Focal Mechanisms of Microseismicity in the San Jacinto Fault Zone Region of Southern California

Date: 4/26/2019

Time: 04:45 PM

Room: Vashon

Earthquake focal mechanisms contain important information on the geometry of fault zones and rupture kinematics, but remain challenging to derive for small earthquakes. Despite recent progress toward applying full-waveform inversion techniques to small earthquakes, such methods remain intractable for most detectable events. Using automated processing procedures, we develop a catalog of >100,000 earthquakes in the magnitude range -1.8 to 5.4 for the San Jacinto fault zone region in Southern California, and then derive focal mechanisms by analyzing P-wave first-motion polarities (FMPs). To overcome the challenge of reliably determining FMP of low-SNR waveforms from small earthquakes, we gather and stack similar waveforms (minimum cross-correlation coefficient of 0.8) to increase SNR. A bootstrap sampling procedure yields a set of stacked waveforms for each similarity-gather (with at least 10 traces), and each stack is independently analyzed for FMP using PhasePApy (Chen and Holland, 2016). All arrivals in a single similarity-gather are assigned the same FMP, and the bootstrap procedure yields a measure of stability for the observation. Picks deemed unstable are omitted from further processing, and stable picks are inverted for focal mechanisms using HASH (Hardebeck and Shearer, 2002). We will present details of the method, comparisons with conventional single-trace analysis, and features of the focal mechanisms derived for the study region.

Presenting Author: Malcolm C. A. White

Authors