Steep-dip seismic imaging at the SAFOD (San Andreas Fault Observatory at Depth) site

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

0106534 Hole SAFOD, the San Andreas Fault Observatory at Depth, intends to drill through the San Andreas fault to study internal fault properties that control earthquakes. Parkfield, in central California, was chosen as the drill site because it is the best characterized seismically active segment of the fault. Interesting observations include high electrical conductivity, low seismic velocity, high Poisson's ratio, and seismic anisotropy within the fault zone. However, other than a thin low velocity zone identified by fault-zone guided waves, the earthquake data only constrain seismic velocity structure on the scale of 2 km and larger. Earthquakes are well located in a relative sense, but have up to 1 km of absolute error and do not line up with the active surface fault trace. Better site characterization can improve the drilling plan and its ability to accurately target a specific patch of repeating earthquakes on the fault. This project will perform a high resolution, 2-D seismic profile of the SAFOD site with the specific aim of imaging steeply dipping structures in and near the fault at depth. Previous work by the Principal Investigator produced the first-ever migrated image of seismic reflections from the near-vertical San Andreas fault, allowing direct imaging of the fault trace at a depth of about 1 km. This project aims to repeat this success beyond the 4 km depth at which drilling will penetrate the fault. The Principal Investigator plans a 50-km combined seismic reflection and refraction line across the San Andreas fault at the drill site. Very dense shot and receiver spacings and very long recording offsets will allow the survey to obtain detailed seismic velocity structure at a resolution of ~0.5 km horizontally and ~0.25 km vertically down to about 5 km depth. These images will better constrain the location of the fault trace and fault zone at depth, its relation to anomalous fault-zone properties, and the overall structure of surrounding geologic units and related faults. ***