Along-Arc Heterogeneity of the Seismic Structure Around a Large Coseismic Shallow Slip Area of the 2011 Tohoku-Oki Earthquake: 2-D Vp Structural Estimation Through an Air Gun-Ocean Bottom Seismometer Experiment in the Japan Trench Subduction Zone

The 2011 Tohoku-oki earthquake occurred with a rupture length of 500 km along the Japan Trench, causing a large slip (>30 m) at the shallowest portion of the plate boundary fault south of 39°N off Miyagi and a smaller slip (~10 m) north of 39°N off Sanriku, the northern part of the source area. We estimated the P wave velocity (Vp) structure around the shallowest portion of the plate boundary along the trench to investigate the spatial correlation between the structural variation and coseismic shallow slip distribution of the 2011 earthquake. For this purpose, we analyzed data from an air gun-ocean bottom seismometer survey on an along-arc line on the lower part of the landward slope of the Japan Trench. We detected a high Vp zone in the hanging wall of the plate interface off Miyagi, which corresponds to the Cretaceous backstop. The Vp model also showed the absence of the high Vp zone off Sanriku. This suggests that the low-Vp deformed prism is wider off Sanriku than off Miyagi. Therefore, the backstop is close to the trench particularly off Miyagi. We suggest a correlation between this along-arc variation of the hanging wall side structure and the extent of the coseismic shallow slip in the trench axial region; the large slip reached the trench off Miyagi but not off Sanriku.