Stress orientation inferred from shear wave splitting in basement rock at Cajon Pass



S-waves generated by mode conversion of P-waves from surface compressional sources and recorded by oriented geophones at depths between 1300 and 1820 meters in the Cajon Pass scientific drillhole arrive at higher apparent velocities when polarized along the direction N70°E than when polarized in the orthogonal direction. The observed polarization dependent velocities are consistent with seismic anisotropy due to in situ fractures or cracks that are preferen tially aligned N70°E (subnormal to the San Andreas fault). This result is consistent with the maximum principal horizontal stress direction at Cajon Pass also being subnormal to the San Andreas fault.