Supercycles along the Japan Trench and Foreseeability of the 2011 Tohoku Earthquake

The devastating Tohoku earthquake of magnitude (M) 9.0 occurred on 11 March 2011 UTC along the Japan trench, where the Pacific plate is subducting beneath the Tohoku district. The national program of seismic hazard assessment, which was initiated by the Japanese government after the 1995 Kobe earthquake, failed to foresee this earthquake, because no supercycle of megathrust events had been identified along the Japan trench. For example, the program identified a regular cycle of six M7 to 8 characteristic earthquakes in the land side of the Miyagi-oki region, and only reported the high probability of having another M7 earthquake there. The Japanese government also built nation-wide dense arrays of seismometers and GPS receivers after the Kobe earthquake. Nishimura et al. recovered annual rates of back slip, which is the drag of the overriding plate by interplate coupling, using GPS data during a calm period before the Tohoku earthquake. We then recovered coseismic slips through a joint inversion of ground motion and GPS data during the earthquake. The distributions of recovered coseismic slips and back slip rates bear a close resemblance to each other. An area of large back slip rate was previously thought to be related to a regular cycle of characteristic earthquakes. However, our result demonstrates that the area is related to a supercycle of megathrust earthquakes. From the coseismic slips and back slip rates in the Miyagi-oki region, we calculated the coseismic moment release and moment accumulation rate of the Tohoku earthquake to be 143 x 10**20 Nm and 0.434 x 10**20 Nm/year, respectively. Since characteristic earthquakes occasionally release some part of accumulated seismic moment, those in the Miyagi-oki region were compiled. We then calculated the moment releases by them to be 47 x 10**20 Nm. These moment releases and accumulation rate lead to a supercycle period of 438 years. However, this period is too short, if the 869 Jogan earthquake is the only documented event to have occurred with a possible magnitude and location similar to that of the Tohoku earthquake. Within the compilation, the 1611 Keicho earthquake can be a hidden candidate between the 869 Jogan and 2011 Tohoku earthquakes. Extensive tsunami damage caused by this earthquake was documented over the Tohoku district. The time series, which was drawn using the moment releases and accumulation rate, is mostly controlled by the moment releases of megathrust earthquakes. Supercycles were found in the Mentawai region along the Sunda trench, but the rupture pattern of two large earthquakes in 2007 does not completely coincide with the back slip distribution. Similar disagreement was also found for the 2010 Maule earthquake in the Chilean subduction zone. These imply that they are not megathrust earthquakes of supercycles. Therefore, we cannot foresee characteristic earthquakes in regular cycles using a back slip distribution, but the 2011 Tohoku earthquake could be foreseen with respect to at least its location and extent, if we monitored GPS array data considering their relation to megathrust earthquakes of supercycles.