Namadu Journal March 2008
Assessment of Bathymetry Effects on Tsunami Propagation in Viet NamNguyen Anh Duong(a)*, Prof. D.Sc. Fumiaki Kimata(b), D.Sc. Irwan Meilano(b)(a) Institute of Geophysics, Vietnamese Academy of Science and Technology, Vietnam (b) Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Japan
Abstract The bathymetry off-coast of East Vietnam varies from region toregion. The distribution of various tsunami heights raises the possibility of significant effects by the focusing on tsunami wave propagation. No experiences were available regarding the propagation and the impact of tsunami along the coast of Vietnam such that potential tsunamigenic sources determined are the deciding factor for tsunami hazard mitigation. The tsunami spreading characteristic in the South China Sea is studied by numerical simulations for tsunami scenarios with different locations of fault source along the Manila Trench in Western Luzon, Philippines. The tsunami spreads over the South China Sea and separates into two ways with a direction towards Vietnam coastal area. The ocean waves focus and resonate each other as they reach gradual bathymetry slope area around 15oN latitude. It is more obvious in scenarios of tsunami sources shifted southward along the trench according to maximum tsunami height maps, tsunami travel time maps and tsunami animations. From the results of this research it is possible to identify the most vulnerable area for the purposes of tsunami hazard mitigations and preparation along the coast of Vietnam. Introduction In consequence of recent tsunami impacts on human life and ecosystem along coastal areas, it has emphasized the need of a better understanding of tsunami characteristics on each specific area. The Sumatra tsunami on 26 December 2004 is one of the major natural disasters of the last centuries which caused extremely damage to the South Asian countries which suffered directly from hazard for which they were unprepared. The 1998 Papua New Guinea tsunami is good example for not understanding the way that tsunami might attack. The tsunami was greater than expected from its earthquake magnitude. That was also surveyed on the June 3, 1994 East Java earthquake tsunami occurred off the southeast coast of Java Island, Indonesia (Tsuji, 1995). That was the consequence of shallow bathymetry influenced on the tsunami behavior (Satake, 1988; Walsh et al., 1999). Tsunami earthquake progression, affected by various factors which were researched, can roughly be separated into three steps: First is the initial tsunami generation related to source, common_______________
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tsunami caused by deep seafloor deformations: movements and shifts. Source orientation is one of the factors which affected arrival time and amplitude of tsunami (Kajiuka, 1972; Satake, 1988). The second step is tsunami propagation from the generation site to the coastline which is affected by ocean bathymetry. The velocity of tsunami in deep ocean is proportional to square root of water depth, so that tsunami reaches the shallower coastal area it begins to slow in speed and build in height, because waves conserve energy and build up until breakpoint in general. Tsunami waves will reach much greater heights and cause the greatest damage on the coastal areas instead of the deep sea. It is the third step that is affected by shore and the resonance of bay or harbor (Murty, 1977 referred to Satake, 1988). The waves reflected from various coastlines and shelf slopes interfere with each other in the later phases (Kajiuka, 1972). Tsunamis generated mainly depend on their size, directivity, and peculiarities of the bathymetry between source and observation point. The anomalous local bathymetry in the middle coastal area of Viet Nam will possibly
N. A. Duong, F. Kimata, and I. Meilano
2 The bathymetry data are made from ETOPO2 provided by National Geophysical Data Center. The study area is showed in figure 1. The grid interval of bathymetry and topography was 2.5km x 2.5km. The time step was 5 seconds used for calculations. There are some off-shore islands located on the eastern coast of Vietnam namely Paracel Islands which serves as a natural barrier for tsunami generated from the Manila Trench. Looking at the bathymetry off-coast of East Vietnam, it is evident that two possible propagation directions will be generated from the two deep ocean ways for tsunamis attacking Vietnam coast. The shallow part is about 2.5km depth located between off-shore of South China and Paracel Islands and the deeper part at 4 km depth is lager and laying on the southern part of Paracel Islands. They are referred as Fronts A and B on the figure 1, respectively. Near shore bathymetry raises gradually up and creates Front A curves upward to coast area spreading from 14o to 16oN latitude. On the other hand, Front B looks like V-shape and its crest pointing towards the central region of the shore of Vietnam (Fig.1). Both Fronts A and B can be illustrated like funnels or conduits for tsunami wave. Near shore bathymetry between 14o and 16oN latitude also exhibits exclusively a shallow dipping compared with surrounding areas. All these factors may amplify tsunami amplitude as it reaches nearer coast line. Therefore, it is anticipated in this Tsunami Numerical Simulation to confirm the focus of off-ocean waves. On the western Luzon, Philippines, the South China Sea sub-plate belongs to Eurasia plate which subducts eastward beneath Luzon creating the Manila Trench (Yu et al., 1999; Sibuet, 2004; Galgana, 2007). Regional GPS and focal mechanism data were utilized to estimate the absorption velocity of the plate convergence showing that the Manila trench is 2-10cm/yr (Galgana, 2007). The western boundary of the Taiwan deformed zone is considered as northward prolongation of Manila trench (Sibuet, 2004). In this study, mainly concern is a segment which extended southward along the western margin of Luzon to Mindoro Island (Fig. 1).
cause the focusing or defocusing of the tsunami waves coming from different directions into a certain place on the shoreline. To date, there is not any scientific confirmation about palaeo-tsunami evidences along shore of Vietnam. Therefore, no experience is available concerning the propagation and the impact of a tsunami wave to the coast. Above all, under the present circumstances of potential earthquake and tsunami activity over the world, it is necessary to consider and understand thoroughly the tsunami characteristics in Viet Nam. We also need to take a judicious look at possible sources of coastal hazards and adopt a concerted approach towards our preparedness. In this paper, confirming the focusing and resonance of tsunami affected by near shore bathymetry of Vietnam and tsunami source location by using numerical simulations for tsunami scenarios will be carried out. The different scenarios will be compared and the impact of tsunami to Vietnam coast will be determined from the maximum wave level along coastline. The objective of the study is also to understand the propagation of a tsunami wave in the South China Sea. The scenarios are going to take place on a distant source, Manila trench in Western Luzon, Philippines. It is also of encouragement for construction of a tsunami warning system and evaluation of tsunami hazard mitigation in Viet Nam. Some papers discuss the historical documented materials of tsunamis for Asian region (e.g. Heck, 1947; Berninghausen, 1969; Nakamura, 1978) the results show there was a tsunami that occurred on this subducting slab. Tsunami Modeling and Data Modeling of propagation and impact tsunami could be applied for near-shore region run-up specifically by using a non-linear shallow water equations (Satake, 1995) and based on the condition of Iwasaki and Mano (1979). The open boundary condition is used at the edge of the computational area (refer Nakamura, 2006). Run-up calculations using a code that is modeled by Nakamura (2006) using the finite-difference method was applied to simulate for the 1771 Yaeyama tsunami, southern Ryukyu islands, Japan.
Assessment of Bathymetry Effects on Tsunami Propagation in Viet Nam
Figure 1. Fronts A and B pointing towards Vietnam coastal area caused bynear shore bathymetry. To the East is location of Manila trench which is the distant source of tsunami. The red star shows the Feb. 14th 1934 earthquake with magnitude of 7.5 on northwest coast of Luzon island which generated tsunami. It will be simulated to estimate how high it might attack the coastline of Viet Nam.
The focal mechanism solutions for shallow focus earthquakes (0-65km) were determined in the western Philippines show Manila trench is thrust faulting and its whole segment from 13o to 22oN latitude is still very active (Bautista et al., 2001). Moreover, Bautista et al., 2001 made nine cross-sections through the trench from 13o to 22oN latitude and the summary shows the slab dip is about from 10o to 45o at shallow depths and it steepens gradually southward. It means that the focal depth gradually deepens from the north to the south. The subduction is being bended as it subducts downward in the depth of 200 km (Bautista et al., 2001; Zhu Jun-Jiang, 2005). The generation mechanism for a tsunami on Manila trench is mainly the static sea-oor uplift caused by abrupt slip at the Eurasia plate / the Philippine plate interface. To determine the fault model of scenarios for Manila trench, earthquake catalogue of Engdahls catalogue (2002) within period of 1900-1963 and U.S.
Geological Survey (USGS) catalogue within period of 1964-July 2007 were used. According to Hamburger et al. (1983), two earthquakes with M7.5 in 1934 and M7.1 in 1948 were detected during the early instrumental years so that their mechanisms are unavailable a