A temporal change of shear wave anisotropy within the marine sedimentary layer associated with the 2011 Tohoku‐Oki earthquake

Abstract

We found persistent reflections of S waves from the bottom of a ∼350 m thick marine sedimentary layer on the outer rise of the Japan Trench, just to the east of the source area of the 2011 great Tohoku‐Oki earthquake (Mw9.0), by auto‐correlating ambient seismic noise recorded on 1 year continuous records of broadband ocean bottom seismometers. The two‐way travel times of reflected S waves, which vary as a function of the polarization direction, indicate a velocity anisotropy of ~1.7% in the sedimentary layer. The fast direction is estimated to be trench parallel, possibly due to cracks or normal faults formed by bending of the plate in the outer rise. The travel time also shows a coseismic velocity reduction of ~2%, with slightly reduced anisotropy, within the layer. The change gradually recovered to pre‐earthquake conditions through 4 months after the earthquake, although recovery was not complete during the period of the observation. We also detected a similar anisotropic structure and magnitude of coseismic velocity reduction in this layer based on S coda of earthquakes with magnitudes greater than 5.0. Such coseismic changes can be explained either by increases of crack density and crack sphericity within the suddenly stressed sedimentary layer or by channeling and networking of water flow in the strongly shaken sedimentary layer.