The southern limit of the 2011 Tohoku earthquake is considered to be located around off Ibaraki. However, it is not well constrained how far south the large slip extended. To give better constraints, we investigated seismicity including small earthquakes before and after the Tohoku earthquake around off Ibaraki using dense ocean bottom seismic array data. We automatically identified epicenters by backprojecting semblance values resulting in a considerable increase in the number of detected events compared with those listed in the catalog based on onshore observation. The results revealed a couple of seismicity-activated region. The largest aftershock also occurred ~30 min after the main shock. Our detailed results suggest that this highly activated seismicity was initiated by the largest aftershock instead of the main shock. It, then, suggests that the large coseismic slip zone of the Tohoku earthquake may not have extended off Ibaraki.