The San Jacinto Fault Zone (SJFZ) is a component of the San Andreas transform system, the most seismically active fault zone in southern California, and accommodates a large portion of the plate motion in the region. Paleoseismic and historic records indicate that the SJFZ is capable of producing Mw > 7 earthquakes and poses a significant seismic hazard to large urban areas nearby.

Fault zone structures contain important information on various aspects of earthquake and fault mechanics ranging from long-term evolutionary processes to brittle rock rheology and dynamic stress fields operating during the occurrence of earthquakes. Thus, in the first part of this talk, we image the internal structure of the SJFZ using seismograms recorded by dense linear arrays (with station spacing < 100 m) crossing the surface trace of the fault. Analyses of fault zone head waves, delay times of P wave, and fault zone trapped waves illuminate the fault zone internal structure as a combination of an impedance contrast and low-velocity zones. The resulting high-resolution fault zone image (e.g., cross-fault velocity contrast, fault zone geometry, and attenuation), in complement to regional tomographic models with a nominal resolution of 1-2 km, can have profound implications for seismic hazard assessment associated with the SJFZ.

Measuring velocity changes within seismically active fault zones, particularly at depth, has been a long-sought goal of seismology. In the second part of this talk, we study temporal changes in seismic velocities associated with a moderate (Mw 5.2) earthquake that occurred in the SJFZ. Coda waves in daily cross-correlations of ambient noise recorded by two dense linear arrays close to the epicenter are used to estimate the co- and post-seismic structural perturbations in the fault zone. The high-resolution 4-D imaging of the internal structure of the SJFZ provides extra constraints for better simulations of dynamic rupture and ground motion for future earthquakes on the SJFZ.