Collaborative Research: Waveform Analysis of Repeating Earthquakes - Implications for Fault Damage and Healing Processes

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Recent development in earthquake relocation techniques has led to many sets of repeating clusters identified in different tectonic environments. These earthquakes are most likely produced by the failure of asperities that are loaded by aseismic creep on the surrounding fault plane. Since they rupture the same fault patch repeatedly and generate nearly identical waveforms, they provide invaluable sources for detecting subtle temporal changes in fault zone properties associated with the occurrence of major earthquakes.This project aims to quantify damage and healing processes in major fault zones that are recently ruptured in moderate to large earthquakes based on the waveform analysis of repeating earthquakes. The following two types of calculations are performed to target for specific kinds of wave propagation: 1) measuring subtle changes in travel times accumulated during source-receiver paths and near receivers from cross-correlation of S and early S-coda waveforms generated by repeating earthquakes; and 2) quantifying variations in the source properties of repeating earthquakes (e.g., seismic moment, corner frequency, stress drop, and rupture velocities) near the hypocentral regions of large earthquakes. Both measurements are critical for improving our knowledge of how fault zone properties evolve during a large earthquake cycle.A better understanding of damage and healing processes inside active fault zones is expected to have significant implications in the physics of earthquakes and faults. Clarifying the spatial distribution and especially the depth extent of the damage and healing processes in fault zones help to better understand the origins of on and off-fault damage, and from that the energy budget during large earthquakes. The time-dependent changes of fault zone properties provide critical information on the rheology and mechanics of faulting under in situ conditions. An improved understanding of the spatio-temporal evolution of earthquake source and fault zone properties may provide crucial new information for deciphering when and where the next major earthquake might strike.