Interplate thrust earthquakes are usually followed by afterslips, and they let the fore-arc move slowly trenchward. However, we do not know if the subducting oceanic plate is accelerated landward after such earthquakes. The westward velocity of Global Positioning System (GPS) stations in NE Japan show gradient decreasing from east to west reflecting the E-W contractional strain built up by the inter-plate coupling. Here we show that such coupling significantly enhanced (~1.5 times) after the 2003 Tokachi-Oki earthquake (M w 8.0), Hokkaido, in the segments adjacent to the ruptured fault. The coupling seems to be further enhanced (~3 times) after the 2011 Tohoku-Oki earthquake (M w 9.0). It is unlikely that interplate friction suddenly increased over such a large region, and relatively strong pre-2003 coupling there would not allow such enhancements even if full coupling is attained. Hence they are attributable to the temporary acceleration of the Pacific Plate subduction. We propose a simple 2-dimensional model in which down-dip acceleration of the slab let the force balance rapidly recover promoted by a thin low-viscosity layer on the slab surface. The accelerated subduction would account for temporary activations of regional interplate seismicity after megathrust earthquakes.

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For the 2011 Tohoku earthquake, we propose a mechanical model to explain rare giant (M9-class) and frequent large (M7-class) earthquakes on a thrust fault in the subduction zone. Observations implied, in the M9 Tohoku earthquake, that extremely large slip on the order of tens of meters occurs in a shallower part to release a slip deficit, as well as substantial slip about ten meters or so in a deeper part including the source area of the M7-class earthquakes.Here, we present a model in which the extremely large slip is caused by hydrothermal weakening (dynamic thermal pressurization of pore fluid) on the fault plane, not by contrast of frictional properties in terms of rate-and statedependent friction. The model explains that the Tohoku earthquake followed a M7-class earthquake in two days, but M7-class earthquakes are not always followed by a giant earthquake. In a giant event, large coseismic slip can occur over an area where quasistatic slip, namely, afterslip of M7-class earthquakes or spontaneous slow slip events, takes place. Slight differences of stress state in the shallow part can result in drastically different coseismic slip. We further perform numerical experiments varying hydraulic parameters and the length of effective hydrothermal weakening area. The experiments imply that observations for monitoring the effective hydrothermal weakening area need spatial resolution on the order of 10 km or finer.