oa A New Method to Seek the Process of Tsunami Generation - Newly Discovered Waves at the 2003 Tokachi-Oki Earthquake

At the 2003 Tokachi-oki earthquake of M8, seafloor phenomena such as a generation process of tsunami, seafloor uplifts, etc., were observed using a cabled observatory installed on the seafloor. The seafloor uplifts were observed not before the main shock but continuously after the main shock. The uplifts were 0.35，0.37, and 0.12 m for epicentral distances of 25.5, 31.4, and 81.8 km, respectively. Pressure fluctuations that took place co-seismically show about 100 times in amplitude to those observed as the uplifts. The uplift of the seafloor generated not only tsunami but high amplitude acoustic waves. Both the tsunami and acoustic waves were generated by the uplift and superposed to each other. After the main shock, a continuous uplift of the seafloor is observed at the all three pressure gauge locations and the rate of uplift was about 0.004 m/day. These phenomena may imply that there was a change in the state of friction on the plate boundary interface by the main shock. In this paper, we demonstrate what was observed using ocean bottom pressure gauges installed right above the focal area of the earthquake, and then discuss these phenomena in tsunami generation and in post-seismic slip processes.