INITIAL SHEAR-WAVE PARTICLE MOTIONS AND STRESS CONSTRAINTS AT THE ANZA SEISMIC NETWORK

We use focal plane solutions to constrain principal stress directions in the vicinity of six Anza Network stations which show evidence for shallow shear wave anisotropy in the vicinity of the Anza seismic gap region of the San Jacinto fault. Faulting near all stations is consistent with approximately N-S maximum compressive stress. Five of these stations show nearly N-S initial particle motion alignment, consistent with the anisotropy-stress relationship expected for stress-aligned microcracks. However, one station (KNW) has a well-defined preferred initial shear wave polarization direction of N40-degrees-W. Although this polarization direction differs dramatically from the local maximum compressive stress direction, it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals, particularly biotite. Thus, anisotropy observed at this station most likely reflects a fixed, palaeostrain alignment of anisotropic minerals and/or microcracks and does not require a dependence on the current stress field.