Bimaterial Interfaces, Damage Zones, Earthquake Directivities and Signatures of Rupture Opening in the Parkfield Section of the San Andreas Fault

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The studies focus on systematic high-resolution imaging of bimaterial interfaces, low velocity fault zone layers, rupture directivities of earthquakes, and seismic signatures of opening rupture mode along various fault segments of the San Andreas Fault system in the Parkfield area. The overall goal is to provide multi-signal tests, based on large seismic waveform and catalog data sets, for the hypothesis that earthquakes on large strike-slip faults have preferred propagation directions that are controlled by the velocity contrasts across the faults, and for the possibility that earthquake ruptures on such faults have tensile component of faulting. The imaging of bimaterial interfaces employ fault zone head waves that refract along bimaterial faults and direct P waves. The fault zone head waves provide the most diagnostic signal for the existence of sharp bimaterial interfaces and the highest-resolution tool for imaging their seismic properties at depth. Fault zone trapped waves are used to quantify symmetry properties with respect to the main rupture surface and other characteristics of damaged fault zone layers at different locations. The analysis of rupture directivities involves stacking aftershock rates at the opposite along-strike directions of each event and observing symmetry properties of dynamic triggering patterns of the stacked data at different space-time scales. The examination for opening rupture mode utilizes several potential seismic signals that are suggested by recent theoretical calculations. The studies also focus on clarifying the structural and earthquake properties around the SAFOD site, clarifying properties of the main SAF between the hypocenters of the 1966 and 2004 M6 events, and clarifying the connectivity between the main SAF and the Southwest Fracture zone at depth. The studies can have profound implications for key aspects of earthquake and fault mechanics including effective constitutive laws, earthquake source and rupture properties, generated frictional heat, interaction between faults and more. The analyses can help resolving existing controversies. Positive results on statistical preference of earthquake propagation directions may lead to refined estimates of seismic shaking hazard near major faults.