Do not call them foreshocks

One of the most intriguing issues in earthquake science concerns the discrimination between foreshocks and swarms. We investigate relocated seismic catalogues in California and Italy and provide a theoretical explanation of our results. Foreshocks and swarms share the same scaling behaviours and are likely generated by the same physical mechanism; however, our statistical analyses highlight that foreshocks spread over larger areas, are featured by larger and more energetic clusters with also higher variance of magnitudes and relative Tsallis and Shannon entropies (Shannon, 1948; Tsallis, 1988). On the other hand, foreshocks have duration, seismic rates, and moment rates, as well as magnitude trends and clustering properties indistinguishable from swarms. Our results prove that mainshocks can occur with or without foreshocks with extremely variable magnitudes. In fact, in crustal volumes, the value of stress at a certain time depends on the history of recently happened variations of the stress itself, depending on memory kernels, and fine-scale structural details of fault interfaces and tectonic forces. This means that even two identical seismic clusters can flow into a large mainshock, moderate events or a swarm depending on the action of tiny details in the evolution of stress gradients. On the other hand, two completely different seismic patterns can give rise to seismic events with similar features. This result strongly challenges the possibility of accurate earthquake prediction, both in terms of time to failure and magnitude, at least just considering past seismic activity. A mathematical model is realized to explain our observations. Clusters covering large areas are displays of long-range correlations within larger crustal volumes. As tectonic strain increases the level of stress, faults become more and more unstable, until a spontaneous rupture develops on the weakest interface. Static and dynamic stress variations trigger further events afterwards within the crustal volume showing significant correlations with the hypocenter, i.e., sensitivity to stress perturbations. The larger the region close to instability, the more seismic events can be triggered and with statistically higher magnitudes. This is the reason why mainshocks tend to happen after clusters spread over larger areas, with higher number of events and magnitudes not because such seismic activity ultimately triggers them (Zaccagnino et al., 2024). However, foreshocks are not “fore-shocks''; they are not informative about the magnitude or time-to-failure of the eventually impending earthquake. Earthquakes ultimately grow to become giant events because of fine details of differential stress patterns and fault strength, regardless of previous seismic activity, if the extension of the prone-to-failure volumes is large enough. References Shannon C.E.; 1948: A mathematical theory of communication. Bell Syst. Tech. J., 27(3), 379-423. Tsallis C.; 1988: Possible generalization of Boltzmann-Gibbs statistics. J. Stat. Phys., 52, 479-487. Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are Foreshocks Fore‐Shocks?. Journal of Geophysical Research: Solid Earth, 129(2), e2023JB027337.