SHEAR-WAVE SPLITTING OBSERVATIONS IN THE NORTHERN LOS-ANGELES BASIN, SOUTHERN CALIFORNIA

Shear-wave splitting with 20 to 120 msec time separation is observed o n 3-component seismograms from earthquakes occurring at depths of 5 to 18 km in the crystalline basement beneath the Los Angeles basin. Shal low events that occur in the basement at the base of the 5-km-thick se dimentary section exhibit little splitting, while deeper events show p rogressively greater splitting with depth. The polarization direction of the fast shear wave is N-S, independent of the azimuth of ray path between the event and station. We interpret that the shear-wave splitt ing occurs mainly in the crystalline basement and is the result of ver tical crustal microcracks aligned in N-S direction. Under the assumpti on that the alignment of microcracks is due to the subsurface stress r egime, the results from the shear-wave splitting data yield a maximum principal stress direction of N-S +/- 15-degrees at depth beneath the northern Los Angeles basin, consistent with results from geological ma pping and fault-plane solutions. Ray trace modeling indicates that the observations of shear-wave splitting can be explained in terms of an anisotropic basement containing vertical microcracks aligned in the N- S direction and with a crack density of approximately 0.04, overlain b y a weakly anisotropic sedimentary section with a crack density less t han 0.02. During the last 4 yr, three major earthquakes occurred in th e Los Angeles basin area. They are the M 4.6 and M 4.4 Montebello doub le earthquakes of 1989, the M 5.5 Upland earthquake of 1990, and the M 5.8 Sierra Madre earthquake of 1991, with epicenters of 10, 55, and 4 0 km away from the recorder site SCS located in the northern Los Angel es basin, respectively. We did not observe significant variations of s hear-wave splitting at SCS before and after the Upland and Sierra Madr e earthquakes. However, we found fairly rapid variations of shear-wave splitting following the Montebello events. The time separation betwee n the split shear waves sharply decreased from 70 msec to less than 20 msec after the mainshock and then returned to the average level gradu ally. This rapid decrease in shear-wave splitting may be a temporal va riation or a result of location variations of the Montebello events th at were located approximately where the line singularity is expected.