Pervasive Foreshock Activity Across Southern California

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. Plain Language Summary Earthquakes often occur without warning or detectable precursors. Here we use a new, highly complete earthquake catalog to show that most mainshock earthquakes in southern California are preceded by elevated seismicity rates-foreshocks-in the days and weeks leading up to the event. Many of these foreshock earthquakes are small in magnitude and hence were previously undetected by the seismic network. These observations help bridge the gap between observations of real earth fault systems and laboratory earthquake experiments, where foreshock occurrence is commonly observed.