Numerical simulation of the 1771 Yaeyama tsunami in the southwestern Ryukyu arc

Numerical simulation of 1771 Yaeyama tsunami was computed and showed that M7-class earthquake would cause very high run-up locally without landslide. The 1771 Yaeyama tsunami occurred on April 24, 1771 in southwestern Ryukyu area, Japan. Maximum run-up heights reached over 30 m. About 12000 people were dead by the tsunami. Previous tsunami source models for this event have included both seismological faults along the Ryukyu Trench and submarine landslides. However, no evidence of landslides in the source has been obtained, despite marine surveying of the area. The fault along the Ryukyu trench also cannot have reproduced the distribution of run-up heights. Recently, CMT solutions of earthquakes and GPS measurement showed that the Ryukyu arc was extending parallel to the Ryukyu Trench. This suggests that the normal faulting cutting the island arc would be possible source for tsunami. Although the arc-cutting normal faults had been observed previously, they had not been examined as a source of the tsunami. Then the numerical tsunami simulation was employed and the possibility of the source of the 1771 Yaeyama tsunami was investigated. The tsunami propagation was computed using nonlinear shallow-water equations. The area for computation extended from 23.7N to 25.5N, and from 123.7E to 125.75E. The bathymetry and topography data were gridded at 75 m intervals over the entire area, and a finer grid interval of 25 m was employed for six areas for detailed analysis. The time interval for computation was 0.1 s. The model was computed for a wave travel time of two hours. Assumed fault length is 50 km (Mw7.6). As a result, the arc-cutting normal fault model proposed in this study successfully reproduced the distribution of tsunami run-up. Since the fault is located at the shallow area (about 300m in water depth), the tsunami waves refract at the shelf and concentrate locally to coast of islands. The arc-cutting normal fault and focusing effect would have caused very high run-up height in this area.