Possible occurrence of a giant interplate earthquake in northeast Japan greater than the 2011 Tohoku earthquake

We supposed there occur M7-class earthquakes and the co-rupturing M8 earthquakes on the Pacific plate interface subducting beneath northeast Japan. Against our speculation, the 2011 Tohoku earthquake grew up to Mw9.0. We have so far constructed cycle models of this giant earthquake to understand why this grew up to Mw9.0. Next question is; is there any possibility that a much larger earthquake occurs in this region? In this study, we explore this possibility through quasi-dynamic earthquake cycle simulations. The 2011 Tohoku earthquake ruptured a large region of 200km x 500km. The rupture region includes a confined area with huge coseismic slip over 50 m in the shallow Off-Miyagi region close to the Japan Trench, and several M7 asperities in Off-Miyagi and Ibaraki regions which have been ruptured repeatedly at intervals of several ten years. The tsunami deposit surveys suggest this giant earthquake has the recurrence time of several hundred years. The afterslip occurs mainly in the deeper region of the coseismic slip region, except the Off-Miyagi region [Ozawa et al., 2012]. At Off-Kamaishi and Off-Fukushima regions located in the northern and southern sides of the Off-Miyagi region, we can find the local maximum of the afterslip. The Off-Kamaishi region did not produce much coseismic slip, and has not experienced historical large earthquakes. And no large afterslip extended to the northern region beyond Off-Kamaishi. Then, the Off-Kamaishi region is a kind of boundary between the 2011 Tohoku earthquake and its adjacent northern regions. In the northern region, there occurred the 1968 Off-Tokachi Mw8.3 earthquake, which has three M7 asperities with recurrence times of several ten years [Yamanaka & Kikuchi, 2004]. An aftershock of the 2011 Tohoku earthquake, which occurred 22 minutes after the main shock, is located at the southern asperity area. And there is a region close to the Japan Trench, where the 1897 Meiji-sanriku tsunami earthquake occurred. We performed quasi-dynamic earthquake cycle simulations. We use a 3D plate interface geometry and assume the frictional stress obeys the rate and state composite law [Kato & Tullis, 2001]. We have so far constructed the hierarchical asperity model of 2011 Tohoku earthquake cycle [Ohtani et al., 2012] which includes a M9 source region with conditionally unstable friction property and M7 asperities with unstable ones within the source region. In this study, we extend this model to include the northern region with the 1968 Off-Tokachi earthquake and the 1897 Meiji-sanriku tsunami earthquake. After several simulations with frictional properties assigned at the Off-Kamaishi region and the asperities in the northern region, we got the case where the giant earthquake occurs at the Tohoku earthquake location and the rupture reaches the northern region, and the case where the Tohoku earthquake does not rupture the northern region as in the 2011 case. In the former case the northern region is enough stressed, and in the latter the northern area is just after the whole rupture of the northern area. Our simulations indicate that the condition of the other region adjacent to the giant earthquake may control to which extent the giant earthquake grows, though we have obtained so far no definite historical evidences for the occurrence of much larger earthquakes than the 2011 Tohoku earthquake.