Combined GPS/Acoustic Seafloor Geodetic Observation System for Monitoring Off-shore Active Seismic Regions Near Japan

M. Fujita, Y. Matsumoto, T. Ishikawa, M. Mochizuki, M. Sato, S-I. Toyama, K. Kawai, T. Yabuki, A. Asada, O.L. Colombo

Abstract:	We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method has been adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. The primary purpose of this paper is to describe the methodology of data analysis that we are currently adopting, in particular, the linear inversion method to determine the position of seafloor stations from the kinematic GPS and acoustic ranging results. Then we present our results of analysis with this method obtained from the observed data at the seafloor reference point situated off Miyagi Prefecture, in northeast Japan, where huge earthquakes have repeated with a relatively regular interval of 30 to 40 years. We also discuss the precision of our technique in relation with observation conditions, such as the stability of the undersea acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a large earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from ten campaign observations for the period 2002–2005 exhibits a linear trend with a scattering RMS of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	592 - 603
Cite this article:	Fujita, M., Matsumoto, Y., Ishikawa, T., Mochizuki, M., Sato, M., Toyama, S-I., Kawai, K., Yabuki, T., Asada, A., Colombo, O.L., "Combined GPS/Acoustic Seafloor Geodetic Observation System for Monitoring Off-shore Active Seismic Regions Near Japan," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 592-603.
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