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Catastrophic Tsunamis In the Indonesian Archipelago

Tsunamis are not rare events for the Indonesian Archipelago as a consequence of four major plate boundaries that meet and collide, and producing highly active seismic zone that is mostly located under the sea. The ‘tsunami season’ within this region started in 1992 at Hading Bay, Flores Island, with casualties of more than 2000 people, followed almost every two years with the 1994 East Java Tsunami, 1996 Tonggolobibi Sulawesi Tsunami, 1996 Biak Tsunamis, 1998 Papua New Guinea Tsunami, and the Banggai Tsunami in 2000. These represent the tsunami season for the eastern part of the Archipelago, since the western part of Archipelago was mostly quiet until the 26 December 2004 Great Sumatra Earthquake and Tsunami, which occurred at a place that never been thought before on the western-most part of the Archipelago. This earthquake and accompanying tsunamis not only a starting point of tsunami season for the western part of the Indonesian Archipelago, but also as a defining moment for the people who lived in the region in looking at the constellation of the archipelago, that changed the development paradigm into natural hazards based development program. The 26 December 2004 event (Mw > 9.0) is a turning point for the tsunamigenic earthquake studies along the subduction zone. Intensive research of this event provides a new insight into tsunami dynamics and characteristics as reflected by the erosional and deposition patterns of the coastal areas, wave run-up height, flow depth and inundation, wave front and bore formation, wave-structure interactions, coastal protection and management of the low-lying areas, and the importance of consistent education and local knowledge about natural hazards (earthquake and tsunamis). The following event on 28 March 2005 (Mw = 8.5), occurred only 250 km distant from the 26 December 2004 event, and continued further south to Java Island on 17 July 2006 (Mw = 7.6) and 12 September 2007 (Mw = 8.4) on southwest Sumatra Island (Bengkulu) along the Java Trench. Field surveys results, and analysis of the source mechanism and distribution of the resulting tsunami waves along the coast, shows that each near-field tsunami generated within the subduction zone is unique and complex. Scientists have urged the public and policy makers to consider all subduction-type tectonic boundaries to be “locked, loaded and dangerous” zones that possess potential tsunami threats. Reliable and comprehensive databases for past and recent events and subsequent scientific analysis are needed in mitigating the tsunami hazards. Fourteen (14) segments of potential catastrophic tsunamigenic earthquake and 21 volcanogenic tsunami sources were identified based on seismotectonic assessment, historical record, paleotsunami deposits and micro-atoll studies, as well as volcanic type and activities. Most of the volcanogenic tsunamis sources are located in the eastern archipelago around the Banda Arc and the Molluca Sea. Tsunamis from volcanic sources had different characteristic to tsunamis generated by an earthquake mechanism, both in the near field and also far field as revealed by numerical modelling assessment. A numerical modeling of tsunami based on scenarios developed, shows the region is very susceptible to tsunamis with elevation at the shoreline greater than 8 m. With this elevation, there is no structural mitigation that is economically feasible to protect long a coastline based on the assessment of the 26 December 2004 event. The nonstructural mitigation measures such as mangroves and coastal forest or in combination with other soft options such as sand dunes, provides protection to some extent. However, further research needs to be carried out in defining appropriate mitigation measures. These high hazard zones require ‘sacrifice zone’ of at least 1 km from the shoreline, and vertical evacuation is needed to save lives. iii Detailed assessment of tsunami inundation based on the 26 December 2004 event revealed that the distributions of the flow depths are not always inline with distribution of the flow speed. The areas that experienced the deepest flooding does not necessarily experience the fastest flows, while the damage within urban and rural areas mostly coincided with the flow speed distribution rather than runup and inundation depth distribution. Consequently, in assessing the tsunami hazards, especially when making inundation maps, the overland flow speed should be taken into account or incorporated into the inundation map. However, the problem is that not all coastal areas have nearshore bathymetry and topography data at a resolution needed to represent the nearshore and overland flow dynamics. Results from assessment of the tsunami field survey and damage data from recent events provide the necessity information to derive the hazards level that correlate the tsunami elevation at a shoreline with destruction scale inland. This provides enough information to permit the construction of hazard maps for the region where detailed nearshore bathymetry and topography data are not available. The tsunami elevation at the shoreline can be derived from numerical models. As demonstrated during the 26 December 2004 event, the impacts of tsunamis on the coastal areas include not only the destruction of the infrastructure, buildings, housing, coastal landforms as well as a massive casualties, but also the resulting waste and debris that mixes with other flotsam during wave runup and backwash. This may create another huge problem that leads to serious long term adverse environmental consequences. Debris dispersal modelling is applied to the Banda Aceh region based on that event, and shows that understanding the pathway and distribution of the suspended materials and flotsam caused by tsunamis is important for proper hazard mitigation planning and waste management action. In assessing the potential future events, there is uncertainty and some disagreement from results of the tsunamigenic earthquake recurrence interval based on the empirical formula used. These need to be refined with more data such as from continuous Global Position System measurements. Likewise for volcanogenic tsunamis sources, which are better defined by their location but difficult to determine which processes are dominant to generate catastrophic tsunamis for the next events. The rule of thumb of the sea receding as a sign for impending tsunamis from the subduction zone earthquake source is not applicable for most of the volcanogenic tsunamis. For a tsunami generated by volcanic eruption, the warning is the eruption itself, which could be several days before a tsunami event. More research is required to better understand the characteristic of volcanogenic tsunamis. In general, the arrival time of tsunamis along the subduction zone within the Indonesian Archipelago is within 10 – 30 minutes. The best lesson learned is from the people in Simeulue, who recognized a simple messaged, if a significant ground shaking was felt, and the sea recedes; then evacuate to higher ground. This type of community warning and self-evacuation are a challenge for modern life style in the city. Integration of life-long efforts to educate the population about the hazards and preparedness for an extreme event is needed. The most favorable way is to include earthquake and tsunami hazards, and preparedness as part of educational curricula taught at schools.
Type of thesis
Prasetya, G. (2010). Catastrophic Tsunamis In the Indonesian Archipelago (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/4167
University of Waikato
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