HR: 0830h
AN: OS41C-0816    [PDF]
TI: Semidiurnal variation of the ocean sound velocity associated with the M$_{2}$ internal tide
AU: * Sugioka, H
EM: hikari@jamstec.go.jo
AF: IFREE, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061 Japan
AU: Fukao, Y
EM: fukao@eri.u-tokyo.ac.jp
AF: Eearthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hibiya, T
EM: hibiya@eps.s.u-tokyo.ac.jp
AF: Dept. of Earth and Planetary Science, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: Swarm of hydroacoustic waves (T-waves) associated with submarine volcanic activity has often been detected by ocean bottom seismometers (OBSs). In three months from September to December, 1999, a series of large submarine volcanic events occurred in the Izu-Bonin-Mariana volcanic region. We analyzed the waveform data recorded on the 3 submarine cabled OBS arrays (off-Boso, off-Sanriku and off-Kushiro) and the 9-month term broadband OBS array of the Ocean Hemisphere network Project with a few stations deployed near the swarm region. One of the off-Boso array stations was chosen as a reference, which locates at a depth near the SOFAR channel axis. At this reference station, we selected the largest 40 waveforms in everyhour and stacked them to obtain a reference waveform at every hour. Assuming that the relative arrival time differences among these 40 events are common to all the other stations, the one-hour time window is shifted at each station to obtain the stacked waveform in the best correlation with the reference waveform. This time shift represents a delay or advance of the T-wave arrival relative to the reference station. We calculated the average of the time shifts at each station to determine the average epicenter of the swarm and the average T-wave speed. The epicenter was located at (20.3729$\deg$N, 144.9508$\deg$E) and the sound speed was determined to be 1.5021 km/s. We found that the every-hour time shift shows a semidiurnal variation with an amplitude of $\pm$0.1 to 0.2 s in a consistent way among the stations. We interpret this as the consequence of the path length change due to vertical distortion of the SOFAR axis in response to the semidiurnal internal tides. This path length change is evaluated from the path length change of a constant density plane near the SOFAR depth. We calculate for this purpose the distribution of the isopycnal vertical displacement and phase associated with the M$_{2}$ component of the internal tide. The result shows that the isopycnal vertical displacement along the Boso path is an order of magnitude larger than those along other paths so that the path length change is also an order of magnitude larger. The amplitude and phase of this change are consistent with the observed ones.
DE: 4500 OCEANOGRAPHY: PHYSICAL
DE: 4544 Internal and inertial waves
DE: 4560 Surface waves and tides (1255)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting