Stress pertubations, seismicity and fault stability: an overall view

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 preseismic 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 on the light of diffuse nucleation phases throughout the crust due to incoming large-scale destabilization. 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 showing 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 similarities between different manifestations of fault instability. Our results are interpreted on the light of a toy-model which explains spatial patterns in the correlation between tidal stress and seismic rate during the interseismic period; conversely, a to have a drop as the stress is released through aftershocks. Different scaling exponents of spatial clustering and magnitudes relationships are found for slow and fast earthquakes being the early are more clustered; furthermore, they showcase long-lasting memory of past magnitudes. On the other hand, regular earthquakes are featured by faster decaying of memory after the mainshock, while swift, exponential drop occurs after small to moderate events. This implies that earthquakes are likely to be triggered at the edge of episodic tremor and slip regions and not far away from them, on the contrary, tremors can be triggered remotely. Moreover, energy nucleation rates become more and more correlated with tidal stress before major failures. Our results are compatible with previous literature (e.g., Tanaka 2012). At last, the responsiveness of seismicity to additional stress sources is negatively related to the temporal and spatial variation coefficients, so that the most elevated responsiveness to tidal stress is found in partially locked fault regions regularly alternating stable phases to diffuse seismic activity. Since tidal stress estimation is affected by large uncertainties, it is not reliable to define risk levels and only remote hope exists that error bars can be significantly reduced. Therefore, our approach cannot be of practical use for seismic hazard; nevertheless, it may be useful to better understand the role of slow hidden processes involving regular, slow events and tectonic tremor in the destabilization of crust volumes.