Conclusion
In our OBS observation period, almost no earthquakes occurred
along the plate interface where large coseismic slip occurred during
the 2011 Tohoku-Oki earthquake. Nearly complete stress drop
due to the large coseismic slip may be the reason. The earthquakes
beneath the trench inner-slope near the trench axis occurred both
within the overriding and subducting plates. There is a 45 km
wide aseismic wedge at the toe of the overriding prism. As reported
from the borehole breakout of the IODP JFAST drilling (Lin
et al., 2013), low differential stress within the overriding plate
near the trench axis results in the aseismic wedge near the trench
axis. The focal mechanisms of the earthquakes within the overriding
and subducting plates show that the tensional stress normal
to the trench axis is pre-dominant in the both plates. The tensional
stress within the subducting plate extends to a depth of
about 30 km at least and caused the Mw 7.0 strike-slip earthquake.
Furthermore, some trench-normal compressional earthquakes occurred
within the overriding plate at the landward end of the
aseismic wedge. These earthquakes suggest horizontal compression
of the overriding prism possibly caused by a strengthening of the
shallowest part of the megathrust, which may facilitate subduction
erosion.