Seismic clusters as markers of crustal stability

Seismic activity is the mirror of local instability in the brittle crust. Each seismic event occurs as a consequence of stress accumulation at weak interfaces within crustal volumes and static and dynamic perturbations, so that earthquakes tend to be clustered in space and time. So, while on one hand, seismicity trivially provides information about the state of stability of the fault patch where it takes place; on the other side, it is rather difficult to assess whether the occurrence of seismic events may be somewhat connected to the large-scale state of stability of a fault. Are foreshocks “fore-shocks” or simply markers of local instability without prognostic value? Is there difference between foreshocks and swarms? We analyse several parameters describing seismic clusters until the mainshocks in relocated seismic catalogues in relationship with the magnitude of the incoming mainshock in the range Mw 2.5-7.3 (the involved area, duration, the p-value of the Omori-Utsu law, the b-value of the Gutenberg-Richter law, the 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; foreshocks tend to spread over larger areas, they are featured by larger and more energetic clusters with also higher variance of magnitudes and relative entropy. Others are not distinguishable. Moreover, foreshocks and swarms are found to share the same scaling behaviour. Theoretical modelling suggests that the combined effect of long-range correlations and fault memory makes the system sensitive to both mechanical details, structural properties of the fault system and its history. Therefore, it produces different future evolutions because of tiny changes in internal parameters or in past occurrences. On the base of our results, we believe that foreshocks can be hardly distinguished by swarms using the structural and statistical properties of clusters. In this sense, foreshocks are likely of limited usefulness for short-term earthquake prediction, but they could be useful for the identification of regions featured by correlated seismic activity, likely a mark of crustal instability.