Spatial partitioning between co- and post-seismic slip of the 2011 Tohoku Earthquake based on seafloor and terrestrial geophysical observations

Abstract:

Large interplate earthquakes are often followed by postseismic slip that is interpreted to occur in areas surrounding the coseismic ruptures on the plate interfaces. Such spatial partitioning is expected from the difference in frictional property that controls slip behavior during inter-, co-, and post-seismic periods. However, the estimation of high-resolution slip in offshore area is usually difficult due to the lack of seafloor geodetic data. An Mw9.0 earthquake, the 2011 Tohoku Earthquake, ruptured vast area on the plate interface offshore Tohoku district, northeastern Japan where Pacific plate is subducting beneath the continental plate from Japan Trench. We investigated the postseismic slip using both seafloor and terrestrial geodetic data, namely seafloor GPS/Acoustic ranging, ocean bottom pressure, and terrestrial GPS, to examine whether the co- and post-seismic slip of the 2011 Tohoku Earthquake are spatially partitioned. The postseismic slip distribution with applying a comprehensive FEM model to eliminate the viscoelastic components from observed postseismic displacements time series with taking landward movements on the seafloor into account. The high-resolution co- and post-seismic slip distributions clarified that they are partitioned spatially which also agree with the activities of interplate and repeating earthquakes. These results mean that the conventional frictional property model on the plate interface based on the rate- and slip-dependent friction law is still valid for the source region of the megathrust earthquake. The results also imply higher probabilities of the interplate earthquakes that rupture on the plate interface north and south to the main shock than previously expected, because the postseismic slip at the areas that surrounds the rupture areas of past interplate large earthquakes are significantly faster than the slip in the source regions of the potential earthquakes where subducting and overriding plates must be still coupled.