土木学会論文集B2（海岸工学），Vol. 72, No. 2, I_331─I_336, 2016.

Influence of Small Islands against Tsunami Wave Impact along Island

Teuku Muhammad RASYIF1, Shigeru KATO2, SYAMSIDIK3, and Takumi OKABE4

1Reserach Student, Dept. of Architecture and Civil Eng., Toyohashi University of Technology (1-1 Tempaku-cho, Toyohashi, Aichi 441-8580, Japan) E-mail:[email protected] 2Member of JSCE, Professor, Dept. of Architecture and Civil Eng., Toyohashi University of Technology (1-1 Tempaku-cho, Toyohashi, Aichi 441-8580, Japan) E-mail:[email protected] 3Lecturer at Civil Engineering Department and Researcher at Tsunami Computation and Visualization Laboratory of Tsunami and Disaster Mitigation Research Center (TDMRC), Syiah Kuala University (jl. Prof. Dr. Ibrahim Hasan, Gampong Pie, Banda , 23233, ) E-mail:[email protected] 4Member of JSCE,Assistant Professor, Dept. of Architecture and Civil Eng., Toyohashi University of Technology (1-1 Tempaku-cho, Toyohashi, Aichi 441-8580, Japan) E-mail: [email protected]

The big earthquake at Sumatra subduction zone has become yearly event in Indonesia since the Earthquake on 2004. The last earthquake which has been occurred near Mentawai archipelago on March 2, 2016 caused panic at the several big cities such as Padang and Meulaboh. Seismic gap in Sumatera subduction zone still has energy to cause tsunamigenic earthquake. Then the west coast of Sumatera Island has been at high risk for tsunami disaster. However, some cities, such as Tapaktuan, were not damaged by 2004 tsunami and others after 2004. These cities locate behind small islands. Therefore many residents believe that the islands will protect the cities against tsunami. Disaster management and education in these cities are not enough because of no damage experience. However, the positional relationship between small islands and a rupture area will result in different impact to the coast along Sumatra Island. In this paper, the possibility of tsunami disaster in the cities which were not damaged by the past tsunami is investigated from the viewpoint of influence of small islands against tsunami propagation. The COMCOT model was used for tsunami simulation in this study. Firstly, the numerical simulations of 2004, 2005 and 2012 tsunami were carried out to investigate the tsunami height along the northern part of Sumatra Island. Secondly, the several hypothetical fault conditions are implemented as future tsunami scenarios to investigate the possibility of tsunami damage at Tapaktuan. Small changes of the location of epicenter in future tsunami scenarios will result in different tsunami impact along Sumatra Island due to the influence of small islands. This result indicates that it will be dispensable for the appropriate disaster management and education to grasp the function of small islands along Sumatra Island about future scenarios.

Key Words : Small island, tsunami wave, COMCOT

1. INTRODUCTION around the Indian Ocean. Less than 3 months after the big tsunami in 2004, an earthquake occurrence The big earthquake that generates destructive does not start to stop. The stress given by the great tsunami was occurred on December, 26th 2004 with earthquake on 2004 activated an adjacent segment of magnitude about Mw= 9.1. This quake was occurred the fault on 2005. In just a decade, about seven big after about 100 year since the big earthquake in 1907 tsunamigenic earthquakes with magnitude over Mw at the Sumatra subduction zone. Along that time, the > 7.5 were occurred along Sumatra subduction zone quake with magnitude Mw > 7.5 did not occurred. such as the and Simeulu islands tsunami on Less information and education regarding this mega March 21, 2005; the Mentawai island tsunami on disaster had caused massive damage in coastal area October 25, 2010; on November 4, 2012 which is 1) and lot of casualties in fourteen different countries also known as the biggest strike-slip earthquake .

I_331

The earthquakes that occurred at Sumatra tsunami5). The fault model based on Okada theory6) is subduction zone itself caused different impact along available in this model to generate the tsunami wave the Sumatra coastal area, especially at the western by assumption that the sea surface will deform cities of Aceh namely: Meulaboh, Tapaktuan, and following the sea floor motion. In order to obtain Singkil. The tsunami wave in 2004 and 2005 did not detail result of simulation at the specific coastal area, give significant damage on Tapaktuan city. This the model implements the multi grid system. The condition caused difficulty to carry out any disaster multi grid system allows the simulation using mitigation program such as prepares an escape route, different grid size and time step with ratio up to 10 in an escape building, etc in order to prevent the different sub-region. The tsunami wave is solved by tsunami hazard at Tapaktuan city2). Minor mitigation numerical method of shallow water equation. Both of activity from the local government can decrease the linear and nonlinear shallow water equations are awareness and preparedness of local community at possible to combine simultaneously. It is also Tapaktuan toward the tsunami disaster. A study to possible for this model to incorporate Spherical and understand the impact of the tsunami wave at the Cartesian coordinate systems in the simulation. The mainland due to existing of small islands is necessary set of LSWE (linear shallow water equation) in to be resolved. In order to achieve this goal, the COMCOT is as follows. The details of equations can numerical simulation method was applied in this be seen in the COMCOT manual7). study to estimate the tsunami wave height at the coastal area, which is directly located behind the ∂ζ 1  ∂P ∂  +  + ()ϕQ  = 0cos (1) small islands. ∂ Rt cos ∂ ∂ψψϕ  The numerical experiment has been extensively ∂P gh ∂ζ applied to understand the tsunami wave impact, in + fQ =− 0 (2) ∂t R cosϕ ∂ψ particular with regard toward a small island. ∂Q gh ∂ζ However, little research to date has discussed on the + fP =+ 0 (3) influence of an isle group especially an island with a ∂t R ∂ϕ long shape such as the Simeulu Island located in front of Tapaktuan toward the tsunami wave. The where ζ is free surface elevation; R is radius of earth; recent researches showed that the small offshore φ and ψ are latitude and longitude of the earth; P and island can amplify the wave energy to the mainland. Q are volume fluxes in West-East and South-North This condition was proven in the case of the direction; g is the gravitational acceleration; h is still Mentawai Island tsunami in 20103). The aim of the water; and f represents the Coriolis force coefficient. present study is at performing a comprehensive investigation of the effects of the small island on the tsunami wave height at the mainland which is 3. STUDY AREA AND MODEL SETUP directly protected by the isle. Past tsunami events and hypothetical events are implemented in this (1) Study area research. The results of this study will be useful as The western coast of Aceh, especially at three basic information in order to inform local community cities namely Meulaboh, Tapaktuan, and Singkil, is to be aware and be prepared for future hazards. used as a case study to investigate the influence of Furthermore, the local government is expected to use small islands against the tsunami wave. This area is these results for disaster risk reduction around their located inside the Sumatra subduction area. The area coastal area. had experienced the tsunami disaster in 2004 and 2005. Although this area has potential to be exposed the tsunami attack, several cities in this area 2. TSUNAMI MODEL experienced the different tsunami impact. For instance, Tapaktuan city, which is located nearby the Cornell multi-grid coupled tsunami model earthquake epicenter as shown in Fig.1a, received (COMCOT) was used to perform the tsunami insignificant damage by the tsunami wave compared numerical simulation in this study. COMCOT is the to other cities along it. model that is capable to simulate the tsunami wave These cities have a unique morphological from its generation to inundation. This model has condition that able to affect the tsunami wave. There been benchmarked with analytical, laboratory, and are the presence of a small island, bathymetric field measurement data for validation4). This model profile, and dimension offshore island, as shown in was used to investigate the 2004 Indian Ocean Fig.1. Meulaboh city is facing directly the Indian

I_332

Ocean. On the other hand, Tapaktuan and Singkil used in this study. The magnitude of the hypothetical cities are obstructed by islands, namely Simeulu and conditions is Mw=8.5. The dimension of estimated Banyak Islands. The both islands have different fault plane is 80 km wide and 100 km long by using dimensions. The length of Simeulu Island is about 98 an analysis of relationship between a magnitude and km, and that of Banyak Island is about 37 km. a rupture dimension11) for the hypothetical cases. The Meulaboh and Singkil have a flat slope in a coastal difference between these cases is on the epicenter region. In other hand, Tapaktuan has a steep slope in location. The epicenters of last earthquake events front of a coast. Therefore, this bathymetric feature around Simeulu Island were used for A1, B1, and C1. can result in an amplification and a reduction of The location of case A1 is the same as 2005 event. tsunami wave in this area. Case B1 is located on the slightly northern side of the 2005 tsunami case. It is the same location of 2010 earthquake which did not cause high tsunami wave along Sumatra Island because of small magnitude. Case C1 is located on off Simeulu Island as same as the quake in 2002.

Fig.2 Initial condition for past tsunami in 2004, 2005 and 2012.

Fig.1 (a) Study area and recent large ruptures and (b) bathymetry (3) Grid setup and input data profile from land to sea. Multi layers were applied in this study. The biggest layer of computation domain, which covers (2) Fault model Meulaboh, Tapaktuan, and Singkil cities, is from Focal mechanism of the real and hypothetical 89.15E to 102.80E and from 3.80S to 7.45N as tsunami events is used in this study to investigate the shown in Fig.3. The GEBCO digital bathymetry influence of the small island on the city behind of it. /topography with resolution 0.5 min and the nautical Firstly, the numerical simulations of 2004, 2005 and chart data with scale 1:100000 were used to prepare 2012 tsunami were carried out to investigate the the computational domain for all grids. The several tsunami height along the northern part of Sumatra grid sizes from 1851 to 124m were applied in the Island. Several researchers proposed a multi-fault simulation. In order to resize the data resolution, model to represent the complex rupture in 2004 and Generic Mapping Tools (GMT) program was used to 2005 based on the analysis of the satellite altimetry extract and interpolate the data. Details of grid setup and tide gauge data. The multi fault parameters were in this study can be seen in Table 1. In order to applied to generate the tsunami wave in 20048) and 9) obtain the maximum tsunami wave height, several 2005 . The initial surface elevation of each event is observation points are put along the shoreline. shown in Fig.2. The magnitude of each tsunami is Mw=9.0, 8.6, and 8.6, respectively. The source of tsunami in 2005 was nearest to Tapaktuan in three cases. In order to prevent future hazard at Tapaktuan city, three hypothetical scenarios were used in this study namely case A1, B1, and C1. Based on the United State Geological Survey (USGS) data, the past big earthquake that occurred in Sumatra subduction zone 10) had similar focal mechanism . For example, the values of strike, slip, and dip are about 329, 7, and Fig.3 Simulation area around the Sumatra Island (Layer1) and a 110, respectively. Therefore, these parameters are small island (layer3).

I_333

Table 1 Model parameters of nested grid employed in the blue color, were propagated to Singkil city through numerical simulations. Banyak Island. The wave was divided by Banyak Layer01 Layer02 Layer31 Island and caused the wave propagated to the right 89.15E-102.8 94.82E-98.38 95.01E-95.52 Lat(º) and left sides of the island. After the wave passed E E E through the island, the wave that comes from the Long(º) 3.8S-7.45N 0.33N-4.56N 4.72N-5.21N both sides of the island focused at the behind of Grid 12) 1851 371 123.667 size (m) island . Then, the focused wave caused the higher Coordinat run-up on Singkil city that located just behind the e Spherical Spherical Spherical isle, as shown in Fig.6b. System SWE Linear Linear Nonlinear

4. RESULT AND DISCUSSIONS

(1) Case of tsunami in 2004 Fig.4 shows the tsunami propagation process during the Indian Ocean Tsunami on 2004. Meulaboh was a nearest city to the earthquake’s epicenter with distance 94 km in 2004. Without the present of a small island in the front, Meulaboh city was directly facing the fault area in 2004. In case of parameters used to generate the tsunami in 2004, the fault area is extended from the northern of Simeulu Island. Therefore, the tsunami wave did not come Fig.4 Snapshots of tsunami propagation process in 2004 at: directly in front of the Simeulu Island. The tsunami (a)t=14 mins, (b)t=33 mins, and (c) t=51 mins. wave reaches to the Tapaktuan coast after propagation through the strait between the Simeulu and Sumatra Island, as shown in Fig.4c. The tsunami wave trapped behind the island arrived at Tapaktuan. Therefore, the amplification process by the island did not happened. According to in-depth interview with local people around Tapaktuan, they witnessed that the tsunami wave inundated to the land. However, that tsunami did not cause massive damage at Tapaktuan. They also said that the wave was similar to storm waves which could lift up fishing boats. The tsunami wave energy was reduced during propagating toward the Singkil city. The tsunami also did not cause massive damage at Singkil and the Banyak Island which located in front of the city. As shown in Fig.5a, tsunami wave was reach bellow 3 m at Tapaktuan and Singkil. Fig.5 The maximum tsunami wave height along the southwestern coast of Aceh for tsunami case in: (a) 2004; (b) 2005; (2) Case of tsunami in 2005 and (c) 2012. After the tsunami in 2004, another earthquake occurred in Sumatra subduction zone near , On the other hand, the amplified wave due to Nias, and Banyak Islands with magnitude Mw = 8.6 focused by the Simeulue Island did not occur at on March 28, 2005. Tsunami was generated due to Tapaktuan city. The initial condition showed that the earthquake movement, which caused dislocation tsunami wave was triggered from northwest of Nias with about 12.3 m. Firstly tsunami propagated to Island to northwest of Simeulue Island. The tsunami Simeulue and Nias Islands about 14 min after the waves from the northwest of Simeulue as shown in earthquake and caused tsunami around it, as shown Fig.6b and Fig.6c were propagated to the northwest in Fig.6. The tsunami waves, which described with of Aceh and turn into the lee side of Simeulue Island. However before the wave completely turns, the

I_334 tsunami wave which was propagated from the strait simulations, the tsunami waves had occurred in between the Simeulue Island and Tapaktuan has Meulaboh and had not caused damage in coastal area already propagated through it. The tsunami waves as shown in Fig.5c. The tsunami wave reached at the that hit the Tapaktuan city have just come from the cities with height about less than 1 m. While in strait between Simeulue and Banyak Island. Tapaktuan and Singkil, tsunami had not occurred. Tsunami wave was blocked by the Simeulue Island and the wave was reflected by the island toward the southern part of Sumatra. Half of tsunami wave that were not blocked was propagated through strait between Sumatra and Simeulue Islands. However, the massive damage due to the tsunami waves was not caused at Singkil coastal area even thought the wave can propagate the strait. That can be seen in Fig.7b.

Fig.6 Snapshots of tsunami propagation proces in 2005 at southwestern coast of Aceh at: (a) t = 14 min, (b) t = Fig.7 Snapshots of tsunami propagation process in 2012 at 33 min, and (c) t = 51 min. southwestern coast of Aceh at: (a) t = 30 min and (b) t = 58 min, after the earthquake Fig.5b shows the maximum tsunami height along Sumatra Island by 2005 tsunami. As it has been (4) Hypothetic model for future hazard confirmed by field measurements, the tsunami height Based on the simulation results, the tsunami at Tapaktuan was calculated lower than the propagation process for case A1 is similar with the neighboring coast, especially Singkil. In case of 2005 tsunami in 2005 as shown in Fig.8a. Singkil become tsunami, the wave height at Singkil which is located the worst area that was impacted by the tsunami on the southern area of Tapaktuan became higher by wave. On other hand, the tsunami wave is lower in going around the southern end of the Simeulue Tapaktuan and Meulaboh. However, the amplified Island. On the other hand, the high wave did not wave which focused by the Island is occurred in the arrive at Tapaktuan coast. case of model B1 and C1. That indicated by the propagation process as shown in Fig.8. The tsunami (3) Case of tsunami in 2012 waves for model B1 and C1 were propagated faster Two big earthquakes occurred again on April 11, than model A1 because almost the entire rupture area 2012 after 8 years of Indian Ocean Tsunami in 2004. for model A1 was blocked by the Simeulue Island. They occurred at the west of Aceh Province Fig.9 shows the results of maximum tsunami outside the Sumatra subduction zone with height at the cities under the past and hypothetical earthquake’s epicenter at (2.311°N, 93.063°E) and scenarios. Case B1 indicates higher than Case A1 (0.773°N, 92.452°E) with magnitudes Mw = 8.6 and and C1 for Tapaktuan city. By the examination of Mw = 8.2, respectively. The focal mechanism of the tsunami propagation process, the Simeulue Island earthquakes was strike slip. However, those seemed to act as a shelter for the area around earthquakes had been predicted not to cause damage Tapaktuan in Case A1 and C1. Banyak Islands and such as the case of in 2004. Tsunami waves were flat topography caused the tsunami amplification propagated faster to the northwestern coast of Aceh around the Singkil city. Meulaboh city, which province because the rupture area was extended directly faces the Indian Ocean, becomes a northeast. Hence, several people around there such vulnerable area towards the tsunami disaster. When as in Banda Aceh had saw receding water after the fault area is located exceed to northwest of the several minutes of the main shock. Based on the Simeulue island. The tsunami wave will be able to

I_335 propagate directly to the Meulaboh city as same as Island due to the influence of small islands. This the tsunami in 2004. result indicates that it will be dispensable for the appropriate disaster management and education to grasp the function of small islands along Sumatra Island about future scenarios.

REFERENCES 1) Duputel, Z., Kanamori, H., Tsai, V.C., Rivera, L., Meng, L., Ampuero, J.P. & Stock, J.M. : The 2012 Sumatra Great Earthquake Sequence. Earth and Planetary Science Letters 351 (2012) 247-257. 2) Syamsidik, Rasyif, T. M., and Kato, S. : Development of accurate tsunami estimated times of arrival for tsunami-prone cities in Aceh, Indonesia. International Journal of Disaster Risk Reduction 14 (2015) 403-410. 3) Hill, E.M., Borrero, J.C., Huang, Z., Qiu, Q., Banerjee, P., Natawidjaja, D.H., Elosegui, P., Fritz, H.M., Suwargadi, B.W., Pranantyo, I.R., Li, L., Macpherson, K.A., Skanavis, V., Synolakis, C.E., and Sieh, K. : The 2010 Mw 7.8 Fig.8 Snapshots of tsunami propagation proces for case: A1 (a) t Mentawai earthquake: Very shallow source of a rare = 3 min and (b) t = 37 min, B1 (c) t = 5 min and (d) t = tsunami earthquake determined from tsunami field survey 41 min, and C1 (e) t = 2.5 min and (f) t =35 min. and near-field GPS data. J. Geophys. Res., 117(B6):B06402-, June 2012. 4) Liu PLF, Yong-Sik C, Briggs MJ, Kanoglu U, Synolakis CE. : Runup of solitary waves on a circular Island, JFM 302:259-285, 1995. 5) Wijetunge, J.J., Wang, X., & Liu, P.L.-F. : Indian Ocean Tsunami on 26 December 2004: Numerical Modeling of Inundation in Three Cities on The South Coast of Sri Lanka. Journal of Earthquake and Tsunami, Vol. 2, No. 2 (2008) 133-155. 6) Okada, Y. : Surface Deformation due to Shear and Tensile Faults in a Half-space. Bulletin of the Seismological Society of America, 75(4), 1135-1154, 1985. 7) Wang, X. : User manual for Cornell multi-grid coupled ts Fig.9 Tsunami wave height at Singkil, Tapaktuan, and Meulaboh name model-COMCOT V1.7. COMCOT website: http://ceeserver.cee.cornell.edu/pll-group/doc/COMCOT User Manual v1 7.pdf. Accessed 11 Oct 2011, 2009. 8) Piatanesi A and Lorito S. : Rupture process of the 2004 5. CONCLUSIONS Sumatra-Andaman earthquake from tsunami waveform inversion. Bulletin of the Seismological Society of America, The COMCOT model was used to investigate the 97 (1 A suppl):S223-S231, 2007. influence of a small island against the tsunami wave. 9) Banerjee, B., Pollitz, F., Nagarajam, B., & B¨urgmann, R. : Several past and hypothetical events were used to Coseismic Slip Distributions of the 26 December 2004 SumatraAndaman and 28 March 2005 Nias Earthquakes investigate the influence of a small island during the from GPS Static Offsets. Bulletin of the Seismological tsunami event. Main results obtained by this study Society of America, Vol. 97, No. 1A, S86-S102. are summarized as follows: (1) The amplification 10) Geist, E., Bilek, S., Arcas, D. & Titov, V., 2006. Differences due to the offshore island happens when the tsunami in Tsunami Generation Between the December 26, 2004 and March 28, 2005 Sumatra Earthquakes. Earth Planets Space, wave propagates from both sides of the island. Then, 58, pp.185-193. the wave, which focused on the behind of the island, 11) Wells, D.L., & Coppersmith, K.J. : New empirical will cause higher run-up at the area behind of the relationships among magnitude, rupture length, rupture island. That condition is shown in case of tsunami width, rupture area, and surface displacement. Bulletin - 2005 at Singkil city. (2) However, this small island Seismological Society of America, 84(4), 974-1002, 1994. 12) Stefanakis, T.S., Contal, E., Vayatis, N., Dias, F. & itself has a possibility to cause an amplification. Synolakis, C. E, : Can Small Islands Protect Nearby Coasts Based on the hypothetical scenario, when the fault From Tsunamis? An Active Experimental Design Approach. area is symmetry in front of the island. The tsunami arXiv:1305.7385v1 [physics.flu-dyn] May 31, 2013. wave is able to propagate through the island and (Received March 16, 2016) focused its wave behind it. Small changes of the location of epicenter in future tsunami scenarios will result in different tsunami impact along Sumatra

I_336