TSUNAMI HAZARD MITIGATION:  LESSONS LEARNED FROM SUMATRAN EARTHQUAKES
	Hery Harjono
	Research Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Jln. Pasir Putih 1, Ancol Timur, Jakarta, Indonesia hharjono@geotek.lipi.go.id
	The Earth's crust actually consists of several major plates and many micro plates. Each of the plate moves in different rates and directions. All of the contacts between two or more plates, as the results of those movements, produce the tectonic activities on the Earth. Those tectonic activities include earthquakes, volcano eruptions and land slides. All of those might be the trigger of tsunamis. Unfortunately, earthquakes, as the major cause of the tsunami, are not predictable. We can show you the hazard map, showing places where the earthquakes will likely to happen. But no scientist in the world has been able to predict exactly  when or where an earthquake will occur. Scientists from LIPI have already identified the tectonic segments of the western Sumatra. One of the results is the 200 years recurrent period of great earthquakes with deviation 20-30 years. Also already recorded in the last 10 years of research, several giant earthquakes in certain zones that followed by tsunamis. Giant earthquake and devastating tsunami hazard are still lurking the west coast of Sumatra (including several islands along the coast) in the next decades. Most people just started to see the significance of a planned earthquake/tsunami mitigation effort only after the catastrophe in Aceh at the end of the year 2004. As we all aware of, a devastating earthquake (magnitude 9 on Richter scale) followed by a giant tsunami hit the northern Sumatra, along the coast of Nanggroe Aceh Darussalam (NAD) Province on December 26, 2004. This catastrophe made us realized the need of national preparedness to deal with any natural hazards in the future to decrease the risk and the casualty. Accordingly, it is very essential to increase the preparedness to deal with future possible disaster. The mitigation program, ranging from improved building codes to public education to develop a tsunami early warning system, has a purpose to reduce losses from big earthquakes and tsunami hazards. One method to get ready for a future tsunami attack is to put a protection along the coast that has a high risk of tsunami destruction. Seashore protection can be a breakwater or a seawall. These types of protections were built in some places in Japan. However, physical constructions like those are very expensive. In addition, it also generates the environmental problem. For instance, it creates a gap between the ecosystem of the sea and of the land; it is also unappealing for people who live along the coast. Therefore, vegetation as the coast protection is preferable. Mangrove that has strong roots has been considered as the right choice to hold back a big wave such as a tsunami. LIPI is also conducting palaeotsunami study that focused on the study of tsunami events occurred in the past based on the sedimentation found along the coast. The objectives of this study are to gain information of tsunami recurrence pattern and characteristics of palaeotsunami such as wave height, run-up and flooded area. Preliminary result of palaeotsunami study in West Sumatra shows that there is no clear evidence related to great tsunami compare to those of tsunami has probably hit Simeuleu Island several times in the past. One possible explanation is the mangrove along the coast that may exist hundred years ago and the tsunami height was less than the recent tsunami. Researches using analytic, numeric and also laboratory experiments had shown that mangrove has an optimal capacity to reduce the wave energy caused by a tsunami.The use of mangrove as protection is effective in the long-term. It is relatively cheaper than building a seawall, so it is affordable for most of developing country. Furthermore, it also has many other functions, for example, as a great biota-breeding place, as a filter of seawater penetration, and as wild bird habitat.
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