Numerical Study of Potential Extreme Tsunami Hazard in South China Sea

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Procedia Engineering 126 ( 2015 ) 332 – 338

7th International Conference on Fluid Mechanics, ICFM7 Numerical study of potential extreme tsunami hazard in South China Sea

Zhi-Yuan Rena, Hua Liua, *

aMOE Key Laboratory of Hydrodynamics, School of NAOCE, Shanghai Jiao Tong University, Shanghai, 200240, China

Abstract

In recent one decade, after two extreme tsunamis have produced devastating damage around the Indian Ocean and Pacific Ocean, it is necessary to investigate the tsunami hazard triggered by the worst case scenario in South China Sea, while the Manila Trench is becoming the most tsunami-hazardous subduction zone. In this study, the tsunami source of a new worst case scenario (Mw=9.3) is used for hazards assessment. The Okada model is taken to generate tsunami from Manila Trench based on the tsunami source parameters, while the shallow water equations are used to simulate the tsunami propagation from earthquake source to far-field region around South China Sea. The time series of surface elevation at measured locations near important cities along coasts and islands are investigated. The tsunami propagation scenarios, the arrival time, and tsunami wave distribution are obtained and analyzed for the assessment of tsunami hazard near coasts around South China Sea. Nested finer grid is used to simulate tsunami propagation to Hainan Island, Taiwan Island, and Lingding Bay for the worst case scenario. The regions with high level hazard are identified. ©© 2015 2015 The The Authors. Authors. Published Published by byElsevier Elsevier Ltd. Ltd This. is an open access article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of The Chinese). Society of Theoretical and Applied Mechanics (CSTAM). Peer-review under responsibility of The Chinese Society of Theoretical and Applied Mechanics (CSTAM) Keywords: Extreme tsunami; South China Sea; Manila Trench; Shallow water equations.

1. Introduction

Considering 2004 Indian Ocean Tsunami and 2011 Japan Tohoku Tsunami have produced devastating damage to coasts, it is necessary to investigate the tsunami hazard triggered by the worst case scenario in South China Sea. The Manila subduction zone, Ryukyu subduction zone and N. Sulawesi subduction zone are proposed as potential tsunami sources by Kirby [1]. The Manila subduction zone is regarded to be a highly hazardous tsunamigenic

* Corresponding author. Tel.:+86-21-34204295; fax:+86-21-34204472. E-mail address: [email protected]

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The Chinese Society of Theoretical and Applied Mechanics (CSTAM) doi: 10.1016/j.proeng.2015.11.201 Zhi-Yuan Ren and Hua Liu / Procedia Engineering 126 ( 2015 ) 332 – 338 333 earthquake source, since the Eurasian Plate subducts the Philippine Sea Plate at a speed of 70 mm/year. More work has been done in numerical simulation of local tsunamis in China seas using the Boussinesq equations [2] and understanding the propagation and deformation of the potential tsunamis over a continental shelf with a gentle slope [3]. Following the USGS Workshop’s outputs on six hypothetical fault planes along Manila Trench based on the trench azimuth, the width of sources based on empirical formula to compose the source parameters of Mw=8.0 was proposed by [4]. We can also find a source model of Mw=9.35 for Manila Trench, due to the fault geometry of three largest historical tsunami in [5]. Considering ten seismic cross section and GPS data, the Manila Trench is divided into 33 rectangular elements for a source model [6]. A recently developed worst case source model along Manila Trench has been used in this study, which is developed by [7]. They incorporated the advantages of two former worst case tsunami source derived by [5] and [6]. Geometrically, this new source model represents Manila Trench in terms of six segments, with the parameters modified to fit the characteristic of Manila Trench and the dislocation of each segment is assigned with the slide values defined by [5]. The parameters of this worst case source are listed in Table 1. The shallow water equations are used for assessment of extreme tsunami hazard triggered by the worst case scenario source (Mw=9.3) in the present study.

Table 1. Fault parameters of worst case scenario [7] Fault Lon. Lat. Length Width Strike Dip Rake Slip (deg.) (deg.) (km.) (km.) (deg.) (deg.) (deg.) (m) 1 120.5 20.2 190 120 354 10 90 25 2 119.8 18.7 250 160 22 20 90 40 3 119.3 17.0 220 160 2 28 90 40 4 119.2 15.1 170 90 356 20 90 28 5 119.6 13.7 140 110 344 22 90 12 6 120.5 12.9 95 80 331 26 90 5

2. Numerical model

The Okada model [8] is used to calculate the seafloor deformation based on earthquake source parameters. The numerical model (GeoClaw) [9] is based on the nonlinear shallow water equations, taking into account the nonlinear effect of the tsunami wave propagation in coastal oceans. This model has been validated by [10,11]. The depth averaged continuity equation and momentum equations read: wwh w hu hv 0 (1) wwtx wy

ww§·221 w wb hu¨¸ hu gh huv gh W x (2) wwtx©¹2 wy w x

ww w§·221 wb hv huv¨¸ hv gh gh W y (3) wwtx wy©¹2 wy in which, hxyt ,, is total water depth, t is time, bxy , is the bottom elevation describing real bathymetry, uxyt ,, and vxyt ,, are the depth-averaged velocities in x and y directions respectively. The gravitational acceleration is denoted by g . The components of the non-linear bottom friction term in x and y direction are: 2 gn 22 °W x 7/3 hu hu hv ° h ® 2 gn 22 (4) °W hv hu hv ¯° y h7/3 where n is the Manning coefficient, representing the roughness of the bottom. 334 Zhi-Yuan Ren and Hua Liu / Procedia Engineering 126 ( 2015 ) 332 – 338

The finite volume method is used to discretize the governing equations, adding additional terms to prevent numerical oscillations and reaches second order accuracy. The adaptive mesh refinement method (AMR) is implemented in this numerical model. This method firstly simulates the tsunami on coarse mesh for preliminary results and, if wave heights in some region exceed a threshold value, discretizes this region with a finer mesh, and then updates the results. This study will use uniform grid resolution, but this method can help to achieve coupled grid.

3. Numerical results and analysis

3.1. Computational domain

In this study, the bathymetry data from ETOPO1 with one minute resolution is used, while the computational domain ranges from 99E to 130E, and 1S to 33N, shown in Fig. 1, denotes by grid 1. Three areas with black line frame denote coupled grids in the simulation with bathymetry resolution of 0.5 minute, including Hainai Island, Taiwan Island, and Lingding Bay, denotes by grid 2. Seven locations near important cities along China Coast (Xiamen, Shantou, Hong Kong, Zhanjiang, Sanya, Kaohsiung, and Kenting), three locations near islands in South China Sea (Yongshu Island, Huangyan Island, and Zhongsha Island), and other three locations (Quy Nhon, Brunei, and Manila) are selected to investigate the tsunami wave along the coast in South China Sea. The grid resolution of grid 1 is 1 minute, while grid 2 is 0.25 minute. The Manning coefficient is 0.024 in the region for the computation.

Xiamen 25 Shantou Hong Kong Kaohsiung -500 Zhanjiang Kenting 20 -2000

Sanya Zhongsha Island -4000 15 Huangyan Island Manila Quy Nhon

Latitude (degree) -6000 10 Yongshu Island -8000

5 -10000 Brunei

0 100 105 110 115 120 125 130 Longitude (degree) Fig. 1. Computational domain and measured locations

3.2. Tsunami source

The worst case scenario used in this study is developed by Nguyen et al. (2014). This source model divides Manila Trench into six plates. Every plate is calculated by Okada model, then the seafloor deformation triggered every plate has been added to be the total seafloor deformation, as shown in Fig. 2. It starts from Bashi Channel to middle of Philippines, with length around 1000km and width 400km. The uplift part locates on west, while subsidence is on east.

Zhi-Yuan Ren and Hua Liu / Procedia Engineering 126 ( 2015 ) 332 – 338 335

30 20 18 25 16 14 12 20 10 8 6 15 4 2 Latitude (degree) 0 10 -2 -4 -6 5 -8

0 100 105 110 115 120 125 130 Longitude (degree) Fig. 2. Seafloor deformation

3.3. Tsunami hazard in South China Sea

The tsunami propagation scenarios in six hours are described in Fig. 3. After the earthquake, the tsunami wave spread to South China Sea quickly, while some waves of tsunami propagate into Pacific Ocean. At time 2 h after earthquake, the tsunami will affect most region of South China Sea. Tsunami spreads into Gulf of Thailand, but has less influence.

Fig. 3. Tsunami propagation scenarios in South China Sea

Fig. 4 presents time series of surface elevation at measured locations. The tsunami will attack most China coasts in around 2 h. The maximum wave amplitude near Hong Kong is 8.2 m, while Xiamen, Shantou, Zhanjiang, and Sanya endure the highest wave are 2.31 m, 4.99 m, 4.22 m, 5.39 m, respectively. Due to the close distance to 336 Zhi-Yuan Ren and Hua Liu / Procedia Engineering 126 ( 2015 ) 332 – 338

tsunami source, Taiwan will be affected by tsunami in a very short time. The tsunami waves exceed 5 m. Although Quy Nhon is far-field for the tsunami source, the maximum tsunami wave is measured 6.18 m. The waveform at some points looks like N-wave, such as Xiamen and Hong Kong. Some are irregular N-wave (Sanya, Kaohsiung, etc.).

3 6 10 Xiamen Shantou Hong Kong Zhanjiang 2 8 4 4 6 1 4 2 2 0 2 0 0 -1 0 -2 -2 -2 -4 Surface elevation (m) Surface elevation (m) Surface elevation (m) Surface elevation (m) -2 0246810 0246810 0246810 0246810

Time (hr) Time (hr) Time (hr) Time (hr)

12 8 6 Sanya Yongshu Zhongsha 6 Kaohsiung 2 10 4 4 8 2 2 6 1 0 4 0 -2 2 -2 0 -4 0 -6 -4 -2 Surface elevation (m) Surface elevation (m) Surface elevation (m) Surface elevation (m) -1 -8 0246810 0246810 0246810 0246810 Time (hr) Time (hr) Time (hr) Time (hr)

8 4 8 Kenting Quy Nhon Brunei 8 Manila ) 6 6 6 m ( 4 2 4 2 4 on 2 i 0 2 0

0 evat -2

l 0 -4 -2 -2 -6 -2 -4 ace e f -8 -4

-6 ur Surface elevation (m) S -10 Surface elevation (m) -4 Surface elevation (m) -6 0246810 0246810 0246810 0246810 Time (hr) Time (hr) Time (hr) Time (hr) Fig. 4. Surface elevation at measured locations

Fig. 5 illustrates the contours of maximum wave distribution and arrival times. It can be found that tsunami wave arrive in south Taiwan and Philippines within 0.5 h, reach Hainan at 2 h, and reach south China Coast between 2 h to 3 h. Vietnam receives tsunami within 2 h. The most energy of tsunami wave distributes in north part of South China Sea. Therefore, the most tsunami-hazardous regions are southeast of Hainan, Guangdong, Taiwan, Philippines, and central Vietnam.

Fig. 5. Contours of maximum amplitude distribution and arrival times

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3.4. Impact on Hainan Island, Taiwan Island, and Lingding Bay

Nested finer grid with high resolution topography data is used to study the impact of potential tsunami from Manila Trench on Hainan Island, Taiwan Island, and Lingding Bay. The resolution of nested grid and topography data is 0.5 min and 0.25 min, respectively. The simulation results in terms of maximum amplitude and arrival times are presented in Fig. 6. The tsunami wave arrives in Hainan Island in 2 h, attacks most southeast coasts in 3 h. The tsunami produces more impact on southeast coasts of Hainan Island, while maximum wave height of nearby sea is 5m. The wave height exceeds 8m in some regions. The tsunami has less influence on Beibu Gulf and Qiongzhou Strait, due to effect of Hainan Island. Because of closer distance to tsunami source, the tsunami arrives in Taiwan Island in less than 0.5 h. The south and east of Taiwan Island endure more risks. The maximum wave amplitude from Tainan to Kenting exceeds 5m. The north of Bashi Channel and south of Taiwan Strait own wave height of 3 m, while maximum wave in other regions is 1 m. The wave height outside Lingding Bay exceed 5 m, however, due to the obstruction of numbers of islands, it is hard for tsunami to propagate into Lingding Bay.

Fig. 6. Contours of maximum amplitude distribution and arrival times at Hainan Island, Taiwan Island, and Lingding Bay (From left to right)

4. Concluding remarks

As an aid to assess the extreme tsunami hazard in South China Sea, the nonlinear shallow water equations are used to simulate the potential tsunami source of Mw=9.3 from Manila Trench. The numerical results show that the most energy of tsunami wave distributes in north part of South China Sea. South China Coast, Hainan Island, and Vietnam have high tsunami hazard level, and the tsunami will arrive in 2 h. South Taiwan Province is also a high tsunami hazard, and may receive tsunami in less than 0.5 h. Finer nested grids are used to analyze the impact of tsunami on Hainan Island, Taiwan Island, and Lingding Bay. The southeast coasts of Hainan Island and the south and east of Taiwan Island endure more tsunami hazard, while Lingding Bay receives less influence. Considering the lesson from recent giant earthquake tsunami, it is important to consider such extreme events. The tsunami warning system and technique should be developed and applied using the tsunami buoy technology [12] or the magnetic anomaly measurement technology [13].

Acknowledgements

The project was supported by National Natural Science Foundation of China (Grant No. 51379123, 11202130), the Natural Science Foundation of Shanghai Municipality (Grant No. 14DZ1205203) and the State Key Laboratory of Ocean Engineering (Grant No. GKZD010063, SJTU). 338 Zhi-Yuan Ren and Hua Liu / Procedia Engineering 126 ( 2015 ) 332 – 338

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