with decreasing shear-wave velocity measured in the top 30 m (Vs 30). However, larger amplifications at 0.5 and 1 Hz for sites with similar Vs 30 values are observed for sites in the Seattle Basin, illustrating the amplification from the deeper (>30 m) sediments and the contribution from basin surface waves. Record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the Seattle Basin and often dominate the amplitude at 1 Hz and lower frequencies.

The 7.9 Denali Fault, Alaska Earthquake of November 3, 2002: Aftershock Locations, Moment Tensors and Focal Mechanisms from the Regional Seismic Network Data

NASA Astrophysics Data System (ADS)

Ratchkovski, N. A.; Hansen, R. A.; Kore, K. R.

2003-04-01

The largest earthquake ever recorded on the Denali fault system (magnitude 7.9) struck central Alaska on November 3, 2002. It was preceded by a magnitude 6.7 earthquake on October 23. This earlier earthquake and its zone of aftershocks were located ~20 km to the west of the 7.9 quake. Aftershock locations and surface slip observations from the 7.9 quake indicate that the rupture was predominately unilateral in the eastward direction. The geologists mapped a ~300-km-long rupture and measured maximum offsets of 8.8 meters. The 7.9 event ruptured three different faults. The rupture began on the northeast trending Susitna Glacier Thrust fault, a splay fault south of the Denali fault. Then the rupture transferred to the Denali fault and propagated eastward for 220 km. At about 143W the rupture moved onto the adjacent southeast-trending Totschunda fault and propagated for another 55 km. The cumulative length of the 6.7 and 7.9 aftershock zones along the Denali and Totschunda faults is about 380 km. The earthquakes were recorded and processed by the Alaska Earthquake Information Center (AEIC). The AEIC acquires and processes data from the Alaska Seismic Network, consisting of over 350 seismograph stations. Nearly 40 of these sites are equipped with the broad-band sensors, some of which also have strong motion sensors. The rest of the stations are either 1 or 3-component short-period instruments. The data from these stations are collected, processed and archived at the AEIC. The AEIC staff installed a temporary seismic network of 6 instruments following the 6.7 earthquake and an additional 20 stations following the 7.9 earthquake. Prior to the 7.9 Denali Fault event, the AEIC was locating 35 to 50 events per day. After the event, the processing load increased to over 300 events per day during the first week following the event. In this presentation, we will present and interpret the aftershock location patterns, first motion focal mechanism solutions, and regional seismic moment tensors for the larger events. We used the double difference method to relocate aftershocks of both the 6.7 and 7.9 events. The relocated aftershocks indicate complex faulting along the rupture zone. The aftershocks are located not only along the main rupture zone, but also illuminate multiple splay faults north and south of the Denali fault. We calculated principal stress directions along the Denali fault both before and after the 7.9 event from the focal mechanisms. The stress orientations before and after the event are nearly identical. The maximum horizontal compressive stress is nearly normal to the trace of the Denali fault and rotates gradually from NW orientation at the western end of the rupture zone to NE orientation near the junction with the Totschunda fault.

Automatic Classification of Extensive Aftershock Sequences Using Empirical Matched Field Processing

NASA Astrophysics Data System (ADS)

Gibbons, Steven J.; Harris, David B.; Kværna, Tormod; Dodge, Douglas A.

2013-04-01

The aftershock sequences that follow large earthquakes create considerable problems for data centers attempting to produce comprehensive event bulletins in near real-time. The greatly increased number of events which require processing can overwhelm analyst resources and reduce the capacity for analyzing events of monitoring interest. This exacerbates a potentially reduced detection capability at key stations, due the noise generated by the sequence, and a deterioration in the quality of the fully automatic preliminary event bulletins caused by the difficulty in associating the vast numbers of closely spaced arrivals over the network. Considerable success has been enjoyed by waveform correlation methods for the automatic identification of groups of events belonging to the same geographical source region, facilitating the more time-efficient analysis of event ensembles as opposed to individual events. There are, however, formidable challenges associated with the automation of correlation procedures. The signal generated by a very large earthquake seldom correlates well enough with the signals generated by far smaller aftershocks for a correlation detector to produce statistically significant triggers at the correct times. Correlation between events within clusters of aftershocks is significantly better, although the issues of when and how to initiate new pattern detectors are still being investigated. Empirical Matched Field Processing (EMFP) is a highly promising method for detecting event waveforms suitable as templates for correlation detectors. EMFP is a quasi-frequency-domain technique that calibrates the spatial structure of a wavefront crossing a seismic array in a collection of narrow frequency bands. The amplitude and phase weights that result are applied in a frequency-domain beamforming operation that compensates for scattering and refraction effects not properly modeled by plane-wave beams. It has been demonstrated to outperform waveform correlation as a classifier of ripple-fired mining blasts since the narrowband procedure is insensitive to differences in the source-time functions. For sequences in which the spectral content and time-histories of the signals from the main shock and aftershocks vary greatly, the spatial structure calibrated by EMFP is an invariant that permits reliable detection of events in the specific source region. Examples from the 2005 Kashmir and 2011 Van earthquakes demonstrate how EMFP templates from the main events detect arrivals from the aftershock sequences with high sensitivity and exceptionally low false alarm rates. Classical waveform correlation detectors are demonstrated to fail for these examples. Even arrivals with SNR below unity can produce significant EMFP triggers as the spatial pattern of the incoming wavefront is identified, leading to robust detections at a greater number of stations and potentially more reliable automatic bulletins. False EMFP triggers are readily screened by scanning a space of phase shifts relative to the imposed template. EMFP has the potential to produce a rapid and robust overview of the evolving aftershock sequence such that correlation and subspace detectors can be applied semi-autonomously, with well-chosen parameter specifications, to identify and classify clusters of very closely spaced aftershocks.

The Mw=8.8 Maule earthquake aftershock sequence, event catalog and locations

NASA Astrophysics Data System (ADS)

Meltzer, A.; Benz, H.; Brown, L.; Russo, R. M.; Beck, S. L.; Roecker, S. W.

2011-12-01

The aftershock sequence of the Mw=8.8 Maule earthquake off the coast of Chile in February 2010 is one of the most well-recorded aftershock sequences from a great megathrust earthquake. Immediately following the Maule earthquake, teams of geophysicists from Chile, France, Germany, Great Britain and the United States coordinated resources to capture aftershocks and other seismic signals associated with this significant earthquake. In total, 91 broadband, 48 short period, and 25 accelerometers stations were deployed above the rupture zone of the main shock from 33-38.5°S and from the coast to the Andean range front. In order to integrate these data into a unified catalog, the USGS National Earthquake Information Center develop procedures to use their real-time seismic monitoring system (Bulletin Hydra) to detect, associate, location and compute earthquake source parameters from these stations. As a first step in the process, the USGS has built a seismic catalog of all M3.5 or larger earthquakes for the time period of the main aftershock deployment from March 2010-October 2010. The catalog includes earthquake locations, magnitudes (Ml, Mb, Mb_BB, Ms, Ms_BB, Ms_VX, Mc), associated phase readings and regional moment tensor solutions for most of the M4 or larger events. Also included in the catalog are teleseismic phases and amplitude measures and body-wave MT and CMT solutions for the larger events, typically M5.5 and larger. Tuning of automated detection and association parameters should allow a complete catalog of events to approximately M2.5 or larger for that dataset of more than 164 stations. We characterize the aftershock sequence in terms of magnitude, frequency, and location over time. Using the catalog locations and travel times as a starting point we use double difference techniques to investigate relative locations and earthquake clustering. In addition, phase data from candidate ground truth events and modeling of surface waves can be used to calibrate the velocity structure of central Chile to improve the real-time monitoring.

3D P-Wave Velocity Structure of the Crust and Relocation of Earthquakes in 21 the Lushan Source Area

NASA Astrophysics Data System (ADS)

Yu, X.; Wang, X.; Zhang, W.

2014-12-01

The double difference seismic tomography method is applied to the absolute first arrival P wave arrival times and high quality relative P arrival times of the Lushan seismic sequence to determine the detailed crustal 3D P wave velocity structure and the hypocenter parameters in the Lushan seismic area. The results show that the Lushan mainshock locates at 30.28 N, 103.98 E, with the depth of 16.38 km. The leading edge of aftershock in the northeast of mainshock present a spade with a steep dip angle, the aftershocks' extended length is about 12 km. In the southwest of the Lushan mainshock, the leading edge of aftershock in low velocity zone slope gently, the aftershocks' extended length is about 23 km. The P wave velocity structure of the Lushan seismic area shows obviously lateral heterogeneity. The P wave velocity anomalies represent close relationship with topographic relief and geological structure. In Baoxing area the complex rocks correspond obvious high-velocity anomalies extending down to 15 km depth，while the Cenozoic rocks are correlated with low-velocity anomalies. Our high-resolution tomographic model not only displays the general features contained in the previous models, but also reveals some new features. An obvious high-velocity anomaly is visible in Daxing area. The high-velocity anomalies beneath Baoxing and Daxing connect each other in 10 km depth, which makes the contrast between high and low velocity anomalies more sharp. Above 20 km depth the velocity structure in southwest and northeast segment of the mainshock shows a big difference: low-velocity anomalies are dominated the southwest segment, while high-velocity anomalies rule the northeast segment. The Lushan mainshock locates at the leading edge of a low-velocity anomaly surrounded by the Baoxing and Daxing high-velocity anomalies. The Lushan aftershocks in southwest are distributed in low-velocity anomalies or the transition belt: the footwall represents low-velocity anomalies, while the hanging wall shows high-velocity anomalies. The northeastern aftershocks are distributed at the boundary between high-velocity anomalies in Baoxing and Daxing area. The main seismogenic layer dips to northwest.

Investigation of the 27 February 2010 Mw 8.8 Chilean earthquake integrating aftershock analysis, back-projection imaging and cGPS results

NASA Astrophysics Data System (ADS)

Clévédé, E.; Satriano, C.; Bukchin, B.; Lancieri, M.; Fuenzalida, A.; Vilotte, J.; Lyon-Caen, H.; Vigny, C.; Socquet, A.; Aranda, C.; Campos, J. A.; Scientific Team of the Lia Montessus de Ballore (Cnrs-Insu, U. Chile)

2010-12-01

The Mw 8.8 earthquake in central Chile ruptured more than 400 km along the subduction bound between the Nazca and the South American plates. The aftershock distribution clearly shows that this earthquake filled a well-known seismic gap, corresponding to rupture extension of the 1835 earthquake. The triggered post-seismic activity extends farther north of the gap, partially overlapping the 1985 and the 1960 Valparaiso earthquakes. However, the analysis of continuous GPS (cGPS) recordings, and back projection imaging of teleseismic body wave energy, indicate that the rupture stopped south of Valparaiso, around -33.5 degrees of latitude. An important question is how far the rupture actually extended to the north and the potential relation between the northernmost aftershock activity and remaining asperities within the ruptured zone of the previous Valparaiso earthquakes. The extension of the rupture offshore, towards west, also deserves further investigation. The aftershock distribution and the back propagation analysis support the hypothesis that, in the northern part, the rupture may have reached the surface at the trench. In this work, we performed a CMT and depth location study for more than 10 of the immediate largest aftershocks using teleseismic surface wave analysis constrained by P-wave polarity. In parallel, a detailed analysis of aftershocks in the northern part of the rupture, between 2010-03-11 and 2010-05-13, have been performed using the data from the station of the Chilean Servicio Sismológico Nacional (SSN), and of the post-seismic network, deployed by the French CNRS-INSU, GFZ, IRIS, and Caltech. We accurately hand-picked 153 larger events, which have been located using a non-linear probabilistic code, with improved depth location. Focal mechanisms have been computed for the larger events. Those results have been integrated with the analysis of cGPS and teleseismic back projection, and the overall kinematic of the Maule earthquake is discussed as well as the extension of the rupture along strike and dip.

Source Process of the Mw 5.0 Au Sable Forks, New York, Earthquake Sequence from Local Aftershock Monitoring Network Data

NASA Astrophysics Data System (ADS)

Kim, W.; Seeber, L.; Armbruster, J. G.

2002-12-01

On April 20, 2002, a Mw 5 earthquake occurred near the town of Au Sable Forks, northeastern Adirondacks, New York. The quake caused moderate damage (MMI VII) around the epicentral area and it is well recorded by over 50 broadband stations in the distance ranges of 70 to 2000 km in the Eastern North America. Regional broadband waveform data are used to determine source mechanism and focal depth using moment tensor inversion technique. Source mechanism indicates predominantly thrust faulting along 45° dipping fault plane striking due South. The mainshock is followed by at least three strong aftershocks with local magnitude (ML) greater than 3 and about 70 aftershocks are detected and located in the first three months by a 12-station portable seismographic network. The aftershock distribution clearly delineate the mainshock rupture to the westerly dipping fault plane at a depth of 11 to 12 km. Preliminary analysis of the aftershock waveform data indicates that orientation of the P-axis rotated 90° from that of the mainshock, suggesting a complex source process of the earthquake sequence. We achieved an important milestone in monitoring earthquakes and evaluating their hazards through rapid cross-border (Canada-US) and cross-regional (Central US-Northeastern US) collaborative efforts. Hence, staff at Instrument Software Technology, Inc. near the epicentral area joined Lamont-Doherty staff and deployed the first portable station in the epicentral area; CERI dispatched two of their technical staff to the epicentral area with four accelerometers and a broadband seismograph; the IRIS/PASSCAL facility shipped three digital seismographs and ancillary equipment within one day of the request; the POLARIS Consortium, Canada sent a field crew of three with a near real-time, satellite telemetry based earthquake monitoring system. The Polaris station, KSVO, powered by a solar panel and batteries, was already transmitting data to the central Hub in London, Ontario, Canada within a day after the field crew arrived in the Au Sable Forks area. This collaboration allowed us to maximize the scarce resources available for monitoring this damaging earthquake and its aftershocks in the Northeastern U.S.

Automatic recovery of aftershock sequences at the International Data Centre: from concept to pipeline

NASA Astrophysics Data System (ADS)

Kitov, I.; Bobrov, D.; Rozhkov, M.

2016-12-01

Aftershocks of larger earthquakes represent an important source of information on the distribution and evolution of stresses and deformations in pre-seismic, co-seismic and post-seismic phases. For the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Organization (CTBTO) largest aftershocks sequences are also a challenge for automatic and interactive processing. The highest rate of events recorded by two and more seismic stations of the International Monitoring System from a relatively small aftershock area may reach hundreds per hour (e.g. Sumatra 2004 and Tohoku 2011). Moreover, there are thousands of reflected/refracted phases per hour with azimuth and slowness within the uncertainty limits of the first P-waves. Misassociation of these later phases, both regular and site specific, as the first P-wave results in creation of numerous wrong event hypotheses in automatic IDC pipeline. In turn, interactive review of such wrong hypotheses is direct waste of analysts' resources. Waveform cross correlation (WCC) is a powerful tool to separate coda phases from actual P-wave arrivals and to fully utilize the repeat character of waveforms generated by events close in space. Array seismic stations of the IMS enhance the performance of the WCC in two important aspects - they reduce detection threshold and effectively suppress arrivals from all sources except master events. An IDC specific aftershock tool has been developed and merged with standard IDC pipeline. The tool includes several procedures: creation of master events consisting of waveform templates at ten and more IMS stations; cross correlation (CC) of real-time waveforms with these templates, association of arrivals detected at CC-traces in event hypotheses; building events matching IDC quality criteria; and resolution of conflicts between events hypotheses created by neighboring master-events. The final cross correlation standard event lists (XSEL) is a start point of interactive analysis. Since global monitoring of underground nuclear tests is based on historical and synthetic data, each aftershock sequence can be tested for the CTBT violation with big earthquakes as an evasion scenario.

Integrated Geophysical Characteristics of the 2015 Illapel, Chile, Earthquake

NASA Astrophysics Data System (ADS)

Herman, M. W.; Yeck, W. L.; Nealy, J. L.; Hayes, G. P.; Barnhart, W. D.; Benz, H.; Furlong, K. P.

2015-12-01

On September 16th, 2015, an Mw 8.3 earthquake (USGS moment magnitude) ruptured offshore of central Chile, 50 km west of the city of Illapel and 200 km north of Santiago. The earthquake occurred just north of where the Juan Fernandez Ridge enters the subduction zone. In this study, we integrate multiple seismic and geodetic datasets, including multiple-event earthquake relocations; moment tensors of the Illapel mainshock, aftershocks, and prior regional seismicity; finite fault models (FFMs) of the mainshock rupture; subduction zone geometry; Coulomb stress transfer calculations; and co-seismic GPS offsets and InSAR images. These datasets allow us to (a) assess the context of the Illapel earthquake sequence with respect to historical seismicity in central Chile; (b) constrain the relationship between subduction geometry and the kinematic characteristics of the earthquake sequence; and (c) understand the distribution of aftershocks with respect to the rupture zone. Double source W-phase moment tensor analysis indicates the Illapel mainshock rupture began as a smaller Mw ~7.2 thrusting event before growing into a great-sized Mw 8.3 earthquake. Relocated aftershock seismicity is concentrated around the main region of slip, and few aftershocks occur on the megathrust shallower than ~15 km, despite the FFM indicating slip near the trench. This distribution is consistent with the aftershock behavior following the 2010 Maule and 2014 Iquique earthquakes: aftershocks primarily surround the rupture zones and are largely absent from regions of greatest slip. However, in contrast to the recent 2014 Iquique and 2010 Maule events, which ruptured in regions of the Chilean subduction zone that had not had large events in over a century, this earthquake occurred in a section of the subduction zone that hosted a large earthquake as recently as 1943, as well as earlier significant events in 1880 and 1822. At this section of the subduction zone, in addition to the impinging Juan Fernandez Ridge, the slab geometry changes from steeply dipping south of the Illapel earthquake to a nearly horizontal dip adjacent to the event. Combining these various observations provides insight into the links between regional tectonics and the timing and distribution of megathrust earthquakes at this segment of the central Chilean subduction zone.

Preliminary Results From the Chile-Illapel Aftershock Experiment (CHILLAX)

NASA Astrophysics Data System (ADS)

Roecker, S. W.; Russo, R. M.; Comte, D.; Carrizo, D.; Peyrat, S.; Opazo, T.; Peña, G.; Farrell, M. E.; Moore, J.; Glick, R.; Rodriguez, E. E.

2016-12-01

On September 16, 2015, the Mw 8.3 Illapel earthquake ruptured a segment of the Nazca-South America subduction zone directly to the north of the 2010 Maule Mw 8.8 earthquake. Soon afterwards, a team from the Departamento de Geofisica, University of Chile, installed 18 short period sensors on land above the rupture to record aftershocks. A month later, the network was upgraded and expanded with funding from NSF RAPID to 20 broad band stations, loaned by IRIS PASSCAL. The installation of the Chile-Illapel Aftershock Experiment (CHILLAX) was completed in mid-November, 2015, and will operate until November, 2016. Preliminary analysis of data collected to date indicates an average detection rate of about 1000 locatable aftershocks per month. The combined CHILLAX and Maule aftershock deployments will yield the first modern-instrumentation observations of the zone of along-strike rupture termination that separates these temporally related and spatially adjacent megathrust rupture zones. Additionally, seismic observations of this part of the Nazca subduction zone are relatively sparse, and an aftershock sequence provides an opportunity to fill this gap efficiently. Preliminary analysis of CHILLAX network data revealed unexpected patterns in seismicity down dip from the rupture zone, in the unusual "flat slab" region to the east. Compared to the Maule event, the Illapel rupture apparently generated a more significant increase in seismicity in the 60-200 km depth range, suggesting that it "lit up" the subducted Nazca plate. Although high strain rates due to rupture might extend brittle failure into normally ductile regions, such an effect at these depths by the relatively low magnitude Illapel event is unusual. A perhaps more intriguing result is the frequent occurrence of events at depths significantly below that of the "flat slab". Attribution of this apparent second, deeper slab segment to event mislocations would require unrealistic seismic heterogeneity, We hypothesize that either the subduction of thick, buoyant crust of the Juan Fernandez Ridge has resulted in a slab tear, or that the "flat" seismicity is not occuring within the Nazca slab, but instead within the overriding South American lithosphere. In the latter case, the shallower flat slab earthquakes may represent a nascent delamination event.

Investigation of the high-frequency attenuation parameter, κ (kappa), from aftershocks of the 2010 Mw 8.8 Maule, Chile earthquake

USGS Publications Warehouse

Neighbors, Corrie; Liao, E. J.; Cochran, Elizabeth S.; Funning, G. J.; Chung, A. I.; Lawrence, J. F.; Christensen, C. M.; Miller, M.; Belmonte, A.; Sepulveda, H. H. Andrés

2014-01-01

The Bío Bío region of Chile experienced a vigorous aftershock sequence following the 2010 February 27 Mw 8.8 Maule earthquake. The immediate aftershock sequence was captured by two temporary seismic deployments: the Quake Catcher Network Rapid Aftershock Mobilization Program (QCN RAMP) and the Incorporated Research Institutions for Seismology CHile Aftershock Mobilization Program (IRIS CHAMP). Here, we use moderate to large aftershocks (ML ≥ 4.0) occurring between 2010 March 1 and June 30 recorded by QCN RAMP and IRIS CHAMP stations to determine the spectral decay parameter, kappa (κ). First, we compare waveforms and κ estimates from the lower-resolution QCN stations to the IRIS CHAMP stations to ensure the QCN data are of sufficient quality. We find that QCN stations provide reasonable estimates of κ in comparison to traditional seismic sensors and provide valuable additional observations of local ground motion variation. Using data from both deployments, we investigate the variation in κ for the region to determine if κ is influenced primarily by local geological structure, path attenuation, or source properties (e.g. magnitude, mechanism and depth). Estimates of κ for the Bío Bío region range from 0.0022 to 0.0704 s with a mean of 0.0295 s and are in good agreement with κ values previously reported for similar tectonic environments. κ correlates with epicentral distance and, to a lesser degree, with source magnitude. We find little to no correlation between the site kappa, κ0, and mapped geology, although we were only able to compare the data to a low-resolution map of surficial geology. These results support an increasing number of studies that suggest κobservations can be attributed to a combination of source, path and site properties; additionally, measured κ are often highly scattered making it difficult to separate the contribution from each of these factors. Thus, our results suggest that contributions from the site, path and source should be carefully considered when interpreting κ values.

Tectonic Deformation Pattern along the Longmen Shan Fault Zone in Eastern Tibet: Insights from Focal Mechanisms of the Wenchuan and Lushan Earthquake Sequences, Southwestern China

NASA Astrophysics Data System (ADS)

Yi, G.; Vallage, A.; Klinger, Y.; Long, F.; Wang, S.

2017-12-01

760 ML≥3.5 aftershocks of the 2008 Wenchuan earthquake, the 2013 Lushan mainshock and its 87 ML≥3.5 aftershocks were selected to obtain focal mechanism solutions from CAP waveform inversion method (Zhu and Helmberger, 1996), along with strain rosette (Amelung and King, 1997) and Areal strain (As) (Vallage et al., 2014), we aimed to analyze the tectonic deformation pattern along the Longmen Shan (LMS) fault zone, southwestern China. The As values show that 93% compressional earthquakes for the Lushan sequence are of pure thrust for the southern segment of the LMS fault zone, while only 50% compressional and nearly 40% of strike-slip and oblique-thrust events for the Wenchuan sequence reflect the strike-slip component increase on the central-northern segment of the LMS fault zone, meaning many different faults responsible for the Wenchuan aftershock activity. The strain rosettes with purely NW-trending compressional white lobe for the entire 87 aftershocks and 4 different classes of magnitudes are very similar to that of the Lushan mainshock. We infer that the geological structures for the southern segment are of thrust faulting under NW compressional deformation. The strain rosettes exhibit self-similarity in terms of orientation and shape for all classes, reflecting that the deformation pattern of the southern segment is independent with earthquake size, and suggesting that each class is representative of the overall deformation for the southern segment. We obtained EW-oriented pure compressional strain rosette of the entire 760 aftershocks and NW-oriented white lobe with small NE-oriented black lobe of the Wenchuan mainshock, and this difference may reflect different tectonic deformation pattern during the co-seismic and post-seismic stages. The deformation segmentation along the Wenchuan coseismic surface rupture is also evidenced from the different orientation of strain rosettes, i.e., NW for the southern area, NE for the central and NNW for the northern parts. The above inferences indicate a very complicated tectonic deformation pattern related to the complex geological structure. The segment of the northern aftershock area without ruptures behaves an oblique compressional deformation.

Double-difference Relocation of the Aftershocks of the Tecomán, Colima, Mexico Earthquake of 22 January 2003

NASA Astrophysics Data System (ADS)

Andrews, Vanessa; Stock, Joann; Ramírez Vázquez, Carlos Ariel; Reyes-Dávila, Gabriel

2011-08-01

On 22 January 2003, the M w = 7.6 Tecomán earthquake struck offshore of the state of Colima, Mexico, near the diffuse triple junction between the Cocos, Rivera, and North American plates. Three-hundred and fifty aftershocks of the Tecomán earthquake with magnitudes between 2.6 and 5.8, each recorded by at least 7 stations, are relocated using the double difference method. Initial locations are determined using P and S readings from the Red Sismológica Telemétrica del Estado de Colima (RESCO) and a 1-D velocity model. Because only eight RESCO stations were operating immediately following the Tecomán earthquake, uncertainties in the initial locations and depths are fairly large, with average uncertainties of 8.0 km in depth and 1.4 km in the north-south and east-west directions. Events occurring between 24 January and 31 January were located using not only RESCO phase readings but also additional P and S readings from 11 temporary stations. Average uncertainties decrease to 0.8 km in depth, 0.3 km in the east-west direction, and 0.7 km in the north-south direction for events occurring while the temporary stations were deployed. While some preliminary studies of the early aftershocks suggested that they were dominated by shallow events above the plate interface, our results place the majority of aftershocks along the plate interface, for a slab dipping between approximately 20° and 30°. This is consistent with the slab positions inferred from geodetic studies. We do see some upper plate aftershocks that may correspond to forearc fault zones, and faults inland in the upper plate, particularly among events occurring more than 3 months after the mainshock.

Kinematic Rupture Process of the 2015 Gorkha (Nepal) Earthquake Sequence from Joint Inversion of Teleseismic, hr-GPS, Strong-Ground Motion, InSAR interferograms and pixel offsets

NASA Astrophysics Data System (ADS)

Yue, H.; Simons, M.; Jiang, J.; Fielding, E. J.; Owen, S. E.; Moore, A. W.; Riel, B. V.; Polet, J.; Duputel, Z.; Samsonov, S. V.; Avouac, J. P.

2015-12-01

The April 2015 Gorkha, Nepal (Mw 7.8) earthquake ruptured the front of Himalaya thrust belt, causing more than 9,000 fatalities. 17 days after the main event, a large aftershock (Mw 7.2) ruptured to down-dip and east of the main rupture area. To investigate the kinematic rupture process of this earthquake sequence, we explored linear and non-linear inversion techniques using a variety of datasets including teleseismic, high rate and conventional GPS, InSAR interferograms and pixel-offsets. InSAR interferograms from ALOS-2, RADARSAT-2 and Sentinel-1a satellites are used in the joint inversion. The main event is characterized by unilateral rupture extending along strike approximately 70 km to the southeast and 40 km along dip direction. The rupture velocity is well resolved to be lie between 2.8 and 3.0 km/s, which is consistent with back-projection results. An emergent initial phase is observed in teleseismic body wave records, which is consistent with a narrow area of rupture initiation near the hypocenter. The rupture mode of the main event is pulse like. The aftershock ruptured down-dip to the northeast of the main event rupture area. The aftershock rupture area is compact and contained within 40 km of its hypocenter. In contrast to the main event, teleseismic body wave records of the aftershock suggest an abrupt initial phase, which is consistent with a crack like rupture mode. The locations of most of the aftershocks (small and large) surround the rupture area of the main shock with little, if any, spatial overlap.

Crustal structure and fault geometry of the 2010 Haiti earthquake from temporary seismometer deployments

USGS Publications Warehouse

Douilly, Roby; Haase, Jennifer S.; Ellsworth, William L.; Bouin, Marie‐Paule; Calais, Eric; Symithe, Steeve J.; Armbruster, John G.; Mercier de Lépinay, Bernard; Deschamps, Anne; Mildor, Saint‐Louis; Meremonte, Mark E.; Hough, Susan E.

2013-01-01

Haiti has been the locus of a number of large and damaging historical earthquakes. The recent 12 January 2010 Mw 7.0 earthquake affected cities that were largely unprepared, which resulted in tremendous losses. It was initially assumed that the earthquake ruptured the Enriquillo Plantain Garden fault (EPGF), a major active structure in southern Haiti, known from geodetic measurements and its geomorphic expression to be capable of producing M 7 or larger earthquakes. Global Positioning Systems (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data, however, showed that the event ruptured a previously unmapped fault, the Léogâne fault, a north‐dipping oblique transpressional fault located immediately north of the EPGF. Following the earthquake, several groups installed temporary seismic stations to record aftershocks, including ocean‐bottom seismometers on either side of the EPGF. We use data from the complete set of stations deployed after the event, on land and offshore, to relocate all aftershocks from 10 February to 24 June 2010, determine a 1D regional crustal velocity model, and calculate focal mechanisms. The aftershock locations from the combined dataset clearly delineate the Léogâne fault, with a geometry close to that inferred from geodetic data. Its strike and dip closely agree with the global centroid moment tensor solution of the mainshock but with a steeper dip than inferred from previous finite fault inversions. The aftershocks also delineate a structure with shallower southward dip offshore and to the west of the rupture zone, which could indicate triggered seismicity on the offshore Trois Baies reverse fault. We use first‐motion focal mechanisms to clarify the relationship of the fault geometry to the triggered aftershocks.

Relaxation of the south flank after the 7.2-magnitude Kalapana earthquake, Kilauea Volcano, Hawaii

USGS Publications Warehouse

Dvorak, John J.; Klein, Fred W.; Swanson, Donald A.

1994-01-01

An M = 7.2 earthquake on 29 November 1975 caused the south flank of Kilauea Volcano, Hawaii, to move seaward several meters: a catastrophic release of compression of the south flank caused by earlier injections of magma into the adjacent segment of a rift zone. The focal mechanisms of the mainshock, the largest foreshock, and the largest aftershock suggest seaward movement of the upper block. The rate of aftershocks decreased in a familiar hyperbolic decay, reaching the pre-1975 rate of seismicity by the mid-1980s. Repeated rift-zone intrusions and eruptions after 1975, which occurred within 25 km of the summit area, compressed the adjacent portion of the south flank, apparently masking continued seaward displacement of the south flank. This is evident along a trilateration line that continued to extend, suggesting seaward displacement, immediately after the M = 7.2 earthquake, but then was compressed during a series of intrusions and eruptions that began in September 1977. Farther to the east, trilateration measurements show that the portion of the south flank above the aftershock zone, but beyond the area of compression caused by the rift-zone intrusions and eruptions, continued to move seaward at a decreasing rate until the mid-1980s, mimicking the decay in aftershock rate. Along the same portion of the south flank, the pattern of vertical surface displacements can be explained by continued seaward movement of the south flank and development of two eruptive fissures along the east rift zone, each of which extended from a depth of ∼3 km to the surface. The aftershock rate and continued seaward movement of the south flank are reminiscent of crustal response to other large earthquakes, such as the 1966 M = 6 Parkfield earthquake and the 1983 M = 6.5 Coalinga earthquake.

Iterative Strategies for Aftershock Classification in Automatic Seismic Processing Pipelines

NASA Astrophysics Data System (ADS)

Gibbons, Steven J.; Kværna, Tormod; Harris, David B.; Dodge, Douglas A.

2016-04-01

Aftershock sequences following very large earthquakes present enormous challenges to near-realtime generation of seismic bulletins. The increase in analyst resources needed to relocate an inflated number of events is compounded by failures of phase association algorithms and a significant deterioration in the quality of underlying fully automatic event bulletins. Current processing pipelines were designed a generation ago and, due to computational limitations of the time, are usually limited to single passes over the raw data. With current processing capability, multiple passes over the data are feasible. Processing the raw data at each station currently generates parametric data streams which are then scanned by a phase association algorithm to form event hypotheses. We consider the scenario where a large earthquake has occurred and propose to define a region of likely aftershock activity in which events are detected and accurately located using a separate specially targeted semi-automatic process. This effort may focus on so-called pattern detectors, but here we demonstrate a more general grid search algorithm which may cover wider source regions without requiring waveform similarity. Given many well-located aftershocks within our source region, we may remove all associated phases from the original detection lists prior to a new iteration of the phase association algorithm. We provide a proof-of-concept example for the 2015 Gorkha sequence, Nepal, recorded on seismic arrays of the International Monitoring System. Even with very conservative conditions for defining event hypotheses within the aftershock source region, we can automatically remove over half of the original detections which could have been generated by Nepal earthquakes and reduce the likelihood of false associations and spurious event hypotheses. Further reductions in the number of detections in the parametric data streams are likely using correlation and subspace detectors and/or empirical matched field processing.

Stress Regime in the Nepalese Himalaya from Recent Earthquakes.

NASA Astrophysics Data System (ADS)

Pant, M.; Karplus, M. S.; Velasco, A. A.; Nabelek, J.; Kuna, V. M.; Ghosh, A.; Mendoza, M.; Adhikari, L. B.; Sapkota, S. N.; Klemperer, S. L.; Patlan, E.

2017-12-01

The two recent earthquakes, April 25, 2015 Mw 7.8 (Gorkha earthquake) and May 12, 2015 Mw 7.2, at the Indo-Eurasian plate margin killed thousands of people and caused billion dollars of property loss. In response to these events, we deployed a dense array of seismometers to record the aftershocks along Gorkha earthquake rupture area. Our network NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake) included 45 different seismic stations (16 short period, 25 broadband, and 4 strong motion sensors) covering a large area from north-central Nepal to south of the Main Frontal Thrust at a spacing of 20 km. The instruments recorded aftershocks from June 2015 to May 2016. We used time domain short term average (STA) and long term average (LTA) algorithms (1/10s and 4/40s) respectively to detect the arrivals and then developed an earthquake catalog containing 9300 aftershocks. We are manually picking the P-wave first motion arrival polarity to develop a catalog of focal mechanisms for the larger magnitude (>M3.0) events with adequate (>10) arrivals. We hope to characterize the seismicity and stress mechanisms of the complex fault geometries in the Nepalese Himalaya and to address the geophysical processes controlling seismic cycles in the Indo-Eurasian plate margin.

Focal Mechanisms and Stress Environment of the 12 May 2008 Wenchuan, China, Earthquake Sequence

NASA Astrophysics Data System (ADS)

Zhao, L.; Luo, Y.; Ni, S.

2012-12-01

The 12 May 2008 Wenchuan earthquake (Mw=7.9) was the largest earthquake in China ever recorded by modern seismic instruments. It generated numerous moderate sized aftershocks that were well recorded by both permanent stations as well as portable instruments deployed after the mainshock. These waveform records yield high-quality data for the determination of focal mechanisms of aftershocks, which in turn provide important information for the investigation of regional stress field and the seismogenic environment in the Wenchuan earthquake source region. In this study, we determine the focal mechanisms, depths and moment magnitudes of moderate-sized (Mw ≥ 4.0) Wenchuan aftershocks using broadband waveform records. The focal mechanism results are then used to obtain the orientation and ratio of the principle stresses by the damped linear stress inversion method of Hardebeck & Michael (2006). Our results show that the majority of the moderate aftershocks occur at a depth range of 10-20 km and outside of the major rupture zones of the mainshock. The Wenchuan source region remains under a nearly horizontal compression with mostly thrust and occasional strike-slip faulting, especially towards the two ends of the rupture of the main shock. There is also clearly local variations in the orientation of the principle stresses.

Estimation of Static Coulomb Stress Change and Strong Motion Simulation for Jiuzhaigou 7.0 Earthquake Base on SENTINEL-1 Insar Data Inversion

NASA Astrophysics Data System (ADS)

Shen, W. H.; Luo, Y.; Jiao, Q. S.

2018-04-01

On August 8, 2017, an earthquake of M 7.0 occurred at Jiuzhaigou. Based on the Sentinel-1 satellite InSAR data, we obtained coseismic deformation field and inverted the source slip model. Results show that this event is dominated by strike slip, and the total released seismic moment is 8.06 × 1018 Nm, equivalent to an earthquake of Mw 6.57. We calculated static stress changes along strike and dip direction, and the static stress analysis show that the average stress drop are at low level, which may be responsible for the low level of ground motion during Jiuzhaigou earthquake. The coseismic Coulomb stress changes are calculated base on the inverted slip model, which revealed that 82.59 % of aftershocks are located in the Coulomb stress increasing area, 78.42 % of total aftershocks may be triggered by the mainshock aftershock, indicating that the mainshock has a significant triggering effect on the subsequent aftershocks. Based on stochastic finite fault model, we simulated regional peak ground acceleration (PGA), peak ground velocity (PGV) and the intensity, and results could capture basic features associated with the ground motion patterns. Moreover, the simulated results reflect the obvious rupture directivity effect.

Modeling Aftershocks and Foreshocks by Time-Dependent Friction Laws

NASA Astrophysics Data System (ADS)

Lippiello, E.; Landes, F.

2017-12-01

The transition with depth from rate-weakening to rate-strengthening rheology represents a viable mechanism to explain both afterslip and the temporal and spatial organization of aftershocks(Avouac, Annu. Rev. Eart Planet Sci. 2015).On the other hand, elastic models for seismic faults, as the Burridge-Knopoff model, are able to reproduce the Gutenberg-Richter (GR) law (de Arcangelis et al., Phys. Rep. 2016). Here we show that the two approaches can be combined in a minimal model containing only a parameter controlling the heterogeneities of the friction force. The key ingredient is the presence of a time-dependent friction on a temporal scale intermediate between the instantaneous scale of fracture propagation and the very slow one of the driving rate. Several features of aftershocks as the GR law, the productivity law, the spatial clustering and the temporal decay of the aftershock number, appear universal properties independent of details of model parameters and friction law. Quantitative agreement with the Omori law constraints the friction law according to a velocity strengthening rheology. The model also provides agreement with recent experimental results on the statistical properties of foreshock occurrence (Lippiello et al. , Pageoph, 2017). We then obtain insights on the nucleation phase preceding mainshocks which we compare with existing models (Ohnaka, Tectonophysics 1992).

Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

PubMed Central

Yao, Huajian; Shearer, Peter M.; Gerstoft, Peter

2013-01-01

Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above ∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (∼35–45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

Seismotectonics of the 2014 Chiang Rai, Thailand, earthquake sequence

NASA Astrophysics Data System (ADS)

Pananont, P.; Herman, M. W.; Pornsopin, P.; Furlong, K. P.; Habangkaem, S.; Waldhauser, F.; Wongwai, W.; Limpisawad, S.; Warnitchai, P.; Kosuwan, S.; Wechbunthung, B.

2017-08-01

On 5 May 2014, a Mw 6.2 strike-slip earthquake occurred in the Mae Lao region of Chiang Rai province in Thailand. This earthquake took place in a region of known faults and caused substantial damage and injuries, although the region had been previously identified as having a relatively low earthquake hazard. Detailed field reconnaissance and deployment of a dense, temporary, network of broadband seismometers allowed details of the damage and its relationship to seismicity to be analyzed. The aftershock sequence associated with this main shock occurs on two well-defined trends, reflecting the two potential fault planes in earthquake mechanisms for the main shock and the majority of the aftershocks. The damage area was relatively large for an event of this magnitude, but building damage was largely limited to the primary rupture region, while liquefaction and other ground failure are spatially associated with the rupture area and along regional rivers. Stress modeling, combined with the time series and pattern of aftershock activity, leads us to propose that slip near the northern termination of the main shock rupture continued slightly onto a conjugate fault, helping to trigger the distinct pattern of two discrete, conjugate trends of aftershock activity that mirror the kinematics of the main shock fault mechanism.

Seismic source study of the Racha-Dzhava (Georgia) earthquake from aftershocks and broad-band teleseismic body-wave records: An example of active nappe tectonics

USGS Publications Warehouse

Fuenzalida, H.; Rivera, L.; Haessler, H.; Legrand, D.; Philip, H.; Dorbath, L.; McCormack, D.; Arefiev, S.; Langer, C.; Cisternas, A.

1997-01-01

The Racha-Dzhava earthquake (Ms = 7.0) that occurred on 1991 April 29 at 09:12:48.1 GMT in the southern border of the Great Caucasus is the biggest event ever recorded in the region, stronger than the Spitak earthquake (Ms = 6.9) of 1988. A field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was deployed to record the aftershock activity. A very precise image of the aftershock distribution is obtained, showing an elongated cloud oriented N105??, with one branch trending N310?? in the western part. The southernmost part extends over 80 km, with the depth ranging from 0 to 15 km, and dips north. The northern branch, which is about 30 km long, shows activity that ranges in depth from 5 to 15 km. The complex thrust dips northwards. A stress-tensor inversion from P-wave first-motion polarities shows a state of triaxial compression, with the major principal axis oriented roughly N-S, the minor principal axis being vertical. Body-waveform inversion of teleseismic seismograms was performed for the main shock, which can be divided into four subevents with a total rupture-time duration of 22 s. The most important part of the seismic moment was released by a gentle northerly dipping thrust. The model is consistent with the compressive tectonics of the region and is in agreement with the aftershock distribution and the stress tensor deduced from the aftershocks. The focal mechanisms of the three largest aftershocks were also inverted from body-wave records. The April 29th (Ms = 6.1) and May 5th (Ms = 5.4) aftershocks have thrust mechanisms on roughly E-W-oriented planes, similar to the main shock. Surprisingly, the June 15th (Ms = 6.2) aftershock shows a thrust fault striking N-S. This mechanism is explained by the structural control of the rupture along the east-dipping geometry of the Dzirula Massif close to the Borzhomi-Kazbeg strike-slip fault. In fact, the orientation and shape of the stress tensor produce a thrust on a N-S oriented plane. Nappe tectonics has been identified as an important feature in the Caucasus, and the source mechanism is consistent with this observation. A hidden fault is present below the nappe, and no large surface breaks were observed due to the main shock. The epicentral region is characterized by sediments that are trapped between two crystalline basements: the Dzirula Massif, which crops out south of Chiatoura, and the Caucasus Main Range north of Oni. Most, if not all, of the rupture is controlled by the thrusting of overlapping, deformed and folded sediments over the Dzirula Massif. This event is another example of blind active faults, with the distinctive feature that the fault plane dips at a gentle angle. The Racha Range is one of the surface expressions of this blind thrust, and its growth is the consequence and evidence of similar earthquakes in the past.

Relationship between large slip area and static stress drop of aftershocks of inland earthquake :Example of the 2007 Noto Hanto earthquake

NASA Astrophysics Data System (ADS)

Urano, S.; Hiramatsu, Y.; Yamada, T.

2013-12-01

The 2007 Noto Hanto earthquake (MJMA 6.9; hereafter referred to the main shock) occurred at 0:41(UTC) on March 25, 2007 at a depth of 11km beneath the west coast of Noto Peninsula, central Japan. The dominant slip of the main shock was on a reverse fault with a right-lateral slip and the large slip area was distributed from hypocenter to the shallow part on the fault plane (Horikawa, 2008). The aftershocks are distributed not only in the small slip area but also in the large slip area (Hiramatsu et al., 2011). In this study, we estimate static stress drops of aftershocks on the fault plane of the main shock. We discuss the relationship between the static stress drops of the aftershocks and the large slip area of the main shock by investigating spatial pattern of the values of the static stress drops. We use the waveform data obtained by the group for the joint aftershock observations of the 2007 Noto Hanto Earthquake (Sakai et al., 2007). The sampling frequency of the waveform data is 100 Hz or 200 Hz. Focusing on similar aftershocks reported by Hiramatsu et al. (2011), we analyze static stress drops by using the method of empirical Green's function (EGF) (Hough, 1997) as follows. The smallest earthquake (MJMA≥2.0) of each group of similar earthquakes is set to the EGF earthquake, and the largest earthquake (MJMA≥2.5) is set to the target earthquake. We then deconvolve the waveform of an interested earthquake with that of the EGF earthquake at each station and obtain the spectral ratio of the sources that cancels the propagation effects (path and site effects). Following the procedure of Yamada et al. (2010), we finally estimate static stress drops for P- and S-waves from corner frequencies of the spectral ratio by using a model of Madariaga (1976). The estimated average value of static stress drop is 8.2×1.3 MPa (8.6×2.2 MPa for P-wave and 7.8×1.3 MPa for S-wave). These values are coincident approximately with the static stress drop of aftershocks of other inland earthquakes in Japan (Ito et al., 2005; Iio et al., 2006) and independent of the seismic moment. We then compare the values with the coseismic slip distribution of the main shock reported by Horikawa (2008). If we define large slip areas as areas with a slip exceeding 1 m, the average value of static stress drop is 12×2.3 (MPa) in the area. On the other hand, the average value is 5.7×0.9 (MPa) outside the large slip area. These results suggest that aftershocks in the large slip area likely have larger values of static stress drop, which would reflect the spatial heterogeneity of shear strength and dynamic stress level. Our results are coincident with the result of Yamada et al. (2010).

Disease aftershocks - The health effects of natural disasters

USGS Publications Warehouse

Guptill, S.C.

2001-01-01

While the initial activity of a natural disaster event may directly injure or kill a number of people, it is possible that a significant number of individuals will be affected by disease outbreaks that occur after the first effects of the disaster have passed. Coupling the epidemiologist's knowledge of disease outbreaks with geographic information systems and remote sensing technology could help natural disaster relief workers to prevent additional victims from disease aftershocks.

Adapting Pipeline Architectures to Track Developing Aftershock Sequences and Recurrent Explosions

DTIC Science & Technology

2014-02-14

Sumatra earthquake was used to study the performance of subspace detectors to detect and classify events from within a very large (Area = ~250,000 km2... detectors to identify and organize repeating waveforms discovered in multichannel seismic data streams. The framework has been tested and evaluated on...a variety of different test cases from mining blasts in Central Asia to moderate and large earthquake aftershock sequences. The framework performs

Dynamic triggering of deep earthquakes within a fossil slab

NASA Astrophysics Data System (ADS)

Cai, Chen; Wiens, Douglas A.

2016-09-01

The 9 November 2009 Mw 7.3 Fiji deep earthquake is the largest event in a region west of the Tonga slab defined by scattered seismicity and velocity anomalies. The main shock rupture was compact, but the aftershocks were distributed along a linear feature at distances of up to 126 km. The aftershocks and some background seismicity define a sharp northern boundary to the zone of outboard earthquakes, extending westward toward the Vitiaz deep earthquake cluster. The northern earthquake lineament is geometrically similar to tectonic reconstructions of the relict Vitiaz subduction zone at 8-10 Ma, suggesting the earthquakes are occurring in the final portion of the slab subducted at the now inactive Vitiaz trench. A Coulomb stress change calculation suggests many of the aftershocks were dynamically triggered. We propose that fossil slabs contain material that is too warm for earthquake nucleation but may be near the critical stress susceptible to dynamic triggering.

A study on off-fault aftershock pattern at N-Adria microplate

NASA Astrophysics Data System (ADS)

Bressan, Gianni; Barnaba, Carla; Magrin, Andrea; Rossi, Giuliana

2018-03-01

The spatial features of the aftershock sequences triggered by three moderate magnitude events with coda-duration magnitudes 4.1, 5.1 and 5.6, which occurred in Northeastern Italy and Western Slovenia, were investigated. The fractal dimension and the orientations of the planar features fitting the hypocentral data have been inferred. The spatial organization is articulated through two temporal phases. The first phase is characterized by the decreasing of the fractal dimension and by vertically oriented planes fitting the hypocentral foci. The second phase is marked by an increase of the fractal dimension and by the activation of different planes, with more widespread orientation. The aftershock temporal distribution is analysed with a model based on a static fatigue process. The process is favoured by the decrease of the overburden pressure, the sharp variations of the mechanical properties of the medium and the unclamping effect resulting from positive normal stress changes caused by the mainshock stress step.

The 7.9 Denali Fault Earthquake: Aftershock Locations, Moment Tensors and Focal Mechanisms from the Regional Seismic Network Data

NASA Astrophysics Data System (ADS)

Ratchkovski, N. A.; Hansen, R. A.; Christensen, D.; Kore, K.

2002-12-01

The largest earthquake ever recorded on the Denali fault system (magnitude 7.9) struck central Alaska on November 3, 2002. It was preceded by a magnitude 6.7 foreshock on October 23. This earlier earthquake and its zone of aftershocks were located slightly to the west of the 7.9 quake. Aftershock locations and surface slip observations from the 7.9 quake indicate that the rupture was predominately unilateral in the eastward direction. Near Mentasta Lake, a village that experienced some of the worst damage in the quake, the surface rupture scar turns from the Denali fault to the adjacent Totschunda fault, which trends toward more southeasterly toward the Canadian border. Overall, the geologists found that measurable scarps indicate that the north side of the Denali fault moved to the east and vertically up relative to the south. Maximum offsets on the Denali fault were 8.8 meters at the Tok Highway cutoff, and were 2.2 meters on the Totschunda fault. The Alaska regional seismic network consists of over 250 station sites, operated by the Alaska Earthquake Information Center (AEIC), the Alaska Volcano Observatory (AVO), and the Pacific Tsunami Warning Center (PTWC). Over 25 sites are equipped with the broad-band sensors, some of which have in addition the strong motion sensors. The rest of the stations are either 1 or 3-component short-period instruments. The data from these stations are collected, processed and archived at the AEIC. The AEIC staff installed a temporary network with over 20 instruments following the 6.7 Nenana Mountain and the 7.9 events. Prior to the M 7.9 Denali Fault event, the automatic earthquake detection system at AEIC was locating between 15 and 30 events per day. After the event, the system had over 200-400 automatic locations per day for at least 10 days following the 7.9 event. The processing of the data is ongoing with the priority given to the larger events. The cumulative length of the 6.7 and 7.9 aftershock locations along the Denali and Totschunda faults is about 300 km. We will present the aftershock locations, first motion focal mechanisms for M4+ events and regional moment tensors for M4.5+ events. The first motion focal mechanism for the main event indicates thrusting on the NE-trending plane with a dip of 48 degrees. We will present results of the double difference relocation of the aftershocks of the M7.9 event. The relocated aftershocks indicate a NW-dipping fault plane in the epicentral area of the event and a vertical plane along the rest of the rupture length.

Imaging inhomogeneous seismic velocity structure in and around the fault plane of the 2008 Iwate-Miyagi, Japan, Nairiku Earthquake (M7.2) - spatial variation in depth of seismic-aseismic transition and possible high-T/overpressurized fluid distribution

NASA Astrophysics Data System (ADS)

Okada, T.; Umino, N.; Hasegawa, A.; 2008 Iwate-Miyagi Nairiku Earthquake, G. O.

2008-12-01

A large shallow earthquake (named the 2008 Iwate-Miyagi Nairiku Earthquake) with a JMA magnitude of 7.2 occurred in the central part of NE Japan on June 14, 2008. Focal area of the present earthquake is located in the Tohoku backbone range strain concentration zone (Miura et al., 2004) along the volcanic front. Just after the occurrence of this earthquake, Japanese universities (Hokkaido, Hirosaki, Tohoku, Tokyo, Nagoya, Kyoto, Kochi, Kyusyu, Kagoshima) and NIED deployed a dense aftershock observation network in and around the focal area. Total number of temporal stations is 128. Using data from this dense aftershock observation and other temporary and routinely operated stations, we estimate hypocenter distribution and seismic velocity structure of the crust in and around the focal area of the present earthquake. We determined three-dimensional seismic velocity structure and relocated hypocenters simultaneously using the double- difference tomography method (Zhang and Thurber, 2003). Spatial extent of the aftershock area is about 45 km (NNE-SSW) by 15 km (WNW-ESE). Most of aftershocks are aligned in westward dipping. Shallower extensions of aftershock alignments seem to be located nearly at the coseismic surface deformations, which are along a geological fault, and the surface trace of the active fault (Detana fault). Note that some aftershocks seem to occur off the fault plane of the mainshock. The focal area of the present earthquake is located at a high Vs area. In the lower crust, we found some distinct low-Vs areas. These low velocity zones are located just beneath the strain concentration zones / seismic belts along the backbone range and in the northern Miyagi region. Focal area of the present earthquake is also located just above the low velocity zone in the lower crust. Beneath active volcanoes, these low velocity zones become more distinct and shallower, and aftershocks tend to occur shallower and not occur within such low-velocity zones. These low-velocity zones in the lower crust might be caused by high temperature upwelling flow of fluid originating from the mantle wedge. The present observation supports the hypothesis by Hasegawa et al. (2005) that anelastic deformation of the crust weakened by fluid forms the strain concentration zone and promotes the occurrence of large shallow inland earthquakes. We used data from JMA, Hi-net/NIED, NAO-Mizusawa and TITECH. We also used data from JNES. This work was conducted under the support of Grant-in-Aid for Special Purposes, MEXT, Japan. We thank Prof. Cliff Thurber and Dr. Haijiang Zhang for providing their programs and valuable discussions.

Focal Depth of the WenChuan Earthquake Aftershocks from modeling of Seismic Depth Phases

NASA Astrophysics Data System (ADS)

Luo, Y.; Zeng, X.; Chong, J.; Ni, S.; Chen, Y.

2008-12-01

After the 05/12/2008 great WenChuan earthquake in Sichuan Province of China, tens of thousands earthquakes occurred with hundreds of them stronger than M4. Those aftershocks provide valuable information about seismotectonics and rupture processes for the mainshock, particularly accurate spatial distribution of aftershocks is very informational for determining rupture fault planes. However focal depth can not be well resolved just with first arrivals recorded by relatively sparse network in Sichuan Province, therefore 3D seismicity distribution is difficult to obtain though horizontal location can be located with accuracy of 5km. Instead local/regional depth phases such as sPmP, sPn, sPL and teleseismic pP,sP are very sensitive to depth, and be readily modeled to determine depth with accuracy of 2km. With reference 1D velocity structure resolved from receiver functions and seismic refraction studies, local/regional depth phases such as sPmP, sPn and sPL are identified by comparing observed waveform with synthetic seismograms by generalized ray theory and reflectivity methods. For teleseismic depth phases well observed for M5.5 and stronger events, we developed an algorithm in inverting both depth and focal mechanism from P and SH waveforms. Also we employed the Cut and Paste (CAP) method developed by Zhao and Helmberger in modeling mechanism and depth with local waveforms, which constrains depth by fitting Pnl waveforms and the relative weight between surface wave and Pnl. After modeling all the depth phases for hundreds of events , we find that most of the M4 earthquakes occur between 2-18km depth, with aftershocks depth ranging 4-12km in the southern half of Longmenshan fault while aftershocks in the northern half featuring large depth range up to 18km. Therefore seismogenic zone in the northern segment is deeper as compared to the southern segment. All the aftershocks occur in upper crust, given that the Moho is deeper than 40km, or even 60km west of the Longmenshan fault. Absence of mid-lower crustal shocks supports the model of lower crustal flow beneath eastern Tibetan plateau, which is probably responsible for Longmenshan uplifting and hence the Wenchuan earthquake.

Spatial stress variations in the aftershock sequence following the 2008 M6 earthquake doublet in the South Iceland Seismic Zone

NASA Astrophysics Data System (ADS)

Hensch, M.; Árnadóttir, Th.; Lund, B.; Brandsdóttir, B.

2012-04-01

The South Iceland Seismic Zone (SISZ) is an approximately 80 km wide E-W transform zone, bridging the offset between the Eastern Volcanic Zone and the Hengill triple junction to the west. The plate motion is accommodated in the brittle crust by faulting on many N-S trending right-lateral strike-slip faults of 2-5 km separation. Major sequences of large earthquakes (M>6) has occurred repeatedly in the SISZ since the settlement in Iceland more than thousand years ago. On 29th May 2008, two M6 earthquakes hit the western part of the SISZ on two adjacent N-S faults within a few seconds. The intense aftershock sequence was recorded by the permanent Icelandic SIL network and a promptly installed temporary network of 11 portable seismometers in the source region. The network located thousands of aftershocks during the following days, illuminating a 12-17 km long region along both major fault ruptures as well as several smaller parallel faults along a diffuse E-W trending region west of the mainshock area without any preceding main rupture. This episode is suggested to be the continuation of an earthquake sequence which started with two M6.5 and several M5-6 events in June 2000. The time delay between the 2000 and 2008 events could be due to an inflation episode in Hengill during 1993-1998, that potentially locked N-S strike slip faults in the western part of the SISZ. Around 300 focal solutions for aftershocks have been derived by analyzing P-wave polarities, showing predominantly strike-slip movements with occasional normal faulting components (unstable P-axis direction), which suggests an extensional stress regime as their driving force. A subsequent stress inversion of four different aftershock clusters reveals slight variations of the directions of the average σ3 axes. While for both southern clusters, including the E-W cluster, the σ3 axes are rather elongated perpendicular to the overall plate spreading axis, they are more northerly trending for shallower clusters located further north. In this study we will try to shed light into whether the azimuth variations of σ3 is caused by stress changes due to the inflation-deflation episode in Hengill (NW of the activated fault zone) or solely depending to the depth of the aftershock clusters.

Foreshocks and Aftershocks Detected from Stick-slip Events on a 3 m Biaxial Apparatus and their Relationship to Quasistatic Nucleation and Wear Processes

NASA Astrophysics Data System (ADS)

Wu, S.; Mclaskey, G.

2017-12-01

We investigate foreshocks and aftershocks of dynamic stick-slip events generated on a newly constructed 3 m biaxial friction apparatus at Cornell University (attached figure). In a typical experiment, two rectangular granite blocks are squeezed together under 4 or 7 MPa of normal pressure ( 4 or 7 million N on a 1 m2 fault surface), and then shear stress is increased until the fault slips 10 - 400 microns in a dynamic rupture event similar to a M -2 to M -3 earthquake. Some ruptures nucleate near the north end of the fault, where the shear force is applied, other ruptures nucleate 2 m from the north end of the fault. The samples are instrumented with 16 piezoelectric sensors, 16 eddy current sensors, and 8 strain gage rosettes, evenly placed along the fault to measure vertical ground motion, local slip, and local stress, respectively. We studied sequences of tens of slip events and identified a total of 194 foreshocks and 66 aftershocks located within 6 s time windows around the stick-slip events and analyzed their timing and locations relative to the quasistatic nucleation process. We found that the locations of the foreshocks and aftershocks were distributed all along the length of the fault, with the majority located at the ends of the fault where local normal and shear stress is highest (caused by both edge effects and the finite stiffness of the steel frame surrounding the granite blocks). We also opened the laboratory fault and inspected the fault surface and found increased wear at the sample ends. To explore the foreshocks' and aftershocks' relationship to the nucleation and afterslip, we compared the occurrence of foreshocks to the local slip rate on the laboratory fault closest to each foreshock in space and time. We found that that majority of foreshocks were generated from local slip rates between 1 and 100 microns/s, though we were not able to resolve slip rate lower than about 1 micron/s. Our experiments provide insight into how foreshocks and aftershocks in natural earthquakes may be influenced both by fault structure and slow slip associated with nucleation or afterslip.

Aftershocks of the western Argentina (Caucete) earthquake of 23 November 1977: some tectonic implications

USGS Publications Warehouse

Langer, C.J.; Bollinger, G.A.

1988-01-01

An aftershock survey, using a network of eight portable and two permanent seismographs, was conducted for the western Argentina (Caucete) earthquake (MS 7.3) of November 23, 1977. Monitoring began December 6, almost 2 weeks after the main shock and continued for 11 days. The data set includes 185 aftershock hypocenters that range in the depth from near surface to more than 30 km. The spatial distribution of those events occupied a volume of about 100 km long ??50 km wide ??30 km thick. The volumnar nature of the aftershock distribution is interpreted to be a result of a bimodal distribution of foci that define east- and west-dipping planar zones. Efforts to select which of those zones was associated with the causal faulting include special attention to the determination of the mainshock focal depth and dislocation theory modeling of the coseismic surface deformation in the epicentral region. Our focal depth (25-35 km) and modeling studies lead us to prefer an east-dipping plane as causal. A previous interpretation by other investigators used a shallower focal depth (17 km) and similar modeling calculations in choosing a west-dipping plane. Our selection of the east-dipping plane is physically more appealing because it places fault initiation at the base of the crustal seismogenic layer (rather than in the middle of that layer) which requires fault propagation to be updip (rather than downdip). ?? 1988.

Observation of aftershocks of the 2003 Tokachi-Oki earthquake for estimation of local site effects

NASA Astrophysics Data System (ADS)

Yamanaka, Hiroaki; Motoki, Kentaro; Etoh, Kiminobu; Murayama, Masanari; Komaba, Nobuhiko

2004-03-01

Observation of aftershocks of the 2003 Tokachi-Oki earthquake was conducted in the southern part of the Tokachi basin in Hokkaido, Japan for estimation of local site effects. We installed accelerographs at 12 sites in Chokubetsu, Toyokoro, and Taiki areas, where large strong motion records were obtained during the main shock at stations of the K-NET and KiK-net. The stations of the aftershock observation are situated with different geological conditions and some of the sites were installed on Pleistocene layers as reference sites. The site amplifications are investigated using spectral ratio of S-waves from the aftershocks. The S-wave amplification factor is dominant at a period of about 1 second at the site near the KiK-net site in Toyokoro. This amplification fits well with calculated 1D amplification of S-wave in alluvial layers with a thickness of 50 meters. In addition to the site effects, we detected nonlinear amplification of the soft soils only during the main shock. The site effects at the strong motion site of the K-NET at Chokubetsu have a dominate peak at a period of 0.4 seconds. This amplification is due to soft soils having a thickness of about 13 meters. Contrary to the results at the two areas, site effects are not significantly different at the stations in the Taiki area, because of similarity on surface geological conditions.

Structure and tectonics of the Main Himalayan Thrust and associated faults from recent earthquake and seismic imaging studies using the NAMASTE array

NASA Astrophysics Data System (ADS)

Karplus, M. S.; Pant, M.; Velasco, A. A.; Nabelek, J.; Kuna, V. M.; Sapkota, S. N.; Ghosh, A.; Mendoza, M.; Adhikari, L. B.; Klemperer, S. L.

2017-12-01

The India-Eurasia collision zone presents a significant earthquake hazard, as demonstrated by the recent, devastating April 25, 2015 M=7.8 Gorkha earthquake and the following May 12, 2015 M=7.3 earthquake. Important questions remain, including distinguishing possible geometries of the Main Himalayan Thrust (MHT), the role of other regional faults, the crustal composition and role of fluids in faulting, and the details of the rupture process, including structural causes and locations of rupture segmentation both along-strike and down-dip. These recent earthquakes and their aftershocks provide a unique opportunity to learn more about this collision zone. In June 2015, funded by NSF, we deployed the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake (NAMASTE) array of 46 seismic stations distributed across eastern and central Nepal, spanning the region with most of the aftershocks. This array remained in place for 11 months from June 2015 to May 2016. We combine new results from this aftershock network in Nepal with previous geophysical and geological studies across the Himalaya to derive a new understanding of the tectonics of the Himalaya and southern Tibet in Nepal and surrounding countries. We focus on structure and composition of the Main Himalayan Thrust and compare this continent-continent subduction megathrust with megathrusts in other subduction zones.

Seismic Parameters of Mining-Induced Aftershock Sequences for Re-entry Protocol Development

NASA Astrophysics Data System (ADS)

Vallejos, Javier A.; Estay, Rodrigo A.

2018-03-01

A common characteristic of deep mines in hard rock is induced seismicity. This results from stress changes and rock failure around mining excavations. Following large seismic events, there is an increase in the levels of seismicity, which gradually decay with time. Restricting access to areas of a mine for enough time to allow this decay of seismic events is the main approach in re-entry strategies. The statistical properties of aftershock sequences can be studied with three scaling relations: (1) Gutenberg-Richter frequency magnitude, (2) the modified Omori's law (MOL) for the temporal decay, and (3) Båth's law for the magnitude of the largest aftershock. In this paper, these three scaling relations, in addition to the stochastic Reasenberg-Jones model are applied to study the characteristic parameters of 11 large magnitude mining-induced aftershock sequences in four mines in Ontario, Canada. To provide guidelines for re-entry protocol development, the dependence of the scaling relation parameters on the magnitude of the main event are studied. Some relations between the parameters and the magnitude of the main event are found. Using these relationships and the scaling relations, a space-time-magnitude re-entry protocol is developed. These findings provide a first approximation to concise and well-justified guidelines for re-entry protocol development applicable to the range of mining conditions found in Ontario, Canada.

The 2000 Nemuro-Hanto-Oki earthquake, off eastern Hokkaido, Japan, and the high intraslab seismic activity in the southwestern Kuril Trench

USGS Publications Warehouse

Takahashi, H.; Hirata, K.

2003-01-01

The 2000 Nemuro-Hanto-Oki earthquake (Mw6.8) occurred in the southwestern part of the Kuril Trench. The hypocenter was located close to the aftershock region of the great 1994 Kuril earthquake (Mw8.3), named "the 1994 Hokkaido-Toho-Oki earthquake" by the Japan Meteorological Agency, for which the fault plane is still in debate. Analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event. The hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage. They clearly show that the 2000 event was an intraslab event occurring on a shallow-dipping fault plane between 55 and 65 km in depth. The well-focused aftershock distribution of the 2000 event, the relative location of the 1994 event with respect to the 2000 event, and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow-dipping fault plane in the subducting slab. The recent hypocenter distribution around the 1994 aftershock region also supports this result. Large intraslab earthquakes occuring to the southeast of Hokkaido may occur due to a strong coupling on the plate boundary, which generates relatively large stress field within the subducting Pacific plate.

Detecting aseismic strain transients from seismicity data

USGS Publications Warehouse

Llenos, A.L.; McGuire, J.J.

2011-01-01

Aseismic deformation transients such as fluid flow, magma migration, and slow slip can trigger changes in seismicity rate. We present a method that can detect these seismicity rate variations and utilize these anomalies to constrain the underlying variations in stressing rate. Because ordinary aftershock sequences often obscure changes in the background seismicity caused by aseismic processes, we combine the stochastic Epidemic Type Aftershock Sequence model that describes aftershock sequences well and the physically based rate- and state-dependent friction seismicity model into a single seismicity rate model that models both aftershock activity and changes in background seismicity rate. We implement this model into a data assimilation algorithm that inverts seismicity catalogs to estimate space-time variations in stressing rate. We evaluate the method using a synthetic catalog, and then apply it to a catalog of M???1.5 events that occurred in the Salton Trough from 1990 to 2009. We validate our stressing rate estimates by comparing them to estimates from a geodetically derived slip model for a large creep event on the Obsidian Buttes fault. The results demonstrate that our approach can identify large aseismic deformation transients in a multidecade long earthquake catalog and roughly constrain the absolute magnitude of the stressing rate transients. Our method can therefore provide a way to detect aseismic transients in regions where geodetic resolution in space or time is poor. Copyright 2011 by the American Geophysical Union.

Constraints on Dynamic Triggering from very Short term Microearthquake Aftershocks at Parkfield

NASA Astrophysics Data System (ADS)

Ampuero, J.; Rubin, A.

2004-12-01

The study of microearthquakes helps bridge the gap between laboratory experiments and data from large earthquakes, the two disparate scales that have contributed so far to our understanding of earthquake physics. Although they are frequent, microearthquakes are difficult to analyse. Applying high precision relocation techniques, Rubin and Gillard (2000) observed a pronounced asymmetry in the spatial distribution of the earliest and nearest aftershocks of microearthquakes along the San Andreas fault (they occur more often to the NW of the mainshock). It was suggested that this could be related to the velocity contrast across the fault. Preferred directivity of dynamic rupture pulses running along a bimaterial interface (to the SE in the case of the SAF) is expected on theoretical grounds. Our numerical simulations of crack-like rupture on such interfaces show a pronounced asymmetry of the stress histories beyond the rupture ends, and suggest two possible mechanisms for the observed asymmetry: First, that it results from an asymmmetry in the static stress field following arrest of the mainshock (closer to failure to the NW), or second, that it is due to a short-duration tensile pulse that propagates to the SE, which could reduce the number of aftershocks to the SE by dynamic triggering of any nucleation site close enough to failure to have otherwise produced an aftershock. To distinguish betwen these mechanisms we need observations of dynamic triggering in microseismicity. For small events triggered at a distance of some mainshock radii, triggering time scales are so short that seismograms of both events overlap. To detect the occurrence of compound events and very short term aftershocks in the HRSN Parkfield archived waveforms we have developed an automated search algorithm based on empirical Green's function (EGF) deconvolution. Optimal EGFs are first selected by the coherency of the cross-component convolution with respect to the target event. Then Landweber deconvolution is applied. The resulting source time functions (STF) are often noisy and corrupted by sidelobes due to finite frequency band of the data. They are scanned for subevents, exploiting the consistency of the occurrence of secondary peaks (outliers among the STF maxima) throughout the 30 network channels. Subevents are picked, in many cases to sub-sample precision, by waveform fitting using all the EGFs available. We have detected a total of 30 such multiple or compound eve nts with inter-event delays of less than one second, in a catalog that spans over 10 years of seismicity in Parkfield (2300 cataloged events in our working box). Most of them are not detectable by visual inspection of the seismograms. In most cases, their timing and relative location are consistent with dynamic triggering. Also, the seismicity rate at very early times (less than 0.1 seconds) seems higher than expected from the longer term aftershock seismicity rate observed in the region. This points to dynamic effects in very short term aftershock decay. Finally, more of these immediate aftershocks occur to the NW, as with the earlier NCSN results, although the number of events analysed so far is small. We will discuss these and ongoing observations from the standpoint of dynamic rupture on bimaterial interfaces, supported by numerical simulations.

Iterative Strategies for Aftershock Classification in Automatic Seismic Processing Pipelines

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gibbons, Steven J.; Kvaerna, Tormod; Harris, David B.

We report aftershock sequences following very large earthquakes present enormous challenges to near-real-time generation of seismic bulletins. The increase in analyst resources needed to relocate an inflated number of events is compounded by failures of phase-association algorithms and a significant deterioration in the quality of underlying, fully automatic event bulletins. Current processing pipelines were designed a generation ago, and, due to computational limitations of the time, are usually limited to single passes over the raw data. With current processing capability, multiple passes over the data are feasible. Processing the raw data at each station currently generates parametric data streams thatmore » are then scanned by a phase-association algorithm to form event hypotheses. We consider the scenario in which a large earthquake has occurred and propose to define a region of likely aftershock activity in which events are detected and accurately located, using a separate specially targeted semiautomatic process. This effort may focus on so-called pattern detectors, but here we demonstrate a more general grid-search algorithm that may cover wider source regions without requiring waveform similarity. Given many well-located aftershocks within our source region, we may remove all associated phases from the original detection lists prior to a new iteration of the phase-association algorithm. We provide a proof-of-concept example for the 2015 Gorkha sequence, Nepal, recorded on seismic arrays of the International Monitoring System. Even with very conservative conditions for defining event hypotheses within the aftershock source region, we can automatically remove about half of the original detections that could have been generated by Nepal earthquakes and reduce the likelihood of false associations and spurious event hypotheses. Lastly, further reductions in the number of detections in the parametric data streams are likely, using correlation and subspace detectors and/or empirical matched field processing.« less

Exponential decline of aftershocks of the M7.9 1868 great Kau earthquake, Hawaii, through the 20th century

USGS Publications Warehouse

Klein, F.W.; Wright, Tim

2008-01-01

The remarkable catalog of Hawaiian earthquakes going back to the 1820s is based on missionary diaries, newspaper accounts, and instrumental records and spans the great M7.9 Kau earthquake of April 1868 and its aftershock sequence. The earthquake record since 1868 defines a smooth curve complete to M5.2 of the declining rate into the 21st century, after five short volcanic swarms are removed. A single aftershock curve fits the earthquake record, even with numerous M6 and 7 main shocks and eruptions. The timing of some moderate earthquakes may be controlled by magmatic stresses, but their overall long-term rate reflects one of aftershocks of the Kau earthquake. The 1868 earthquake is, therefore, the largest and most controlling stress event in the 19th and 20th centuries. We fit both the modified Omori (power law) and stretched exponential (SE) functions to the earthquakes. We found that the modified Omori law is a good fit to the M ??? 5.2 earthquake rate for the first 10 years or so and the more rapidly declining SE function fits better thereafter, as supported by three statistical tests. The switch to exponential decay suggests that a possible change in aftershock physics may occur from rate and state fault friction, with no change in the stress rate, to viscoelastic stress relaxation. The 61-year exponential decay constant is at the upper end of the range of geodetic relaxation times seen after other global earthquakes. Modeling deformation in Hawaii is beyond the scope of this paper, but a simple interpretation of the decay suggests an effective viscosity of 1019 to 1020 Pa s pertains in the volcanic spreading of Hawaii's flanks. The rapid decline in earthquake rate poses questions for seismic hazard estimates in an area that is cited as one of the most hazardous in the United States.

Static stress changes and the triggering of earthquakes

USGS Publications Warehouse

King, Geoffrey C.P.; Stein, Ross S.; Lin, Jian

1994-01-01

To understand whether the 1992 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the general problem of how one earthquake might trigger another. The tendency of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally orientated for failure as a result of the prevailing regional stress field and the stress change caused by the mainshock. We find that the distribution of aftershocks for the Landers earthquake, as well as for several other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulomb stress on optimally orientated faults rose by more than one-half bar, and aftershocks are sparse where the Coulomb stress dropped by a similar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 centuries. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but depends on the fault geometry, sense of slip, and the coefficient of friction. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the Landers and Big Bear earthquakes raised the stress along the San Bernardino segment of the southern San Andreas fault by 2 to 6 bars, hastening the next great earthquake there by about a decade.

Reexamination of the subsurface fault structure in the vicinity of the 1989 moment-magnitude-6.9 Loma Prieta earthquake, central California, using steep-reflection, earthquake, and magnetic data

USGS Publications Warehouse

Zhang, Edward; Fuis, Gary S.; Catchings, Rufus D.; Scheirer, Daniel S.; Goldman, Mark; Bauer, Klaus

2018-06-13

We reexamine the geometry of the causative fault structure of the 1989 moment-magnitude-6.9 Loma Prieta earthquake in central California, using seismic-reflection, earthquake-hypocenter, and magnetic data. Our study is prompted by recent interpretations of a two-part dip of the San Andreas Fault (SAF) accompanied by a flower-like structure in the Coachella Valley, in southern California. Initially, the prevailing interpretation of fault geometry in the vicinity of the Loma Prieta earthquake was that the mainshock did not rupture the SAF, but rather a secondary fault within the SAF system, because network locations of aftershocks defined neither a vertical plane nor a fault plane that projected to the surface trace of the SAF. Subsequent waveform cross-correlation and double-difference relocations of Loma Prieta aftershocks appear to have clarified the fault geometry somewhat, with steeply dipping faults in the upper crust possibly connecting to the more moderately southwest-dipping mainshock rupture in the middle crust. Examination of steep-reflection data, extracted from a 1991 seismic-refraction profile through the Loma Prieta area, reveals three robust fault-like features that agree approximately in geometry with the clusters of upper-crustal relocated aftershocks. The subsurface geometry of the San Andreas, Sargent, and Berrocal Faults can be mapped using these features and the aftershock clusters. The San Andreas and Sargent Faults appear to dip northeastward in the uppermost crust and change dip continuously toward the southwest with depth. Previous models of gravity and magnetic data on profiles through the aftershock region also define a steeply dipping SAF, with an initial northeastward dip in the uppermost crust that changes with depth. At a depth 6 to 9 km, upper-crustal faults appear to project into the moderately southwest-dipping, planar mainshock rupture. The change to a planar dipping rupture at 6–9 km is similar to fault geometry seen in the Coachella Valley.

Implications of diverse fault orientations imaged in relocated aftershocks of the Mount Lewis, ML 5.7, California, earthquake

NASA Astrophysics Data System (ADS)

Kilb, D.; Rubin, A. M.

2002-11-01

We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 ML5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the approximately north striking main shock, defining an hourglass with the long axis aligned approximately with the main shock. Surprisingly, our relocation demonstrates that many of these aftershocks illuminate a series of near-vertical east-west faults that are ˜0.5-1 km long and separated by as little as ˜200 m. We propose that these structures result from the growth of a relatively young fault in which displacement across a right-lateral approximately north striking fault zone is accommodated by slip on secondary left-lateral approximately east striking faults. We derive the main shock-induced static Coulomb failure function (Δσf) on the dominant fault orientation in our study area using a three-dimensional (3-D) boundary element program. To bound viable friction coefficients, we measure the correlation between the rank ordering of relative amplitudes of Δσf and seismicity rate change. We find that likely friction coefficients are 0.2-0.6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values. We obtain similar results from a visual correlation of calculated Δσf contours with the distribution of aftershocks. Viable rate-and-state constitutive parameters bound the observed relationship between magnitude of Δσf and seismicity rate change, and for our favored main shock model a maximum correlation is achieved when Δσf is computed with friction coefficients of 0.3-0.6. These values are below those previously cited for young faults.

Source modeling of the 2015 Mw 7.8 Nepal (Gorkha) earthquake sequence: Implications for geodynamics and earthquake hazards

NASA Astrophysics Data System (ADS)

McNamara, D. E.; Yeck, W. L.; Barnhart, W. D.; Schulte-Pelkum, V.; Bergman, E.; Adhikari, L. B.; Dixit, A.; Hough, S. E.; Benz, H. M.; Earle, P. S.

2017-09-01

The Gorkha earthquake on April 25th, 2015 was a long anticipated, low-angle thrust-faulting event on the shallow décollement between the India and Eurasia plates. We present a detailed multiple-event hypocenter relocation analysis of the Mw 7.8 Gorkha Nepal earthquake sequence, constrained by local seismic stations, and a geodetic rupture model based on InSAR and GPS data. We integrate these observations to place the Gorkha earthquake sequence into a seismotectonic context and evaluate potential earthquake hazard. Major results from this study include (1) a comprehensive catalog of calibrated hypocenters for the Gorkha earthquake sequence; (2) the Gorkha earthquake ruptured a 150 × 60 km patch of the Main Himalayan Thrust (MHT), the décollement defining the plate boundary at depth, over an area surrounding but predominantly north of the capital city of Kathmandu (3) the distribution of aftershock seismicity surrounds the mainshock maximum slip patch; (4) aftershocks occur at or below the mainshock rupture plane with depths generally increasing to the north beneath the higher Himalaya, possibly outlining a 10-15 km thick subduction channel between the overriding Eurasian and subducting Indian plates; (5) the largest Mw 7.3 aftershock and the highest concentration of aftershocks occurred to the southeast the mainshock rupture, on a segment of the MHT décollement that was positively stressed towards failure; (6) the near surface portion of the MHT south of Kathmandu shows no aftershocks or slip during the mainshock. Results from this study characterize the details of the Gorkha earthquake sequence and provide constraints on where earthquake hazard remains high, and thus where future, damaging earthquakes may occur in this densely populated region. Up-dip segments of the MHT should be considered to be high hazard for future damaging earthquakes.

The 12th June 2017 Mw = 6.3 Lesvos earthquake from detailed seismological observations

NASA Astrophysics Data System (ADS)

Papadimitriou, P.; Kassaras, I.; Kaviris, G.; Tselentis, G.-A.; Voulgaris, N.; Lekkas, E.; Chouliaras, G.; Evangelidis, C.; Pavlou, K.; Kapetanidis, V.; Karakonstantis, A.; Kazantzidou-Firtinidou, D.; Fountoulakis, I.; Millas, C.; Spingos, I.; Aspiotis, T.; Moumoulidou, A.; Skourtsos, E.; Antoniou, V.; Andreadakis, E.; Mavroulis, S.; Kleanthi, M.

2018-04-01

A major earthquake (Mwö=ö6.3) occurred on the 12th of June 2017 (12:28 GMT) offshore, south of the SE coast of Lesvos Island, at a depth of 13ökm, in an area characterized by normal faulting with an important strike-slip component in certain cases. Over 900 events of the sequence between 12 and 30 June 2017 were manually analyzed and located, employing an optimized local velocity model. Double-difference relocation revealed seven spatially separated groups of events, forming two linear branches, roughly aligned N130°E, compatible with the strike of known mapped faults along the southern coast of Lesvos Island. Spatiotemporal analysis indicated gradual migration of seismicity towards NW and SE from the margins of the main rupture, while a strong secondary sequence at a separate fault patch SE of the mainshock, oriented NW-SE, was triggered by the largest aftershock (Mwö=ö5.2) that occurred on 17 June. The focal mechanisms of the mainshock (φö=ö122°, δö=ö40° and λö=ö-83°) and of the major aftershocks were determined using regional moment tensor inversion. In most cases normal faulting was revealed with the fault plane oriented in a NW-SE direction, dipping SW, with the exception of the largest aftershock that was characterized by strike-slip faulting. Stress inversion revealed a complex stress field south of Lesvos, related both to normal, in an approximate E-W direction, and strike-slip faulting. All aftershocks outside the main rupture, where gradual seismicity migration was observed, are located within the positive lobes of static stress transfer determined by applying the Coulomb criterion for the mainshock. Stress loading on optimal faults under a strike-slip regime explains the occurrence of the largest aftershock and the seismicity that was triggered at the eastern patch of the rupture zone.

Long-lived aftershock sequences around Beijing, China

NASA Astrophysics Data System (ADS)

Wang, Jian; Main, Ian G.; Musson, Roger M. W.

2017-04-01

SUMMARY Most aftershock sequences are relatively transient, decaying over months or years to background levels. However, in some intra-plate areas, persistent clusters of events can occur over much greater time scales, for example the ongoing sequence in the New Madrid zone of the eastern US. Here we examine the evidence for such long-lived aftershock sequences around Beijing, China. First we introduce a metric known as the 'seismic density index' that quantifies the degree of clustering of seismic energy release. For a given map location, this multi-dimensional index depends on the number of events, their magnitudes, and the distances to the locations of the surrounding population of earthquakes. We apply the index to modern instrumental catalogue data between 1970 and 2014, and identify six clear candidate zones for long-lived aftershocks. We then compare these locations to earthquake epicenter and seismic intensity data for the six largest historical earthquakes. Each candidate zone contains one of the six historical events, and the location of peak intensity is within 5km or so of the reported epicenter in five of these cases. In one case - the great Ms 8 earthquake of 1679 - the peak is closer to the area of strongest shaking (Intensity XI or more) than the reported epicenter. These observations are consistent with the hypothesis that the modern clusters are long-lived aftershocks. However, there is no systematic reduction in the seismic event rate in these candidate zones with time since 1970, as one might expect from a transient decay by the Omori law. This could be due to the decay rate being too slow to be detected, or that the index is instead picking out the location of persistent weaknesses in the lithosphere. In either case the results imply that areas of high seismic density index could be used in principle to indicate the location of unrecorded historical of palaeo-seismic events in areas of intra-plate continental seismicity.

Source modeling of the 2015 Mw 7.8 Nepal (Gorkha) earthquake sequence: Implications for geodynamics and earthquake hazards

USGS Publications Warehouse

McNamara, Daniel E.; Yeck, William; Barnhart, William D.; Schulte-Pelkum, V.; Bergman, E.; Adhikari, L. B.; Dixit, Amod; Hough, S.E.; Benz, Harley M.; Earle, Paul

2017-01-01

The Gorkha earthquake on April 25th, 2015 was a long anticipated, low-angle thrust-faulting event on the shallow décollement between the India and Eurasia plates. We present a detailed multiple-event hypocenter relocation analysis of the Mw 7.8 Gorkha Nepal earthquake sequence, constrained by local seismic stations, and a geodetic rupture model based on InSAR and GPS data. We integrate these observations to place the Gorkha earthquake sequence into a seismotectonic context and evaluate potential earthquake hazard.Major results from this study include (1) a comprehensive catalog of calibrated hypocenters for the Gorkha earthquake sequence; (2) the Gorkha earthquake ruptured a ~ 150 × 60 km patch of the Main Himalayan Thrust (MHT), the décollement defining the plate boundary at depth, over an area surrounding but predominantly north of the capital city of Kathmandu (3) the distribution of aftershock seismicity surrounds the mainshock maximum slip patch; (4) aftershocks occur at or below the mainshock rupture plane with depths generally increasing to the north beneath the higher Himalaya, possibly outlining a 10–15 km thick subduction channel between the overriding Eurasian and subducting Indian plates; (5) the largest Mw 7.3 aftershock and the highest concentration of aftershocks occurred to the southeast the mainshock rupture, on a segment of the MHT décollement that was positively stressed towards failure; (6) the near surface portion of the MHT south of Kathmandu shows no aftershocks or slip during the mainshock. Results from this study characterize the details of the Gorkha earthquake sequence and provide constraints on where earthquake hazard remains high, and thus where future, damaging earthquakes may occur in this densely populated region. Up-dip segments of the MHT should be considered to be high hazard for future damaging earthquakes.

Iterative Strategies for Aftershock Classification in Automatic Seismic Processing Pipelines

DOE PAGES

Gibbons, Steven J.; Kvaerna, Tormod; Harris, David B.; ...

2016-06-08

We report aftershock sequences following very large earthquakes present enormous challenges to near-real-time generation of seismic bulletins. The increase in analyst resources needed to relocate an inflated number of events is compounded by failures of phase-association algorithms and a significant deterioration in the quality of underlying, fully automatic event bulletins. Current processing pipelines were designed a generation ago, and, due to computational limitations of the time, are usually limited to single passes over the raw data. With current processing capability, multiple passes over the data are feasible. Processing the raw data at each station currently generates parametric data streams thatmore » are then scanned by a phase-association algorithm to form event hypotheses. We consider the scenario in which a large earthquake has occurred and propose to define a region of likely aftershock activity in which events are detected and accurately located, using a separate specially targeted semiautomatic process. This effort may focus on so-called pattern detectors, but here we demonstrate a more general grid-search algorithm that may cover wider source regions without requiring waveform similarity. Given many well-located aftershocks within our source region, we may remove all associated phases from the original detection lists prior to a new iteration of the phase-association algorithm. We provide a proof-of-concept example for the 2015 Gorkha sequence, Nepal, recorded on seismic arrays of the International Monitoring System. Even with very conservative conditions for defining event hypotheses within the aftershock source region, we can automatically remove about half of the original detections that could have been generated by Nepal earthquakes and reduce the likelihood of false associations and spurious event hypotheses. Lastly, further reductions in the number of detections in the parametric data streams are likely, using correlation and subspace detectors and/or empirical matched field processing.« less

Seismic moment tensor inversion using 3D velocity model and its application to the 2013 Lushan earthquake sequence

NASA Astrophysics Data System (ADS)

Zhu, Lupei; Zhou, Xiaofeng

2016-10-01

Source inversion of small-magnitude events such as aftershocks or mine collapses requires use of relatively high frequency seismic waveforms which are strongly affected by small-scale heterogeneities in the crust. In this study, we developed a new inversion method called gCAP3D for determining general moment tensor of a seismic source using Green's functions of 3D models. It inherits the advantageous features of the ;Cut-and-Paste; (CAP) method to break a full seismogram into the Pnl and surface-wave segments and to allow time shift between observed and predicted waveforms. It uses grid search for 5 source parameters (relative strengths of the isotropic and compensated-linear-vector-dipole components and the strike, dip, and rake of the double-couple component) that minimize the waveform misfit. The scalar moment is estimated using the ratio of L2 norms of the data and synthetics. Focal depth can also be determined by repeating the inversion at different depths. We applied gCAP3D to the 2013 Ms 7.0 Lushan earthquake and its aftershocks using a 3D crustal-upper mantle velocity model derived from ambient noise tomography in the region. We first relocated the events using the double-difference method. We then used the finite-differences method and reciprocity principle to calculate Green's functions of the 3D model for 20 permanent broadband seismic stations within 200 km from the source region. We obtained moment tensors of the mainshock and 74 aftershocks ranging from Mw 5.2 to 3.4. The results show that the Lushan earthquake is a reverse faulting at a depth of 13-15 km on a plane dipping 40-47° to N46° W. Most of the aftershocks occurred off the main rupture plane and have similar focal mechanisms to the mainshock's, except in the proximity of the mainshock where the aftershocks' focal mechanisms display some variations.

Comparison of Observed Spatio-temporal Aftershock Patterns with Earthquake Simulator Results

NASA Astrophysics Data System (ADS)

Kroll, K.; Richards-Dinger, K. B.; Dieterich, J. H.

2013-12-01

Due to the complex nature of faulting in southern California, knowledge of rupture behavior near fault step-overs is of critical importance to properly quantify and mitigate seismic hazards. Estimates of earthquake probability are complicated by the uncertainty that a rupture will stop at or jump a fault step-over, which affects both the magnitude and frequency of occurrence of earthquakes. In recent years, earthquake simulators and dynamic rupture models have begun to address the effects of complex fault geometries on earthquake ground motions and rupture propagation. Early models incorporated vertical faults with highly simplified geometries. Many current studies examine the effects of varied fault geometry, fault step-overs, and fault bends on rupture patterns; however, these works are limited by the small numbers of integrated fault segments and simplified orientations. The previous work of Kroll et al., 2013 on the northern extent of the 2010 El Mayor-Cucapah rupture in the Yuha Desert region uses precise aftershock relocations to show an area of complex conjugate faulting within the step-over region between the Elsinore and Laguna Salada faults. Here, we employ an innovative approach of incorporating this fine-scale fault structure defined through seismological, geologic and geodetic means in the physics-based earthquake simulator, RSQSim, to explore the effects of fine-scale structures on stress transfer and rupture propagation and examine the mechanisms that control aftershock activity and local triggering of other large events. We run simulations with primary fault structures in state of California and northern Baja California and incorporate complex secondary faults in the Yuha Desert region. These models produce aftershock activity that enables comparison between the observed and predicted distribution and allow for examination of the mechanisms that control them. We investigate how the spatial and temporal distribution of aftershocks are affected by changes to model parameters such as shear and normal stress, rate-and-state frictional properties, fault geometry, and slip rate.

Aftershock Analysis of the 2016 Mw7.8 Pedernales (Ecuador) Earthquake: Seismotectonics, Seismicity Distribution and Relationship with Coseismic Slip Distribution

NASA Astrophysics Data System (ADS)

Agurto-Detzel, H.; Font, Y.; Charvis, P.; Ambrois, D.; Cheze, J.; Courboulex, F.; De Barros, L.; Deschamps, A.; Galve, A.; Godano, M.; Laigle, M.; Maron, C.; Martin, X.; Monfret, T.; Oregioni, D.; Peix, F., Sr.; Regnier, M. M.; Yates, B.; Mercerat, D.; Leon Rios, S.; Rietbrock, A.; Acero, W.; Alvarado, A. P.; Gabriela, P.; Ramos, C.; Ruiz, M. C.; Singaucho, J. C.; Vasconez, F.; Viracucha, C.; Beck, S. L.; Lynner, C.; Hoskins, M.; Meltzer, A.; Soto-Cordero, L.; Stachnik, J.

2017-12-01

0n April 2016, a Mw 7.8 megathrust earthquake struck the coast of Ecuador causing vast human and material losses. The earthquake ruptured a 100 km-long segment of the subduction interface between Nazca and South America, spatially coinciding with the 1942 M 7.8 earthquake rupture area. Shortly after the mainshock, an international effort made by institutions from Ecuador, France, UK and USA, deployed a temporary network of +60 land and ocean-bottom seismometers to capture the aftershock sequence for the subsequent year. These stations came to join the local Ecuadorian national network already monitoring in place. Here we benefit from this dataset to produce a suite of automatic locations and a subset of regional moment tensors for high quality events. Over 2900 events were detected for the first month of postseismic activity alone, and a subset of 600 events were manually re-picked and located. Similarly, thousands of aftershocks were detected using the temporary deployment over the following months, with magnitudes ranging between 1 to 7. As expected, moment tensors show mostly thrust faulting at the interface, but we also observe sparse normal and strike-slip faulting at shallow depths in the forearc. The spatial distribution of seismicity delineates the coseismic rupture area, but extends well beyond it over a 300 km long segment. Main features include three seismicity alignments perpendicular to the trench, at the north, center and south of the mainshock rupture. Preliminary results comparing quantitatively the distribution of aftershocks to the distribution of the coseismic rupture show that the bulk of the aftershock seismicity occurs at intermediate levels of coseismic slip, while areas of maximum coseismic slip are mostly devoid of events M>3. Our results shed light on the interface processes occurring mainly during the early post-seismic period of large megathrust earthquakes, and implications on the earthquake cycle.

Aftershock Forecasting: Recent Developments and Lessons from the 2016 M5.8 Pawnee, Oklahoma, Earthquake

NASA Astrophysics Data System (ADS)

Michael, A. J.; Field, E. H.; Hardebeck, J.; Llenos, A. L.; Milner, K. R.; Page, M. T.; Perry, S. C.; van der Elst, N.; Wein, A. M.

2016-12-01

After the Mw 5.8 Pawnee, Oklahoma, earthquake of September 3, 2016 the USGS issued a series of aftershock forecasts for the next month and year. These forecasts were aimed at the emergency response community, those making decisions about well operations in the affected region, and the general public. The forecasts were generated manually using methods planned for automatically released Operational Aftershock Forecasts. The underlying method is from Reasenberg and Jones (Science, 1989) with improvements recently published in Page et al. (BSSA, 2016), implemented in a JAVA Graphical User Interface and presented in a template that is under development. The methodological improvements include initial models based on the tectonic regime as defined by Garcia et al. (BSSA, 2012) and the inclusion of both uncertainty in the clustering parameters and natural random variability. We did not utilize the time-dependent magnitude of completeness model from Page et al. because it applies only to teleseismic events recorded by NEIC. The parameters for Garcia's Generic Active Continental Region underestimated the modified-Omori decay parameter and underestimated the aftershock rate by a factor of 2. And the sequence following the Mw 5.7 Prague, Oklahoma, earthquake of November 6, 2011 was about 3 to 4 times more productive than the Pawnee sequence. The high productivity for these potentially induced sequences is consistent with an increase in productivity in Oklahoma since 2009 (Llenos and Michael, BSSA, 2013) and makes a general tectonic model inapplicable to sequences in this region. Soon after the mainshock occurred, the forecasts relied on the sequence specific parameters. After one month, the Omori decay parameter p is less than one, implying a very long-lived sequence. However, the decay parameter is known to be biased low at early times due to secondary aftershock triggering, and the p-value determined early in the sequence may be inaccurate for long-term forecasting.

Tectonic implications and seismicity triggering during the 2008 Baluchistan, Pakistan earthquake sequence

NASA Astrophysics Data System (ADS)

Yadav, R. B. S.; Gahalaut, V. K.; Chopra, Sumer; Shan, Bin

2012-02-01

A damaging and widely felt moderate earthquake (Mw 6.4) hit the rural, mountainous region of southwestern Pakistan on October 28, 2008. The main shock was followed by another earthquake of identical magnitude (Mw 6.4) on the next day. The spatial distribution of aftershocks and focal mechanism revealed a NW-SE striking rupture with right-lateral strike-slip motion which is sympathetic to the NNW-SSE striking active mapped Urghargai Fault. The occurrence of strike-slip earthquakes suggests that along with the thrust faults, strike slip faults too are present beneath the fold-and-thrust belt of Sulaiman-Kirthar ranges and accommodates some of the relative motion of the Indian and Eurasian plates. To assess the characteristics of this sequence, the statistical parameters like aftershocks temporal decay, b-value of G-R relationship, partitioning of radiated seismic energy due to aftershocks, and spatial fractal dimension (D-value) have been examined. The b-value is estimated as 1.03 ± 0.42 and suggests the tectonic genesis of the sequence and crustal heterogeneity within rock mass. The low p-value of 0.89 ± 0.07 implies slow decay of aftershocks activity which is probably an evidence for low surface heat flow. A value of spatial fractal dimension of 2.08 ± 0.02 indicates random spatial distribution and that the source is a two-dimensional plane filled-up by fractures. The static coseismic Coulomb stress changes due to the foreshock (Mw 5.3) were found to increase stress by more than 0.04 bars at the hypocenter of the main shock, thus promoting the failure. The cumulative coseismic Coulomb stress changes due to the foreshock and mainshocks suggest that most of the aftershocks occurred in the region of increased Coulomb stress, and to the SE to the mainshock rupture.

Modeling of the Nano- and Picoseismicity Rate Changes Resulting from Static Stress Triggering due to Small (MW2.2) Event Recorded at Mponeng Deep Gold Mine, South Africa

NASA Astrophysics Data System (ADS)

Kozlowska, M.; Orlecka-Sikora, B.; Kwiatek, G.; Boettcher, M. S.; Dresen, G. H.

2014-12-01

Static stress changes following large earthquakes are known to affect the rate and spatio-temporal distribution of the aftershocks. Here we utilize a unique dataset of M ≥ -3.4 earthquakes following a MW 2.2 earthquake in Mponeng gold mine, South Africa, to investigate this process for nano- and pico- scale seismicity at centimeter length scales in shallow, mining conditions. The aftershock sequence was recorded during a quiet interval in the mine and thus enabled us to perform the analysis using Dietrich's (1994) rate and state dependent friction law. The formulation for earthquake productivity requires estimation of Coulomb stress changes due to the mainshock, the reference seismicity rate, frictional resistance parameter, and the duration of aftershock relaxation time. We divided the area into six depth intervals and for each we estimated the parameters and modeled the spatio-temporal patterns of seismicity rates after the stress perturbation. Comparing the modeled patterns of seismicity with the observed distribution we found that while the spatial patterns match well, the rate of modeled aftershocks is lower than the observed rate. To test our model, we used four metrics of the goodness-of-fit evaluation. Testing procedure allowed rejecting the null hypothesis of no significant difference between seismicity rates only for one depth interval containing the mainshock, for the other, no significant differences have been found. Results show that mining-induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive Coulomb stress change. Furthermore, we demonstrate that the main features of the temporal and spatial distribution of very small, mining-induced earthquakes at shallow depths can be successfully determined using rate- and state-based stress modeling.

Comparisons of ground motions from five aftershocks of the 1999 Chi-Chi, Taiwan, earthquake with empirical predictions largely based on data from California

USGS Publications Warehouse

Wang, G.-Q.; Boore, D.M.; Igel, H.; Zhou, X.-Y.

2004-01-01

The observed ground motions from five large aftershocks of the 1999 Chi-Chi, Taiwan, earthquake are compared with predictions from four equations based primarily on data from California. The four equations for active tectonic regions are those developed by Abrahamson and Silva (1997), Boore et al. (1997), Campbell (1997, 2001), and Sadigh et al. (1997). Comparisons are made for horizontal-component peak ground accelerations and 5%-damped pseudoacceleration response spectra at periods between 0.02 sec and 5 sec. The observed motions are in reasonable agreement with the predictions, particularly for distances from 10 to 30 km. This is in marked contrast to the motions from the Chi-Chi mainshock, which are much lower than the predicted motions for periods less than about 1 sec. The results indicate that the low motions in the mainshock are not due to unusual, localized absorption of seismic energy, because waves from the mainshock and the aftershocks generally traverse the same section of the crust and are recorded at the same stations. The aftershock motions at distances of 30-60 km are somewhat lower than the predictions (but not nearly by as small a factor as those for the mainshock), suggesting that the ground motion attenuates more rapidly in this region of Taiwan than it does in the areas we compare with it. We provide equations for the regional attenuation of response spectra, which show increasing decay of motion with distance for decreasing oscillator periods. This observational study also demonstrates that ground motions have large earthquake-location-dependent variability for a specific site. This variability reduces the accuracy with which an earthquake-specific prediction of site response can be predicted. Online Material: PGAs and PSAs from the 1999 Chi-Chi earthquake and five aftershocks.

Earthquakes induced by fluid injection and explosion

USGS Publications Warehouse

Healy, J.H.; Hamilton, R.M.; Raleigh, C.B.

1970-01-01

Earthquakes generated by fluid injection near Denver, Colorado, are compared with earthquakes triggered by nuclear explosion at the Nevada Test Site. Spatial distributions of the earthquakes in both cases are compatible with the hypothesis that variation of fluid pressure in preexisting fractures controls the time distribution of the seismic events in an "aftershock" sequence. We suggest that the fluid pressure changes may also control the distribution in time and space of natural aftershock sequences and of earthquakes that have been reported near large reservoirs. ?? 1970.

SOME NEW PROCESSING TECHNIQUES FOR THE IMPERIAL VALLEY 1979 AFTERSHOCKS.

USGS Publications Warehouse

Brady, A. Gerald; ,

1983-01-01

This paper describes some of the features of the latest processing improvements that the U. S. Geological Survey (USGS) is currently applying to strong-motion accelerograms from the national network of permanent stations. At the same time it introduces the application of this processing to the set of Imperial Valley aftershocks recorded following the main shock of October 15, 1979. Earlier processing of the 22 main shock recordings provided corrected accelerations, velocity and displacement, response spectra, and Fourier spectra.

The 10 April 2014 Nicaraguan Crustal Earthquake: Evidence of Complex Deformation of the Central American Volcanic Arc

NASA Astrophysics Data System (ADS)

Suárez, Gerardo; Muñoz, Angélica; Farraz, Isaac A.; Talavera, Emilio; Tenorio, Virginia; Novelo-Casanova, David A.; Sánchez, Antonio

2016-10-01

On 10 April 2014, an M w 6.1 earthquake struck central Nicaragua. The main event and the aftershocks were clearly recorded by the Nicaraguan national seismic network and other regional seismic stations. These crustal earthquakes were strongly felt in central Nicaragua but caused relatively little damage. This is in sharp contrast to the destructive effects of the 1972 earthquake in the capital city of Managua. The differences in damage stem from the fact that the 1972 earthquake occurred on a fault beneath the city; in contrast, the 2014 event lies offshore, under Lake Managua. The distribution of aftershocks of the 2014 event shows two clusters of seismic activity. In the northwestern part of Lake Managua, an alignment of aftershocks suggests a northwest to southeast striking fault, parallel to the volcanic arc. The source mechanism agrees with this right-lateral, strike-slip motion on a plane with the same orientation as the aftershock sequence. For an earthquake of this magnitude, seismic scaling relations between fault length and magnitude predict a sub-surface fault length of approximately 16 km. This length is in good agreement with the extent of the fault defined by the aftershock sequence. A second cluster of aftershocks beneath Apoyeque volcano occurred simultaneously, but spatially separated from the first. There is no clear alignment of the epicenters in this cluster. Nevertheless, the decay of the number of earthquakes beneath Apoyeque as a function of time shows the typical behavior of an aftershock sequence and not of a volcanic swarm. The northeast-southwest striking Tiscapa/Ciudad Jardín and Estadio faults that broke during the 1972 and 1931 Managua earthquakes are orthogonal to the fault where the 10 April earthquake occurred. These orthogonal faults in close geographic proximity show that Central Nicaragua is being deformed in a complex tectonic setting. The Nicaraguan forearc sliver, between the trench and the volcanic arc, moves to the northwest relative to the Caribbean plate at a rate of 14 mm/year. Part of the deformation is apparently accommodated by strain partitioning in the form of bookshelf faulting, on a system of orthogonal faults. The sinistral faults striking northeast-southwest rotate blocks of the Caribbean plate in a clockwise manner. The recent crustal earthquakes in central Nicaragua in 1931, 1972 and 2005 earthquakes took place on these left-lateral faults. The motion of the forearc sliver is also accommodated by a second set of right-lateral, strike-slip faults oriented parallel to the volcanic arc. Faults with this orientation and direction of motion are responsible for the 2014 and possibly the 1955 earthquakes. The presence of this geometry of orthogonal crustal faults highlights the seismic hazard posed by this complex faulting system, not only in the capital city of Managua, but also to the major Nicaraguan cities, which lie close to the volcanic arc.

Computer simulation of earthquakes

NASA Technical Reports Server (NTRS)

Cohen, S. C.

1976-01-01

Two computer simulation models of earthquakes were studied for the dependence of the pattern of events on the model assumptions and input parameters. Both models represent the seismically active region by mechanical blocks which are connected to one another and to a driving plate. The blocks slide on a friction surface. In the first model elastic forces were employed and time independent friction to simulate main shock events. The size, length, and time and place of event occurrence were influenced strongly by the magnitude and degree of homogeniety in the elastic and friction parameters of the fault region. Periodically reoccurring similar events were frequently observed in simulations with near homogeneous parameters along the fault, whereas, seismic gaps were a common feature of simulations employing large variations in the fault parameters. The second model incorporated viscoelastic forces and time-dependent friction to account for aftershock sequences. The periods between aftershock events increased with time and the aftershock region was confined to that which moved in the main event.

Seismic Observations Indicating That the 2015 Ogasawara (Bonin) Earthquake Ruptured Beneath the 660 km Discontinuity

NASA Astrophysics Data System (ADS)

Kuge, Keiko

2017-11-01

The termination of deep earthquakes at a depth of 700 km is a key feature for understanding the physical mechanism of deep earthquakes. The 680 km deep 30 May 2015, Ogasawara (Bonin) earthquake (Mw 7.9) and its aftershocks were recorded by seismic stations at distances from 7° to 19°. Synthetic seismograms indicate that the P waveforms depend on whether the earthquake is located above or below the 660 km discontinuity. In this study, I show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660 km velocity discontinuity. Recordings of the P wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter. Theoretical waveforms computed with a 660 km discontinuity above the aftershock and mainshock match the observed waveforms more accurately. These observations may indicate earthquake ruptures due to mantle minerals other than olivine or strong deformation of the 660 km phase transition.

Afterslip Enhanced Aftershock Activity During the 2017 Earthquake Sequence Near Sulphur Peak, Idaho

DOE PAGES

Koper, Keith D.; Pankow, Kristine L.; Pechmann, James C.; ...

2018-05-29

An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal–faulting M w 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes ≥4.0 M L. High–accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10 km) crust of the Aspen Range, east of the northern section of the range–bounding, west–dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike–slipmore » faulting with a summed aftershock moment that is 1.8–2.4 times larger than the mainshock moment. Here, we propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning ~1 day after the mainshock.« less

Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Ranch and Superstition Hills, California, earthquakes

USGS Publications Warehouse

Mori, J.

1991-01-01

Event record sections, which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations, show multiple free-surface reflections (PP, PPP, PPPP) of the P wave in the Imperial Valley. The relative timing of these arrivals is used to estimate the strength of the P-wave velocity gradient within the upper 5 km of the sediment layer. Consistent with previous studies, a velocity model with a value of 1.8 km/sec at the surface increasing linearly to 5.8 km/sec at a depth of 5.5 km fits the data well. The relative amplitudes of the P and PP arrivals are used to estimate the source depth for the aftershock distributions of the Elmore Ranch and Superstition Hills main shocks. Although the depth determination has large uncertainties, both the Elmore Ranch and Superstition Hills aftershock sequencs appear to have similar depth distribution in the range of 4 to 10 km. -Author

Aftershocks, earthquake effects, and the location of the large 14 December 1872 earthquake near Entiat, central Washington

USGS Publications Warehouse

Brocher, Thomas M.; Hopper, Margaret G.; Algermissen, S.T. Ted; Perkins, David M.; Brockman, Stanley R.; Arnold, Edouard P.

2017-01-01

Reported aftershock durations, earthquake effects, and other observations from the large 14 December 1872 earthquake in central Washington are consistent with an epicenter near Entiat, Washington. Aftershocks were reported for more than 3 months only near Entiat. Modal intensity data described in this article are consistent with an Entiat area epicenter, where the largest modified Mercalli intensities, VIII, were assigned between Lake Chelan and Wenatchee. Although ground failures and water effects were widespread, there is a concentration of these features along the Columbia River and its tributaries in the Entiat area. Assuming linear ray paths, misfits from 23 reports of the directions of horizontal shaking have a local minima at Entiat, assuming the reports are describing surface waves, but the region having comparable misfit is large. Broadband seismograms recorded for comparable ray paths provide insight into the reasons why possible S–P times estimated from felt reports at two locations are several seconds too small to be consistent with an Entiat area epicenter.

Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults.

PubMed

McGuire, Jeffrey J; Boettcher, Margaret S; Jordan, Thomas H

2005-03-24

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominantly aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion.

Including foreshocks and aftershocks in time-independent probabilistic seismic hazard analyses

USGS Publications Warehouse

Boyd, Oliver S.

2012-01-01

Time‐independent probabilistic seismic‐hazard analysis treats each source as being temporally and spatially independent; hence foreshocks and aftershocks, which are both spatially and temporally dependent on the mainshock, are removed from earthquake catalogs. Yet, intuitively, these earthquakes should be considered part of the seismic hazard, capable of producing damaging ground motions. In this study, I consider the mainshock and its dependents as a time‐independent cluster, each cluster being temporally and spatially independent from any other. The cluster has a recurrence time of the mainshock; and, by considering the earthquakes in the cluster as a union of events, dependent events have an opportunity to contribute to seismic ground motions and hazard. Based on the methods of the U.S. Geological Survey for a high‐hazard site, the inclusion of dependent events causes ground motions that are exceeded at probability levels of engineering interest to increase by about 10% but could be as high as 20% if variations in aftershock productivity can be accounted for reliably.

Afterslip Enhanced Aftershock Activity During the 2017 Earthquake Sequence Near Sulphur Peak, Idaho

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koper, Keith D.; Pankow, Kristine L.; Pechmann, James C.

An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal–faulting M w 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes ≥4.0 M L. High–accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10 km) crust of the Aspen Range, east of the northern section of the range–bounding, west–dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike–slipmore » faulting with a summed aftershock moment that is 1.8–2.4 times larger than the mainshock moment. Here, we propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning ~1 day after the mainshock.« less

Strong motion observations and recordings from the great Wenchuan Earthquake

USGS Publications Warehouse

Li, X.; Zhou, Z.; Yu, H.; Wen, R.; Lu, D.; Huang, M.; Zhou, Y.; Cu, J.

2008-01-01

The National Strong Motion Observation Network System (NSMONS) of China is briefly introduced in this paper. The NSMONS consists of permanent free-field stations, special observation arrays, mobile observatories and a network management system. During the Wenchuan Earthquake, over 1,400 components of acceleration records were obtained from 460 permanent free-field stations and three arrays for topographical effect and structural response observation in the network system from the main shock, and over 20,000 components of acceleration records from strong aftershocks occurred before August 1, 2008 were also obtained by permanent free-field stations of the NSMONS and 59 mobile instruments quickly deployed after the main shock. The strong motion recordings from the main shock and strong aftershocks are summarized in this paper. In the ground motion recordings, there are over 560 components with peak ground acceleration (PGA) over 10 Gal, the largest being 957.7 Gal. The largest PGA recorded during the aftershock exceeds 300 Gal. ?? 2008 Institute of Engineering Mechanics, China Earthquake Administration and Springer-Verlag GmbH.

Co-seismic slip, post-seismic slip, and largest aftershock associated with the 1994 Sanriku-haruka-oki, Japan, earthquake

NASA Astrophysics Data System (ADS)

Yagi, Yuji; Kikuchi, Masayuki; Nishimura, Takuya

2003-11-01

We analyzed continuous GPS data to investigate the spatio-temporal distribution of co-seismic slip, post-seismic slip, and largest aftershock associated with the 1994 Sanriku-haruka-oki, Japan, earthquake (Mw = 7.7). To get better resolution for co-seismic and post-seismic slip distribution, we imposed a weak constraint as a priori information of the co-seismic slip determined by seismic wave analyses. We found that the post-seismic slip during 100 days following the main-shock amount to as much moment release as the main-shock, and that the sites of co-seismic slip and post-seismic slip are partitioning on a plate boundary region in complimentary fashion. The major post-seismic slip was triggered by the mainshock in western side of the co-seismic slip, and the extent of the post-seismic slip is almost unchanged with time. It rapidly developed a shear stress concentration ahead of the slip area, and triggered the largest aftershock.

Testing the Predictive Power of Coulomb Stress on Aftershock Sequences

NASA Astrophysics Data System (ADS)

Woessner, J.; Lombardi, A.; Werner, M. J.; Marzocchi, W.

2009-12-01

Empirical and statistical models of clustered seismicity are usually strongly stochastic and perceived to be uninformative in their forecasts, since only marginal distributions are used, such as the Omori-Utsu and Gutenberg-Richter laws. In contrast, so-called physics-based aftershock models, based on seismic rate changes calculated from Coulomb stress changes and rate-and-state friction, make more specific predictions: anisotropic stress shadows and multiplicative rate changes. We test the predictive power of models based on Coulomb stress changes against statistical models, including the popular Short Term Earthquake Probabilities and Epidemic-Type Aftershock Sequences models: We score and compare retrospective forecasts on the aftershock sequences of the 1992 Landers, USA, the 1997 Colfiorito, Italy, and the 2008 Selfoss, Iceland, earthquakes. To quantify predictability, we use likelihood-based metrics that test the consistency of the forecasts with the data, including modified and existing tests used in prospective forecast experiments within the Collaboratory for the Study of Earthquake Predictability (CSEP). Our results indicate that a statistical model performs best. Moreover, two Coulomb model classes seem unable to compete: Models based on deterministic Coulomb stress changes calculated from a given fault-slip model, and those based on fixed receiver faults. One model of Coulomb stress changes does perform well and sometimes outperforms the statistical models, but its predictive information is diluted, because of uncertainties included in the fault-slip model. Our results suggest that models based on Coulomb stress changes need to incorporate stochastic features that represent model and data uncertainty.

Rifting mechanisms constrained by InSAR, seismicity, GPS, and surface rupture from the Karonga earthquake sequence in northern Lake Malawi (Nyasa)

NASA Astrophysics Data System (ADS)

Zheng, W.; Pritchard, M. E.; Henderson, S. T.; Gaherty, J. B.; Shillington, D. J.; Oliva, S. J.; Ebinger, C.; Nooner, S. L.; Elliott, J.; Saria, E.; Ntambila, D.; Chindandali, P. R. N.

2017-12-01

The Malawi rift is part of the archetypal East African rift where early-stage crustal extension is dominated by faulting. In the Karonga region of northern Malawi, a sequence of earthquakes in late 2009, with 15 teleseismically detected (Mw 4.5-6.0) over 13 days, provides a uniqueopportunity to evaluate faulting processes controlling present-day extension in an early-stage rift. We describe observations of this sequence including hundreds of aftershocks located by a temporary seismic array installed in 2010, ground deformation from satellite interferograms, and surface rupture from field surveys published by others. We use all of these data to model fault geometry and slip. The aftershocks from January-May 2010 suggest the involvement of multiple faults, and we test the extent that this can be resolved by the InSAR data. The InSAR and surface rupture both suggest that the major slip occurred at shallow depth (<5 km). Our preferred aftershock locations appear to correlate with this principal slip zone, although uncertainty in the shallow velocity structure can allow for a bulk of the events to fall down-dip of the geodetically constrained slip. Subsequent deformation, including that associated with a December 2014 Mw 5.1 earthquake, can be constrained from multidisciplinary data collected during the SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) project, which includes the Karonga region and spans 2013-2015. We find 3 cm of potential ground movement at the location of the earthquake as determined by the SEGMeNT seismic array from Sentinel-1. Geodetic fault slip is consistent with the focal mechanism and depth determined by the local array. The location is at the northern end of the 2009-2010 aftershock zone, and aftershocks suggest some linkage with faults that slipped in 2009. InSAR observations do not provide any evidence for large aseismic slip or fluid movements during or after the 2014 sequence, which had <200 aftershocks above the network threshold. For example, we do not observe any deformation at Rungwe volcano above the 2 cm/yr detection threshold with InSAR time series from ALOS (2007-2010) or Sentinel-1 (10/2014 - 04/2017). The time series from SEGMeNT and other continuous GPS stations do not show transients related to the earthquakes, but are not optimally located in space or time.

Seismicity in the Wake of the April 2016 Pedernales Earthquake

NASA Astrophysics Data System (ADS)

Meltzer, A.; Beck, S. L.; Ruiz, M. C.; Hernandez, S.; Alvarado, A. P.; Regnier, M. M.; Rietbrock, A.; Font, Y.; Charvis, P.; Yepes, H. A.; Lynner, C.; Porritt, R. W.

2016-12-01

On April 16th 2016 a Mw7.8 earthquake struck along the Colombia-Ecuador trench near Pedernales, Ecuador. The epicentral region lies just north of the intersection of the Carnegie Ridge with the subduction zone where the orientation of the trench shifts from N20°E to N32°E. This portion of the subduction zone has ruptured on decadal time scales; Mw7.8 (1942), Mw 7.7 (1958), and Mw 8.8 (1906). The rupture zone of the 2016 Pedernales earthquake falls within the rupture area of the 1906 event and appears to overlap with the previous 1942 event. In the wake of the earthquake an international response coordinated by the Instituto Geofisico EPN in Quito deployed accelerometers, seismometers, OBS, and GPS receivers to record aftershocks and post-seismic deformation. These data provide the opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities in promoting or inhibiting rupture propagation, and the relationship between locked and creeping parts of the subduction interface. Onland, a dense array of 64 broadband and intermediate period seismometers cover the 2016 rupture zone, extending north to the section that ruptured in 1958 and covers the contiguous area that ruptured in 1906. The US portion of the response supported by the NSF includes 19 broadband seismic stations to record aftershocks for a year, an eGPS survey, and five cGPS stations to enhance the existing network in Ecuador. Data from the NSF supported stations are open access. We examine the distribution of seismicity from the aftershock deployment in relationship to the 1942, 1958, and 1906 ruptures. Preliminary locations from the IG-EPN aftershock catalog outline the rupture zone and initially exhibited an abrupt termination to the north at 1°N. Along this northern boundary a series of aftershocks that took place over a period of several hours along a very linear trend culminated in one of several ≥Mw 6.0 aftershocks. To the south, a significant cluster of events is observed 100 km south of the area that ruptured in the mainshock. This area lies south of an area of low coupling observed in GPS data suggesting that stress is being transmitted across a zone that is freely slipping. In July a series of events occurred north of the rupture zone close to the edge of the segment that ruptured in 1958.

An Improved Source-Scanning Algorithm for Locating Earthquake Clusters or Aftershock Sequences

NASA Astrophysics Data System (ADS)

Liao, Y.; Kao, H.; Hsu, S.

2010-12-01

The Source-scanning Algorithm (SSA) was originally introduced in 2004 to locate non-volcanic tremors. Its application was later expanded to the identification of earthquake rupture planes and the near-real-time detection and monitoring of landslides and mud/debris flows. In this study, we further improve SSA for the purpose of locating earthquake clusters or aftershock sequences when only a limited number of waveform observations are available. The main improvements include the application of a ground motion analyzer to separate P and S waves, the automatic determination of resolution based on the grid size and time step of the scanning process, and a modified brightness function to utilize constraints from multiple phases. Specifically, the improved SSA (named as ISSA) addresses two major issues related to locating earthquake clusters/aftershocks. The first one is the massive amount of both time and labour to locate a large number of seismic events manually. And the second one is to efficiently and correctly identify the same phase across the entire recording array when multiple events occur closely in time and space. To test the robustness of ISSA, we generate synthetic waveforms consisting of 3 separated events such that individual P and S phases arrive at different stations in different order, thus making correct phase picking nearly impossible. Using these very complicated waveforms as the input, the ISSA scans all model space for possible combination of time and location for the existence of seismic sources. The scanning results successfully associate various phases from each event at all stations, and correctly recover the input. To further demonstrate the advantage of ISSA, we apply it to the waveform data collected by a temporary OBS array for the aftershock sequence of an offshore earthquake southwest of Taiwan. The overall signal-to-noise ratio is inadequate for locating small events; and the precise arrival times of P and S phases are difficult to determine. We use one of the largest aftershocks that can be located by conventional methods as our reference event to calibrate the controlling parameters of ISSA. These parameters include the overall Vp/Vs ratio (because a precise S velocity model was unavailable), the length of scanning time window, and the weighting factor for each station. Our results show that ISSA is not only more efficient in locating earthquake clusters/aftershocks, but also capable of identifying many events missed by conventional phase-picking methods.

Detection and location of small aftershocks using waveform cross correlation

NASA Astrophysics Data System (ADS)

Kitov, Ivan; Sanina, Irina; Sergeev, Sergey

2017-04-01

Aftershock sequences of earthquakes with magnitudes 5.0 and lower are difficult to detect and locate by sparse regional networks. Signals from aftershocks with magnitudes 2 to 3 are usually below detection thresholds of standard 3-C seismic stations at near regional distances. For seismic events close in space, the method waveform cross correlation (WCC) allows to reduce detection threshold by at least a unit of magnitude and to improve location precision to a few kilometers. Therefore, the WCC method is directly applicable to weak aftershock sequences. Here, we recover seismic activity after the earthquake near the town of Mariupol (Ukraine) occurred on August 7, 2016. The main shock was detected by many stations of the International monitoring system (IMS), including the closest primary IMS array stations AKASG (6.62 deg.) and BRTR (7.81), as well as 3-C station KBZ (5.00). The International data centre located this event (47.0013N, 37.5427E), estimated its origin time (08:15:4.1 UTC), magnitude (mb=4.5), and depth (6.8 km). This event was also detected by two array stations of the Institute for Dynamics of Geospheres (IDG) of the Russian Academy of Sciences: portable 3-C array RDON (3.28), which is the closest station, and MHVAR (7.96). Using signals from the main shock at five stations as waveform templates, we calculated continuous traces of cross correlation coefficient (CC) from the 7th to the 11th of August. We found that the best templates should include all regional phases, and thus, have the length from 80 s to 180 s. For detection, we used standard STA/LTA method with threshold depending on station. The accuracy of onset time estimation by the STA/LTA detector based on CC-traces is close to one sample, which varies from 0.05 s at BRTR to 0.005 s for RDON and MHVAR. Arrival times of all detected signals were reduced to origin times using the observed travel times from the main shock. Clusters of origin times are considered as event hypotheses in the phase association procedure. As a result, we found 12 aftershocks with magnitudes between 1.5 and 3.5. These small events were detected neither by the IDC nor by the near regional network of the Geophysical Survey of RAS, which has three closest 3-C stations at distances of 2.2 to 3.5 degrees from the studied earthquake. We also applied procedure of relative location and all aftershocks were found within a few km from the main shock.

Determination of Focal Depths of Earthquakes in the Mid-Oceanic Ridges from Amplitude Spectra of Surface Waves

DTIC Science & Technology

1969-06-01

Foreshock , mainshock and aftershock of the Parkfield, California earthquake of June 28, 1966. b. The Denver earthquake of August 9, 1967. Let us look...into the results of these tests in more details. (1) Test on the main shock, foreshock and aftershock of the Parkfield earthquake of June 28, 1966...According to McEvilly et. al. (1967), the origin times and locations of.these events were the following: Foreshock June 28, 1966, 04:08:56.2 GMT; 350 57.6

Seismic amplitude measurements suggest foreshocks have different focal mechanisms than aftershocks

USGS Publications Warehouse

Lindh, A.; Fuis, G.; Mantis, C.

1978-01-01

The ratio of the amplitudes of P and S waves from the foreshocks and aftershocks to three recent California earthquakes show a characteristic change at the time of the main events. As this ratio is extremely sensitive to small changes in the orientation of the fault plane, a small systematic change in stress or fault configuration in the source region may be inferred. These results suggest an approach to the recognition of foreshocks based on simple measurements of the amplitudes of seismic waves. Copyright ?? 1978 AAAS.

Geotechnical effects of the 2015 magnitude 7.8 Gorkha, Nepal, earthquake and aftershocks

USGS Publications Warehouse

Moss, Robb E. S.; Thompson, Eric M.; Kieffer, D Scott; Tiwari, Binod; Hashash, Youssef M A; Acharya, Indra; Adhikari, Basanta; Asimaki, Domniki; Clahan, Kevin B.; Collins, Brian D.; Dahal, Sachindra; Jibson, Randall W.; Khadka, Diwakar; Macdonald, Amy; Madugo, Chris L M; Mason, H Benjamin; Pehlivan, Menzer; Rayamajhi, Deepak; Uprety, Sital

2015-01-01

This article summarizes the geotechnical effects of the 25 April 2015 M 7.8 Gorkha, Nepal, earthquake and aftershocks, as documented by a reconnaissance team that undertook a broad engineering and scientific assessment of the damage and collected perishable data for future analysis. Brief descriptions are provided of ground shaking, surface fault rupture, landsliding, soil failure, and infrastructure performance. The goal of this reconnaissance effort, led by Geotechnical Extreme Events Reconnaissance, is to learn from earthquakes and mitigate hazards in future earthquakes.

The Mw6.5 17 November 2015 Lefkada (Greece) Earthquake: Structural Interpretation by Means of the Aftershock Analysis

NASA Astrophysics Data System (ADS)

Papadimitriou, E.; Karakostas, V.; Mesimeri, M.; Chouliaras, G.; Kourouklas, Ch.

2017-10-01

The 2015 Mw6.5 Lefkada main shock occurred at the south western part of Lefkada Island (Greece), less than 2 years after the occurrence of a doublet along the western part of the nearby Kefalonia Island, Paliki peninsula (on 25/01/2014, with Mw6.1 and 03/02/2014 with Mw6.0) and 12 years after the 2003 Mw6.2 main shock that struck the northwestern part of Lefkada Island. The four failed dextral strike slip fault segments belong to the Kefalonia transform fault zone (KTFZ), the major active boundary that bounds from the west the area of central Ionian Islands, namely Lefkada and Kefalonia. It is associated with several known historical earthquakes and is considered the most hazardous area in the Greek territory. The KTFZ fault segments are characterized by high slip rates (of the order of tens of millimeters per year), with maximum earthquake magnitudes up to 6.7 for Lefkada and 7.2 for Kefalonia fault zone, respectively. The double difference location technique was employed for relocating the aftershocks revealing a seismogenic layer extending from 3 to 16 km depth and multiple activation on well-defined fault planes, with strikes that differ than the main rupture and dips either to east or to west. This implies that strain energy was not solely released on a main fault only, but on secondary and adjacent fault segments as well. The reliable definition of their geometry forms the basis for the structural interpretation of the local fault network. The aftershock spatial distribution indicates three main clusters of the seismic activity, along with activation of smaller faults to an extent of more than 50 km. A northeasterly striking cluster is observed to the north of the main shock epicenter, with a remarkable aftershock density. The central cluster is less dense than the previous one with an epicentral alignment in full accordance with the strike provided by the main shock centroid moment tensor solution, and is considered as the main rupture with a length of 17 km. The third cluster, encompassing a large number of aftershocks, is located in the offshore area between Lefkada and Kefalonia Islands with a NE-SW epicentral alignment, alike the first cluster. The northeast-southwest striking secondary faults positioned obliquely and in continuation of the main fault segment, reveal that the KTFZ is being deformed in a complex tectonic setting. The presence of faults with this geometry implies strain partitioning and sheds light to new components necessary to be taken into account in the seismic hazard assessment. Stress transfer models of the M ≥6.0 main shocks were investigated and the calculated static stress changes may well explain their sequential occurrence. Static stress changes due to the 2015 coseismic slip were also calculated with the main objective of exploring the aftershock occurrence pattern and it was found as the driving mechanism that triggered the vast majority of the off-fault aftershocks.

The 2004 Sumatra Earthquake Mw 9.3: Seismological and Geophysical Investigations in the Andaman-Nicobar Islands

NASA Astrophysics Data System (ADS)

Mooney, W. D.; Kayal, J.

2007-05-01

The December 26, 2004 Sumatra-Andaman earthquake (MW 9.3) is the fourth largest event (M>9.0) in the world during the last 100 years. It occurred by thrust faulting on the interplate thrust zone of the subducting India plate and overriding Burma platelet. The main shock rupture, ~1300 km long and ~200 km wide, propagated from north of Sumatra to Andaman - Nicobar Islands; the slow rupture generated Tsunami which killed about 300,000 people. The epicenter of the earthquake is located at 3.90N and 94.260E with a focal depth at 28 km (USGS). This mega seismic event triggered giant tsunamis that devastated the coastal regions of Indonesia, Malaysia, Thailand, Sri Lanka, India, Maldives and even the east coast of Africa. The impact of the tsunami was quite severe in India, in the coasts of Andaman and Nicobar Islands. The Air-base in the Car- Nicobar island was totally devastated by the tsunami and killed about 200 people. Macroseismic survey was carried out by different teams of GSI in North Andaman, Middle Andaman, South Andaman, Havelock Hut Bay and also in the Nicobar Islands. A maximum intensity VIII was recorded in the Andaman Islands. The mega thrust event was followed by an intense aftershock activity spreading over an area extending between 30-140N along the Andaman - Nicobar - Sumatra Island arc region. The aftershocks are distributed northwards from the epicenter of the main shock suggesting a unilateral rupture propagation. The aftershock (M >4.5) area covers a length of about 1300 km and a width of about 200 km, in a 'banana' shape. The national network (IMD) recorded almost all aftershocks M >5.0; about 350 were recorded till 31.01.2005. The Geological Survey of India (GSI) deployed six temporary seismograph stations in the Andaman and Nicobar Islands and also in Havelok and Narkunda (volcanic) islands. About 20,000 aftershocks (M >3.0) were recorded until end of March, 2005. About 1000 aftershocks (M >3.0) located by the GSI network until January 31, 2005 are studied. The aftershocks are still continuing; frequency of occurrence is, however, reduced now. Fault plane solutions suggest predominant thrust faulting in the fore arc region, and normal/strike ship in the back arc region, consistent with the regional tectonics. Crustal deformation study was carried out by various organizations. Pre- and -post earthquake vectors clearly show that islands have moved 2 to 6 meters in horizontal position towards mainland, and also there is anti-clockwise rotation. The GPS stations move southwesterly, 2 to 3 m in the Andaman Islands and 5 to 6 m in the Nicobar islands. Tidal observations indicate that there is a rise in local mean sea level of an order of 1.05 m at the Port Blair observatory. This observation is conformable with the GPS/levelling measurements that show a subsidence of the observatory to an extent of 1.1 m. The uplift and subsidence are explained by the thrust faulting involving reverse slip; uplift at the up dip edge and subsidence at the down dip on the coseismic rupture.

Mapping b-value for 2009 Harrat Lunayyir earthquake swarm, western Saudi Arabia and Coulomb stress for its mainshock

NASA Astrophysics Data System (ADS)

Abdelfattah, Ali K.; Mogren, Saad; Mukhopadhyay, Manoj

2017-01-01

The Harrat Lunayyir (HL) earthquake swarm of 2009 originated in the HL volcanic field and attracted global attention mainly due to three factors: (i) its relatively short life span that ushered a large frequency of the swarm population (30,000 events in < 2 years), (ii) the swarm epicenter zone was contained within a small crustal volume under the HL and (iii) the migratory nature of the swarm following the tectonic trend of a normal fault zone beneath HL. The HL belongs to the Large Igneous Province of Saudi Arabia (LIP-SA) where it correlates to the Great Dikes locally. Our aim in this study is to describe the spatial distribution of the hypocenters, b-value character, and Coulomb stress failure (CSF) in an attempt to analyze the underlying geodynamic process that caused the swarm. We utilize the relocated hypocenters monitored by local networks to examine the b-value characteristics for the swarm. This is best represented in a cross section showing two domains of higher b-value anomalies: two patches occurring at shallow depth and at the deeper crust to the SE from the mainshock originated at the shallower depth northwestward. Consistently positive ΔCFF pattern with a large percentage of aftershocks imply how the mainshock rupture controlled the aftershocks activity. This implies that the failure along the NNW fault trend is due to the prevailing ambient stress field imparted to the swarm. We model this by CSF associated with the mainshock for three time dependent situations: (a) foreshock and aftershock epicenters, (b) foreshock hypocenters, and (c) aftershock hypocenters. In actuality, multiple factors might have controlled the aftershock activity as we speculate that positive Coulomb stress was associated in an area where the higher b-value prevails. The CSF produced by the mainshock illustrates how the stress dissipated along the NNW normal fault zone that interrupts the Great Dykes along the Red Sea coast. These results further suggest that the crustal heterogeneity under HL act as an asperity in the epicentral area, whose origin may relate to magma intrusion into upper crust. However, seismic survey is required for detailing this geologic inference.

Improving the resolution of the 2010 Haiti earthquake fault geometry using temporary seismometer deployments

NASA Astrophysics Data System (ADS)

Douilly, R.; Haase, J. S.; Ellsworth, W. L.; Bouin, M.; Calais, E.; Armbruster, J. G.; Mercier De Lepinay, B. F.; Deschamps, A.; Saint Louis, M.; Meremonte, M. E.; Hough, S. E.

2011-12-01

Haiti has several active faults that are capable of producing large earthquakes such as the 2010 Mw 7.0 Haiti earthquake. This earthquake was not unexpected, given geodetic measurements showing strain accumulation on the Enriquillo Plantain Garden Fault Zone, the major fault system in southern Haiti (Manaker et al. 2008). GPS and INSAR data (Calais et al., 2010) show, however, that this rupture occurred on the previously unmapped Léogâne fault, a 60° north dipping oblique blind thrust located immediately north of the Enriquillo Fault. Following the earthquake, several groups installed temporary seismic stations to record aftershocks. Natural Resources Canada installed three broadband seismic stations, Géoazur installed 21 ocean bottom seismometers, L'Institut de Physique du Globe de Paris installed 5 broadband seismometers, and the United States Geological Survey deployed 17 short period and strong motion seismometers in and around Port-au-Prince. We use data from this complete set of stations, along with data from permanent regional stations, to relocate all of the events from March 17 to June 24, to determine the regional one-dimensional crustal structure and determine focal mechanisms. The aftershock locations from the combined data set clearly delineate the Léogâne fault. The strike and dip closely agrees with that of the global centroid moment tensor solution, but appears to be more steeply dipping than the finite fault inversions. The aftershocks also delineate a flat structure on the west side of the rupture zone and may indicate triggered seismicity on the Trois Baies fault, although the depths of these events are not as well constrained. There is no clear evidence for aftershocks on the other rupture segments inferred in the Hayes et al. (2010) mainshock rupture model. There is a cluster of aftershocks in the hanging wall near the western patch of high slip identified by Calais et al. (2010) and Meng et al. (2011), or central patch in the Hayes et al. (2010) model. We use first-motion focal mechanism solutions to clarify the relationship of the fault geometry to the mechanisms of the larger events.

The 2015 April 25 Gorkha Earthquake and its Aftershocks: Implications for lateral heterogeneity on the Main Himalayan Thrust

NASA Astrophysics Data System (ADS)

Mitra, S.; Kumar, A.; Priestley, K. F.

2016-12-01

The 2015 Gorkha earthquake (Mw 7.8) occurred by thrust faulting on a ˜150 km long and ˜70 km wide, locked downdip segment of the Main Himalayan Thrust (MHT), causing the Himalaya to slip SSW over the Indian Plate, and was followed by major-to-moderate aftershocks. Back projection of teleseismic P-wave and inversion of teleseismic body waves provide constraints on the geometry and kinematics of the mainshock rupture and source mechanism of aftershocks. The mainshock initiated ˜80 km west of Katmandu, close to the locking line on the MHT and propagated eastwards, along ˜117° azimuth, for a duration of ˜70 s, in multi-stage rupture. The mainshock has been modeled using four sub-events, propagating from west-to-east. The first sub-event (0-20 s) ruptured at a velocity of ˜3.5 km/s on a ˜6° N dipping flat segment of the MHT with thrust motion. The second sub-event (20-35 s) ruptured a ˜18° W dipping lateral ramp on the MHT in oblique thrust motion. The rupture velocity dropped from 3.5 km/s to 2.5 km/s, as a result of updip propagation of the rupture. The third sub-event (35-50 s) ruptured a ˜7° N dipping, eastward flat segment of the MHT with thrust motion and resulted in the largest amplitude arrivals at teleseismic distances. The fourth sub-event (50-70 s) occurred by left-lateral strike-slip motion on a steeply dipping transverse fault, at high angle to the MHT and arrested the eastward propagation of the mainshock rupture. Eastward stress build-up following the mainshock resulted in the largest aftershock (Mw 7.3), which occurred on the MHT, immediately east of the mainshock rupture. Source mechanism of moderate aftershocks reveal stress adjustment at the edges of the mainshock fault, flexural faulting on top of the downgoing Indian Plate and extensional faulting in the hanging wall of the MHT.

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

NASA Astrophysics Data System (ADS)

Kumar, Ajay; Singh, Shashwat K.; Mitra, S.; Priestley, K. F.; Dayal, Shankar

2017-02-01

The 2015 Gorkha earthquake (Mw 7.8) occurred by thrust faulting on a ˜150 km long and ˜70 km wide, locked downdip segment of the Main Himalayan Thrust (MHT), causing the Himalaya to slip SSW over the Indian Plate, and was followed by major-to-moderate aftershocks. Back projection of teleseismic P-wave and inversion of teleseismic body waves provide constraints on the geometry and kinematics of the main-shock rupture and source mechanism of aftershocks. The main-shock initiated ˜80 km west of Katmandu, close to the locking line on the MHT and propagated eastwards along ˜117° azimuth for a duration of ˜70 s, with varying rupture velocity on a heterogeneous fault surface. The main-shock has been modelled using four subevents, propagating from west-to-east. The first subevent (0-20 s) ruptured at a velocity of ˜3.5 km s- 1 on a ˜6°N dipping flat segment of the MHT with thrust motion. The second subevent (20-35 s) ruptured a ˜18° W dipping lateral ramp on the MHT in oblique thrust motion. The rupture velocity dropped from 3.5 km s- 1 to 2.5 km s- 1, as a result of updip propagation of the rupture. The third subevent (35-50 s) ruptured a ˜7°N dipping, eastward flat segment of the MHT with thrust motion and resulted in the largest amplitude arrivals at teleseismic distances. The fourth subevent (50-70 s) occurred by left-lateral strike-slip motion on a steeply dipping transverse fault, at high angle to the MHT and arrested the eastward propagation of the main-shock rupture. Eastward stress build-up following the main-shock resulted in the largest aftershock (Mw 7.3), which occurred on the MHT, immediately east of the main-shock rupture. Source mechanisms of moderate aftershocks reveal stress adjustment at the edges of the main-shock fault, flexural faulting on top of the downgoing Indian Plate and extensional faulting in the hanging wall of the MHT.

Observing Triggered Earthquakes Across Iran with Calibrated Earthquake Locations

NASA Astrophysics Data System (ADS)

Karasozen, E.; Bergman, E.; Ghods, A.; Nissen, E.

2016-12-01

We investigate earthquake triggering phenomena in Iran by analyzing patterns of aftershock activity around mapped surface ruptures. Iran has an intense level of seismicity (> 40,000 events listed in the ISC Bulletin since 1960) due to it accommodating a significant portion of the continental collision between Arabia and Eurasia. There are nearly thirty mapped surface ruptures associated with earthquakes of M 6-7.5, mostly in eastern and northwestern Iran, offering a rich potential to study the kinematics of earthquake nucleation, rupture propagation, and subsequent triggering. However, catalog earthquake locations are subject to up to 50 km of location bias from the combination of unknown Earth structure and unbalanced station coverage, making it challenging to assess both the rupture directivity of larger events and the spatial patterns of their aftershocks. To overcome this limitation, we developed a new two-tiered multiple-event relocation approach to obtain hypocentral parameters that are minimally biased and have realistic uncertainties. In the first stage, locations of small clusters of well-recorded earthquakes at local spatial scales (100s of events across 100 km length scales) are calibrated either by using near-source arrival times or independent location constraints (e.g. local aftershock studies, InSAR solutions), using an implementation of the Hypocentroidal Decomposition relocation technique called MLOC. Epicentral uncertainties are typically less than 5 km. Then, these events are used as prior constraints in the code BayesLoc, a Bayesian relocation technique that can handle larger datasets, to yield region-wide calibrated hypocenters (1000s of events over 1000 km length scales). With locations and errors both calibrated, the pattern of aftershock activity can reveal the type of the earthquake triggering: dynamic stress changes promote an increase in the seismicity rate in the direction of unilateral propagation, whereas static stress changes should not be biased by rupture propagation direction. Here we present results from Ahar, Baladeh, Qom, Rigan, Silakhour and Zirkuh clusters, that include early-instrumental and modern mainshock-aftershock sequences. These will in turn provide a greatly improved basis for research into seismic hazards in this region.

Analysis of earthquake clustering and source spectra in the Salton Sea Geothermal Field

NASA Astrophysics Data System (ADS)

Cheng, Y.; Chen, X.

2015-12-01

The Salton Sea Geothermal field is located within the tectonic step-over between San Andreas Fault and Imperial Fault. Since the 1980s, geothermal energy exploration has resulted with step-like increase of microearthquake activities, which mirror the expansion of geothermal field. Distinguishing naturally occurred and induced seismicity, and their corresponding characteristics (e.g., energy release) is important for hazard assessment. Between 2008 and 2014, seismic data recorded by a local borehole array were provided public access from CalEnergy through SCEC data center; and the high quality local recording of over 7000 microearthquakes provides unique opportunity to sort out characteristics of induced versus natural activities. We obtain high-resolution earthquake location using improved S-wave picks, waveform cross-correlation and a new 3D velocity model. We then develop method to identify spatial-temporally isolated earthquake clusters. These clusters are classified into aftershock-type, swarm-type, and mixed-type (aftershock-like, with low skew, low magnitude and shorter duration), based on the relative timing of largest earthquakes and moment-release. The mixed-type clusters are mostly located at 3 - 4 km depth near injection well; while aftershock-type clusters and swarm-type clusters also occur further from injection well. By counting number of aftershocks within 1day following mainshock in each cluster, we find that the mixed-type clusters have much higher aftershock productivity compared with other types and historic M4 earthquakes. We analyze detailed spatial variation of 'b-value'. We find that the mixed-type clusters are mostly located within high b-value patches, while large (M>3) earthquakes and other types of clusters are located within low b-value patches. We are currently processing P and S-wave spectra to analyze the spatial-temporal correlation of earthquake stress parameter and seismicity characteristics. Preliminary results suggest that the mixed-type clusters and high b-value patches are spatially correlated with low stress drop earthquakes, indicating high-productivity microearthquakes within low differential stress region, potentially due to deeper injection activities.

(Multi)fractality of Earthquakes by use of Wavelet Analysis

NASA Astrophysics Data System (ADS)

Enescu, B.; Ito, K.; Struzik, Z. R.

2002-12-01

The fractal character of earthquakes' occurrence, in time, space or energy, has by now been established beyond doubt and is in agreement with modern models of seismicity. Moreover, the cascade-like generation process of earthquakes -with one "main" shock followed by many aftershocks, having their own aftershocks- may well be described through multifractal analysis, well suited for dealing with such multiplicative processes. The (multi)fractal character of seismicity has been analysed so far by using traditional techniques, like the box-counting and correlation function algorithms. This work introduces a new approach for characterising the multifractal patterns of seismicity. The use of wavelet analysis, in particular of the wavelet transform modulus maxima, to multifractal analysis was pioneered by Arneodo et al. (1991, 1995) and applied successfully in diverse fields, such as the study of turbulence, the DNA sequences or the heart rate dynamics. The wavelets act like a microscope, revealing details about the analysed data at different times and scales. We introduce and perform such an analysis on the occurrence time of earthquakes and show its advantages. In particular, we analyse shallow seismicity, characterised by a high aftershock "productivity", as well as intermediate and deep seismic activity, known for its scarcity of aftershocks. We examine as well declustered (aftershocks removed) versions of seismic catalogues. Our preliminary results show some degree of multifractality for the undeclustered, shallow seismicity. On the other hand, at large scales, we detect a monofractal scaling behaviour, clearly put in evidence for the declustered, shallow seismic activity. Moreover, some of the declustered sequences show a long-range dependent (LRD) behaviour, characterised by a Hurst exponent, H > 0.5, in contrast with the memory-less, Poissonian model. We demonstrate that the LRD is a genuine characteristic and is not an effect of the time series probability distribution function. One of the most attractive features of wavelet analysis is its ability to determine a local Hurst exponent. We show that this feature together with the possibility of extending the analysis to spatial patterns may constitute a valuable approach to search for anomalous (precursory?) patterns of seismic activity.

The 2015 Gorkha (Nepal) earthquake sequence: I. Source modeling and deterministic 3D ground shaking

NASA Astrophysics Data System (ADS)

Wei, Shengji; Chen, Meng; Wang, Xin; Graves, Robert; Lindsey, Eric; Wang, Teng; Karakaş, Çağıl; Helmberger, Don

2018-01-01

To better quantify the relatively long period (< 0.3 Hz) shaking experienced during the 2015 Gorkha (Nepal) earthquake sequence, we study the finite rupture processes and the associated 3D ground motion of the Mw7.8 mainshock and the Mw7.2 aftershock. The 3D synthetics are then used in the broadband ground shaking in Kathmandu with a hybrid approach, summarized in a companion paper (Chen and Wei, 2017, submitted together). We determined the coseismic rupture process of the mainshock by joint inversion of InSAR/SAR, GPS (static and high-rate), strong motion and teleseismic waveforms. Our inversion for the mainshock indicates unilateral rupture towards the ESE, with an average rupture speed of 3.0 km/s and a total duration of 60 s. Additionally, we find that the beginning part of the rupture (5-18 s) has about 40% longer rise time than the rest of the rupture, as well as slower rupture velocity. Our model shows two strong asperities occurring 24 s and 36 s after the origin and located 30 km to the northwest and northeast of the Kathmandu valley, respectively. In contrast, the Mw7.2 aftershock is more compact both in time and space, as revealed by joint inversion of teleseismic body waves and InSAR data. The different rupture features between the mainshock and the aftershock could be related to difference in fault zone structure. The mainshock and aftershock ground motions in the Kathmandu valley, recorded by both strong motion and high-rate GPS stations, exhibited strong amplification around 0.2 Hz. A simplified 3D basin model, calibrated by an Mw5.2 aftershock, can match the observed waveforms reasonably well at 0.3 Hz and lower frequency. The 3D simulations indicate that the basin structure trapped the wavefield and produced an extensive ground vibration. Our study suggests that the combination of rupture characteristics and propagational complexity are required to understand the ground shaking produced by hazardous earthquakes such as the Gorkha event.

Low-angle normal faulting in the Basin and Range-Colorado Plateau transition zone during the January 3, 2011 Circleville, UT earthquake sequence

NASA Astrophysics Data System (ADS)

Gammans, Christine Naomi Louise

On January 3, 2011, an Mw 4.5 earthquake occurred in the Tushar Mountains near Circleville, Utah (38.248°N, -112.329°W, 7.75 km depth, and origin time of 12:06:36.58). The Tushar Mountains are located in the transition zone between the stable Colorado Plateau (CP) to the east and the deforming Basin and Range (BR) province to the west. In this area, seismicity associated with the Intermountain Seismic Belt is relatively common. The University of Utah Seismograph Stations (UUSS) detected and located 97 aftershocks in the 33 weeks following the mainshock. On January 6, UUSS installed a portable station in the source region. Using three aftershocks recorded by the portable station as master events, including the largest (Mw 3.8), we relocated the mainshock/aftershock sequence. These refined locations were used as initial locations for the HypoDD method of Waldhauser and Ellsworth [2001] to produce a second, improved set of relocations. In addition to P- and S-arrival time picks, we used the lag-times from waveform cross-correlations as input to HypoDD. We analyzed the fault geometry apparent in the final locations by comparing them to known moment-tensor focal planes and by applying principal component analysis to measure the degree of planarity and orientation of the sequence as a whole. Additionally, using cross-correlation analysis, we identified aftershocks best suited for an empirical Green's function analysis of the mainshock and a strike-slip aftershock that occurred on January 6. From the events chosen by cross-correlation, we were able to obtain source-time functions that were used to obtain fault dimensions, stress drops, and evidence for or against directivity. Lastly, we determined focal mechanisms for ten of the events using first-motion methods. The results of the combined analyses indicate that the mainshock occurred on a low-angle normal fault and that the entire sequence occurred on at least two different fault planes.

Source and Aftershock Analysis of a Large Deep Earthquake in the Tonga Flat Slab

NASA Astrophysics Data System (ADS)

Cai, C.; Wiens, D. A.; Warren, L. M.

2013-12-01

The 9 November 2009 (Mw 7.3) deep focus earthquake (depth = 591 km) occurred in the Tonga flat slab region, which is characterized by limited seismicity but has been imaged as a flat slab in tomographic imaging studies. In addition, this earthquake occurred immediately beneath the largest of the Fiji Islands and was well recorded by a temporary array of 16 broadband seismographs installed in Fiji and Tonga, providing an excellent opportunity to study the source mechanism of a deep earthquake in a partially aseismic flat slab region. We determine the positions of main shock hypocenter, its aftershocks and moment release subevents relative to the background seismicity using a hypocentroidal decomposition relative relocation method. We also investigate the rupture directivity by measuring the variation of rupture durations at different azimuth [e.g., Warren and Silver, 2006]. Arrival times picked from the local seismic stations together with teleseismic arrival times from the International Seismological Centre (ISC) are used for the relocation. Teleseismic waveforms are used for directivity study. Preliminary results show this entire region is relatively aseismic, with diffuse background seismicity distributed between 550-670 km. The main shock happened in a previously aseismic region, with only 1 small earthquake within 50 km during 1980-2012. 11 aftershocks large enough for good locations all occurred within the first 24 hours following the earthquake. The aftershock zone extends about 80 km from NW to SE, covering a much larger area than the mainshock rupture. The aftershock distribution does not correspond to the main shock fault plane, unlike the 1994 March 9 (Mw 7.6) Fiji-Tonga earthquake in the steeply dipping, highly seismic part of the Tonga slab. Mainshock subevent locations suggest a sub-horizontal SE-NW rupture direction. However, the directivity study shows a complicated rupture process which could not be solved with simple rupture assumption. We will present the result of this example earthquake and some other deep earthquakes at the fall meeting. Warren, L. M., and P. G. Silver (2006), Measurement of differential rupture durations as constraints on the source finiteness of deep earthquakes, J. Geophys. Res., 111, B06304, doi:10.1029/2005JB004001.

Compound earthquakes on a bimaterial interface and implications for rupture mechanics

NASA Astrophysics Data System (ADS)

Wang, E.; Rubin, A. M.

2012-12-01

Earthquake ruptures on the San Andreas are affected by the material contrast across the fault. Previous observations of microearthquakes in the northern creeping section have found strong signals of asymmetry in both rupture directivity (preferential propagation to the SE; Wang & Rubin, 2011), and aftershock asymmetry (many more to the NW, on timescales from 10 to 1000 s; Rubin & Gillard, 2000; Rubin, 2002). To understand the aftershock asymmetry, Rubin & Ampuero [2007] simulated slip-weakening ruptures on a bimaterial interface and observed differences in the timescales for the two edges to experience their peak stress after being slowed by barriers. While the barrier on the "positive" side (SE in the case of the San Andreas fault) reached the peak stress almost instantaneously, the "negative" side (NW) has to wait for the arrival of the P-wave stopping phase from the opposite end. For a 100 m main event the delay between the two potential ruptures is 20 ms, which may be long enough for the secondary rupture to be observed as a distinct subevent. A second possible source of subevent asymmetry is that when slowed by barriers, a significant tensile stress pulse propagates in the positive but not the negative direction. The events in the region where previous asymmetries were found have high spatial density and similar focal mechanisms, which makes it possible to use an Empirical Green's Function's method to do a comprehensive search. About 200 candidate compound events were identified in an 8300-event catalog from 1984 to 2009. Most delays between the two subevents cluster around the shear-wave transit time over the subevent separation, although with considerable scatter. For subevents separated by 0.7 to 2 estimated mainshock radii (the same spatial separation found by Rubin [2002] to maximize the longer-term aftershock asymmetry), about twice as many second events occurred on the SE (positive) side of the first, and the origin time delays also tend to be shorter for SE aftershocks. One interpretation is that these subevents represent the events "missing" from the aftershock population because they became part of the mainshock.

Automatic analysis of the 2015 Gorkha earthquake aftershock sequence.

NASA Astrophysics Data System (ADS)

Baillard, C.; Lyon-Caen, H.; Bollinger, L.; Rietbrock, A.; Letort, J.; Adhikari, L. B.

2016-12-01

The Mw 7.8 Gorkha earthquake, that partially ruptured the Main Himalayan Thrust North of Kathmandu on the 25th April 2015, was the largest and most catastrophic earthquake striking Nepal since the great M8.4 1934 earthquake. This mainshock was followed by multiple aftershocks, among them, two notable events that occurred on the 12th May with magnitudes of 7.3 Mw and 6.3 Mw. Due to these recent events it became essential for the authorities and for the scientific community to better evaluate the seismic risk in the region through a detailed analysis of the earthquake catalog, amongst others, the spatio-temporal distribution of the Gorkha aftershock sequence. Here we complement this first study by doing a microseismic study using seismic data coming from the eastern part of the Nepalese Seismological Center network associated to one broadband station in Everest. Our primary goal is to deliver an accurate catalog of the aftershock sequence. Due to the exceptional number of events detected we performed an automatic picking/locating procedure which can be splitted in 4 steps: 1) Coarse picking of the onsets using a classical STA/LTA picker, 2) phase association of picked onsets to detect and declare seismic events, 3) Kurtosis pick refinement around theoretical arrival times to increase picking and location accuracy and, 4) local magnitude calculation based amplitude of waveforms. This procedure is time efficient ( 1 sec/event), reduces considerably the location uncertainties ( 2 to 5 km errors) and increases the number of events detected compared to manual processing. Indeed, the automatic detection rate is 10 times higher than the manual detection rate. By comparing to the USGS catalog we were able to give a new attenuation law to compute local magnitudes in the region. A detailed analysis of the seismicity shows a clear migration toward the east of the region and a sudden decrease of seismicity 100 km east of Kathmandu which may reveal the presence of a tectonic feature acting as a seismic barrier. Comparison of the aftershock distribution with respect to the coseismic slip distribution will be discussed.d.

Seismicity associated with the Sumatra-Andaman Islands earthquake of 26 December 2004

USGS Publications Warehouse

Dewey, J.W.; Choy, G.; Presgrave, B.; Sipkin, S.; Tarr, A.C.; Benz, H.; Earle, P.; Wald, D.

2007-01-01

The U.S. Geological Survey/National Earthquake Information Center (USGS/ NEIC) had computed origins for 5000 earthquakes in the Sumatra-Andaman Islands region in the first 36 weeks after the Sumatra-Andaman Islands mainshock of 26 December 2004. The cataloging of earthquakes of mb (USGS) 5.1 and larger is essentially complete for the time period except for the first half-day following the 26 December mainshock, a period of about two hours following the Nias earthquake of 28 March 2005, and occasionally during the Andaman Sea swarm of 26-30 January 2005. Moderate and larger (mb ???5.5) aftershocks are absent from most of the deep interplate thrust faults of the segments of the Sumatra-Andaman Islands subduction zone on which the 26 December mainshock occurred, which probably reflects nearly complete release of elastic strain on the seismogenic interplate-thrust during the mainshock. An exceptional thrust-fault source offshore of Banda Aceh may represent a segment of the interplate thrust that was bypassed during the mainshock. The 26 December mainshock triggered a high level of aftershock activity near the axis of the Sunda trench and the leading edge of the overthrust Burma plate. Much near-trench activity is intraplate activity within the subducting plate, but some shallow-focus, near-trench, reverse-fault earthquakes may represent an unusual seismogenic release of interplate compressional stress near the tip of the overriding plate. The interplate-thrust Nias earthquake of 28 March 2005, in contrast to the 26 December aftershock sequence, was followed by many interplate-thrust aftershocks along the length of its inferred rupture zone.

Estimating seismic site response in Christchurch City (New Zealand) from dense low-cost aftershock arrays

USGS Publications Warehouse

Kaiser, Anna E.; Benites, Rafael A.; Chung, Angela I.; Haines, A. John; Cochran, Elizabeth S.; Fry, Bill

2011-01-01

The Mw 7.1 September 2010 Darfield earthquake, New Zealand, produced widespread damage and liquefaction ~40 km from the epicentre in Christchurch city. It was followed by the even more destructive Mw 6.2 February 2011 Christchurch aftershock directly beneath the city’s southern suburbs. Seismic data recorded during the two large events suggest that site effects contributed to the variations in ground motion observed throughout Christchurch city. We use densely-spaced aftershock recordings of the Darfield earthquake to investigate variations in local seismic site response within the Christchurch urban area. Following the Darfield main shock we deployed a temporary array of ~180 low-cost 14-bit MEMS accelerometers linked to the global Quake-Catcher Network (QCN). These instruments provided dense station coverage (spacing ~2 km) to complement existing New Zealand national network strong motion stations (GeoNet) within Christchurch city. Well-constrained standard spectral ratios were derived for GeoNet stations using a reference station on Miocene basalt rock in the south of the city. For noisier QCN stations, the method was adapted to find a maximum likelihood estimate of spectral ratio amplitude taking into account the variance of noise at the respective stations. Spectral ratios for QCN stations are similar to nearby GeoNet stations when the maximum likelihood method is used. Our study suggests dense low-cost accelerometer aftershock arrays can provide useful information on local-scale ground motion properties for use in microzonation. Preliminary results indicate higher amplifications north of the city centre and strong high-frequency amplification in the small, shallower basin of Heathcote Valley.

On near-source earthquake triggering

USGS Publications Warehouse

Parsons, T.; Velasco, A.A.

2009-01-01

When one earthquake triggers others nearby, what connects them? Two processes are observed: static stress change from fault offset and dynamic stress changes from passing seismic waves. In the near-source region (r ??? 50 km for M ??? 5 sources) both processes may be operating, and since both mechanisms are expected to raise earthquake rates, it is difficult to isolate them. We thus compare explosions with earthquakes because only earthquakes cause significant static stress changes. We find that large explosions at the Nevada Test Site do not trigger earthquakes at rates comparable to similar magnitude earthquakes. Surface waves are associated with regional and long-range dynamic triggering, but we note that surface waves with low enough frequency to penetrate to depths where most aftershocks of the 1992 M = 5.7 Little Skull Mountain main shock occurred (???12 km) would not have developed significant amplitude within a 50-km radius. We therefore focus on the best candidate phases to cause local dynamic triggering, direct waves that pass through observed near-source aftershock clusters. We examine these phases, which arrived at the nearest (200-270 km) broadband station before the surface wave train and could thus be isolated for study. Direct comparison of spectral amplitudes of presurface wave arrivals shows that M ??? 5 explosions and earthquakes deliver the same peak dynamic stresses into the near-source crust. We conclude that a static stress change model can readily explain observed aftershock patterns, whereas it is difficult to attribute near-source triggering to a dynamic process because of the dearth of aftershocks near large explosions.

A scale-invariant cellular-automata model for distributed seismicity

NASA Technical Reports Server (NTRS)

Barriere, Benoit; Turcotte, Donald L.

1991-01-01

In the standard cellular-automata model for a fault an element of stress is randomly added to a grid of boxes until a box has four elements, these are then redistributed to the adjacent boxes on the grid. The redistribution can result in one or more of these boxes having four or more elements in which case further redistributions are required. On the average added elements are lost from the edges of the grid. The model is modified so that the boxes have a scale-invariant distribution of sizes. The objective is to model a scale-invariant distribution of fault sizes. When a redistribution from a box occurs it is equivalent to a characteristic earthquake on the fault. A redistribution from a small box (a foreshock) can trigger an instability in a large box (the main shock). A redistribution from a large box always triggers many instabilities in the smaller boxes (aftershocks). The frequency-size statistics for both main shocks and aftershocks satisfy the Gutenberg-Richter relation with b = 0.835 for main shocks and b = 0.635 for aftershocks. Model foreshocks occur 28 percent of the time.

Seismotectonics of Marasesti region (Eastern Romania) revealed by earthquake relocations and moment tensor determinations

NASA Astrophysics Data System (ADS)

Borleanu, Felix; Rogozea, Maria; Placinta, Anca; Popa, Mihaela; Radulian, Mircea

2017-04-01

A large seismic sequence occurred between 22 November 2014 and 31 January 2015 in the Foredeep area of the South-Eastern Carpathians at a distance of about 10 km north-east relative to Marasesti city. The sequence was located in the lower crust, close to 40 km depth. Although the moment magnitude of the largest event was 5.4 according to Romplus (Romanian earthquakes catalog) the largest aftershock did not exceed 4.0 (Mw) and most of the aftershocks were weak (magnitude below 3). From a total of 230 well-located events, we relocated 178 using more than 17000 P and S differential travel times. The results show a NW-SE alignment consistent with the focal mechanism solution computed through the broadband seismic waveforms inversion. An important aspect of this sequence is the distribution in time of the seismic events, which reveals an aftershocks migration with an average velocity of about 3 km/day. This seismicity behavior might be due to the presence of the fluids. We interpret all these features in terms of the seismotectonics of the region.

Influence of fault geometry and tectonic driving stress orientation on the mechanics of multifault earthquakes

NASA Astrophysics Data System (ADS)

Madden, E. H.; Maerten, F.; Pollard, D. D.

2012-12-01

The M 7.3 28 June 1992 Landers, California earthquake was a well-documented event that highlighted the complex relationship between the earthquake and the multiple faults on which it occurred. Not only was fault slip data mapped in the field in detail, due to good exposure in the arid conditions of the Mojave Desert, but also it was one of the first earthquakes for which the surface displacement field was captured by satellite technology. In addition, precise aftershock relocations and fault plane solutions provide information about stress and fault behavior at depth. Study of fault interactions leading to the linkage of five right-lateral, strike-slip faults at Landers is aided by this abundance of available surface and subsurface data. While mapped near-field surface data often are restricted to the realm of the geologist, and subsurface data, such as aftershocks, often are restricted to the realm of the geophysicist, we find that integrating these data in mechanical forward models provides good constraint on the three-dimensional structures of the faults involved. Mechanical models also reveal that fault geometry and the orientation of the tectonic driving stress greatly influence whether or not slip is promoted across the extensional step between two of the faults along the southern-central rupture and elucidate the role of a crossing fault located within the step. Unfortunately, the orientation of the principal stresses are not well constrained near Landers or in many regions around the world. Previous determinations of the tectonic driving stress at Landers range from 7 degrees to 45 degrees, measured clockwise from North. We introduce a new stress inversion method that honors mechanical relationships among the remote stress state that is being inverted for, mainshock fault slip, the resulting total stress field following fault slip, and aftershocks. Use of the principal of superposition in this new algorithm obviates the need for the prohibitive computation times associated with running successive forward models. We apply the inverse method using aftershock, Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data associated with the Landers earthquake and address how fault geometry and aftershock size, timing, and focal mechanism quality influence inversion results. The advantages of this new method are that: (1) coseismic displacement data can be used, (2) the underlying model is better constrained to find a solution in the parameter space in the presence of fault slip perturbations, (3) absolute magnitudes can be recovered when using data with magnitude information such as GPS, InSAR and stress tensors inferred from aftershocks with known magnitudes. In addition, while one can choose to invert for an Andersonian fault regime, the method is not restricted to that particular case with one vertical principal stress.

The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

NASA Astrophysics Data System (ADS)

Uchida, Naoki; Kirby, Stephen H.; Umino, Norihito; Hino, Ryota; Kazakami, Tomoe

2016-09-01

The aftershock distribution of the 1933 Sanriku-oki outer trench earthquake is estimated by using modern relocation methods and a newly developed velocity structure to examine the spatial extent of the source-fault and the possibility of a triggered interplate seismicity. In this study, we first examined the regional data quality of the 1933 earthquake based on smoked-paper records and then relocated the earthquakes by using the 3-D velocity structure and double-difference method. The improvements of hypocentre locations using these methods were confirmed by the examination of recent earthquakes that are accurately located based on ocean bottom seismometer data. The results show that the 1933 aftershocks occurred under both the outer- and inner-trench-slope regions. In the outer-trench-slope region, aftershocks are distributed in a ˜280-km-long area and their depths are shallower than 50 km. Although we could not constrain the fault geometry from the hypocentre distribution, the depth distribution suggests the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks under the inner trench slope was also confirmed by an investigation of waveform frequency difference between outer and inner trench earthquakes as recorded at Mizusawa. The earthquakes under the inner trench slope were shallow (depth ≦30 km) and the waveforms show a low-frequency character similar to the waveforms of recent, precisely located earthquakes in the same area. They are also located where recent activity of interplate thrust earthquakes is high. These suggest that the 1933 outer-trench-slope main shock triggered interplate earthquakes, which is an unusual case in the order of occurrence in contrast with the more common pairing of a large initial interplate shock with subsequent outer-slope earthquakes. The off-trench earthquakes are distributed about 80 km width in the trench perpendicular direction. This wide width cannot be explained from a single high-angle fault confined at a shallow depth (depth ≦50 km). The upward motion of the 1933 tsunami waveform records observed at Sanriku coast also cannot be explained from a single high-angle west-dipping normal fault. If we consider additional fault, involvement of high-angle, east-dipping normal faults can better explain the tsunami first motion and triggering of the aftershock in a wide area under the outer trench slope. Therefore multiple off-trench normal faults may have activated during the 1933 earthquake. We also relocated recent (2001-2012) seismicity by the same method. The results show that the present seismicity in the outer-trench-slope region can be divided into several groups along the trench. Comparison of the 1933 rupture dimensions based on our aftershock relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited to the region where fault scarps are largely trench parallel and cross cut the seafloor spreading fabric. These findings imply that bending geometry and structural segmentation of the incoming plate largely controls the spatial extent of the 1933 seismogenic faulting. In this shallow rupture model for this largest outer trench earthquake, triggered seismicity in the forearc and structural control of faulting represent an important deformation styles for off-trench and shallow megathrust zones.

On comprehensive recovery of an aftershock sequence with cross correlation

NASA Astrophysics Data System (ADS)

Kitov, I.; Bobrov, D.; Coyne, J.; Turyomurugyendo, G.

2012-04-01

We have introduced cross correlation between seismic waveforms as a technique for signal detection and automatic event building at the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization. The intuition behind signal detection is simple - small and mid-sized seismic events close in space should produce similar signals at the same seismic stations. Equivalently, these signals have to be characterized by a high cross correlation coefficient. For array stations with many individual sensors distributed over a large area, signals from events at distances beyond, say, 50 km, are subject to destructive interference when cross correlated due to changing time delays between various channels. Thus, any cross correlation coefficient above some predefined threshold can be considered as a signature of a valid signal. With a dense grid of master events (spacing between adjacent masters between 20 km and 50 km corresponds to the statistically estimated correlation distance) with high quality (signal-to-noise ratio above 10) template waveforms at primary array stations of the International Monitoring System one can detect signals from and then build natural and manmade seismic events close to the master ones. The use of cross correlation allows detecting smaller signals (sometimes below noise level) than provided by the current IDC detecting techniques. As a result it is possible to automatically build from 50% to 100% more valid seismic events than included in the Reviewed Event Bulletin (REB). We have developed a tentative pipeline for automatic processing at the IDC. It includes three major stages. Firstly, we calculate cross correlation coefficient for a given master and continuous waveforms at the same stations and carry out signal detection as based on the statistical behavior of signal-to-noise ratio of the cross correlation coefficient. Secondly, a thorough screening is performed for all obtained signals using f-k analysis and F-statistics as applied to the cross-correlation traces at individual channels of all included array stations. Thirdly, local (i.e. confined to the correlation distance around the master event) association of origin times of all qualified signals is fulfilled. These origin times are calculated from the arrival times of these signals, which are reduced to the origin times by the travel times from the master event. An aftershock sequence of a mid-size earthquake is an ideal case to test cross correlation techniques for autiomatic event building. All events should be close to the mainshock and occur within several days. Here we analyse the aftershock sequence of an earthquake in the North Atlantic Ocean with mb(IDC)=4.79. The REB includes 38 events at distances less than 150 km from the mainshock. Our ultimate goal is to excersice the complete iterative procedure to find all possible aftershocks. We start with the mainshock and recover ten aftershocks with the largest number of stations to produce an initial set of master events with the highest quality templates. Then we find all aftershocks in the REB and many additional events, which were not originally found by the IDC. Using all events found after the first iteration as master events we find new events, which are also used in the next iteration. The iterative process stops when no new events can be found. In that sense the final set of aftershocks obtained with cross correlation is a comprehensive one.

Systematic Study of Foreshocks and Triggered Earthquakes During the 2010 Mw7.2 El Mayor-Cucapah Earthquake Sequence

NASA Astrophysics Data System (ADS)

Meng, X.; Peng, Z.; Deng, S.; Castro, R. R.

2015-12-01

The 2010 Mw7.2 El Mayor-Cucapah earthquake occurred southwest of the Pacific-North America plate boundary in north Baja California. It was preceded by an intensive foreshock sequence, and was followed by numerous aftershocks both on and off the mainshock rupture zone, hence providing us a great opportunity to study the physical mechanisms of foreshock and aftershock triggering. In our previously published work (Meng and Peng, GJI, 2014), we focused on the seismicity rate changes around the Salton Sea Geothermal Field (SSGF) and along the San Jacinto Fault (SJF) following the mainshock. Based on a recently developed matched filter technique, we were able to detect up to 20 times more events than listed in the SCSN catalog. We found that the seismicity rate near SSGF and SJF both experienced significant increase immediately following the mainshock. However, the seismicity rate near SSGF, where static Coulomb stress decreased, dropped below the pre-mainshock level after ~50 days. On the other hand, the seismicity rate near SJF, where static Coulomb stress increased, remained high till the end of our detecting time window. Such pattern indicates that both static and dynamic triggering may coexist, but dominate in different time scales. Motivated by this success, we shift our focus to the foreshock and aftershock sequence of the El Mayor-Cucapah event. We utilize available seismic stations immediately north to US-Mexico boarder and a few stations within Mexico to conduct a similar detection ~40 days before to 40 days after the mainshock. We aim to obtain a complete foreshock sequence and investigate its spatio-temporal evolutions before the mainshock. Moreover, we plan to study similar patterns for aftershocks and the corresponding triggering mechanisms. Updated results will be presented at the meeting.

The May 29 2008 earthquake aftershock sequence within the South Iceland Seismic Zone: Fault locations and source parameters of aftershocks

NASA Astrophysics Data System (ADS)

Brandsdottir, B.; Parsons, M.; White, R. S.; Gudmundsson, O.; Drew, J.

2010-12-01

The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. The South Iceland Seismic Zone (SISZ) is a complex transform zone where left-lateral E-W shear between the Reykjanes Peninsula Rift Zone and the Eastern Volcanic Zone is accommodated by bookshelf faulting along N-S lateral strike-slip faults. The SISZ is also a transient feature, migrating sideways in response to the southward propagation of the Eastern Volcanic Zone. Sequences of large earthquakes (M > 6) lasting from days to years and affecting most of the seismic zone have occurred repeatedly in historical time (last 1100 years), separated by intervals of relative quiescence lasting decades to more than a century. On May 29 2008, a Mw 6.1 earthquake struck the western part of the South Iceland Seismic Zone, followed within seconds by a slightly smaller event on a second fault ~5 km further west. Aftershocks, detected by a temporal array of 11 seismometers and three permanent Icelandic Meteorological Office stations were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two major and several smaller N-S faults as well as an E-W zone of activity stretching further west into the Reykjanes Peninsula Rift Zone. Fault plane solutions show both right lateral and oblique strike slip mechanisms along the two major N-S faults. The aftershocks deepen from 3-5 km in the north to 8-9 km in the south, suggesting that the main faults dip southwards. The faulting is interpreted to be driven by the local stress due to transform motion between two parallel segments of the divergent plate boundary crossing Iceland.

Detailed Spatio-temporal Distribution of the 16 April 2016 M7.8 Pedernales Ecuador Aftershock Sequence

NASA Astrophysics Data System (ADS)

Hernandez, S.; Font, Y.; Rolandone, F.; Nocquet, J. M.; Beck, S. L.; Meltzer, A.; Gabriela, P.; Plain, M.; Ruiz, M. C.; Alvarado, A. P.

2016-12-01

On 16 April 2016, a magnitude 7.8 earthquake occurred along the Nazca-South America plate interface near Pedernales, Ecuador. In this study, we present a detailed characterization of the spatial and temporal distribution of aftershocks and how they relate to the inter-, co-, and post-seismic deformation patterns. Aside from a magnitude 5.1 earthquake 10 minutes prior to the main event, no extended foreshock sequence was observed in the vicinity of the mainshock hypocenter. However, an analysis of the frequency-magnitude distributions of the seismicity near Pedernales in the years preceding the mainshock indicates a significant decrease in the b-value leading up to the mainshock origin time. This same area is also the location of elevated (>0.7) interseismic coupling as gleaned from GPS. Various coseismic slip models also seem to correlate quite well with plate interface coupling. Temporally, the aftershocks nearest to the areas of peak coseismic displacement show a power law decay with a p-value of 0.5. Within days of the mainshock, a southwesterly migration of aftershocks became apparent and likely represents a seismic manifestation of a triggered slow slip event. To date, at least 10 events of magnitude greater than M6 have been triggered. Curiously, 8 of these 10 events have occurred in the form of doublets, with the largest pair (M6.7,M6.9) occurring one month after the mainshock and separated by 9 hours. After several months of quiescence, seismicity north of the Atacames Promontory has begun to show elevated rate increases, especially near the city of Esmeraldas. This seismicity is likely related to stress changes imparted by the mainshock, but their delayed nature and likely location in the overriding plate requires further analysis to establish a definitive link.

A Report Of The December 6, 2016 Mw 6.5 Pidie Jaya, Aceh Earthquake

NASA Astrophysics Data System (ADS)

Muzli, M.; Daniarsyad, G.; Nugraha, A. D.; Muksin, U.; Widiyantoro, S.; Bradley, K.; Wang, T.; Jousset, P. G.; Erbas, K.; Nurdin, I.; Wei, S.

2017-12-01

The December 6, 2016 Mw 6.5 earthquake in Pidie Jaya, Aceh was one of the devastating inland earthquakes in Sumatra that took away more than 100 people's life. Here we present our seismological analysis of the earthquake sequence. The earthquake focal mechanism inversions using regional BMKG broadband data and teleseismic waveform data all indicate a strike-slip focal mechanism with a centroid depth of 15 km. Preliminary finite fault inversion using teleseismic body waves prefers the fault plane with strike of 45 degree and dip of 50 degree, in agreement with the surface geology and USGS aftershock distributions. Nine broadband seismic stations were installed in the source region along the coast one week after the earthquake and have collected the data for one month. The data have been used to locate aftershocks with grid search and double-difference algorithm, which results in the lineup of the seismicity in NE-SW direction, in agreement with the fault inversion and geology results. Using the M4.0 calibration earthquake that was recorded by the temporally network, we relocated the mainshock epicenter, which is also consistent with fault geometry defined by the well located aftershocks. In addition, a portion of the seismicity shows a lineation in E-W direction, indicating a secondary fault that has not been identified before. Aftershock focal mechanisms determined by the first motion reveal similar solutions as the mainshock. The observed macro intensity data shows most of the damaged buildings are distributed along the coast, approximately perpendicular to the preferred fault strike instead of parallel with it. It appears that the distribution of damage is strongly related to the site conditions, since these strong shaking/damage regions are mainly located on the costal sedimentary soils.

Static stress drop of the largest recorded M 4.6 hydraulic fracturing induced earthquake and its aftershock pattern in the northern Montney Play, British Columbia, Canada

NASA Astrophysics Data System (ADS)

Wang, B.; Harrington, R. M.; Liu, Y.; Kao, H.

2016-12-01

The largest suspected fracking-induced earthquake to date occurred near Fort St. John, British Columbia on August 17, 2015, with a reported magnitude of Mw 4.6. Here we estimate the static stress released by the mainshock and the five cataloged aftershocks using new data from eight broadband seismometers installed approximately 50km from the hypocenter of the mainshock, at distances much closer than the Natural Resources Canada regional seismic stations. The estimated cross-correlation coefficient among the 5 cataloged earthquakes is 0.35 or greater. We will present seismic moment (M0) and spectral corner frequency (fc) values estimated using both individual earthquake spectra and spectral ratios to correct for travel-path attenuation and site effects. Static stress drop and scaled energy value calculations based on the estimated moment and corner frequency values will be presented, as well as focal mechanisms for the largest events with adequate station coverage. We will also use a multi-station matched-filter approach to detect additional uncataloged earthquakes on continuous waveforms for a period of two months after the mainshock. Using the results of the matched-filter approach, we will present the aftershock magnitude distribution and locations. The results of our detection and location calculations will be compared to reported fracking parameters, such as fluid injection pressure and duration, to determine their correlation with the spatial and temporal distribution of aftershocks. The objective of this study is to relate operational parameters to earthquake occurrence in order to help to develop procedures to understand the mechanisms responsible for fracking induced earthquakes, their relation to the maximum induced magnitude, and to reduce potential hazards of anthropogenically induced seismic activity.

Nanoseismicity and picoseismicity rate changes from static stress triggering caused by a Mw 2.2 earthquake in Mponeng gold mine, South Africa

NASA Astrophysics Data System (ADS)

Kozłowska, Maria; Orlecka-Sikora, Beata; Kwiatek, Grzegorz; Boettcher, Margaret S.; Dresen, Georg

2015-01-01

Static stress changes following large earthquakes are known to affect the rate and distribution of aftershocks, yet this process has not been thoroughly investigated for nanoseismicity and picoseismicity at centimeter length scales. Here we utilize a unique data set of M ≥ -3.4 earthquakes following a Mw 2.2 earthquake in Mponeng gold mine, South Africa, that was recorded during a quiet interval in the mine to investigate if rate- and state-based modeling is valid for shallow, mining-induced seismicity. We use Dieterich's (1994) rate- and state-dependent formulation for earthquake productivity, which requires estimation of four parameters: (1) Coulomb stress changes due to the main shock, (2) the reference seismicity rate, (3) frictional resistance parameter, and (4) the duration of aftershock relaxation time. Comparisons of the modeled spatiotemporal patterns of seismicity based on two different source models with the observed distribution show that while the spatial patterns match well, the rate of modeled aftershocks is lower than the observed rate. To test our model, we used three metrics of the goodness-of-fit evaluation. The null hypothesis, of no significant difference between modeled and observed seismicity rates, was only rejected in the depth interval containing the main shock. Results show that mining-induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive Coulomb stress change. Furthermore, we demonstrate that the main features of the temporal and spatial distributions of very small, mining-induced earthquakes can be successfully determined using rate- and state-based stress modeling.

Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer

USGS Publications Warehouse

Toda, S.; Stein, R.S.; Richards-Dinger, K.; Bozkurt, S.B.

2005-01-01

We develop a forecast model to reproduce the distibution of main shocks, aftershocks and surrounding seismicity observed during 1986-200 in a 300 ?? 310 km area centered on the 1992 M = 7.3 Landers earthquake. To parse the catalog into frames with equal numbers of aftershocks, we animate seismicity in log time increments that lengthen after each main shock; this reveals aftershock zone migration, expansion, and densification. We implement a rate/state algorithm that incorporates the static stress transferred by each M ??? 6 shock and then evolves. Coulomb stress changes amplify the background seismicity, so small stress changes produce large changes in seismicity rate in areas of high background seismicity. Similarly, seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates. Thus a key constituent of the model is the background seismicity rate, which we smooth from 1981 to 1986 seismicity. The mean correlation coefficient between observed and predicted M ??? 1.4 shocks (the minimum magnitude of completeness) is 0.52 for 1986-2003 and 0.63 for 1992-2003; a control standard aftershock model yields 0.54 and 0.52 for the same periods. Four M ??? 6.0 shocks struck during the test period; three are located at sites where the expected seismicity rate falls above the 92 percentile, and one is located above the 75 percentile. The model thus reproduces much, but certainly not all, of the observed spatial and temporal seismicity, from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades. Finally, we offer a M ??? 5 earthquake forecast for 2005-2015, assigning probabilities to 324 10 ?? 10 km cells.

High-precision relocation for aftershocks of the 2016 ML 5.8 Gyeongju earthquake in South Korea: Stress partitioning controlled by complex fault systems

NASA Astrophysics Data System (ADS)

Woo, J. U.; Rhie, J.; Kang, T. S.; Kim, S.; Chai, G.; Cho, E.

2017-12-01

Complex inherent fault system is one of key factors controlling the main shock occurrence and the pattern of aftershock sequence. Many field studies have shown that the fault systems in the Korean Peninsula are complex because they formed by various tectonic events since Proterozoic. Apart from that the mainshock is the largest one (ML 5.8) ever recorded in South Korea, the Gyeongju earthquake sequence shows particularly interesting features: ML 5.1 event preceded ML 5.8 event by 50 min and they are located closely to each other ( 1 km). In addition, ML 4.5 event occurred 2 3 km away from the two events after a week of the mainshock. Considering reported focal mechanisms and hypocenters of the three major events, it is unlikely that the earthquake sequence occurs on a single fault plane. To depict the detailed fault geometry associated with the sequence, we precisely determine the relative locations of 1,400 aftershocks recorded by 27 broadband stations, which started to be deployed less than one hour after the mainshock. Double difference algorithm is applied using relative travel time measurements by a waveform cross-correlation method. Relocated hypocenters show that a major fault striking NE-SW and some minor faults get involved in the sequence. In particular, aftershocks immediately following ML 4.5 event seem to occur on a fault striking NW-SE, which is orthogonal to the strike of a major fault. We expect that the Gyeongju earthquake sequence resulted from the stress transfer controlled by the complex inherent fault system in this region.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sweeney, J. J.; Ford, S. R.

The experience of IFE14 emphasizes the need for a better way to simulate aftershocks during an OSI exercise. The obvious approach is to develop a digital model of aftershocks that can be used either for a real field exercise or for a computer simulation that can be done in an office, for training for example. However, this approach involves consideration of several aspects, such as how and when to introduce waveforms in a way that maximizes the realism of the data and that will be convincing to a savvy, experienced seismic analyst. The purpose of this report is to outlinemore » a plan for how this approach can be implemented.« less

North Korea's 2017 Test and its Nontectonic Aftershock

NASA Astrophysics Data System (ADS)

Liu, J.; Li, L.; Zahradník, J.; Sokos, E.; Liu, C.; Tian, X.

2018-04-01

Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus-far strongest North Korean test of 3 September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 min. Here we provide evidence that this aftershock was a nontectonic event which radiated seismic waves as a buried horizontal closing crack. This vigorous crack closure, occurring shortly after the blast, is studied in the North Korea test site for the first time. The event can be qualitatively explained as rapid destruction of an explosion-generated cracked rock chimney due to cavity collapse, although other compaction processes cannot be ruled out.

Using the USGS Seismic Risk Web Application to estimate aftershock damage

USGS Publications Warehouse

McGowan, Sean M.; Luco, Nicolas

2014-01-01

The U.S. Geological Survey (USGS) Engineering Risk Assessment Project has developed the Seismic Risk Web Application to combine earthquake hazard and structural fragility information in order to calculate the risk of earthquake damage to structures. Enabling users to incorporate their own hazard and fragility information into the calculations will make it possible to quantify (in near real-time) the risk of additional damage to structures caused by aftershocks following significant earthquakes. Results can quickly be shared with stakeholders to illustrate the impact of elevated ground motion hazard and earthquake-compromised structural integrity on the risk of damage during a short-term, post-earthquake time horizon.

Imaging the source region of the 2003 San Simeon earthquake within the weak Franciscan subduction complex, central California

USGS Publications Warehouse

Hauksson, E.; Oppenheimer, D.; Brocher, T.M.

2004-01-01

Data collected from the 2003 Mw6.5 San Simeon earthquake sequence in central California and a 1986 seismic refraction experiment demonstrate that the weak Franciscan subduction complex suffered brittle failure in a region without significant velocity contrast across a slip plane. Relocated hypocenters suggest a spatial relationship between the seismicity and the Oceanic fault, although blind faulting on a nearby, unknown fault is an equally plausible alternative. The aftershock volume is sandwiched between the Nacimiento and Oceanic faults and is characterized by rocks of low compressional velocity (Vp) abutted to the east and west by rocks of higher Vp. This volume of inferred Franciscan rocks is embedded within the larger Santa Lucia anticline. Pore fluids, whose presence is implied by elevated Vp/Vs values, may locally decrease normal stress and limit the aftershock depth distribution between 3 to 10 km within the hanging wall. The paucity of aftershocks along the mainshock rupture surface may reflect either the absence of a damage zone or an almost complete stress drop within the low Vp or weak rock matrix surrounding the mainshock rupture. Copyright 2004 by the American Geophysical Union.

Identifying the dynamic characteristics of a dual core-wall and frame building in Chile using aftershocks of the 27 February 2010 (Mw=8.8) Maule, Chile, earthquake

USGS Publications Warehouse

Çelebi, Mehmet; Sereci, Mark; Boroschek, Ruben; Carreño, Rodrigo; Bonelli, Patricio

2013-01-01

Following the 27 February 2010 (Mw = 8.8) Offshore Maule, Chile earthquake, a temporary, 16-channel, real-time data streaming array was installed in a recently constructed building in Viña del Mar to capture its responses to aftershocks. The cast-in-place, reinforced concrete building is 16 stories high, with 3 additional basement levels, and has dual system comprising multiple structural walls and perimeter frames. This building was not damaged during the main-shock, but other buildings of similar design in Viña del Mar and other parts of Chile were damaged, although none collapsed. Dynamic characteristics of the building identified from the low-amplitude (PGA of about 2 Gal) response recordings of aftershocks are found to compare well with those determined from modal analyses using a design level FEM model. Distinct “major-axes” translational and torsional fundamental frequencies, as well as frequencies of secondary modes, are identified. Evidence of beating is consistently observed in the response data for each earthquake. Results do not match well with U.S. code formulas.

The LVD signals during the early-mid stages of the L'Aquila seismic sequence and the radon signature of some aftershocks of moderate magnitude.

PubMed

Cigolini, C; Laiolo, M; Coppola, D

2015-01-01

The L'Aquila seismic swarm culminated with the mainshock of April 6, 2009 (ML = 5.9). Here, we report and analyze the Large Volume Detector (LVD, used in neutrinos research) low energy traces (∼0.8 MeV), collected during the early-mid stages of the seismic sequence, together with the data of a radon monitoring experiment. The peaks of LVD traces do not correlate with the evolution and magnitude of earthquakes, including major aftershocks. Conversely, our radon measurements obtained by utilizing three automatic stations deployed along the regional NW-SE faulting system, seem to be, in one case, more efficient. In fact, the timeseries collected on the NW-SE Paganica fracture recorded marked variations and peaks that occurred during and prior moderate aftershocks (with ML > 3). The Paganica monitoring station (PGN) seems to better responds to active seismicity due to the fact that the radon detector was placed directly within the bedrock of an active fault. It is suggested that future networks for radon monitoring of active seismicity should preferentially implement this setting. Copyright © 2014 Elsevier Ltd. All rights reserved.

Source parameters of the 2014 Ms6.5 Ludian earthquake sequence and their implications on the seismogenic structure

NASA Astrophysics Data System (ADS)

Zheng, Y.

2015-12-01

On August 3, 2014, an Ms6.5 earthquake struck Ludian county, Zhaotong city in Yunnan province, China. Although this earthquake is not very big, it caused abnormal severe damages. Thus, study on the causes of the serious damages of this moderate strong earthquake may help us to evaluate seismic hazards for similar earthquakes. Besides the factors which directly relate to the damages, such as site effects, quality of buildings, seismogenic structures and the characteristics of the mainshock and the aftershocks may also responsible for the seismic hazards. Since focal mechanism solution and centroid depth provide key information of earthquake source properties and tectonic stress field, and the focal depth is one of the most important parameters which control the damages of earthquakes, obtaining precise FMSs and focal depths of the Ludian earthquake sequence may help us to determine the detailed geometric features of the rupture fault and the seismogenic environment. In this work we obtained the FMSs and centroid depths of the Ludian earthquake and its Ms>3.0 aftershocks by the revised CAP method, and further verified some focal depths using the depth phase method. Combining the FMSs of the mainshock and the strong aftershocks, as well as their spatial distributions, and the seismogenic environment of the source region, we can make the following characteristics of the Ludian earthquake sequence and its seismogenic structure: (1) The Ludian earthquake is a left-lateral strike slip earthquake, with magnitude of about Mw6.1. The FMS of nodal plane I is 75o/56o/180o for strike, dip and rake angles, and 165o/90o/34ofor the other nodal plane. (2) The Ludian earthquake is very shallow with the optimum centroid depth of ~3 km, which is consistent with the strong ground shaking and the surface rupture observed by field survey and strengthens the damages of the Ludian earthquake. (3) The Ludian Earthquake should occur on the NNW trend BXF. Because two later aftershocks occurred close to the fault zone of the ZLF, and their FMSs are similar with the characteristics of the ZLF, the shallower part of the ZLF may also rupture during the aftershock duration of the Ludian earthquake. Since the ZLF is much longer than the BXF, the seismic risk of the ZLF may be high and should be required more attention.

The Seismic Sequence of the 2016 Mw 7.8 Pedernales, Ecuador Sarthquake

NASA Astrophysics Data System (ADS)

Leon, S.; Fuenzalida, A.; Bie, L.; Garth, T.; Gonzalez, P. J.; Holt, J.; Rietbrock, A.; Edwards, B.; Regnier, M. M.; Pernoud, M.; Mercerat, E. D.; Perrault, M.; Font, Y.; Alvarado, A. P.; Charvis, P.; Beck, S. L.; Meltzer, A.

2016-12-01

On the 16th April 2016, a Mw 7.8 mega-thrust earthquake occurred in Northern Ecuador, close to the city of Pedernales. The event ruptured an area of 120 x 60 km and was preceded by a Mw 5.0 foreshock, located only 15 km south of the epicentre, and registered 10 minutes before the main event.A few weeks after the main event a large array of instruments was deployed by a collaborative project between the Geophysical Institute of Ecuador (IGEPN), IRIS (USA), Géoazur (France) and the University of Liverpool (UK). This dense seismic network, with more than 70 stations, includes broadband, short period and strong motion instruments and is currently recording the aftershock activity of the earthquake. It is hoped that this data set will give further insights into the structure of the subduction zone mega thrust beneath Ecuador.Using data recorded both on the permanent and the recently deployed network we located and calculated the moment tensor solutions for the foreshock event, and the large aftershocks (M > 5). We analyse the spatial distribution of the seismicity and its relation with the co-seismic slip, estimated by inverting radar satellite interferometry data, and with previous models of inter-seismic coupling (e.g. Chlieh et al., 2014). It is possible to identify two lineations in the aftershock activity located to the north and south of the rupture. Moreover, the geodetic slip model shows that the boundaries of the maximum coseismic slip coincides with the observed lineaments in the aftershocks and with the rupture area of a previous Mw 7.8 event in 1942. This suggests that the features to the north and south may impose a barrier to rupture propagation, creating different segments in the subduction zone beneath Ecuador. In addition, we model the Coulomb stress change caused by the foreshock and mainshock in order to investigate whether this could explain the aftershock distribution and potential earthquake interactions. Previous activity has presented a northward-propagating series of ruptures greater than Mw 7 spaced approximately 20 years apart. An open question is therefore whether the present event is the start of a further series of large magnitude events in northern Ecuador, and/or whether slow slip events/creep south of the rupture have partly accommodated the strain due to subduction.

Complex rupture process of the Mw 7.8, 2016, Kaikoura earthquake, New Zealand, and its aftershock sequence

NASA Astrophysics Data System (ADS)

Cesca, S.; Zhang, Y.; Mouslopoulou, V.; Wang, R.; Saul, J.; Savage, M.; Heimann, S.; Kufner, S.-K.; Oncken, O.; Dahm, T.

2017-11-01

The M7.8 Kaikoura Earthquake that struck the northeastern South Island, New Zealand, on November 14, 2016 (local time), is one of the largest ever instrumentally recorded earthquakes in New Zealand. It occurred at the southern termination of the Hikurangi subduction margin, where the subducting Pacific Plate transitions into the dextral Alpine transform fault. The earthquake produced significant distributed uplift along the north-eastern part of the South Island, reaching a peak amplitude of ∼8 m, which was accompanied by large (≥10 m) horizontal coseismic displacements at the ground surface along discrete active faults. The seismic waveforms' expression of the main shock indicate a complex rupture process. Early automated centroid moment tensor solutions indicated a strong non-double-couple term, which supports a complex rupture involving multiple faults. The hypocentral distribution of aftershocks, which appears diffuse over a broad region, clusters spatially along lineaments with different orientations. A key question of global interest is to shed light on the mechanism with which such a complex rupture occurred, and whether the underlying plate-interface was involved in the rupture. The consequences for seismic hazard of such a distributed, shallow faulting is important to be assessed. We perform a broad seismological analysis, combining regional and teleseismic seismograms, GPS and InSAR, to determine the rupture process of the main shock and moment tensors of 118 aftershocks down to Mw 4.2. The joint interpretation of the main rupture and aftershock sequence allow reconstruction of the geometry, and suggests sequential activation and slip distribution on at least three major active fault domains. We find that the rupture nucleated as a weak strike-slip event along the Humps Fault, which progressively propagated northward onto a shallow reverse fault, where most of the seismic moment was released, before it triggered slip on a second set of strike-slip faults at the northern end of the rupture. The northern and southern strike-slip fault domains have the same orientation but are spatially separated by >15 km. In our model, the low angle splay thrust fault is located above the slab and connects the strike-slip faults kinematically. During the aftershock phase, the entire fault system remained active.

The 2008 Wells, Nevada Earthquake Sequence: Application of Subspace Detection and Multiple Event Relocation Techniques

NASA Astrophysics Data System (ADS)

Nealy, J. L.; Benz, H.; Hayes, G. P.; Bergman, E.; Barnhart, W. D.

2016-12-01

On February 21, 2008 at 14:16:02 (UTC), Wells, Nevada experienced a Mw 6.0 earthquake, the largest earthquake in the state within the past 50 years. Here, we re-analyze in detail the spatiotemporal variations of the foreshock and aftershock sequence and compare the distribution of seismicity to a recent slip model based on inversion of InSAR observations. A catalog of earthquakes for the time period of February 1, 2008 through August 31, 2008 was derived from a combination of arrival time picks using a kurtosis detector (primarily P arrival times), subspace detector (primarily S arrival times), associating the combined pick dataset, and applying multiple event relocation techniques using the 19 closest USArray Transportable Array stations, permanent regional seismic monitoring stations in Nevada and Utah, and temporary stations deployed for an aftershock study. We were able to detect several thousand earthquakes in the months following the mainshock as well as several foreshocks in the days leading up to the event. We reviewed the picks for the largest 986 earthquakes and relocated them using the Hypocentroidal Decomposition (HD) method. The HD technique provides both relative locations for the individual earthquakes and an absolute location for the earthquake cluster, resulting in absolute locations of the events in the cluster having minimal bias from unknown Earth structure. A subset of these "calibrated" earthquake locations that spanned the duration of the sequence and had small uncertainties in location were used as prior constraints within a second relocation effort using the entire dataset and the Bayesloc approach. Accurate locations (to within 2 km) were obtained using Bayesloc for 1,952 of the 2,157 events associated over the seven-month period of the study. The final catalog of earthquake hypocenters indicates that the aftershocks extend for about 20 km along the strike of the ruptured fault. The aftershocks occur primarily updip and along the southwestern edge of the zone of maximum slip as modeled by seismic waveform inversion (Dreger et al., 2011) and by InSAR. The aftershock locations illuminate areas of post-mainshock strain increase and their depths are consistent with InSAR imaging, which showed that the Wells earthquake was a buried source with no observable near-surface offset.

Rupture behaviors of the 2010 Jiashian and 2016 Meinong Earthquakes: Implication for interaction of two asperities on the Chishan Transfer Fault Zone in SW Taiwan.

NASA Astrophysics Data System (ADS)

Jian, P. R.; Hung, S. H.; Chen, Y. L.; Meng, L.; Tseng, T. L.

2017-12-01

After about 45 years of seismic quiescence, southwest Taiwan was imperiled by two strong earthquakes, the 2010 Mw 6.2 Jiashian and deadly 2016 Mw 6.4 Meinong earthquakes in the last decade. The focal mechanisms and their aftershock distributions imply that both events occurred on NW-SE striking, shallow-dipping fault planes but at different depths of 21 and 16 km, respectively. Here we present the MUSIC back projection images using high-frequency P- and sP-waves recorded in the European and Australian seismic networks, the directivity analysis using global teleseismic P waves and relocated aftershocks to characterize the rupture behaviors of the two mainshocks and explore the potential connection between them. The results for the Meinong event indicate a unilateral, subhorizontal rupture propagating NW-ward 17 km and lasting for 6-7 s [Jian et al., 2017]. For the Jiashian event, the rupture initiated at a greater depth of 21 km and then propagated both NW-ward and up-dip ( 16o) on the fault plane, with a shorter rupture length of 10 km and duration of 4-5 s. The up-dip propagation is corroborated by the 3-D directivity analysis that leads to the widths of P-wave pulses increasing linearly with the directivity parameter. Moreover, relocation of aftershocks reveals that the Jiashian sequence is confined in a NW-SE elongated zone extending 15 km and 5 km shallower than the hypocenter. The Meinong aftershock sequence shows three clusters: one surrounding the mainshock hypocenter, another one distributed northwestern and deeper (>20 km) off the rupture plane beneath Tainan, and the other distant shallow-focus one (<10 km) beneath the southern Central Mountain Range. As evidenced by similar focal mechanism, rupture behaviors, as well as the spatial configuration of the mainshock rupture zones and aftershock distributions, we attribute the Jiashian and Meinong earthquakes to two asperities on a buried oblique fault that has been reactivated recently, the NW-SE striking Chishan Transfer Fault Zone as a likely candidate rupture plane. In 2010, the Jiasian earthquake initiated at the deeper NE asperity and propagated NW-ward and up-dip. Six years later, the stronger shallower asperity responsible for the Meinong event was statically triggered, which consequently caused the ruinous destruction in SW Taiwan.

The Hellenic Seismological Network Of Crete (HSNC): Validation and results of the 2013 aftershock sequences

NASA Astrophysics Data System (ADS)

Chatzopoulos, Georgios; Papadopoulos, Ilias; Vallianatos, Filippos

2015-04-01

The number and quality of seismological networks in Europe has increased in the past decades. Nevertheless, the need for localized networks monitoring areas of great seismic and scientific interest is constant. Hellenic Seismological Network of Crete (HSNC) covers this need for the vicinity of the South Aegean Sea and Crete Island. In the present work with the use of Z-map software (www.seismo.ethz.ch) the spatial variability of Magnitude of Completeness (Mc) is calculated from HSNC's manual analysis catalogue of events for the period 2011 until today, proving the good coverage of HSNC in the areas. Furthermore the 2013, South Aegean seismicity where two large shallow earthquakes occurred in the vicinity of Crete Island, is discussed. The first event takes place on 15th June 2013 in the front of the Hellenic Arc, south from central Crete, while the second one on 12th October, 2013 on the western part of Crete. The two main shocks and their aftershock sequences have been relocated with the use of hypoinverse earthquake location software and an appropriate crust model. The HSNC identified more than 500 and 300 aftershocks respectively followed after the main events. The detailed construction of aftershocks catalogue permits the applicability of modern theories based on complexity sciences as described recently in the frame of non extensive statistical physics. In addition site effects in the stations locations are presented using event and noise recordings. This work was implemented through the project IMPACT-ARC in the framework of action "ARCHIMEDES III-Support of Research Teams at TEI of Crete" (MIS380353) of the Operational Program "Education and Lifelong Learning" and is co-financed by the European Union (European Social Fund) and Greek national funds References A. Tzanis and F. Vallianatos, "Distributed power-law seismicity changes and crustal deformation in the EW Hellenic Arc", Natural Hazards and Earth Systems Sciences, 3, 179-195, 2003 F. Vallianatos, G. Michas, G. Papadakis and P. Sammonds " A non extensive statistical physics view to the spatiotemporal properties of the June 1995, Aigion earthquake (M6.2) aftershock sequence (West Corinth rift, Greece)", Acta Geophysica, 60(3), 758-768, 2012. G. Hloupis, I. Papadopoulos, J. P. Makris and F. Vallianatos, "The South Aegean seismological network - HSNC" Advances in Geosciences, 34, 15-21, doi:10.5194/adgeo-34-15-2013, 2013.

The 1995 November 22, Mw 7.2 Gulf of Elat earthquake cycle revisited

NASA Astrophysics Data System (ADS)

Baer, Gidon; Funning, Gareth J.; Shamir, Gadi; Wright, Tim J.

2008-12-01

The 1995 November 22, Mw = 7.2 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform (DST) in the Gulf of Elat (Aqaba). It was the largest earthquake along the DST in at least 160 yr. The main shock was preceded by earthquake swarms north and south of its NE-striking rupture since the early 1980s, and was followed by about 6 months of intense aftershock activity, concentrated mainly northwest and southeast of the main rupture. In this study we re-analyse ERS-1 and ERS-2 InSAR data for the period spanning the main shock and 5 post-seismic years. Because the entire rupture was under the Gulf water, surface observations related to the earthquake are limited to distances greater than 5 km away from the rupture zone. Coseismic interferograms were produced for the earthquake +1 week, +4 months and +6 months. Non-linear inversions were carried out for fault geometry and linear inversions were made for slip distribution using an ascending-descending 2-frame data set. The moment calculated from our best-fitting model is in agreement with the seismological moment, but trade-offs exist among several fault parameters. The present model upgrades previous InSAR models of the Nuweiba earthquake, and differs from recent teleseismic waveform inversion results mainly in terms of slip magnitude and distribution. The moment released by post-seismic deformation in the period of 6 months to 2 yr after the Nuweiba earthquake is about 15 per cent of the coseismic moment release. Our models suggest that this deformation can be represented by slip along the lower part of the coseismic rupture. Localised deformation along the Gulf shores NW of the main rupture in the first 6 months after the earthquake is correlated with surface displacements along active Gulf-parallel normal faults and possibly with shallow M > 3.9, D < 6 km aftershocks. The geodetic moment calculated by modelling this deformation is more than an order of magnitude larger than expected for a single M ~ 4 aftershock, but could be a result of a sequence of aftershocks and/or aseismic slip. The major aftershocks and the slip along Gulf-parallel normal faulting NW of the main rupture are associated with positive Coulomb stress changes induced by the main event.

The Great Maule earthquake: seismicity prior to and after the main shock from amphibious seismic networks

NASA Astrophysics Data System (ADS)

Lieser, K.; Arroyo, I. G.; Grevemeyer, I.; Flueh, E. R.; Lange, D.; Tilmann, F. J.

2013-12-01

The Chilean subduction zone is among the seismically most active plate boundaries in the world and its coastal ranges suffer from a magnitude 8 or larger megathrust earthquake every 10-20 years. The Constitución-Concepción or Maule segment in central Chile between ~35.5°S and 37°S was considered to be a mature seismic gap, rupturing last in 1835 and being seismically quiet without any magnitude 4.5 or larger earthquakes reported in global catalogues. It is located to the north of the nucleation area of the 1960 magnitude 9.5 Valdivia earthquake and to the south of the 1928 magnitude 8 Talca earthquake. On 27 February 2010 this segment ruptured in a Mw=8.8 earthquake, nucleating near 36°S and affecting a 500-600 km long segment of the margin between 34°S and 38.5°S. Aftershocks occurred along a roughly 600 km long portion of the central Chilean margin, most of them offshore. Therefore, a network of 30 ocean-bottom-seismometers was deployed in the northern portion of the rupture area for a three month period, recording local offshore aftershocks between 20 September 2010 and 25 December 2010. In addition, data of a network consisting of 33 landstations of the GeoForschungsZentrum Potsdam were included into the network, providing an ideal coverage of both the rupture plane and areas affected by post-seismic slip as deduced from geodetic data. Aftershock locations are based on automatically detected P wave onsets and a 2.5D velocity model of the combined on- and offshore network. Aftershock seismicity analysis in the northern part of the survey area reveals a well resolved seismically active splay fault in the accretionary prism of the Chilean forearc. Our findings imply that in the northernmost part of the rupture zone, co-seismic slip most likely propagated along the splay fault and not the subduction thrust fault. In addition, the updip limit of aftershocks along the plate interface can be verified to about 40 km landwards from the deformation front. Prior to the Great Maule earthquake the Collaborative Research Center SFB 574 'Volatiles and Fluids in Subduction Zones' shot several wide-angle profiles and operated a network, also consisting of OBS and land stations for six months in 2008. Both projects provide a great opportunity to study the evolution of a subduction zone within the seismic cycle of a great earthquake. The most profound features are (i) a sharp reduction in intraslab seismic activity after the Maule earthquake and (ii) a sharp increase in seismic activity at the slab interface above 50 km depth, where large parts of the rupture zone were largely aseismic prior to the Maule earthquake. Further, the aftershock seismicity shows a broader depth distribution above 50 km depth.

Impact of Short-term Changes In Earthquake Hazard on Risk In Christchurch, New Zealand

NASA Astrophysics Data System (ADS)

Nyst, M.

2012-12-01

The recent Mw 7.1, 4 September 2010 Darfield, and Mw 6.2, 22 February 2011 Christchurch, New Zealand earthquakes and the following aftershock activity completely changed the existing view on earthquake hazard of the Christchurch area. Not only have several faults been added to the New Zealand fault database, the main shocks were also followed by significant increases in seismicity due to high aftershock activity throughout the Christchurch region that is still on-going. Probabilistic seismic hazard assessment (PSHA) models take into account a stochastic event set, the full range of possible events that can cause damage or loss at a particular location. This allows insurance companies to look at their risk profiles via average annual losses (AAL) and loss-exceedance curves. The loss-exceedance curve is derived from the full suite of seismic events that could impact the insured exposure and plots the probability of exceeding a particular loss level over a certain period. Insurers manage their risk by focusing on a certain return period exceedance benchmark, typically between the 100 and 250 year return period loss level, and then reserve the amount of money needed to account for that return period loss level, their so called capacity. This component of risk management is not too sensitive to short-term changes in risk due to aftershock seismicity, as it is mostly dominated by longer-return period, larger magnitude, more damaging events. However, because the secondairy uncertainties are taken into account when calculating the exceedance probability, even the longer return period losses can still experience significant impact from the inclusion of time-dependent earthquake behavior. AAL is calculated by summing the product of the expected loss level and the annual rate for all events in the event set that cause damage or loss at a particular location. This relatively simple metric is an important factor in setting the annual premiums. By annualizing the expected losses due to events of varying severities and recurrence intervals, annual premium rates can be set with some longer term risk planning in mind. However, this metric is particularly sensitive to high frequency, moderate magnitude events. Inclusion of earthquake aftershock sequence characteristics into the stochastic event set may have a strong impact on the AAL, depending on the time window of aftershocks that is taken into account. We will present our model of the aftershock-derived, time-dependent hazard for the region of the two earthquakes and will bring about a detailed view on regional, short-term hazard. Dealing with this short-term hazard poses a challenge to the earthquake insurance business. In this presentation we will look at these short-term hazard changes from a risk perspective and quantify the impact on earthquake risk in terms of the main risk metrics used in the industry.

Ductile Gap between the Wenchuan and Lushan Earthquakes Revealed from the Two-dimensional Pg Seismic Tomography

PubMed Central

Pei, Shunping; Zhang, Haijiang; Su, Jinrong; Cui, Zhongxiong

2014-01-01

A high-resolution two-dimensional Pg-wave velocity model is obtained for the upper crust around the epicenters of the April 20, 2013 Ms7.0 Lushan earthquake and the May 12, 2008 Ms8.0 Wenchuan earthquake, China. The tomographic inversion uses 47235 Pg arrival times from 6812 aftershocks recorded by 61 stations around the Lushan and Wenchuan earthquakes. Across the front Longmenshan fault near the Lushan earthquake, there exists a strong velocity contrast with higher velocities to the west and lower velocities to the east. Along the Longmenshan fault system, there exist two high velocity patches showing an “X” shape with an obtuse angle along the near northwest-southeast (NW-SE) direction. They correspond to the Precambrian Pengguan and Baoxing complexes on the surface but with a ~20 km shift, respectively. The aftershock gap of the 2008 Wenchuan and the 2013 Lushan earthquakes is associated with lower velocities. Based on the theory of maximum effective moment criterion, this suggests that the aftershock gap is weak and the ductile deformation is more likely to occur in the upper crust within the gap under the near NW-SE compression. Therefore our results suggest that the large earthquake may be hard to happen within the gap. PMID:25267344

Ductile gap between the Wenchuan and Lushan earthquakes revealed from the two-dimensional Pg seismic tomography.

PubMed

Pei, Shunping; Zhang, Haijiang; Su, Jinrong; Cui, Zhongxiong

2014-09-30

A high-resolution two-dimensional Pg-wave velocity model is obtained for the upper crust around the epicenters of the April 20, 2013 Ms7.0 Lushan earthquake and the May 12, 2008 Ms8.0 Wenchuan earthquake, China. The tomographic inversion uses 47235 Pg arrival times from 6812 aftershocks recorded by 61 stations around the Lushan and Wenchuan earthquakes. Across the front Longmenshan fault near the Lushan earthquake, there exists a strong velocity contrast with higher velocities to the west and lower velocities to the east. Along the Longmenshan fault system, there exist two high velocity patches showing an "X" shape with an obtuse angle along the near northwest-southeast (NW-SE) direction. They correspond to the Precambrian Pengguan and Baoxing complexes on the surface but with a ~20 km shift, respectively. The aftershock gap of the 2008 Wenchuan and the 2013 Lushan earthquakes is associated with lower velocities. Based on the theory of maximum effective moment criterion, this suggests that the aftershock gap is weak and the ductile deformation is more likely to occur in the upper crust within the gap under the near NW-SE compression. Therefore our results suggest that the large earthquake may be hard to happen within the gap.

Aftershocks halted by static stress shadows

USGS Publications Warehouse

Toda, Shinji; Stein, Ross S.; Beroza, Gregory C.; Marsan, David

2012-01-01

Earthquakes impart static and dynamic stress changes to the surrounding crust. Sudden fault slip causes small but permanent—static—stress changes, and passing seismic waves cause large, but brief and oscillatory—dynamic—stress changes. Because both static and dynamic stresses can trigger earthquakes within several rupture dimensions of a mainshock, it has proven difficult to disentangle their contributions to the triggering process1–3. However, only dynamic stress can trigger earthquakes far from the source4,5, and only static stress can create stress shadows, where the stress and thus the seismicity rate in the shadow area drops following an earthquake6–9 . Here we calculate the stress imparted by the magnitude 6.1 Joshua Tree and nearby magnitude 7.3 Landers earthquakes that occurred in California in April and June 1992, respectively, and measure seismicity through time. We show that, where the aftershock zone of the first earthquake was subjected to a static stress increase from the second, the seismicity rate jumped. In contrast, where the aftershock zone of the first earthquake fell under the stress shadow of the second and static stress dropped, seismicity shut down. The arrest of seismicity implies that static stress is a requisite element of spatial clustering of large earthquakes and should be a constituent of hazard assessment.

The 2013 Crete (Hellenic Arc) Earthquake Sequence

NASA Astrophysics Data System (ADS)

Karakostas, V. G.; Papadimitriou, E. E.; Vallianatos, F.

2014-12-01

The western Hellenic Arc is a well known place of active interplate deformation, where the convergence motion vector is perpendicular to the subduction front. On 12 October 2013 this area was hit by a strong (Mw=6.7) earthquake, occurred on a thrust fault onto the coupled part of the overriding and descending plates, with the compression axis being oriented in the direction of plate convergence. This was the first strong (M>6.0) event to have occurred onto this segment of the descending slab, which has accommodated the largest (M8.3) known earthquake in the Mediterranean area, and to be recorded by the Hellenic Unified Seismological Network (HUSN) that has been considerably improved in the last five years. The first 2-days relocated seismicity shows activation of the upper part of the descending slab, downdip of the plate interface and forming a relatively narrow aftershock area on map view. The less densely visited by aftershocks area, where the main shock is also encompassed, is considered as the high-slip area along the downdip portion of the subducting plane. Dense concentration of the intraslab aftershocks are probably due to the increase of static stress generated by the main shock. A spectacular feature of the aftershock activity concerns the lateral extension of the slipped area, which appears very sharply defined. This provides evidence on localized coupling and aseismically creeping areas, explaining the low coupling ratio in the Hellenic Arc, as it derives from comparison between relative plate motion and seismic energy release. Elucidating the issue of how far the associated large-slip zone might be extended along the plate interface during the main rupture is crucial in assessing future earthquake hazards from subduction events in the study area. This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the "THALES Program: SEISMO FEAR HELLARC" project.

Persistent fine-scale fault structures control rupture development in Parkfield, CA.

NASA Astrophysics Data System (ADS)

Perrin, C.; Waldhauser, F.; Scholz, C. H.

2016-12-01

We investigate the fine-scale geometry and structure of the San Andreas Fault (SAF) near Parkfield, CA, and their role in the development of the 1966 and 2004 M6 earthquakes. Both events broke the fault mainly unilaterally with similar length ( 30 km) but in opposite directions. Seismic slip occurred in a narrow zone between 5 and 10 km depth, as outlined by the concentration of aftershocks along the edge of the slip area. Across fault distribution of the 2004 aftershocks show a rapid decrease of event density away from the fault core. The damage zone is narrower in the Parkfield section (few 100 meters) than in the creeping section ( 1 km). We observe a similar but broader distribution during the interseismic periods. This implies that stress accumulates in a volume around the fault during interseismic periods, whereas coseismic deformation is more localized on the mature SAF. Large aftershocks are concentrated at both rupture tips, characterized by strong heterogeneities in the fault structure at the surface and at depth: i) in the south near Gold Hill-Cholame, a large releasing bend (>25°) separates the Parkfield section from the southern section of the SAF; ii) in the north at Middle Mountain, the surface fault trace goes through an ancient restraining step-over connecting the Parkfield and creeping sections. Fine-scale analysis of the 2004 aftershocks reveals a change in the fault dip and local variations of the fault strike (up to 25°) beneath Middle Mountain, in good agreement with focal mechanisms, which show oblique normal and reverse faulting. We observe these variations during the interseismic periods before and after the 2004 event, suggesting that the structural heterogeneities persisted through at least two earthquake cycles. These heterogeneities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, but also as stress concentrations where rupture initiates.

The 2012 Strike-slip Earthquake Sequence in Black Sea and its Link to the Caucasus Collision Zone

NASA Astrophysics Data System (ADS)

Tseng, T. L.; Hsu, C. H.; Legendre, C. P.; Jian, P. R.; Huang, B. S.; Karakhanian, A.; Chen, C. W.

2016-12-01

The Black Sea formed as a back-arc basin in Late Cretaceous to Paleogene with lots of extensional features. However, the Black Sea is now tectonically stable and absent of notable earthquakes except for the coastal region. In this study we invert regional waveforms of a new seismic array to constrain the focal mechanisms and depths of the 2012/12/23 earthquake sequence occurred in northeastern Black Sea basin that can provide unique estimates on the stress field in the region. The results show that the focal mechanisms for the main shock and 5 larger aftershocks are all strike-slip faulting and resembling with each other. The main rupture fall along the vertical dipping, NW-SE trending sinistral fault indicated by the lineation of most aftershocks. The fault strike and aftershock distribution are both consistent with the Shatsky Ridge, which is continental in nature but large normal faults was created by previous subsidence. The occurrence of 2012 earthquakes can be re-activated, as strike-slip, on one of the pre-existing normal fault cutting at depth nearly 20-30 km in the extended crust. Some of the aftershocks, including a larger one occurred 5 days later, are distributed toward NE direction 20 km away from main fault zone. Those events might be triggered by the main shock along a conjugate fault, which is surprisingly at the extension of proposed transform fault perpendicular to the rift axis of eastern Black Sea Basin. The focal mechanisms also indicate that the maximum compression in northeast Black Sea is at E-W direction, completely different from the N-S compression in the Caucasus and East Turkey controlled by Arabia-Eurasia collision. The origin of E-W maximum compression is probably the same as the secondary stress inferred from earthquakes in Racha region of the Greater Caucasus.

Aftershock source properties of events following the 2013 Craig Earthquake: new evidence for structural heterogeneity on the northern Queen Charlotte Fault

NASA Astrophysics Data System (ADS)

Roland, E. C.; Walton, M. A. L.; Ruppert, N. A.; Gulick, S. P. S.; Christeson, G. L.; Haeussler, P. J.

2014-12-01

In January 2013, a Mw 7.5 earthquake ruptured a segment of the Queen Charlotte Fault offshore the town of Craig in southeast Alaska. The region of the fault that slipped during the Craig earthquake is adjacent to and possibly overlapping with the northern extent of the 1949 M 8.1 Queen Charlotte earthquake rupture (Canada's largest recorded earthquake), and is just south of the rupture area of the 1972 M 7.6 earthquake near Sitka, Alaska. Here we present aftershock locations and focal mechanisms for events that occurred four months following the mainshock using data recorded on an Ocean Bottom Seismometer (OBS) array that was deployed offshore of Prince of Wales Island. This array consisted of 9 short period instruments surrounding the fault segment, and recorded hundreds of aftershocks during the months of April and May, 2013. In addition to highlighting the primary mainshock rupture plane, aftershocks also appear to be occurring along secondary fault structures adjacent to the main fault trace, illuminating complicated structure, particularly toward the northern extent of the Craig rupture. Focal mechanisms for the larger events recorded during the OBS deployment show both near-vertical strike slip motion consistent with the mainshock mechanism, as well as events with varying strike and a component of normal faulting. Although fault structure along this northern segment of the QCF appears to be considerably simpler than to the south, where a higher degree of oblique convergence leads to sub-parallel compressional deformation structures, secondary faulting structures apparent in legacy seismic reflection data near the Craig rupture may be consistent with the observed seismicity patterns. In combination, these data may help to characterize structural heterogeneity along the northern segment of the Queen Charlotte Fault that contributes to rupture segmentation during large strike slip events.

Analysis of November 3, 2010 Kraljevo Earthquake (Mw=5.4) and Its Aftershock Sequence

NASA Astrophysics Data System (ADS)

Knezevic Antonijevic, S.; Arroucau, P.; Vlahovic, G.

2011-12-01

A Mw=5.4 earthquake occurred on November 3, 2010 near the City of Kraljevo, Serbia (lat. 43.765 N, long. 20.713 E) and was followed by a sequence of more than 650 aftershocks with magnitude greater than 1.0. Despite the moderate magnitude of the event, two people were killed, many other were injured, and the total damage to the city is estimated to more than 150 million dollars. Changes in ground water circulation, liquefaction features and rockfalls have also been reported in some places. The earthquake occurred on the southern rim of the Pannonian Basin, in SE-NW-trending Čačak-Kraljevo Basin, also known as West Morava graben. This basin was formed by activation of several deep and secondary shallower faults during Lower Miocene and represents the largest of the intradinaric depressions. Depths proposed by different agencies for the mainshock range between 2 and 30 km. Moment tensor solutions show a mostly strike-slip component on an EW or NS trending fault, with either normal or reverse component depending on the solutions. In order to better characterize the location and source characteristics of that earthquake, we obtained data from seismological institutions of Serbia, Montenegro, Croatia, Greece, Albania, Romania and Italy and we manually picked P and S wave arrival times and first motion polarities on the available seismograms for the entire mainshock-aftershock sequence. More than 100 events were precisely relocated and focal mechanisms were determined in the best cases. Our results confirm that Kraljevo earthquake probably involved the activation in strike-slip regime of an EW-trending fault located in the northern rim of the West Morava Graben, while the seismicity of the past decades was mostly confined to the southern rim of that basin. Key words: Seismotectonic, Balkan region, Serbia, Čačak-Kraljevo Basin, aftershock sequence, earthquake location, focal mechanism

Sequential combination of multi-source satellite observations for separation of surface deformation associated with serial seismic events

NASA Astrophysics Data System (ADS)

Chen, Qiang; Xu, Qian; Zhang, Yijun; Yang, Yinghui; Yong, Qi; Liu, Guoxiang; Liu, Xianwen

2018-03-01

Single satellite geodetic technique has weakness for mapping sequence of ground deformation associated with serial seismic events, like InSAR with long revisiting period readily leading to mixed complex deformation signals from multiple events. It challenges the observation capability of single satellite geodetic technique for accurate recognition of individual surface deformation and earthquake model. The rapidly increasing availability of various satellite observations provides good solution for overcoming the issue. In this study, we explore a sequential combination of multiple overlapping datasets from ALOS/PALSAR, ENVISAT/ASAR and GPS observations to separate surface deformation associated with the 2011 Mw 9.0 Tohoku-Oki major quake and two strong aftershocks including the Mw 6.6 Iwaki and Mw 5.8 Ibaraki events. We first estimate the fault slip model of major shock with ASAR interferometry and GPS displacements as constraints. Due to the used PALSAR interferogram spanning the period of all the events, we then remove the surface deformation of major shock through forward calculated prediction thus obtaining PALSAR InSAR deformation associated with the two strong aftershocks. The inversion for source parameters of Iwaki aftershock is conducted using the refined PALSAR deformation considering that the higher magnitude Iwaki quake has dominant deformation contribution than the Ibaraki event. After removal of deformation component of Iwaki event, we determine the fault slip distribution of Ibaraki shock using the remained PALSAR InSAR deformation. Finally, the complete source models for the serial seismic events are clearly identified from the sequential combination of multi-source satellite observations, which suggest that the major quake is a predominant mega-thrust rupture, whereas the two aftershocks are normal faulting motion. The estimated seismic moment magnitude for the Tohoku-Oki, Iwaki and Ibaraki evens are Mw 9.0, Mw 6.85 and Mw 6.11, respectively.

Earthquakes in Oita triggered by the 2016 M7.3 Kumamoto earthquake

NASA Astrophysics Data System (ADS)

Yoshida, Shingo

2016-11-01

During the passage of the seismic waves from the M7.3 Kumamoto, Kyushu, earthquake on April 16, 2016, a M5.7 [semiofficial value estimated by the Japan Meteorological Agency (JMA)] event occurred in the central part of Oita prefecture, approximately 80 km far away from the mainshock. Although there have been a number of reports that M < 5 earthquakes were remotely triggered during the passage of seismic waves from mainshocks, there has been no evidence for M > 5 triggered events. In this paper, we firstly confirm that this event is a M6-class event by re-estimating the magnitude using the strong-motion records of K-NET and KiK-net, and crustal deformation data at the Yufuin station observed by the Geospatial Information Authority of Japan. Next, by investigating the aftershocks of 45 mainshocks which occurred over the past 20 years based on the JMA earthquake catalog (JMAEC), we found that the delay time of the 2016 M5.7 event in Oita was the shortest. Therefore, the M5.7 event could be regarded as an exceptional M > 5 event that was triggered by passing seismic waves, unlike the usual triggered events and aftershocks. Moreover, a search of the JMAEC shows that in the 2016 Oita aftershock area, swarm earthquake activity was low over the past 30 years compared with neighboring areas. We also found that in the past, probably or possibly triggered events frequently occurred in the 2016 Oita aftershock area. The Oita area readily responds to remote triggering because of high geothermal activity and young volcanism in the area. The M5.7 Oita event was triggered by passing seismic waves, probably because large dynamic stress change was generated by the mainshock at a short distance and because the Oita area was already loaded to a critical stress state without a recent energy release as suggested by the past low swarm activity.[Figure not available: see fulltext.

Coseismic fault slip associated with the 1992 M(sub w) 6.1 Joshua Tree, California, earthquake: Implications for the Joshua Tree-Landers earthquake sequence

NASA Technical Reports Server (NTRS)

Bennett, Richard A.; Reilinger, Robert E.; Rodi, William; Li, Yingping; Toksoz, M. Nafi; Hudnut, Ken

1995-01-01

Coseismic surface deformation associated with the M(sub w) 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near-fault displacement estimates exceeding 40 mm. In order to determine the long-wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right-lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7 x 10(exp 18) N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's functions analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6-8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2-4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the M(sub w) 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree-induced stress perturbations played a role in the spatio-temporal development of the earth sequence culminating in the Landers event.

Toggling of seismicity by the 1997 Kagoshima earthquake couplet: A demonstration of time-dependent stress transfer

USGS Publications Warehouse

Toda, S.; Stein, R.

2003-01-01

Two M ??? 6 well-recorded strike-slip earthquakes struck just 4 km and 48 days apart in Kagoshima prefecture, Japan, in 1997, providing an opportunity to study earthquake interaction. Aftershocks are abundant where the Coulomb stress is calculated to have been increased by the first event, and they abruptly stop where the stress is dropped by the second event. This ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks, whereas the dynamic Coulomb stresses, which should only promote seismicity, appear to play a secondary role. If true, the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution. However, adding the stress changes from the two Kagoshima events does not fully capture the ensuing seismicity, such as its rate change, temporal decay, or migration away from the ends of the ruptures. We therefore implement a stress transfer model that incorporates rate/state friction, in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip, slip rate, and elapsed time since the last event. The model reproduces the temporal response of seismicity to successive stress changes, including toggling, decay, and aftershock migration. Nevertheless, the match of observed to predicted seismicity is quite imperfect, due perhaps to inadequate knowledge of several model parameters. However, to demonstrate the potential of this approach, we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks, taking advantage of the large statistical sample the small shocks afford. Not surprisingly, such probabilities are highly time- and location-dependent: During the first decade after the main shocks, the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones. Copyright 2003 by the American Geophysical Uion.

Estimating Rupture Directivity of Aftershocks of the 2014 Mw8.1 Iquique Earthquake, Northern Chile

NASA Astrophysics Data System (ADS)

Folesky, Jonas; Kummerow, Jörn; Timann, Frederik; Shapiro, Serge

2017-04-01

The 2014 Mw8.1 Iquique earthquake was accompanied by numerous fore- and aftershocks of magnitudes up to M ˜ 7.6. While the rupture processes of the main event and its largest aftershock were already analysed in great detail, this study focusses on the rupture processes of about 230 smaller aftershocks that occurred during the first two days after the main event. Since the events are of magnitudes 4.0 ≤ M ≤ 6.5 it is not trivial which method is most suitable. Thus we apply and compare here three different approaches attempting to extract a possible rupture directivity for each single event. The seismic broadband recordings of the Integrated Plate Boundary Observatory Chile (IPOC) provide an excellent database for our analysis. Their high sampling rate (100 Hz) and a well distributed station selection that cover an aperture of about 180 ° are a great advantage for a thorough directivity analysis. First, we apply a P wave polarization analysis (PPA) where we reconstruct the direction of the incoming wave-field by covariance analysis of the first particle motions. Combined with a sliding time window the results from different stations are capable of identifying first the hypocentre of the events and also a migration of the rupture front, if the event is of unilateral character. A second approach is the back projection imaging (BPI) technique, which illuminates the rupture path by back-projecting the recorded seismic energy to its source. A propagating rupture front would be reconstructed from the migration of the zone of high constructive amplitude stacks. In a third step we apply the empirical Green's function (EGF) method, where events of high waveform similarity, hence co-located and of similar mechanisms, are selected in order to use the smaller event as the Green's function of the larger event. This approach results in an estimated source time function, which is compared station wise and whose azimuthal variations are analysed for complexities and directivity.

Deep Tectonic Tremor in Haiti triggered by the 2010/02/27 Mw8.8 Maule, Chile earthquake

NASA Astrophysics Data System (ADS)

Aiken, C.; Peng, Z.; Douilly, R.; Calais, E.; Deschamps, A.; Haase, J. S.

2013-05-01

Tectonic tremors have been observed along major plate-boundary faults around the world. In most of these regions, tremors occur spontaneously (i.e. ambient) or as a result of small stress perturbations from passing surface waves (i.e. triggered). Because tremors are located below the seismogenic zone, a detailed study of their behavior could help to better understand how tectonic movement is accommodated in the deep root of major faults, and the relationship with large earthquakes. Here, we present evidence of triggered tremor in southern Haiti around the aftershock zone of the 2010/01/12 Mw7.0 Haiti earthquake. Following the January mainshock, several groups have installed land and ocean bottom seismometers to record aftershock activity (e.g., De Lepinay et al., 2011). In the following month, the 2010/02/27 Mw8.8 Maule, Chile earthquake occurred and was recorded in the southern Haiti region by these seismic stations. We apply a 5-15 Hz band-pass filter to all seismograms to identify local high-frequency signals during the Chile teleseismic waves. Tremor is identified as non-impulsive bursts with 10-20 s durations that is coherent among different stations and is modulated by surface waves. We also convert the seismic data into audible sounds and use them to distinguish between local aftershocks and deep tremor. We locate the source of the tremor bursts using an envelope cross-correlation method based on travel time differences. Because tremor depth is not well constrained with this method, we set it to 20 km, close to the recent estimate of Moho depth in this region (McNamara et al., 2012). Most tremors are located south of the surface expression of the Enriquillo-Plantain Garden Fault (EPGF), a high-angle southward dipping left-lateral strike-slip fault that marks the boundary between the Gonave microplate and the Caribbean plate, although the location errors are large. Tremor peaks are mostly modulated by Love wave velocity, which is consistent with left-lateral shear motion induced by the normal incidence of Love wave on a near-vertical strike-slip fault. Our ongoing efforts include comparing tremor and aftershock locations with the same envelope techniques, and identifying tremor at other times. If the tremor locations are reliable, the results pose interesting questions about stress changes following the Haiti mainshock that lead to triggered seismicity on the shallow south dipping Trois Baies fault (De Lepinay et al., 2011, Douilly et al, 2013), and triggered tremor on the EPGF, where no aftershocks were recorded.

A Chicken-Or-Egg Riddle: Why Do Many Large Basin and Range Ruptures Terminate in Seismically Active Geothermal Zones?

NASA Astrophysics Data System (ADS)

Stein, R. S.; Sevilgen, V.

2016-12-01

The 26 March 1872 M 7.6 Owens Valley, CA, rupture extended between two seismically active geothermal areas: Ridgecrest-Coso to the south and Mammoth-Long Valley to the north. Is this a coincidence, or is the rupture extent related to the geothermal and seismic activity? The 1872 rupture might have been confined between these two shattered, fluid-saturated zones if, because of very low friction, they do not accumulate stress. Alternatively, the 1872 earthquake could have activated these zones, as they are located where the rupture is calculated to have increased the Coulomb stress by 1-2 bars. Cause or effect, this phenomenon may be common in trans-tensional regimes: There are geothermal areas near or just beyond the ends of the 1954 M=7.1 Fairview Peak and M=6.8 Dixie Valley, NV, ruptures, with some seismicity clusters beyond the rupture tips. The 2008 Mw=6.0 South Iceland earthquake, another site of trans-tensional faulting and geothermal activity, shows intense aftershock clusters off the ends of the main rupture, resembling the 1872 earthquake. This chicken-or-egg riddle would be easy to solve if it were known whether the earthquake rate in the geothermal areas increased as a result of the 1872 shock. There is only one recorded quake before the mainshock, a July 1871 M 5.5 event at the southern tip of the 1872 rupture; this suggests the clusters might have been active before the mainshock. On the other hand, one of the two largest aftershocks of the 1872 event, a M 6.8 near Bishop two weeks after the mainshock, stuck in the center of the northern cluster, which supports the clusters were activated by the mainshock. We walk away with a tie. If the 1872 rupture did trigger the activity in the clusters, it begs a harder question: Could aftershocks or triggered seismicity have continued almost 150 years after the mainshock? The Owens Valley Fault slips 2-3 mm/yr, about a tenth of the San Andreas Fault rate. Rate/state theory and observations from the central US and Japan indicate that an Owens Valley aftershock sequence should last about ten times longer than one on the San Andreas. The Owens Valley Fault appears to produce M 7.5 shocks roughly every 3,500 yr, and so 150 yr is 4% of the interevent time, equivalent to about 10 yr on the San Andreas. Therefore, one cannot exclude the possibility that these earthquakes are aftershocks.

Testing the stress shadow hypothesis

NASA Astrophysics Data System (ADS)

Felzer, Karen R.; Brodsky, Emily E.

2005-05-01

A fundamental question in earthquake physics is whether aftershocks are predominantly triggered by static stress changes (permanent stress changes associated with fault displacement) or dynamic stresses (temporary stress changes associated with earthquake shaking). Both classes of models provide plausible explanations for earthquake triggering of aftershocks, but only the static stress model predicts stress shadows, or regions in which activity is decreased by a nearby earthquake. To test for whether a main shock has produced a stress shadow, we calculate time ratios, defined as the ratio of the time between the main shock and the first earthquake to follow it and the time between the last earthquake to precede the main shock and the first earthquake to follow it. A single value of the time ratio is calculated for each 10 × 10 km bin within 1.5 fault lengths of the main shock epicenter. Large values of the time ratio indicate a long wait for the first earthquake to follow the main shock and thus a potential stress shadow, whereas small values indicate the presence of aftershocks. Simulations indicate that the time ratio test should have sufficient sensitivity to detect stress shadows if they are produced in accordance with the rate and state friction model. We evaluate the 1989 MW 7.0 Loma Prieta, 1992 MW 7.3 Landers, 1994 MW 6.7 Northridge, and 1999 MW 7.1 Hector Mine main shocks. For each main shock, there is a pronounced concentration of small time ratios, indicating the presence of aftershocks, but the number of large time ratios is less than at other times in the catalog. This suggests that stress shadows are not present. By comparing our results to simulations we estimate that we can be at least 98% confident that the Loma Prieta and Landers main shocks did not produce stress shadows and 91% and 84% confident that stress shadows were not generated by the Hector Mine and Northridge main shocks, respectively. We also investigate the long hypothesized existence of a stress shadow following the 1906 San Francisco Bay area earthquake. We find that while Bay Area catalog seismicity rates are lower in the first half of the twentieth century than in the last half of the nineteenth, this seismicity contrast is also true outside of the Bay Area, in regions not expected to contain a stress shadow. This suggests that the rate change is due to a more system wide effect, such as errors in the historical catalog or the decay of aftershocks of the larger 1857 Fort Tejon earthquake.

Deepest Depth of Seismogenic Layer Within the Crust Beneath Japanese Islands on the Japan Sea Side Using High Resolved Earthquake Catalog and Heat Flux Data

NASA Astrophysics Data System (ADS)

Matsubara, M.; Yano, T. E.

2017-12-01

Understanding the deepest depth of seismogenic layer is important parameter for the earthquake hazard assessment because this relates to the size of earthquakes caused by the active fault. Using the indexes D90 and D95, defined as the depth above which 90% and 95 % of the whole crustal earthquakes occurred from the surface, as the lower limits of the seismogenic layer. We verified the seismogenic depth for particular earthquakes on the Japan Sea side occurred after the year of 2001. We compared with the actual main shock hypocenter depth, their aftershocks, main slip region on the fault, and depth where the temperature estimated to be 250, 300, and 450 degrees. For D90 and D95, we used two different earthquake catalogs. First, the catalog in which we relocated hypocenters for 12 years between 2001 and 2012 from the NIED Hi-net catalog (JUICE catalog, Yano et al. 2017) for high resolution hypocenter locations (Depth <40 km, M>0.0). This catalog is used to get D95 values. Second, the earthquake catalog redetermined with the 3D velocity structure (Matsubara and Obara, 2011) particularly for getting the D90 value around the costal region. In order to satisfy Gutenberg-Richter magnitude-frequency relation, we chose events M>1.5. We then calculated the D90 and D95 using the same method as Matsubara and Sato (2015). For depths where the temperatures are 250, 300, and 450 degrees are estimated from heat flux measured at Hi-net boreholes (Matsumoto, 2007) and other additional data Sakagawa et al. (2005). Depths are calculated using the steady-state, one-dimensional, heat conduction equation with an exponential decrease in the radioactivity heat generation introduced in Tanaka (2004). The general pattern of our results is consistent with previous studies of D90 as very deep D95 beneath the northern Hokkaido and northern Honshu and very shallow D95 along the volcanic front. We found that our D90/D95 showed the deepest boundary of hypocenter of mainshock, majority of aftershocks, main co-seismic slip regions for many large events except the aftershocks of 2016 Kumamoto Earthquake. We concluded that the deep aftershocks off the Kumamoto event occurred within the brittle-ductile transition zoon since the aftershocks region has too high P-wave velocity as 7.4 km/s for crustal brittle layer, with temperature higher than 300 degrees.

Earthquake Triggering in the September 2017 Mexican Earthquake Sequence

NASA Astrophysics Data System (ADS)

Fielding, E. J.; Gombert, B.; Duputel, Z.; Huang, M. H.; Liang, C.; Bekaert, D. P.; Moore, A. W.; Liu, Z.; Ampuero, J. P.

2017-12-01

Southern Mexico was struck by four earthquakes with Mw > 6 and numerous smaller earthquakes in September 2017, starting with the 8 September Mw 8.2 Tehuantepec earthquake beneath the Gulf of Tehuantepec offshore Chiapas and Oaxaca. We study whether this M8.2 earthquake triggered the three subsequent large M>6 quakes in southern Mexico to improve understanding of earthquake interactions and time-dependent risk. All four large earthquakes were extensional despite the the subduction of the Cocos plate. The traditional definition of aftershocks: likely an aftershock if it occurs within two rupture lengths of the main shock soon afterwards. Two Mw 6.1 earthquakes, one half an hour after the M8.2 beneath the Tehuantepec gulf and one on 23 September near Ixtepec in Oaxaca, both fit as traditional aftershocks, within 200 km of the main rupture. The 19 September Mw 7.1 Puebla earthquake was 600 km away from the M8.2 shock, outside the standard aftershock zone. Geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and time-series analysis of GPS station data constrain finite fault total slip models for the M8.2, M7.1, and M6.1 Ixtepec earthquakes. The early M6.1 aftershock was too close in time and space to the M8.2 to measure with InSAR or GPS. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite. Our preliminary geodetic slip model for the M8.2 quake shows significant slip extended > 150 km NW from the hypocenter, longer than slip in the v1 finite-fault model (FFM) from teleseismic waveforms posted by G. Hayes at USGS NEIC. Our slip model for the M7.1 earthquake is similar to the v2 NEIC FFM. Interferograms for the M6.1 Ixtepec quake confirm the shallow depth in the upper-plate crust and show centroid is about 30 km SW of the NEIC epicenter, a significant NEIC location bias, but consistent with cluster relocations (E. Bergman, pers. comm.) and with Mexican SSN location. Coulomb static stress change calculations by Toda et al. (Temblor.net) from NEIC FFM showed significant positive change at epicenter of Ixtepec quake, but very small change for Puebla quake. Our analysis will update these calculations. We are planning to add seismic waveforms to our inversions to estimate kinematic fault slip evolution models that will enable dynamic stress transfer calculations.

Magnetic, gravity, radiometric, LiDAR, and seismic data to characterize the region of the M5.8 August 23, 2011 earthquake near Mineral, Virginia

NASA Astrophysics Data System (ADS)

Shah, A. K.; McNamara, D. E.; Odum, J. K.; Stephenson, W. J.; Kayen, R. E.; Emmett, P. F.; Herrmann, R. B.; Snyder, S. L.; Horton, J. W.; Williams, R. A.

2012-12-01

In response to the M5.8 August 23, 2011 Mineral, VA earthquake, the U.S. Geological Survey and partner organizations acquired or sponsored collection of several geophysical datasets to assist characterization of the earthquake region. Goals include the mapping of the main shock causative and aftershock faults as well as associated geologic features which may be buried or difficult to access, estimates of regional ground motion attenuation, and measurements describing local amplification of seismic energy. The deployment of 46 portable seismic stations by several organizations within days following the earthquake, along with public availability of the resulting data, has greatly aided site characterization efforts. The aftershock data recorded by these stations have allowed delineation of the probable causative fault and other faults that were active afterwards. Using the portable seismograph network and regional permanent stations, S and Lg waves were analyzed to estimate crustal attenuation characteristics. Active- and passive-source seismic experiments were also conducted at many of the portable and permanent stations to characterize site conditions and constrain local response models via estimates of Vs30 and bedrock depth. In March 2012, a LiDAR survey with 8pt/m2 resolution was flown over a ~20x35 km area covering the epicenters of the earthquake and most aftershocks. In July 2012, a high-resolution airborne magnetic, gravity, and radiometric survey was flown over a similar but slightly smaller area. Supplementary ground gravity data have been collected inside and outside of the airborne survey areas. The gravity and magnetic data reflect subsurface features in a region where outcropping rocks are sparse, while LiDAR and radiometric data, respectively, delineate subtle features at the land surface and upper few centimeters. Each of these datasets reflects the regional NE-SW striking fabric. Seismic wave analyses show preferential attenuation in a NW-SE direction, perpendicular to major structures and consistent with estimates of motion intensity such as USGS "Did you feel it?" website responses and local damage reports (Virginia DMME). Preliminary analyses of strong ground motions show measurable differences in shaking over scales of kilometers. Magnetic data show numerous NE-SW lineations, some which extend several tens of km to the northeast from near the aftershock plane, suggesting continuity of some features in that direction. Near the earthquake epicenter and primary aftershock plane, small changes in the orientation of major structures (< 20°) are apparent in magnetic, gravity and topographic data. Detailed analyses to understand possible relations are underway.

Geological controls on hillslope-failure mechanisms during the 2010-2011 earthquake sequence in suburban Christchurch, New Zealand

NASA Astrophysics Data System (ADS)

McSaveney, M. J.; Massey, C. I.; Wang, G.

2012-12-01

Parts of the city of Christchurch NZ were severely damaged when a series of shallow aftershocks from the Mw 7.1 Darfield earthquake of 4 Sept. 2010 occurred beneath the city. Hillside suburbs are on the flanks of an extinct and deeply dissected Miocene basalt volcano which is widely mantled by Pleistocene loess. Portions of the lower flanks of the volcano have been cliffed by late Holocene marine erosion. Highly prized cliff-top homes overlook the city with a backdrop of the Southern Alps, or a seascape of the south Pacific Ocean. Here we discuss how various hillslope materials responded in different ways to the unusually high ground accelerations in the hillside suburbs. Most ground damage occurred in two major aftershocks: the Mw 6.2 Christchurch earthquake of 22 Feb. 2011, in which 184 people died, and a Mw 6.2 aftershock of 13 June 2011. Ground accelerations in these earthquakes were recorded locally in excess of 2 g. Significant ground damage occurred in 3 other strong aftershocks, although minor rockfalls were observed in many smaller aftershocks of which there were thousands. In the highest ground accelerations, many loose hillside objects were thrown into the air. More than 5000 loose basalt boulders were thrown or toppled from steep slopes to roll through residential properties on the slopes below. In addition, cliff tops and faces crumbled into debris avalanches. In the 13 June earthquake a geotechnical witness on a cliff top described ground cracks opening and closing around him as he scrambled off the collapsing cliff edge during the strong cyclic shaking. Loess landslides were a relative minor but damaging component. Dynamic ring-shear testing showed that the loess fails during amplified strong ground shaking, but shear displacement stops soon after, so that the earthquake-triggered loess landslides move only during strong earthquakes. Detected minor lateral movement in strongly weathered basaltic tuff and basaltic lava breccia is a cause of concern because deep weathering has left a rock composed largely of plagioclase, goethite, and pore space, with minor clay minerals. Dynamic ring-shear testing revealed some alarming properties. A remolded sample was initially unresponsive to amplified strong earthquake loading, but failed catastrophically (shear strain continuing long after the end of strong shaking) on the third seismic challenge. Shear resistance was strongly rate dependent, with shear resistance reducing with increasing strain rate. No catastrophic landslides of these weathered materials have occurred in the brief historic period, but the potential for them to occur gives new emphasis to their further investigation.

The influence of one earthquake on another

NASA Astrophysics Data System (ADS)

Kilb, Deborah Lyman

1999-12-01

Part one of my dissertation examines the initiation of earthquake rupture. We study the initial subevent (ISE) of the Mw 6.7 1994 Northridge, California earthquake to distinguish between two end-member hypotheses of an organized and predictable earthquake rupture initiation process or, alternatively, a random process. We find that the focal mechanisms of the ISE and mainshock are indistinguishable, and both events may have nucleated on and ruptured the same fault plane. These results satisfy the requirements for both end-member models, and do not allow us to distinguish between them. However, further tests show the ISE's waveform characteristics are similar to those of typical nearby small earthquakes (i.e., dynamic ruptures). The second part of my dissertation examines aftershocks of the M 7.1 1989 Loma Prieta, California earthquake to determine if theoretical models of static Coulomb stress changes correctly predict the fault plane geometries and slip directions of Loma Prieta aftershocks. Our work shows individual aftershock mechanisms cannot be successfully predicted because a similar degree of predictability can be obtained using a randomized catalogue. This result is probably a function of combined errors in the models of mainshock slip distribution, background stress field, and aftershock locations. In the final part of my dissertation, we test the idea that earthquake triggering occurs when properties of a fault and/or its loading are modified by Coulomb failure stress changes that may be transient and oscillatory (i.e., dynamic) or permanent (i.e., static). We propose a triggering threshold failure stress change exists, above which the earthquake nucleation process begins although failure need not occur instantaneously. We test these ideas using data from the 1992 M 7.4 Landers earthquake and its aftershocks. Stress changes can be categorized as either dynamic (generated during the passage of seismic waves), static (associated with permanent fault offsets caused by fault slip) or complete (including both static and dynamic). We examine theoretically calculated Coulomb failure stress changes for the static (DeltaCFS) and complete (DeltaCFS(t)) cases, and statistically test for a correlation with spatially varying post-Landers seismicity rate changes. We find that directivity, which was required to model waveforms of the 1992 Landers earthquake, creates an asymmetry in mapped peak DeltaCFS(t). A similar asymmetry is apparent in the seismicity rate change map but not in the DeltaCFS map. Statistical analyses show that peak DeltaCFS(t) correlates as well or better with seismicity rate change as DeltaCFS, and qualitatively peak DeltaCFS(t) is the preferred model. (Abstract shortened by UMI.)

The source parameters, surface deformation and tectonic setting of three recent earthquakes: thessalonki (Greece), tabas-e-golshan (iran) and carlisle (u.k.).

PubMed

King, G; Soufleris, C; Berberian, M

1981-03-01

Abstract- Three earthquakes have been studied. These are the Thessaloniki earthquake of 20th June 1978 (Ms = 6.4, Normal faulting), the Tabase-Golshan earthquake of 16th September 1978 (Ms = 7.7 Thrust faulting) and the Carlisle earth-quake of 26th December 1979 (Mb = 5.0, Thrust faulting). The techniques employed to determine source parameters included field studies of SUP face deformation, fault breaks, locations of locally recorded aftershocks and teleseismic studies including joint hypocentral location, first motion methods and waveform modelling. It is clear that these techniques applied together provide more information than the same methods used separately. The moment of the Thessaloniki earthquake determined teleseismically (Force moment 5.2 times 10(25) dyne cm. Geometric moment 1.72 times 10(8) m(3) ) is an order of magnitude greater than that determined using field data (surface ruptures and aftershock depths) (Force moment 4.5 times 10(24) dyne cm. Geometric moment 0.16 times 10(8) m(3) ). It is concluded that for this earthquake the surface rupture only partly reflects the processes on the main rupture plane. This view i s supported by a distribution of aftershocks and damage which extends well outside the region of ground rupture. However, the surface breaks consistently have the same slip vector direction as the fault plane solutions suggesting that they are in this respect related to to the main faulting and are not superficial slumping. Both field studies and waveform studies suggest a low stress drop which may explain the relatively little damage and loss of life as a result of the Thessaloniki earthquake. In contrast, the teleseismic moment of the Tabas-e-Golshan earthquake (Force moment 4.4 times 10(26) dyne cm. Geometric moment 1.5 times 10(9) m(3) ) is similar t o that determined from field studies (Force moment 10.2 times 10(26) dyne cm. Geometric moment 3.4 times 10(9) m(3) ) and the damage and after-shock distributions clearly relate to the surface faulting. It h a s also been observed that high aftershock activity appears beneath gaps in the surface rupture system. The Carlisle earthquake (Force moment 9 times 10(23) dyne cm. Geometric moment 3 times 10(6) m(3) ) produced no surface ruptures. However, dislocation model-ling suggests that surface deformation will be visible on a first order levelling line which passes very close t o the epicentre. A well controlled fault plane solution, the first in the British Isles, derived from an aftershock study shows north-south compression. All three studied earthquakes occurred along major faults which had been reactivated in geological times. The fault on which the Tabas-e-Golshan earthquake occurred could have been identified a s active from evidence of Quaternary motion and previous smaller earthquakes. However, there were no perceptible events in the 12 months preceeding the catastrophic earthquake. In both Thessaloniki and Carlisle, significant foreshocks did occur within 6 months prior to the main shock*

Seismotectonic Models of the Three Recent Devastating SCR Earthquakes in India

NASA Astrophysics Data System (ADS)

Mooney, W. D.; Kayal, J.

2007-12-01

During the last decade, three devastating earthquakes, the Killari 1993 (Mb 6.3), Jabalpur 1997 (Mb 6.0) and the Bhuj 2001 (Mw 7.7) occurred in the Stable Continental Region (SCR), Peninsular India. First, the September 30, 1993 Killari earthquake (Mb 6.3) occurred in the Deccan province of central India, in the Latur district of Maharashtra state. The local geology in the area is obscured by the late Cretaceous-Eocene basalt flows, referred to as the Deccan traps. This makes it difficult to recognize the geological surface faults that could be associated with the Killari earthquake. The epicentre was reported at 18.090N and 76.620E, and the focal depth at 7 +/- 1 km was precisely estimated by waveform inversion (Chen and Kao, 1995). The maximum intensity reached to VIII and the earthquake caused a loss of about 10,000 lives and severe damage to property. The May 22, 1997 Jabalpur earthquake (Mb 6.0), epicentre at 23.080N and 80.060E, is a well studied earthquake in the Son-Narmada-Tapti (SONATA) seismic zone. A notable aspects of this earthquake is that it was the first significant event in India to be recorded by 10 broadband seismic stations which were established in 1996 by the India Meteorological Department (IMD). The focal depth was well estimated using the "converted phases" of the broadband seismograms. The focal depth was given in the lower crust at a depth of 35 +/- 1 km, similar to the moderate earthquakes reported from the Amazona ancient rift system in SCR of South America. Maximum MSK intensity of the Jabalpur earthquake reached to VIII in the MSK scale and this earthquake killed about 50 people in the Jabalpur area. Finally, the Bhuj earthquake (MW 7.7) of January 26, 2001 in the Gujarat state, northwestern India, was felt across the whole country, and killed about 20,000 people. The maximum intensity level reached X. The epicenter of the earthquake is reported at 23.400N and 70.280E, and the well estimated focal depth at 25 km. A total of about 3000 aftershocks (M> 1.0) were recorded until mid April, 2001. About 500 aftershocks (M>2.0) are well located; the epicenter map shows an aftershock cluster area, about 60 km x 30 km, between 70.0-70.60E and 23.3-23.60N; almost all the aftershocks occurred within the high intensity (IX) zone. The source area of the main shock and most of the aftershocks are at a depth range of 20-25 km. The fault-plane solutions suggest that the main shock originated at the base of the paleo-rift zone by a south dipping, hidden reverse fault; the rupture propagated both NE and NW. The aftershocks occurred by left-lateral strike-slip motion along the NE trending fault, compatible with the main shock solution, and by pure reverse to right-lateral, strike-slip motion along the NW trending conjugate fault. Understanding these earthquake sequences may shed new light in on the tectonics and active faults in the source regions.

Maximal radius of the aftershock zone in earthquake networks

NASA Astrophysics Data System (ADS)

Mezentsev, A. Yu.; Hayakawa, M.

2009-09-01

In this paper, several seismoactive regions were investigated (Japan, Southern California and two tectonically distinct Japanese subregions) and structural seismic constants were estimated for each region. Using the method for seismic clustering detection proposed by Baiesi and Paczuski [M. Baiesi, M. Paczuski, Phys. Rev. E 69 (2004) 066106; M. Baiesi, M. Paczuski, Nonlin. Proc. Geophys. (2005) 1607-7946], we obtained the equation of the aftershock zone (AZ). It was shown that the consideration of a finite velocity of seismic signal leads to the natural appearance of maximal possible radius of the AZ. We obtained the equation of maximal radius of the AZ as a function of the magnitude of the main event and estimated its values for each region.

Seismic tomography of the area of the 2010 Beni-Ilmane earthquake sequence, north-central Algeria.

PubMed

Abacha, Issam; Koulakov, Ivan; Semmane, Fethi; Yelles-Chaouche, Abd Karim

2014-01-01

The region of Beni-Ilmane (District of M'sila, north-central Algeria) was the site of an earthquake sequence that started on 14 May 2010. This sequence, which lasted several months, was triggered by conjugate E-W reverse and N-S dextral faulting. To image the crustal structure of these active faults, we used a set of 1406 well located aftershocks events and applied the local tomography software (LOTOS) algorithm, which includes absolute source location, optimization of the initial 1D velocity model, and iterative tomographic inversion for 3D seismic P- and S-wave velocities (and the Vp/Vs ratio), and source parameters. The patterns of P-wave low-velocity anomalies correspond to the alignments of faults determined from geological evidence, and the P-wave high-velocity anomalies may represent rigid blocks of the upper crust that are not deformed by regional stresses. The S-wave low-velocity anomalies coincide with the aftershock area, where relatively high values of Vp/Vs ratio (1.78) are observed compared with values in the surrounding areas (1.62-1.66). These high values may indicate high fluid contents in the aftershock area. These fluids could have been released from deeper levels by fault movements during earthquakes and migrated rapidly upwards. This hypothesis is supported by vertical sections across the study area show that the major Vp/Vs anomalies are located above the seismicity clusters.

Coseismic and Postseismic Deformation Following the 2011 Mw 9.0 Tohoku Earthquake and its Mw 7.9 Aftershock: Searching for Fault-localized Relaxation of Coseismic Stress Increments

NASA Astrophysics Data System (ADS)

Wang, F.; Bevis, M. G.; Blewitt, G.; Gomez, D.

2017-12-01

We study the postseismic transient displacements following the 2011 Mw 9.0 Tohoku earthquake using the Nevada Geodetic Laboratory's daily and 5-minute interval PPP solutions for 1,272 continuous GPS stations in Japan, with particular emphasis on the early transient displacements of these stations. One significant complication is the Mw 7.9 aftershock that occurred just 29.3 minutes after the main shock, since the coseismic (and postseismic) displacements driven by the aftershock are superimposed on the postseismic transients driven by the main shock. We address the question of whether or not the stresses induced by the Mw 9.0 main shock were relaxed by any major faults within Japan. The notion is that significant stress relaxation which is localized on a fault system should be manifested in the spatial pattern of the postseismic transient displacement field in the vicinity of that system. This would provide a basis for distinguishing between faults that engage in stick-slip behavior and those that creep instead. The distinction is important in that it has implications for the seismic risk associated with upper plate faulting. We will make the case that we do detect localized fault creeping in response to the coseismic stress field produced by the Mw 9 event.

Real time drilling mud gas response to small-moderate earthquakes in Wenchuan earthquake Scientific Drilling Hole-1 in SW China

NASA Astrophysics Data System (ADS)

Gong, Zheng; Li, Haibing; Tang, Lijun; Lao, Changling; Zhang, Lei; Li, Li

2017-05-01

We investigated the real time drilling mud gas of the Wenchuan earthquake Fault Scientific Drilling Hole-1 and their responses to 3918 small-moderate aftershocks happened in the Longmenshan fault zone. Gas profiles for Ar, CH4, He, 222Rn, CO2, H2, N2, O2 are obtained. Seismic wave amplitude, energy density and static strain are calculated to evaluate their power of influence to the drilling site. Mud gases two hours before and after each earthquake are carefully analyzed. In total, 25 aftershocks have major mud gas response, the mud gas concentrations vary dramatically immediately or minutes after the earthquakes. Different gas species respond to earthquakes in different manners according to local lithology encountered during the drill. The gas variations are likely controlled by dynamic stress changes, rather than static stress changes. They have the seismic energy density between 10-5 and 1.0 J/m3 whereas the static strain are mostly less than 10-8. We suggest that the limitation of the gas sources and the high hydraulic diffusivity of the newly ruptured fault zone could have inhibited the drilling mud gas behaviors, they are only able to respond to a small portion of the aftershocks. This work is important for the understanding of earthquake related hydrological changes.

Stress triggering of the 1994 M = 6.7 Northridge, California, Earthquake by its predecessors

USGS Publications Warehouse

Stein, R.S.; King, G.C.P.; Lin, J.

1994-01-01

A model of stress transfer implies that earthquakes in 1933 and 1952 increased the Coulomb stress toward failure at the site of the 1971 San Fernando earthquake. The 1971 earthquake in turn raised stress and produced aftershocks at the site of the 1987 Whittier Narrows and 1994 Northridge ruptures. The Northridge main shock raised stress in areas where its aftershocks and surface faulting occurred. Together, the earthquakes with moment magnitude M ??? 6 near Los Angeles since 1933 have stressed parts of the Oak Ridge, Sierra Madre, Santa Monica Mountains, Elysian Park, and Newport-Inglewood faults by more than 1 bar. Although too small to cause earthquakes, these stress changes can trigger events if the crust is already near failure or advance future earthquake occurrence if it is not.

Near-field investigations of the Landers earthquake sequence, April to July 1992

USGS Publications Warehouse

Sieh, K.; Jones, L.; Hauksson, E.; Hudnut, K.; Eberhart-Phillips, D.; Heaton, T.; Hough, S.; Hutton, K.; Kanamori, H.; Lilje, A.; Lindvall, Scott; McGill, S.F.; Mori, J.; Rubin, C.; Spotila, J.A.; Stock, J.; Thio, H.K.; Treiman, J.; Wernicke, B.; Zachariasen, J.

1993-01-01

The Landers earthquake, which had a moment magnitude (Mw) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its Mw 6.1 preshock and Mw 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

Use of QuakeSim and UAVSAR for Earthquake Damage Mitigation and Response

NASA Technical Reports Server (NTRS)

Donnellan, A.; Parker, J. W.; Bawden, G.; Hensley, S.

2009-01-01

Spaceborne, airborne, and modeling and simulation techniques are being applied to earthquake risk assessment and response for mitigation from this natural disaster. QuakeSim is a web-based portal for modeling interseismic strain accumulation using paleoseismic and crustal deformation data. The models are used for understanding strain accumulation and release from earthquakes as well as stress transfer to neighboring faults. Simulations of the fault system can be used for understanding the likelihood and patterns of earthquakes as well as the likelihood of large aftershocks from events. UAVSAR is an airborne L-band InSAR system for collecting crustal deformation data. QuakeSim, UAVSAR, and DESDynI (following launch) can be used for monitoring earthquakes, the associated rupture and damage, and postseismic motions for prediction of aftershock locations.

Stress shadows - a controversial topic

NASA Astrophysics Data System (ADS)

Lasocki, Stanislaw; Karakostas, Vassilis G.; Papadimitriou, Eletheria E.; Orlecka-Sikora, Beata

2010-05-01

The spatial correlation between the positive Coulomb stress changes and the subsequent seismic activity has been firmly confirmed in many recent studies. If, however, the static stress transfer is a consistent expression of interaction between earthquakes one should also observe a decrease of the activity in the zones of negative stress changes. Instead, the existence of stress shadows is poorly evidenced and may be questioned. We tested the influence of the static stress changes associated with the coseismic slip of the 1995 Mw6.5 Kozani-Grevena (Greece) earthquake on locations of its aftershocks. The study was based on a detailed slip model for the main shock and accurate locations and reliable fault plane solutions of an adequate number of the aftershocks. We developed a statistical testing method, which tested whether the proportions of aftershocks located inside areas determined by a selected criterion on the static stress change could be attained if there were no effect of the stress change due to the main shock on aftershock locations. The areas of stress change were determined at the focus of every aftershock. The distribution of test statistic was constructed with the use of a two-dimensional nonparametric, kernel density estimator of the reference epicenter distribution. The tests highly confidently indicated a rise in probability to locate aftershocks inside areas of positive static stress change, which supported the hypothesis on the triggering effect in these areas. Furthermore, it was evidenced that a larger stress increase caused a stronger triggering effect. The analysis, however, did not evidence the existence of stress shadows inside areas of negative stress change. Contrary to expectations, the tests indicated a significant increase of the probability of event location in the areas of a stress decrease of more than or equal to 5.0 and 10.0 bar. It turned out that for areas of larger absolute stress change this probability increased regardless of the sign of the change though distinctly more in areas of positive than of negative change. In the case of seismicity accompanying underground mining exploitation the coseismic stress changes expressed in terms of the Coulomb failure function are at least of one order smaller than those for earthquakes. Furthermore, they are only a small component of the total stress field variations in mining rockmass, which are mainly controlled by the mining process. Nevertheless, our studies of the induced seismicity in the Rudna mine in the Legnica-Głogow Copper District in Poland showed that the influence of the Coulomb stress changes on locations of subsequent events was statistically significant. We analyzed series of seismic events quantifying the triggering and inhibiting effect by the proportion of events in the series whose locations were consistent with the stress increased and stress decreased zones, respectively. It was found out that more than 60 per-cent of the analyzed seismic events occurred in areas where stress was enhanced due to the occurrence of previous events. The significance of this result was determined by comparing it with 2000 results of the same analysis carried out on the random permutations of the original series of events. The test indicated that the locations in positive stress changes areas were preferred statistically significantly when the stress changes exceeded 0.05 bar. However, no statistically significant inhibiting effect of negative static stress changes, within the considered range of these changes, was ascertained. Here we present details of these two studies and discuss possible reasons behind the negative conclusions on the existence of stress shadows.

3D P-wave velocity structure of the crust and relocation of earthquakes in the Lushan, China, source area

NASA Astrophysics Data System (ADS)

Yu, Xiangwei; Wang, Xiaona; Zhang, Wenbo

2016-04-01

Many researchers have investigated the Lushan source area with geological and geophysical approaches since the 2013 Lushan, China, earthquake happened. Compared with the previous tomographic studies, we have used a much large data set and an updated tomographic method to determine a small scale three-dimensional P wave velocity structure with spatial resolution less than 5km, which plays the important role for understanding the deep structure and the genetic mechanism beneath the Lushan area. The double difference seismic tomography method is applied to 50,711 absolute first arrival P wave arrival times and 7,294,691 high quality relative P arrival times of 5,285 events of Lushan seismic sequence to simultaneously determine the detailed crustal 3D P wave velocity structure and the hypocenter parameters in the Lushan seismic area. This method takes account of the path anomaly biases explicitly by making full use of valuable information of seismic wave propagation jointly with absolute and relative arrival time data. Our results show that the Lushan mainshock locates at 30.28N, 103.98E, with the depth of 16.38km. The front edge of aftershock in the northeast of mainshock present a spade with a steep dip angle, the aftershocks' extended length is about 12km. In the southwest of Lushan mainshock, the front edge of aftershock in low velocity zone slope gently, the aftershocks' extended length is about 23km. Our high-resolution tomographic model not only displays the general features contained in the previous models, but also reveals some new features. The Tianquan, Shuangshi and Daguan line lies in the transition zone between high velocity anomalies to the southeast and low velocity anomalies to the northwest at the ground surface. An obvious high-velocity anomaly is visible in Daxing area. With the depth increasing, Baoxing high velocity anomaly extends to Lingguan, while the southeast of the Tianquan, Shuangshi and Daguan line still shows low velocity. The high-velocity anomalies beneath Baoxing and Daxing connect each other in 10km depth, which makes the contrast between high and low velocity anomalies more sharp. Above 20km depth the velocity structure in southwest and northeast segment of mainshock shows a big difference: low-velocity anomalies are dominated the southwest segment, while high-velocity anomalies rule the northeast segment. Lushan aftershocks in southwest are distributed in low-velocity anomalies or the transition belt: the footwall represents low-velocity anomalies, while the hanging wall shows high-velocity anomalies. The northeastern aftershocks are distributed at the boundary between high-velocity anomalies in Baoxing and Daxing area. The P wave velocity structure of Lushan seismic area shows obviously lateral heterogeneity. The P wave velocity anomalies represent close relationship with topographic relief and geological structure. In Baoxingarea the complex rocks correspond obvious high-velocity anomalies extending down to 15km depth，while the Cenozoic rocks are correlated with low-velocity anomalies. Lushan mainshock locates at the leading edge of a low-velocity anomaly surrounded by the Baoxing and Daxing high-velocity anomalies. The main seismogenic layer dips to northwest. Meanwhile, a recoil seismic belt dips to southeast above the main seismogenic layer exists at the lower boundary of Baoxing high-velocity anomaly.

The DSeis Project: Drilling into Seismogenic zones of M2.0 to M5.5 earthquakes in South African gold mines

NASA Astrophysics Data System (ADS)

Yabe, Y.; Ogasawara, H.; Ito, T.; van Aswegen, G.; Durrheim, R. J.; Cichowicz, A.; Onstott, T. C.; Kieft, T. L.; Boettcher, M. S.; Wiemer, S.; Ziegler, M.; Shapiro, S. A.; Gupta, H. K.; Dight, P.

2017-12-01

The DSeis project under ICDP consists of drilling in three mines; MK, TT and C4 mines. Common scientific targets among them are the stress state and the microstructure in the seismogenic zone. In addition to these targets, specific targets in individual mines are detailed below. A M5.5 earthquake occurred beneath the MK mine on 5 August 2014. The hypocenter of this event was 5km depth from the surface. In contrast to the normal faulting of induced earthquakes in mining horizons (<4km depth), the M5.5 event was a strike-slip one with an N-S striking, sub-vertical nodal plane along which aftershocks aligned. Aftershocks extend up to 3.5km depth. We established a drilling site at 2.8km depth in the mine, from where two boreholes 800m-long penetrate into the areas of high and low aftershock densities. Targets of these drilling are 1) to investigate a depth variation in the stress state from the normal faulting to the strike-slip one, 2) to know what controls the spatial variation in the aftershock activity, and 3) to explore a limit of deep life that might be trapped in Archean sediments. Our site in the TT mine is 50m under the hypocenter of a M3.2 earthquake which occurred on 28 January 2017 at 3.6km depth. Although aftershock activity recorded by the seismic network operated by the mine is low, the source fault looks to extend along or parallel to a pre-existing, N-S striking fault. Three boreholes go through the fault at the hypocenter and the northern and the southern margins of the fault to compare the stress states and the microfracture distributions. Further, monitoring of microseismicity down to M -4 and geochemistry is planned to evaluate how much is a ratio of microseismicity associated with creation of new fractures. In the C4 mine, there was the site of a previous project, in which the microseismicity monitoring and the stress measurement by the CCBO technique were carried out. A M2.8 earthquake occurred 1 year after the CCBO and its hypocenter was only 100m away from the CCBO site. Due to little mining activity in the source region, the stress state just after the M2.8 event should be preserved. We will measure the stress again. Damage zones that evolved quasi-statically were seen by the microseismicity monitoring. Drilling into these zones would provide a clue to see a difference between faults evolved dynamically and quasi-statically.

The global aftershock zone

USGS Publications Warehouse

Parsons, Thomas E.; Margaret Segou,; Warner Marzocchi,

2014-01-01

The aftershock zone of each large (M ≥ 7) earthquake extends throughout the shallows of planet Earth. Most aftershocks cluster near the mainshock rupture, but earthquakes send out shivers in the form of seismic waves, and these temporary distortions are large enough to trigger other earthquakes at global range. The aftershocks that happen at great distance from their mainshock are often superposed onto already seismically active regions, making them difficult to detect and understand. From a hazard perspective we are concerned that this dynamic process might encourage other high magnitude earthquakes, and wonder if a global alarm state is warranted after every large mainshock. From an earthquake process perspective we are curious about the physics of earthquake triggering across the magnitude spectrum. In this review we build upon past studies that examined the combined global response to mainshocks. Such compilations demonstrate significant rate increases during, and immediately after (~ 45 min) M > 7.0 mainshocks in all tectonic settings and ranges. However, it is difficult to find strong evidence for M > 5 rate increases during the passage of surface waves in combined global catalogs. On the other hand, recently published studies of individual large mainshocks associate M > 5 triggering at global range that is delayed by hours to days after surface wave arrivals. The longer the delay between mainshock and global aftershock, the more difficult it is to establish causation. To address these questions, we review the response to 260 M ≥ 7.0 shallow (Z ≤ 50 km) mainshocks in 21 global regions with local seismograph networks. In this way we can examine the detailed temporal and spatial response, or lack thereof, during passing seismic waves, and over the 24 h period after their passing. We see an array of responses that can involve immediate and widespread seismicity outbreaks, delayed and localized earthquake clusters, to no response at all. About 50% of the catalogs that we studied showed possible (localized delayed) remote triggering, and ~ 20% showed probable (instantaneous broadly distributed) remote triggering. However, in any given region, at most only about 2–3% of global mainshocks caused significant local earthquake rate increases. These rate increases are mostly composed of small magnitude events, and we do not find significant evidence of dynamically triggered M > 5 earthquakes. If we assume that the few observed M > 5 events are triggered, we find that they are not directly associated with surface wave passage, with first incidences being 9–10 h later. We note that mainshock magnitude, relative proximity, amplitude spectra, peak ground motion, and mainshock focal mechanisms are not reliable determining factors as to whether a mainshock will cause remote triggering. By elimination, azimuth, and polarization of surface waves with respect to receiver faults may be more important factors.

Kinect Technology Game Play to Mimic Quake Catcher Network (QCN) Sensor Deployment During a Rapid Aftershock Mobilization Program (RAMP)

NASA Astrophysics Data System (ADS)

Kilb, D. L.; Yang, A.; Rohrlick, D.; Cochran, E. S.; Lawrence, J.; Chung, A. I.; Neighbors, C.; Choo, Y.

2011-12-01

The Kinect technology allows for hands-free game play, greatly increasing the accessibility of gaming for those uncomfortable using controllers. How it works is the Kinect camera transmits invisible near-infrared light and measures its "time of flight" to reflect off an object, allowing it to distinguish objects within 1 centimeter in depth and 3 mm in height and width. The middleware can also respond to body gestures and voice commands. Here, we use the Kinect Windows SDK software to create a game that mimics how scientists deploy seismic instruments following a large earthquake. The educational goal of the game is to allow the players to explore 3D space as they learn about the Quake Catcher Network's (QCN) Rapid Aftershock Mobilization Program (RAMP). Many of the scenarios within the game are taken from factual RAMP experiences. To date, only the PC platform (or a Mac running PC emulator software) is available for use, but we hope to move to other platforms (e.g., Xbox 360, iPad, iPhone) as they become available. The game is written in programming language C# using Microsoft XNA and Visual Studio 2010, graphic shading is added using High Level Shader Language (HLSL), and rendering is produced using XNA's graphics libraries. Key elements of the game include selecting sensor locations, adequately installing the sensor, and monitoring the incoming data. During game play aftershocks can occur unexpectedly, as can other problems that require attention (e.g., power outages, equipment failure, and theft). The player accrues points for quickly deploying the first sensor (recording as many initial aftershocks as possible), correctly installing the sensors (orientation with respect to north, properly securing, and testing), distributing the sensors adequately in the region, and troubleshooting problems. One can also net points for efficient use of game play time. Setting up for game play in your local environment requires: (1) the Kinect hardware ( $145); (2) a computer with a Windows operating system (Mac users can use a Windows emulator); and (3) our free QCN game software (available from http://quakeinfo.ucsd.edu/ dkilb/WEB/QCN/Downloads.html).

Seismic and Aseismic Slip Surrounding the 2014 Mw 8.2 Pisagua (Chile) rupture

NASA Astrophysics Data System (ADS)

Shrivastava, M. N.; Gonzalez, G.; Salazar, P.; Moreno, M.; Baez, J. C.

2017-12-01

On April 1 2014, the Mw 8.2 Pisagua earthquake occurred in a part of standing seismic gap northern Chile. We inverted the cGPS time series for coseismic slip of mainshock and big aftershock Mw 7.6 on April 3, 2014 individually. The big aftershock Mw 7.6 occurred in south of the coseismic slip region of main shock. The coseismic slip model suggest that it has an asperity of single patch has slipped during the Pisagua earthquake and confined in the inter-seismically highly locked region. It seems that Iquique ridge fragmented northern Chile seismic gap region. It could provide small asperity, which has broken partial discretely. The spatial distribution of afterslip from the inversion of cGPS time series is consistent and appears in two patches to cover north and south of the mainshock coseismic slip patch. The afterslip patches suggest that the maximum cumulative afterslip 50cm is located at ˜19.0°S to the north of the mainshock rupture zone and down dip 38km. The estimated postseismic moment released in the first 60-days of afterslip is equivalent to an earthquake of Mw 7.43. The 60-days 306 aftershocks (Mw>3.0) has confined in the mainshock slip and southern afterslip region. With the afterslip model 60-days, it concludes that aftershocks distribution not only depends on Coulomb stress changes but also the locking degree of region/heterogeneity of the plate interface. Seismic slip is restricted in the southern locked region, but aseismic slip in the northern low locking region. It means the southern portion of the northern Chile is still non-ruptured. It can break with great megathrust earthquake in future. Therefore, it is essential to scrutinize the slip deficit by using a space geodetic technique to assess earthquake potential. The northern patch of the afterslip is purely aseismic as stable sliding confined in the low locking region, however the southern patch afterslip seismically as unstable stick-slip dominating. It suggests that afterslip illuminates the velocity strengthening in the northern patch and velocity weakening in the southern patch.

Frictional constraints on crustal faulting

USGS Publications Warehouse

Boatwright, J.; Cocco, M.

1996-01-01

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate- and state-dependent friction law, where we differentiate velocity-weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity-strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two "intermediate" fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This "mixed" frictional behavior can be obtained from a sufficiently heterogenous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

Statistical discrimination of induced and tectonic earthquake sequences in Central and Eastern US based on waveform detected catalogs

NASA Astrophysics Data System (ADS)

Meng, X.; Daniels, C.; Smith, E.; Peng, Z.; Chen, X.; Wagner, L. S.; Fischer, K. M.; Hawman, R. B.

2015-12-01

Since 2001, the number of M>3 earthquakes increased significantly in Central and Eastern United States (CEUS), likely due to waste-water injection, also known as "induced earthquakes" [Ellsworth, 2013]. Because induced earthquakes are driven by short-term external forcing and hence may behave like earthquake swarms, which are not well characterized by branching point-process models, such as the Epidemic Type Aftershock Sequence (ETAS) model [Ogata, 1988]. In this study we focus on the 02/15/2014 M4.1 South Carolina and the 06/16/2014 M4.3 Oklahoma earthquakes, which likely represent intraplate tectonic and induced events, respectively. For the South Carolina event, only one M3.0 aftershock is identified by the ANSS catalog, which may be caused by a lack of low-magnitude events in this catalog. We apply a recently developed matched filter technique to detect earthquakes from 02/08/2014 to 02/22/2014 around the epicentral region. 15 seismic stations (both permanent and temporary USArray networks) within 100 km of the mainshock are used for detection. The mainshock and aftershock are used as templates for the initial detection. Newly detected events are employed as new templates, and the same detection procedure repeats until no new event can be added. Overall we have identified more than 10 events, including one foreshock occurred ~11 min before the M4.1 mainshock. However, the numbers of aftershocks are still much less than predicted with the modified Bath's law. For the Oklahoma event, we use 1270 events from the ANSS catalog and 182 events from a relocated catalog as templates to scan through continuous recordings 3 days before to 7 days after the mainshock. 12 seismic stations within the vicinity of the mainshock are included in the study. After obtaining more complete catalogs for both sequences, we plan to compare the statistical parameters (e.g., b, a, K, and p values) between the two sequences, as well as their spatial-temporal migration pattern, which may shed light on the underlying physics of tectonic and induced earthquakes.

Analysis of the Seismic Events Apparently Associated with the 3 September 2017 DPRK Declared Nuclear Explosion

NASA Astrophysics Data System (ADS)

Walter, W. R.; Dodge, D. A.; Ichinose, G.; Myers, S. C.; Ford, S. R.; Pitarka, A.; Pyle, M. L.; Pasyanos, M.; Matzel, E.; Rodgers, A. J.; Mellors, R. J.; Hauk, T. F.; Kroll, K.

2017-12-01

On September 3, 2017, an mb 6.3 seismic event was reported by the USGS in the vicinity of the DPRK nuclear test site at Punggye-ri. Shortly afterwards DPRK declared it had conducted a nuclear explosion. The seismic signals indicate this event is roughly an order of magnitude larger than the largest of the previous five DPRK declared nuclear tests. In addition to its size, this explosion was different from previous DPRK tests in being associated with a number of additional seismic events. Approximately eight and a half minutes after the explosion a seismic event reported as ML 4.0 by the USGS occurred. Regional waveform modeling indicated this event had a collapse mechanism (e.g. Ichinose et al., 2017, written communication). On September 23 and again on October 12, 2017, seismic events were reported near the DPRK test site by the USGS and the CTBTO (on 9/23/17 two events: USGS ML 3.6 and USGS ML 2.6; and on 10/12/17 one event: USGS mb(Lg) 2.9). Aftershocks following underground nuclear testing are expected, though at much lower magnitudes and rates than for comparably sized earthquakes. This difference in aftershock production has been proposed by Ford and Walter (2010), and others as a potential source-type discriminant. Seismic signals from the collapse of cavities formed by underground nuclear testing have also been previously observed. For example, the mb 5.7 nuclear test ATRISCO in Nevada in 1982 was followed twenty minutes later by a collapse with an mb of 4.0. Here we examine the seismic characteristics of nuclear tests, post-test collapses and post-test aftershocks from both the former Nevada test site and the DPRK test site to better understand the differences between these different source-type signals. In particular we look at discriminants such as P/S ratios, to see if there are unique characteristics to post-test collapses and aftershocks. Finally, we apply correlation methods to continuous data at regional stations to look for additional seismic signals that might have an apparent association with the DPRK nuclear testing, post-testing collapses and post-test induced seismicity.

Employing the Quake-Catcher Network (QCN) to Investigate Site Effects in Christchurch, New Zealand Using the κ Method

NASA Astrophysics Data System (ADS)

Neighbors, C.; Liao, E. J.; Cochran, E. S.; Chung, A. I.; Lawrence, J.; Kaiser, A. E.; Fry, B.; Christensen, C. M.

2011-12-01

The 3 September 2010 Mw 7.1 Darfield earthquake was felt over 900 km from the source. The maximum felt-intensity was estimated to be at Modified Mercalli Intensity (MMI) 9 and measured near-field accelerations were found to exceed 1 g. The mainshock damaged or destroyed over 100,000 buildings and spatially variable effects (such as liquefaction, slumping, and amplification) were observed throughout the city of Christchurch. Following the mainshock, a vigorous aftershock sequence has continued for months in the region. To record the aftershocks for early warning and other hazard mitigation efforts, a network of over 192 low-cost, 14-bit accelerometers were deployed in local buildings as part of the Quake-Catcher Network Rapid Aftershock Mobilization Project (RAMP). With a large number of sensors covering a city area of ~300 km2, the RAMP collected vastly more data and at a finer scale relative to the deployment of costly traditional broadband sensors. Recent comparison of the signal-to-noise quality of the 14-bit QCN sensors to the strong motion 24-bit New Zealand GeoNet sensors show similar responses. Initial analyses of the data show that aftershocks of magnitude >M4.5 within 30 km of the hypocentral distance were well recorded by QCN sensors. Utilizing the dense coverage, we investigate local site amplification by analyzing the spectra decay parameter, kappa (κ). Following the routine outlined in Douglas et al. (2010, Pure Appl. Geophys.), whereby a 5-sec S-wave window is used to calculate the Fourier spectra and κ for each station, investigations of κ values between stations are used to estimate site conditions on seismic wave behavior at each location. Results from this study will be compared to work performed by New Zealand researchers using other methods to calculate site response, such as the spectral ratio method. Together, these studies will highlight areas of the Christchurch region that may be more susceptible to ground shaking, which can be used to mitigate loss prior to future large earthquakes.

Location of early aftershocks of the 2004 Mid-Niigata Prefecture Earthquake (M = 6.8) in central Japan using seismogram envelopes as templates

NASA Astrophysics Data System (ADS)

Kosuga, M.

2013-12-01

The location of early aftershocks is very important to obtain information of mainshock fault, however, it is often difficult due to the long-lasting coda wave of mainshock and successive occurrence of afterrshocks. To overcome this difficulty, we developed a method of location using seismogram envelopes as templates, and applied the method to the early aftershock sequence of the 2004 Mid-Niigata Prefecture (Chuetsu) Earthquake (M = 6.8) in central Japan. The location method composes of three processes. The first process is the calculation of cross-correlation coefficients between a continuous (target) and template envelopes. We prepare envelopes by taking the logarithm of root-mean-squared amplitude of band-pass filtered seismograms. We perform the calculation by shifting the time window to obtain a set of cross-correlation values for each template. The second process is the event detection (selection of template) and magnitude estimate. We search for the events in descending order of cross-correlation in a time window excluding the dead times around the previously detected events. Magnitude is calculated by the amplitude ratio of target and template envelopes. The third process is the relative event location to the selected template. We applied this method to the Chuetsu earthquake, a large inland earthquake with extensive aftershock activity. The number of detected events depends on the number of templates, frequency range, and the threshold value of cross-correlation. We set the threshold as 0.5 by referring to the histogram of cross-correlation. During a period of one-hour from the mainshock, we could detect more events than the JMA catalog. The location of events is generally near the catalog location. Though we should improve the methods of relative location and magnitude estimate, we conclude that the proposed method works adequately even just after the mainshock of large inland earthquake. Acknowledgement: We thank JMA, NIED, and the University of Tokyo for providing arrival time data, and waveform data. This work was supported by JSPS KAKENHI Grant Number 23540487.

Joint inversion of teleseismic body-waves and geodetic data for the Mw6.8 aftershock of the Balochistan earthquake with refined epicenter location

NASA Astrophysics Data System (ADS)

Wei, S.; Wang, T.; Jonsson, S.; Avouac, J. P.; Helmberger, D. V.

2014-12-01

Aftershocks of the 2013 Balochistan earthquake are mainly concentrated along the northeastern end of the mainshock rupture despite of much larger coseismic slip to the southwest. The largest event among them is an Mw6.8 earthquake which occurred three days after the mainshock. A kinematic slip model of the mainshock was obtained by joint inversion of the teleseismic body-waves and horizontal static deformation field derived from remote sensing optical and SAR data, which is composed of seven fault segments with gradually changing strikes and dips [Avouac et al., 2014]. The remote sensing data provide well constraints on the fault geometry and spatial distribution of slip but no timing information. Meanwhile, the initiation of the teleseismic waveform is very sensitive to fault geometry of the epicenter segment (strike and dip) and spatial slip distribution but much less sensitive to the absolute location of the epicenter. The combination of the two data sets allows a much better determination of the absolute epicenter location, which is about 25km to the southwest of the NEIC epicenter location. The well located mainshock epicenter is used to establish path calibrations for teleseismic P-waves, which are essential for relocating the Mw6.8 aftershock. Our grid search shows that the refined epicenter is located right at the northeastern end of the mainshock rupture. This is confirmed by the SAR offsets calculated from images acquired after the mainshock. The azimuth and range offsets display a discontinuity across the rupture trace of the mainshock. Teleseismic only and static only, as well as joint inversions all indicate that the aftershock ruptured an asperity with 25km along strike and range from 8km to 20km in depth. The earthquake was originated in a positive Coulomb stress change regime due to the mainshock and has complementary slip distribution to the mainshock rupture at the northeastern end, suggesting that the entire seismic generic zone in the crust was ruptured during the earthquake sequence.

Site effects in Avcilar, West of Istanbul, Turkey, from strong- and weak-motion data

USGS Publications Warehouse

Ozel, O.; Cranswick, E.; Meremonte, M.; Erdik, M.; Safak, E.

2002-01-01

Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during the 17 August 1999 I??zmit earthquake, whose epicenter was roughly 90 km east of the city. Most of the fatalities and damage occurred in the suburb of Avcilar that is 20 km further west of the epicenter than the city proper. To investigate this pattern of damage, the U.S. Geological Survey, in cooperation with the Kandilli Observatory and Earthquake Research Institute (KOERI), deployed portable digital seismographs at seven free-field sites in western Istanbul, to record aftershocks during the period from 24 August to 2 September. The primary objective of this deployment was to study the site effects by comparing the aftershock ground motions recorded at sites inside and outside the damaged area, and to correlate site effects with the distribution of the damaged buildings. In addition to using weak-motion data, mainshock and aftershock acceleration records from the KOERI permanent strong-motion array were also used in estimating the site effects. Site effects were estimated using S waves from both types of records. For the weak-motion data set, 22 events were selected according to the criteria of signal-to-noise ratio (S/N ratio) and the number of stations recording the same event. The magnitudes of these events ranged from 3.0 to 5.2. The acceleration data set consisted of 12 events with magnitudes ranging from 4.3 to 5.8 and included two mainshock events. Results show that the amplifying frequency band is, in general, less than 4 Hz, and the physical properties of the geologic materials are capable of amplifying the motions by a factor of 5-10. In this frequency band, there is a good agreement among the spectral ratios obtained from the two mainshocks and their aftershocks. The damage pattern for the 17 August I??zmit earthquake is determined by several factors. However, our study suggests that the site effects in Avcilar played an important role in contributing to the damage.

Stress Interactions Between the 1976 Magnitude 7.8 Tangshan Earthquake and Adjacent Fault Systems in Northern China

NASA Astrophysics Data System (ADS)

Zhang, Z.; Lin, J.; Chen, Y. J.

2004-12-01

The 28 July 1976 ML = 7.8 Tangshan earthquake struck a highly populated metropolitan center in northern China and was one of the most devastating earthquakes in modern history. Its occurrence has significantly changed the Coulomb stresses on a complex network of strike-slip, normal, and thrust faults in the region, potentially heightened the odds of future earthquakes on some of these fault segments. We have conducted a detailed analysis of the 3D stress effects of the Tangshan earthquake on its neighboring faults, the relationship between stress transfer and aftershock locations, and the implications for future seismic hazard in the region. Available seismic and geodetic data, although limited, indicate that the Tangshan main shock sequence is composed of complex rupture on 2-3 fault segments. The dominant rupture mode is right-lateral strike-slip on two adjoining sub-segments that strike N5¡aE and N35¡aE, respectively. We calculated that the Tangshan main shock sequence has increased the Coulomb failure stress by more than 1 bar in the vicinity of the Lunanxian district to the east, where the largest aftershock (ML = 7.1) occurred 15 hours after the Tangshan main event. The second largest aftershock (ML = 6.8) occurred on the Ninghe fault to the southwest of the main rupture, in a transitional region between the calculated Coulomb stress increase and decrease. The majority of the ML > 5.0 aftershocks also occurred in areas of calculated Coulomb stress increase. Our analyses further indicate that the Coulomb stress on portions of other fault segments, including the Leting and Lulong fault to the east and Yejito fault to the north, may also have been increased. Thus it is critical to obtain estimates of earthquake repeat times on these and other tectonic faults and to acquire continuous GPS and space geodetic measurements. Investigation of stress interaction and earthquake triggering in northern China is not only highly societal relevant but also important for advancing our understanding of the fundamental characteristics of earthquakes in regions of diffuse continental deformation.

Finite-fault inversion of the Mw 5.9 2012 Emilia-Romagna earthquake (Northern Italy) using aftershocks as near-field Green's function approximations

NASA Astrophysics Data System (ADS)

Causse, Mathieu; Cultrera, Giovanna; Herrero, André; Courboulex, Françoise; Schiappapietra, Erika; Moreau, Ludovic

2017-04-01

On May 29, 2012 occurred a Mw 5.9 earthquake in the Emilia-Romagna region (Po Plain) on a thrust fault system. This shock, as well as hundreds of aftershocks, were recorded by 10 strong motion stations located less than 10 km away from the rupture plane, with 4 stations located within the surface rupture projection. The Po Plain is a very large EW trending syntectonic alluvial basin, delimited by the Alps and Apennines chains to the North and South. The Plio-Quaternary sedimentary sequence filling the Po Plain is characterized by an uneven thickness, ranging from several thousands of meters to a few tens of meters. This particular context results especially in a resonance basin below 1 Hz and strong surface waves, which makes it particularly difficult to model wave propagation and hence to obtain robust images of the rupture propagation. This study proposes to take advantage of the large set of recorded aftershocks, considered as point sources, to model wave propagation. Due to the heterogeneous distribution of the aftershocks on the fault plane, an interpolation technique is proposed to compute an approximation of the Green's function between each fault point and each strong motion station in the frequency range [0.2-1Hz]. We then use a Bayesian inversion technique (Monte Carlo Markov Chain algorithm) to obtain images of the rupture propagation from the strong motion data. We propose to retrieve the slip distribution by inverting the final slip value at some control points, which are allowed to move on the fault plane, and by interpolating the slip value between these points. We show that the use of 5 control points to describe the slip, coupled with the hypothesis of spatially constant rupture velocity and rise-time (that is 18 free source parameters), results in a good level of fit with the data. This indicates that despite their complexity, the strong motion data can be properly modeled up to 1 Hz using a relatively simple rupture. The inversion results also reveal that the rupture propagated slowly, at a speed of about 45% of the shear wave velocity.

Pore Pressure Pulse Drove the 2012 Emilia (Italy) Series of Earthquakes

NASA Astrophysics Data System (ADS)

Pezzo, Giuseppe; De Gori, Pasquale; Lucente, Francesco Pio; Chiarabba, Claudio

2018-01-01

The 2012 Emilia earthquakes sequence is the first debated case in Italy of destructive event possibly induced by anthropic activity. During this sequence, two main earthquakes occurred separated by 9 days on contiguous thrust faults. Scientific commissions engaged by the Italian government reported complementary scenarios on the potential trigger mechanism ascribable to exploitation of a nearby oil field. In this study, we combine a refined geodetic source model constrained by precise aftershock locations and an improved tomographic model of the area to define the geometrical relation between the activated faults and investigate possible triggering mechanisms. An aftershock decay rate that deviates from the classical Omori-like pattern and Vp/Vs changes along the fault system suggests that natural pore pressure pulse drove the space-time evolution of seismicity and the activation of the second main shock.

Computational Software for Fitting Seismic Data to Epidemic-Type Aftershock Sequence Models

NASA Astrophysics Data System (ADS)

Chu, A.

2014-12-01

Modern earthquake catalogs are often analyzed using spatial-temporal point process models such as the epidemic-type aftershock sequence (ETAS) models of Ogata (1998). My work introduces software to implement two of ETAS models described in Ogata (1998). To find the Maximum-Likelihood Estimates (MLEs), my software provides estimates of the homogeneous background rate parameter and the temporal and spatial parameters that govern triggering effects by applying the Expectation-Maximization (EM) algorithm introduced in Veen and Schoenberg (2008). Despite other computer programs exist for similar data modeling purpose, using EM-algorithm has the benefits of stability and robustness (Veen and Schoenberg, 2008). Spatial shapes that are very long and narrow cause difficulties in optimization convergence and problems with flat or multi-modal log-likelihood functions encounter similar issues. My program uses a robust method to preset a parameter to overcome the non-convergence computational issue. In addition to model fitting, the software is equipped with useful tools for examining modeling fitting results, for example, visualization of estimated conditional intensity, and estimation of expected number of triggered aftershocks. A simulation generator is also given with flexible spatial shapes that may be defined by the user. This open-source software has a very simple user interface. The user may execute it on a local computer, and the program also has potential to be hosted online. Java language is used for the software's core computing part and an optional interface to the statistical package R is provided.

High-frequency spectral falloff of earthquakes, fractal dimension of complex rupture, b value, and the scaling of strength on faults

USGS Publications Warehouse

Frankel, A.

1991-01-01

The high-frequency falloff ??-y of earthquake displacement spectra and the b value of aftershock sequences are attributed to the character of spatially varying strength along fault zones. I assume that the high frequency energy of a main shock is produced by a self-similar distribution of subevents, where the number of subevents with radii greater than R is proportional to R-D, D being the fractal dimension. In the model, an earthquake is composed of a hierarchical set of smaller earthquakes. The static stress drop is parameterized to be proportional to R??, and strength is assumed to be proportional to static stress drop. I find that a distribution of subevents with D = 2 and stress drop independent of seismic moment (?? = 0) produces a main shock with an ??-2 falloff, if the subevent areas fill the rupture area of the main shock. By equating subevents to "islands' of high stress of a random, self-similar stress field on a fault, I relate D to the scaling of strength on a fault, such that D = 2 - ??. Thus D = 2 corresponds to constant stress drop scaling (?? = 0) and scale-invariant fault strength. A self-similar model of aftershock rupture zones on a fault is used to determine the relationship between the b value, the size distribution of aftershock rupture zones, and the scaling of strength on a fault. -from Author

Aftershock triggering by complete Coulomb stress changes

USGS Publications Warehouse

Kilb, Debi; Gomberg, J.; Bodin, P.

2002-01-01

We examine the correlation between seismicity rate change following the 1992, M7.3, Landers, California, earthquake and characteristics of the complete Coulomb failure stress (CFS) changes (??CFS(t)) that this earthquake generated. At close distances the time-varying "dynamic" portion of the stress change depends on how the rupture develops temporally and spatially and arises from radiated seismic waves and from permanent coseismic fault displacement. The permanent "static" portion (??CFS) depends only on the final coseismic displacement. ??CFS diminishes much more rapidly with distance than the transient, dynamic stress changes. A common interpretation of the strong correlation between ??CFS and aftershocks is that load changes can advance or delay failure. Stress changes may also promote failure by physically altering properties of the fault or its environs. Because it is transient, ??CFS(t) can alter the failure rate only by the latter means. We calculate both ??CFS and the maximum positive value of ??CFS(t) (peak ??CFS(t)) using a reflectivity program. Input parameters are constrained by modeling Landers displacement seismograms. We quantify the correlation between maps of seismicity rate changes and maps of modeled ??CFS and peak ??CFS(t) and find agreement for both models. However, rupture directivity, which does not affect ??CFS, creates larger peak ??CFS(t) values northwest of the main shock. This asymmetry is also observed in seismicity rate changes but not in ??CFS. This result implies that dynamic stress changes are as effective as static stress changes in triggering aftershocks and may trigger earthquakes long after the waves have passed.

Tomographic models and seismotectonics of the Reggio Emilia region, Italy

NASA Astrophysics Data System (ADS)

Ciaccio, M. G.; Chiarabba, C.

2002-02-01

The aim of this study is to define the Vp and Vp/Vs structure of the fault zone ruptured by the M L 5.1 earthquake of October 15, 1996 which occurred near Reggio Emilia (central-northern Italy). A 1-month-long seismic sequence followed the mainshock and occurred in a small region along the outer border of the northern Apenninic belt, at depth ranging between 10 and 17 km. P- and S-wave arrival times from 304 aftershocks recorded by two local dense seismic arrays installed in the epicentral region have been inverted to obtain one- and three-dimensional velocity models by using state of the art local earthquake tomographic techniques. Velocity models and aftershock relocation help us to infer the seismotectonic of the region. Earthquakes originated along a NW-dipping backthrust of a NE-trending main thrust, composing the western part of the broad Ferrara Arc. A main high Vp and high Vp/Vs region delineates a pop-up structure in the center of the area. The high Vp/Vs within the pop-up structure supports the presence of a zone with increased pore pressure. The hypocentral depth of both mainshock and aftershocks is greater than those usually found for the main seismogenic regions of the Apenninic belt. P-wave velocity values in the seismogenic area, obtained by tomography, are compatible with rocks of the Mesozoic cover and suggest that seismicity occurred within the Mesozoic units stack at present by compressional tectonics.

Modeling earthquake rate changes in Oklahoma and Arkansas: possible signatures of induced seismicity

USGS Publications Warehouse

Llenos, Andrea L.; Michael, Andrew J.

2013-01-01

The rate of ML≥3 earthquakes in the central and eastern United States increased beginning in 2009, particularly in Oklahoma and central Arkansas, where fluid injection has occurred. We find evidence that suggests these rate increases are man‐made by examining the rate changes in a catalog of ML≥3 earthquakes in Oklahoma, which had a low background seismicity rate before 2009, as well as rate changes in a catalog of ML≥2.2 earthquakes in central Arkansas, which had a history of earthquake swarms prior to the start of injection in 2009. In both cases, stochastic epidemic‐type aftershock sequence models and statistical tests demonstrate that the earthquake rate change is statistically significant, and both the background rate of independent earthquakes and the aftershock productivity must increase in 2009 to explain the observed increase in seismicity. This suggests that a significant change in the underlying triggering process occurred. Both parameters vary, even when comparing natural to potentially induced swarms in Arkansas, which suggests that changes in both the background rate and the aftershock productivity may provide a way to distinguish man‐made from natural earthquake rate changes. In Arkansas we also compare earthquake and injection well locations, finding that earthquakes within 6 km of an active injection well tend to occur closer together than those that occur before, after, or far from active injection. Thus, like a change in productivity, a change in interevent distance distribution may also be an indicator of induced seismicity.

Open Source Tools for Seismicity Analysis

NASA Astrophysics Data System (ADS)

Powers, P.

2010-12-01

The spatio-temporal analysis of seismicity plays an important role in earthquake forecasting and is integral to research on earthquake interactions and triggering. For instance, the third version of the Uniform California Earthquake Rupture Forecast (UCERF), currently under development, will use Epidemic Type Aftershock Sequences (ETAS) as a model for earthquake triggering. UCERF will be a "living" model and therefore requires robust, tested, and well-documented ETAS algorithms to ensure transparency and reproducibility. Likewise, as earthquake aftershock sequences unfold, real-time access to high quality hypocenter data makes it possible to monitor the temporal variability of statistical properties such as the parameters of the Omori Law and the Gutenberg Richter b-value. Such statistical properties are valuable as they provide a measure of how much a particular sequence deviates from expected behavior and can be used when assigning probabilities of aftershock occurrence. To address these demands and provide public access to standard methods employed in statistical seismology, we present well-documented, open-source JavaScript and Java software libraries for the on- and off-line analysis of seismicity. The Javascript classes facilitate web-based asynchronous access to earthquake catalog data and provide a framework for in-browser display, analysis, and manipulation of catalog statistics; implementations of this framework will be made available on the USGS Earthquake Hazards website. The Java classes, in addition to providing tools for seismicity analysis, provide tools for modeling seismicity and generating synthetic catalogs. These tools are extensible and will be released as part of the open-source OpenSHA Commons library.

Potentially induced earthquakes in Oklahoma, USA: links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

USGS Publications Warehouse

Keranen, Katie M.; Savage, Heather M.; Abers, Geoffrey A.; Cochran, Elizabeth S.

2013-01-01

Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ~200 m of active injection wells and within ~1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Physics-based and statistical earthquake forecasting in a continental rift zone: the case study of Corinth Gulf (Greece)

NASA Astrophysics Data System (ADS)

Segou, Margarita

2016-01-01

I perform a retrospective forecast experiment in the most rapid extensive continental rift worldwide, the western Corinth Gulf (wCG, Greece), aiming to predict shallow seismicity (depth <15 km) with magnitude M ≥ 3.0 for the time period between 1995 and 2013. I compare two short-term earthquake clustering models, based on epidemic-type aftershock sequence (ETAS) statistics, four physics-based (CRS) models, combining static stress change estimations and the rate-and-state laboratory law and one hybrid model. For the latter models, I incorporate the stress changes imparted from 31 earthquakes with magnitude M ≥ 4.5 at the extended area of wCG. Special attention is given on the 3-D representation of active faults, acting as potential receiver planes for the estimation of static stress changes. I use reference seismicity between 1990 and 1995, corresponding to the learning phase of physics-based models, and I evaluate the forecasts for six months following the 1995 M = 6.4 Aigio earthquake using log-likelihood performance metrics. For the ETAS realizations, I use seismic events with magnitude M ≥ 2.5 within daily update intervals to enhance their predictive power. For assessing the role of background seismicity, I implement a stochastic reconstruction (aka declustering) aiming to answer whether M > 4.5 earthquakes correspond to spontaneous events and identify, if possible, different triggering characteristics between aftershock sequences and swarm-type seismicity periods. I find that: (1) ETAS models outperform CRS models in most time intervals achieving very low rejection ratio RN = 6 per cent, when I test their efficiency to forecast the total number of events inside the study area, (2) the best rejection ratio for CRS models reaches RN = 17 per cent, when I use varying target depths and receiver plane geometry, (3) 75 per cent of the 1995 Aigio aftershocks that occurred within the first month can be explained by static stress changes, (4) highly variable performance on behalf of both statistical and physical models is suggested by large confidence intervals of information gain per earthquake and (5) generic ETAS models can adequately predict the temporal evolution of seismicity during swarms. Furthermore, stochastic reconstruction of seismicity makes possible the identification of different triggering processes between specific seismic crises (2001, 2003-04, 2006-07) and the 1995 aftershock sequence. I find that: (1) seismic events with M ≥ 5.0 are not a part of a preceding earthquake cascade, since they are characterized by high probability being a background event (average Pback > 0.8) and (2) triggered seismicity within swarms is characterized by lower event productivity when compared with the corresponding value during aftershock sequences. I conclude that physics-based models contribute on the determination of the `new-normal' seismicity rate at longer time intervals and that their joint implementation with statistical models is beneficial for future operational forecast systems.

The Somalian earthquakes of May, 1980, East Africa

NASA Astrophysics Data System (ADS)

Ruegg, J. C.; Lépine, J. C.; Tarantola, A.; Lévêque, J. J.

1981-04-01

A seismic crisis, with a mb = 5.3 main shock, occured in the Somali Republic, East Africa (10°N, 43°E) from April to November 1980. Up to 2000 earthquakes with ML > 2 have been recorded during this period. This earthquake sequence is of particular interest because it occured in a seismically inactive zone and include a rather long aftershock sequence. Two groups of epicenters were identified using a relative location procedure. Aftershocks observed during the first two weeks fall very close to the Borama city, while latter shocks are situated 10km west. This may suggest that the second group of earthquakes has been induced by the first one. These data show that the continental margin between the Somalian Plateau shield and the quasi-oceanic crust of the Afar-Gulf of Aden region, remains active to day and is relevant to intraplate seismicity.

The Western Guerrero, Mexico, seismogenic zone from the microseismicity associated to the 1979 Petatlan and 1985 Zihuatanejo earthquakes

NASA Astrophysics Data System (ADS)

Valdés-González, C.; Novelo-Casanova, D. A.

1998-03-01

The Western Guerrero, Mexico, seismogenic zone was completely ruptured by the 1979 ( Ms 7.6) Petatlan and 1985 ( Ms 7.5) Zihuatanejo earthquakes. Hypocenters of the Petatlan aftershocks define an approximately 10-km-thick Wadati— Benioff zone of high seismic activity and a thinner seaward region that is primarily an extension of the deeper part of the 10-km-thick zone. The aftershocks of the Zihuatanejo earthquake occurred in the seaward portion of the same epicentral region but the hypocenters were shallower. The spatial distribution of the closely timed microseismicity following the two earthquakes outlines a seismogenic zone which begins at about 40 km from the trench axis of the Western Guerrero subduction region and extends approximately 90 km. These results indicate that the maximum possible size of thrust earthquakes in the Guerrero seismic gap is of Mw ˜8.4.

A data-based model to locate mass movements triggered by seismic events in Sichuan, China.

PubMed

de Souza, Fabio Teodoro

2014-01-01

Earthquakes affect the entire world and have catastrophic consequences. On May 12, 2008, an earthquake of magnitude 7.9 on the Richter scale occurred in the Wenchuan area of Sichuan province in China. This event, together with subsequent aftershocks, caused many avalanches, landslides, debris flows, collapses, and quake lakes and induced numerous unstable slopes. This work proposes a methodology that uses a data mining approach and geographic information systems to predict these mass movements based on their association with the main and aftershock epicenters, geologic faults, riverbeds, and topography. A dataset comprising 3,883 mass movements is analyzed, and some models to predict the location of these mass movements are developed. These predictive models could be used by the Chinese authorities as an important tool for identifying risk areas and rescuing survivors during similar events in the future.

Bicentennial of the 1811-1812 New Madrid earthquake sequence December 2011-2012

USGS Publications Warehouse

,

2010-01-01

A series of earthquakes hit the New Madrid seismic zone of southeastern Missouri, northeastern Arkansas, and adjacent parts of Tennessee and Kentucky, in December 1811 to February 1812. Three earthquakes had a magnitude of 7.0 or greater. The first earthquake occurred December 16, 1811, at 2:15 a.m.; the second 9 a.m. on January 23, 1812; and the third on February 7, 1812, at 3:45 a.m. These three earthquakes were among the largest to strike North America since European settlement. The main shocks were followed by many hundreds of aftershocks that lasted for decades. Many of the aftershocks were major earthquakes themselves. The area that was strongly shaken by the three main shocks was 2-3 times as large as the strongly shaken area of the 1964 M9.2 Alaskan earthquake, and 10 times as large as that of the 1906 M7.8 San Francisco earthquake.

Near real-time aftershock hazard maps for earthquakes

NASA Astrophysics Data System (ADS)

McCloskey, J.; Nalbant, S. S.

2009-04-01

Stress interaction modelling is routinely used to explain the spatial relationships between earthquakes and their aftershocks. On 28 October 2008 a M6.4 earthquake occurred near the Pakistan-Afghanistan border killing several hundred and causing widespread devastation. A second M6.4 event occurred 12 hours later 20km to the south east. By making some well supported assumptions concerning the source event and the geometry of any likely triggered event it was possible to map those areas most likely to experience further activity. Using Google earth, it would further have been possible to identify particular settlements in the source area which were particularly at risk and to publish their locations globally within about 3 hours of the first earthquake. Such actions could have significantly focused the initial emergency response management. We argue for routine prospective testing of such forecasts and dialogue between social and physical scientists and emergency response professionals around the practical application of these techniques.

Earthquake Simulator Finds Tremor Triggers

ScienceCinema

Johnson, Paul

2018-01-16

Using a novel device that simulates earthquakes in a laboratory setting, a Los Alamos researcher has found that seismic waves-the sounds radiated from earthquakes-can induce earthquake aftershocks, often long after a quake has subsided. The research provides insight into how earthquakes may be triggered and how they recur. Los Alamos researcher Paul Johnson and colleague Chris Marone at Penn State have discovered how wave energy can be stored in certain types of granular materials-like the type found along certain fault lines across the globe-and how this stored energy can suddenly be released as an earthquake when hit by relatively small seismic waves far beyond the traditional “aftershock zone” of a main quake. Perhaps most surprising, researchers have found that the release of energy can occur minutes, hours, or even days after the sound waves pass; the cause of the delay remains a tantalizing mystery.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Paul

Using a novel device that simulates earthquakes in a laboratory setting, a Los Alamos researcher has found that seismic waves-the sounds radiated from earthquakes-can induce earthquake aftershocks, often long after a quake has subsided. The research provides insight into how earthquakes may be triggered and how they recur. Los Alamos researcher Paul Johnson and colleague Chris Marone at Penn State have discovered how wave energy can be stored in certain types of granular materials-like the type found along certain fault lines across the globe-and how this stored energy can suddenly be released as an earthquake when hit by relatively smallmore » seismic waves far beyond the traditional “aftershock zone” of a main quake. Perhaps most surprising, researchers have found that the release of energy can occur minutes, hours, or even days after the sound waves pass; the cause of the delay remains a tantalizing mystery.« less

The Mara Rosa 2010 GT-5 earthquake and its possible relationship with the continental-scale transbrasiliano lineament

NASA Astrophysics Data System (ADS)

Barros, Lucas V.; Assumpção, Marcelo; Chimpliganond, Cristiano; Carvalho, Juraci M.; Von Huelsen, Mônica G.; Caixeta, Daniel; França, George Sand; de Albuquerque, Diogo F.; Ferreira, Vinicius M.; Fontenele, Darlan P.

2015-07-01

On October 8th, 2010, a 5.0 mb earthquake with intensity VI (MM) occurred close to Mara Rosa, in the North of Goiás State, central Brazil, in an area where previous low magnitude seismicity had been observed. This earthquake was felt up to 300 km away from the epicenter, and was the biggest event ever detected in Central Brazil Seismic Zone. Despite the difficulty of associating earthquakes in Stable Continental Interior with geological structures, this event is possibly related to the reactivation of a geological fault of the continental-scale Transbrasiliano Lineament (TBL): the aftershock activity observed with an 8-station seismic network, indicates a NW dipping, SW-NE trending reverse fault, parallel to the TBL. The P axis is NW-SE oriented, consistent with expected stress direction in the region. Cross correlation technique was used to synchronize the weak P- and S- wave phases of some of the aftershocks, recorded at regional stations, with the corresponding arrivals of the main shock producing a consistent set of relative arrival times. The use of regional station corrections allowed the mainshock to be located with uncertainties small enough to qualify for a GT5 event, which will help to constrain 3D velocity models in South America. We found that the aftershocks were distributed around a circular area about 1.5-2.0 km across, with no events in the middle. This is interpreted as the mainshock rupture completely releasing all stresses. The rupture area and the mainshock moment correspond to a stress-drop of about 2 MPa.

Slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1) along the Kuril Trench deduced from tsunami waveform inversion

USGS Publications Warehouse

Hirata, K.; Geist, E.; Satake, K.; Tanioka, Y.; Yamaki, S.

2003-01-01

We inverted 13 tsunami waveforms recorded in Japan to estimate the slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1), which occurred southeast off Hokkaido along the southern Kuril subduction zone. The previously estimated source area determined from tsunami travel times [Hatori, 1973] did not coincide with the observed aftershock distribution. Our results show that a large amount of slip occurred in the aftershock area east of Hatori's tsunami source area, suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock. We also found more than 5 m of slip along the deeper part of the seismogenic interface, just below the central part of Hatori's tsunami source area. This region, which also has the largest stress drop during the main shock, had few aftershocks. Large tsunami heights on the eastern Hokkaido coast are better explained by the heterogeneous slip model than previous uniform-slip fault models. The total seismic moment is estimated to be 1.87 ?? 1021 N m, giving a moment magnitude of Mw = 8.1. The revised tsunami source area is estimated to be 25.2 ?? 103 km2, ???3 times larger than the previous tsunami source area. Out of four large earthquakes with M ??? 7 that subsequently occurred in and around the rupture area of the 1952 event, three were at the edges of regions with relatively small amount of slip. We also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface.

Delayed seismicity rate changes controlled by static stress transfer

USGS Publications Warehouse

Kroll, Kayla A.; Richards-Dinger, Keith B.; Dieterich, James H.; Cochran, Elizabeth S.

2017-01-01

On 15 June 2010, a Mw5.7 earthquake occurred near Ocotillo, California, in the Yuha Desert. This event was the largest aftershock of the 4 April 2010 Mw7.2 El Mayor-Cucapah (EMC) earthquake in this region. The EMC mainshock and subsequent Ocotillo aftershock provide an opportunity to test the Coulomb failure hypothesis (CFS). We explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative CFS change imparted by the Ocotillo event. Based on simple CFS calculations we divide the Yuha Desert into three subregions, one triggering zone and two stress shadow zones. We find the nominal triggering zone displays immediate triggering, one stress shadowed region experiences immediate quiescence, and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown. We quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of CFS change with the rate-state earthquake rate formulation of Dieterich (1994), assuming that each subregion contains a mixture of nucleation sources that experienced a CFS change of differing signs. Our modeling reproduces the observations, including the observed delay in the stress shadow effect in the third region following the Ocotillo aftershock. The delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations. We find that rate-state models of time-dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity.

The 1998 earthquake sequence south of Long Valley Caldera, California: Hints of magmatic involvement

USGS Publications Warehouse

Hough, S.E.; Dollar, R.S.; Johnson, P.

2000-01-01

A significant episode of seismic and geodetic unrest took place at Long Valley Caldera, California, beginning in the summer of 1997. Activity through late May of 1998 was concentrated in and around the south moat and the south margin of the resurgent dome. The Sierran Nevada block (SNB) region to the south/southeast remained relatively quiet until a M 5.1 event occurred there on 9 June 1998 (UT). A second M 5.1 event followed on 15 July (UT); both events were followed by appreciable aftershock sequences. An additional, distinct burst of activity began on 1 August 1998. The number of events in the August sequence (over the first week or two) was similar to the aftershock sequence of the 15 July 1998 M 5.1 event, but the later sequence was not associated with any events larger than M 4.3. All of the summer 1998 SNB activity was considered tectonic rather than magmatic; in general the SNB is considered an unlikely location for future eruptions. However, the August sequence-an 'aftershock sequence without a mainshock'-is suggestive of a strain event larger than the cumulative seismotectonic strain release. Moreover, a careful examination of waveforms from the August sequence reveals a small handful of events whose spectral signature is strikingly harmonic. We investigate the waveforms of these events using spectral, autocorrelation, and empirical Green's function techniques and conclude that they were most likely associated with a fluid-controlled source. Our observations suggest that there may have been some degree of magma or magma-derived fluid involvement in the 1998 SNB sequence.

Delayed Seismicity Rate Changes Controlled by Static Stress Transfer

NASA Astrophysics Data System (ADS)

Kroll, Kayla A.; Richards-Dinger, Keith B.; Dieterich, James H.; Cochran, Elizabeth S.

2017-10-01

On 15 June 2010, a Mw5.7 earthquake occurred near Ocotillo, California, in the Yuha Desert. This event was the largest aftershock of the 4 April 2010 Mw7.2 El Mayor-Cucapah (EMC) earthquake in this region. The EMC mainshock and subsequent Ocotillo aftershock provide an opportunity to test the Coulomb failure hypothesis (CFS). We explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative CFS change imparted by the Ocotillo event. Based on simple CFS calculations we divide the Yuha Desert into three subregions, one triggering zone and two stress shadow zones. We find the nominal triggering zone displays immediate triggering, one stress shadowed region experiences immediate quiescence, and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown. We quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of CFS change with the rate-state earthquake rate formulation of Dieterich (1994), assuming that each subregion contains a mixture of nucleation sources that experienced a CFS change of differing signs. Our modeling reproduces the observations, including the observed delay in the stress shadow effect in the third region following the Ocotillo aftershock. The delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations. We find that rate-state models of time-dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity.

Assessment of teleseismically-determined source parameters for the April 25, 2015 MW 7.9 Gorkha, Nepal earthquake and the May 12, 2015 MW 7.2 aftershock

NASA Astrophysics Data System (ADS)

Lay, Thorne; Ye, Lingling; Koper, Keith D.; Kanamori, Hiroo

2017-09-01

On April 25, 2015 a major (MW 7.9) thrust earthquake ruptured the deeper portion of the seismogenic plate boundary beneath Nepal along which India is underthrusting Eurasia. An MW 7.2 aftershock on May 12, 2015 extended the eastern, down-dip edge of the rupture. These destructive events caused about 9000 fatalities and 23,000 injuries. The overall rupture zone is about 170 km long and 40-80 km wide. This region of the plate boundary previously experienced a large earthquake in 1833, and in 1934 a larger MS 8.0 event located to the east ruptured all the way to the surface. The Main Himalayan Thrust (MHT) on which slip occurred in 2015 has a very low dip angle of 6°, and the depth of the mainshock slip distribution is very shallow, extending from 7 to 18 km. The shallow dip and depth present challenges for resolving faulting characteristics using teleseismic data. We analyze global teleseismic signals for the mainshock and aftershock to estimate source parameters, evaluating the stability of various procedures used for remotely characterizing kinematics of such shallow faulting. Back-projection and finite-fault slip inversion are used to assess the spatio-temporal rupture history and evidence for frequency-dependent radiation along dip. Slip zone width constraints from near-field geodetic observations are imposed on the preferred models to overcome some limitations of purely teleseismic methods. Radiated energy, stress drop and moment rate functions are determined for both events.

Pattern recognition approach to the subsequent event of damaging earthquakes in Italy

NASA Astrophysics Data System (ADS)

Gentili, S.; Di Giovambattista, R.

2017-05-01

In this study, we investigate the occurrence of large aftershocks following the most significant earthquakes that occurred in Italy after 1980. In accordance with previous studies (Vorobieva and Panza, 1993; Vorobieva, 1999), we group clusters associated with mainshocks into two categories: ;type A; if, given a main shock of magnitude M, the subsequent strongest earthquake in the cluster has magnitude ≥M - 1 or type B otherwise. In this paper, we apply a pattern recognition approach using statistical features to foresee the class of the analysed clusters. The classification of the two categories is based on some features of the time, space, and magnitude distribution of the aftershocks. Specifically, we analyse the temporal evolution of the radiated energy at different elapsed times after the mainshock, the spatio-temporal evolution of the aftershocks occurring within a few days, and the probability of a strong earthquake. An attempt is made to classify the studied region into smaller seismic zones with a prevalence of type A and B clusters. We demonstrate that the two types of clusters have distinct preferred geographic locations inside the Italian territory that likely reflected key properties of the deforming regions, different crustal domains and faulting style. We use decision trees as classifiers of single features to characterize the features depending on the cluster type. The performance of the classification is tested by the Leave-One-Out method. The analysis is performed on different time-spans after the mainshock to simulate the dependence of the accuracy on the information available as data increased over a longer period with increasing time after the mainshock.

Source Characterization of the 2015 Collapse in Gypsum Mine in Shandong, China

NASA Astrophysics Data System (ADS)

Yang, H.; Chu, R.; Sheng, M.

2016-12-01

Source parameters of mining earthquakes are essential to investigating pressure redistribution and accumulation due to underground excavation. On 25 December 2015, a local magnitude 4.0 earthquake occurred at 07:56:12 BJT in Pingyi County, China (latitude: 35.5°N, longitude: 117.7°E) with a depth of 0 km. This earthquake is caused by underground cave collapse. In this paper, we used sliding-window cross-correlation method to detect aftershocks of this event. The result indicates there are at least six aftershocks within ten minutes after the earthquake. Then we inverted focal mechanisms and depths of the mainshock and the largest aftershock with three-component broadband seismic waveform data recorded by the National Seismic Network. We use the generalized Cut-and-Paste (gCAP) method to obtain their moment tensors, which allows for a characterization of the relative amounts of deviatoric and isotropic source components. This gCAP method divides three component waveforms into Pnl and surface wave segments, and allows adjustable time shifts between observed and synthetic data, so that it reduces the influence of uncertainties in the 1-D velocity model. The results show that both events have similar focal mechanisms, which contains obvious non-double-couple component with a large proportion of isotropic source component. The mechanisms are dominated by 80% implosive isotropic energy and 20% thrusting double couple energy. Such mechanisms might be explained by an asymmetric collapse of the mine cavity due to unevenly distributed in situ stresses, sympathetic shear on a roof fault, or between the roof and floor of the mine.

Fault Zone Imaging from Correlations of Aftershock Waveforms

NASA Astrophysics Data System (ADS)

Hillers, Gregor; Campillo, Michel

2018-03-01

We image an active fault zone environment using cross correlations of 154 15 s long 1992 Landers earthquake aftershock seismograms recorded along a line array. A group velocity and phase velocity dispersion analysis of the reconstructed Rayleigh waves and Love waves yields shear wave velocity images of the top 100 m along the 800 m long array that consists of 22 three component stations. Estimates of the position, width, and seismic velocity of a low-velocity zone are in good agreement with the findings of previous fault zone trapped waves studies. Our preferred solution indicates the zone is offset from the surface break to the east, 100-200 m wide, and characterized by a 30% velocity reduction. Imaging in the 2-6 Hz range resolves further a high-velocity body of similar width to the west of the fault break. Symmetry and shape of zero-lag correlation fields or focal spots indicate a frequency and position dependent wavefield composition. At frequencies greater than 4 Hz surface wave propagation dominates, whereas at lower frequencies the correlation field also exhibits signatures of body waves that likely interact with the high-velocity zone. The polarization and late arrival times of coherent wavefronts observed above the low-velocity zone indicate reflections associated with velocity contrasts in the fault zone environment. Our study highlights the utility of the high-frequency correlation wavefield obtained from records of local and regional seismicity. The approach does not depend on knowledge of earthquake source parameters, which suggests the method can return images quickly during aftershock campaigns to guide network updates for optimal coverage of interesting geological features.

The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide

USGS Publications Warehouse

Pollitz, Fred F.; Stein, Ross S.; Sevilgen, Volkan; Burgmann, Roland

2012-01-01

Large earthquakes trigger very small earthquakes globally during passage of the seismic waves and during the following several hours to days1, 2, 3, 4, 5, 6, 7, 8, 9, 10, but so far remote aftershocks of moment magnitude M≥5.5 have not been identified11, with the lone exception of an M=6.9 quake remotely triggered by the surface waves from an M=6.6 quake 4,800 kilometres away12. The 2012 east Indian Ocean earthquake that had a moment magnitude of 8.6 is the largest strike-slip event ever recorded. Here we show that the rate of occurrence of remote M≥5.5 earthquakes (>1,500 kilometres from the epicentre) increased nearly fivefold for six days after the 2012 event, and extended in magnitude to M≥7. These global aftershocks were located along the four lobes of Love-wave radiation; all struck where the dynamic shear strain is calculated to exceed 10-7 for at least 100 seconds during dynamic-wave passage. The other M≥8.5 mainshocks during the past decade are thrusts; after these events, the global rate of occurrence of remote M≥5.5 events increased by about one-third the rate following the 2012 shock and lasted for only two days, a weaker but possibly real increase. We suggest that the unprecedented delayed triggering power of the 2012 earthquake may have arisen because of its strike-slip source geometry or because the event struck at a time of an unusually low global earthquake rate, perhaps increasing the number of nucleation sites that were very close to failure.

Aftershocks and triggering processes in rock fracture

NASA Astrophysics Data System (ADS)

Davidsen, J.; Kwiatek, G.; Goebel, T.; Stanchits, S. A.; Dresen, G.

2017-12-01

One of the hallmarks of our understanding of seismicity in nature is the importance of triggering processes, which makes the forecasting of seismic activity feasible. These triggering processes by which one earthquake induces (dynamic or static) stress changes leading to potentially multiple other earthquakes are at the core relaxation processes. A specic example of triggering are aftershocks following a large earthquake, which have been observed to follow certain empirical relationships such as the Omori-Utsu relation. Such an empirical relation should arise from the underlying microscopic dynamics of the involved physical processes but the exact connection remains to be established. Simple explanations have been proposed but their general applicability is unclear. Many explanations involve the picture of an earthquake as a purely frictional sliding event. Here, we present experimental evidence that these empirical relationships are not limited to frictional processes but also arise in fracture zone formation and are mostly related to compaction-type events. Our analysis is based on tri-axial compression experiments under constant displacement rate on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. More importantly, we show that event-event triggering plays an important role in the presence of large-scale or macrocopic imperfections while such triggering is basically absent if no signicant imperfections are present. We also show that spatial localization and an increase in activity rates close to failure do not necessarily imply triggering behavior associated with aftershocks. Only if a macroscopic crack is formed and its propagation remains subcritical do we observe significant triggering.

Characterizing potentially induced earthquake rate changes in the Brawley Seismic Zone, southern California

USGS Publications Warehouse

Llenos, Andrea L.; Michael, Andrew J.

2016-01-01

The Brawley seismic zone (BSZ), in the Salton trough of southern California, has a history of earthquake swarms and geothermal energy exploitation. Some earthquake rate changes may have been induced by fluid extraction and injection activity at local geothermal fields, particularly at the North Brawley Geothermal Field (NBGF) and at the Salton Sea Geothermal Field (SSGF). We explore this issue by examining earthquake rate changes and interevent distance distributions in these fields. In Oklahoma and Arkansas, where considerable wastewater injection occurs, increases in background seismicity rate and aftershock productivity and decreases in interevent distance were indicative of fluid‐injection‐induced seismicity. Here, we test if similar changes occur that may be associated with fluid injection and extraction in geothermal areas. We use stochastic epidemic‐type aftershock sequence models to detect changes in the underlying seismogenic processes, shown by statistically significant changes in the model parameters. The most robust model changes in the SSGF roughly occur when large changes in net fluid production occur, but a similar correlation is not seen in the NBGF. Also, although both background seismicity rate and aftershock productivity increased for fluid‐injection‐induced earthquake rate changes in Oklahoma and Arkansas, the background rate increases significantly in the BSZ only, roughly corresponding with net fluid production rate increases. Moreover, in both fields the interevent spacing does not change significantly during active energy projects. This suggests that, although geothermal field activities in a tectonically active region may not significantly change the physics of earthquake interactions, earthquake rates may still be driven by fluid injection or extraction rates, particularly in the SSGF.

Source characteristics of the 2015 MW 7.8 Gorkha (Nepal) earthquake and its MW 7.2 aftershock from space geodesy

NASA Astrophysics Data System (ADS)

Feng, Wanpeng; Lindsey, Eric; Barbot, Sylvain; Samsonov, Sergey; Dai, Keren; Li, Peng; Li, Zhenhong; Almeida, Rafael; Chen, Jiajun; Xu, Xiaohua

2017-08-01

On April 25, 2015, a destructive MW 7.8 earthquake struck the capital of Nepal, Kathmandu, killing more than 8800 people and destroying numerous historical structures. We analyze six coseismic interferograms from several satellites (ALOS-2, Sentinel-1 A, and RADARSAT-2), as well as three-dimensional displacements at six GPS stations to investigate fault structure and slip distribution of the Gorkha earthquake. Using a layered crustal structure, the best-fit slip model shows that the preferred dip angle of the mainshock fault is 6 ± 3.5° and the major slip is concentrated within depths of 8-15 km. The maximum slip of 6.0 m occurs at a depth of 11 km, 70 km south east of the epicenter. The coseismic rupture extends 150 km eastward of the epicentre with a cumulative geodetic moment of 7.8 × 1020 Nm, equivalent to an earthquake of MW 7.84. We also investigate the MW 7.2 aftershock on 12 May 2015 using another three postseismic interferograms from ALOS2, RADARSAT-2, and Sentinel-1 A. The InSAR-based best-fit slip model of the largest aftershock implies that its major slip is next to the eastern lower end of the mainshock rupture with a similar maximum slip of 6 m at a depth of 13 km. This study generates various coseismic geodetic measurements to determine the source parameters of the MW 7.8 Gorkha earthquake and 12 May MW 7.2 afershock, providing an additional chance to understand the local fault structure and slip extent.

The 2008 Mw 7.2 North Pagai earthquake sequence: Partial rupture of a fully locked Mentawai patch

NASA Astrophysics Data System (ADS)

Salman, R.; Hill, E.; Feng, L.; Wei, S.; Barbot, S.; Lindsey, E.; WANG, X.; Chen, W.; Bannerjee, P.; Hermawan, I.; Natawidjaja, D. H.

2016-12-01

The Mentawai patch is a seismic gap along the Sumatra subduction zone that has not ruptured completely over the last decade. This is worrying because coral colonies of the Mentawai islands show that over the last 700 years the Mentawai patch ruptured in a sequence of great earthquake (Mw > 8.5) about every two centuries. In September 2007, the Mw 8.4 Bengkulu earthquake ruptured the southern section of the Mentawai patch. The event was then followed by two Mw >= 7 aftershocks. Five months later, the 2008 Mw 7.2 earthquake ruptured a small asperity a little further north. The event ruptured a small area in the middle portion of the Mentawai patch, where the megathrust had been estimated as highly coupled. The mainshock was preceded by a foreshock of Mw 6.5 one day before and two M 6 aftershocks that occurred on the same day as the mainshock event. However, the whole earthquake sequence ruptured only a confined area on the megathrust and failed to wake up the sleeping giant. We have yet to explain why the 2008 event did not break more asperities and develop into one gargantuan earthquake. In this study, we use geodetic and seismic data to investigate the 2008 earthquake, its following afterslip, and its fore- and after-shocks. First, we jointly invert static and high-rate cGPS, InSAR and teleseismic data in a joint inversion for a co-seismic slip distribution of the mainshock. Second, we invert teleseismic data alone to develop slip models for the foreshock, mainshock and aftershock events. Third, we use the Cut-And-Paste (CAP) technique to estimate a more accurate depths for the 2008 earthquake sequence. Finally, we use six years of cGPS data, from 2008 to 2013, to develop a model for afterslip. Our preliminary results show 2 meters of peak coseismic slip for the mainshock. In addition, 1 meter of peak afterslip overlap with the coseismic slip model. The total estimated slip is far smaller than expected from the accumulated strain that has been stored in the Mentawai patch since the last earthquake in 1833. Thus, the likelihood that the Mentawai patch will generate another great earthquake in the near future remains high. But the possibility of releasing the accumulated strain piecemeal in smaller earthquakes cannot be ruled out.

Observations of Static Coulomb Stress Triggering During the Mw 5.7 Pawnee Earthquake Sequence

NASA Astrophysics Data System (ADS)

Pennington, C.; Chen, X.; Nakata, N.; Chang, J. C.

2016-12-01

The Pawnee earthquake occurred at 12:02 UTC on September 3 and was felt throughout Oklahoma and is the largest event recorded in Oklahoma instrumented history. The earthquake occurred near the junction of two previously mapped faults (Watchorn Fault and Labette Fault), but the actual fault that ruptured was a left-lateral unmapped basement fault (now known as the Sooner Lake Fault) with a strike of 107°, which is conjugate to a segment of the Labette fault that is optimally oriented (referred as OOF). We located 634 events from both before and after the mainshock (updated on September 15, 2016) and use these locations to map other seismogenic faults in the area. Examining the catalog, we found two episodes of seismicity, which started at 100 days and 40 days prior to mainshock, each episode has two clusters occurring two days apart on both OOF and near the mainshock. The near-simultaneous occurrence of clusters suggests possible stress interaction between the Sooner Lake Fault and the Labette fault. We examined the Coulomb stress changes on the surrounding faults caused by the mainshock and have found an increase of coulomb stress along the rakes of mapped faults in the area, the highest being along the Sooner Lake fault and the OOF segment of the Labette fault (see fig 1). These faults experienced up to 5 bars of positive coulomb stress increase, which matched the areas that experience the most aftershocks. To better understand the effect of the coulomb stress on the aftershocks, we plan on refining the catalogs for both aftershocks over a longer period and focal mechanisms to obtain accurate nodal planes, which will be used to see how and if the aftershocks were triggered by the Coulomb stress changes. We will also examine and refine the focal mechanisms that were produced for the events that occurred both before and after the main shock to investigate Coulomb stress interaction. Fig 1. (a) Is a map of faults in the Pawnee area with the red line being the source fault, which is part of the Sooner Lake Fault (green and red line segments.) The opitimally oriented segment of the Labette Fault (OOF) is shown in blue. (b) Shows the coulomb stress change for individual rakes after the rupture along the source fault.

Source parameters of the 1999 Osa peninsula (Costa Rica) earthquake sequence from spectral ratios analysis

NASA Astrophysics Data System (ADS)

Verdecchia, A.; Harrington, R. M.; Kirkpatrick, J. D.

2017-12-01

Many observations suggest that duration and size scale in a self-similar way for most earthquakes. Deviations from the expected scaling would suggest that some physical feature on the fault surface influences the speed of rupture differently at different length scales. Determining whether differences in scaling exist between small and large earthquakes is complicated by the fact that duration estimates of small earthquakes are often distorted by travel-path and site effects. However, when carefully estimated, scaling relationships between earthquakes may provide important clues about fault geometry and the spatial scales over which it affects fault rupture speed. The Mw 6.9, 20 August 1999, Quepos earthquake occurred on the plate boundary thrust fault along southern Costa Rica margin where the subducting seafloor is cut by numerous normal faults. The mainshock and aftershock sequence were recorded by land and (partially by) ocean bottom (OBS) seismic arrays deployed as part of the CRSEIZE experiment. Here we investigate the size-duration scaling of the mainshock and relocated aftershocks on the plate boundary to determine if a change in scaling exists that is consistent with a change in fault surface geometry at a specific length scale. We use waveforms from 5 short-period land stations and 12 broadband OBS stations to estimate corner frequencies (the inverse of duration) and seismic moment for several aftershocks on the plate interface. We first use spectral amplitudes of single events to estimate corner frequencies and seismic moments. We then adopt a spectral ratio method to correct for non-source-related effects and refine the corner frequency estimation. For the spectral ratio approach, we use pairs of earthquakes with similar waveforms (correlation coefficient > 0.7), with waveform similarity implying event co-location. Preliminary results from single spectra show similar corner frequency values among events of 0.5 ≤ M ≤ 3.6, suggesting a decrease in static stress drop with magnitude. Our next step is to refine corner frequency estimates using spectral ratios to see if the trend in corner frequency persists with small events, and to extend the magnitude range of the estimations using land-based recordings of the mainshock and two largest aftershocks, which occurred prior to the Osa array deployment.

Along-strike Variations in the Himalayas Illuminated by the Aftershock Sequence of the 2015 Mw 7.8 Gorkha Earthquake Using the NAMASTE Local Seismic Network

NASA Astrophysics Data System (ADS)

Mendoza, M.; Ghosh, A.; Karplus, M. S.; Nabelek, J.; Sapkota, S. N.; Adhikari, L. B.; Klemperer, S. L.; Velasco, A. A.

2016-12-01

As a result of the 2015 Mw 7.8 Gorkha earthquake, more than 8,000 people were killed from a combination of infrastructure failure and triggered landslides. This earthquake produced 4 m of peak co-seismic slip as the fault ruptured 130 km east under densely populated cities, such as Kathmandu. To understand earthquake dynamics in this part of the Himalayas and help mitigate similar future calamities by the next destructive event, it is imperative to study earthquake activities in detail and improve our understanding of the source and structural complexities. In response to the Gorkha event, multiple institutions developed and deployed a 10-month long dense seismic network called NAMASTE. It blanketed a 27,650 km2 area, mainly covering the rupture area of the Gorkha earthquake, in order to capture the dynamic sequence of aftershock behavior. The network consisted of a mix of 45 broadband, short-period, and strong motion sensors, with an average spacing of 20 km. From the first 6 months of data, starting approximately 1.5 after the mainshock, we develop a robust catalog containing over 3,000 precise earthquake locations, and local magnitudes that range between 0.3 and 4.9. The catalog has a magnitude of completeness of 1.5, and an overall low b-value of 0.78. Using the HypoDD algorithm, we relocate earthquake hypocenters with high precision, and thus illustrate the fault geometry down to depths of 25 km where we infer the location of the gently-dipping Main Frontal Thrust (MFT). Above the MFT, the aftershocks illuminate complex structure produced by relatively steeply dipping faults. Interestingly, we observe sharp along-strike change in the seismicity pattern. The eastern part of the aftershock area is significantly more active than the western part. The change in seismicity may reflect structural and/or frictional lateral heterogeneity in this part of the Himalayan fault system. Such along-strike variations play an important role in rupture complexities and arresting the mainshock from rupturing further east. This catalog serves as a starting point for not only identifying the physical processes controlling the earthquake cycles, but also areas of increased stress, in this segment of the Himalayas.

­Tectonic and geomorphic setting of the Pamir Plateau: Insights from InSAR and teleseismic analysis of the 2015 Lake Saurez and 2016 Muji fault earthquake sequence

NASA Astrophysics Data System (ADS)

Nanjundiah, P.; Barbot, S.; Wei, S.; Tapponnier, P.; Feng, W.; Wang, T.

2017-12-01

The Pamir Plateau lies on the western edge of the India- Eurasia collision zone and has been the sight of complex subduction regime in the past 50 Ma. In our study, we focus on two earthquakes and their aftershocks that occurred between December 2015 and December 2016. The first earthquake (Mw7.2), on 7 December 2015 between the Karakoram and Darwas fault systems, was sinstral strike slip in nature. The earthquake on 25 November (Mw6.6) occurred on the western end of Muji Fault, a dextral strike slip fault with an avg slip rate of 4mm/yr. We aim to better understand the structure, stress and deformation patterns in the northern and central Pamir plateau by analyzing InSAR, teleseismic, and optical data for these events and their aftershocks. We aim to better understand the structure, stress and deformation patterns in the northern and central Pamir plateau by analysing InSAR, teleseismic, and optical data for these events and their aftershocks. We constrain the fault geometry by precisely relocating aftershocks using the double difference technique implemented in HypoDD (Waldhauser & Ellsworth 2000). We used the Green's functions of Okada (1992) to invert for slip on the fault with rectangular dislocation and edgreen to numerically invert for the slip in a layered medium (Wang et al. 2003). The surface rupture of the December 2015 Lake Saurez earthquake shows evidence of multiple segments and step-overs. The combination of data sets used in this study highlights the existence of a seismic gap south of Lake Karakul as well as coupling between the Muji and Darwas-Karakoram fault systems. Mapping of past ruptures shows that the Sarez fault continues along the eastern coast of Lake Karakul almost until the Muji fault. With near field geodetic data in the form of InSAR, we can get a better insight into complex fault structures as well as post seismic slip and strain along the faults and its surroundings. We emphasize the role of smaller faults and their interactions in accommodating the overall strain in the Pamir region and their effect on estimating local seismic hazard.

Application of Second-Moment Source Analysis to Three Problems in Earthquake Forecasting

NASA Astrophysics Data System (ADS)

Donovan, J.; Jordan, T. H.

2011-12-01

Though earthquake forecasting models have often represented seismic sources as space-time points (usually hypocenters), a more complete hazard analysis requires the consideration of finite-source effects, such as rupture extent, orientation, directivity, and stress drop. The most compact source representation that includes these effects is the finite moment tensor (FMT), which approximates the degree-two polynomial moments of the stress glut by its projection onto the seismic (degree-zero) moment tensor. This projection yields a scalar space-time source function whose degree-one moments define the centroid moment tensor (CMT) and whose degree-two moments define the FMT. We apply this finite-source parameterization to three forecasting problems. The first is the question of hypocenter bias: can we reject the null hypothesis that the conditional probability of hypocenter location is uniformly distributed over the rupture area? This hypothesis is currently used to specify rupture sets in the "extended" earthquake forecasts that drive simulation-based hazard models, such as CyberShake. Following McGuire et al. (2002), we test the hypothesis using the distribution of FMT directivity ratios calculated from a global data set of source slip inversions. The second is the question of source identification: given an observed FMT (and its errors), can we identify it with an FMT in the complete rupture set that represents an extended fault-based rupture forecast? Solving this problem will facilitate operational earthquake forecasting, which requires the rapid updating of earthquake triggering and clustering models. Our proposed method uses the second-order uncertainties as a norm on the FMT parameter space to identify the closest member of the hypothetical rupture set and to test whether this closest member is an adequate representation of the observed event. Finally, we address the aftershock excitation problem: given a mainshock, what is the spatial distribution of aftershock probabilities? The FMT representation allows us to generalize the models typically used for this purpose (e.g., marked point process models, such as ETAS), which will again be necessary in operational earthquake forecasting. To quantify aftershock probabilities, we compare mainshock FMTs with the first and second spatial moments of weighted aftershock hypocenters. We will describe applications of these results to the Uniform California Earthquake Rupture Forecast, version 3, which is now under development by the Working Group on California Earthquake Probabilities.

Quantifying the Earthquake Clustering that Independent Sources with Stationary Rates (as Included in Current Risk Models) Can Produce.

NASA Astrophysics Data System (ADS)

Fitzenz, D. D.; Nyst, M.; Apel, E. V.; Muir-Wood, R.

2014-12-01

The recent Canterbury earthquake sequence (CES) renewed public and academic awareness concerning the clustered nature of seismicity. Multiple event occurrence in short time and space intervals is reminiscent of aftershock sequences, but aftershock is a statistical definition, not a label one can give an earthquake in real-time. Aftershocks are defined collectively as what creates the Omori event rate decay after a large event or are defined as what is taken away as "dependent events" using a declustering method. It is noteworthy that depending on the declustering method used on the Canterbury earthquake sequence, the number of independent events varies a lot. This lack of unambiguous definition of aftershocks leads to the need to investigate the amount of clustering inherent in "declustered" risk models. This is the task we concentrate on in this contribution. We start from a background source model for the Canterbury region, in which 1) centroids of events of given magnitude are distributed using a latin-hypercube lattice, 2) following the range of preferential orientations determined from stress maps and focal mechanism, 3) with length determined using the local scaling relationship and 4) rates from a and b values derived from the declustered pre-2010 catalog. We then proceed to create tens of thousands of realizations of 6 to 20 year periods, and we define criteria to identify which successions of events in the region would be perceived as a sequence. Note that the spatial clustering expected is a lower end compared to a fully uniform distribution of events. Then we perform the same exercise with rates and b-values determined from the catalog including the CES. If the pre-2010 catalog was long (or rich) enough, then the computed "stationary" rates calculated from it would include the CES declustered events (by construction, regardless of the physical meaning of or relationship between those events). In regions of low seismicity rate (e.g., Canterbury before 2010), it is hard to know how long is long enough. Using simulations, we can look at the apparent activity rate for the region over a few years (for example for events above M5.8), see how often it exceeds some level and also long those high activity rate periods last on average.

Importance of model parameterization in finite fault inversions: Application to the 1974 Mw 8.0 Peru Earthquake

USGS Publications Warehouse

Hartzell, Stephen; Langer, Charley

1993-01-01

The spatial and temporal slip distributions for the October 3, 1974 (Mw = 8.0), Peru subduction zone earthquake and its largest aftershock on November 9 (Ms = 7.1) are calculated and analyzed in terms of the inversion parameterization and tectonic significance. Teleseismic, long-period World-Wide Standard Seismograph Network, P and SH waveforms are inverted to obtain the rupture histories. We demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source, involving spatial variation in slip amplitude, risetime, and rupture time. The inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time. Well-located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone. For the main shock a hinged fault is preferred having a shallow plane with a dip of 11° and a deeper, landward plane with a dip of 30°. The preferred nucleation depth lies between 11 and 15 km. A bilateral rupture is obtained with two major concentrations of slip, one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter. For these source regions, risetimes vary from 6 to 18 s. Our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity. The slip distribution for the November 9 aftershock falls within a conspicuous hole in the main shock rupture pattern, near the hypocenter of the main shock. The November 9 event has a simple risetime function with a duration of 2 s. Aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending November 9 event and downdip from the largest main shock source region. Slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis. The large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments, and the relatively young age and high convergence rate of the subducted plate, which results in good seismic coupling near the trench axis.

Tehuantepec and Morelos-Puebla earthquakes lived and reported by the Servicio Sismológico Nacional, Mexico

NASA Astrophysics Data System (ADS)

Perez-Campos, X.

2017-12-01

On September 2017, Mexico experienced two significant inslab earthquakes with only 11 days apart from each other. Both caused severe damage in the epicentral states: Chiapas, Oaxaca, Puebla, Morelos, and Mexico City. In all senses, they tested the capabilities of the Servicio Sismológico Nacional (SSN, Mexican National Seismological Service), from the acquisition, processing, and reporting systems (both, automatic and manual), to social network and media response. In this work, we present the various aspects of the performance of the SSN and the results obtained real-time and the days after. The first earthquake occurred on 8 September within the Gulf of Tehuantepec. The SSN estimated its magnitude as Mww8.2, from W-phase inversion of local and regional data. Forty days later, it has had more than 7750 aftershocks with magnitudes larger than 2.5, making restless to inhabitants in the epicentral area. A preliminary hypo-DD relocation of the aftershocks shows two parallel SE-NW alignments. The mainshock seemed to have triggered seismicity in central Mexico, an effect previously observed by Singh et al. (1998) for coastal earthquakes. Barely 11 days had passed since this major quake. The SSN was in the middle of an intense aftershock sequence and conducting several outreach activities due to the anniversary of the 19 September 1985 (Mw8.0) earthquake, when the second quake hit. SSN located its epicenter at the border of the states of Morelos and Puebla and estimated its magnitude as Mww7.1. In this case, SSN identified only eight aftershocks, which was a similar behavior for previous inslab earthquakes in the region. Important aspects that these events have highlighted are the media and social network responses. Immediately after the first quake, SSN faced misinformation due to viral videos and social media messages predicting massive earthquakes and their relation to a solar storm that took place days before. Outreach to the public and the media became essential. Despite the excellent performance of SSN during this seismic crisis, there are challenges and areas of opportunity that should be addressed shortly; for instance, automatization of detection, location and magnitude estimation of small earthquakes, and a new approach to interact with social media.

Stress transferred by the 1995 Mw = 6.9 Kobe, Japan, shock: Effect on aftershocks and future earthquake probabilities

USGS Publications Warehouse

Toda, S.; Stein, R.S.; Reasenberg, P.A.; Dieterich, J.H.; Yoshida, A.

1998-01-01

The Kobe earthquake struck at the edge of the densely populated Osaka-Kyoto corridor in southwest Japan. We investigate how the earthquake transferred stress to nearby faults, altering their proximity to failure and thus changing earthquake probabilities. We find that relative to the pre-Kobe seismicity, Kobe aftershocks were concentrated in regions of calculated Coulomb stress increase and less common in regions of stress decrease. We quantify this relationship by forming the spatial correlation between the seismicity rate change and the Coulomb stress change. The correlation is significant for stress changes greater than 0.2-1.0 bars (0.02-0.1 MPa), and the nonlinear dependence of seismicity rate change on stress change is compatible with a state- and rate-dependent formulation for earthquake occurrence. We extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of Kobe and calculating the change in probability caused by these stress changes. Transient effects of the stress changes are incorporated by the state-dependent constitutive relation, which amplifies the permanent stress changes during the aftershock period. Earthquake probability framed in this manner is highly time-dependent, much more so than is assumed in current practice. Because the probabilities depend on several poorly known parameters of the major faults, we estimate uncertainties of the probabilities by Monte Carlo simulation. This enables us to include uncertainties on the elapsed time since the last earthquake, the repeat time and its variability, and the period of aftershock decay. We estimate that a calculated 3-bar (0.3-MPa) stress increase on the eastern section of the Arima-Takatsuki Tectonic Line (ATTL) near Kyoto causes fivefold increase in the 30-year probability of a subsequent large earthquake near Kyoto; a 2-bar (0.2-MPa) stress decrease on the western section of the ATTL results in a reduction in probability by a factor of 140 to 2000. The probability of a Mw = 6.9 earthquake within 50 km of Osaka during 1997-2007 is estimated to have risen from 5-6% before the Kobe earthquake to 7-11% afterward; during 1997-2027, it is estimated to have risen from 14-16% before Kobe to 16-22%.

Magnitude Scaling of the early displacement for the 2007, Mw 7.8 Tocopilla sequence (Chile)

NASA Astrophysics Data System (ADS)

Lancieri, M.; Fuenzalida, A.; Ruiz, S.; Madariaga, R. I.

2009-12-01

We investigate the empirical relationships between the initial portion of P and S-phase and the final event magnitude, on the Tocopilla (Chile) event and its aftershocks. Such correlations, on which real-time magnitude estimation for seismic early warning is founded, have been widely studied on several data sets, merging earthquakes generated in different tectonic settings and recorded with very different networks. The Tocopilla (Mw 7.8) earthquake, occurred along the northern Chile seismic gap on 14 November 2007, provides, together with its aftershocks, a unique opportunity of studying a homogeneous data set in terms of tectonic environment, focal mechanism, and recording network. The preliminary analysis required to build the seismic catalogue includes the automatic identification of more than 570 aftershocks using an automatic phase detector and picker algorithm, and the subsequent location of the events through a non-linear and probabilistic code. The seismic moment (M0) has been calculated by spectral modeling of P and S waves, assuming a Brune omega-square model. This analysis also yields values for the corner frequency and quality factor. The estimated range of moment magnitude for the aftershocks sequence is [2.8 - 6.8]. The correlation between the low pass filtered peak displacement (PD) and the final magnitude has been investigated for 90 events with magnitude greater than 4. These include the main event, its larger aftershock (Mw 6.8 occurred twenty-four hours after the main shock), and seven events with magnitude greater than 5.7. The recovered relationships confirm the observations of Zollo et al. [2006, 2007] of a clear correlation between distance corrected PD and final magnitude in the magnitude range [4.0 - 7.4], when considering time windows of 4 sec of P- or 2 sec of S- wave. In contrast with the previous studies, when examining time windows of 2 sec of P-wave, we surprisingly do not observe any saturation effect for magnitudes greater than 6.5, but rather a slope change in the regression curve. To explain the causative link between the PD and the final magnitude, it has been argued that an earthquake fracture that initiates with a higher flow rate of the elastic energy has an increased probability of propagating to longer distances. As consequence, the stress drop and/or active slip surface have to scale with seismic moment in the initial stage of seismic ruptures. To test this hypothesis, we investigated the correlation between radiated energy (E), corner frequency (fc) and seismic moment (M0). A first analysis performed on the entire S-phase confirms that M0 is proportional to fc^3; the apparent stress drop does not depend from M0 and that the E and M0 scale with a slope of 1.5. Therefore it appears to be no clear violation of the self-similarity. The same relationships will be investigated on signal windows of 2 sec of P- and 2 sec S-wave, used for early warning.

Nonlinear soil response in the vicinity of the Van Norman Complex following the 1994 Northridge, California, earthquake

USGS Publications Warehouse

Cultrera, G.; Boore, D.M.; Joyner, W.B.; Dietel, C.M.

1999-01-01

Ground-motion recordings obtained at the Van Norman Complex from the 1994 Northridge, California, mainshock and its aftershocks constitute an excellent data set for the analysis of soil response as a function of ground-motion amplitude. We searched for nonlinear response by comparing the Fourier spectral ratios of two pairs of sites for ground motions of different levels, using data from permanent strong-motion recorders and from specially deployed portable instruments. We also compared the amplitude dependence of the observed ratios with the amplitude dependence of the theoretical ratios obtained from 1-D linear and 1-D equivalent-linear transfer functions, using recently published borehole velocity profiles at the sites to provide the low-strain material properties. One pair of sites was at the Jensen Filtration Plant (JFP); the other pair was the Rinaldi Receiving Station (RIN) and the Los Angeles Dam (LAD). Most of the analysis was concentrated on the motions at the Jensen sites. Portable seismometers were installed at the JFP to see if the motions inside the structures housing the strong-motion recorders differed from nearby free-field motions. We recorded seven small earthquakes and found that the high-frequency, low-amplitude motions in the administration building were about 0.3 of those outside the building. This means that the lack of high frequencies on the strong-motion recordings in the administration building relative to the generator building is not due solely to nonlinear soil effects. After taking into account the effects of the buildings, however, analysis of the suite of strong- and weak-motion recordings indicates that nonlinearity occurred at the JFP. As predicted by equivalent-linear analysis, the largest events (the mainshock and the 20 March 1994 aftershock) show a significant deamplification of the high-frequency motion relative to the weak motions from aftershocks occurring many months after the mainshock. The weak-motion aftershocks recorded within 12 hours of the mainshock, however, show a relative deamplification similar to that in the mainshock. The soil behavior may be a consequence of a pore pressure buildup during large-amplitude motion that was not dissipated until sometime later. The motions at (RIN) and (LAD) are from free-field sites. The comparison among spectral ratios of the mainshock, weak-motion coda waves of the mainshock, and an aftershock within ten minutes of the mainshock indicate that some nonlinearity occurred, presumably at (RIN) because it is the softer site. The spectral ratio for the mainshock is between that calculated for pure linear response and that calculated from the equivalent-linear method, using commonly used modulus reduction and damping ratio curves. In contrast to the Jensen sites, the ratio of motions soon after the high-amplitude portion of the mainshock differs from the ratio of the mainshock motions, indicating the mechanical properties of the soil returned to the low-strain values as the high-amplitude motion ended. This may indicate a type of nonlinear soil response different from that affecting motion at the Jensen administration building.

The 2016 south Alboran earthquake (Mw = 6.4): A reactivation of the Ibero-Maghrebian region?

NASA Astrophysics Data System (ADS)

Buforn, E.; Pro, C.; Sanz de Galdeano, C.; Cantavella, J. V.; Cesca, S.; Caldeira, B.; Udías, A.; Mattesini, M.

2017-08-01

On 25 January 2016, an earthquake of magnitude Mw = 6.4 occurred at the southern part of the Alboran Sea, between southern Spain and northern Morocco. This shock was preceded by a foreshock (Mw = 5.1) and followed by a long aftershock sequence. Focal mechanism of main shock has been estimated from slip inversion of body waves at teleseismic distances. Solution corresponds to left-lateral strike-slip motion, showing a complex bilateral rupture, formed by two sub-events, with most energy propagating along a plane oriented N30°E plane dipping to the NW. Relocation of larger events of the aftershock series, show two alignments of epicentres in NE-SW and NNE-SSW direction that intersect at the epicentre of the main shock. We have estimated the focal mechanisms of the largest aftershocks from moment tensor inversion at regional distances. We have obtained two families of focal mechanisms corresponding to strike slip for the NNE-SSW alignment and thrusting motion for the NE-SW alignment. Among the faults present in the area the Al Idrisi fault (or fault zone) may be a good candidate for the source of this earthquake. The study of Coulomb Failure Stress shows that it is possible that the 2016 earthquake was triggered by the previous nearby earthquakes of 1994 (Mw = 5.8) and 2004 (Mw = 6.3). The possible seismic reactivation of the central part of the Ibero-Maghrebian region is an open question, but it is clear that the occurrence of the 2016 earthquake confirms that from 1994 the seismicity of central part of IMR is increasing and that focal mechanism of largest earthquakes in this central part correspond to complex ruptures (or zone of fault).

Insights on the 1990 Bohol Tsunamigenic Earthquake, Bohol Island, Philippines

NASA Astrophysics Data System (ADS)

Besana, G. M.; Daligdig, J. A.; Abigania, M. T.; Talisic, J. E.; Evangelista, N.

2004-12-01

The February 8, 1990 earthquake at Bohol area is one of the few strong earthquakes that have affected central Philippines since the early 1900's. This M6.0 1990 Bohol event nonetheless wrought havoc to at least 16 municipalities, caused numerous casualties, injured about three hundred people, rendered several thousand homeless and evacuated from the coastal areas, and damaged at least P154 million worth of properties. The epicenter of this earthquake was initially placed onshore at 17km east of Tagbilaran City and was attributed to the movement along the Alicia Thrust Fault- a fault trending northeast-southwest. Noticeably, there was no surface rupture and the succeeding aftershocks clustered along a northeast-southwest trend off the eastern shore of Bohol island. In addition, the southeastern part of Bohol island experienced tsunami inundation particularly the municipalities of Jagna, Duero, Guindulman, Garcia Hernandez, and Valencia. In this study, several issues were resolved regarding this seismic event. First, the 1990 Bohol earthquake was generated along an offshore thrust fault based on the reviews of seismicity data from the NEIC. -Post-determined plots of the mainshock and aftershocks indicate offshore event with focal mechanism solutions that imply thrust fault activity. Intensity data likewise indicates that intense ground shaking was mainly felt in the southeastern part of the island. Second, recent field investigations undertaken clearly indicated a widespread tsunami inundation wherein the southeastern shorelines of Bohol likewise experienced a regional retreat in sea level several minutes after the strong ground shaking. Lastly, such tsunamigenic structure could somehow explain the anomalously large waves that impacted Camiguin island, an island more than 50km southeast of Bohol. A reconstruction of true tsunami heights and runup distances was also undertaken based from eyewitness accounts. Future works would involve relocation of aftershocks and numerical modeling of tsunami.

Repeating Earthquakes Following an Mw 4.4 Earthquake Near Luther, Oklahoma

NASA Astrophysics Data System (ADS)

Clements, T.; Keranen, K. M.; Savage, H. M.

2015-12-01

An Mw 4.4 earthquake on April 16, 2013 near Luther, OK was one of the earliest M4+ earthquakes in central Oklahoma, following the Prague sequence in 2011. A network of four local broadband seismometers deployed within a day of the Mw 4.4 event, along with six Oklahoma netquake stations, recorded more than 500 aftershocks in the two weeks following the Luther earthquake. Here we use HypoDD (Waldhauser & Ellsworth, 2000) and waveform cross-correlation to obtain precise aftershock locations. The location uncertainty, calculated using the SVD method in HypoDD, is ~15 m horizontally and ~ 35 m vertically. The earthquakes define a near vertical, NE-SW striking fault plane. Events occur at depths from 2 km to 3.5 km within the granitic basement, with a small fraction of events shallower, near the sediment-basement interface. Earthquakes occur within a zone of ~200 meters thickness on either side of the best-fitting fault surface. We use an equivalency class algorithm to identity clusters of repeating events, defined as event pairs with median three-component correlation > 0.97 across common stations (Aster & Scott, 1993). Repeating events occur as doublets of only two events in over 50% of cases; overall, 41% of earthquakes recorded occur as repeating events. The recurrence intervals for the repeating events range from minutes to days, with common recurrence intervals of less than two minutes. While clusters occur in tight dimensions, commonly of 80 m x 200 m, aftershocks occur in 3 distinct ~2km x 2km-sized patches along the fault. Our analysis suggests that with rapidly deployed local arrays, the plethora of ~Mw 4 earthquakes occurring in Oklahoma and Southern Kansas can be used to investigate the earthquake rupture process and the role of damage zones.

Postearthquake relaxation and aftershock accumulation linearly related after the 2003 M 6.5 Chengkung, Taiwan, and the 2004 M 6.0 Parkfield, California, earthquakes

USGS Publications Warehouse

Savage, J.C.; Yu, S.-B.

2007-01-01

We treat both the number of earthquakes and the deformation following a mainshock as the superposition of a steady background accumulation and the post-earthquake process. The preseismic displacement and seismicity rates ru and rE are used as estimates of the background rates. Let t be the time after the mainshock, u(t) + u0 the postseismic displacement less the background accumulation rut, and ??N(t) the observed cumulative number of postseismic earthquakes less the background accumulation rE t. For the first 160 days (duration limited by the occurrence of another nearby earthquake) following the Chengkung (M 6.5, 10 December 2003, eastern Taiwan) and the first 560 days following the Parkfield (M 6.0, 28 September 2004, central California) earthquakes u(t) + u0 is a linear function of ??N(t). The aftershock accumulation ??N(t) for both earthquakes is described by the modified Omori Law d??N/dt ?? (1 + t/??)-p with p = 0.96 and ?? = 0.03 days. Although the Chengkung earthquake involved sinistral, reverse slip on a moderately dipping fault and the Parkfield earthquake right-lateral slip on a near-vertical fault, the earthquakes share an unusual feature: both occurred on faults exhibiting interseismic fault creep at the surface. The source of the observed postseismic deformation appears to be afterslip on the coseismic rupture. The linear relation between u(t) + u0 and N(t) suggests that this afterslip also generates the aftershocks. The linear relation between u(t) + u0 and ??N(t) obtains after neither the 1999 M 7.1 Hector Mine (southern California) nor the 1999 M 7.6 Chi-Chi (central Taiwan) earthquakes, neither of which occurred on fault segments exhibiting fault creep.

Foreshocks and aftershocks of the 2014 M8.1 Iquique, northern Chile, megathrust earthquake

NASA Astrophysics Data System (ADS)

Soto, Hugo; Sippl, Christian; Schurr, Bernd; Asch, Günter; Tilmann, Frederik; Comte, Diana; Ruiz, Sergio; Oncken, Onno

2017-04-01

The M8.1 2014 Iquique earthquake broke a central piece of the long-standing, >500 km long northern Chile seismic gap. The Iquique earthquake sequence started off with a M6.7 thrust event presumably in the upper plate seaward of the Chilean coastline. Deformation was quickly transferred onto the megathrust with three more events of M>6 until it culminated in the mainshock that broke a compact asperity with possibly up to 12 m of slip two weeks later. The mainshock was followed by vigorous aftershock sequence, including a M7.7 event just south of the main slip patch approx. two days later. The whole sequence of events was well recorded by the Integrated Plate Boundary Observatory Chile (IPOC). The IPOC network was complemented quickly after the first large foreshock by 60 additional temporary seismic stations deployed by the University of Chile and the German Research Centre for Geosciences - GFZ. Processing the continuous data with an automated multi-step process for event detection, association and phase picking, we located more than 25,000 events for one month preceding and nine months following the Iquique mainshock. Whereas the foreshocks skirt around the updip limit of the mainshock asperity, the aftershocks agglomerate in two belts, one updip and one downdip of the main asperity offshore the Chilean coast. The deepest events on the plate interface reach 65 km depth in two separated clusters under the coastal cordillera, which show a significant difference in dip, indicating strong long-wavelength slab topography or a slab tear. We will also analyze upper- and deeper intra-plate seismicity and in particular its changes following the Iquique mainshock.

The 2016 Mw 7.8 Pedernales, Ecuador earthquake: Minimum 1D Velocity Model and Regional Moment Tensors Based on the Aftershock Sequence

NASA Astrophysics Data System (ADS)

Leon-Rios, S.; Aguiar, A. L.; Bie, L.; Edwards, B.; Fuenzalida Velasco, A. J.; Holt, J.; Garth, T.; González, P. J.; Rietbrock, A.; Agurto-Detzel, H.; Charvis, P.; Font, Y.; Nocquet, J. M.; Regnier, M. M.; Renouard, A.; Mercerat, D.; Pernoud, M.; Beck, S. L.; Meltzer, A.; Soto-Cordero, L.; Alvarado, A. P.; Perrault, M.; Ruiz, M. C.; Santo, J.

2017-12-01

On 16th April 2016, a Mw 7.8 mega-thrust earthquake occurred in northern Ecuador, close to the city of Pedernales. The event that ruptured an area of 120 x 60 km led to a deployment of a large array of seismic instruments as part of a collaborative project between the Geophysical Institute of Ecuador (IGEPN), Lehigh University (USA), University of Arizona (USA), Geoazur (France) and the University of Liverpool (UK). This dense seismic network, with more than 80 stations, includes broadband, short period, strong motion and OBS instruments were recording up to one year after the mainshock. Using the recorded data set, we manually analysed and located 450 events. Selection was based on the largest aftershocks (Ml > 3.5 from the IGEPN catalogue) and additional preliminary automatic locations to increase the observation density in the southern part of the network. High quality P and S arrival times plus several reference velocity structures were used to create more than 80.000 input models in order to obtain a minimum 1D velocity model and associated P and S waves station correction terms. Aftershock locations are concentrated in NW-SE striking lineaments reaching the trench. Additionally, we computed moment tensor solutions for a subset of earthquakes to independently confirm hypocentre depths using a full waveform simulation approach. Based on this analysis we can identify normal and strike-slip events located in the marine forearc and close to the trench. This type of activity has been observed in previous megathrust earthquakes (e.g. Maule 2010 and Tohoku-Oki 2011), and might be associated with extensional re-activation of existing fault systems due to a large event located on the megathrust fault.

Response of seismicity to Coulomb stress triggers and shadows of the 1999 Mw=7.6 Chi-Chi, Taiwan, earthquake

USGS Publications Warehouse

Ma, K.-F.; Chan, C.-H.; Stein, R.S.

2005-01-01

The correlation between static Coulomb stress increases and aftershocks has thus far provided the strongest evidence that stress changes promote seismicity, a correlation that the Chi-Chi earthquake well exhibits. Several studies have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the aftershocks are optimally oriented for failure. Here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi-Chi we find a 28% increase for thrust and an 18% increase for strike-slip mechanisms, commensurate with increases reported for other large main shocks. However, perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows, or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate, conditions that are met at Chi-Chi. We find four lobes with statistically significant seismicity rate declines of 40-90% for 50 months, and they coincide with the stress shadows calculated for strike-slip faults, the dominant faulting mechanism. The rate drops are evident in uniform cell calculations, 100-month time series, and by visual inspection of the M ??? 3 seismicity. An additional reason why detection of such declines has proven so rare emerges from this study: there is a widespread increase in seismicity rate during the first 3 months after Chi-Chi, and perhaps many other main shocks, that might be associated with a different mechanism. Copyright 2005 by the American Geophysical Union.

The Chiloé Mw 7.6 earthquake of 2016 December 25 in Southern Chile and its relation to the Mw 9.5 1960 Valdivia earthquake

NASA Astrophysics Data System (ADS)

Lange, Dietrich; Ruiz, Javier; Carrasco, Sebastián; Manríquez, Paula

2018-04-01

On 2016 December 25, an Mw 7.6 earthquake broke a portion of the Southern Chilean subduction zone south of Chiloé Island, located in the central part of the Mw 9.5 1960 Valdivia earthquake. This region is characterized by repeated earthquakes in 1960 and historical times with very sparse interseismic activity due to the subduction of a young (˜15 Ma), and therefore hot, oceanic plate. We estimate the coseismic slip distribution based on a kinematic finite-fault source model, and through joint inversion of teleseismic body waves and strong motion data. The coseismic slip model yields a total seismic moment of 3.94 × 1020 N.m that occurred over ˜30 s, with the rupture propagating mainly downdip, reaching a peak slip of ˜4.2 m. Regional moment tensor inversion of stronger aftershocks reveals thrust type faulting at depths of the plate interface. The fore- and aftershock seismicity is mostly related to the subduction interface with sparse seismicity in the overriding crust. The 2016 Chiloé event broke a region with increased locking and most likely broke an asperity of the 1960 earthquake. The updip limit of the main event, aftershocks, foreshocks and interseismic activity are spatially similar, located ˜15 km offshore and parallel to Chiloé Islands west coast. The coseismic slip model of the 2016 Chiloé earthquake suggests a peak slip of 4.2 m that locally exceeds the 3.38 m slip deficit that has accumulated since 1960. Therefore, the 2016 Chiloé earthquake possibly released strain that has built up prior to the 1960 Valdivia earthquake.

Passive seismic imaging based on seismic interferometry: method and its application to image the structure around the 2013 Mw6.6 Lushan earthquake

NASA Astrophysics Data System (ADS)

Gu, N.; Zhang, H.

2017-12-01

Seismic imaging of fault zones generally involves seismic velocity tomography using first arrival times or full waveforms from earthquakes occurring around the fault zones. However, in most cases seismic velocity tomography only gives smooth image of the fault zone structure. To get high-resolution structure of the fault zones, seismic migration using active seismic data needs to be used. But it is generally too expensive to conduct active seismic surveys, even for 2D. Here we propose to apply the passive seismic imaging method based on seismic interferometry to image fault zone detailed structures. Seismic interferometry generally refers to the construction of new seismic records for virtual sources and receivers by cross correlating and stacking the seismic records on physical receivers from physical sources. In this study, we utilize seismic waveforms recorded on surface seismic stations for each earthquake to construct zero-offset seismic record at each earthquake location as if there was a virtual receiver at each earthquake location. We have applied this method to image the fault zone structure around the 2013 Mw6.6 Lushan earthquake. After the occurrence of the mainshock, a 29-station temporary array is installed to monitor aftershocks. In this study, we first select aftershocks along several vertical cross sections approximately normal to the fault strike. Then we create several zero-offset seismic reflection sections by seismic interferometry with seismic waveforms from aftershocks around each section. Finally we migrate these zero-offset sections to create seismic structures around the fault zones. From these migration images, we can clearly identify strong reflectors, which correspond to major reverse fault where the mainshock occurs. This application shows that it is possible to image detailed fault zone structures with passive seismic sources.

Postseismic relaxation and aftershocks

USGS Publications Warehouse

Savage, J.C.; Svarc, J.L.; Yu, S.-B.

2007-01-01

Perfettini et al. (2005) suggested that the temporal dependence of surface displacements u(t) measured in the epicentral area following an earthquake is related to N(t), the cumulative number of aftershocks, by the equation u(t) = a + bt + cN(t) + d(1 - e-??t), where a, b, c, d, and ?? are constants chosen to fit the data and t is the postearthquake time. N(t) appears in the expression for u(t) because both the aftershocks and a portion of u(t) are thought to be driven by the same source, postseismic fault creep at subseismogenic depths on the downdip extension of the coseismic rupture. We show that this equation with the actually observed N(t) fits the postseismic displacements recorded on several baselines following each of five earthquakes: 1999 M7.6 Chi-Chi (Taiwan), 1999 M7.1 Hector Mine (southern California), 2002 M7.9 Denali (central Alaska), 2003 M6.5 San Simeon (central California), and 2004 M6.0 Parkfield (central California) earthquakes. Although there are plausible physical interpretations for each of the terms in the expression for u(t), the large number of adjustable constants (a, b, c, d, and ??) involved in fitting the rather simple postseismic displacements diminishes the significance of the fit. Because the observed N(t) is well fit by the modified Omori's law, fault creep at depth presumably exhibits the same temporal dependence. That dependence could be explained if the rheology of the fault downdip from the coseismic rupture is consistent with ordinary transient creep. Montesi (2004) demonstrated that power law creep across a shear zone at depth would also produce that temporal signal.

Teleseismic depth estimation of the 2015 Gorkha-Nepal aftershocks

NASA Astrophysics Data System (ADS)

Letort, Jean; Bollinger, Laurent; Lyon-Caen, Helene; Guilhem, Aurélie; Cano, Yoann; Baillard, Christian; Adhikari, Lok Bijaya

2016-12-01

The depth of 61 aftershocks of the 2015 April 25 Gorkha, Nepal earthquake, that occurred within the first 20 d following the main shock, is constrained using time delays between teleseismic P phases and depth phases (pP and sP). The detection and identification of these phases are automatically processed using the cepstral method developed by Letort et al., and are validated with computed radiation patterns from the most probable focal mechanisms. The events are found to be relatively shallow (13.1 ± 3.9 km). Because depth estimations could potentially be biased by the method, velocity model or selected data, we also evaluate the depth resolution of the events from local catalogues by extracting 138 events with assumed well-constrained depth estimations. Comparison between the teleseismic depths and the depths from local and regional catalogues helps decrease epistemic uncertainties, and shows that the seismicity is clustered in a narrow band between 10 and 15 km depth. Given the geometry and depth of the major tectonic structures, most aftershocks are probably located in the immediate vicinity of the Main Himalayan Thrust (MHT) shear zone. The mid-crustal ramp of the flat/ramp MHT system is not resolved indicating that its height is moderate (less than 5-10 km) in the trace of the sections that ruptured on April 25. However, the seismicity depth range widens and deepens through an adjacent section to the east, a region that failed on 2015 May 12 during an Mw 7.3 earthquake. This deeper seismicity could reflect a step-down of the basal detachment of the MHT, a lateral structural variation which probably acted as a barrier to the dynamic rupture propagation.

Low stress drops observed for aftershocks of the 2011 Mw 5.7 Prague, Oklahoma, earthquake

NASA Astrophysics Data System (ADS)

Sumy, Danielle F.; Neighbors, Corrie J.; Cochran, Elizabeth S.; Keranen, Katie M.

2017-05-01

In November 2011, three Mw ≥ 4.8 earthquakes and thousands of aftershocks occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that wastewater injection induced a Mw 4.8 foreshock, which subsequently triggered a Mw 5.7 mainshock. We examine source properties of aftershocks with a standard Brune-type spectral model and jointly solve for seismic moment (M0), corner frequency (f0), and kappa (κ) with an iterative Gauss-Newton global downhill optimization method. We examine 934 earthquakes with initial moment magnitudes (Mw) between 0.33 and 4.99 based on the pseudospectral acceleration and recover reasonable M0, f0, and κ for 87 earthquakes with Mw 1.83-3.51 determined by spectral fit. We use M0 and f0 to estimate the Brune-type stress drop, assuming a circular fault and shear-wave velocity at the hypocentral depth of the event. Our observations suggest that stress drops range between 0.005 and 4.8 MPa with a median of 0.2 MPa (0.03-26.4 MPa with a median of 1.1 MPa for Madariaga-type), which is significantly lower than typical eastern United States intraplate events (>10 MPa). We find that stress drops correlate weakly with hypocentral depth and magnitude. Additionally, we find the stress drops increase with time after the mainshock, although temporal variation in stress drop is difficult to separate from spatial heterogeneity and changing event locations. The overall low median stress drop suggests that the fault segments may have been primed to fail as a result of high pore fluid pressures, likely related to nearby wastewater injection.

Aftershock Sequences and Seismic-Like Organization of Acoustic Events Produced by a Single Propagating Crack

NASA Astrophysics Data System (ADS)

Alizee, D.; Bonamy, D.

2017-12-01

In inhomogeneous brittle solids like rocks, concrete or ceramics, one usually distinguish nominally brittle fracture, driven by the propagation of a single crack from quasibrittle one, resulting from the accumulation of many microcracks. The latter goes along with intermittent sharp noise, as e.g. revealed by the acoustic emission observed in lab scale compressive fracture experiments or at geophysical scale in the seismic activity. In both cases, statistical analyses have revealed a complex time-energy organization into aftershock sequences obeying a range of robust empirical scaling laws (the Omori-Utsu, productivity and Bath's law) that help carry out seismic hazard analysis and damage mitigation. These laws are usually conjectured to emerge from the collective dynamics of microcrack nucleation. In the experiments presented at AGU, we will show that such a statistical organization is not specific to the quasi-brittle multicracking situations, but also rules the acoustic events produced by a single crack slowly driven in an artificial rock made of sintered polymer beads. This simpler situation has advantageous properties (statistical stationarity in particular) permitting us to uncover the origins of these seismic laws: Both productivity law and Bath's law result from the scale free statistics for event energy and Omori-Utsu law results from the scale-free statistics of inter-event time. This yields predictions on how the associated parameters are related, which were analytically derived. Surprisingly, the so-obtained relations are also compatible with observations on lab scale compressive fracture experiments, suggesting that, in these complex multicracking situations also, the organization into aftershock sequences and associated seismic laws are also ruled by the propagation of individual microcrack fronts, and not by the collective, stress-mediated, microcrack nucleation. Conversely, the relations are not fulfilled in seismology signals, suggesting that additional ingredient should be taken into account.

CMP reflection imaging via interferometry of distributed subsurface sources

NASA Astrophysics Data System (ADS)

Kim, D.; Brown, L. D.; Quiros, D. A.

2015-12-01

The theoretical foundations of recovering body wave energy via seismic interferometry are well established. However in practice, such recovery remains problematic. Here, synthetic seismograms computed for subsurface sources are used to evaluate the geometrical combinations of realistic ambient source and receiver distributions that result in useful recovery of virtual body waves. This study illustrates how surface receiver arrays that span a limited distribution suite of sources, can be processed to reproduce virtual shot gathers that result in CMP gathers which can be effectively stacked with traditional normal moveout corrections. To verify the feasibility of the approach in practice, seismic recordings of 50 aftershocks following the magnitude of 5.8 Virginia earthquake occurred in August, 2011 have been processed using seismic interferometry to produce seismic reflection images of the crustal structure above and beneath the aftershock cluster. Although monotonic noise proved to be problematic by significantly reducing the number of usable recordings, the edited dataset resulted in stacked seismic sections characterized by coherent reflections that resemble those seen on a nearby conventional reflection survey. In particular, "virtual" reflections at travel times of 3 to 4 seconds suggest reflector sat approximately 7 to 12 km depth that would seem to correspond to imbricate thrust structures formed during the Appalachian orogeny. The approach described here represents a promising new means of body wave imaging of 3D structure that can be applied to a wide array of geologic and energy problems. Unlike other imaging techniques using natural sources, this technique does not require precise source locations or times. It can thus exploit aftershocks too small for conventional analyses. This method can be applied to any type of microseismic cloud, whether tectonic, volcanic or man-made.

The 2013, Mw 7.7 Balochistan earthquake, energetic strike-slip reactivation of a thrust fault

NASA Astrophysics Data System (ADS)

Avouac, Jean-Philippe; Ayoub, Francois; Wei, Shengji; Ampuero, Jean-Paul; Meng, Lingsen; Leprince, Sebastien; Jolivet, Romain; Duputel, Zacharie; Helmberger, Don

2014-04-01

We analyse the Mw 7.7 Balochistan earthquake of 09/24/2013 based on ground surface deformation measured from sub-pixel correlation of Landsat-8 images, combined with back-projection and finite source modeling of teleseismic waveforms. The earthquake nucleated south of the Chaman strike-slip fault and propagated southwestward along the Hoshab fault at the front of the Kech Band. The rupture was mostly unilateral, propagated at 3 km/s on average and produced a 200 km surface fault trace with purely strike-slip displacement peaking to 10 m and averaging around 6 m. The finite source model shows that slip was maximum near the surface. Although the Hoshab fault is dipping by 45° to the North, in accordance with its origin as a thrust fault within the Makran accretionary prism, slip was nearly purely strike-slip during that earthquake. Large seismic slip on such a non-optimally oriented fault was enhanced possibly due to the influence of the free surface on dynamic stresses or to particular properties of the fault zone allowing for strong dynamic weakening. Strike-slip faulting on thrust fault within the eastern Makran is interpreted as due to eastward extrusion of the accretionary prism as it bulges out over the Indian plate. Portions of the Makran megathrust, some thrust faults in the Kirthar range and strike-slip faults within the Chaman fault system have been brought closer to failure by this earthquake. Aftershocks cluster within the Chaman fault system north of the epicenter, opposite to the direction of rupture propagation. By contrast, few aftershocks were detected in the area of maximum moment release. In this example, aftershocks cannot be used to infer earthquake characteristics.

On the possible effect of round-the-world surface seismic waves in the dynamics of repeated shocks after strong earthquakes

NASA Astrophysics Data System (ADS)

Zotov, O. D.; Zavyalov, A. D.; Guglielmi, A. V.; Lavrov, I. P.

2018-01-01

Based on the observation data for hundreds of the main shocks and thousands of aftershocks, the existence of effect of round-the-world surface seismic waves is demonstrated (let us conditionally refer to them as a round-the-world seismic echo) and the manifestations of this effect in the dynamics of the repeated shocks of strong earthquakes are analyzed. At the same time, we by no means believe this effect has been fully proven. We only present a version of our own understanding of the physical causes of the observed phenomenon and analyze the regularities in its manifestation. The effect is that the surface waves excited in the Earth by the main shock make a full revolution around the Earth and excite a strong aftershock in the epicentral zone of the main shock. In our opinion, the physical nature of this phenomenon consists in the fact that the superposition leads to a concentration of wave energy when the convergent surface waves reach the epicentral zone (cumulative effect). The effect of the first seismic echo is most manifest. Thus, the present work supports our hypothesis of the activation of rock failure under the cumulative impact of an round-the-world seismic echo on the source area which is releasing ("cooling") after the main shock. The spatial regularities in the manifestations of this effect are established, and the independence of the probability of its occurrence on the main shock magnitude is revealed. The effect of a round-the-world seismic echo can be used to improve the reliability of the forecasts of strong aftershocks in determining the scenario for the seismic process developing in the epicentral zone of a strong earthquake that has taken place.

Multifractal Omori law for earthquake triggering: new tests on the California, Japan and worldwide catalogues

NASA Astrophysics Data System (ADS)

Ouillon, G.; Sornette, D.; Ribeiro, E.

2009-07-01

The Multifractal Stress-Activated model is a statistical model of triggered seismicity based on mechanical and thermodynamic principles. It predicts that, above a triggering magnitude cut-off M0, the exponent p of the Omori law for the time decay of the rate of aftershocks is a linear increasing function p(M) = a0M + b0 of the main shock magnitude M. We previously reported empirical support for this prediction, using the Southern California Earthquake Center (SCEC) catalogue. Here, we confirm this observation using an updated, longer version of the same catalogue, as well as new methods to estimate p. One of this methods is the newly defined Scaling Function Analysis (SFA), adapted from the wavelet transform. This method is able to measure a mathematical singularity (hence a p-value), erasing the possible regular part of a time-series. The SFA also proves particularly efficient to reveal the coexistence and superposition of several types of relaxation laws (typical Omori sequences and short-lived swarms sequences) which can be mixed within the same catalogue. Another new method consists in monitoring the largest aftershock magnitude observed in successive time intervals, and thus shortcuts the problem of missing events with small magnitudes in aftershock catalogues. The same methods are used on data from the worldwide Harvard Centroid Moment Tensor (CMT) catalogue and show results compatible with those of Southern California. For the Japan Meteorological Agency (JMA) catalogue, we still observe a linear dependence of p on M, but with a smaller slope. The SFA shows however that results for this catalogue may be biased by numerous swarm sequences, despite our efforts to remove them before the analysis.

Broadband Ground Motion Synthesis of the 1999 Turkey Earthquakes Based On: 3-D Velocity Inversion, Finite Difference Calculations and Emprical Greens Functions

NASA Astrophysics Data System (ADS)

Gok, R.; Kalafat, D.; Hutchings, L.

2003-12-01

We analyze over 3,500 aftershocks recorded by several seismic networks during the 1999 Marmara, Turkey earthquakes. The analysis provides source parameters of the aftershocks, a three-dimensional velocity structure from tomographic inversion, an input three-dimensional velocity model for a finite difference wave propagation code (E3D, Larsen 1998), and records available for use as empirical Green's functions. Ultimately our goal is to model the 1999 earthquakes from DC to 25 Hz and study fault rupture mechanics and kinematic rupture models. We performed the simultaneous inversion for hypocenter locations and three-dimensional P- and S- wave velocity structure of Marmara Region using SIMULPS14 along with 2,500 events with more than eight P- readings and an azimuthal gap of less than 180\\deg. The resolution of calculated velocity structure is better in the eastern Marmara than the western Marmara region due to the dense ray coverage. We used the obtained velocity structure as input into the finite difference algorithm and validated the model by using M < 4 earthquakes as point sources and matching long period waveforms (f < 0.5 Hz). We also obtained Mo, fc and individual station kappa values for over 500 events by performing a simultaneous inversion to fit these parameters with a Brune source model. We used the results of the source inversion to deconvolve out a Brune model from small to moderate size earthquakes (M < 4.0) to obtain empirical Green's function (EGF) for the higher frequency range of ground motion synthesis (0.5 < f > 25 Hz). We additionally obtained the source scaling relation (energy-moment) of these aftershocks. We have generated several scenarios constrained by a priori knowledge of the Izmit and Duzce rupture parameters to validate our prediction capability.

Spatio-temporal analysis of Modified Omori law in Bayesian framework

NASA Astrophysics Data System (ADS)

Rezanezhad, V.; Narteau, C.; Shebalin, P.; Zoeller, G.; Holschneider, M.

2017-12-01

This work presents a study of the spatio temporal evolution of the modified Omori parameters in southern California in then time period of 1981-2016. A nearest-neighbor approach is applied for earthquake clustering. This study targets small mainshocks and corresponding big aftershocks ( 2.5 ≤ mmainshocks ≤ 4.5 and 1.8 ≤ maftershocks ≤ 2.8 ). We invert for the spatio temporal behavior of c and p values (especially c) all over the area using a MCMC based maximum likelihood estimator. As parameterizing families we use Voronoi cells with randomly distributed cell centers. Considering that c value represents a physical character like stress change we expect to see a coherent c value pattern over seismologically coacting areas. This correlation of c valus can actually be seen for the San Andreas, San Jacinto and Elsinore faults. Moreover, the depth dependency of c value is studied which shows a linear behavior of log(c) with respect to aftershock's depth within 5 to 15 km depth.

Topographical and geological amplification: case studies and engineering implications

USGS Publications Warehouse

Celebi, M.

1991-01-01

Topographical and geological amplification that occurred during past earthquakes are quantified using spectral ratios of recorded motions. Several cases are presented from the 1985 Chilean and Mexican earthquakes as well as the 1983 Coalinga (California) and 1987 Supersition Hills (California) earthquake. The strong motions recorded in Mexico City during the 1985 Michoacan earthquake are supplemented by ambient motions recorded within Mexico City to quantify the now well known resonating frequencies of the Mexico City lakebed. Topographical amplification in Canal Beagle (Chile), Coalinga and Superstition Hills (California) are quantified using the ratios derived from the aftershocks following the earthquakes. A special dense array was deployed to record the aftershocks in each case. The implications of both geological and topographical amplification are discussed in light of current code provisions. The observed geological amplifications has already influenced the code provisions. Suggestions are made to the effect that the codes should include further provisions to take the amplification due to topography into account. ?? 1991.

Evaluating the co-production of a near real time Earthquake Aftershock forecasting tool for humanitarian risk assessment and emergency planning

NASA Astrophysics Data System (ADS)

Quinn, Keira; Hope, Max; McCloskey, John; NicBhloscaidh, Mairead; Jimenez, Abigail; Dunlop, Paul

2015-04-01

Concern Worldwide and the University of Ulster Geophysics Research Group are engaged in a project to co-produce a suite of software and mapping tools to assess aftershock hazard in near real-time during the emergency response phase of earthquake disaster, and inform humanitarian emergency planning and response activities. This paper uses a social learning approach to evaluate this co-production process. Following Wenger (1999) we differentiate between the earthquake science and humanitarian communities of practice (CoP) along three dimensions: enterprise (the purpose of CoPs and the problems participants are working to address), repertoire (knowledge, skills, language), and identity (values and boundaries). We examine the effectiveness of learning between CoP, focusing on boundary work and objects, and various organisational structures and aspects of the wider political economy of learning that enable and hinder the co-production process. We conclude by identifying a number of ways to more effectively integrate earthquake science into humanitarian decision-making, policy development and programme design.

The Parkfield-Cholame, California, earthquakes of June-August, 1966; instrumental seismic studies

USGS Publications Warehouse

Eaton, Jerry P.

1967-01-01

U.S. Geological Survey instrumental seismic studies in the Parkfield-Cholame area consist of three related parts that were undertaken as pilot studies in a program designed to develop improved tools and concepts for investigating the properties and behavior of the San Andreas fault. These studies include: 1. The long=term monitoring of the seismic background on the San Andreas fault in Cholame Valley by means of a short-period Benioff seismograph station at Gold Hill. 2. The investigation of the geometry of the zone of aftershocks of the June 27 earthquakes by means of a small portable cluster of short-period, primarily vertical-component seismographs. 3. The seismic-refraction calibration of the region enclosing the aftershock source by means of three short reversed refraction profiles and a "calibration shot" near the epicenter of the main June 27 earthquake. This brief report outlines the work that has been completed and presents some preliminary results obtained from analysis of records from Gold Hill and the portable cluster.

Tidal triggering of earthquakes in the Ning'er area of Yunnan Province, China

NASA Astrophysics Data System (ADS)

Xie, Chaodi; Lei, Xinglin; Zhao, Xiaoyan; Ma, Qingbo; Yang, Simeng; Wang, Yingnan

2017-05-01

To investigate the potential effect of tidal modulation on the seismicity in the Ning'er area, a seismically and geothermally active zone in Yunnan Province, China, we studied the correlation between Earth tides and the occurrence of M ≥ 6.0 earthquakes dating back to 1970, as well as their aftershock sequences, using theoretically calculated tidal stresses and a statistical test. The results show a significant correlation between Earth tides and the occurrence of earthquakes. Six of seven main events occurred when the Earth tide increased the Coulomb failure stress on the source fault. Four main events occurred in a narrow range of phase angle corresponding to the maximum loading rate of tidal stress. Furthermore, the histories of the aftershock sequence as a function of the tidal phases demonstrate clear tidal modulation with a high significance. Thus, we conclude that Earth tides have a clear role in triggering (or modulating) the rupture of the fault systems in the Ning'er area.

Post-earthquake building safety assessments for the Canterbury Earthquakes

USGS Publications Warehouse

Marshall, J.; Barnes, J.; Gould, N.; Jaiswal, K.; Lizundia, B.; Swanson, David A.; Turner, F.

2012-01-01

This paper explores the post-earthquake building assessment program that was utilized in Christchurch, New Zealand following the Canterbury Sequence of earthquakes beginning with the Magnitude (Mw.) 7.1 Darfield event in September 2010. The aftershocks or triggered events, two of which exceeded Mw 6.0, continued with events in February and June 2011 causing the greatest amount of damage. More than 70,000 building safety assessments were completed following the February event. The timeline and assessment procedures will be discussed including the use of rapid response teams, selection of indicator buildings to monitor damage following aftershocks, risk assessments for demolition of red-tagged buildings, the use of task forces to address management of the heavily damaged downtown area and the process of demolition. Through the post-event safety assessment program that occurred throughout the Canterbury Sequence of earthquakes, many important lessons can be learned that will benefit future response to natural hazards that have potential to damage structures.

The Iquique earthquake sequence of April 2014: Bayesian modeling accounting for prediction uncertainty

USGS Publications Warehouse

Duputel, Zacharie; Jiang, Junle; Jolivet, Romain; Simons, Mark; Rivera, Luis; Ampuero, Jean-Paul; Riel, Bryan; Owen, Susan E; Moore, Angelyn W; Samsonov, Sergey V; Ortega Culaciati, Francisco; Minson, Sarah E.

2016-01-01

The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On 1 April 2014, this region was struck by a large earthquake following a two week long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami, and seismic data, to produce a reliable kinematic slip model of the Mw=8.1 main shock and a static slip model of the Mw=7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Green's functions, both static and dynamic, while avoiding nonphysical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and updip of high-frequency sources inferred by back-projection analysis. Both the main shock and the Mw=7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region.

Intraslab rupture triggering megathrust rupture coseismically in the 17 December 2016 Solomon Islands Mw 7.9 earthquake

NASA Astrophysics Data System (ADS)

Lay, Thorne; Ye, Lingling; Ammon, Charles J.; Kanamori, Hiroo

2017-02-01

The 17 December 2016 Solomon Islands earthquake (Mw 7.9) initiated 103 km deep in the subducting Solomon Sea slab near the junction of the Solomon Islands and New Britain trenches. Most aftershocks are located near the Solomon Islands plate boundary megathrust west of Bougainville, where previous large interplate thrust faulting earthquakes occurred in 1995 (Mw 7.7) and 1971 (Mw 8.0). Teleseismic body wave modeling and aftershock relocations indicate that the initial 30 s of the 2016 rupture occurred over depths of 90 to 120 km on an intraslab fault dipping 30° to the southwest, almost perpendicular to the dipping slab interface. The next 50 s of rupture took place at depths of 32 to 47 km in the deeper (Domain C) portion of the overlying megathrust fault dipping 35° to the northeast. High susceptibility to triggering in the region accounts for this compound rupture of two separate fault planes.

Omori Law After Exogenous Shocks on Supplier-Customer Network

NASA Astrophysics Data System (ADS)

Fujiwara, Yoshi

We study the relaxation process of a supplier-customer network after mass destruction due to two giant earthquakes, Kobe 1995 and East Japan 2011, by investigating the number of chained failures. Firstly, a mass destruction and intervention of business activities in the damaged areas can be considered as a main shock. The exogenous shock was propagated on the supplier-customer network deteriorating financial states of other firms, even if they are not located in geographical neighbors. To quantify such aftershocks, we use chained failures on the network assuming that they indicate the trace of propagation of shocks. We show that the number of chained failures in its temporal change obeys an Omori-law, a power-law relaxation. This finding implies that the relaxation is much more sluggish than one would naively expect, and that it might be possible to estimate the extent and duration of aftershocks by using the empirical law. Several issues are discussed including the origin of the long-time relaxation.

The 2015 Illapel earthquake, central Chile: A type case for a characteristic earthquake?

NASA Astrophysics Data System (ADS)

Tilmann, F.; Zhang, Y.; Moreno, M.; Saul, J.; Eckelmann, F.; Palo, M.; Deng, Z.; Babeyko, A.; Chen, K.; Baez, J. C.; Schurr, B.; Wang, R.; Dahm, T.

2016-01-01

On 16 September 2015, the MW = 8.2 Illapel megathrust earthquake ruptured the Central Chilean margin. Combining inversions of displacement measurements and seismic waveforms with high frequency (HF) teleseismic backprojection, we derive a comprehensive description of the rupture, which also predicts deep ocean tsunami wave heights. We further determine moment tensors and obtain accurate depth estimates for the aftershock sequence. The earthquake nucleated near the coast but then propagated to the north and updip, attaining a peak slip of 5-6 m. In contrast, HF seismic radiation is mostly emitted downdip of the region of intense slip and arrests earlier than the long period rupture, indicating smooth slip along the shallow plate interface in the final phase. A superficially similar earthquake in 1943 with a similar aftershock zone had a much shorter source time function, which matches the duration of HF seismic radiation in the recent event, indicating that the 1943 event lacked the shallow slip.

Coulomb stress analysis of the 21 February 2008 Mw= 6.0 Wells, Nevada, earthquake

USGS Publications Warehouse

Sevilgen, Volkan

2011-01-01

Static Coulomb stress changes imparted by the February 21, 2008 Wells, Nevada earthquake are calculated, using an 8 x 6 km rectangular patch with a uniform slip as a source fault. Stress changes are resolved on nearby active faults using their rake, dip, and strike direction, assuming a fault friction of 0.4. The largest Coulomb stress increase (0.2 bars) imparted to surrounding major active faults from the Wells earthquake occurs on the Clover Hill fault, which may be the southern continuation of the ruptured fault. A 0.1 bar Coulomb stress increase is calculated on the western Snake Mountains fault. Coulomb stress decreases of 0.5 bars are calculated for the northern parts of the Independence and Ruby Mountains faults. The Coulomb stress change is calculated on relocated aftershocks assuming that they have the same strike, dip, and rake, as the source fault. Under this assumption, 75% of the aftershocks received a Coulomb stress increase.

Source Complexity of an Injection Induced Event: The 2016 Mw 5.1 Fairview, Oklahoma Earthquake

NASA Astrophysics Data System (ADS)

López-Comino, J. A.; Cesca, S.

2018-05-01

Complex rupture processes are occasionally resolved for weak earthquakes and can reveal a dominant direction of the rupture propagation and the presence and geometry of main slip patches. Finding and characterizing such properties could be important for understanding the nucleation and growth of induced earthquakes. One of the largest earthquakes linked to wastewater injection, the 2016 Mw 5.1 Fairview, Oklahoma earthquake, is analyzed using empirical Green's function techniques to reveal its source complexity. Two subevents are clearly identified and located using a new approach based on relative hypocenter-centroid location. The first subevent has a magnitude of Mw 5.0 and shows the main rupture propagated toward the NE, in the direction of higher pore pressure perturbations due to wastewater injection. The second subevent appears as an early aftershock with lower magnitude, Mw 4.7. It is located SW of the mainshock in a region of increased Coulomb stress, where most aftershocks relocated.

Microaftershock survey of the 1978 Bermuda rise earthquake

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nishenko, S.P.; Purdy, G.M.; Ewing, J.I.

1982-12-10

On March 24, 1978, a magnitude 6.0 intraplate earthquake occurred 380 km southwest of Bermuda near magnetic anomaly M4 (roughly-equal118 m.y.B.P.). A catalog of seismicity for the Bermuda rise indicates that this is an area of significant intraplate seismicity in the western North Atlantic Ocean. The fault plane solution for the 1978 event is of thrust type and strikes 340/sup 0/, in an intermediate direction to the trends of major fracture zones (300/sup 0/) and abyssal hill topography (035/sup 0/) in the area. The P axis of this mechanism is nearly horizontal and trends 259/sup 0/, subparallel to the absolutemore » plate motion vector for North America. Aftershock activity was detected teleseismically for approximately 8 months after March 24, and the entire sequence is best described as a prolonged mainshock-aftershock series. During June 18--28, 1978, we conducted a microaftershock survey of the area using ocean bottom hydrophones and recorded 250 events (0« less

Frequency-dependent seismic attenuation in the eastern United States as observed from the 2011 central Virginia earthquake and aftershock sequence

USGS Publications Warehouse

McNamara, Daniel E.; Gee, Lind; Benz, Harley M.; Chapman, Martin

2014-01-01

Ground shaking due to earthquakes in the eastern United States (EUS) is felt at significantly greater distances than in the western United States (WUS) and for some earthquakes it has been shown to display a strong preferential direction. Shaking intensity variation can be due to propagation path effects, source directivity, and/or site amplification. In this paper, we use S and Lg waves recorded from the 2011 central Virginia earthquake and aftershock sequence, in the Central Virginia Seismic Zone, to quantify attenuation as frequency‐dependent Q(f). In support of observations based on shaking intensity, we observe high Q values in the EUS relative to previous studies in the WUS with especially efficient propagation along the structural trend of the Appalachian mountains. Our analysis of Q(f) quantifies the path effects of the northeast‐trending felt distribution previously inferred from the U.S. Geological Survey (USGS) “Did You Feel It” data, historic intensity data, and the asymmetrical distribution of rockfalls and landslides.

The Seismic Aftershock Monitoring System (SAMS) for OSI - Experiences from IFE14

NASA Astrophysics Data System (ADS)

Gestermann, Nicolai; Sick, Benjamin; Häge, Martin; Blake, Thomas; Labak, Peter; Joswig, Manfred

2016-04-01

An on-site inspection (OSI) is the third of four elements of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The sole purpose of an OSI is to confirm whether a nuclear weapon test explosion or any other nuclear explosion has been carried out in violation of the treaty and to gather any facts which might assist in identifying any possible violator. It thus constitutes the final verification measure under the CTBT if all other available measures are not able to confirm the nature of a suspicious event. The Provisional Technical Secretariat (PTS) carried out the Integrated Field Exercise 2014 (IFE14) in the Dead Sea Area of Jordan from 3 November to 9. December 2014. It was a fictitious OSI whose aim was to test the inspection capabilities in an integrated manner. The technologies allowed during an OSI are listed in the Treaty. The aim of the Seismic Aftershock Monitoring System (SAMS) is to detect and localize aftershocks of low magnitudes of the triggering event or collapses of underground cavities. The locations of these events are expected in the vicinity of a possible previous explosion and help to narrow down the search area within an inspection area (IA) of an OSI. The success of SAMS depends on the main elements, hardware, software, deployment strategy, the search logic and not least the effective use of personnel. All elements of SAMS were tested and improved during the Built-Up Exercises (BUE) which took place in Austria and Hungary. IFE14 provided more realistic climatic and hazardous terrain conditions with limited resources. Significant variations in topography of the IA of IFE14 in the mountainous Dead Sea Area of Jordan led to considerable challenges which were not expected from experiences encountered during BUE. The SAMS uses mini arrays with an aperture of about 100 meters and with a total of 4 elements. The station network deployed during IFE14 and results of the data analysis will be presented. Possible aftershocks of the triggering event are expected in a very low magnitude range. Therefore the detection threshold of the network is one of the key parameters of SAMS and crucial for the success of the monitoring. One of the objectives was to record magnitude values down to -2.0 ML. The threshold values have been compared with historical seismicity in the region and those monitored during IFE14. Results of the threshold detection estimation and experiences of the exercise will be presented.

Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults

USGS Publications Warehouse

Lin, J.; Stein, R.S.

2004-01-01

We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface-cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 Mw = 6.7 Coalinga earthquake brought the subsequent 1983 Mw = 6.0 Nunez and 1985 Mw = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust-faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 Mw = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 Mw = 9.5 and 1995 Mw = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2-20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike-slip systems modulates the stress acting on nearby thrust and strike-slip faults. We calculate that the 1857 Mw = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault ???1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw = 7.3 Kern County shock but inhibited slip on the left-lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales. Copyright 2004 by the American Geophysical Union.

An intraslab earthquake (M7.1) along a buried hydrated fault in the Pacific plate, triggered by the 2011 M9 Tohoku earthquake

NASA Astrophysics Data System (ADS)

Nakajima, J.; Hasegawa, A.; Kita, S.

2011-12-01

A M9.0 megathrust earthquake, the 2011 off the Pacific Coast of Tohoku Earthquake, occurred on 11 March 2011 on the plate boundary east off northeastern (NE) Japan. After this great earthquake, seismicity has been activated in the Pacific plate as well as along its upper surface, and a large earthquake (M7.1) occurred on April 7 in the Pacific slab at a depth of 66 km, located near the down-dip limit of the large interplate slip of the M9 event. Here we perform travel-time tomography to reveal heterogeneous seismic velocity structures around the focal area of the 2011 M7.1 intraslab event, and discuss the occurrence of the 2011 M7.1 event in terms of dehydration embrittlement hypothesis. We applied the double-difference tomography method (Zhang and Thurber, 2003) to large number of arrival-time data obtained at a nation-wide seismograph network in Japan. Arrival-time data were produced from 8911 earthquakes and 188 stations, and comprised 247,504 P waves and 196,057 S waves, with differential data of 1,608,230 for P waves and 1,114,068 for S waves. Grid intervals were set at 10-20 km in the along-arc direction, 5-10 km perpendicular to the arc, and 5-10 km in the vertical direction The final results were obtained after eight iterations, which reduced the travel-time residual from 0.17 s to 0.11 s for P waves, and from 0.33 s to 0.19 s for S waves. The results show a low-velocity zone around the focal area of the M7.1 event, and that the aftershock activity is limited to the upper 15 km of the oceanic mantle. The lateral extent of the low-velocity zone is comparable to the distribution of aftershocks, suggesting a concentration of fluids in the aftershock area. The angle between the aftershock alignment and the dip of the slab surface is estimated to be ~60°, which is consistent with the dip of an oceanward-dipping normal fault observed at the outer-trench slope. These observations suggest that the M7.1 intraslab event occurred as a result of reactivation of a buried hydrated fault that formed prior to subduction. The upper ~15 km of the oceanic mantle may be locally hydrated by bending-related tensional faulting at the outer-trench slope.

Teacher Guidelines for Helping Students after an Earthquake

ERIC Educational Resources Information Center

National Child Traumatic Stress Network, 2013

2013-01-01

Being in an earthquake is very frightening, and the days, weeks, and months following are very stressful. Most families recover over time, especially with the support of relatives, friends, and their community. But different families may have different experiences during and after the earthquake, including the experience of aftershocks which may…

Using structures of the August 24, 2016 Amatrice earthquake affected area as seismoscopes for assessing ground motion characteristics and parameters of the main shock and its largest aftershocks

NASA Astrophysics Data System (ADS)

Carydis, Panayotis; Lekkas, Efthymios; Mavroulis, Spyridon

2017-04-01

On August 24, 2016 an Mw 6.0 earthquake struck Central Apennines (Italy) resulting in 299 fatalities, 388 injuries and about 3000 homeless in Amatrice wider area. Normal faulting surface ruptures along the western slope of Mt Vettore along with provided focal mechanisms demonstrated a NW-SE striking and SE dipping causative normal fault. The dominant building types in the affected area are unreinforced masonry (URM) and reinforced concrete (RC) buildings. Based on our macroseismic survey in the affected area immediately after the earthquake, RC buildings suffered non-structural damage including horizontal cracking of infill and internal partition walls, detachment of infill walls from the surrounding RC frame and detachment of large plaster pieces from infill walls as well as structural damage comprising soft story failure, symmetrical buckling of rods, compression damage at midheight of columns and bursting of over-stressed columns resulting in partial or total collapse. Damage in RC buildings was due to poor quality of concrete, inadequacy of reinforcement, inappropriate foundation close to the edge of slopes leading to differential settlements, poor workmanship and the destructive effect of vertical ground motions. Damage in URM buildings ranged from cracks and detachment of large plaster pieces from load-bearing walls to destruction due to poor workmanship with randomly placed materials bound by low-strength mortars, the effect of the vertical ground motion, inadequate repair and/or strengthening after previous earthquakes as well as inadequate interventions, additions and extensions to older URM buildings. During field surveying, the authors had the opportunity to observe damage induced not only by the main shock but also by its largest aftershocks (Mw 4.5-5.3) during the first three days of the aftershock sequence (August 24-26). Bearing in mind that: (a) soil conditions in foundations of the affected villages were more or less similar, (b) building damage induced by the studied earthquakes indicated the predominant effect of the vertical ground motion on buildings based on already reported building damage induced by recent destructive events in the Mediterranean region, (c) the conventional dynamic parameters of buildings did not play a significant role in their seismic response against the vertical component, due to its impact type of loading, (d) structures and materials presented similar response to ground motions almost independent from type and existing quality, and carried memories from previous large shocks of this sequence, (e) the main shock and its largest aftershocks caused building damage including spatial homothetic motions that reached statistically significant levels, it is concluded that the main shock and its largest aftershocks had similar focal mechanism parameters (normal faulting), were shallow events and were near-field earthquakes with short duration but high amplitude and the vertical component of the earthquakes' ground motion has prevailed. The aforementioned approach based solely on macroseismic observations was applied in the case of the 1755 Great Lisbon earthquake in order to determine its mechanism and epicenter location. Thus, it is suggested that the aforementioned methodology can be applied either in past historic earthquakes or complementarily in cases when the available seismological data are insufficient.

Nurturing Institutional Cultures of Caring

ERIC Educational Resources Information Center

Spano, David B.

2008-01-01

The aftershocks of the Virginia Tech incident have included numerous committee meetings, task forces, congressional hearings, and ad hoc debates focused on what went wrong and what educators can do to make their suddenly scary campuses safer. The issue of safety on their campuses has been thrust front and center. The author found it remarkable and…

Sediment-induced amplification and the collapse of the Nimitz Freeway

USGS Publications Warehouse

Hough, S.E.; Friberg, P.A.; Busby, R.; Field, E.F.; Jacob, K.H.; Borcherdt, R.D.

1990-01-01

THE amplification of ground motion by low-seismic-velocity surface sediments is an important factor in determining the seismic hazard specific to a given site. The Ms = 7.1 Loma Prieta earthquake of 17 October 1989 was the largest event in the contiguous United States in 37 years, and yielded an unparalleled volume of seismic data from the main shock and aftershock sequence1. These data can be used to image the seismic source, to study detailed Earth structure, and to study the propagation of seismic waves both through bedrock at depth and through sediment layers near the surface. Near the edge of San Francisco Bay, site conditions vary considerably on scales of hundreds of metres. The collapsed section of the two-tiered Nimitz Freeway in Oakland was built on San Francisco Bay mud, whereas stiffer alluvial sediments underlie a southern section that was damaged but did not collapse. Here we analyse high-quality, digital aftershock recordings from several sites near the Nimitz Freeway, and conclude that soil conditions and resulting ground-motion amplification may have contributed significantly to the failure of the structure.

The Constantine (northeast Algeria) earthquake of October 27, 1985: surface ruptures and aftershock study

NASA Astrophysics Data System (ADS)

Bounif, A.; Haessler, H.; Meghraoui, M.

1987-10-01

An earthquake of magnitude Ms = 6.0 (CSEM, Strasbourg) occurred at Constantine (Algeria) on 27 October 1985. This seismic event is the strongest felt in the Tellian Atlas since the El Asnam seismic crisis of October 10, 1980. A team from the Centre de Recherche d'Astronomie, d'Astrophysique et de Géophysique (CRAAG, Algeria), utilising 8 portable stations, registered the activity a few days after the main shock. The aftershocks follow a N045° direction, and show the existence of three ruptured segments. Cross sections display a remarkable vertical fault plane and suggest asperities in the rupture process. Surface breaks were found affecting Quaternary deposits. The principal segment is about 3.8 km long showing “enéchelon” cracks with left-lateral displacement while the main direction of the rupture is N055°. Although the vertical motion is small, the northwestern block shows a normal component of the main surface faulting, while the left-lateral displacement is about 10 cm. The strike-slip focal mechanism solution determined from the global seismic network and field observations are in good agreement.

Comparison of main-shock and aftershock fragility curves developed for New Zealand and US buildings

USGS Publications Warehouse

Uma, S.R.; Ryu, H.; Luco, N.; Liel, A.B.; Raghunandan, M.

2011-01-01

Seismic risk assessment involves the development of fragility functions to express the relationship between ground motion intensity and damage potential. In evaluating the risk associated with the building inventory in a region, it is essential to capture 'actual' characteristics of the buildings and group them so that 'generic building types' can be generated for further analysis of their damage potential. Variations in building characteristics across regions/countries largely influence the resulting fragility functions, such that building models are unsuitable to be adopted for risk assessment in any other region where a different set of building is present. In this paper, for a given building type (represented in terms of height and structural system), typical New Zealand and US building models are considered to illustrate the differences in structural model parameters and their effects on resulting fragility functions for a set of main-shocks and aftershocks. From this study, the general conclusion is that the methodology and assumptions used to derive basic capacity curve parameters have a considerable influence on fragility curves.

Rupture process of 2016, 25 January earthquake, Alboran Sea (South Spain, Mw= 6.4) and aftershocks series

NASA Astrophysics Data System (ADS)

Buforn, E.; Pro, C.; del Fresno, C.; Cantavella, J.; Sanz de Galdeano, C.; Udias, A.

2016-12-01

We have studied the rupture process of the 25 January 2016 earthquake (Mw =6.4) occurred in South Spain in the Alboran Sea. Main shock, foreshock and largest aftershocks (Mw =4.5) have been relocated using the NonLinLoc algorithm. Results obtained show a NE-SW distribution of foci at shallow depth (less than 15 km). For main shock, focal mechanism has been obtained from slip inversion over the rupture plane of teleseismic data, corresponding to left-lateral strike-slip motion. The rupture starts at 7 km depth and it propagates upward with a complex source time function. In order to obtain a more detailed source time function and to validate the results obtained from teleseismic data, we have used the Empirical Green Functions method (EGF) at regional distances. Finally, results of the directivity effect from teleseismic Rayleigh waves and the EGF method, are consistent with a rupture propagation to the NE. These results are interpreted in terms of the main geological features in the region.

Characteristics of foreshock activity inferred from the JMA earthquake catalog

NASA Astrophysics Data System (ADS)

Tamaribuchi, Koji; Yagi, Yuji; Enescu, Bogdan; Hirano, Shiro

2018-05-01

We investigated the foreshock activity characteristics using the Japan Meteorological Agency Unified Earthquake Catalog for the last 20 years. Using the nearest-neighbor distance approach, we systematically and objectively classified the earthquakes into clustered and background seismicity. We further categorized the clustered events into foreshocks, mainshocks, and aftershocks and analyzed their statistical features such as the b-value of the frequency-magnitude distribution. We found that the b-values of the foreshocks are lower than those of the aftershocks. This b-value difference suggested that not only the stochastic cascade effect but also the stress changes/aseismic processes may contribute to the mainshock-triggering process. However, forecasting the mainshock based on b-value analysis may be difficult. In addition, the rate of foreshock occurrence in all clusters (with two or more events) was nearly constant (30-40%) over a wide magnitude range. The difference in the magnitude, time, and epicentral distance between the mainshock and largest foreshock followed a power law. We inferred that the distinctive characteristics of foreshocks can be better revealed using the improved catalog, which includes the micro-earthquake information.

Seismicity Controlled by a Frictional Afterslip During a Small-Magnitude Seismic Sequence (ML < 5) on the Chihshang Fault, Taiwan

NASA Astrophysics Data System (ADS)

Canitano, Alexandre; Godano, Maxime; Hsu, Ya-Ju; Lee, Hsin-Ming; Linde, Alan T.; Sacks, Selwyn

2018-02-01

We report evidence for frictional afterslip at shallow depths (about 5 to 7 km) during a small-magnitude seismic sequence (with ML<5) along the Chihshang Fault, a main active structure of the Longitudinal Valley, in southeast Taiwan. The afterslip, which was recorded by a nearby borehole dilatometer, lasted about a month with a cumulative geodetic moment magnitude of 4.8 ± 0.2. The afterslip comprised two stages and controlled the aftershock sequence. The first postseismic stage, which followed a ML 4.6 earthquake, lasted about 6 h and mostly controlled the ruptures of neighboring asperities (e.g., multiplets) near the hypocenter. Then, a 4 week duration large afterslip event following a ML 4.9 earthquake controlled the rate of aftershocks during its first 2 days through brittle creep. The study presents a rare case of simultaneous seismological and geodetic observations for afterslip following earthquakes with magnitude lower than 5. Furthermore, the geodetic moment of the postseismic phase is at least equivalent to the coseismic moment of the sequence.

Aftershock identification problem via the nearest-neighbor analysis for marked point processes

NASA Astrophysics Data System (ADS)

Gabrielov, A.; Zaliapin, I.; Wong, H.; Keilis-Borok, V.

2007-12-01

The centennial observations on the world seismicity have revealed a wide variety of clustering phenomena that unfold in the space-time-energy domain and provide most reliable information about the earthquake dynamics. However, there is neither a unifying theory nor a convenient statistical apparatus that would naturally account for the different types of seismic clustering. In this talk we present a theoretical framework for nearest-neighbor analysis of marked processes and obtain new results on hierarchical approach to studying seismic clustering introduced by Baiesi and Paczuski (2004). Recall that under this approach one defines an asymmetric distance D in space-time-energy domain such that the nearest-neighbor spanning graph with respect to D becomes a time- oriented tree. We demonstrate how this approach can be used to detect earthquake clustering. We apply our analysis to the observed seismicity of California and synthetic catalogs from ETAS model and show that the earthquake clustering part is statistically different from the homogeneous part. This finding may serve as a basis for an objective aftershock identification procedure.

Dense array recordings in the San Bernardino Valley of landers-big bear aftershocks: Basin surface waves, Moho reflections, and three-dimensional simulations

USGS Publications Warehouse

Frankel, Arthur

1994-01-01

Fourteen GEOS seismic recorders were deployed in the San Bernardino Valley to study the propagation of short-period (T ≈ 1 to 3 sec) surface waves and Moho reflections. Three dense arrays were used to determine the direction and speed of propagation of arrivals in the seismograms. The seismograms for a shallow (d ≈ 1 km) M 4.9 aftershock of the Big Bear earthquake exhibit a very long duration (60 sec) of sustained shaking at periods of about 2 sec. Array analysis indicates that these late arrivals are dominated by surface waves traveling in various directions across the Valley. Some energy is arriving from a direction 180° from the epicenter and was apparently reflected from the edge of the Valley opposite the source. A close-in aftershock (Δ = 25 km, depth = 7 km) displays substantial short-period surface waves at deep-soil sites. A three-dimensional (3D) finite difference simulation produces synthetic seismograms with durations similar to those of the observed records for this event, indicating the importance of S-wave to surface-wave conversion near the edge of the basin. Flat-layered models severely underpredict the duration and spectral amplification of this deep-soil site. I show an example where the coda wave amplitude ratio at 1 to 2 Hz between a deep-soil and a rock site does not equal the S-wave amplitude ratio, because of the presence of surface waves in the coda of the deep-soil site. For one of the events studied (Δ ≈ 90 km), there are sizable phases that are critically reflected from the Moho (PmP and SmS). At one of the rock sites, the SmS phase has a more peaked spectrum that the direct S wave.

Fundamental questions of earthquake statistics, source behavior, and the estimation of earthquake probabilities from possible foreshocks

USGS Publications Warehouse

Michael, Andrew J.

2012-01-01

Estimates of the probability that an ML 4.8 earthquake, which occurred near the southern end of the San Andreas fault on 24 March 2009, would be followed by an M 7 mainshock over the following three days vary from 0.0009 using a Gutenberg–Richter model of aftershock statistics (Reasenberg and Jones, 1989) to 0.04 using a statistical model of foreshock behavior and long‐term estimates of large earthquake probabilities, including characteristic earthquakes (Agnew and Jones, 1991). I demonstrate that the disparity between the existing approaches depends on whether or not they conform to Gutenberg–Richter behavior. While Gutenberg–Richter behavior is well established over large regions, it could be violated on individual faults if they have characteristic earthquakes or over small areas if the spatial distribution of large‐event nucleations is disproportional to the rate of smaller events. I develop a new form of the aftershock model that includes characteristic behavior and combines the features of both models. This new model and the older foreshock model yield the same results when given the same inputs, but the new model has the advantage of producing probabilities for events of all magnitudes, rather than just for events larger than the initial one. Compared with the aftershock model, the new model has the advantage of taking into account long‐term earthquake probability models. Using consistent parameters, the probability of an M 7 mainshock on the southernmost San Andreas fault is 0.0001 for three days from long‐term models and the clustering probabilities following the ML 4.8 event are 0.00035 for a Gutenberg–Richter distribution and 0.013 for a characteristic‐earthquake magnitude–frequency distribution. Our decisions about the existence of characteristic earthquakes and how large earthquakes nucleate have a first‐order effect on the probabilities obtained from short‐term clustering models for these large events.

Earthquake Rate Models for Evolving Induced Seismicity Hazard in the Central and Eastern US

NASA Astrophysics Data System (ADS)

Llenos, A. L.; Ellsworth, W. L.; Michael, A. J.

2015-12-01

Injection-induced earthquake rates can vary rapidly in space and time, which presents significant challenges to traditional probabilistic seismic hazard assessment methodologies that are based on a time-independent model of mainshock occurrence. To help society cope with rapidly evolving seismicity, the USGS is developing one-year hazard models for areas of induced seismicity in the central and eastern US to forecast the shaking due to all earthquakes, including aftershocks which are generally omitted from hazards assessments (Petersen et al., 2015). However, the spatial and temporal variability of the earthquake rates make them difficult to forecast even on time-scales as short as one year. An initial approach is to use the previous year's seismicity rate to forecast the next year's seismicity rate. However, in places such as northern Oklahoma the rates vary so rapidly over time that a simple linear extrapolation does not accurately forecast the future, even when the variability in the rates is modeled with simulations based on an Epidemic-Type Aftershock Sequence (ETAS) model (Ogata, JASA, 1988) to account for earthquake clustering. Instead of relying on a fixed time period for rate estimation, we explore another way to determine when the earthquake rate should be updated. This approach could also objectively identify new areas where the induced seismicity hazard model should be applied. We will estimate the background seismicity rate by optimizing a single set of ETAS aftershock triggering parameters across the most active induced seismicity zones -- Oklahoma, Guy-Greenbrier, the Raton Basin, and the Azle-Dallas-Fort Worth area -- with individual background rate parameters in each zone. The full seismicity rate, with uncertainties, can then be estimated using ETAS simulations and changes in rate can be detected by applying change point analysis in ETAS transformed time with methods already developed for Poisson processes.

Seismicity Increase in North China After the 2008 Mw7.9 Wenchuan Earthquake.

NASA Astrophysics Data System (ADS)

Goldhagen, G.; Li, C.; Peng, Z.; Wu, J.; Zhao, L.

2016-12-01

A large mainshock is capable of setting off an increase in seismicity in areas thousands of kilometers away. This phenomenon, known as remote triggering, is more likely to occur along active fault lines, aftershock zones, or regions with anthropogenic activities (e.g., mining, reservoirs, and fluid injections). By studying these susceptible areas, we can gain a better understanding of subsurface stress conditions, and long-range earthquake interactions. In this study we conduct a systematic search for remotely triggered seismicity in North China along two linear dense arrays (net code 1A and Z8) deployed by Chinese Academy of Sciences (CAS) following the 2008 Mw7.9 Wenchuan earthquake. A 5 Hz high pass filter is applied to the broadband seismogram recorded at the 1A array, which is more than 2,000 km away from the mainshock, in order to manually pick local events with double peaks. These local events have higher frequencies than earthquakes in the aftershock zone of the Wenchuan earthquake. An STA/LTA method is then employed as a way to automatically detect microseismicity in a section of the array that showed preliminary evidence of remote triggering. We find a clear increase of small earthquakes, right after the surface waves of the Wenchuan mainshock. These events, were recorded at stations close to the north section of the Tanlu fault and aftershock zones of the 1975, Ms7.3 Haicheng earthquake. This result suggests that remote triggering is more likely near active fault zones or other specific regions, as previous studies have proposed. Future work includes applying a waveform matching method to both arrays and automatically detecting micro-earthquakes missed on the catalog, and using them to better confirm the existence (or lack of) remote triggering following the Wenchuan mainshock. Our finding helps to better classify conditions that lead to the occurrence of remotely triggered earthquakes at intraplate regions.

Aftershock Distribution of the Mw=7.8 April 16, 2016 Pedernales Ecuador Subduction Earthquake: Constraints from 3D Earthquake Locations

NASA Astrophysics Data System (ADS)

Font, Y.; Agurto-Detzel, H.; Alvarado, A. P.; Regnier, M. M.; Rolandone, F.; Charvis, P.; Mothes, P. A.; Nocquet, J. M.; Jarrin, P.; Ambrois, D.; Maron, C.; Deschamps, A.; Cheze, J.; Peix, F., Sr.; Ruiz, M. C.; Gabriela, P.; Acero, W.; Singaucho, J. C.; Viracucha, C.; Vasconez, F.; De Barros, L.; Mercerat, D.; Courboulex, F.; Galve, A.; Godano, M.; Monfret, T.; Ramos, C.; Martin, X.; Rietbrock, A.; Beck, S. L.; Metlzer, A.

2017-12-01

The Mw7.8 Pedernales earthquake is associated with the subduction of the Nazca Plate beneath the South American Plate. The mainshock caused many casualties and widespread damage across the Manabi province. The 150 km-long coseismic rupture area extends beneath the coastline, near 25 km depth. The rupture propagated southward and involved the successive rupture of two discrete asperities, with a maximum slip ( 5 m) on the southern patch. The rupture area is consistent with the highly locked regions observed on interseismic coupling models, overlaps the 7.2 Mw rupture zone, and terminates near where the 1906 Mw 8.8 megathrust earthquake rupture zone is estimated to have ended. Two neighboring highly coupled patches remain locked: (A) south and updip of the coseismic rupture zone and (B) north and downdip. In this study, we are working on the earthquake locations of the first month of aftershocks and compare the seismicity distribution to the interseismic coupling, the rupture area and to early afterslip. We use continuous seismic traces recorded on the permanent network partly installed in the framework of the collaboration between l'Institut de Recherche pour le Développement (France) and the Instituto Geofísico, Escuela Politécnica Nacional (IGEPN), Quito, Ecuador. Detections are conducted using Seiscomp in play-back mode and arrival-times are manually picked. To improve earthquake locations, we use the MAXi technique and a heterogeneous a priori P-wave velocity model that approximates the large velocity variations of the Ecuadorian subduction system. Aftershocks align along 3 to 4 main clusters that strike perpendicularly to the trench, and mostly updip of the co-seismic rupture. Seismicity develops over portions of plate interface that are known to be strongly locked or almost uncoupled. The seismicity pattern is similar to the one observed during a decade of observation during the interseismic period with swarms such as the Galera alignment, Jama and Cabo Pasado, Manta to Puerto Lopez.

Fluid‐driven seismicity response of the Rinconada fault near Paso Robles, California, to the 2003 M 6.5 San Simeon earthquake

USGS Publications Warehouse

Hardebeck, Jeanne L.

2012-01-01

The 2003 M 6.5 San Simeon, California, earthquake caused significant damage in the city of Paso Robles and a persistent cluster of aftershocks close to Paso Robles near the Rinconada fault. Given the importance of secondary aftershock triggering in sequences of large events, a concern is whether this cluster of events could trigger another damaging earthquake near Paso Robles. An epidemic‐type aftershock sequence (ETAS) model is fit to the Rinconada seismicity, and multiple realizations indicate a 0.36% probability of at least one M≥6.0 earthquake during the next 30 years. However, this probability estimate is only as good as the projection into the future of the ETAS model. There is evidence that the seismicity may be influenced by fluid pressure changes, which cannot be forecasted using ETAS. The strongest evidence for fluids is the delay between the San Simeon mainshock and a high rate of seismicity in mid to late 2004. This delay can be explained as having been caused by a pore pressure decrease due to an undrained response to the coseismic dilatation, followed by increased pore pressure during the return to equilibrium. Seismicity migration along the fault also suggests fluid involvement, although the migration is too slow to be consistent with pore pressure diffusion. All other evidence, including focal mechanisms and b‐value, is consistent with tectonic earthquakes. This suggests a model where the role of fluid pressure changes is limited to the first seven months, while the fluid pressure equilibrates. The ETAS modeling adequately fits the events after July 2004 when the pore pressure stabilizes. The ETAS models imply that while the probability of a damaging earthquake on the Rinconada fault has approximately doubled due to the San Simeon earthquake, the absolute probability remains low.

Rupture processes of the 2010 Canterbury earthquake and the 2011 Christchurch earthquake inferred from InSAR, strong motion and teleseismic datasets

NASA Astrophysics Data System (ADS)

Yun, S.; Koketsu, K.; Aoki, Y.

2014-12-01

The September 4, 2010, Canterbury earthquake with a moment magnitude (Mw) of 7.1 is a crustal earthquake in the South Island, New Zealand. The February 22, 2011, Christchurch earthquake (Mw=6.3) is the biggest aftershock of the 2010 Canterbury earthquake that is located at about 50 km to the east of the mainshock. Both earthquakes occurred on previously unrecognized faults. Field observations indicate that the rupture of the 2010 Canterbury earthquake reached the surface; the surface rupture with a length of about 30 km is located about 4 km south of the epicenter. Also various data including the aftershock distribution and strong motion seismograms suggest a very complex rupture process. For these reasons it is useful to investigate the complex rupture process using multiple data with various sensitivities to the rupture process. While previously published source models are based on one or two datasets, here we infer the rupture process with three datasets, InSAR, strong-motion, and teleseismic data. We first performed point source inversions to derive the focal mechanism of the 2010 Canterbury earthquake. Based on the focal mechanism, the aftershock distribution, the surface fault traces and the SAR interferograms, we assigned several source faults. We then performed the joint inversion to determine the rupture process of the 2010 Canterbury earthquake most suitable for reproducing all the datasets. The obtained slip distribution is in good agreement with the surface fault traces. We also performed similar inversions to reveal the rupture process of the 2011 Christchurch earthquake. Our result indicates steep dip and large up-dip slip. This reveals the observed large vertical ground motion around the source region is due to the rupture process, rather than the local subsurface structure. To investigate the effects of the 3-D velocity structure on characteristic strong motion seismograms of the two earthquakes, we plan to perform the inversion taking 3-D velocity structure of this region into account.

Imaging and Understanding Foreshock and Aftershock Behavior Around the 2014 Iquique, Northern Chile, Earthquake

NASA Astrophysics Data System (ADS)

Yang, H.; Meng, X.; Peng, Z.; Newman, A. V.; Hu, S.; Williamson, A.

2014-12-01

On April 1st, 2014, a moment magnitude (MW) 8.2 earthquake occurred offshore Iquique, Northern Chile. There were numerous smaller earthquakes preceding and following the mainshock, making it an ideal case to study the spatio-temporal relation among these events and their association with the mainshock. We applied a matched-filter technique to detect previously missing foreshocks and aftershocks of the 2014 Iquique earthquake. Using more than 900 template events recorded by 19 broadband seismic stations (network code CX) operated by the GEOFON Program of GFZ Potsdam, we found 4392 earthquakes between March 1st and April 3rd, 2014, including more than 30 earthquakes with magnitude larger than 4 that were previously missed in the catalog from the Chile National Seismological Center. Additionally, we found numerous small earthquakes with magnitudes between 1 and 2 preceding the largest foreshock, an MW 6.7 event occurring on March 16th, approximately 2 weeks before the Iquique mainshock. We observed that the foreshocks migrated northward at a speed of approximately 6 km/day. Using a finite fault slip model of the mainshock determined from teleseismic waveform inversion (Hayes, 2014), we calculated the Coulomb stress changes in the nearby regions of the mainshock. We found that there was ~200% increase in seismicity in the areas with increased Coulomb stress. Our next step is to evaluate the Coulomb stress changes associated with earlier foreshocks and their roles in triggering later foreshocks, and possibly the mainshock. For this, we plan to create a fault model of the temporal evolution of the Coulomb behavior along the interface with time, assuming Wells and Coppersmith (1994) type fault parameters. These results will be compared with double-difference relocations (using HypoDD), presenting a more accurate understanding of the spatial-temporal evolution of foreshocks and aftershocks of the 2014 Iquique earthquake.

Coulomb Stress Change and Seismic Hazard of Rift Zones in Southern Tibet after the 2015 Mw7.8 Nepal Earthquake and Its Mw7.3 Aftershock

NASA Astrophysics Data System (ADS)

Dai, Z.; Zha, X.; Lu, Z.

2015-12-01

In southern Tibet (30~34N, 80~95E), many north-trending rifts, such as Yadong-Gulu and Lunggar rifts, are characterized by internally drained graben or half-graben basins bounded by active normal faults. Some developed rifts have become a portion of important transportation lines in Tibet, China. Since 1976, eighty-seven >Mw5.0 earthquakes have happened in the rift regions, and fifty-five events have normal faulting focal mechanisms according to the GCMT catalog. These rifts and normal faults are associated with both the EW-trending extension of the southern Tibet and the convergence between Indian and Tibet. The 2015 Mw7.8 Nepal great earthquake and its Mw7.3 aftershock occurred at the main Himalayan Thrust zone and caused tremendous damages in Kathmandu region. Those earthquakes will lead to significant viscoelastic deformation and stress changes in the southern Tibet in the future. To evaluate the seismic hazard in the active rift regions in southern Tibet, we modeled the slip distribution of the 2015 Nepal great earthquakes using the InSAR displacement field from the ALOS-2 satellite SAR data, and calculated the Coulomb failure stress (CFS) on these active normal faults in the rift zones. Because the estimated CFS depends on the geometrical parameters of receiver faults, it is necessary to get the accurate fault parameters in the rift zones. Some historical earthquakes have been studied using the field data, teleseismic data and InSAR observations, but results are in not agreement with each other. In this study, we revaluated the geometrical parameters of seismogenic faults occurred in the rift zones using some high-quality coseismic InSAR observations and teleseismic body-wave data. Finally, we will evaluate the seismic hazard in the rift zones according to the value of the estimated CFS and aftershock distribution.

An Examination of Upper Plate Aftershocks of the Tohoku-oki Earthquake: Are They Caused by a Long-term Change in the Dip of the Subducting Plate?

NASA Astrophysics Data System (ADS)

Oryan, B.; Buck, W. R.

2017-12-01

The Tohoku-oki earthquake was one of the strongest earthquakes ever recorded. 50-80 meters of lateral motion of the sloping seafloor resulted in a tsunami that exceeded predictions and caused one of the costliest natural disasters in history. It was also the first time extensional aftershocks were observed in the upper plate over a region as wide as 250km. Inspired by these findings, researchers found similar upper plate extensional earthquakes after reexamining seismic data from past earthquakes that had also produced large tsunamis. Such extensional aftershocks are difficult to explain in terms of standard subduction models. Most models assume that the dip of the subducting plate remains constant with time. However, geological evidence indicates that the dip angle of the subducting plate changes. We hypothesize that a reduction in the dip angle of the subducting plate can cause upper plate extensional earthquakes. This change in dip angle adds extensional bending stress to the upper plate. During an inter-seismic period, the interface is `locked' causing regional compression that prevents the release of extensional energy. Relief of compressional stresses during a megathrust event can trigger the release of the accumulated extensional energy, explaining why extensional earthquakes were observed after some megathrust events. Numerical models will be used to test our hypothesis. First, we will model long term subduction with a nearly constant dip angle. Then, we will impose a `mantle wind' to reduce the dip angle of the subducting plate. Eventually, we will model a full seismic cycle of the subduction resulting in a megathrust event. The generation of extensional earthquakes in the upper plate of our model following the megathrust event will allow us to determine whether a causal link exists between these earthquakes and a reduction in the dip angle of the subducting plate.

Strain rates, stress markers and earthquake clustering (Invited)

NASA Astrophysics Data System (ADS)

Fry, B.; Gerstenberger, M.; Abercrombie, R. E.; Reyners, M.; Eberhart-Phillips, D. M.

2013-12-01

The 2010-present Canterbury earthquakes comprise a well-recorded sequence in a relatively low strain-rate shallow crustal region. We present new scientific results to test the hypothesis that: Earthquake sequences in low-strain rate areas experience high stress drop events, low-post seismic relaxation, and accentuated seismic clustering. This hypothesis is based on a physical description of the aftershock process in which the spatial distribution of stress accumulation and stress transfer are controlled by fault strength and orientation. Following large crustal earthquakes, time dependent forecasts are often developed by fitting parameters defined by Omori's aftershock decay law. In high-strain rate areas, simple forecast models utilizing a single p-value fit observed aftershock sequences well. In low-strain rate areas such as Canterbury, assumptions of simple Omori decay may not be sufficient to capture the clustering (sub-sequence) nature exhibited by the punctuated rise in activity following significant child events. In Canterbury, the moment release is more clustered than in more typical Omori sequences. The individual earthquakes in these clusters also exhibit somewhat higher stress drops than in the average crustal sequence in high-strain rate regions, suggesting the earthquakes occur on strong Andersonian-oriented faults, possibly juvenile or well-healed . We use the spectral ratio procedure outlined in (Viegas et al., 2010) to determine corner frequencies and Madariaga stress-drop values for over 800 events in the sequence. Furthermore, we will discuss the relevance of tomographic results of Reyners and Eberhart-Phillips (2013) documenting post-seismic stress-driven fluid processes following the three largest events in the sequence as well as anisotropic patterns in surface wave tomography (Fry et al., 2013). These tomographic studies are both compatible with the hypothesis, providing strong evidence for the presence of widespread and hydrated regional upper crustal cracking parallel to sub-parallel to the dominant transverse failure plane in the sequence. Joint interpretation of the three separate datasets provide a positive first attempt at testing our fundamental hypothesis.

Tuning critical failure with viscoelasticity: How aftershocks inhibit criticality in an analytical mean field model of fracture.

NASA Astrophysics Data System (ADS)

Baro Urbea, J.; Davidsen, J.

2017-12-01

The hypothesis of critical failure relates the presence of an ultimate stability point in the structural constitutive equation of materials to a divergence of characteristic scales in the microscopic dynamics responsible of deformation. Avalanche models involving critical failure have determined universality classes in different systems: from slip events in crystalline and amorphous materials to the jamming of granular media or the fracture of brittle materials. However, not all empirical failure processes exhibit the trademarks of critical failure. As an example, the statistical properties of ultrasonic acoustic events recorded during the failure of porous brittle materials are stationary, except for variations in the activity rate that can be interpreted in terms of aftershock and foreshock activity (J. Baró et al., PRL 2013).The rheological properties of materials introduce dissipation, usually reproduced in atomistic models as a hardening of the coarse-grained elements of the system. If the hardening is associated to a relaxation process the same mechanism is able to generate temporal correlations. We report the analytic solution of a mean field fracture model exemplifying how criticality and temporal correlations are tuned by transient hardening. We provide a physical meaning to the conceptual model by deriving the constitutive equation from the explicit representation of the transient hardening in terms of a generalized viscoelasticity model. The rate of 'aftershocks' is controlled by the temporal evolution of the viscoelastic creep. At the quasistatic limit, the moment release is invariant to rheology. Therefore, the lack of criticality is explained by the increase of the activity rate close to failure, i.e. 'foreshocks'. Finally, the avalanche propagation can be reinterpreted as a pure mathematical problem in terms of a stochastic counting process. The statistical properties depend only on the distance to a critical point, which is universal for any parametrization of the transient hardening and a whole category of fracture models.

Space-Time Earthquake Rate Models for One-Year Hazard Forecasts in Oklahoma

NASA Astrophysics Data System (ADS)

Llenos, A. L.; Michael, A. J.

2017-12-01

The recent one-year seismic hazard assessments for natural and induced seismicity in the central and eastern US (CEUS) (Petersen et al., 2016, 2017) rely on earthquake rate models based on declustered catalogs (i.e., catalogs with foreshocks and aftershocks removed), as is common practice in probabilistic seismic hazard analysis. However, standard declustering can remove over 90% of some induced sequences in the CEUS. Some of these earthquakes may still be capable of causing damage or concern (Petersen et al., 2015, 2016). The choices of whether and how to decluster can lead to seismicity rate estimates that vary by up to factors of 10-20 (Llenos and Michael, AGU, 2016). Therefore, in order to improve the accuracy of hazard assessments, we are exploring ways to make forecasts based on full, rather than declustered, catalogs. We focus on Oklahoma, where earthquake rates began increasing in late 2009 mainly in central Oklahoma and ramped up substantially in 2013 with the expansion of seismicity into northern Oklahoma and southern Kansas. We develop earthquake rate models using the space-time Epidemic-Type Aftershock Sequence (ETAS) model (Ogata, JASA, 1988; Ogata, AISM, 1998; Zhuang et al., JASA, 2002), which characterizes both the background seismicity rate as well as aftershock triggering. We examine changes in the model parameters over time, focusing particularly on background rate, which reflects earthquakes that are triggered by external driving forces such as fluid injection rather than other earthquakes. After the model parameters are fit to the seismicity data from a given year, forecasts of the full catalog for the following year can then be made using a suite of 100,000 ETAS model simulations based on those parameters. To evaluate this approach, we develop pseudo-prospective yearly forecasts for Oklahoma from 2013-2016 and compare them with the observations using standard Collaboratory for the Study of Earthquake Predictability tests for consistency.

Rupture Processes of the Mw8.3 Sea of Okhotsk Earthquake and Aftershock Sequences from 3-D Back Projection Imaging

NASA Astrophysics Data System (ADS)

Jian, P. R.; Hung, S. H.; Meng, L.

2014-12-01

On May 24, 2013, the largest deep earthquake ever recorded in history occurred on the southern tip of the Kamchatka Island, where the Pacific Plate subducts underneath the Okhotsk Plate. Previous 2D beamforming back projection (BP) of P- coda waves suggests the mainshock ruptured bilaterally along a horizontal fault plane determined by the global centroid moment tensor solution. On the other hand, the multiple point source inversion of P and SH waveforms argued that the earthquake comprises a sequence of 6 subevents not located on a single plane but actually distributed in a zone that extends 64 km horizontally and 35 km in depth. We then apply a three-dimensional MUSIC BP approach to resolve the rupture processes of the manishock and two large aftershocks (M6.7) with no a priori setup of preferential orientations of the planar rupture. The maximum pseudo-spectrum of high-frequency P wave in a sequence of time windows recorded by the densely-distributed stations from US and EU Array are used to image 3-D temporal and spatial rupture distribution. The resulting image confirms that the nearly N-S striking but two antiparallel rupture stages. The first subhorizontal rupture initially propagates toward the NNE direction, while at 18 s later it directs reversely to the SSW and concurrently shifts downward to 35 km deeper lasting for about 20 s. The rupture lengths in the first NNE-ward and second SSW-ward stage are about 30 km and 85 km; the estimated rupture velocities are 3 km/s and 4.25 km/s, respectively. Synthetic experiments are undertaken to assess the capability of the 3D MUSIC BP for the recovery of spatio-temporal rupture processes. Besides, high frequency BP images based on the EU-Array data show two M6.7 aftershocks are more likely to rupture on the vertical fault planes.

Problems of the active tectonics of the Eastern Black Sea

NASA Astrophysics Data System (ADS)

Javakhishvili, Z.; Godoladze, T.; Dreger, D. S.; Mikava, D.; Tvaliashvili, A.

2016-12-01

The Black Sea Basin is the part of the Arabian Eurasian Collision zone and important unit for understanding the tectonic process of the region. This complex basin comprises two deep basins, separated by the mid-Black Sea Ridge. The basement of the Black Sea includes areas with oceanic and continental crust. It was formed as a "back-arc" basin over the subduction zone during the closing of the Tethys Ocean. In the past decades the Black Sea has been the subject of intense geological and geophysical studies. Several papers were published about the geological history, tectonics, basement relief and crustal and upper mantle structure of the basin. New tectonic schemes were suggested (e. g. Nikishin et al 2014, Shillington et al. 2008, Starostenko et al. 2004 etc.). Nevertheless, seismicity of the Black Sea is poorly studied due to the lack of seismic network in the coastal area. It is considered, that the eastern basin currently lies in a compressional setting associated with the uplift of the Caucasus and structural development of the Caucasus was closely related to the evolution of the Eastern Black Sea Basin. Analyses of recent sequence of earthquakes in 2012 can provide useful information to understand complex tectonic structure of the Eastern Black Sea region. Right after the earthquake of 2012/12/23, National Seismic monitoring center of Georgia deployed additional 4 stations in the coastal area of the country, close to the epicenter area, to monitor aftershock sequence. Seismic activity in the epicentral area is continuing until now. We have relocated approximately 1200 aftershocks to delineate fault scarf using data from Georgian, Turkish and Russian datacenters. Waveforms of the major events and the aftershocks were inverted for the fault plane solutions of the events. For the inversion were used green's functions, computed using new 1D velocity model of the region. Strike-slip mechanism of the major events of the earthquake sequence indicates extensional features in the Eastern Black Sea Region as well.

Subsurface fault damage zone of the 2014 Mw 6.0 South Napa, California, earthquake viewed from fault‐zone trapped waves

USGS Publications Warehouse

Li, Yong-Gang; Catchings, Rufus D.; Goldman, Mark R.

2016-01-01

The aftershocks of the 24 August 2014 Mw 6.0 South Napa earthquake generated prominent fault‐zone trapped waves (FZTWs) that were recorded on two 1.9‐km‐long seismic arrays deployed across the northern projection (array 1, A1) and the southern part (A2) of the surface rupture of the West Napa fault zone (WNFZ). We also observed FZTWs on an array (A3) deployed across the intersection of the Franklin and Southampton faults, which appear to be the southward continuations of the WNFZ. A1, A2, and A3 consisted of 20, 20, and 10 L28 (4.5 Hz) three‐component seismographs. We analyzed waveforms of FZTWs from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with this Mw 6.0 earthquake. Post‐S coda durations of FZTWs increase with epicentral distances and focal depths from the recording arrays, suggesting a low‐velocity waveguide along the WNFZ to depths in excess of 5–7 km. Locations of the aftershocks showing FZTWs, combined with 3D finite‐difference simulations, suggest the subsurface rupture zone having an S‐wave speed reduction of ∼40%–50% between A1 and A2, coincident with the ∼14‐km‐long mapped surface rupture zone and at least an ∼500‐m‐wide deformation zone. The low‐velocity waveguide along the WNFZ extends further southward to at least A3, but with a more moderate‐velocity reduction of 30%–35% at ray depth. This last FZTW observation suggests continuity between the WNFZ and Franklin fault. The waveguide effect may have localized and amplified ground shaking along the WNFZ and the faults farther to the south (see a companion paper by Catchings et al., 2016).

Low stress drops observed for aftershocks of the 2011 Mw 5.7 Prague, Oklahoma, earthquake

USGS Publications Warehouse

Sumy, Danielle F.; Neighbors, Corrie J.; Cochran, Elizabeth S.; Keranen, Katie M.

2017-01-01

In November 2011, three Mw ≥ 4.8 earthquakes and thousands of aftershocks occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that wastewater injection induced a Mw 4.8 foreshock, which subsequently triggered a Mw 5.7 mainshock. We examine source properties of aftershocks with a standard Brune-type spectral model and jointly solve for seismic moment (M0), corner frequency (f0), and kappa (κ) with an iterative Gauss-Newton global downhill optimization method. We examine 934 earthquakes with initial moment magnitudes (Mw) between 0.33 and 4.99 based on the pseudospectral acceleration and recover reasonable M0, f0, and κ for 87 earthquakes with Mw 1.83–3.51 determined by spectral fit. We use M0 and f0 to estimate the Brune-type stress drop, assuming a circular fault and shear-wave velocity at the hypocentral depth of the event. Our observations suggest that stress drops range between 0.005 and 4.8 MPa with a median of 0.2 MPa (0.03–26.4 MPa with a median of 1.1 MPa for Madariaga-type), which is significantly lower than typical eastern United States intraplate events (>10 MPa). We find that stress drops correlate weakly with hypocentral depth and magnitude. Additionally, we find the stress drops increase with time after the mainshock, although temporal variation in stress drop is difficult to separate from spatial heterogeneity and changing event locations. The overall low median stress drop suggests that the fault segments may have been primed to fail as a result of high pore fluid pressures, likely related to nearby wastewater injection.

Advances in Geophysical Methods at Parkfield, California

NASA Astrophysics Data System (ADS)

Bennington, Ninfa

The Parkfield segment of the San Andreas fault (SAF) is one of the most highly monitored fault sites in the world. I carry out two studies, taking advantage of the dense set of geophysical observations obtained for this segment of the fault. In the first study, I use geodetic data to had a model of coseismic slip for the 2004 Parkfield earthquake with the constraint that the edges of coseismic slip patches preferentially align with aftershocks. Application of the aftershock distribution constraint on coseismic slip yields a model that agrees in location and amplitude with features observed in previous geodetic studies and the majority of strong motion studies. The curvature-constrained solution shows slip primarily between aftershock "streaks" with the continuation of moderate levels of slip towards the 2004 Parkfield earthquake hypocenter. The observed continuation of coseismic slip towards the hypocenter is in good agreement with strong motion studies but is not observed in the majority of published geodetic slip models, which I attribute to resolution limitations. In the second study, I develop tomoDDMT, a joint inversion code that simultaneously inverts for resistivity and seismic velocity models under the cross- gradient constraint. This constraint uses a weighted penalty function to encourage areas where the two models are changing to be structurally similar. I present jointly inverted models of P-wave velocity (Vp) and resistivity for a cross-section centered on the San Andreas Fault Observatory at Depth (SAFOD). The joint inversion scheme achieves structurally similar Vp and resistivity images that adequately fit the seismic and MT data without forcing model similarity where none exists. Using tomoDDMT, I obtain models or resistivity and Vp that yield increased insight into the geologic structure at Parkfield. I address key issues including: the location of the Franciscan formation at depth, the spatial extent of the Upper Great Valley sequence, the validity of the eastern wall as a fluid pathway, the distribution of the eastern conductor, and the distribution of the Salinian block at depth.

Machine-Learning Inspired Seismic Phase Detection for Aftershocks of the 2008 MW7.9 Wenchuan Earthquake

NASA Astrophysics Data System (ADS)

Zhu, L.; Li, Z.; Li, C.; Wang, B.; Chen, Z.; McClellan, J. H.; Peng, Z.

2017-12-01

Spatial-temporal evolution of aftershocks is important for illumination of earthquake physics and for rapid response of devastative earthquakes. To improve aftershock catalogs of the 2008 MW7.9 Wenchuan earthquake in Sichuan, China, Alibaba cloud and China Earthquake Administration jointly launched a seismological contest in May 2017 [Fang et al., 2017]. This abstract describes how we handle this problem in this competition. We first used Short-Term Average/Long-Term Average (STA/LTA) and Kurtosis function to obtain over 55000 candidate phase picks (P or S). Based on Signal to Noise Ratio (SNR), about 40000 phases (P or S) are selected. So far, these 40000 phases have a hit rate of 40% among the manually picks. The causes include that 1) there exist false picks (neither P nor S); 2) some P and S arrivals are mis-labeled. To improve our results, we correlate the 40000 phases over continuous waveforms to obtain the phases missed by during the first pass. This results in 120,000 events. After constructing an affinity matrix based on the cross-correlation for newly detected phases, subspace clustering methods [Vidal 2011] are applied to group those phases into separated subspaces. Initial results show good agreement between empirical and clustered labels of P phases. Half of the empirical S phases are clustered into the P phase cluster. This may be a combined effect of 1) mislabeling isolated P phases to S phases and 2) clustering errors due to a small incomplete sample pool. Phases that were falsely detected in the initial results can be also teased out. To better characterize P and S phases, our next step is to apply subspace clustering methods directly to the waveforms, instead of using the cross-correlation coefficients of detected phases. After that, supervised learning, e.g., a convolutional neural network, can be employed to improve the pick accuracy. Updated results will be presented at the meeting.

Complex faulting in the Quetta Syntaxis: fault source modeling of the October 28, 2008 earthquake sequence in Baluchistan, Pakistan, based on ALOS/PALSAR InSAR data

NASA Astrophysics Data System (ADS)

Usman, Muhammad; Furuya, Masato

2015-09-01

The Quetta Syntaxis in western Baluchistan, Pakistan, is the result of an oroclinal bend of the western mountain belt and serves as a junction for different faults. As this area also lies close to the left-lateral strike-slip Chaman fault, which marks the boundary between the Indian and Eurasian plates, the resulting seismological behavior of this regime is very complex. In the region of the Quetta Syntaxis, close to the fold and thrust belt of the Sulaiman and Kirthar Ranges, an earthquake with a magnitude of 6.4 (Mw) occurred on October 28, 2008, which was followed by a doublet on the very next day. Six more shocks associated with these major events then occurred (one foreshock and five aftershocks), with moment magnitudes greater than 4. Numerous researchers have tried to explain the source of this sequence based on seismological, GPS, and Environmental Satellite (ENVISAT)/Advanced Synthetic Aperture Radar (ASAR) data. Here, we used Advanced Land Observing Satellite (ALOS)/Phased Array-type L-band Synthetic Aperture Radar (PALSAR) InSAR data sets from both ascending and descending orbits that allow us to more completely detect the deformation signals around the epicentral region. The results indicated that the shock sequence can be explained by two right-lateral and two left-lateral strike-slip faults that also included reverse slip. The right-lateral faults have a curved geometry. Moreover, whereas previous studies have explained the aftershock crustal deformation with a different fault source, we found that the same left-lateral segment of the conjugate fault was responsible for the aftershocks. We thus confirmed the complex surface deformation signals from the moderate-sized earthquake. Intra-plate crustal bending and shortening often seem to be accommodated as conjugate faulting, without any single preferred fault orientation. We also detected two possible landslide areas along with the crustal deformation pattern.

The 1911 M ~6.6 Calaveras earthquake: Source parameters and the role of static, viscoelastic, and dynamic coulomb stress changes imparted by the 1906 San Francisco earthquake

USGS Publications Warehouse

Doser, D.I.; Olsen, K.B.; Pollitz, F.F.; Stein, R.S.; Toda, S.

2009-01-01

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated 0.2-2.5 bar static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strikeslip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 Mw 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4-5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of ???3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for ~17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes.

The Evolution of the Seismic-Aseismic Transition During the Earthquake Cycle: Constraints from the Time-Dependent Depth Distribution of Aftershocks

NASA Astrophysics Data System (ADS)

Rolandone, F.; Bürgmann, R.; Nadeau, R.; Freed, A.

2003-12-01

We have demonstrated that in the aftermath of large earthquakes, the depth extent of aftershocks shows an immediate deepening from pre-earthquake levels, followed by a time-dependent postseismic shallowing. We use these seismic data to constrain the variation of the depth of the seismic-aseismic transition with time throughout the earthquake cycle. Most studies of the seismic-aseismic transition have focussed on the effect of temperature and/or lithology on the transition either from brittle faulting to viscous flow or from unstable to stable sliding. They have shown that the maximum depth of seismic activity is well correlated with the spatial variations of these two parameters. However, little has been done to examine how the maximum depth of seismogenic faulting varies locally, at the scale of a fault segment, during the course of the earthquake cycle. Geologic and laboratory observations indicate that the depth of the seismic-aseismic transition should vary with strain rate and thus change with time throughout the earthquake cycle. We quantify the time-dependent variations in the depth of seismicity on various strike-slip faults in California before and after large earthquakes. We specifically investigate (1) the deepening of the aftershocks relative to the background seismicity, (2) the time constant of the postseismic shallowing of the deepest earthquakes, and (3) the correlation of the time-dependent pattern with the coseismic slip distribution and the expected stress increase. Together with geodetic measurements, these seismological observations form the basis for developing more sophisticated models for the mechanical evolution of strike-slip shear zones during the earthquake cycle. We develop non-linear viscoelastic models, for which the brittle-ductile transition is not fixed, but varies with assumed temperature and calculated stress gradients. We use them to place constraints on strain rate at depth, on time-dependent rheology, and on the partitioning of deformation between brittle faulting and distributed viscous flow associated with the earthquake cycle.

Constraints on recent earthquake source parameters, fault geometry and aftershock characteristics in Oklahoma

NASA Astrophysics Data System (ADS)

McNamara, D. E.; Benz, H.; Herrmann, R. B.; Bergman, E. A.; McMahon, N. D.; Aster, R. C.

2014-12-01

In late 2009, the seismicity of Oklahoma increased dramatically. The largest of these earthquakes was a series of three damaging events (Mw 4.8, 5.6, 4.8) that occurred over a span of four days in November 2011 near the town of Prague in central Oklahoma. Studies suggest that these earthquakes were induced by reactivation of the Wilzetta fault due to the disposal of waste water from hydraulic fracturing ("fracking") and other oil and gas activities. The Wilzetta fault is a northeast trending vertical strike-slip fault that is a well known structural trap for oil and gas. Since the November 2011 Prague sequence, thousands of small to moderate (M2-M4) earthquakes have occurred throughout central Oklahoma. The most active regions are located near the towns of Stillwater and Medford in north-central Oklahoma, and Guthrie, Langston and Jones near Oklahoma City. The USGS, in collaboration with the Oklahoma Geological Survey and the University of Oklahoma, has responded by deploying numerous temporary seismic stations in the region in order to record the vigorous aftershock sequences. In this study we use data from the temporary seismic stations to re-locate all Oklahoma earthquakes in the USGS National Earthquake Information Center catalog using a multiple-event approach known as hypo-centroidal decomposition that locates earthquakes with decreased uncertainty relative to one another. Modeling from this study allows us to constrain the detailed geometry of the reactivated faults, as well as source parameters (focal mechanisms, stress drop, rupture length) for the larger earthquakes. Preliminary results from the November 2011 Prague sequence suggest that subsurface rupture lengths of the largest earthquakes are anomalously long with very low stress drop. We also observe very high Q (~1000 at 1 Hz) that explains the large felt areas and we find relatively low b-value and a rapid decay of aftershocks.

Vertical deformation of the Ubaye valley (South East part of the French Alps) monitored by precise leveling

NASA Astrophysics Data System (ADS)

Gilbert, Ferhat; Gilles, Ménard; Malet, Jean-Philippe

2017-04-01

The Ubaye valley (South East French Alps) is one of the most seismically active region of the massif (Saint-Paul-sur-Ubaye Ml=5.5 earthquake of 1959). Since at least 1977, several seismic crisis occurred. In 1989, 250 seismic events are reported. In 2003-2004, a seismic swarm generated 16000 micro-earthquakes East of the town of Barcelonnette. The observed migration of events suggested that overpressure of fluids was responsible for the activity (Jenatton et al., 2007; Daniel et al., 2011). A few kilometers to the North, in February 2012, a Ml=4.5 earthquake affected the region. This earthquake was followed by thousands of aftershocks which rate decreased very slowly in time. On April 7th 2014, an Ml=5.2 earthquake occurred (Thouvenot et al., 2016). Again it was followed by thousands of aftershocks; the present-day activity is still very high, much higher than a classic mainshock-aftershock sequence. Since, the seismic activity has never really decreased, being still very active in October 2016. As a matter of fact, the 2012-2016 seismic sequence shows a dual seismogenic behavior, including both sustained migrating seismic swarms (Ml<3) and short-term mainshock-aftershocks sequences (Ml>4). To understand the seismic activity and its effects on surface deformation, we used precise leveling along a 30 km long-profile from the municipality of Barcelonnette (to the West) to the municipality of Meyronnes and Saint-Paul-sur-Ubaye (to the East). Several historical leveling sections were observed in 1909, 1949 and 1969 by the French Mapping Agency (IGN), and by research institutes in 2005, 2006 and 2014. We present the results of the comparison of the leveling over one century. We discuss its possible links to the regional seismic activity or to the local presence of unstable slopes. We observe a change of tilting of the N-S section of Jausiers / Saint-Paul-sur-Ubaye (subsidence of 1 mm.yr-1 before 2005, and uplift of 3.5 mm.yr-1 after 2005), which may be linked to a change of stress regime in the valley as suggested by fluid pressures changes and its response in terms of seismic swarm activity. Moreover in the Meyronnes sector, we observe changes in displacement rates over the period 1909 - 1949 - 1969 - 2005 - 2014. These variations may be linked to unstable slopes in the village. The acceleration between 1949 and 1969 may be related to the Ml-5.5 earthquake of 1959. The deceleration after 2005 might be related to the mitigation works (surface drainage) installed in 2002.

Foreshocks and aftershocks of Pisagua 2014 earthquake: time and space evolution of megathrust event.

NASA Astrophysics Data System (ADS)

Fuenzalida Velasco, Amaya; Rietbrock, Andreas; Wollam, Jack; Thomas, Reece; de Lima Neto, Oscar; Tavera, Hernando; Garth, Thomas; Ruiz, Sergio

2016-04-01

The 2014 Pisagua earthquake of magnitude 8.2 is the first case in Chile where a foreshock sequence was clearly recorded by a local network, as well all the complete sequence including the mainshock and its aftershocks. The seismicity of the last year before the mainshock include numerous clusters close to the epicentral zone (Ruiz et al; 2014) but it was on 16th March that this activity became stronger with the Mw 6.7 precursory event taking place in front of Iquique coast at 12 km depth. The Pisagua earthquake arrived on 1st April 2015 breaking almost 120 km N-S and two days after a 7.6 aftershock occurred in the south of the rupture, enlarging the zone affected by this sequence. In this work, we analyse the foreshocks and aftershock sequence of Pisagua earthquake, from the spatial and time evolution for a total of 15.764 events that were recorded from the 1st March to 31th May 2015. This event catalogue was obtained from the automatic analyse of seismic raw data of more than 50 stations installed in the north of Chile and the south of Peru. We used the STA/LTA algorithm for the detection of P and S arrival times on the vertical components and then a method of back propagation in a 1D velocity model for the event association and preliminary location of its hypocenters following the algorithm outlined by Rietbrock et al. (2012). These results were then improved by locating with NonLinLoc software using a regional velocity model. We selected the larger events to analyse its moment tensor solution by a full waveform inversion using ISOLA software. In order to understand the process of nucleation and propagation of the Pisagua earthquake, we also analysed the evolution in time of the seismicity of the three months of data. The zone where the precursory events took place was strongly activated two weeks before the mainshock and remained very active until the end of the analysed period with an important quantity of the seismicity located in the upper plate and having variations in its focal mechanisms. The evolution of the Pisagua sequence point out a rupture by steps, that we suggest to be related to the properties of the upper plate, as well as along in the subduction interface. The spatial distribution of seismicity was compared to the inter-seismic coupling of previous studies, the regional bathymetry and the slip distribution of both the mainshock and the Magnitude 7.6 event. The results show an important relation between the low coupling zones and the areas lacking large magnitude events

Earthquake statistics, spatiotemporal distribution of foci and source mechanisms - a key to understanding of the West Bohemia/Vogtland earthquake swarms

NASA Astrophysics Data System (ADS)

Horálek, Josef; Čermáková, Hana; Fischer, Tomáš

2016-04-01

Earthquake swarms are sequences of numerous events closely clustered in space and time and do not have a single dominant mainshock. A few of the largest events in a swarm reach similar magnitudes and usually occur throughout the course of the earthquake sequence. These attributes differentiate earthquake swarms from ordinary mainshock-aftershock sequences. Earthquake swarms occur worldwide, in diverse geological units. The swarms typically accompany volcanic activity at margins of the tectonic plate but also occur in intracontinental areas where strain from tectonic-plate movement is small. The origin of earthquake swarms is still unclear. The swarms typically occur at the plate margins but also in intracontinental areas. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe. It is characterised by a frequent reoccurrence of ML < 4.0 swarms and by high activity of crustal fluids. West Bohemia-Vogtland is one of the most active intraplate earthquake-swarm areas in Europe which also exhibits high activity of crustal fluids. The Nový Kostel focal zone (NK) dominates the recent seismicity, there were swarms in 1997, 2000, 2008 and 20011, and a striking non-swarm activity (mainshock-aftershock sequences) up to magnitude ML= 4.5 in May to August 2014. The swarms and the 2014 mainshock-aftershock sequences are located close to each other at depths between 6 and 13 km. The frequency-magnitude distributions of all the swarms show bimodal-like character: the most events obey the b-value = 1.0 distribution, but a group of the largest events depart significantly from it. All the ML > 2.8 swarm events are located in a few dense clusters which implies step by step rupturing of one or a few asperities during the individual swarms. The source mechanism patters (moment-tensor description, MT) of the individual swarms indicate several families of the mechanisms, which fit well geometry of respective fault segments. MTs of the most events signify pure shears except for the 1997-swarm events the MTs of which indicates a combine sources including both shear and tensile components. The origin of earthquake swarms is still unclear. Nevertheless, we infer that the individual earthquake swarms in West Bohemia-Vogtland are mixture of the mainshock-aftershock sequences which correspond to step by step rupturing of one or a few asperities. The swarms occur on short fault segments with heterogeneous stress and strength, which may be affected by pressurized crustal fluids reducing normal component of the tectonic stress and lower friction. This way critically loaded faults are brought to failure and the swarm activity is driven by the differential local stress.

Megacity Megaquakes: Two Near-misses, and the Clues they Leave for Earthquake Interaction

NASA Astrophysics Data System (ADS)

Shapiro, S. A.; Krüger, O.; Dinske, C.; Langenbruch, C.

2011-12-01

Two recent earthquakes left their mark on cities lying well beyond the mainshock rupture zones, raising questions of their future vulnerability, and about earthquake interaction broadly. The 27 February 2010 M=8.8 Maule earthquake struck the Chilean coast, killing 550 people. Chile's capital of Santiago lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. The 11 March 2011 M=9.0 Tohoku oki earthquake struck the coast of Japan, its massive tsunami claiming most of its 18,564 victims. Reminiscent of Santiago, Japan's capital of Tokyo lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. Because of this distance, both cities largely escaped damage. But it may not have been a clean get-away: The rate of small shocks beneath each city jumped by a factor of about 10 immediately after its megaquake. At Santiago, the quake rate remains two times higher today than it was before the Maule shock; at Tokyo it is three times higher. What this higher rate of moderate (M<6) quakes portends for the likelihood of large ones is difficult--but imperative--to answer, as Tokyo and Santiago are probably just the most striking cases of a common phenomenon: Seismicity increases well beyond the rupture zone, as also seen in the 1999 Izmit-Düzce and 2010 Darfield-Christchurch sequences. Are the Tokyo and Santiago earthquakes, 100 km from the fault rupture, aftershocks? The seismicity beneath Santiago is occurring on the adjacent unruptured section of the Chile-Peru trench megathrust, whereas shocks beneath Tokyo illuminate a deeper, separate fault system. In both cases, the rate of shocks underwent an Omori decay, although the decay ceased beneath Tokyo about a year after the mainshock. Coulomb calculations suggest that the stress imparted by the nearby megaquakes brought the faults beneath Santiago and Tokyo closer to failure (Lorito et al, Nature Geoscience 2010; Toda and Stein, GRL 2013). So, they are aftershocks in the sense that they are contingent on the mainshock, and underwent at least an initial decay. But aftershocks do not necessarily signal a heightened likelihood of large shocks. They could instead accompany post-seismic creep, with the creep shedding the stress imposed by the megaquakes. These aftershocks are too deep for GPS observations to reveal unequivocally whether the faults are locked or creeping. But one clue is that the ratio of small to large shocks was not changed by the megaquakes. This distribution could be a reliable pointer for the probability of lager quakes, and so large shocks may now indeed be more probable than before the megaquakes--by a factor of at least two.

Megacity Megaquakes: Two Near-misses, and the Clues they Leave for Earthquake Interaction

NASA Astrophysics Data System (ADS)

Stein, R. S.; Toda, S.

2013-12-01

Two recent earthquakes left their mark on cities lying well beyond the mainshock rupture zones, raising questions of their future vulnerability, and about earthquake interaction broadly. The 27 February 2010 M=8.8 Maule earthquake struck the Chilean coast, killing 550 people. Chile's capital of Santiago lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. The 11 March 2011 M=9.0 Tohoku oki earthquake struck the coast of Japan, its massive tsunami claiming most of its 18,564 victims. Reminiscent of Santiago, Japan's capital of Tokyo lies 400 km from the high-slip portion of the rupture, and 100 km beyond its edge. Because of this distance, both cities largely escaped damage. But it may not have been a clean get-away: The rate of small shocks beneath each city jumped by a factor of about 10 immediately after its megaquake. At Santiago, the quake rate remains two times higher today than it was before the Maule shock; at Tokyo it is three times higher. What this higher rate of moderate (M<6) quakes portends for the likelihood of large ones is difficult--but imperative--to answer, as Tokyo and Santiago are probably just the most striking cases of a common phenomenon: Seismicity increases well beyond the rupture zone, as also seen in the 1999 Izmit-Düzce and 2010 Darfield-Christchurch sequences. Are the Tokyo and Santiago earthquakes, 100 km from the fault rupture, aftershocks? The seismicity beneath Santiago is occurring on the adjacent unruptured section of the Chile-Peru trench megathrust, whereas shocks beneath Tokyo illuminate a deeper, separate fault system. In both cases, the rate of shocks underwent an Omori decay, although the decay ceased beneath Tokyo about a year after the mainshock. Coulomb calculations suggest that the stress imparted by the nearby megaquakes brought the faults beneath Santiago and Tokyo closer to failure (Lorito et al, Nature Geoscience 2010; Toda and Stein, GRL 2013). So, they are aftershocks in the sense that they are contingent on the mainshock, and underwent at least an initial decay. But aftershocks do not necessarily signal a heightened likelihood of large shocks. They could instead accompany post-seismic creep, with the creep shedding the stress imposed by the megaquakes. These aftershocks are too deep for GPS observations to reveal unequivocally whether the faults are locked or creeping. But one clue is that the ratio of small to large shocks was not changed by the megaquakes. This distribution could be a reliable pointer for the probability of lager quakes, and so large shocks may now indeed be more probable than before the megaquakes--by a factor of at least two.

Seismic subduction of the Nazca Ridge as shown by the 1996-97 Peru earthquakes

USGS Publications Warehouse

Spence, W.; Mendoza, C.; Engdahl, E.R.; Choy, G.L.; Norabuena, E.

1999-01-01

By rupturing more than half of the shallow subduction interface of the Nazca Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically. The mainshock's rupture has a length of about 200 km and has an average slip of about 1.4 m. Its moment is 1.5 x 1028 dyne-cm and the corresponding M(w) is 8.0. The mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves. About 55% of the mainshock's total moment release occurred south of the Nazca Ridge, and the remaining moment release occurred at the southern half of the subduction interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle to the plate convergence direction, the subducting Nazca Ridge has a large southwards component of motion, 5 cm/yr parallel to the coast. The 900-1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a 'rigid indenter,' causing the greatest coupling south of the ridge's leading edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster' occurred at the northern part of the subducting Nazca Ridge. Aftershocks were notably lacking at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km downstrike of the mainshock's epicenter, implies that conditions existed for rupture into that zone. The 1996 earthquake ruptured much of the inferred source zone of the M(w) 7.9-8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes (M(w) ~8.8-9.1) of 1868 and 1877. The M(w) 8.0 Antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap. The M(w) 8.2 deep-focus Bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap. The recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events, among other factors, may signal an increased seismic potential for that zone.

Moment tensor inversion of recent local moderate sized Van Earthquakes: seismicity and active tectonics of the Van region : Eastern Turkey

NASA Astrophysics Data System (ADS)

Kalafat, D.; Suvarikli, M.; Ogutcu, Z.; Kekovali, K.; Ocal, M. F.; Gunes, Y.; Pinar, A.

2013-12-01

The study area of the present research, the Van Region is located at the norththern end of the collision zone between the Anatolia and Arabian plates. Therefore, the southeast border of the Anatolian plate collides with the Arabian plate along the Bitlis Suture Zone. This zone is formed by collision of Arabian and in large scale Eurasian plates at mid-Miocen age. This type of thrust generation as a result of compressional regime extends east-west. The largest recorded earthquakes have all taken place along Southern Turkey (e.g. Lice, 1971; Varto, 1966; Caldiran, 1976). On the 23th of October 2011, an earthquake shook the Van Lake, Eastern Turkey, following a seismic sequence of more than three months in an unprecedented episode for this region characterized by null or low seismicity. The October 23, 2011 Van-Ercis Earthquake (Mw=7.1) was the most devastating resulting in loss of life and destruction. In order to study the aftershocks' activity of this main event, we installed and kept a seismic network of 10 broad-band (BB) stations in the area for an interval of nearly fifteen months. We characterized the seismogenic structure of the zone by calculating a minimum 1-D local velocity model and obtaining precise hypocentre locations. We also calculated fault plane solutions for more than 200 moderate sized earthquakes based on first motion polarities and commonly Moment Tensor Inversion Methods. The seismogenic zone would be localized at aproximately 10 km depth. Generally, the distribution of the important moderate earthquakes and the aftershock distribution shows that the E-W and NE-SW oriented fault segments cause the earthquake activities. Aftershock events are located along the eastern border of Lake Van and mainly between 5 and 10 km depth and disposed in two alignments: a ~E-W-trending alignment that matches with the trace of the Van Trust fault Zone and a NE-trending which could correspond to an structure not previously seen. Selected focal mechanisms show a strong trust faulting which coincides with the nature of the Van fault. We were currently analysing an archive of over 5000 local events recorded by the KOERI seismic network of over 20 broadband stations between 2010 and 2013 in the whole Van Region. The Van Earthquake initiated and caused an increase in seismic activity of the region. Van Earthquake and its important aftershocks fault mechanism solutions show that the region is under compression and reverse faulting is a result of this regime which is effective on the active compressional tectonics of the region. This study was supported by Bogazici University Research Projects Commission under SRP/BAP project No. 6040.

Aftershocks: The Role of State Labor Policies in Shaping Teacher Sensemaking, Satisfaction and Exit Decisions in North Carolina

ERIC Educational Resources Information Center

Feldman, Rachel Carly

2017-01-01

This dissertation examines how teachers respond to state-wide labor reforms. In 2013 and 2014, the state of North Carolina drastically altered compensation for teachers. Policymakers affected labor by attempting to revoke tenure for all teachers (and succeeded in eliminating it for new teachers), discontinuing supplemental pay for advanced…

Coping with Natural Disasters: Lessons Learnt by a Head of Department

ERIC Educational Resources Information Center

Lord, Beverley

2011-01-01

Since the first of the 29 significant earthquakes and thousands of aftershocks that the University of Canterbury (New Zealand) community has endured in the last year, Beverly Lord has learned a few lessons as a departmental head in a university during a time of natural disaster. Herein, she organizes and describes these lessons under five…

Aftershocks of Chile's Earthquake for an Ongoing, Large-Scale Experimental Evaluation

ERIC Educational Resources Information Center

Moreno, Lorenzo; Trevino, Ernesto; Yoshikawa, Hirokazu; Mendive, Susana; Reyes, Joaquin; Godoy, Felipe; Del Rio, Francisca; Snow, Catherine; Leyva, Diana; Barata, Clara; Arbour, MaryCatherine; Rolla, Andrea

2011-01-01

Evaluation designs for social programs are developed assuming minimal or no disruption from external shocks, such as natural disasters. This is because extremely rare shocks may not make it worthwhile to account for them in the design. Among extreme shocks is the 2010 Chile earthquake. Un Buen Comienzo (UBC), an ongoing early childhood program in…

Tsunami: Hope in the Midst of Disaster

ERIC Educational Resources Information Center

Thirumurthy, Vidya; Uma, V.; Muthuram, R. N.

2008-01-01

The lives of many were changed forever when a tsunami struck on the morning of December 26, 2004, as a result of an earthquake off the coast of Indonesia registering 9.0 on the Richter scale. Aftershocks in the nearby Andaman and Nicobar Islands sent waves of fear among the survivors, further debilitating their spirits. The aim of this article is…

Theory of time-dependent rupture in the Earth

NASA Technical Reports Server (NTRS)

Das, S.; Scholz, C. H.

1980-01-01

Fracture mechanics is used to develop a theory of earthquake mechanism which includes the phenomenon of subcritical crack growth. The following phenomena are predicted: slow earthquakes, multiple events, delayed multiple events (doublets), postseismic rupture growth and afterslip, foreshocks, and aftershocks. The theory predicts a nucleation stage prior to an earthquake, and suggests a physical mechanism by which one earthquake may 'trigger' another.

FORESHOCK AND ATERSHOCK SEQUENCES OF SOME LARGE EARTHQUAKES IN THE REGION OF GREECE,

DTIC Science & Technology

or more foreshocks of magnitude larger than 3.8 occurred in forty per cent of the cases. The probability for an earthquake to be preceded by a large... foreshock not much smaller than the main shock is 10%. It is shown that some properties of the earth’s material in the aftershock region can be