A combination of tides and nontidal variations in ocean bottom pressure may generate interannual slip fluctuations in the transition zone along a subduction plate interface

The tidal triggering of earthquakes has been studied for many years. The discovery of slow earthquakes in the early 2000s, including slow slip, has urged scientists to investigate the tidal responses of these earthquakes due to their sensitivity to weak stress perturbations. Previous studies have shown that slow earthquakes correlate with diurnal and semidiurnal tides and seasonal variations in surface loads more clearly than ordinary earthquakes. However, little is known about long-term responses to external stresses. In this paper, based on a widely accepted frictional law for faults, a mechanism is proposed by which nontidal variations in ocean bottom pressure, when combined with tides, promote the occurrence of slow earthquakes. Because slow earthquakes accompany a slip on the plate interface, this mechanism allows one to estimate slip modulations. A one-degree-of-freedom slip model is constructed and applied to Ise Bay in the Tonankai region of southwestern Japan, where large-scale ocean mass redistributions have occurred. The model calculated with parameters determined from the observation of tectonic tremors is quantitatively consistent with the slip during 1997–2013 inferred from GNSS data, suggesting that the decrease in the sea-level change in approximately 2006 could cause the acceleration of a slip observed after that. This result implies that the decreases in sea level in approximately 1996 and 2014 could also cause subsequent slip accelerations. These three slip acceleration periods temporally coincide with the increases in background seismicity in a shallower portion of the plate interface. These changes in seismicity are common to shallow earthquakes in the Tokai area, and a similar model can reproduce them. Further studies are expected to reveal causality between shallow earthquakes and long-term slip fluctuations based on modeling that considers changes in the frictional property along the plate interface.