SEISMOTECTONIC ANALYSIS OF SEISMICITY PATTERNS AND FOCAL MECHANISMS IN THE EAST LOS ANGELES BASIN, CALIFORNIA

The eastern edge of the Los Angeles basin (eastLA) abuts the Whittier fault and the Puente and Chino Hills. The seismicity patterns that are complex along the Whittier fault imply the presence of slip partitioning, with predominantly strike-slip faulting along the Whittier fault. A broad zone of thrust faulting exists to the west of the Whittier fault, extending from the Peralta Hills fault into the Whittier Narrows. Similarly, a zone of mixed strike-slip and thrust faulting exists to the east of the Whittier fault. In contrast, with increasing depth, the spatial distribution of earthquakes becomes more lineated in the north-south direction and the spatial heterogeneity of stress gradually decreases. The results of the stress drop inversion show that stress drop increases with depth. The two moderate-sized earthquakes, the 1987 ML 5.9 and the 2008 Mw5.4 Chino Hills that occurred close to the bottom of seismogenic zone, both exhibited high stress drop. Stress field inversion from focal mechanisms results show that the trend of the maximum principal stress strikes north-south. The spatial variations in the stress field, depth distribution of seismicity, and stress drops are consistent with a regional decollment near the bottom of the seismogenic zone. With abundant earthquake data recorded by the Southern California Seismic Network (SCSN) in the eastLA region, we systematically analyze seismicity and focal mechanisms from earthquakes over a broaden range of magnitudes that occurred during the last twenty years. We use the wavefrom based relocation methods to relocate the seismicity. We determine focal mechanisms by using both first motions and P/S amplitude ratios. We also invert focal mechanism data for a spatially/temporally varying stress field. We use an iterative-stacking method to calculate seismic stress drop from P- wave spectrum.