Stress accumulation rate on source faults around the junction of Ryukyu and Southwest Japan arcs using finite element model

Akinori Hashima, Hiroshi Sato, Tatsuya Ishiyama, Andrew Freed, & Thorsten W. Becker

Published August 15, 2018, SCEC Contribution #8622, 2018 SCEC Annual Meeting Poster #120

The Kyushu island of Japan is located at the hinge of the Ryukyu and the Southwest Japan arcs, which are formed by the subduction of the Philippine Sea plate along the Ryukyu trench and the Nankai trough, respectively. The tectonics of the island gained attention by the occurrence of the 2016 Kumamoto earthquake (Mw 7.1). To understand why this earthquake was triggered and assess the risk of future crustal earthquakes in this region, it is necessary to model stress loading on the source faults due to the dominant subduction system, which can be well constrained by the regional GPS data. Several studies obtained slip-rate deficit under the Nankai Trough. However, little attention has been given to the retreat of the Ryukyu trench.

In the present study, we developed a 3D finite element model (FEM) to quantify the evolution of plate stresses from the Ryukyu trench to the Nankai trough based on our previous model. The slip region along the Ryukyu trench–Nankai trough is divided into 8 x 27 subfaults where unit slip responses are calculated for input into an inversion analysis. Since the interseismic deformation includes the effect of the concurrent viscous relaxation in the asthenosphere, we used responses after complete relaxation of unit slips. For crustal deformation data, we used 10-year time series before the 2011 Tohoku-oki earthquake at 453 stations from the daily coordinates by GSI of Japan. We revised the source fault model by the Headquarters for Earthquake Research Promotion based on recent geophysical and geological data and added new faults especially in the Sea of Japan.

Our interseismic inversion suggests a 4-8 cm/year slip-rate deficit along the Nankai trough and ~4 cm/year slip-rate excess along the Ryukyu trench. While the slip-rate deficit along the Nankai trough is similar to the previous studies, the slip-rate excess along the Ryukyu trench is consistent with slab rollback at the Ryukyu trench and necessary to explain the characteristic anti-clockwise motion on the Kyushu island. Using the slip distribution, we can also calculate the evolution of the crustal stress field. Coulomb stress is positive in the western part of the EW trending Median Tectonic Line, the source faults in the Sea of Japan, and those in Kyushu associated with historic earthquakes. In particular, positive Coulomb stress on the source faults of the 2016 Kumamoto earthquake and the other M7 earthquakes is consistent with their occurrence.

Key Words
Ryukyu, Southwest Japan, Source fault, slip deficit, Finite element model

Citation
Hashima, A., Sato, H., Ishiyama, T., Freed, A., & Becker, T. W. (2018, 08). Stress accumulation rate on source faults around the junction of Ryukyu and Southwest Japan arcs using finite element model. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy