Coseismic fault rupture at the trench axis during the 2011 Tohoku-oki earthquake

Fault rupture during the 2011 Tohoku-oki earthquake, which generated a huge tsunami, is thought to have propagated to a shallow part of the subduction zone(1-3). 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(4,5). However, the available observations of the earthquake and tsunami do not have sufficient resolution near to the subduction trench to determine whether coseismic fault slip extended all the way to the trench axis. Here we use seismic reflection data to image the subduction trench axis seawards of the Tohoku-oki earthquake epicentre. We compare an image of a profile taken in 1999 with one acquired along the same profile 11 days after the earthquake. Before the earthquake, we observe a triangular wedge of sediments at the trench axis. After the earthquake, we observe a deformed upheaval structure in the sedimentary layer that is 3 km long and 350 m thick. We suggest that this remarkable deformation structure formed as a result of compression during coseismic slip on the shallow plate interface, implying that fault rupture during the Tohoku-oki earthquake did reach the sea floor at the trench axis. We conclude that the shallow plate interface at the subduction trench axis can slip seismically.