Observation of shear wave splitting from natural events: South Moat of Long Valley Caldera, California, June 29 to August 12, 1982

Abstract

Three‐component seismograms from local earthquakes recorded by three University of Wisconsin stations in the south moat of Long Valley caldera in 1982 have been analyzed for shear wave splitting. The average polarization direction of the first‐arriving shear waves observed at the two stations south of the resurgent dome is NNW–SSE. Time delays between the fast and slow shear waves are proportional to hypocentral distances, and the maximum velocity anisotropy inferred is 6.4%. The average shear wave polarization observed at the station above the resurgent dome is north‐south. The time delays there are not a function of hypocentral distance, and the normalized time delays (ms/km) from events at the southwest boundary of the caldera are larger than those at other azimuths, indicating maximum 9.6% velocity anisotropy. The east‐west to ENE‐WSW variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the Sierra Nevada range‐front faults under the generally WNW–ESE tension expected from the regional tectonics.