Abstract
The 2011 Tohoku-oki earthquake resulted in substantial slip relatively near the coast from Miyagi to Ibaraki prefectures as well as extremely large slip near the trench. The moment-rate function has a shoulder around 35 s and a large peak about 70 s. The former corresponds to a significant moment release in the source area of the 1978 Miyagi-oki earthquake, and the latter was caused by extremely large slip near the trench. The main rupture process is characterized was by large maximum slip (50 m), long slip duration in the large slip area (90 s), and a relatively large stress drop in the up-dip source region (10 MPa). These characteristics mean that dislocation across the plate interface continued to release elastic stress and strain during the earthquake, which strongly suggests significant weakening of frictional strength due to thermal pressurization or other mechanisms on the fault plane. A drastic change in earthquake mechanisms from compression to extension above the large-slip area together with the occurrence of low-angle normal-fault aftershocks at approximately the depth of the plate interface also supports the idea that the earthquake did release roughly all of the accumulated strain on the plate interface owing to dynamic weakening of the fault. The stress accumulated on the plate interface before the earthquake is estimated to have been about 10 MPa from the hypocenter to the trench. If dynamic weakening on a fault plane plays an essential role in large earthquakes, then the periodicity of large earthquakes is questioned, as fault weakening mechanisms are non-linear in nature. Pseudo-cyclical records of the occurrence of large interplate earthquakes can be understood by constant accumulation of elastic strain due to steady plate motion and accidental release of elastic strain due to dynamic fault weakening that strongly depends on the conditions immediately prior to the earthquake.
