GPS/Acoustic seafloor geodetic observation: method of data analysis and its application

We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is 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. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven 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.