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

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. The second phase of our experimental investigations re-examine the 2002, Mw7.9, Denali fault earthquake and the remarkable set of near-source ground motion records obtained at (PS10), located approximately 85 km east of the epicenter and just 3 km north of the fault along the Alaska pipeline. Motivated by the analysis and interpretation of these records by (Ellsworth et al., 2004; Dunham and Archuleta, 2004, 2005), we attempt to mimic the Denali strike-slip rupture scenario and replicate the PS10 ground motion signatures using a laboratory earthquake experiment. The experiments feature a left-to-right (west-to-east) propagating right lateral rupture within a Homalite-100 test specimen with particle velocity data collected at a near-field station situated just above (north of) the fault. 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. Supershear laboratory records capture all of the prominent features displayed within the PS10 ground motion records. Noted velocity signatures are correlated to the location of the rupture fronts and their noted arrival times in the synchronized photoelastic image sequence. Scaling relationships are also presented which transform the laboratory records through six orders of magnitude in time, to match the scale of the PS10 ground motion records. The strong correlation between the scaled experimental records and the actual PS10 ground motion records supports the hypothesis that the Denali strike-slip fault exhibited a supershear burst. Finally, we present a 2D steady state, stress-velocity formulation that relates the FP and FN particle velocity records measured close to the fault, to the evolution of the stress tensor at the same location. A locally steady-state condition is assumed within a restricted time interval in order to invoke these relationships and estimate the dynamic stresses, sigma and tau, at the near-fault station. Dynamic stress measurements enable a new class of friction investigations using the laboratory earthquake configuration. Experimental findings are presented, which capture the temporal and spatial distributions of sigma and tau, evolution of the dynamic friction coefficient, and velocity weakening behavior of a supershear slip-pulse.