Systematic analysis of foreshock sequences in southern California

Foreshocks are one of the few recognized precursors to earthquakes, but they do not precede every earthquake nor are foreshock sequences readily recognizable as foreshocks until after the mainshock occurs. We examine all earthquakes of M ≥ 5 in southern California between 1981 and 2010, using a recently updated catalog with improved locations computed from waveform cross-correlation. 62 out of 131 M ≥ 5 mainshocks have one or more foreshocks occurring within 5 days and 2 km of the mainshock hypocenter. We examine these events to see if their locations, focal mechanisms, and estimated stress drops are consistent with earthquake-to-earthquake triggering or if they appear to result from an underlying physical process, such as fluid flow or slow slip, that might also have triggered the mainshock. We also attempt to compare foreshock sequences with comparable sets of events that do not produce mainshocks, to see if any distinguishing features can be resolved in foreshocks compared to that seen in background seismic activity. These results should help constrain earthquake triggering models and theories of earthquake nucleation.