The 2011 off the Pacific coast of Tohoku Earthquake related to a strong velocity gradient with the Pacific plate

We conduct seismic tomography using arrival time data picked by NIED Hi-net, including earthquakes off the coast, outside the seismic network. For these offshore events, we use the NIED F-net focal depth. We detect two low-V. zones in the uppermost subducting oceanic crust. The landward low-V zone with a large anomaly corresponds to the western edge of the coseismic slip zone of the 2011 off the Pacific coast of Tohoku Earthquake. The asperities of the previously known Off-Miyagi and Off-Fukushima earthquakes with magnitudes around 7.0 are also located at the boundary of the low-V and the eastern high-V zones. The initial break point (hypocenter) is associated with the edge of a slightly low-V and low-V p /V s zone. The trenchward low-V and low-V p /V s zone extending southwestward from the hypocenter may indicate the existence of a subducted seamount. The high-V zone and low-V p /V s zone might have accumulated the strain and resulted in the huge coseismic slip zone of the 2011 Tohoku Earthquake. The low-V and low-V p /V s zone is a slight fluctuation within the high-V zone and might have acted as the initial break point of the 2011 Tohoku Earthquake.