GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.3 ______ ___ GNGTS 2023 Stress perturbations, seismicity and fault stability: an overall view D. Zaccagnino 1, * , L. Telesca 2 & C. Doglioni 1,3 1 Sapienza University, Earth Sciences Department, Rome, Italy. 2 Consiglio Nazionale delle Ricerche - Istituto di Metodologie per l’Analisi Ambientale (CNR-IMAA), Tito Scalo, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy. Several studies have so far investigated the relationship between fault slip and stress perturbations. Seismic activity becomes more and more sensitive to perturbations as background stress accumulates, so that earthquakes tend to occur, on average, during phases close to stress peak. We analyse the effect of solid and liquid tides in modulating seismicity (Zaccagnino et al., 2021). Our study shows that the correlation between the amplitude of tidal CFS and seismic energy rate usually increases before large shocks (Fig. 1), while it undergoes drops during foreshock activity and after the mainshock. Swift drops are also observed if pre-slip occurs. A pre-seismic phase, featured by increasing correlation, is detected before large and intermediate (Mw > 4.5) shallow earthquakes in about 2/3 of cases (Zaccagnino et al., 2022a). The duration of the anomaly T appears to be related to the seismic moment M of the future mainshock via the relationship T ∝ M^0.3 if the magnitude of the largest event below 6.5. This power exponent, 1/3, is typical of seismic nucleation scaling of single seismic events; moreover, in the framework of disordered critical systems, it can be interpreted as a marker of development of a critical state in the brittle crust before the seismic sequence. Therefore, the increase of correlation between seismic rates and tidal stress on fault may be understood in light of diffuse nucleation phases throughout the crust due to incoming large-scale destabilisation. Large seismic events show lower responsiveness to tidal stress than smaller ones, which agrees with previous results (e.g., Petrelis et al., 2021). We also investigate some selected settings more in detail: the Cascadia region along the West coasts of British Columbia, Washington, Oregon and Northern California and the Nankai thrust in Japan. We identify seismically silent interfaces which show elevated values of correlation between seismic activity and tidal stress perturbations (Fig. 2). We also study how fault slip evolves over time along the trench as a function of the position along dip and along strike and distance from the epicenter of the earthquake. In the same areas, we also consider tremors and LFEs (Zaccagnino et al., 2022b). The joint analysis of both slow and fast events time series allows us to identify patterns and peculiar behaviours of seismicity in response to stress accumulation. Tremors become more and more sensitive to stress perturbations as the surrounding fault interface is seismically locked, showing analogous response of fast seismic events. New insights into seismic clustering are also provided by self-correlation analysis of spatial and magnitude time series, showing differences and