The narrow path to the discrimination of precursory seismic activity

Whenever seismic activity occurs before large earthquakes seismologists refer to such “precursory” events as “foreshocks”. Seismicity tends to take place clustered in space and time regardless of the happening of major events; a vast majority of earthquake clusters is indeed made up of small events; so, it does not flow into a mainshock and the following seismic sequence. Therefore, the discrimination of foreshocks is an important topic, being usually investigated a-posteriori and still poorly understood by a theoretical point of view, mostly in the framework of ETAS models. We analyse relocated seismic activity in Southern California and Italy during the last three decades to clarify whether foreshocks are statistically prevalent before large shocks with respect to smaller ones and to provide a theoretical explanation of our results (Zaccagnino et al., 2024). We show that foreshocks share the same scaling behaviour with clusters that do not precede large (Mw ≥4.5) events, and they are likely generated by the same physical mechanism; however, they tend to spread over wider areas, are featured by larger and more energetic clusters with also higher magnitude fluctuations and entropy. Conversely, foreshocks are not distinguishable on the base of their duration and seismic rate; no clear magnitude trends are clearly observed within them, and clustering coefficients are not found to be statistically different from non-precursory events. The comparison of empirical cumulative distributions of various features of clusters of swarms and foreshocks is performed using the non-parametric two-sample Smirnov test. Reference Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are Foreshocks Fore‐Shocks?. Journal of Geophysical Research: Solid Earth, 129(2), e2023JB027337.