[S-CG67] Ocean area observation to detect crustal activity under the seafloor: Present and future

[SCG67-04] Interplate coupling and strain partitioning along the Nankai Trough estimated from GNSS and GPS-A data

*Takuya NISHIMURA¹, Yusuke Yokota², Keiichi Tadokoro³, Tadafumi Ochi⁴ (1.Disaster Prevention Research Institute, Kyoto University, 2.Hydrographic and Oceanographic Department, Japan Coast Guard, 3.Graduate School of Environmental Studies, Nagoya University, 4.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

Keywords:Interplate coupling, Nankai Trough, GNSS, GPS-A

Southwest Japan is located in the subduction margin between the continental Amurian and oceanic Philippine Sea plates. Recent land GNSS (Global Navigation Satellite System) and offshore GPS (Global Positioning System)-Acoustic geodetic measurements were used to clarify the deformation in and around these plate margins. We examined strain partitioning and interplate coupling using a block modeling approach on the observed velocities. Although the main plate boundary is the Nankai and Sagami Troughs, our results suggest that one-third of the relative plate motion between the two plates is accommodated by several block boundaries in the southeastern margin of the Amurian plate. The most active boundaries, with a slip rate of >8 mm/yr, cross southwest Japan from the Okinawa Trough through the Median Tectonic Line (MTL) and Niigata Kobe Tectonic Zone (NKTZ), to the eastern margin of the Japan Sea. A subparallel boundary with a slip rate of 4–5 mm/yr lies along the coastline of Japan.
These two boundaries have a right-lateral shear motion that accommodates part of the interplate motion, with a boundary across the Korean Peninsula and Japan Sea. The slip partitioning results in an eastward decrease of relative block motion from 78 to 4 mm/yr along the Nankai and Suruga Troughs. Interplate coupling is moderate to strong at 10–25 km depth along the Nankai Trough, but it is lower at ~132ºE, ~136ºE, and ~137ºE than in the surrounding regions, corresponding to the segment boundaries of past megathrust earthquakes, suggesting that regions of insufficient strain accumulation act as a barrier for earthquake rupture.