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

Abstract

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.

Accelerated pacific plate subduction following interplate thrust earthquakes at the Japan trench

“…This point has been monitored since 2002 and was found to move landward by 8.5 cm/yr (Fujita et al, 2006). Sato et al (2011) reported its coseismic trenchward movement of ~24 m by the 2011 earthquake.…”

Slip distribution of the 2011 off the Pacific coast of Tohoku Earthquake inferred from geodetic data

“…1). In addition, we use horizontal displacement data from three seafloor geodetic sites just above the source region, which were measured by the Japan Coast Guard using the GPS/acoustic measurement (Fujita et al, 2006). One of the seafloor sites off the Miyagi prefecture moved about 24 m in the ESE direction associated with the earthquake (Sato et al, 2011).…”

Heterogeneous interplate coupling along the Nankai Trough, Japan, detected by GPS-acoustic seafloor geodetic observation

“…We spent approximately 24 h performing an observation. More details on the analytical methodology are presented in Fujita et al (2006), Matsumoto et al (2008), Sato et al (2013a), and Watanabe et al (2014).…”

“…In the early stages of our observations, we performed this inversion analysis constrained by the height of the grouped transponders over all epochs (e.g., Fujita et al 2006). The present analysis was constrained by the positional relationship of the grouped transponders for all epochs (e.g., Watanabe et al 2014).…”