Evidence for Vertical Partitioning of Strike-Slip and Compressional Tectonics from Seismicity, Focal Mechanisms, and Stress Drops in the East Los Angeles Basin Area, California

Abstract

We have synthesized the characteristics of the seismogenic zone in the east Los Angeles basin by analyzing earthquake data recorded during the past 30 years (1981–2010). The seismicity is distributed along the Whittier fault, with the majority of earthquakes located adjacent to the south side, in the depth range from 0 to 9 km, with b value of 1.1±0.05 and mostly normal and strike-slip faulting. Within the depth range of 9–12 km, the seismicity is scattered uniformly across the region, the b value is 1.0±0.05, and all three faulting styles are present. At the deepest depths (12–18 km), seismicity is sparse and primarily limited to a few clusters striking north; these deeper earthquakes primarily have reverse fault motion, and the b value is 0.78±0.04. Inversion of high-quality focal-mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from N12°W at shallow depth to due north at the bottom of the seismogenic zone. Similarly, a depth dependence is observed in stress drops calculated from P-wave source spectra, which indicate stress drop generally increases from ~7 MPa at shallow depth (3 km) to ~53 MPa at the base of the seismogenic zone (17 km). Overall, our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east Los Angeles basin.