Oscillating tidal stress loading on the lithosphere

It is well known that oscillating stress sources play a relevant role in the stability of mechanical systems. The Earth is routinely subject to stress loading due to tides, hydrological cycles, atmospheric pressure variations and anthropical activities. However, the shallow part of our planet is far from being a simple system, so each component showcases a different response to perturbations depending on its physical properties. Macroscopically, the outer layers of the Earth form a two-tier system with respect to periodic stress changes: the brittle crust reacts forthwith to additional loads; conversely, the viscous lithosphere behaves as a low-pass filter. Such a dichotomy produces a wide range of different geodynamic, tectonic, and seismological processes. Seismicity becomes more and more sensitive to stress perturbations as strain 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 during the seismic cycle in several regions of tectonic interest. Our study shows that the correlation between the amplitude of tidal CFS and seismic energy rate usually increases before large shocks, while it undergoes drops during foreshock activity and after the mainshock. A preseismic phase, featured by increasing correlation, is detected before large and intermediate (Mw > 4.5) shallow earthquakes in about 2/3 of cases. 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 is below 6.5. This power exponent, 1/3, is typical of seismic nucleation scaling of single seismic events; therefore, the increase of correlation between seismic rates and tidal stress on fault may be understood in the light of diffuse nucleation phases throughout the crust due to incoming large-scale destabilization. We also consider tremors and low-frequency earthquakes in the Cascadia region along the West coasts of British Columbia, Washington, Oregon and Northern California and the Nankai thrust in Japan. Their sensitivity to stress perturbations increases as the surrounding fault interface is seismically locked, showing an analogous response to fast seismic events. On the other hand, viscous layers of the lithosphere are almost unresponsive to high-frequency stress perturbations (e.g., at least up to annual periods); however, they can flow plastically under the action of long-lasting loading: it is the case of low-frequency Earth tides (e.g., lunar nodal 18.61-years-long cycle) which can be detected as millimetric modulations in relative plate velocities using single-station- and baseline- modes GNSS time series. On the light of thin ultralow viscosity zones spreading at the lithosphere-asthenosphere boundary and inside the asthenosphere, and of thermally active small-cell stratified convection in the super-adiabatic zones of the upper mantle, it is reasonable that such modulations may have geodynamic implications. This conclusion is also supported by several observations proving a worldwide asymmetry in global geodynamics such as the westerly oriented motions of plates which follow a mainstream with a 0.2-1.2°/Myr drift relative to the sub-asthenospheric mantle in the hotspot reference frame.