Shear wave splitting in the Parkfield pilot hole from cross-correlation of seismic noise

We use cross-correlation of seismic noise recorded at stations in the San Andreas Fault Observatory at Depth (SAFOD) pilot hole to extract P and S waves from a source at depth and measure shear-wave splitting on the horizontal components. The data is recorded at 7 three-component stations at depths from 1857-2097 m in the pilot hole. In late September and early October 2004 drilling noise underneath the stations generates propagating waves in the noise cross-correlations between stations, which are absent in the rest of October. Noise cross-correlations between the deepest station and the other six stations in the pilot hole produce estimates of the P and S wave velocities on the vertical and horizontal components respectively which are consistent with velocity measurements taken directly in the borehole. We observe polarization of the S wave, with a fast polarization direction of 120-130° that is 4% faster than the slow direction. Cross-correlation of seismic noise cannot only accurately determine seismic velocities but also S wave anisotropy.