Hiroyuki Matsumoto, Shusaku Inoue, and Tatsuo Ohmachi

Abstract:
Water pressure changes have been recorded during the 2011 Tohoku earthquake (Mw9.0) by two oceanbottom pressure gauges of the JAMSTEC cabled observatory located approximately 400 km north of the tsunami source. In the present study, observed water pressure fluctuations have been interpreted by timefrequency processing analysis, followed by a comparison with those fluctuations of ocean-bottom seismometers. Acoustic resonance has been involved due to seismic waves even in the intermediate-field. The present study suggests that water pressure fluctuations during the 2011 Tohoku earthquake are dominated by oceanbottom acceleration and velocity for the intermediate frequency range and the high-frequency range, respectively, and its threshold roughly coincides with the fundamental acoustic resonant frequency. Water pressure amplitude is independent, in contrast, on bottom oscillations in the low-frequency range, for which the threshold is related to intermediate waves rather than shallow water waves.

Keywords: the 2011 Tohoku earthquake, tsunami, oceanbottom pressure gauge, ocean-bottom seismometer