An Abnormal Tsunami Generated By October 25th Mentawai Earthquake

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An Abnormal Tsunami Generated By October 25th Mentawai Earthquake Powered By Docstoc
                          MENTAWAI EARTHQUAKE

             Bambang Sunardi1, Suci Dewi Anugrah2, Thomas Hardy1 , Drajat Ngadmanto1
     Research and Development Center, Indonesia Meteorological Climatological and Geophysical Agency
             Tsunami Mitigation, Indonesia Meteorological Climatological and Geophysical Agency


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
                                                        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
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
                                                       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
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
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.

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
                      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
                                        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
       Location                         Position              Observational                              Simulation
                                                                  (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.

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