Seismic Reflection and Diffraction Imaging of the San Andreas Fault at SAFOD

A 2D seismic reflection and refraction survey across the San Andreas Fault (SAF) near Parkfield at the location of the San Andreas Fault Observatory at Depth (SAFOD) provides a detailed characterization of upper crustal structure. We present results from prestack migration and from waveform tomography of these data. Three-component stations at 50m spacing recorded 63 explosive shots along a 46 km long line perpendicular to the surface trace of the SAF. P-wave velocities derived by first-break travel time tomography were used in a Kirchhoff prestack migration that included steep dips. The resulting seismic reflection section shows several steep reflectors down to ~5 km depth. A strong reflector ~10 km to the northeast of the SAF is interpreted to mark the faulted transition from the Franciscan accretionary sediments to the Great Valley Sequence. It dips steeply to the southwest down to 2 km depth, where it overturns to dip to the northeast to at least 5 km depth. Its unusual shape is interpreted to be the result of transpressional forces related to the plate motions and backthrust faulting. Weaker, but still prominent steep reflections are imaged at the SAF, and from the steep edge of the Salinian granite 1.6 km southwest of the SAF. A vertical reflection between 0.5 and 1.0 km below the surface trace of the SAF is interpreted to be a direct image of the fault. A second vertical reflection from 1.0 to 2.5 km depth is offset 300 m towards the northeast, and coincides with the mapped location of the Gold Hill Fault (GHF). This reflector either marks an offset to the northeast of the active SAF or marks the inactive GHF. At ~2.5 km depth, the SAF/GHF steps at least 500 m to the southwest, where two active fault strands are constrained by seismic activity. The reflections from the steep edge of the Salinian granite and from below the surface trace of the GHF bound a vertical wedge of sedimentary rocks constrained from previous seismic and magnetotelluric explorations and by SAFOD drilling. In order to improve the spatial resolution of the velocity model, waveform tomography is being applied to the data. Preliminary results from this ongoing work will be presented.