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We have examined the focal mechanism distributions in the source region of the 2011 off the Pacific coast of Tohoku Earthquake, before and after the mainshock, using F-net data, and then performed a stress tensor inversion by using the multiple inverse method. It is found that earthquakes were mainly reverse fault type events all over the source region before the main shock, whereas, after the main shock, many normal fault type events occurred in the hanging wall but focal mechanisms did not change in the deepest part in the source region. It is inferred from the stress tensor inversion that the stress state is a reverse fault type in the deepest part before, and after, the mainshock but that, in the central part, the stress state changed from reverse fault type before the mainshock to normal fault type after the mainshock. These results suggest that the absolute shear stress on the plate boundary fault in this region was almost completely released by the mainshock. Furthermore, we examined, in detail, the focal mechanism distributions and stress states in the shallower part near the trench axis off the Miyagi Prefecture and found that strike-slip events with the P-axis in the NS direction occurred in the footwall east of the mainshock hypocenter. This finding can be well explained by a slip model with a large moment release near the hypocenter. Our analysis constrains the spatial distribution of the moment release of the 2011 off the Pacific coast of Tohoku Earthquake around the mainshock hypocenter rather than in the vicinity of the trench axis.

Detailed stress fields in the focal region of the 2011 off the Pacific coast of Tohoku Earthquake—Implication for the distribution of moment release—