Revisiting the Tidal Activation of Seismicity with a Damage Mechanics and Friction Point of View

Do Earth tides activate seismicity? Since the peak semi-diurnal tidal stresses are as large as the expected tectonic stress changes over a month’s time, they would easily activate seismicity if it were a simple threshold process. The better question to ask might be, “why don’t we see a very obvious tidal correlation?” Could it be that the rate of tectonic stress accumulation is not uniform, but remains quite low until a rapid increase occurs shortly before the earthquake? Or could it be that a self-driven nucleation process occurs on a time scale longer than the diurnal tide, thus destroying the expected synchronization. If there is any nonlinear response to tides, then this purely cyclic loading will follow a hysteresis loop. If tectonic loading is added to the cyclic load, then damage, or seismic response should occur only during that portion of each cycle when the load exceeds the previous maximum. This is known as the Kaiser effect in laboratory studies of acoustic emissions. In the case of Earth tides, this effect should produce a periodicity of roughly 15 days. If there are departures from a strictly linear tectonic loading rate, then the tidal effect on seismicity will not even be periodic, and thus not easily detected with spectral methods