Shear-Wave Splitting Observed in Local Earthquake Data on the Reykjanes Peninsula, SW Iceland

Abstract: Shear-wave splitting is a phenomenon observed in almost all in situ rocks. Due to propagation through stress-aligned and fluid-saturated microcracks and fractures the initial shear wave splits into two almost orthogonal waves which propagate with different velocities along similar ray paths. The process is characterized by the polarization direction of the faster split shear wave, which is parallel to the orientation of the cracks, and the time delay between the onsets of the two waves. The analysis of shear-wave splitting has been conducted over records of 233 microearthquakes in the vicinity of five seismic stations in SW Iceland. Visual methods have been applied to the data to retrieve the final results for polarization directions and time delays. The main polarization azimuth for the leading split wave is N30°- 60°E which is in full agreement with the mapped alignments of normal faults and volcanic fissures in the surface. The time delays measured at different sites vary in the range of 10-100 ms for the events of best quality. In general, splitting times do not show a clear pattern at all recording sites with increasing depth. The only firm conclusion that can be drawn from the time delays is that at station BLF in the Brennisteinsfjöll fissure swarm, the time delays are smaller than in the Hengill area and therefore the strength of anisotropy beneath that station appears to be lower.