On the relationship between foreshocks and mainshock

Foreshocks are clustered seismic events occurring before large earthquakes. They are usually identified a posteriori on the base of significant increases of the seismic rate not compatible with the previous background and static stress transfer simulations. However, seismicity is spontaneously clustered in space and time regardless of the happening of major events; most of earthquake clusters are indeed “swarms”, i.e., they do not flow into major shocks. The discrimination between foreshocks and swarms is therefore an outstanding issue in statistical seismology. We investigate relocated seismic activity in California and Italy during the last three decades to understand whether foreshocks are statistically prevalent before large shocks with respect to smaller ones and to provide a theoretical explanation of our results [1]. We find that clusters happen before small, moderate, and large magnitude earthquakes, even though they seem to spread over larger areas, they are featured by more numerous and energetic clusters with also larger magnitude fluctuations and higher entropy (Fig. 1). We also show that foreshocks and swarms share the same scaling behaviors, so that we believe there are no compelling reasons for proposing different physical mechanisms. Moreover, foreshocks and swarms are not distinguishable on the base of duration, seismic rates, and moment rates; no clear magnitude trends are observed. The comparison of empirical cumulative distributions of various features of clusters of swarms and foreshocks is performed using the Smirnov test to assess whether they can be considered different or not. References [1] Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are Foreshocks Fore‐Shocks?. Journal of Geophysical Research: Solid Earth, 129(2), e2023JB027337.