Shear wave anisotropy and stress direction in and near Long Valley Caldera, California, 1979–1988

摘要: We observed shear wave splitting (birefringence) for two temporary surface deployments of three-component, digital seismographs, that were in place before and after M = 6+ earthquakes near the Long Valley caldera, California. In one these deployments, data sample precedes large events May 1980 Mammoth Lakes earthquake sequence by 6 months cover most active aftershock sequence; stations (WIT) from this deployment was reoccupied with identical instrumentation 1988. Another preceded 1986 Chalfant mainshock 2 days recorded days. The polarization faster changes N30°W at to due north both cases is parallel strike nearby faults mean direction P axes determined focal mechanism groupings. Observations yield fast directions nearly 23° each other, time separations slow waves show considerable station-to-station variation, showing no correlation earthquake-station distance or depth. These observations suggest anisotropy results primarily near-station (presumably shallow) effects rather than widespread aligned microcracks. records station WIT a slight variation average 1979 1980, but limited amount available, difference not statistically significant. Time separation measurements up 0.48 s estimates 15–30% velocity anisotropy, among highest reported local earthquakes. Secular between swamped scatter data, if it present. five 1988 showed similar S polarizations those there are enough statistical comparison. Unlocated MRS increase 6.0+ earthquakes,