Overview of the Tohoku Earthquake: 50-m Fault Slip Reaching the Deep Sea Trench

The Tohoku Earthquake (M9) occurred along the Japan Trench, which is the plate boundary between the subducting Pacific Plate and the overriding plate, at 14:46 on 11^thof March, 2011 (Japan time). The fault rapture of this giant earthquake propagated over a 500 km x 200 km fault zone in three minutes. Occurrence of a magnitude 9 class giant earthquake along the Japan trench have been never expected, although magnitude 7 – 8 class earthquakes have repeatedly occurred in the last 200 years. Based on ground motion, tsunami wave and geodetic data of the Tohoku Earthquake, fault rupture during this earthquake is thought to have propagated to a shallow part of the subduction zone in the Japan Trench. This observation calls into question conceptual models that assume that the shallow part of the plate boundary interface in a seismogenic subduction zone slips aseismically. However, the available observations do not have sufficient resolution near to the subduction trench to determine whether coseismic fault slip extended all the way to the trench axis. In order to solve this problem, many Japanese research institutions carried out geological and geophysical observations in the fault zone soon after the earthquake occurred. Among those observations, a rapid response cruise at the trench-ward of the epicenter provided the most fundamental discovery to examine where and how fault rupture propagated. During the rapid response cruise, high resolution bathymetric data were acquired from the epicenter to the trench axis exactly along the same ship track taken in 1999. Comparing the bathymetric data acquired before and after the earthquake shows that the upper-plate crustal block, on which the Japanese island situated, moved ~50 m toward the east-southeast and ~7-10 m upward during the earthquake. We also fund, from comparison of a sub-seafloor structure acquired before and after the earthquake, that trench-filled sediment in the Japan Trench axis was remarkable deformed due to a co-seismic fault slip reaching to the seafloor during the Tohoku Earthquake. Our examination of the observed structure leads us to propose the following scenario for the rupture propagation to the trench. The fault rupture of the Tohoku Earthquake starting from the epicenter at around 20 km depth propagated up-dip along the plate boundary close to the trench, then the rupture finally broke the sea floor at the trench axis. We conclude from line of our observations that the shallow plate interface at the subduction trench axis can slip seismically.