Spatial distribution of earthquakes off the east coast of the Kanto region along the Japan Trench deduced from ocean bottom seismographic observations and their relations with the aftershock sequence of the 2011 off the Pacific coast of Tohoku Earthquake

The 2011 Tohoku earthquake hit the forearc region of the Japan Trench on March 11, 2011. The rupture zone seemed to reach off the coast of the Kanto region. We had conducted ocean bottom seismographic observations off the coast of the Kanto in 2008 and estimated 851 hypocenter locations around the south part of the 2011 off the Pacific coast of Tohoku Earthquake by using over 50 ocean bottom seismometers (OBSs) and routine data jointly. The hypocenters distributed some clusters, and we found a few seismic gaps at the boundary zones of the clusters. The most remarkable seismic gap was positioned at the edge of intervened Philippine Sea plate (PHS) between the North American plate (NA) and Pacific plate (PAC). We compare the epicenter distributions with the 2011 aftershock distribution determined by routine data. The aftershocks are segmented spatially and there are some seismic gaps among the segments. The remarkable low places are consistent with the boundary zones of each cluster we estimated from the 2008 data. We infer that those regions have strong heterogeneity resulting from strong deformations caused by various subduction processes, such as intervening PHS between NA and PAC, seamount chains and changes in physical properties.