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Computing a large refined catalog of focal mechanisms for southern California (1981 – 2010): Temporal Stability of the Style of Faulting

Wenzheng Yang, Egill Hauksson, & Peter M. Shearer

Published June 2012, SCEC Contribution #1512

Using the HASH method (Hardebeck and Shearer, 2002, 2003), 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. We apply grid search to determine the best-fitting double-couple focal mechanism solution. The data consist of both the P-wave first motion polarities and the S/P amplitude ratios computed from three-component seismograms. We process data from more than 480,000 earthquakes, and analyze the statistical features of the whole data set. As more S/P amplitude ratios become available after 2000, the average nodal plane uncertainty decreases significantly compared with solutions that include only P-wave polarities. We filter a preliminary data set with criteria associated with mean nodal plane uncertainty and azimuthal gap, and obtain a high quality catalog with approximately 179,000 focal mechanisms. 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 for M > 2.5 events. A comparison of 23,000 common earthquakes shows our results generally agree with the focal mechanism catalog obtained by Hardebeck and Shearer (2003). Using 211 moment tensor solutions in Tape et al (2010) as benchmarks, we compare the focal plane rotation angles of common events in the catalog. 69% of common earthquakes in both catalogs match well, with rotation angles less than the typical nodal plane uncertainty in the total first motion and S/P ratio mechanisms. The common events with relatively large rotation angles are either located around the edge of the SCSN network or poorly recorded.

Yang, W., Hauksson, E., & Shearer, P. M. (2012). Computing a large refined catalog of focal mechanisms for southern California (1981 – 2010): Temporal Stability of the Style of Faulting. Bulletin of the Seismological Society of America, 102, 1179-1194. doi: 10.1785/0120110311.