Multiply-polarized shear-wave VSPs from the Cajon Pass Drillhole

A 3‐component VSP data set was acquired in the Cajon Pass Drillhole, with a multiply‐polarized shear‐wave source and a conventional P‐wave source, producing a 9‐component data set. The VSP data show effects on the full wavefield from the sediment‐granite interface. Mode‐converted waves are identified, with significant scattering of S‐wave energy from the leading P‐wavefront. Hodographs are used to display S‐wave particle motions in two dimensions. Shear‐wave splitting is observed in the hodographs. The shear‐wave particle motion is used to infer anisotropy orientation with respect to the local strike of the San Andreas fault. Stress‐induced fracturing is assumed to be a possible cause of the shear‐wave splitting. A variation of particle motion orientation with depth is observed, implying a variation in anisotropy orientation as a function of depth.