How did the 2011 off the Pacific coast of Tohoku Earthquake start and grow? The role of a conditionally stable area

The 2011 off the Pacific coast of Tohoku Earthquake broke a huge area of the plate interface; such a large event has not been observed before off the Pacific coast of Japan. We propose a preliminary scenario of the generation mechanism of the Mw 9 earthquake, referring to geophysical observation data: In the restoration period of the interplate locking after the active period of M 7-class earthquakes and afterslip, one M 7-class earthquake off Miyagi occurred. It could have triggered the 2011 Tohoku Earthquake as in the model proposed by Matsuzawa et al. (2004). The seismic slip from the northern part propagated through a barrier-like region, which had been almost locked except the afterslip of the M 7-class earthquakes, and extended to the southern part. We present a preliminary numerical simulation to show that the extremely large slip in the northern part was able to cause the seismic slip without large-scale nucleation over the whole fault. The interseismic and coseismic slip behavior of the barrier-like region is well explained by a “conditionally stable” condition of frictional instability. Since the present simulation does not allow us to explain the generation mechanism of the extremely large slip in the northern part of the M 9 fault, we need to construct a more detailed model in future studies.