Daniel Trugman, Zachary E. Ross

Foreshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state-of-the-art, high-resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008-2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy-two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought, and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes.