Interplate coupling along the Nankai Trough, southwest Japan, inferred from inversion analyses of GPS data: Effects of subducting plate geometry and spacing of hypothetical ocean-bottom GPS stations

We estimated the slip-deficit rate distribution on the plate boundary between the subducting Philippine Sea plate and the continental Amurian plate along the Nankai Trough, southwest Japan. Horizontal and vertical displacement rates were calculated from land-based Global Positioning System (GPS) data during the 5-year period from 1 January 2005 to 31 December 2009. We employed an inversion analysis of geodetic data using Akaike's Bayesian information criterion (ABIC), including an indirect prior constraint that slip distribution is smooth to some extent and a direct prior constraint that slip is mainly oriented in the plate-convergent direction. The results show that a large slip deficit exists at depths ranging from 15 to 20km on the plate boundary in a belt-like form. The maximum slip-deficit rate was identified off Shikoku and reached 6cm/year. The slip-deficit rate differed by as much as 1cm/year when using a different geometric model of the subducting plate. On the basis of the spatial distribution of estimation errors and the resolution of the obtained slip-deficit rate on the plate boundary, we also found that the offshore slip-deficit rate cannot be estimated with sufficient accuracy using only land-based GPS data. Therefore, we tested the improvement in results when introducing hypothetical ocean-bottom GPS stations. The stations were arranged in four along-arc and across-arc spacings of 80km and 40km. The ocean-bottom data improved the estimation errors and resolutions, and successful results were obtained for a checkerboard with each square 75km×76km. Our results indicate that 40-km along-arc and across-arc two-dimensional spacing of ocean-bottom GPS stations is required to obtain reliable slip-deficit distributions near the trough axis, assuming the current estimation accuracy for ocean-bottom horizontal displacement rates. ► Developing a new inversion source code, we applied it to the Nankai Trough. ► We find belt-like strongly coupled zone off Shikoku in depths from 15 to 20km. ► We evaluate effects of different geometry models of the PHS plate on the results. ► We investigate how ocean-bottom GPS stations improve accuracy of solutions.