Seismic Characterization of Core Structure of the San Andreas Fault at Parkfield, California Using Fault-Zone Guided Waves

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In this research project, the PIs carry out field seismic experiments to acquire fault-zone guided waves (FZGW) generated by microearthquakes and explosions at the San Andreas Fault (SAF) Parkfield, California. Observation and preliminary modeling of 2-5 Hz trapped waves recorded at dense seismic arrays of 55 three-component seismometers deployed in 2002 on 500-1000-m long lines along and across the fault traces for events located within the fault zone shown evidence of a damaged core zone on the main SAF. The zone from the surface to at least 4-5 km depth is marked by a low-velocity waveguide ~150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. This distinct low-velocity zone on the main SAF is interpreted as being a remanent of repeated damage due to M6 episode and historical large earthquakes on the principal fault plane at Parkfield. In the current FZGW experiment conducted closer to the SAFOD (San Andreas Fault Observatory at Depth) drilling site, more valuable data are recorded at along- and across-fault dense arrays for a bunch of shots detonated by USGS and refraction experiments, and microearthquakes occurring at various depths, including SAFOD "target" events at ~4km depths and deep events to the ~11km depths beneath arrays. The quantitative analysis and 3-D finite-difference modeling of fault-zone trapped wave data are to be carried out for a high-resolution delineation of the magnitude and extent of damage on the SAF nearby the drilling site, and draw the depth-dependent and lateral variations in internal structure and material properties of the fault zone. The body wave arrival times inversion for all shots will be used to obtain a quasi-3D local velocity structure for improvement of the numerical modeling of guided waves in terms of a refined structural model as a function of depth and distance along the fault. PIs plan to reoccupy seismic stations and repeat shots within the fault zone in 2004-2005. The data from the repeated experiment will be used to detect if there are the temporal changes in fault-zone material property before a pending M6 earthquake at Parkfield. The shear-wave splitting (SWS) analysis for earthquake data shall provide information on near-fault stresses. The 3-D finite-element dynamic rupture simulations based on the fault-zone structure delineated by trapped waves and the stress pattern from SWS will help better understand the relationship between the fault zone structure and dynamic rupture. Through data collection and analysis efforts in this project, PIs shall characterize the dimensions, magnitude and time dependence of the damaged zone on the SAF at Parkfield. The spatial extent of fault weakness compared to surrounding bedrock, and the loss and recouping of strength across the earthquake cycle are critical ingredients in understanding of fault mechanics and physics, as well as in estimation of losses from future earthquakes. This research project is a supplement to the main program SAFOD among various components of EarthScope. The PIs are responsible on fault-zone trapped wave study while overall coordination between this supplementary project and scheduling of the field work for geophysical site characterization in SAFOD program will be done under the main project.