Seismicity Patterns in Southern California and Their Physical Origin

We investigate the temporal and spatial clustering of earthquakes in Southern California using the relocated catalog of Lin et al. (2007). We first search for well-defined earthquake sequences in spatio-temporal regions around each event with 2 ≤ M ≤ 6.0. The spatial influence window around an earthquake is a sphere with a radius that scales with the magnitude and hence the rupture length of the event. The temporal window is also magnitude-dependent. For all earthquake sequences that have a sufficiently large number of events, we fit the ETAS model (Ogata, 1988) and estimate the five ETAS parameters using a maximum likelihood procedure. In addition, we determine with a gridding technique, for events with magnitudes above the completeness threshold, the spatial variation of the b-value, the earthquake background rate (Hainzl et al., 2006) and earthquake density. Based on the values of the triggering exponent α of the ETAS model, we try to separate swarm- like seismicity from sequences that have an obvious mainshock. We also investigate if the various estimated seismicity parameters show correlating spatial patterns. Finally, we compare the seismicity patterns with the estimated local temperature, estimated local tectonic strain rates, existence of sedimentary cover and mainshock faulting style, in an attempt to understand the underlying physics of the different seismicity patterns.