Characterization of the San Andreas Fault by Fault-Zone Trapped Waves

and operational activities under the various EarthScope components. SAFOD provides us an opportunity to study the major plate boundary, i.e. the SAF, using fault-zone seismic trapped waves. We have successfully recorded fault-zone trapped waves generated by microearthquakes and controlled sources at the Californian active fault zones, including the SAF at Parkfield, the San Jacinto fault near Anza, and rupture zones of recent major earthquakes at Landers and Hector Mine [Li et al., 1990-2001]. Observations and numerical modeling of fault-zone trapped waves allowed us to delineate the fault geometry (width, depth, continuities, barriers, and branches) and physical properties (velocities, Q, crack densities, and Poisson's ratio) of fault-zone rocks with high-resolution at the seismogenic depth. We have observed the post-earthquake fault healing (fault strength regaining) with time at the Landers rupture zone in repeated seismic surveys. The evolving wave velocity increase with time, consistent with the closure of cracks that opened during the 1992 M7.4 earthquake, hints tantalizingly at the governing physics. Knowledge of spatial and temporal variations in fault structure through fault-zone seismic wave study may help resolve these variations and predict the behavior of future earthquakes on the active faults in California and elsewhere. We propose to record fault-zone trapped waves generated by explosions and microearthquakes using the surface and borehole seismic arrays deployed at the San Andreas fault near the SAFOD drilling site Parkfield (Fig. 1). We shall use fault-zone trapped waves to delineate the internal structure of the Parkfield SAF segment in 3-D and monitor th