Foreshocks or swarms? Insights from theory and statistical analysis

Are foreshocks “fore-shocks” or simply markers of local instability without any prognostic value about future large-scale mainshocks? We try to answer this question using both theoretical modelling and statistical analysis of clusters listed in relocated high quality seismic catalogues. Concerning theory, we show that the combined effect of long-range correlations and fault memory make the system sensitive to both details and history. It results in different future evolutions because of tiny changes in internal parameters or in past occurrences. For this reason, even an identical sequence of events can flow into drastically different future because of a tiny variation in the physical conditions. On the other hand, two completely different seismic patterns can give rise to seismic events with similar features, e.g., the 2009 Mw 6.3 L'Aquila and the 2016 Mw 6.0 Amatrice event, which occurred in the same normal-faulting region in Central Apennines in Italy, were preceded by five months of foreshocks (Papadopoulos et al., 2010) and seismic quiescence (Gentili et al., 2017) respectively. To provide observational evidence for answering our question, we investigate the relationships between several parameters of seismic clusters until the mainshocks in relocated seismic catalogues in California and Italy as a function of their mainshock magnitude (e.g., the involved area, duration, seismic rate, number of events, cumulative nucleated seismic moment, Shannon and Tsallis entropy, inter-event times distribution, global coefficient of variation of interevent times, trend, and fluctuations of magnitudes and interevent times). We find differences in the distributions of some features (e.g., Zaccagnino et al., 2023), although large variability is observed, while others cannot be distinguished. Foreshocks and swarms share the same scaling behaviour and are likely generated by the same physical mechanism; however, 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. We suggest possible explanations for them. On the base of our results, we believe that precursory patterns of accelerated seismic activity can be hardly distinguished by more frequent swarms using the structural and statistical properties of clusters (Zaccagnino et al., 2024). In this sense, foreshocks are likely of limited usefulness for short-term earthquake prediction. Conversely, seismic hazard may benefit of the identification of regions featured by strongly correlated seismic activity, which is likely to be a mark of crustal developing instability. References: Gentili, S., Di Giovambattista, R., & Peresan, A. (2017). Seismic quiescence preceding the 2016 central Italy earthquakes. Physics of the Earth and Planetary Interiors, 272, 27-33. Papadopoulos, G. A., Charalampakis, M., Fokaefs, A., & Minadakis, G. (2010). Strong foreshock signal preceding the L'Aquila (Italy) earthquake (M w 6.3) of 6 April 2009. Natural Hazards and Earth System Sciences, 10(1), 19-24. Zaccagnino, D., Telesca, L., & Doglioni, C. (2023). Global versus local clustering of seismicity: Implications with earthquake prediction. Chaos, Solitons & Fractals, 170, 113419. Zaccagnino, D., Vallianatos, F., Michas, G., Telesca, L., & Doglioni, C. (2024). Are Foreshocks Fore‐ Shocks?. Journal of Geophysical Research: Solid Earth, 129(2), e2023JB027337.