Structure, seismicity, and stress along the San Andreas Fault near SAFOD

Abstract

The San Andreas Fault Observatory at Depth (SAFOD) has yielded significant new insights into the nature of the San Andreas fault (SAF). In particular, the recovery of ~ 40 meters of core containing two meters-thick zones of fault gouge and adjacent zones of damage and alteration provides a unique opportunity to characterize the physical and chemical properties of fault zone rocks from a depth where earthquakes occur, although these samples are interpreted to come from a creeping, not seismogenic, part of the fault (Hickman et al., 2007, 2008). We propose to improve our understanding of the context within which these fault zone samples existed in-situ by utilizing arrival times of fault zone head waves (FZHW's) and the associated direct-wave secondary arrivals (DWSA's) to (1) improve the seismic tomography image of the SAF at relatively fine scale, and (2) improve absolute location estimates for earthquakes in the region around SAFOD and in particular the drilling target earthquakes. These two tasks have interrelated goals. Primary among them is to characterize in detail the seismogenic structures on which the earthquakes near SAFOD occur and relate those structures to the borehole and core observations. There is a general consensus that the shallower of the two gouge zones is related to the fault strand along which the so-called "Hawaii" target earthquakes occur. Our improved absolute earthquake locations will either help support or refute this interpretation.