Assessing crustal stability via fault stress perturbation analysis

Assessing the stability state of faults is a crucial issue not only for seismic hazard, but also for understanding how the earthquake machine works. A possible approach consists in perturbing fault systems and studying how seismicity changes after additional stress is provided: 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. Especially, 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 kept; conversely, abrupt drops are observed as foreshocks and preslip occur. A preparatory phase, featured by increasing correlation, is usually detected before large and intermediate (Mw > 5) shallow (depth < 50 km) earthquakes. The duration of the anomaly, T, is suggested to be related to the seismic moment M of the impending mainshock by T ∝ M^(1/3) for M < 10^19 N m. The same power exponent characterizes seismic nucleation scaling of single earthquakes. This analogy may be explained assuming that the physical mechanism behind both these phenomena is the same. Consequently, the anomalies we measure might be interpreted as diffuse nucleation phases throughout the crust. The scaling relation becomes T ∝ M^0.1 for M > 10^19 N m, probably because of preparation processes occurring contemporaneously in interacting faults. We apply this method to dozens of seismic sequences which hit California, Greece, Iceland, Italy and New Zealand, we also analysed seismic activity jointly with slow slip events in the Cascadia subduction zone, Manawatu region and in the Nankai Trough. Even though it is unlikely that our results may ever be of practical use for seismic hazard, the procedure could illuminate slow hidden processes of destabilization taking place within the brittle crust.