Shear-wave Splitting Analysis of Microearthquake Data Observed with Downhole Seismic Network

This paper presents a semi-automatic method to analyse shear-wave splitting of microearthquakes. The rotation-correlation method, co-variance matrix method, aspect ratio method and the measure of polarization strength are combined to provide a stable estimate of a semi-automatic estimation of the shear-wave splitting parameters, namely rotation angle of the fast direction and the delay time. The method is then applied to microearthquakes that have been recorded with deep borehole seismometers in the San Andreas Fault Observatory at Depth in combination with data from the High Resolution Seismic Network. Both networks are borehole installations and the sampling rates are relatively high (4000 Hz and 250 Hz, respectively). The abundance of microearthquakes in the magnitude ranges from about -2 to 2 and the good coverage of high quality seismic stations makes it possible to map the anisotropy with respect to the ray paths between sources and receivers. Preliminary results already show significant differences of shear-wave splitting delay times in some areas of the network.