Large Coseismic Slip to the Trench During the 2011 Tohoku-Oki Earthquake

The strong ground motions, large crustal deformation, and tsunami generated by the 2011 Tohoku-oki earthquake ( Mw 9.1) reveal that a large coseismic slip likely propagated to shallow depth in the Japan Trench. Although data acquired by onshore networks cannot resolve the slip behavior of the updip fault rupture, marine geophysical and geological studies provide direct evidence of coseismic slip to the trench. Differential bathymetry data show ∼50 m of coseismic seafloor displacement extending to the central Japan Trench (38–39.2°N). Seismic data show that coseismic slip ruptured the seafloor within the trench. Pelagic clays may have promoted slip propagation to shallow depths, whereas disturbed/metamorphosed clays may have restricted slip to the main rupture zone. Those observations imply that a smooth, broadly distributed, weak, clay-rich sediment in a shallow part of a subduction zone is a characteristic factor that can foster a large coseismic slip to the trench and, consequently, the generation of a large tsunami. ▪ During the 2011 Tohoku-oki earthquake ( Mw 9.1), more than ∼50 m of slip occurred on a fault that ruptured the seafloor in the central Japan Trench. ▪ The fault rupture reaching the seafloor caused a large tsunami. ▪ Marine geophysical explorations revealed that a clay-rich sediment in the subduction zone was one factor fostering the large fault slip. ▪ Understanding of slip behavior in the shallow portion of a subduction zone will help us prepare for future large tsunamis along the Japan-Kuril Trench.