System Development for Sea-bottom Crustal Deformation Measurement: Main Observational Results at the Suruga-Nankai Trough, Japan, and Future Problems

Our group has developed a system for observing sea-floor crustal deformation with combining the kinematic GPS positioning and accurate acoustic ranging techniques [Tadokoro et al., 2006, GRL; Ikuta et al., 2008, JGR]. The Suruga-Nankai Trough, Japan, is one of the active plate boundaries where the major subduction earthquakes, Nankai and Tonankai earthquakes, repeatedly occur. The source regions of the earthquakes are located beneath the sea bottom, and it is necessary to monitor the crustal activities including crustal deformation. We have already started monitoring of sea-floor crustal deformation with this system along the Suruga-Nankai Trough. We successfully observed the co-seismic displacement caused by the M7 class off-shore earthquakes occurred close to our monitoring sites. We observed also interseismic horizontal deformation of about 4-5 cm/ yr caused by the plate subduction at five sites installed along the trough. The following integrations are necessary for our system to measure the sea-bottom crustal deformation at multi stations for wide ocean area: shortening the observation period and the time for analysis, and integration of positioning accuracy. We need observation periods of 2-3 days with the present system to average the temporal variation in sound speed structure beneath the sea surface. The desirable observation period for each epoch is half to one day. The positioning accuracies for each epoch, which depends on the condition of sound speed variation, are 1-5 cm in the present status; the accuracy of displacement velocity is 2 cm/yr trough the repeated observation for 2-3 years. We need to integrate the accuracy to 1 cm for each epoch in any condition, and to about 1 cm/yr for displacement velocity estimation through the repeated observation for about one year. We need to estimate and/or correct the spatial variation in sound speed structure to integrate the positioning accuracy with short observation period. It is, therefore, necessary to develop a new system with multi, about five or more, sea-surface acoustic instruments.