Datensatz

Zusammenfassung

Detailed characterization of the subsurface geology surrounding the San Andreas Fault Observatory at Depth (SAFOD) drill site is required to plan drilling and to interpret down-hole results. A 46-km long seismic refraction and reflection line was acquired in November 2003 perpendicular to the San Andreas Fault centered on the SAFOD drill site. The line was deployed straight across country assisted by helicopters. A fixed array of 912 three-component stations at a 25-50 m spacing recorded 68 explosive 25-100 kg shots at a 0.5-1 km spacing. Piggyback deployments recorded the shots on seismographs in the SAFOD pilot hole, on permanent and temporary earthquake arrays, and on a high-resolution (5-m) refraction array between the drill site and the fault. First arrival travel times from the main line were inverted to obtain a 2-D seismic velocity model of the upper crust. The model contains strong variations from $<$2 km/s at the surface to $>$6 km/s in granitic basement. Granitic rocks of the Salinian terrane west of the San Andreas Fault are substantially faster than adjacent predominantly sedimentary rock of the Franciscan terrane east of the fault. Salinian basement slopes westward from 0.7 km subsurface at SAFOD to $sim$2.5 km depth at the Salinas River, with suggestions of two faulted steps. A small velocity contrast and reflections indicate the position of the Coast Range fault, which juxtaposes the Great Valley Sequence sedimentary rocks and the Franciscan terrane. A body of late Cenozoic sedimentary rocks indicated by low seismic velocity lies immediately west of the surface trace of the San Andreas Fault, between SAFOD and the fault. This body is bounded to the west by the Buzzard Canyon Fault or similar structure just east of SAFOD. It extends much deeper than sedimentary rocks at the SAFOD site and approaches the depth of the shallowest earthquakes. Saline water in these rocks could explain the previously observed zone of high electrical conductivity. In August, SAFOD drilled laterally from granitic into sedimentary rocks northeast of the drill site. Detailed structure of the San Andreas and Buzzard Canyon faults is critical to linking down-hole observations to surface geology.