Towards Quasi-Automated Estimates of Source Properties of Small to Moderate Southern California Earthquakes with Second Seismic Moments

Haoran Meng, Jeff J. McGuire, & Yehuda Ben-Zion

Published August 13, 2018, SCEC Contribution #8469, 2018 SCEC Annual Meeting Poster #072

We develop a method for quasi-automated estimation of directivity, rupture area, duration, and centroid velocity of earthquakes with second seismic moments. The method is applied to small to moderate earthquakes in southern California. The P and S phase picks are given by a 1-D ray tracing algorithm and cataloged event locations. These are refined for deconvolution by using a grid search on zero-crossings within a short time window around the automated P, S picks. Source Time Functions (STFs) of target events are derived using deconvolution with 3 stacked empirical Green’s function (seGf). The use of seGf suppresses non-generic source effects such as directivity in individual eGf’s. The seGf for each target event is based on stacking individual eGfs (normalized by seismic potencies) selected by spatial and magnitude criteria as well as performances in the projected Landweber deconvolution. The weighted stack of eGfs, with coefficients determined by grid search and waveform fits, helps further to correct inaccuracies of focal mechanisms. For stations near the source we allow using only 2 individual eGfs to increase the take-off angle coverage. Compared with a single eGf, analysis with a weighted stack can significantly improve waveform fit and typically allows getting 5 to 15 more STFs. The method is suitable for analysis of large seismic datasets and it works for target events in southern California with magnitudes as small as 3.5. We analyzed 40 small to moderate earthquakes from 2009 to 2016 in southern California and get stable results for 25 events. Most analyzed events have significant directivities. We also estimate the optimized, minimum and maximum values of the rupture area that are consistent with a particular dataset at the 95% confidence level. This is used to estimate stress drops. Most of the stress drops are well constrained and the optimized stress drops vary in the range 20-100 MPa.

Citation
Meng, H., McGuire, J. J., & Ben-Zion, Y. (2018, 08). Towards Quasi-Automated Estimates of Source Properties of Small to Moderate Southern California Earthquakes with Second Seismic Moments . Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology