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Indonesia merupakan salah satu wilayah dengan aktifitas kegempaan yang sangat tinggi. Sebagai konsekwensi akibat pertemuan lempeng-lempeng raksasa, Indonesia merupakan wilayah dengan tingkat seismisitas yang sangat aktif. Sejarah juga mencatat banyaknya gempa besar serta peristiwa tsunami yang terjadi di wilayah Indonesia. Salah satu gempa terbesar di dunia Mw 9,0 pernah terjadi di wilayah ini pada 26 Desember 2004, yang membangkitkan tsunami maha dasyat dan menimbulkan korban tewas ratusan ribu jiwa.

Risiko tsunami adalah unik karena sebagian masyarakat yang berada jauh dari sumber tsunami dapat memperoleh waktu peringatan lebih lama, sebaliknya masyarakat yang dekat dengan sumber tsunami hanya memiliki waktu peringatan yang pendek . Gambar peta zona potensial tsunami di Indonesia. Sebagaimana diketahui, tsunami dapat dikelompokkan dalam 3 tipe yaitu :

• Tsunami Global
  Tsunami Global merupakan tipe tsunami dengan Warning Time lebih dari 2 jam.


• Tsunami Regional
  Tsunami Regional merupakan tipe tsunami dengan Warning Time lebih antara 30 menit - 2 jam.


• Tsunami Lokal
  Tsunami Lokal merupakan tipe tsunami dengan Warning Time kurang dari 30 menit.

Zona subduksi dari wilayah Indonesia memanjang dari utara barat Sumatera hingga ke Flores bagian timur Indonesia., jarak subduksi 50 – 400 km dari pantai Jika tsunami terjadi gelombang akan mencapai pantai dalam waktu 20 – 40 menit bahkan kurang dari itu.

Dalam beberapa kasus, khususnya wilayah Indonesia timur tsunami dapat dihasilkan di laut dalam, sebagai contoh Flores sea, Banda sea, Ceram sea, Maluku sea, dan lain-lain.

Dalam kasus ini, tsunami dapat mencapai daratan kurang dari 10 menit / very local tsunami. Tidak banyak waktu untuk menyampaikan peringatan dan melakukan penyelamatan dengan baik.

Bagi masyarakat yang dekat dengan sumber tsunami mungkin tidak mempunyai waktu yang cukup untuk evakuasi, sehingga emergency respon perlu memasukkan upaya evakuasi vertikal.

Letusan gunung berapi Krakatau 1883 telah menghasilkan tsunami destruktif lebih tinggi dari 40 m pada pantai di Indonesia dan lebih dari 36 000 orang meninggal. Osilasi permukaan laut akibat letusan tersebut dilaporkan memiliki jarak yang signifikan di Samudera India, Atlantik dan Samudera Pasifik.
Banyak bukti manifestasi dari Tsunami yang diakibatkan oleh letusan gunung Krakatau telah menjadi bahan diskusi intensif, dan beberapa diantaranya tidak terkait langsung dengan penjalaran gelombang tsunami yang diakibatkan oleh letusan gunung Krakatau.
Penyebaran gelombang tsunami yang dihasilkan oleh letusan gunung berapi Krakatau ke seluruh dunia dipelajari dengan melakukan simulasi numerik menggunakan dua model konvensional yaitu metode ray tracing dan model linear shallow-water 2D. Hasil simulasi numerik telah dibandingkan dengan data yang tersedia tentang tsunami akibat letusan gunung Krakatau tersebut

Sumber : B. H. Choi, E. Pelinovsky, K. O. Kim and J. S. Lee
Natural Hazards and Earth System Sciences (2003) 3: 321–332 c
European Geosciences Union 2003

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
Many studies of some great earthquakes in subduction zone of Sumatera had been carried out. Those investigations have contributed significantly on the seismic hazard potential at this area. Natawidjaja, 2007 noted that the potential megathrust earthquake at the area of the subduction depends on the fault segmentation, the dimension of the locked region and the history of the earthquake. These parameters determine the strain energy accumulation area that can generate a big earthquake. Subarya et al, 2006 had been investigated the great earthquake of Aceh-Andaman (2004, Mw 9.15), while Briggs et al, 2006, investigated the Nias Simelue (2005, Mw 8.7). Both of the earthquakes were characterized by the seismic gap zone. The Aceh Earthquake was already signed by the Simeuleu Earthquake of 2002 (Mw7.4), and then the Aceh Earthquake was assumed as an earthquake-triggered of the Nias Earthquake which occurred three months later after that.

After a series of two large earthquakes in the northern zone of Sumatra, Natawidjaja et al, 2007 predicted the megathrust of Mentawai Earthquake in the south of Sumatera. This prediction based on a study of paleogeodesy and paleoseismic (Natawidjaja, 2003) that noted the last major earthquake in Mentawai had occurred in the 1300’s and 1600’s, therefore, the cycle of the Mentawai Megathrust Earthquake is about 200 years.

Most of the big earthquakes occurred at the Sumatera area generated a tsunami. Jaiswal et al., (2006), noted that there were 33 tsunamis occurred at the Sumatera area. As a part of Sunda Arc, Sumatera had been much more active than Java. Table 1 gives a list of Tsunami Occurrence in Sumatera area.

From the list of tsunami events, generally, the tsunamis in the Sumatera area are generated by an earthquake with a magnitude of more than 7 Ms. The Mentawai Earthquake which happened on Oktober 25th, 2010 is an earthquake that generated a tsunami due to its magnitude and depth. This study investigated a Mentawai Tsunami which generated by an earthquake with a magnitude of 7.2 Mw. The earthquake has occurred along the plate interface boundary between the Australia and Sunda plates at Pagai Selatan Sumatera. According to BMKG, the earthquake located at the location of 3.6oS and 99.9oE with 10 km depth. This big earthquake occurred due to the movement of the Australia Plate with respect to the Sunda Plate with a velocity of approximately 50-70 mm/yr.

The Mamoru Nakamura’s Program was applied in this study to run a modeling of the Mentawai tsunami. A field study to the Mentawai Islands after the event had also been carried out in this research to provide the height of tsunami run-up in that area to validate the tsunami modeling

The establishment of the tsunami warning system is based on the fact, that the seismic waves propagate faster then tsunami wave. The tsunami wave arrive in the shore line after the earthquake parameters can be determined, so that there is time left to evaluate the earthquake, whether the earthquake is tsunamigenic or not, disseminated the information or warning to the target area, and finally to do evacuation if necessary. For the tele-tsunami, in which the tsunami will arrive in the beach more then one hour after the origin time of earthquake, generally there is enough time to do all of those processes, so that the warning can be disseminate in 100 % confidence level. For local tsunami, in which the tsunami will arrive in the beach less then one hour, the warning is disseminated in the lower the confidence level. For example, Indonesia Tsunami Warning System (InaTEWS) is designed for local tsunami warning for Indonesian region. Any tsunami-genic earthquake occurs in the Indian Ocean plate boundary, the tsunami wave will sweep the closest shore line within 20-40 minutes after the origin time of the earthquake. The warning is determined just only based on the evaluation of the earthquake parameters and tsunami modeling. It is difficult to add sea level data to increase the confidence level of he warning. Although the confidence level is lower, the tsunami warning could be disseminated. In some cases, especially for the eastern of Indonesia, the tsunami can be generated within inner sea, such as Flores sea, Banda sea, Ceram sea, Maluku sea, and any others. In that cases, tsunami wave will reach the closest shore line within 10 minutes or less. This tsunami may be called as very local tsunami. By evaluating the time line for issuing the tsunami warning, it is clear that very critical to disseminate tsunami warning based on the available technology recently. The effectiveness of the warning is very small, therefore another possibility warning for very local tsunami should be designed. In this presentation, we propose to use natural signs as the warning. As we might know that in general tsunami can be generated by large earthquake.

This kind earthquake could be felt strongly by most peoples. This natural sign could be applied as a tsunami warning. The other possibility is to use rather simple technology, as compliment to the natural warning. A very local tsunami warning system can be designed as the following. The warning system consist of two devices, those are a three component broadband seismograph and a local siren. The seismograph can be used to monitor and estimate the magnitude of the local earthquake and the siren can be used to warn the people if the earthquake is possible to generate tsunami.
Although, the natural signs and the simple technology could be combine to establish the very local tsunami warning system, the important is to educate people so that they understand the earthquake,tsunami, and how they can save from those disasters