Earthquake processes and geologic structure of the San Andreas Fault at Parkfield through the SAFOD seismic array (California)

The San Andreas Fault Observatory at Depth (SAFOD) has the goal of understanding earthquake processes at hypocentral depths. In July 2002 Duke University installed a vertical array of seismometers in the SAFOD Pilot Hole (PH). Seismograms recorded by the array give insights into the structure of the SAFOD site. The ratios of P- and S-wave velocities (Vp/Vs) along the array suggest the presence of two faults intersecting the PH. The Vp/Vs ratios also depend on source location, with high values for sources to the northwest along the San Andreas, and lower ones to the southeast. This distribution correlates with high and low creep rates along the SAF. Since higher Vp/Vs ratios can be produced by increasing fluid saturation, this effect could be the one guiding the frequent seismicity and creep along this segment of the fault.; The SAFOD PH Vertical Seismic Profiling-seismograms from nearby microearthquake and explosion sources also contain secondary signals between the P- and S-waves. These signals are shown to be P and S waves scattered by the local structure. Kirchhoff migration was applied to define the origin points of these scattered signals. Both 2D and 3D analysis of microearthquake and explosion seismograms showed that the collected scattering points form planar surfaces, interpreted as a vertical San Andreas Fault and four other secondary faults forming a flower structure. These structures along with seismicity located in secondary fault strands suggest that stresses along the San Andreas at Parkfield could be distributed in more complex ways, modifying the local earthquake cycle.; Modeling of scattered phases indicates strong geologic contrasts that have recently been drilled by SAFOD. A granite-sediment interface may constitute the boundary of a hanging block with sedimentary materials with low electrical resistivities. Shallow earthquakes at Parkfield take place at the interface of the northeastern boundary of this block, adjacent to the San Andreas Fault, and they may be related to high pore pressures present in the sediments within it.