Analysis of shear waves with respect to splitting near the San Andreas Fault

Summary Shear‐wave recordings acquired by Vertical Seismic Profiling in the Cajon Pass Deep Scientific Drillhole, California, have been investigated with respect to shear‐wave splitting using various methods. The methods used should enable detection of possible anisotropy‐induced (AI) shear‐wave splitting consistent with a system with hexagonal symmetry and a horizontally oriented axis of symmetry. Careful investigation of the direct shear waves leads to the conclusion that AI shear‐wave splitting consistent with this simple model cannot be observed with the methods applied. The lack of such AI shear‐wave splitting is probably partly caused by the low difference between the maximum and minimum principal stress in the area, which is probably too modest to vertically align cracks and microcracks to any significant degree. The propagation and polarization of the shear waves in this area is thus probably more controlled by factors related to the complex geology rather than to the current stress field. The lack of detected AI shear‐wave splitting does not, however, imply that the medium is isotropic; the polarization diagrams show clear evidence of shear‐wave splitting which most likely indicates the presence of an anisotropy of higher order.