Temporal variation of intermediate-depth earthquakes around the time of the M9.0 Tohoku-oki earthquake

The temporal evolution of intermediate-depth seismicity before and after the 2011 M9.0 Tohoku-oki earthquake reveals interactions between plate interface slip and deformation in the subducting slab. We investigate seismicity rate changes in the upper and lower planes of the double seismic zone beneath northeast Japan using both a β statistic approach and a temporal epidemic type aftershock sequence model. We do not observe an anomalous precursory increase in intermediate-depth earthquake activity preceding the mainshock; however, following the mainshock, we observe a rate increase for the intermediate-depth earthquakes in the upper plane. The average ratio of upper plane to lower plane activity and the mean deep aseismic slip rate both increased by factor of 2. An increase of downdip compression in the slab resulting from coseismic and postseismic deformation enhanced seismicity in the upper plane, which is dominated by events accommodating downdip shortening from plate unbending.