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,pro mote 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 accele ration 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.

Dynamics of seasonal and interannual variability of the ocean bottom pressure in the Southern Ocean

Seasonal and interannual variability of ocean bottom pressure(OBP)in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment(GRACE)data and a Pressure Coordinate Ocean Model(PCOM)based on mass conservation.By comparing OBP,steric sea level,and sea level,it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales.The diagnostic OBP based on barotropic vorticity equation has a good correlation with the observations,indicating that wind forcing plays an important role in the variability of the OBP in the Southern Ocean.The unique interannual patterns of OBP in the Southern Ocean are closely associated with El Niño-Southern Oscillation(ENSO)and Southern Annular Mode(SAM).Regression analysis indicates that ENSO and SAM influence the OBP through altering the Ekman transport driven by surface wind.The leading pattern of OBP from PCOM are very similar to observations.Sensitive experiments of PCOM show that surface wind forcing explains the observed OBP variability quite well,confirming the importance of wind forcing and related oceanic processes.In the eastern South Pacific,the averaged OBP shows a decrease(increase)trend before(after)2011,reflecting the reverse trend in westerly wind.In the South Indo-Atlantic Ocean,the averaged OBP has a weak increase trend during 2003–2016.