Repeated Observation of Sea-floor Crustal Deformation at Suruga and Nankai Trough in 2002 and 2003

In order to monitor seafloor crustal deformation is one of the most important roles of global strain measurement because about 70 % of the Earth_fs surface is covered with seawater. Additionally, most part of the plate boundaries and seismogenic zones of interplate earthquakes are located under the sea. To achieve the monitoring of seafloor crustal deformation, we have developed a system with kinematic GPS positioning and acoustic ranging. We have repeatedly tested for the accuracy of our system in both Suruga Bay and Kumano Basin, central Japan in 2002 and 2003. The Suruga Bay is an appropriate site to test the seafloor observation system because it can be well-surrounded by coastal GPS reference stations with short baselines < 15km. In addition, a subduction plate boundary between the Philippine Sea and Eurasian plates lies in the Suruga bay, where a large earthquake has been anticipated to occur in a seismic gap. The Suruga Trough is thus worthy of monitoring the sea-floor crustal deformation. In the Suruga Trough, we installed two sets of three ocean-bottom units at depths about 800 m on both sides of the trough on October 29-30 and November 19-21, 2002, respectively. In the Kumano Basin, eastern Nankai Trough, we installed two arrays at about 2200 m depth on June 12 and July 14, 2003. We measured the slant ranges between the acoustic transducer on the observation vessel and those of the ocean-bottom units. Combining ship positions measured by GPS, vessel_fs attitudes, and acoustic travel-time data, we determine the precise locations of ocean-bottom units. Our system will make it possible to estimate very accurate seafloor position with acoustic ranging data. During the survey we also measured two CTD profiles at the same time over distances 0.1nm to 2 nm, to estimate lateral heterogeneity of the sea-water sound velocity. On the basis of such data, the observations enable us to estimate the strain accumulation at those plate boundaries. We report the evaluation about the locations of ocean-bottom units and these systematic errors in our whole observations. The present observation system will attain a centimeter-level resolution of sea-bottom position.