Probabilistic Tsunami Hazard Assessment for interpolate earthquakes along the Ryukyu Trench

We assessed a probabilistic tsunami hazard for interplate earthquakes along the Ryukyu Trench between Kyushu and Taiwan, in which seismological information is quite inadequate because of very less, large earthquakes. We modeled earthquakes as square-shaped "characterized earthquake fault models" (CEFMs) on the analogy of a point source without fault geometry information, and placed a set of the CEFMs on the 3D upper surface of the subducting Philippine Sea plate. We expressed heterogeneity in slip distribution on CEFMs by introducing a large slip area on a fault. Total of 1,071 square-shape CEFMs were constructed for earthquakes with magnitudes from Mw 8.0 to 9.4, using Mo-S (fault area) scaling law with a static stress drop of 3 MPa . We solved a nonlinear long wave equation, using FDM, including runup calculation, over a nesting grid system with a minimum grid size of 50 meters to obtain maximum tsunami height at every coastal point around all islands in the Ryukyu arc. We assumed that earthquakes in the Ryukyu subduction zone are occurred to follow a stationary Poisson process and that frequency of those follows a G-R law with b=0.9 (Japanese standard). Using JMA earthquake catalog from Jan. 1, 1983 to Dec.31, 2010, we derived an annual frequency of 8.15 x 10^-2 for earthquakes in the whole Ryukyu arc (~1200 km in arc length) except the most southern tip of ~100km in arc length where abnormal high seismicity is being observed; we applied different annual frequency of 3.32 x 10^-2 there. Long-term mean hazard assessment suggested that exceedance probability in the next 30 yrs (starting on Jan.1, 2019) for coastal tsunami height to be greater than 3 m showed 30 % or more in the Pacific side of almost all islands. This exceedance probability over 30% is higher than that for subduction-zone earthquakes along the Nankai Trough. It means that islands in the Ryukyu arc are subject to tsunami potential threats, as the same as the Pacific coast of Honshu and Shikoku, Japan facing the Nankai Trough, in the viewpoint of long-term mean hazard.