A Reverse Tracking Method to Analyze the 1867 Keelung Tsunami Event

The 1867 Keelung tsunami is the only tsunami event verified by Taiwan government. This event caused serious damage and hundreds death toll in northern Taiwan, including the Keelung city, Jinshan and Patoutzu areas (Fig. 1). This event is not only recorded in many literatures, but also unveiled by sedimentary evidence. In addition, this event also indicates that the three nuclear power plants nearby are prone to tsunami attacks (Fig. 1). The previous studies consider that this tsunami might be generated by a Mw 7.0 earthquake which might occur along the Shanchiao Fault (Zheng et al, 2011). However, there is no evidence showing the relationship between these geological activities and the tsunami event. In this study, we intend to find the potential tsunami source through numerical analysis. We conducted series of numerical experiments by using sets of fault parameters from Mw 7.0 to Mw 8.0. However, none of them was able to explain the 7 m tsunami height observed in history and the sedimentary evidence found on the Hoping Island. Considering the steep bathymetry and intense volcanic activity along the Keelung coast, one reasonable hypothesis is that the earthquake or volcanic eruption triggered a submarine landslide which increased the tsunami height dramatically. In order to confirm this scenario, we performed the Reverse Tracking Method (RTM), based on the linear hypothesis of tsunami wave propagation, to find the possible locations of the tsunami sources (Fig. 1). The Cornell Multi-grid Coupled Tsunami Model (COMCOT) was then used to perform the tsunami simulations. We followed the Mw 7.0 earthquake proposed by Lin et al. (2006) and added the landslide disturbance (Watts et al., 2005). The source-scaling relationship proposed by Yen and Ma (2011) was used to determine the fault parameters. In addition to the Shanchiao Fault, five submarine volcanos and three submarine canyons were considered as the potential tsunami sources. The result shows that the 1867 tsunami event was most likely triggered by a near-field submarine landslide just outside the Keelung harbor. The potential tsunami sources from Mien-Hwa Canyon and submarine volcanos should also be noted. The result of this study is important not only for densely populated cities in northern Taiwan, but also for the three nuclear power plants nearby. The detailed scenario results will be presented in the full paper. Fig. 1. The map of Reverse Tracking Method (RTM) in northern Taiwan. Black dots show the relative location between Keelung city, Jinshan and Patouzu areas. Red dots present the nuclear power plants (NPP1, NPP2, and NPP4). Green dots present the sedimentary evidence discovered on Hoping Island. Color indicates the maximum flux of tsunami propagation.