The submarine fault scarp of the 2011 Tohoku-oki Earthquake in the Japan Trench

Rupture of the plate boundary fault during the 2011 Tohoku-oki earthquake with moment magnitude 9.0 is thought to have propagated to the trench. Surface exposure of the fault has not yet been confirmed because of great depths that are challenging to access and study. Using a manned submersible in the Japan Trench, we explored and visually assessed the seafloor near the epicenter. On the eastern slope of a thrust ridge 59 m high, which appeared after the earthquake, we found a 26 m high subvertical cliff regarded as the fault scarp. Cross-section analysis suggests an 80–120 m slip on the fault when assumed to dip at 45–30°, to build up the observed relief. The estimated larger displacement in the trench than in more proximal parts can be attributed to local enhancement of the slip by extension of the wedge above a subducting graben on the Pacific plate. A 26 m high cliff is identified as the fault scarp corresponding to the devastating Mw 9.0 Tohoku-oki Earthquake and indicates 80–120 m of slip occurred on the shallow-dipping thrust fault, according to manned submersible observations at 7500 m water depth.