Strain partitioning and interplate coupling along the northern margin of the Philippine Sea plate, estimated from Global Navigation Satellite System and Global Positioning System-Acoustic data

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 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 boundaries are the Nankai Trough and Sagami trough, 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 and Niigata Kobe tectonic zone, to the eastern margin of the Japan Sea. A subparallel boundary with a slip rate of 4–5 mm/yr is 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 Trough and Suruga trough. Interplate coupling is moderate to strong at 10–25 km depth along the Nankai Trough, but it is lower at ∼132°E, ∼136°E, and ∼137°E 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.