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

Abstract

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, M_w 7.9 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 and 3 km north of the fault along the Alaska Pipeline. Motivated by the analysis and thorough interpretation of these records by Dunham and Archuleta (2004, 2005), we attempt to replicate the Denali ground motion signatures using a laboratory earthquake experiment. The experiments feature a left (west) to right (east) propagating right lateral rupture within the Homalite test specimen with particle velocity data collected at a near-field station situated just above (north of) the fault, (on the compressional quadrant) in order to simulate the PS10 scenario. Both sub-Rayleigh and supershear laboratory earthquake experiments are conducted using the "Denali PS10" configuration in order to compare and contrast the resulting particle velocity signatures. Results from the supershear experiment capture the prominent FN and FP ground motion signatures and corresponding sense of particle motion revealed in the PS10 ground motion records. Most notably, the particle velocity records feature a dominant FP component coinciding with the arrival of the shear Mach front, followed by a pronounced velocity swing in the FN component coinciding with the passage of a trailing Rayleigh disturbance, as independently confirmed by the presence of these features and their noted arrival times in the synchronized photoelastic image sequence.

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