Modeling tsunami hazards from Manila trench to Taiwan
Introduction
The southwest (SW) Taiwan has been considered as a region of tectonic escape because of infrequent intense seismic activities (Lacombe et al., 2001). However, the SW Taiwan is not immune to the attacks of large earthquakes. One piece of the evidence is the Pingtung dual earthquakes on 26 December 2006. Two Mw = 7.0 offshore earthquakes with 8 min offset along with the 40 cm tsunami record highlighted the potential tsunami hazard on SW Taiwan coast.
Motivated by the Pingtung dual earthquakes, we wish to understand the potential devastating tsunami event and the hazard to Taiwan. The epicenters of 2006 dual Pingtung earthquakes is located at the north end of Manila (Luzon) trench, where the Eurasian plate is actively subducting eastward underneath the Luzon volcanic arc on the Philippine Sea plate. The east-dipping is initiated in the Miocene (22–25 Ma) and remains active to the present (Yumul et al., 2003, Queano et al., 2007). Recently, USGS has assessed Manila trench as a high risk zone to be a tsunami source. USGS Tsunami Sources Workshop 2006 (Kirby et al., 2006) further indentified six hypothetical fault planes based on the trench azimuth and the fault geometries (Table 1 and Fig. 1A).
To study the potential devastating hazard to SW Taiwan, the historical tsunami might be able to provide useful information. The 2004 Sumatra–Andaman earthquake with Indian Ocean tsunami is one notorious reference (Titove et al., 2005, Choi et al., 2005). With a similar length to the 1500 km Aceh-Andaman Megathrust, the subduction thrust under Manila trench which has accumulated strain over a 440-year period could become another Megathrust.
In this study, we refer to the three largest earthquakes in history, Global CMT seismic database, as well as the fault parameters issued by USGS, to create the worst-case scenario on the Manila trench and to study the hazard to SW Taiwan. A numerical approach is adopted to perform the simulation. The tsunami initial condition, maximum wave height, arrival time, wave period, and tsunami characteristics in Taiwan water area is addressed and discussed.
Section snippets
Fault parameters of Manila Megathrust
To characterize hypothetical fault planes along the Manila trench in the South China Sea (SCS), three largest historical tsunami earthquakes are referred. The earthquake parameters are shown in Table 2. All three earthquakes have similar length varying from 740 to 1300 km, similar width varying from 200 to 300 km, and similar earthquake magnitude ranging from Mw = 9.0 to 9.5. Using all of the information and referring to the fault geometry, a set of hypothetical fault of Manila Megathrust is nailed
Tsunami propagation model
The numerical approach is applied to this study. The well validated open source code, COMCOT (Cornell Multi-grid Coupled Tsunami Model), is chosen to perform the simulation. The COMCOT model has been used to investigate tsunami events, such as the 1992 Flores Islands (Indonesia) tsunami (Liu et al., 1994, Liu et al., 1995), the 2003 Algeria tsunami (Wang and Liu, 2005), and more recently the 2004 Indian Ocean tsunami (Wang and Liu, 2006). The COMCOT model is capable of solving both linear and
Bathymetry and grid setup
To simulate both far-field and near-field tsunami propagation, four grid layers are adopted and referred as Grids 1, 2, 3A, and 3B (Fig. 3). The grid information as well as the governing equations are shown on Table 5. Finer grid layers, Grids 3A and 3B, are placed in the SW Taiwan to study the tsunami inundation and runup in depth.
Results and discussion
To study the characteristics of the tsunamis in Taiwan, several virtual wave gauges are installed. Fig. 4 shows the virtual gauge location and the time-history free-surface elevation. Time zero denotes the instant when rupture occurs. The gauge record shows that it takes only 20 min for the first tsunami wave to arrive Kenting, a famous tour spot in Taiwan. The wave period is about 25 min at wg_3A_2 and wg_3A_3, and about 20 min at wg_3A_1. The waves merge when they propagate from the deep-water
Conclusion
From the hazard mitigation point of view, we shall clearly understand the potential devastating tsunami hazards. In this study, we create a worst-case scenario of tsunami earthquake excited by Manila Megathrust. The fault parameters are referred to the data issued by USGS, historical tsunamis, and Global CMT. The earthquake magnitude, Mw, is assumed to be 9.35. The total length of the Manila Megathrust is 990 km.
The initial tsunami profile is provided. The maximum initial wave height is 9.3 m (
Acknowledgements
The authors are grateful to Professor Philip L.-F. Liu of Cornell University for providing the latest version of COMCOT and Dr. Xiaoming Wang of Cornell University for providing technical support on using COMCOT.
This work is founded by the National Science Council, Taiwan. Project No: 95-2116-M-008-006-MY3.
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