Tsunami Forerunner of the 2011 Tohoku Earthquake Observed in the Sea of Japan

During the March 11, 2011 Tohoku tsunami, some Japanese and Russian tide gauges in the Sea of Japan recorded sea level disturbances with periods of several to 10 min and amplitudes of several to a few tens of centimeters starting immediately after the earthquake. Tsunami travel times computed from the 2011 source in the Pacific Ocean to these stations are 1–2 h. To examine the cause of the observed tsunami forerunners and to reproduce them, we carried out tsunami numerical computations from three kinds of coseismic seafloor deformation: the vertical displacement in the tsunami source area in the Pacific Ocean, the vertical displacement extending in the Sea of Japan, and the vertical displacement including the effects of the horizontal displacement and the seafloor slope in both the Pacific Ocean and the Sea of Japan. Only the last source reproduced the observed tsunami forerunners. This indicates that the tsunami forerunners were produced by the horizontal displacement of fault motion and the seafloor slope in the Sea of Japan. For the tide gauge records in Japan, the forerunners with periods longer than 2 min were well reproduced by the tsunami simulation. The shorter-period components may be induced by seismic ground shakings. For the tide gauge records in Russia, the agreement between the observed and computed waveforms was less satisfactory, probably because of less accurate and coarser bathymetry data.