SHEAR-WAVE ANISOTROPY - SPATIAL AND TEMPORAL VARIATIONS IN TIME DELAYS AT PARKFIELD, CENTRAL CALIFORNIA

Shear-wave splitting is analysed on data recorded by the High Resoluti on Seismic Network (HRSN) at Parkfield on the San Andreas fault, Centr al California, during the three-year period 1988-1990, Shear-wave pola rizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress, a t some 70 degrees to the fault strike, whereas at station MM in the im mediate fault zone, shear-wave polarizations are aligned approximately parallel to the fault. Normalized time delays at :his station are fou nd to be about twice as large as those in the rock mass either side. T his suggests that fluid-filled cracks and fractures within the fault z one are elastically or seismically different from those in the surroun ding rocks, and that the alignment of fault-parallel shear-wave polari zations are associated with some fault-specific phenomenon, Temporal v ariations in time delays between the two split shear-waves before and after a M-L=4 earthquake can be identified at two stations with suffic ient data: MM within the fault zone and VC outside the immediate fault zone, Time delays between faster and slower split shear waves increas e before the M-L = 4 earthquake and decrease near the time of the even t, The temporal variations are statistically significant at 68 per cen t confidence levels, Earthquake doublets and multiplets also show simi lar temporal variations, consistent with those predicted by anisotropi c poroelasticity theory for stress modifications to the microcrack geo metry pervading the rock mass, This study is broadly consistent with t he behaviour observed before three other earthquakes, suggesting that the build-up of stress before earthquakes may be monitored and interpr eted by the analysis of shear-wave splitting.