HR: 0800h
AN: G21A-0143    [Abstracts]
TI: A System for Observing Sea-floor Deformation: System Configuration and the Monitoring at the Nankai Margin, Japan
AU: * Tadokoro, K
EM: tad@seis.nagoya-u.ac.jp
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Ando, M
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Okuda, T
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Ikuta, R
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Sugimoto, S
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Yada, K
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Takatani, k
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AU: Besana, G M
AF: Nagoya University, Furo-Cho, Chikusa, Nagoya, 464-8602 Japan
AB: \ \ \ The Japanese Islands are located close to the plate boundaries, and large subduction earthquakes repeatedly occur at the plate boundaries. The source regions of the earthquakes are beneath the see bottom. It is, therefore, necessary to monitor the crustal activities, such as seismicity and crustal deformation, for the sake of earthquake prediction and disaster prevention. We have developed a system for observing sea-floor crustal deformation using the acoustic ranging technique. In this presentation, we present the general concept of our system, observation sites for the long-time monitoring of the sea-floor deformation, and preliminary results of the monitoring. \ \ \ The observation system is composed of 1) acoustic measurement between a ship transducer and sea-bottom transponders, and 2) kinematic GPS positioning of the observation vessel when the acoustic signal is transmitted. The sea-bottom transponders are set in a 13-inchs glass sphere, and are equipped with the batteries for five-years-measurements. The acoustic measurement is performed by the following procedure. An m-sequence signal with a length of 14.322 ms is transmitted from the ship transducer to sea-bottom transponders. The frequency of the carrier wave is 12.987 kHz. The sea-bottom transponder returns the same signal as it received after a delay time of 1048.576 ms to diminish the reverberation between the sea-floor and the sea-surface. We record the waveform of the returned signal at the ship transducer. The two-way travel time is measured by means of the cross-correlation computation. The travel time is converted to the path length using the sound speed on the basis of the CTD measurements. Combining the travel time data, ship positions, and attitude of the observation vessel, we determine the position of the sea-bottom transponders. \ \ \ We have installed the transponder networks at the Nankai margin and the adjacent region, Suruga bay; large earthquakes are expected to occur in the regions. The water depths at the regions are 800-2,100 m. Each network is composed of two to four transponder arrays. We have repeatedly measured the sea-floor deformation twice a year. We report the preliminary results of the measurements. We also performed experiments for estimating the error factors in sea-bottom positioning, that is, kinematic GPS and spatial changes in sound speed structure. The error in the kinematic GPS positioning behaves quadric increase with baseline length. The spatial variations in sound speed are conspicuous at a depth range from 0 to 500 m, and are 7 m/s at the two positions with a separation of 2 nautical miles.
DE: 7200 SEISMOLOGY
DE: 4259 Ocean acoustics
DE: 4294 Instruments and techniques
DE: 3094 Instruments and techniques
DE: 1206 Crustal movements--interplate (8155)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting