Seismic response to tidal stress perturbations sheds new light on how fault patches become unstable

The stability state of fault systems is mainly controlled by the frictional properties of weak interfaces and the available energy accumulated in the volumes beside them. Heterogeneities, roughness, and topological features play a key role in driving seismic dynamics and tectonic stress dissipation. However, the physics of the processes fostering mechanical instability in the stages just before failures is still poorly understood. Do peculiar processes occur before major failures? How long do such destabilizations last, if any? Do they share common features or each of them is a one of a kind? A possible approach consists in perturbing fault systems and studying how seismicity changes after additional stress is applied: if the starting energy state is stable, it will oscillate around it; otherwise, the background seismic rate will be modified. Tides provide natural stress sources featured by a wide range of frequencies and amplitudes, which make them a suitable candidate for our needs. Analyses prove that the brittle crust becomes more and more sensible to stress modulations as the critical breaking point comes close. Our results are compatible with past literature (e.g., Métivier et al., 2009; Tanaka, 2012; Varga & Grafarend, 2019). The correlation between the variation of Coulomb failure stress induced by tidal loading, ΔCFS, and seismic energy rate progressively increases as long as seismic stability is preserved; conversely, abrupt drops are observed as foreshocks and pre-slip happen. A “preparatory phase”, characterized by increasing correlation, is detected before large and intermediate (Mw ≳ 5) shallow (depth ≤ 50 km) earthquakes. The duration of the anomaly, T, is suggested to be connected to the seismic moment M_0 of the impending mainshock by the scaling relation T ∝ M_0^(1/3) for M_0 < 10^19 N∙m while T ∝ M_0^0.1 for M_0 > 10^19 N∙m. We apply this method to noteworthy seismic sequences in California, Greece, Iceland, Italy, Japan, and New Zealand. Even though our results cannot be of practical use for seismic hazard because of large error bars and variability of the investigated phenomenon, the procedure may help us to better understand the physics of earthquakes.