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. Plain Language Summary This study estimated the P wave velocity structure to investigate the spatial correlation between the structural variation and coseismic shallow slip distribution of the 2011 Tohoku‐oki earthquake through an air gun‐ocean bottom seismometer survey on an along‐arc line of the deformed sedimentary prism on the landward slope of the Japan Trench. We detected a high P wave velocity zone in the hanging wall of the plate interface off Miyagi, but not off Sanriku, implying that the deformed prism off Sanriku is wider than off Miyagi. This implies that rigid rock would be in contact with the plate interface considerably closer to the trench off Miyagi than off Sanriku. We suggest that this along‐arc variation of the hanging wall side structure is a possible factor controlling the extent of the coseismic shallow slip in the trench axial region. Key Points Seismic structural study land side of Japan Trench showed along‐arc Vp heterogeneity above the plate boundary High Vp backstop localized south of 39°N implies the backstop protrusion in that area, thus closer to the trench than north of 39°N Trench axial zone of 38°–39°N with the backstop protrusion corresponds to the “slip to the trench” area during 2011 Tohoku‐oki event