Tsunami hazard from the subduction Megathrust of the South China Sea: Part II. Hydrodynamic modeling and possible impact on Singapore

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Abstract

USGS has identified certain section of the Manila (Luzon) trench as a high-risk earthquake zone. This zone is where the Eurasian plate is actively subducting eastward underneath the Luzon volcanic arc on the Philippine Sea plate. An earthquake of magnitude 9.0 could be generated based on the worst case scenario rupture of Manila Trench. In this paper, the possible impact of this worst case scenario rupture on Singapore is investigated using a numerical model. It is found that (1) it takes about 12 h for the tsunami waves generated at Manila Trench to arrive at Singapore coastal waters; (2) the wave period of the tsunami wave, i.e., time interval between two peaks, is about 5 h; (3) the maximum water level rise in Singapore water is about 0.8 m; and (4) the maximum velocity associated with the tsunami waves is about 0.5 m/s, which is not likely to have significant impact on the port operations in Singapore.

Introduction

Coastal areas and shorelines are exposed to potentially damaging effects from severe environmental conditions associated with long waves, such as tsunamis and storm surges, which may lead to wave run-up and landward inundation. After the devastating Indian Ocean tsunami in 2004 (see Titov et al., 2005, Choi et al., 2005, Narayan et al., 2005, for example), tsunami forecast and mitigation have been implemented by many coastal nations in and around the Indian and Pacific Oceans. Economic and long-term social impacts of tsunamis, as shown in the 2004 Asian tsunami, have been devastating (see Yalciner et al. (2005)). The goal of Operational Tsunami Prediction and Assessment System (OTPAS) in Singapore is to build Singapore’s capabilities in earthquake and tsunami modeling and forecasting by integrating cutting-edge scientific knowledge, software, equipment, monitoring stations, infrastructure and telecommunications in an automatic/semi-automatic warning system for Singapore. Recently USGS has identified the Manila (Luzon) trench as a high-risk earthquake zone, where the Eurasian plate is actively subducting eastward underneath the Luzon volcanic arc on the Philippine Sea plate. This subduction zone can also rupture and generate large tsunamis in the future that will have significant impacts on the countries in the South China Sea region. The rupture characterization of Manila Trench and the resulting tsunami waves in South China Sea are reported in Megawati et al. (2009). In this paper, we will focus on the numerical modeling of tsunami waves and the possible impact to Singapore.

The tsunami-induced devastation at any particular location is a function of the velocity, acceleration, and the elevation of water level as tsunami waves interact with natural and man-made coastal structures. The economic costs of strengthening local infrastructures and evacuating coastal areas are very high. More importantly, severe tsunamis and storm surges can cause significant losses in human lives. Singapore also has in its southern coastline, an estuarine reservoir – the Marina Bay, which is isolated from the sea by a tidal barrage with gates. Large waves overtopping the tidal barrage/gate could also set up other shock waves which could propagate deeper inland. The impact of the return waves could be equally devastating. Furthermore, the current associated with the tsunami waves could cause collision of ships mooring in the harbor. In this study, we will focus on the possible impact of the tsunami waves generated by the worst case scenario rupture at Manila Trench (see Megawati et al. (2009)), which could trigger an earthquake of magnitude 9.0. A clear understanding of the behavior of tsunami waves in Singapore coastal water is critical in developing appropriate warning system and evacuation strategies for Singapore.

Although field measurements of run-up of several recent tsunamis exist, they are insufficient because of the nature of after-the-event field surveys. Very little information about temporal variations can be obtained (except tide gauge data) and the data are often extremely spatially sparse. Furthermore, the source of tsunami generation cannot be accurately specified, since any information in deep water is difficult to obtain. Physical experimentation is valuable but it is costly and slow, and requires high-resolution, real-time capture of multidimensional data. It is also ephemeral, in that there is only one brief opportunity to capture suitable data for a particular run. Numerical experiments offer an attractive alternative. An excellent recent review on tsunami simulation can be found in Gisler (2008). One recent experimental study of non-breaking tsunami wave run-up on a plan beach is reported in Gedik et al. (2005). One breaking criterion for solitary waves on slopes was given by Grilli et al. (1997). A good discussion on breaking tsunami run-up was provided by Heller et al., 2005, Li and Raichlen, 2003. In this paper, we shall use a numerical model to investigate the characteristics of the tsunami waves in Singapore coastal water.

The outline of this paper is as follows: Section 2 briefly introduces the numerical model for studying tsunami waves in Singapore coastal water; Section 3 discusses the compilation of the geometrical data for the study; Section 4 presents and discusses the main results, and finally Section 5 summaries the main conclusions from this study.

Section snippets

Numerical model

Recently, as a result of a strong effort by the tsunami community, several two- and three-dimensional numerical models have been developed to study the generation and propagation of tsunami waves, and to quantify the interactions of tsunamis with shorelines. Example models include MOST described by Titov et al., 1997, Tang et al., 2006, AnuGA described by Nielsen et al. (2005), COMCOT (Cornell Multi-grid Coupled Tsunami Model) described by COMCOT User Manual, 2007, Wang and Liu, 2006,

Bathymetrical and topographical data

Bathymetrical data for the region outside of Singapore was derived from a public archive, ETOPO2, which is freely available from National Geophysical Data Center (NGDC), National Oceanic Atmospheric Administration (NOAA). ETOPO2 data has visible artifacts due to imperfect data integration. It is necessary to smooth the bottom bathymetrical data by removing rectangular artifacts so that they would not cause anomalous wave reflections. The bathymetrical data in Singapore water was complied from

Results and discussion

Fig. 4 shows the time history of water levels recorded at three virtual wave gages shown in Fig. 3. The time zero denotes the instant when the rupture at Manila Trench occurs. It can be seen that it takes about 12 h for the tsunami waves to arrive at Singapore after traveling over a distance of 8000 km. This gives Singapore enough time to issue a warning and prepare evacuation if necessary. The maximum water level rise for the worst case scenario studied here is about 0.8 m at Marina Bay, about 0.7

Conclusions

In this study, the worse case scenario (earthquake magnitude Mw = 9.0 at Manila Trench) is studied numerically. It takes about 12 h for the first peak of the tsunami waves to travel a distance of 8000 km from Manila Trench to Singapore. This indicates that Singapore has plenty of time to be prepared for the tsunami generated by the rupture of Manila Trench. The maximum water level rise recorded by the virtual wave gauges is 0.8 m in Singapore water for the worst case scenario reported herein. The

Acknowledgement

The authors are grateful to Professor Philip L.-F. Liu of Cornell University, USA, for providing the latest version of COMCOT software package and suggestions on monitoring the wave period evolution along the path of wave propagation. Two anonymous reviewers are acknowledged for their constructive comments, which have greatly improved the quality of this paper. This work is funded by the National Environmental Agency of Singapore. The opinions described in this paper do not necessarily reflect

References (21)

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