This study develops and uses advanced seismological techniques and continuous waveform data recorded by spatially-dense fault zone arrays around the San Jacinto Fault Zone (SJFZ) in southern California. Analyses of these data provide detailed characterization of (potentially time-dependent) seismic velocities and attenuation coefficients of the shallow crust. To provide benchmarks for the new techniques and understand how seismic properties change with location and depth, the investigators use data recorded by shallow borehole seismometers installed within and adjacent to the fault zone, as well as recordings from regional seismic networks. These results complement models of tomographic studies that typically do not resolve seismic properties in the shallow crust, and allow development of seismic structural models around the SJFZ essentially up to the surface. Knowledge of seismic properties of the shallow crust is particularly relevant to observed ground motion and failure processes. These results may be relevant to numerous other applications including crustal hydrology, geothermal energy, subsurface reservoirs and storage, as well as seismic safety of infrastructure facilities. The analysis methods develop by this research can be used in studies of seismic properties of the subsurface material in other locations.
The research uses state-of-the-art data and novel seismic imaging and monitoring techniques to close critical observational gaps. The studies address the following key questions: (1) What are the seismic velocities and attenuation coefficients in the top few hundred meters of the crust at various sections of the SJFZ and at sites that are off the fault? (2) At what stress/strain amplitudes do shallow rocks have nonlinear response and temporal changes of properties in relation to different loading mechanisms and periods (e.g., local and remote earthquakes, tides, and seasonal variations) and location with respect to the fault zone, site condition parameters, and depth? (3) How do amplitudes of temporal changes of seismic properties and recovery timescales produced by various loadings vary in relation to the fault zone, site conditions and depth? (4) Are there fault sections and/or off-fault sites with higher susceptibly to failures (manifested by time-dependent seismic properties), and if they are found, can anomalous properties in these regions be identified that may explain the elevated susceptibility? The studies can augment significantly the available in situ characterization of seismic velocity and attenuation coefficients, nonlinear response and temporal changes of properties in the top few hundred meters of the crust. The results are expected to provide fundamental information that may lead to improved understanding of seismic ground motion (and hence shaking hazard) as well as other applications.
