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Laboratory Earthquakes : New Insights into Earthquake Source Processes


IPGP - Îlot Cuvier


Séminaires de Sismologie

Salle 310

Harsha Bath


Abstract : This is a two part talk focusing on some new insights provided by laboratory earthquake ruptures in understanding earthquake source physics. The first part of the talk will focus on supershear earthquake ruptures. 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. Heterodyne laser interferometers enable continuous, high bandwidth measurements of fault-normal (FN) and fault-parallel (FP) particle velocity ``ground motion" records at focused positions 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 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 ``trailing Rayleigh rupture" in the wake of the primary rupture, which sweeps the measurement station following passage of the shear Mach front. We also examine the 2002, M_w 7.9 Denali fault earthquake and the remarkable set of ground motion records obtained at Pump Station 10 (PS10) and attempt to replicate the Denali ground motion signatures using a laboratory earthquake experiment. The focus of the second part of the talk would be on understanding the effect of off-fault damage on earthquake ruptures. The interaction between a dynamic mode II fracture on a fault plane and off-fault damage has been studied experimentally using high-speed photography and theoretically using finite element numerical simulations. In the experimental studies, fracture damage was created in photoelastic Homalite plates by thermal shock in liquid nitrogen and rupture velocities were measured by imaging fringes at the tips. Two cases were studied: an interface between damaged and undamaged Homalite plates, and an interface between damaged Homalite and undamaged polycarbonate plates. Propagation on the interface between damaged and undamaged Homalite is asymmetric. A ruptures propagating in the direction for which the compressional lobe of its crack-tip stress field is in the damage (which we term the ‘C’ direction) is unaffected by the damage. In the opposite ‘T’ direction, the rupture velocity is significantly slower than the velocity in undamaged plates at the same load. Specifically, transitions to supershear observed using undamaged plates are not observed in the ‘T’ direction. Propagation on the interface between damaged Homalite and undamaged polycarbonate exhibits the same asymmetry, even though the elastically “favored” ‘+’ direction coincides with the ‘T’ direction in this case indicating that the effect of damage is stronger than the effect of elastic asymmetry. This asymmetric propagation was also simulated numerically by incorporating the micromechanical damage mechanics formulated by Ashby and Sammis (PAGEOPH, 1990) into the ABAQUS dynamic finite element code. The quasi-static Ashby/Sammis formulation has been improved to include modern concepts of dynamic fracture mechanics, which become important at the high loading rates in the process zone of a propagating rupture. Séminaire de sismologie