AN ABNORMAL TSUNAMI GENERATED BY OCTOBER 25th, 2010MENTAWAI EARTHQUAKE

Bambang Sunardi1, Suci Dewi Anugrah2, Thomas Hardy1 , Drajat Ngadmanto1

1Research and Development Center, Indonesia Meteorological Climatological and Geophysical Agency2Tsunami Mitigation, Indonesia Meteorological Climatological and Geophysical Agency

ABSTRACT

A research of tsunami generated by the October 25th 2010 earthquake at Mentawai Western ofSumatra has been investigated. The observation of tsunami run up is about 5.7-7.4 m at threelocations in the South and North Pagai. Numerical simulation of tsunami using the sourcemechanism obtained from BMKG results 3.8 m of tsunami wave height, while the propagationmodel shows that tsunami reach Enggano and Padang for about 38 and 58 minutes close to thetsunami travel time observation. It is clearly showed that the result of run up model is lower thanits observation. From the calculation of the magnitude, it is obtained that the tsunami magnitudeis about 8.1. This value is higher than the moment magnitude which is only 7.4. It can beconclude that the tsunami Mentawai can be characterized as an abnormal tsunami. This tsunamican also be categorized as a Tsunami Earthquake (TsE).

INTRODUCTION

Many studies of some great earthquakes insubduction zone of Sumatera had been carried out.Those investigations have contributedsignificantly on the seismic hazard potential atthis area. Natawidjaja, 2007 noted that thepotential megathrust earthquake at the area ofthe subduction depends on the fault segmentation,the dimension of the locked region and the historyof the earthquake. These parameters determine thestrain energy accumulation area that cangenerate a big earthquake. Subarya et al, 2006 hadbeen investigated the great earthquake of Aceh-Andaman (2004, Mw 9.15), while Briggs et al,2006, investigated the Nias Simelue (2005, Mw8.7). Both of the earthquakes were characterizedby the seismic gap zone. The Aceh Earthquakewas already signed by the Simeuleu Earthquake of2002 (Mw7.4), and then the Aceh Earthquake wasassumed as an earthquake-triggered of the NiasEarthquake which occurred three months laterafter that.

After a series of two large earthquakes in thenorthern zone of Sumatra, Natawidjaja et al, 2007

predicted the megathrust of Mentawai Earthquakein the south of Sumatera. This prediction based ona study of paleogeodesy and paleoseismic(Natawidjaja, 2003) that noted the last majorearthquake in Mentawai had occurred in the1300’s and 1600’s, therefore, the cycle of theMentawai Megathrust Earthquake is about 200years.

Most of the big earthquakes occurred at theSumatera area generated a tsunami. Jaiswal et al.,(2006), noted that there were 33 tsunamisoccurred at the Sumatera area. As a part of SundaArc, Sumatera had been much more active thanJava. Table 1 gives a list of Tsunami Occurrencein Sumatera area.

From the list of tsunami events, generally, thetsunamis in the Sumatera area are generated by anearthquake with a magnitude of more than 7 Ms.The Mentawai Earthquake which happened onOktober 25th, 2010 is an earthquake thatgenerated a tsunami due to its magnitude anddepth. This study investigated a MentawaiTsunami which generated by an earthquake with amagnitude of 7.2 Mw. The earthquake has

occurred along the plate interface boundarybetween the Australia and Sunda plates at PagaiSelatan Sumatera. According to BMKG, theearthquake located at the location of 3.6oS and99.9oE with 10 km depth. This big earthquakeoccurred due to the movement of the AustraliaPlate with respect to the Sunda Plate with avelocity of approximately 50-70 mm/yr.

The Mamoru Nakamura’s Program was applied inthis study to run a modeling of the Mentawaitsunami. A field study to the Mentawai Islandsafter the event had also been carried out in thisresearch to provide the height of tsunami run-upin that area to validate the tsunami modeling.

TECTONIC SETTING

The west Sumatera region is a part of the EurasianPlate with a very slow speed of approximately 0.4cm/year that moves relatively to the southeast. Aninteraction between The Eurasian and the IndianOcean plate is occurred in the western part of thisprovince that moved to the north at speeds up to 7cm / year relatively (Minster and Jordan, 1978 inYeats et al., 1997). This interaction produces anoblique subduction, which had been formed sincethe Cretaceous era and still continues up tonow. In addition to subduction, two plates of thisinteraction also resulted in major structural patternof Sumatra, which are known as the Sumatra FaultZone and Mentawai Fault Zone (Figure 1).

The Tectonic evolution of West Sumatra beforethe Age of Neogen tectonics was characterized bythe expansion (Tectonic rifting) followed by thecollision, amalgamation, and akrasi which lead tothe formation of mountains, crumpling, andfaulting (Simanjuntak, 2004). The Unveiling ofmelange rock in North Sumatra and West Sumatraof the Cretaceous age indicates the presence ofsubduction related to the complex akrasi systems(Asikin, 1974; Simanjuntak, 1980; Sukamto,1986; Wajzer et al, 1991 in Simandjuntak,2004). In the Age of Paleogene subduction systemwas relatively shifted to the west. It was provedby the presence of mélange rocks at Nias Island,Pagai and Sipora which are located in west ofSumatra Island (Katili, 1973; Karig et al., 1978;Hamilton, 1979; Djamal et al., 1990 ; Andi-Mango, 1991 in Simandjuntak, 2004). The change

of Melange rock lane associated withakrasi complex is known as the rollback. TheOrogenesaon process in Neogen Age produces theexistence of the four phenomenons in this regionnamely the Bukit Barisan Mountains, an obliquesubduction with angle range from 50o – 65o in thewest of Sumatera, the Sumatera fault, and theSumatra magmatisme activities (Barber, 2005).

The western part of Sumatera Island is an arealocated on the outskirts of the active plate that isreflected by a high frequency occurrence of theearthquakes. The earthquakes distributionin this region is not only caused by the activityof subduction zones, but it also caused by anactive fault systems along the island of Sumatra.Based on the Harvard CMT focal mechanismdata, most of the subduction zone seismicityshows a normal fault type, while most ofseismicity activity on the ground shows amechanism of a strike slip fault (Figure 2).

DATA AND TSUNAMI MODELING

To calculate run-up heights we use a code that ismodeled by Nakamura. This code applied a finite-difference method. The bathymetry data areobtained from ETOPO2 provided by NationalGeophysical Data Center. The study area isshowed in figure 3. The grid interval ofbathymetry was 2.5 km x 2.5 km. In thissimulation we used 5 seconds time step used forcalculations.

A focal mechanism solution from BMKG wasused for this model. An uplift red fault block(Figure 4) represents an earthquake sourcemechanism that reflects an oblique reversefault. This source mechanism triggered a tsunamiafter the earthquake occurrence. According to theearthquake source of BMKG data it was obtainedthat the maximum vertical displacement is 1.3 m.Table 2 is the simulation parameters as a solutionof focal mechanism.

The numerical simulation of tsunami propagationshows that tsunamis arrived the coastlinesarea of Purourogat and Munte, Enggano, andPadang about 11, 38 and 58 minute after theearthquake respectively (Figure 5).

The run-up simulations are showed on theFigure 6. The maximum tsunami height at thePagai Island is about 4 m. Figure 6b shows atsunami height at three locations, while Table 3 isa list of tsunami height at many points along thecoast. It appears that a bay or an estuaryexperiences a higher run-up value than itssurrounding area relatively.

FIELD SURVEY

Post-tsunami field survey was also conducted inthis study. We measured and collected as many aspossible data of tsunami height traces, tsunamidirection, inundation, and also the impact oftsunami on life and material losses.

Three locations were observed in that surveynamely Munte Kecil, Purourogat, and Muntei.The first and two are located at Malakopak, Southof Pagai, and the third are located at Betumonga,North of Pagai. The followings are a briefdescription of observation result at each area.

The first observation point was in the DusunMunte Kecil, Desa Malakopak, South Pagai. Wefound 4 buildings were damage at the distance of70 m, 120 m, 150 m, and 170 m from the beachrespectively. At a distance of 230 m from thebeach, some buildings were found safe. We did aninterview with some local residents to investigatewhether any casualties due to the tsunami. Theysaid that there were no casualties caused by thetsunami. After the earthquake of 2007, thegovernment had relocated the coast resident to thesafe place. This effort, however, had saved thepeople from tsunami hazard successfully. Becauseof the earthquake, this area can be categorized as azone of 2-3 MMI scales (weak shocks). Wemeasured approximately 6.4 m of run-up traces,290 m of inundation, and N15E of tsunamidirection.

The second one was in the di Dusun Purourogat,Desa Malakopak, South Pagai. This place is a bayarea. We found 3 damaged buildings at thelocation of 50, 120, and 130 m from the beach. Ata distance of about 210 m, we found some houseswith a small scale of damage. Not far from thislocation, there was a valley with a depth of about2.3 m. The tsunami measurement results are about

7.4 m of tsunami run-up, 420 m of tsunamiinundation, and N85E of tsunami direction. 71people were reported die and 4 people weremissing.The last observation point was in DusunMuntei, Desa Betumonga, North Pagai. Mostarea of Muntei is located at a bay, while some partis located at the edge of an estuary. Although theMuntei area is more far from the earthquakesource compare to others, this area was the mosttsunami affected. A lot of houses were damagedbecause the buildings were built close to a slopingbeach and confronted the sources of earthquakesdirectly. The geographic condition of Muntei as abay and an estuary is another factor that causedthis area is more vulnerable than others. Thetsunami run-up is higher and its inundation iswidespread. The tsunami measurement results areabout 5.7 m of tsunami run-up, 406 m of tsunamiinundation, and N10E of tsunami direction. 115people were reported die and 34 people weremissing. 73 houses were severely damaged.

This study estimated also a tsunami sourcemagnitude based on the wave height distributionat various places in relation with the source oftectonic earthquake which is known as TsunamiMagnitude (Mt). Abe (1979, 1981, 1989b),formulated a calculation of Mt as follows:

Mt = logΔ + logH + 5.8

where H is the maximum amplitude of tide gaugeobservation, and Δ is the distance of theearthquake source to the tide gauge. In this case8.1 of Mt were estimated.

MODEL VALIDATION

The ocean modeling of tsunami propagation wasnearly appropriate when verified with tide gaugeobservation data. Table 5 shows the comparisonof tsunami travel time between observational data(tide gauge) and numerical simulation results oftsunami propagation using Mamoru Nakamura'sProgram. The model estimated that the tsunamisentered the Pagai island coastline of about 11minutes after the main earthquake. Therefore, it isnot true that the tsunami occurred after BMKGBMKG end tsunami warning at 51 minutes afterthe earthquake.

DISCUSSION AND CONCLUSION

The results of post-tsunami survey at the threeobservation points namely Munte kecil,Purourogat and Muntei noted that the run-upvaried between 5.7 - 7.4 m. The maximum run-upwas occurred in Purourogat of about 7.4 m whichcaptured 420 m of inundation area from the beach.In this location the tsunami moved from thedirection of N850E. The Munte Kecil and Munteiexperienced the maximum run-up of about 6.4 mand 5.7 m respectively, with inundation area ofabout 290 m and 420 m. In that points the tsunamimoved from the direction of N150E and N100E.The geographic condition of the beach influencedthe height of tsunami run-up. A bay as well as anestuary, like the shape of the Purourogat beach,will produce a higher tsunami run-up.

The run-up estimation of the tsunami numericalmodel using the BMKG data was 4 m, lower thanfield measurements of 7.4 m. However, theocean modeling of tsunami propagation wasalmost similar with the tide gauge data. It wasestimated that within 11 minutes after the mainearthquake, the tsunamis began to enter somecoastlines area in the South and North of Pagai.

The Mentawai tsunami has a similar characteristicwith the event of Pangandaran 2006. Both ofthose can be categorized as the TsE, consideringthat the estimated value of the tsunami run-up wasmuch smaller than the actual height of thetsunami. A slow ground shaking had madetsunami magnitude calculation (Mt) of MentawaiEarthquake was much larger than the earthquakemagnitude (Mw). The Mentawai tsunami might bealso influenced by other mechanisms. The sourceof the earthquake was at the point that close to theocean trench where subduction between the Indo-Australian plate with the Eurasian plate in the areawhere the accumulation of sediments experiencesa great pressure forming a continuous ridgesubmarine elongated with steep slopes that tend toshock and vulnerable to earthquake shocks.Possible lifting of sediment above it or a largelandslide that occurred after the earthquake as anadditional factor that triggered the tsunami heightbecomes abnormal (much higher than normalcalculation). However, to prove the source ofresearch takes a more in-depth process.

REFERENCES

Abe, K., 1979. Size of great earthquakes of 1837-1974 inferred from tsunami data. J. Geophys.Res., v. 84, pp. 1561-1568.

Abe, K., 1981. Size of tsunamigenic earthquakesof the northwestern Pacific, Phys. Earth Planet.Inter., v. 27, pp. 194-205.

Abe, K., 1989b. Quantification of tsunamigenicearthquakes by Mt scale, Tectonophys. 166,27-34.

Barber, A. J., Crow, M. J., Milsom, J. S., 2005.Sumatera Geology, Resources and TectonicEvolution. Geological Society Memoir No. 31,The Geological Society, London, 290 p.

Briggs, R., Sieh, K., Meltzer, A.S., Natawidjaja,D., Galetzka, J., Suwargadi, B., Hsu, Y.J.,Simons, M., Hananto, N., Suprihanto, I.,Prayudi, D., Avouac, J.-P., Prawirodirdjo, L.,and Bock, Y. (2006). Deformation and slipalong the Sunda megathrust in the Great 2005Nias-Simeulue earthquake.: Science, v. 311, p.1897-1901.

Jaiswal, R.K., B.K (2006). Tsunamigenic sourcesin the Indian Ocean.

Lasitha, S., Radhakrishna, M., Sanu, T. D., 2006.Seismically active deformation in the Sumatera– Java trench arc region : geodynamicimplications. Current Science, 90 (5), pp. 690 –696.

Natawidjaja, D.H. (2003). Neotectonics of theSumatran fault and paleogeodesy of theSumatran subduction zone. Ph.D thesis,California Institute of Technology (Caltech).

Natawidjaja. (2007). Tectonic Setting Indonesiadan Pemodelan Sumber Gempa dan Tsunami.Presented on Pelatihan pemodelan run-uptsunami, ristek, 20-24 Agustus 2007, Jakarta.

Natawidjaja, D., Sieh, K., Galetzka, J., Suwargadi,B., Cheng, H., and Edwards, R. (2007).Interseismic deformation above the Sundamegathrust recorded in coral microatolls of the

Mentawai Islands, West Sumatra: J. Geophys.Res.

Simandjuntak, T. O., 2004. Tektonika. PublikasiKhusus, Pusat Penelitian dan PengembanganGeologi, 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 associatedwith 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. Oxforduniversity 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 FaultCoordinate

Latitude -3.2

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.

LocationMunte KecilSouth Pagai

PurourogatSouth Pagai

MunteNorth 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.

LocationsTide Gauge

Travel Time (Minute)Simulation

Travel Time (Minute)

Enggano 37 38

Padang 58 58

Table 5: The comparison of tsunami travel time between observational data (tide gauge) and numericalsimulation results of tsunami propagation using Mamoru Nakamura's Program.

Location PositionRun Up

Observational(m)

SimulationPosition

Run UpSimulation

(m)Munte Kecil MalakopakSouth Pagai

3.02185°S 100.22244°E 6.4 3.01797°S 100.2169°E 3

Purourogat MalakopakSouth Pagai

3.03782°S 100.23215°E 7.4 3.03593°S 100.22489°E 3.3

Munte BetumongaNorth Pagai

2.82955°S 100.09409°E 5.7 2.82033°S 100.08096°E 2.5

Table 6: Comparison of tsunami run up between observational and numerical simulation.

Name Lat Long DistanceArival Time

(UTC)Travel Time

(minute)Max WaterHeight (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 surroundingbased on Harvard CMT data (Lasitha et al., 2006).

Figure 1: Tectonic of Sumatra and Java (Lasitha et al., 2006).

Figure 2: Earthquakes source mechanism in Sumatra and surroundingbased on Harvard CMT data (Lasitha et al., 2006).

Figure 1: Tectonic of Sumatra and Java (Lasitha et al., 2006).

Figure 2: Earthquakes source mechanism in Sumatra and surroundingbased on Harvard CMT data (Lasitha et al., 2006).

Figure 3: Research area.

Figure 4: Source modeling.

Figure 3: Research area.

Figure 4: Source modeling.

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 5a: Ocean modeling at 11 minutes.

Figure 5b: Ocean modeling at 38 minutes.

Figure 5c: Ocean modeling at 58 minutes.

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.

Figure 6a: Run up modeling.

Figure 6b: Run up modeling at Muntei Kecil, Purourogat and Munte.

Figure 6a: Run up modeling.

Figure 6b: Run up modeling at Muntei Kecil, Purourogat and Munte.