[Pervasive Foreshock Activity Across Southern California](https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL083725)
> "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. "