AN ABNORMAL TSUNAMI GENERATED BY OCTOBER 25th, 2010 MENTAWAI EARTHQUAKE Bambang Sunardi1, Suci Dewi Anugrah2, Thomas Hardy1 , Drajat Ngadmanto1 1 Research and Development Center, Indonesia Meteorological Climatological and Geophysical Agency 2 Tsunami Mitigation, Indonesia Meteorological Climatological and Geophysical Agency ABSTRACT A research of tsunami generated by the October 25th 2010 earthquake at Mentawai Western of Sumatra has been investigated. The observation of tsunami run up is about 5.7-7.4 m at three locations in the South and North Pagai. Numerical simulation of tsunami using the source mechanism obtained from BMKG results 3.8 m of tsunami wave height, while the propagation model shows that tsunami reach Enggano and Padang for about 38 and 58 minutes close to the tsunami travel time observation. It is clearly showed that the result of run up model is lower than its observation. From the calculation of the magnitude, it is obtained that the tsunami magnitude is about 8.1. This value is higher than the moment magnitude which is only 7.4. It can be conclude that the tsunami Mentawai can be characterized as an abnormal tsunami. This tsunami can also be categorized as a Tsunami Earthquake (TsE). INTRODUCTION predicted the megathrust of Mentawai Earthquake in the south of Sumatera. This prediction based on Many studies of some great earthquakes in a study of paleogeodesy and paleoseismic subduction zone of Sumatera had been carried out. (Natawidjaja, 2003) that noted the last major Those investigations have contributed earthquake in Mentawai had occurred in the significantly on the seismic hazard potential at 1300’s and 1600’s, therefore, the cycle of the this area. Natawidjaja, 2007 noted that the Mentawai Megathrust Earthquake is about 200 potential megathrust earthquake at the area of years. the subduction depends on the fault segmentation, the dimension of the locked region and the history Most of the big earthquakes occurred at the of the earthquake. These parameters determine the Sumatera area generated a tsunami. Jaiswal et al., strain energy accumulation area that can (2006), noted that there were 33 tsunamis generate a big earthquake. Subarya et al, 2006 had occurred at the Sumatera area. As a part of Sunda been investigated the great earthquake of Aceh- Arc, Sumatera had been much more active than Andaman (2004, Mw 9.15), while Briggs et al, Java. Table 1 gives a list of Tsunami Occurrence 2006, investigated the Nias Simelue (2005, Mw in Sumatera area. 8.7). Both of the earthquakes were characterized by the seismic gap zone. The Aceh Earthquake From the list of tsunami events, generally, the was already signed by the Simeuleu Earthquake of tsunamis in the Sumatera area are generated by an 2002 (Mw7.4), and then the Aceh Earthquake was earthquake with a magnitude of more than 7 Ms. assumed as an earthquake-triggered of the Nias The Mentawai Earthquake which happened on Earthquake which occurred three months later Oktober 25th, 2010 is an earthquake that after that. generated a tsunami due to its magnitude and depth. This study investigated a Mentawai After a series of two large earthquakes in the Tsunami which generated by an earthquake with a northern zone of Sumatra, Natawidjaja et al, 2007 magnitude of 7.2 Mw. The earthquake has occurred along the plate interface boundary of Melange rock lane associated with between the Australia and Sunda plates at Pagai akrasi complex is known as the rollback. The Selatan Sumatera. According to BMKG, the Orogenesaon process in Neogen Age produces the earthquake located at the location of 3.6oS and existence of the four phenomenons in this region 99.9oE with 10 km depth. This big earthquake namely the Bukit Barisan Mountains, an oblique occurred due to the movement of the Australia subduction with angle range from 50o – 65o in the Plate with respect to the Sunda Plate with a west of Sumatera, the Sumatera fault, and the velocity of approximately 50-70 mm/yr. Sumatra magmatisme activities (Barber, 2005). The Mamoru Nakamura’s Program was applied in The western part of Sumatera Island is an area this study to run a modeling of the Mentawai located on the outskirts of the active plate that is tsunami. A field study to the Mentawai Islands reflected by a high frequency occurrence of the after the event had also been carried out in this earthquakes. The earthquakes distribution research to provide the height of tsunami run-up in this region is not only caused by the activity in that area to validate the tsunami modeling. of subduction zones, but it also caused by an active fault systems along the island of Sumatra. Based on the Harvard CMT focal mechanism TECTONIC SETTING data, most of the subduction zone seismicity shows a normal fault type, while most of The west Sumatera region is a part of the Eurasian seismicity activity on the ground shows a Plate with a very slow speed of approximately 0.4 mechanism of a strike slip fault (Figure 2). cm/year that moves relatively to the southeast. An interaction between The Eurasian and the Indian Ocean plate is occurred in the western part of this DATA AND TSUNAMI MODELING province that moved to the north at speeds up to 7 cm / year relatively (Minster and Jordan, 1978 in To calculate run-up heights we use a code that is Yeats et al., 1997). This interaction produces an modeled by Nakamura. This code applied a finite- oblique subduction, which had been formed since difference method. The bathymetry data are the Cretaceous era and still continues up to obtained from ETOPO2 provided by National now. In addition to subduction, two plates of this Geophysical Data Center. The study area is interaction also resulted in major structural pattern showed in figure 3. The grid interval of of Sumatra, which are known as the Sumatra Fault bathymetry was 2.5 km x 2.5 km. In this Zone and Mentawai Fault Zone (Figure 1). simulation we used 5 seconds time step used for calculations. The Tectonic evolution of West Sumatra before the Age of Neogen tectonics was characterized by A focal mechanism solution from BMKG was the expansion (Tectonic rifting) followed by the used for this model. An uplift red fault block collision, amalgamation, and akrasi which lead to (Figure 4) represents an earthquake source the formation of mountains, crumpling, and mechanism that reflects an oblique reverse faulting (Simanjuntak, 2004). The Unveiling of fault. This source mechanism triggered a tsunami melange rock in North Sumatra and West Sumatra after the earthquake occurrence. According to the of the Cretaceous age indicates the presence of earthquake source of BMKG data it was obtained subduction related to the complex akrasi systems that the maximum vertical displacement is 1.3 m. (Asikin, 1974; Simanjuntak, 1980; Sukamto, Table 2 is the simulation parameters as a solution 1986; Wajzer et al, 1991 in Simandjuntak, of focal mechanism. 2004). In the Age of Paleogene subduction system was relatively shifted to the west. It was proved The numerical simulation of tsunami propagation by the presence of mélange rocks at Nias Island, shows that tsunamis arrived the coastlines Pagai and Sipora which are located in west of area of Purourogat and Munte, Enggano, and Sumatra Island (Katili, 1973; Karig et al., 1978; Padang about 11, 38 and 58 minute after the Hamilton, 1979; Djamal et al., 1990 ; Andi- earthquake respectively (Figure 5). Mango, 1991 in Simandjuntak, 2004). The change The run-up simulations are showed on the 7.4 m of tsunami run-up, 420 m of tsunami Figure 6. The maximum tsunami height at the inundation, and N85E of tsunami direction. 71 Pagai Island is about 4 m. Figure 6b shows a people were reported die and 4 people were tsunami height at three locations, while Table 3 is missing. a list of tsunami height at many points along the The last observation point was in Dusun coast. It appears that a bay or an estuary Muntei, Desa Betumonga, North Pagai. Most experiences a higher run-up value than its area of Muntei is located at a bay, while some part surrounding area relatively. is located at the edge of an estuary. Although the Muntei area is more far from the earthquake source compare to others, this area was the most FIELD SURVEY tsunami affected. A lot of houses were damaged because the buildings were built close to a sloping Post-tsunami field survey was also conducted in beach and confronted the sources of earthquakes this study. We measured and collected as many as directly. The geographic condition of Muntei as a possible data of tsunami height traces, tsunami bay and an estuary is another factor that caused direction, inundation, and also the impact of this area is more vulnerable than others. The tsunami on life and material losses. tsunami run-up is higher and its inundation is widespread. The tsunami measurement results are Three locations were observed in that survey about 5.7 m of tsunami run-up, 406 m of tsunami namely Munte Kecil, Purourogat, and Muntei. inundation, and N10E of tsunami direction. 115 The first and two are located at Malakopak, South people were reported die and 34 people were of Pagai, and the third are located at Betumonga, missing. 73 houses were severely damaged. North of Pagai. The followings are a brief description of observation result at each area. This study estimated also a tsunami source magnitude based on the wave height distribution The first observation point was in the Dusun at various places in relation with the source of Munte Kecil, Desa Malakopak, South Pagai. We tectonic earthquake which is known as Tsunami found 4 buildings were damage at the distance of Magnitude (Mt). Abe (1979, 1981, 1989b), 70 m, 120 m, 150 m, and 170 m from the beach formulated a calculation of Mt as follows: respectively. At a distance of 230 m from the beach, some buildings were found safe. We did an Mt = logΔ + logH + 5.8 interview with some local residents to investigate whether any casualties due to the tsunami. They where H is the maximum amplitude of tide gauge said that there were no casualties caused by the observation, and Δ is the distance of the tsunami. After the earthquake of 2007, the earthquake source to the tide gauge. In this case government had relocated the coast resident to the 8.1 of Mt were estimated. safe place. This effort, however, had saved the people from tsunami hazard successfully. Because of the earthquake, this area can be categorized as a MODEL VALIDATION zone of 2-3 MMI scales (weak shocks). We measured approximately 6.4 m of run-up traces, The ocean modeling of tsunami propagation was 290 m of inundation, and N15E of tsunami nearly appropriate when verified with tide gauge direction. observation data. Table 5 shows the comparison of tsunami travel time between observational data The second one was in the di Dusun Purourogat, (tide gauge) and numerical simulation results of Desa Malakopak, South Pagai. This place is a bay tsunami propagation using Mamoru Nakamura's area. We found 3 damaged buildings at the Program. The model estimated that the tsunamis location of 50, 120, and 130 m from the beach. At entered the Pagai island coastline of about 11 a distance of about 210 m, we found some houses minutes after the main earthquake. Therefore, it is with a small scale of damage. Not far from this not true that the tsunami occurred after BMKG location, there was a valley with a depth of about BMKG end tsunami warning at 51 minutes after 2.3 m. The tsunami measurement results are about the earthquake. DISCUSSION AND CONCLUSION REFERENCES The results of post-tsunami survey at the three Abe, K., 1979. Size of great earthquakes of 1837- observation points namely Munte kecil, 1974 inferred from tsunami data. J. Geophys. Purourogat and Muntei noted that the run-up Res., v. 84, pp. 1561-1568. varied between 5.7 - 7.4 m. The maximum run-up was occurred in Purourogat of about 7.4 m which Abe, K., 1981. Size of tsunamigenic earthquakes captured 420 m of inundation area from the beach. of the northwestern Pacific, Phys. Earth Planet. In this location the tsunami moved from the Inter., v. 27, pp. 194-205. direction of N850E. The Munte Kecil and Muntei experienced the maximum run-up of about 6.4 m Abe, K., 1989b. Quantification of tsunamigenic and 5.7 m respectively, with inundation area of earthquakes by Mt scale, Tectonophys. 166, about 290 m and 420 m. In that points the tsunami 27-34. moved from the direction of N150E and N100E. The geographic condition of the beach influenced Barber, A. J., Crow, M. J., Milsom, J. S., 2005. the height of tsunami run-up. A bay as well as an Sumatera Geology, Resources and Tectonic estuary, like the shape of the Purourogat beach, Evolution. Geological Society Memoir No. 31, will produce a higher tsunami run-up. The Geological Society, London, 290 p. The run-up estimation of the tsunami numerical Briggs, R., Sieh, K., Meltzer, A.S., Natawidjaja, model using the BMKG data was 4 m, lower than D., Galetzka, J., Suwargadi, B., Hsu, Y.J., field measurements of 7.4 m. However, the Simons, M., Hananto, N., Suprihanto, I., Prayudi, D., Avouac, J.-P., Prawirodirdjo, L., ocean modeling of tsunami propagation was and Bock, Y. (2006). Deformation and slip almost similar with the tide gauge data. It was along the Sunda megathrust in the Great 2005 estimated that within 11 minutes after the main Nias-Simeulue earthquake.: Science, v. 311, p. earthquake, the tsunamis began to enter some 1897-1901. coastlines area in the South and North of Pagai. Jaiswal, R.K., B.K (2006). Tsunamigenic sources The Mentawai tsunami has a similar characteristic in the Indian Ocean. with the event of Pangandaran 2006. Both of those can be categorized as the TsE, considering Lasitha, S., Radhakrishna, M., Sanu, T. D., 2006. that the estimated value of the tsunami run-up was Seismically active deformation in the Sumatera much smaller than the actual height of the – Java trench arc region : geodynamic tsunami. A slow ground shaking had made implications. Current Science, 90 (5), pp. 690 – tsunami magnitude calculation (Mt) of Mentawai 696. Earthquake was much larger than the earthquake magnitude (Mw). The Mentawai tsunami might be Natawidjaja, D.H. (2003). Neotectonics of the also influenced by other mechanisms. The source Sumatran fault and paleogeodesy of the of the earthquake was at the point that close to the Sumatran subduction zone. Ph.D thesis, ocean trench where subduction between the Indo- California Institute of Technology (Caltech). Australian plate with the Eurasian plate in the area where the accumulation of sediments experiences Natawidjaja. (2007). Tectonic Setting Indonesia a great pressure forming a continuous ridge dan Pemodelan Sumber Gempa dan Tsunami. submarine elongated with steep slopes that tend to Presented on Pelatihan pemodelan run-up shock and vulnerable to earthquake shocks. tsunami, ristek, 20-24 Agustus 2007, Jakarta. Possible lifting of sediment above it or a large landslide that occurred after the earthquake as an Natawidjaja, D., Sieh, K., Galetzka, J., Suwargadi, additional factor that triggered the tsunami height B., Cheng, H., and Edwards, R. (2007). becomes abnormal (much higher than normal Interseismic deformation above the Sunda calculation). However, to prove the source of megathrust recorded in coral microatolls of the research takes a more in-depth process. Mentawai Islands, West Sumatra: J. Geophys. Res. Simandjuntak, T. O., 2004. Tektonika. Publikasi Khusus, Pusat Penelitian dan Pengembangan Geologi, 216 p. Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J.P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., and McCaffrey, R. (2006). Plate-boundary deformation associated with the great Sumatra-Andaman earthquake: Nature, v. 440, p. 46-51. Yeats, R. S., Sieh, K., and Allen, C. R., 1997. The Geology of Earthquakes. Oxford university press, 567 p. No Year Location Latitude Longitude Magnitude 1 1681 Sumatera 2 1770 Sw Sumatera 102 -5 3 3 1797 Sw. Sumatera 99 -1 4 4 1799 Se. Sumatera 104.75 -2.983 2 5 1818 Bengkulu, Sumatera 102.267 -3.77 7 6 1833 Bengkulu, Sumatera 7 1833 Sw. Sumatera 102.2 -3.5 8.7 8 1837 Banda Aceh 96 5.5 7.2 9 1843 Sw. Sumatera 98 1.5 7.2 10 1843 Sw. Sumatera 97.33 1.05 11 1852 Sibolga, Sumatera 98.8 1.7 6.8 12 1861 Sw. Sumatera 97.5 -1 6.8 13 1861 Sw. Sumatera 97.5 -1 8.5 14 1861 Sw. Sumatera 99.37 0.3 7 15 1861 Sw. Sumatera 97.5 1 7 16 1861 Sw. Sumatera 107.3 -6.3 17 1864 Sumatera 97.5 1 6.8 18 1896 Sw. Sumatera 100 -1.5 6.5 19 1904 Sumatera 20 1907 Sw. Sumatera 102.5 -3.5 6.8 21 1908 Sw. Sumatera 100 -5 7.5 22 1909 Sumatera 101 -2 7.7 23 1914 W. Coast Of S. Sumatera 102.5 -4.5 8.1 24 1922 Lhoknga, Aceh 95.233 5.467 25 1926 Sw. Sumatera 99.5 -1.5 6.7 26 1931 Sw. Sumatera 102.7 -5 7.5 27 1935 Sw. Sumatera 98.25 .001 8.1 28 1958 Sw. Sumatera 104 -4.5 6.5 29 1984 Off West Coast Of Sumatera 97.95 0.18 7.2 30 1994 Southern Sumatera 104.3 -5 7 31 2000 Off West Coast of Sumatera 102.09 -4.72 7.8 32 2004 Off West Coast of Sumatera 95.947 3.307 9.3 33 2005 Off West Coast Of Sumatera 97.01 2.074 8.7 34 2005 Kepulauan Mentawai 99.607 -1.64 6.7 Table 1: Tsunami catalogue for Sumatera area (Rastogi and Jaiswal, 2006). Simulation Parameters Length (km) 74.5 Width (km) 26.5 Slip (m) 3.3 Mw 7.4 Center Fault Latitude -3.2 Coordinate Longitude 100 Table 2: Simulation Parameters. Longitude Latitude Run Up (m) 100.18891 -3 4.0 100.22489 -3.03593 3.3 100.2169 -3.01797 3.0 100.36883 -3.23357 2.7 100.08096 -2.82033 2.5 100.18891 -3.01797 2.5 100.15293 -2.85627 2.5 100.18891 -2.85627 2.4 100.04498 -2.82033 2.3 100.009 -2.76643 2.2 100.06297 -2.8383 2.1 100.17092 -2.98203 2.1 100.29686 -3.07187 2.1 100.18891 -2.96407 2.0 100.17092 -2.87423 2.0 100.18891 -2.87423 2.0 100.22489 -3.08983 2.0 100.2069 -3.03593 2.0 100.2069 -3.0539 1.9 100.02699 -2.82033 1.9 100.009 -2.7844 1.9 100.18891 -3.03593 1.9 100.24289 -3.1078 1.6 100.27887 -3.1078 1.6 100.24289 -3.07187 1.6 Longitude Latitude Run Up (m) 100.17092 -3.01797 1.6 100.18891 -2.82033 1.6 100.36883 -3.2695 1.6 100.33284 -3.19764 1.6 100.11694 -2.85627 1.6 100.26088 -3.12577 1.6 100.18891 -3.07187 1.6 100.11694 -2.8383 1.6 100.13494 -2.85627 1.5 100.2069 -2.8922 1.5 100.17092 -2.8922 1.5 100.29686 -3.17967 1.5 100.18891 -2.9461 1.5 100.38682 -3.28747 1.5 100.15293 -2.87423 1.5 100.2069 -3.08983 1.5 100.35083 -3.2156 1.7 100.08096 -2.85627 1.6 100.2069 -2.91017 1.6 100.2069 -3.0 1.9 100.06297 -2.8383 1.8 100.27887 -3.12577 1.8 100.24289 -3.08983 1.8 100.17092 -3.0 1.7 100.2069 -3.07187 1.7 100.18891 -3.0539 1.7 100.27887 -3.14373 1.7 100.13494 -2.8383 1.7 100.26088 -3.1078 1.7 100.18891 -2.8922 1.7 100.09895 -2.85627 1.7 100.09895 -2.8383 1.7 100.17092 -2.96407 1.7 100.17092 -2.85627 1.7 100.009 -2.80236 1.7 100.35083 -3.2156 1.7 Longitude Latitude Run Up (m) 100.08096 -2.85627 1.6 100.2069 -2.91017 1.6 100.2069 -2.87423 1.6 100.04498 -2.8383 1.6 100.2069 -2.87423 1.6 100.04498 -2.8383 1.6 100.24289 -3.1078 1.6 100.24289 -3.07187 1.6 100.17092 -3.01797 1.6 100.18891 -2.82033 1.6 100.36883 -3.2695 1.6 100.11694 -2.85627 1.6 100.26088 -3.12577 1.6 100.18891 -3.07187 1.6 100.11694 -2.8383 1.6 100.13494 -2.85627 1.5 100.2069 -2.8922 1.5 100.17092 -2.8922 1.5 100.29686 -3.17967 1.5 100.18891 -2.9461 1.5 100.38682 -3.28747 1.5 100.15293 -2.87423 1.5 100.2069 -3.08983 1.5 Table 3: Tsunami height modeling at many points along the Pagai coast. Munte Kecil Purourogat Munte Location South Pagai South Pagai North Pagai Position 3.02185 S 100.22244 E 3.03782 S 100.23215 E 2.82955 S 100.09409 E Direction N150E N850E N100E Run-Up (m) 6.4 7.4 5.7 Inundation (m) 290 420 406 Table 4: Post Tsunami Survey. Tide Gauge Simulation Locations Travel Time (Minute) Travel Time (Minute) Enggano 37 38 Padang 58 58 Table 5: The comparison of tsunami travel time between observational data (tide gauge) and numerical simulation results of tsunami propagation using Mamoru Nakamura's Program. Run Up Run Up Simulation Location Position Observational Simulation Position (m) (m) Munte Kecil Malakopak 3.02185°S 100.22244°E 6.4 3.01797°S 100.2169°E 3 South Pagai Purourogat Malakopak 3.03782°S 100.23215°E 7.4 3.03593°S 100.22489°E 3.3 South Pagai Munte Betumonga 2.82955°S 100.09409°E 5.7 2.82033°S 100.08096°E 2.5 North Pagai Table 6: Comparison of tsunami run up between observational and numerical simulation. Arival Time Travel Time Max Water Name Lat Long Distance (UTC) (minute) Height (m) Enggano -5.3461 102.2781 316 15:19 37 0.22 Padang -0.9500 100.3661 283 15:40 58 0.38 Tanabalah -0.5326 98.4977 374 15:39 57 0.25 Telukdalam 0.5542 97.8222 516 16:10 88 0.14 Table 7: Observational data (tide gauges data). Figure 1: Tectonic of Sumatra and Java (Lasitha et al., 2006). Figure 2: Earthquakes source mechanism in Sumatra and surrounding based on Harvard CMT data (Lasitha et al., 2006). Figure 3: Research area. Figure 4: Source modeling. Figure 5a: Ocean modeling at 11 minutes. Figure 5b: Ocean modeling at 38 minutes. Figure 5c: Ocean modeling at 58 minutes. Figure 6a: Run up modeling. Figure 6b: Run up modeling at Muntei Kecil, Purourogat and Munte.