Earthquake potential revealed by tidal influence on earthquake size-frequency statistics

The possibility that tidal stress can trigger earthquakes is long debated(1-6). In particular, a clear causal relationship between small earthquakes and the phase of tidal stress is elusive(2-8). However, tectonic tremors deep within subduction zones are highly sensitive to tidal stress levels(9-13), with tremor rate increasing at an exponential rate with rising tidal stress(11-13). Thus, slow deformation and the possibility of earthquakes at subduction plate boundaries may be enhanced during periods of large tidal stress. Here we calculate the tidal stress history, and specifically the amplitude of tidal stress, on a fault plane in the two weeks before large earthquakes globally, based on data from the global(14), Japanese(15), and Californian(16) earthquake catalogues. We find that very large earthquakes, including the 2004 Sumatran, 2010 Maule earthquake in Chile and the 2011 Tohoku-Oki earthquake in Japan, tend to occur near the time of maximum tidal stress amplitude. This tendency is not obvious for small earthquakes. However, we also find that the fraction of large earthquakes increases (the b-value of the Gutenberg-Richter relation decreases) as the amplitude of tidal shear stress increases. The relationship is also reasonable, considering the well-known relationship between stress and the b-value(17-20). This suggests that the probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels. We conclude that large earthquakes are more probable during periods of high tidal stress.