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Sample records for homalite

  1. Constraining friction laws by experimental observations and numerical simulations of various rupture modes

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A. J.

    2006-12-01

    Several different types of friction laws, such as linear slip-weakening law and variants of rate- and state- dependent friction laws, are widely used in earthquake modeling. It is important to understand how much complexity one needs to include in a friction law to properly capture the dynamics of frictional rupture. Observations suggest that earthquake ruptures propagate as slip pulses (Heaton, 1990). In the absence of local heterogeneities and bimaterial effect, only one mechanism, namely strong rate-weakening friction, is shown, theoretically and numerically, to be capable of generating pulses on homogeneous interfaces separating two identical materials. We have observed pulses in our recent experiments designed to reproduce such a setting (Rosakis, Lu, Lapusta, AGU, 2006). By exploring experimental parameter space, we have identified different dynamic rupture modes including pulse-like, crack-like, and mixed modes. This suggests that rate weakening may play an important role in rupture dynamics. The systematic transition between rupture modes in the experiments is consistent with the theoretical and numerical study of Zheng and Rice (1998), who studied the behavior of rate-weakening interfaces. They concluded that whether strong rate weakening results in a pulse-like or crack-like behavior depends on the combination of two parameters: the level of prestress before rupture propagation and the amount of rate weakening on the fault. If we use Dieterich-Ruina rate-and-state friction laws with enhanced rate weakening at high slip rates, as appropriate for flash heating, to describe frictional properties of Homalite, use reasonable friction parameters motivated by previous studies, and apply Zheng and Rice analysis, we can qualitatively explain the rupture modes observed in experiments. Our current work is focused on modeling the experimental setup numerically to confirm that one indeed requires rate dependence of friction to reproduce experimental results. This

  2. Laboratory Experiments and Theoretical Studies of Rupture Modes and Supershear Transition

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A.

    2007-12-01

    Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatio-temporal complexity of fault slip. Early theoretical models often treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed, including strong weakening of the interface with sliding rate, interaction of rupture with local heterogeneities, and normal stress variation due to a bimaterial effect. We observe pulse-like and crack-like rupture modes in the experimental configuration of a Homalite plate with inclined interface prestressed both in compression and in shear, similarly to faults in the Earth's crust. Dynamic rupture is initiated by exploding a 0.1 mm nickel wire. Digital high-speed cameras are used to record photoelastic images. Two interferometry-based velocimeters are used to determine the history of relative sliding velocity at one location along the interface. Our results indicate that pulse-like ruptures can exist on such interfaces in the absence of a bimaterial effect or local heterogeneities. For a set of experiments with increasing ratio of shear to normal prestress, which is achieved by increasing the inclination angle of the interface, we observe a change in rupture modes from pulse-like to crack-like. This systematic variation is consistent with the theoretical study of velocity-weakening interfaces by Zheng and Rice (1998). We also establish experimentally, for the first time, that both pulse-like and crack-like rupture modes can transition to supershear speeds. After the supershear transition, both modes have speeds within the open interval \\sqrt{2} Cs to Cp, where Cs and Cp are the S- and P-wave speeds of Homalite, respectively. However, the rupture

  3. Identifying the unique ground motion signatures of supershear earthquakes: Theory and experiments

    NASA Astrophysics Data System (ADS)

    Mello, M.; Bhat, H. S.; Rosakis, A. J.; Kanamori, H.

    2010-10-01

    The near field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and coworkers (Xia et al., 2004, 2005a; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high bandwidth measurements of fault-normal (FN) and fault-parallel (FP) particle velocity "ground motion" records at discrete locations on the surface of a Homalite test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field. Experimental results confirm that near field ground motion records associated with the passage of a sub-Rayleigh rupture are characterized by a FN velocity swing which dominates over the FP velocity swing. The situation is shown to reverse in the supershear rupture speed regime whereby the motion along the shear Mach front is characterized by a FP particle velocity swing which dominates over the FN velocity swing. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are (1) a pronounced peak in the FP velocity record, induced by the leading dilatational field, which sweeps the measurement station just prior to the arrival of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a "trailing Rayleigh disturbance", which sweeps the measurement station following passage of the shear Mach front. Each of these features are addressed in detail. We conclude by reexamining the 2002, Mw7.9 Denali fault earthquake and the remarkable set of ground motion records obtained at Pump Station 10 (PS10), located approximately 85 km east of the epicenter

  4. Off-fault tensile cracks: A link between geological fault observations, experiments and earthquake rupture models

    NASA Astrophysics Data System (ADS)

    Ngo, D.; Huang, Y.; Rosakis, A.; Griffith, W. A.; Pollard, D. D.

    2009-12-01

    Motivated by the occurrence of high-angle pseudotachylite injection veins along exhumed faults, we use optical experiments and high-speed photography to interpret the origins of tensile fractures that form during dynamic shear rupture in laboratory experiments. Sub-Rayleigh (slower than the Rayleigh wave speed) shear ruptures in Homalite-100 produce damage zones consisting of a periodic array of tensile cracks. These cracks nucleate and grow within cohesive zones behind the tips of shear ruptures that propagate dynamically along interfaces with frictional and cohesive strength. The tensile cracks are produced only along one side of the interface where transient, fault-parallel, tensile stress perturbations are associated with the growing shear rupture tip. We use an analytical, linear velocity weakening, rupture model to examine the local nature of the dynamic stress field in the vicinity of the tip of the main shear rupture which grows along a weak plane (fault) with sub-Rayleigh speed. It is this stress field which is responsible for driving the off-fault mode-I microcracks that grow during the experiments. We show that (1) the orientation of the cracks can be explained by this analytical model; and (2) the cracks can be used to simultaneously constrain the constitutive behavior of the shear rupture tip. In addition, we propose an extension of this model to explain damage structures observed along exhumed faults. Results of this study represent an important bridge between geological observations of structures preserved along exhumed faults, laboratory experiments and theoretical models of earthquake propagation, potentially leading to diagnostic criteria for interpreting velocity, directivity, and static pre-stress state associated with past earthquakes on exhumed faults.

  5. Experimental Investigation of Radiated Ground Motion Due to Supershear Earthquake Ruptures

    NASA Astrophysics Data System (ADS)

    Mello, M.; Bhat, H.; Kanamori, H.; Rosakis, A.

    2009-12-01

    Recent theoretical and numerical investigation of supershear ruptures in 2D (Dunham and Archuleta, 2004 and Bhat et al., 2007) and in 3D (Dunham and Bhat, 2008 ) have shown that ground motion due to the passage of the Mach front is virtually unattenuated at large distances from the fault. In the 2D steady-state supershear rupture model, the Mach front carries the ground motion unattenuated to infinity. Bhat et al. (2007) estimate that the actual distance should be of the order of the depth of the seismogenic zone. This as been partly observed by Bouchon and Karabulut (2008) who showed that the aftershocks cluster in a region away from the fault at distances comparable to the depth of the seismogenic zone following the passage of a supershear rupture. Numerical simulations of supershear earthquake ruptures by Aagaard and Heaton (2004) also show that in the supershear regime the fault parallel component of particle velocity dominates over the fault normal one whereas in the sub-Rayleigh regime the opposite is true. We have recently examined and validated these distinguishing features using an established laboratory earthquake setup (Xia et al., 2004, 2005). Heterodyne laser interferometers are used to obtain continuous particle velocity records at discrete stations on the surface of a Homalite test specimen as a supershear or sub-Rayleigh rupture propagates along the frictional fault. A photoelastic image sequence is simultaneously acquired using high-speed digital photography in order to obtain a synchronized whole field view of the event. Ground motion attenuation in the case of sub-Rayleigh and supershear events is examined by considering the ratio of the measured fault normal and fault parallel particle velocity swings at various distances from the fault. Additional experiments were also conducted to characterize the attenuation of the dominant ground motion component for sub-Rayleigh and supershear ruptures. Last but not least, we also verify several key

  6. Laboratory Earthquake Measurements with the High-speed Digital Image Correlation Method and Applications to Super-shear Transition

    NASA Astrophysics Data System (ADS)

    Rubino, V.; Lapusta, N.; Rosakis, A.

    2012-12-01

    Mapping full-field displacements and strains on the Earth's surface during an earthquake is of paramount importance to enhance our understanding of earthquake mechanics. In this study, the feasibility of such measurements using image correlation methods is investigated in a laboratory earthquake setup. Earthquakes are mimicked in the laboratory by dynamic rupture propagating along an inclined frictional interface formed by two Homalite plates under compression, using the configuration developed by Rosakis and coworkers (e.g., Rosakis et al., 2007). In our study, the interface is partially glued, in order to confine the rupture before it reaches the ends of the specimen. The specimens are painted with a speckle pattern to provide the surface with characteristic features for image matching. Images of the specimens are taken before and after dynamic rupture with a 4 Megapixels resolution CCD camera. The digital images are analyzed with two software packages: VIC-2D (Correlated Solutions Inc.) and COSI-Corr (Leprince et. al, 2007). Both VIC-2D and COSI-Corr are able to characterize the full-field static displacement of a dynamic crack. For example, in a case with secondary mode I cracks, the correlation analysis performed with either software clearly shows (i) the relative displacement (slip) along the frictional interface, (ii) the rupture arrest on the glued boundaries, and (iii) the presence of two wing cracks. The obtained displacement measurements are converted to strains, using de-noising techniques. The digital image correlation method is then used in combination with high-speed photography. We will report our progress on the study of a spontaneously expanding sub-Rayleigh shear crack advancing along an interface containing a patch of favorable heterogeneity, such as a preexisting subcritical crack or a patch with higher prestress. According to the predictions of Liu and Lapusta (2008), intersonic transition and propagation can be achieved in the presence of a

  7. Identifying the Unique Ground Motion Signatures of Supershear Earthquakes: Theory and Experiments

    NASA Astrophysics Data System (ADS)

    Mello, Michael

    The near-field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and co-workers (Xia et al., 2004, 2005; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high-bandwidth measurements of fault-normal (FN), fault-parallel (FP), and vertical (V) particle velocity ``ground motion" records at discrete locations on the surface of a Homalite-100 test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field. The first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures. Experimental results demonstrate that a shear Mach front produced by a stable supershear rupture is characterized by a dominant FP velocity component. The situation is shown to reverse in the sub-Rayleigh rupture speed regime whereby the FN particle velocity component dominates the ground motion record. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are, (1) a pronounced peak in the FP velocity record induced by the leading dilatational field, which sweeps the measurement station in advance of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a trailing Rayleigh sub-Rayleigh (secondary) rupture, which follows the arrival of the shear Mach front. Analysis of the particle velocity records also confirms 2D steady-state theoretical predictions pertaining to the separation, attenuation, and radiation partitioning of the shear and dilatational portions of the rupture velocity field components

  8. Testing Friction Laws by Comparing Simulation Results With Experiments of Spontaneous Dynamic Rupture

    NASA Astrophysics Data System (ADS)

    Lu, X.; Lapusta, N.; Rosakis, A. J.

    2005-12-01

    Friction laws are typically introduced either based on theoretic ideas or by fitting laboratory experiments that reproduce only a small subset of possible behaviors. Hence it is important to validate the resulting laws by modeling experiments that produce spontaneous frictional behavior. Here we simulate experiments of spontaneous rupture transition from sub-Rayleigh to supershear done by Xia et al. (Science, 2004). In the experiments, two thin Homalite plates are pressed together along an inclined interface. Compressive load P is applied to the edges of the plates and the rupture is triggered by an explosion of a small wire. Xia et al. (2004) link the transition in their experiments to the Burridge-Andrews mechanism (Andrews, JGR, 1976) which involves initiation of a daughter crack in front of the main rupture. Xia et al. have measured transition lengths for different values of the load P and compared their results with numerical simulations of Andrews who used linear slip-weakening friction. They conclude that to obtain a good fit they need to assume that the critical slip of the slip-weakening law scales as P-1/2, as proposed by Ohnaka (JGR, 2003). Hence our first goal is to verify whether the dependence of the critical slip on the compressive load P is indeed necessary for a good fit to experimental measurements. To test that, we conducted simulations of the experiments by using boundary integral methodology in its spectral formulation (Perrin et al., 1995; Geubelle and Rice, 1995). We approximately model the wire explosion by temporary normal stress decrease in the region of the interface comparable to the size of the exploding wire. The simulations show good agreement of the transition length with the experimental results for different values of the load P, even though we keep the critical slip constant. Hence the dependence of the critical slip on P is not necessary to fit the experimental measurements. The inconsistency between Andrews' numerical results