Effects of Temporal Variations in Sound Speed Structure at Suruga Bay, Central Japan, on the Observations of Seafloor Crustal Deformation

Observations of seafloor crustal deformation is very important to understand the dynamics of plate boundary that include the strain accumulation processes, great interplate earthquakes mechanisms, and submarine volcanoes activities. Since most of the plate boundaries and seismogenic zones are located under the sea, we have been developing an observation system for monitoring of seafloor crustal deformation. This system consists of the following two main components: (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distance between a transducer equipped to the vessel and a transponder installed on the seafloor. We have repeatedly tested for the repeatability of transponder positioning for two months in 2002 at Suruga Bay, central Japan. Suruga Bay is situated in the northeast part of Nankai Trough, a place where the subduction plate boundary between the Philippine Sea and Eurasian plates. The horizontal and vertical errors in the repeated measurements (bias) are 20 cm and 44 cm, respectively [Sugimoto et al., 2003 AGU Fall Meeting]. We started the repeated observation from October 2002 in Suruga Bay and five-times measurements have been performed until May 2004. During the above measurements, temporal and spatial variations of sound speed structure in seawater are the error sources. Using the CTD (conductivity, temperature and depth) profiler, we repeatedly measured sound speed profiles during each acoustic ranging observation. In this study, we will show that the residuals of the observed acoustic travel-times can be explained by the temporal variations of sound speed structure. To achieve our purpose, using actual undersea velocity profiles measured with CTD profile and observed acoustic travel-time data, we have been trying to calculate acoustic travel-times after estimation of the temporal model of horizontal multi-layer velocity structure, and evaluate the effects of temporal variation of sound speed structure for seafloor positioning. The results of the present evaluation study may provide important information on the seafloor positioning including the temporal variations of sound speed.