Rare Occurrences of Non‐cascading Foreshock Activity in Southern California

Earthquakes preceding large events are commonly referred to as foreshocks. They are often considered as precursory phenomena reflecting the nucleation process of the main rupture. Such foreshock sequences may also be explained by cascades of triggered events. Recent advances in earthquake detection motivates a reevaluation of seismicity variations prior to mainshocks. Based on a highly complete earthquake catalog, previous studies suggested that mainshocks in Southern California are often preceded by anomalously elevated seismicity. In this study, we test the same catalog against the Epidemic Type Aftershock Sequence model that accounts for temporal clustering due to earthquake interactions. We find that 10/53 mainshocks are preceded by a significantly elevated seismic activity compared with our model. This shows that anomalous foreshock activity is relatively uncommon when tested against a model of earthquake interactions. Accounting for the recurrence of anomalies over time, only 3/10 mainshocks present a mainshock‐specific anomaly with a high predictive power.
Key Points We further investigate previous claims of significantly elevated seismic activity prior to large earthquakes in Southern California10 out of 53 mainshocks are preceded by anomalously high seismicity, but only 3 of these anomalies are exclusively related to the mainshockThese selected foreshock sequences are likely due to additional pre‐slip, aseismic processes