Determining when and where the next big earthquake will occur is a fundamental challenge in earthquake forecasting. Although it is reasonable to assume that the next major earthquake will occur in regions where stress has been increased by previous events, the most common and reliable earthquake forecasting models assume that the magnitude of next earthquakes is independent from what happen before and, implicitly, from the stress state. In this study, we investigate the correlation between stress distribution and the occurrence of large earthquakes using a realistic physical model. Our findings reveal that the next big earthquake is more likely to occur on the periphery of previous large earthquakes, where stress has accumulated but not yet been relaxed. Additionally, we explore how stress redistribution influences the magnitude distribution of aftershocks. These results can inform the introduction of correlations between large earthquakes in existing seismic forecasting models, potentially enhancing their accuracy and reliability.

Foreshock analysis promises new insights into the earthquake nucleation process and could potentially improve earthquake forecasting. Well-performing clustering models like the Epidemic-Type Aftershock Sequence (ETAS) model assume that foreshocks and general seismicity are generated by the same physical process, implying that foreshocks can be identified only in retrospect. However, several studies have recently found higher foreshock activity than predicted by ETAS. Here, we revisit the foreshock activity in southern California using different statistical methods and find anomalous foreshock sequences, i.e., those unexplained by ETAS, mostly for mainshock magnitudes below 5.5. The spatial distribution of these anomalies reveals a preferential occurrence in zones of high heat flow, which are known to host swarm-like seismicity. Outside these regions, the foreshocks generally behave as expected by ETAS. These findings show that anomalous foreshock sequences in southern California do not indicate a pre-slip nucleation process, but swarm-like behavior driven by heat flow.