Ocean bottom pressure records of the 2011 Tohoku-Oki earthquake

The 2011 Tohoku-Oki earthquake was recorded by a number of geophysical instruments densely distributed over the Japanese Islands as well as on the seafloor just above the source area. These data contribute to clarify the generation process of massive earthquakes unprecedentedly in detail. In this paper, we will present continuous records of ocean bottom pressure (OBP) obtained in the vicinity of the epicenter of the earthquake. The OBP records provided us with coseismic permanent displacement field associated with the mainshock as well as those with ith largest foreshock and helped to constrain fault slip models of these earthquakes. The amounts of the coseismic displacement of the mainshock monotonically increased towards the Japan Trench, which were strong evidence of the large coseismic fault motion near the trench. The mainshock of the Tohoku-Oki earthquake was preceded by aseismic slip induced by the foreshock which propagated towards the hypocenter of the mainshock. Tsunamis and acoustic vibrations in the seawater layer forced by coseismic seafloor motions were also observed after the occurrence of earthquakes as fluctuations of OBP with very large amplitudes and short period. Tsunami waveforms derived from the data gave strong constraint on the source model of the devastating tsunami caused by the mainshock. Considerably large pressure fluctuation due to the acoustic vibrations were observed reflecting the gigantic size of the earthquake. All these experiences of the OBP measurement demonstrate its importance as monitoring tools of crustal deformation and tsunami generation related to submarine earthquakes.