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GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L12S07, doi:10.1029/2003GL019382, 2004

The SAFOD Pilot Hole seismic array: Wave propagation effects as a function of sensor depth and source location

J. Andres Chavarria

Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina, USA


Peter E. Malin

Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina, USA


Eylon Shalev

Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina, USA


Abstract

In July 2002 we installed a vertical array of seismometers in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH). The bottom of this 32 level, 1240 m long array of 3- components is located at a depth of ∼2100 m below ground. Surface-explosion and microearthquake seismograms recorded by the array give valuable 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 to the NW, and lower ones to the SE, correlating with high and low creep rates along the SAF, respectively. Since higher ratios can be produced by increasing fluid saturation, we suggest that this effect might account for both our observations and their correlation with the creep distribution.

Received 29 December 2003; accepted 9 March 2004; published 19 May 2004.

Index Terms: 7230 Seismology: Seismicity and seismotectonics; 7294 Seismology: Instruments and techniques; 8010 Structural Geology: Fractures and faults; 8149 Tectonophysics: Planetary tectonics (5475).

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Citation: Chavarria, J. A., P. E. Malin, and E. Shalev (2004), The SAFOD Pilot Hole seismic array: Wave propagation effects as a function of sensor depth and source location, Geophys. Res. Lett., 31, L12S07, doi:10.1029/2003GL019382.