Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981–2010): Temporal Stability of the Style of Faulting

Using the method developed by Hardebeck and Shearer (2002, 2003) termed the HASH method, we calculate focal mechanisms for earthquakes that occurred in the southern California region from 1981 to 2010. When available, we use hypocenters refined with differential travel times from waveform cross correlation. Using both the P-wave first motion polarities and the S/P amplitude ratios computed from three-component seismograms, we determine mechanisms for more than 480,000 earthquakes and analyze the statistical features of the whole catalog. We filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high-quality catalog with approximately 179,000 focal mechanisms. As more S/P amplitude ratios become available after 2000, the average nodal plane uncertainty decreases significantly compared with mechanisms that include only P-wave polarities. In general the parameters of the focal mechanisms have been stable during the three decades. The dominant style of faulting is high angle strike-slip faulting with the most likely P axis centered at N5°E. For earthquakes of M <2:5, there are more normal-faulting events than reverse-faulting events, while the opposite holds forM >2:5 events. Using the 210 moment-tensor solutions in Tape et al. (2010) as benchmarks, we compare the focal plane rotation angles of common events in the catalog. Seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty. The common events with relatively large rotation angles are either located around the edge of the (SCSN) network or poorly recorded. Online Material: Table of HASH parameter settings and figures of 1D velocity models, distribution of earthquakes with different focal-mechanism qualities, comparison of YHS2010 and HS2003 catalogs, and distributions of earthquake focal mechanisms.