Stress adjustment revealed by seismicity and earthquake focal mechanisms in northeast China before and after the 2011 Tohoku-Oki earthquake

• Focal mechanism solutions of 69 earthquakes in Northeast China (NEC) are obtained by gCAP.
• Seismicity rate change is noticeable on major NEC faults before and after the mainshock.
• Crustal FMSs fall in two groups: E–W compression strike–slip and N–S extension normal-faulting.
• Static Coulomb Stress calculations confirm the distinct variations of seismicity and FMSs.
• Mantle-depth earthquakes in NEC are influenced by a down-dip transfer of compressional stress along the subducting plate.

In order to understand the influence of the March 11, 2011, MW 9.0 Tohoku-Oki earthquake on regional-scale seismicity, we study the seismicity rate and focal mechanism solutions (FMSs) of earthquakes in northeast China (NEC) before and after the megathrust event. Broadband seismic waveforms from 270 permanent and temporary stations are used to invert for the moment tensors of 69 earthquakes between 2009 and 2013 in the NEC. Our results show that there are distinct changes in seismicity rate on major NEC faults before and after the 2011 Tohoku-Oki event although the seismic moment rate of the whole region remains roughly constant. In comparison to a wide distribution of earthquakes before the Tohoku-Oki event, FMSs of crustal earthquakes in the NEC after the megathrust event can be categorized into two groups: strike–slip events with E–W compression and normal-faulting events with N–S extension. Stress field inversions before and after the Tohoku-Oki event suggest that the variations in seismicity and FMSs are due to a minor adjustment of regional stress state imposed by the megathrust event, which is further confirmed by static Coulomb stress change calculations. Mantle-depth seismicity is also influenced by the megathrust event, possibly via a down-dip transfer of compressional stress along the subducting plate, as manifested by the absence of moderate-sized mantle-depth earthquakes (~ MW 4–5) between May 2011 and April 2013 and the occurrence of deep-focus events with P axes along the dip direction of the subducting Pacific Plate in E–W vertical cross-sectional view and in WNW–ESE direction in map view.

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