Preparation for Future Earthquake and Tsunami Hazards: Lessons Learned from the 2004 Sumatra-Andaman Earthquake and the Asian Tsunami

Transcription

1 First International Conference of Aceh and Indian Ocean Studies Organized by Asia Research Institute, National University of Singapore & Rehabilitation and Construction Executing Agency for Aceh and Nias (BRR), Banda Aceh, Indonesia Preparation for Future Earthquake and Tsunami Hazards: Lessons Learned from the 2004 Sumatra-Andaman Earthquake and the Asian Tsunami Kenji Satake Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Japan Not to be quoted without permission from the author

2 The 2004 Sumatra-Andaman earthquake caused the worst tsunami disaster in history. In order to reduce damage from future natural hazards, important steps include (1) scientific research, (2) capacity building, and (3) education and awareness. Scientific research on earthquakes and tsunamis helps to better understand what happened in the past, what is happening now, and what will happen in the future. Recent seismological developments make it possible to estimate the earthquake source parameters within minutes after its occurrence and utilize it for tsunami warning. Study of past earthquakes, both historic and pre-historic time, would help to estimate the future probabilities. Capacity building includes early warning systems, hazard maps, and infrastructures to disseminate such information. Implementation of tsunami warning system and international coordination in Indian Ocean countries are on the way. Computer simulation helps to make hazard maps and to assess future tsunami damage. Education and awareness to coastal residents are an important component for reducing future casualties. Even if the infrastructure is completed, the disaster will repeat, unless the coastal residents take the evacuation action. Because earthquakes and tsunamis are rare phenomena, the 2004 tsunami memory needs to be transmitted to future generations. 1. Introduction The 2004 Asian tsunami was the worst tsunami disaster in history. The tsunami, caused by the giant Sumatra-Andaman earthquake on December 26, 2004, devastated the shores of Indian Ocean. Within a half hour of the earthquake, the tsunami devastated Banda Aceh and other coastal villages of Sumatra Island. The tsunami arrived at Thai, Sri Lankan and Indian coasts in about two hours after the earthquake. The tsunami further propagated and arrived at the eastern coast of Africa in several hours. The total number of victims, death and missing together, is estimated as 230,000 (International Federation of Red Cross and Red Crescent Societies, 2005); the largest in Indonesia (163,795), followed by Sri Lanka (35,399), India (16,389), Thailand (8,345), and Somalia (298). Fig. 1. Tsunami propagation (contour in hours) in the Indian Ocean. Solid circles indicate aftershocks within one day of the mainshock shown by the star (data by U. S. Geological Survey). Draft Copy Not to be Quoted Without Permission from the Author 2

3 In the Pacific Ocean, a similar, ocean-wide tsunami occurred in This tsunami was generated by the giant Chilean earthquake, which remains the largest instrumentallyrecorded earthquake in the world. The tsunami caused more than 1,000 casualties along the Chilean coast, propagated across the Pacific Ocean, taking 61 lives in Hawaii 15 hours later, and reaching the coast of Japan in 23 hours and claiming 142 more casualties. Following this Pacific-wide tsunami, international coordination started on operational tsunami warning systems and scientific studies of tsunamis around the Pacific Ocean. In this paper, current knowledge about earthquakes and tsunamis are first reviewed, with a brief history how research has contributed to reduce tsunami hazards in the Pacific Ocean. As products of such scientific development, tsunami warning system and tsunami hazard maps are introduced. These technologies existed in 2004, unlike at the time of the 1960 Chilean tsunami, but they could not prevent the worst tsunami disaster in history. The importance of education and awareness is also discussed. 2. Earthquake Recurrence In the source area of the 2004 Sumatra-Andaman earthquake, the Indian plate is sinking beneath the Burma microplate at a rate of 5 cm per year (Lay et al., 2005; Fig. 2). This subduction causes upper plate to be dragged and deformed up to a certain limit. When the strain reaches the limit, the two plates are rebound to cause fault motion and generate an earthquake. This is the mechanism of an interplate earthquake. While the epicenter of the 2004 event was located the west off Sumatra Island, the aftershocks extended through Nicobar to Andaman Islands; the total length of the earthquake fault is estimated as more than 1,000 km. The slip amounts on the fault are estimated to be 20 to 30 m off Sumatra. The earthquake size, or moment magnitude (Mw), is calculated from the fault size and the slip amount; Mw of the 2004 earthquake is estimated as , the largest in the world since the 1960 Chilean earthquake (Mw 9.5). Fig. 2. Map of the source region of the 2004 Sumatra-Andaman earthquake. Draft Copy Not to be Quoted Without Permission from the Author 3

4 Seismology, both theory and observation, has significantly advanced since 1960, at the time of the Chilean earthquake. The plate tectonics theory was introduced to explain the mechanism of great earthquakes. Mathematical models of earthquake source relate seismic moment and fault parameters. On the observational side, global seismograph network was deployed in the 1960 s. Using these theories and observed data, fault parameters of many large earthquakes in the world were determined in the 1970 s. The moment magnitude (Mw) scale, to measure the size of earthquake fault motion, was also introduced. It took more than a decade to accurately estimate the size of the 1960 Chilean earthquake. Theoretical and computational developments made it possible to compute seafloor deformation from fault models and the tsunami propagation on actual bathymetry. In the 1980 s, digital recording and processing of seismograms made it possible to monitor earthquakes in the world, to estimate the basic earthquake source parameters immediately after their occurrence. In the 1990 s, networks of Global Position System made it possible to monitor crustal deformation, plate motion and strain accumulation. Developments of Internet enabled scientist to share information such as the results of seismic wave analysis in real time. Thanks to these developments, it is now possible to estimate the source parameters within minutes after large earthquakes and utilize it for the tsunami warning purposes (see Section 4). The plate tectonics theory predicts that great earthquakes recur at the plate boundaries more or less regularly. If the fault slip occurs only during earthquakes, the 20 to 30 m of fault slip of the 2004 earthquake represents 400 to 600 years of plate motion. In order to estimate how often a 2004-sized earthquake happens, we need to know the past history of such earthquakes. While the records of European visitors exist only after around the 16th century, local history may have recorded past earthquakes and tsunamis. For example, studies of historical records in Japan have shown that large plate-boundary earthquakes have repeated with intervals of about 100 years since the 7th century. The Japanese tsunami damage records also helped to identify the date and size of the 1700 earthquake in North America. In addition, geological evidence of past earthquakes has been used to study past great earthquakes (Sieh, this meeting). Once the cyclic nature of past earthquakes is known, it can be used to calculate the probability of earthquake occurrence. The probability of next earthquake in specific time window can be computed from the average recurrence period and the date of the most recent event, as well as a parameter indicating the irregularity. Such probabilistic earthquake forecast had been made around the Pacific (Nishenko, 1991) or Japan ( 3. Tsunami Generation and Propagation Tsunamis are generated by submarine earthquakes, volcanic eruptions or landslides. Such submarine geological processes produce water surface disturbance, which propagates toward coasts. Depending on the relation between wavelength and water depth, water waves can be classified into shallow-water (or long) waves and deep-water (short) waves. The Indian Ocean or Andaman Sea is deep, up to 4,000 m or 4 km, but the wavelength of seafloor deformation is an order of 100 km, much larger than the water depth. Hence we can use the shallow water approximation for tsunamis generated from earthquakes. The speed of shallow-waters, or tsunamis, is given as a square root of product of water depth and the gravitational acceleration. For the 2004 tsunami, the tsunami travel time to Thailand and Sri Lanka are similar, despite the distance is different, because the Andaman Sea is shallower than Bay of Bengal (Fig. 1). In the deep ocean, at 4,000m, the tsunami velocity is about 700 km/h, a speed of jetliner. In shallow water near coasts, at 40 m depth, the velocity becomes 70 km/h, a speed of automobile, but Draft Copy Not to be Quoted Without Permission from the Author 4

5 becomes larger and more dangerous. Tsunami is a Japanese term, meaning harbor wave. The tsunami propagation and coastal behavior can be modeled by computer simulation, once its source is known (Geist et al., 2006; Satake, 2002). For an earthquake, or fault motion, the elastic dislocation theory shows such deformation that seafloor just above the fault is uplifted while above the deeper end of the fault is subsided (Fig. 3). The water above the fault is vertically moved in a similar way and becomes the source of tsunami. Because the tsunami wave propagates in both directions, those in the east would first observe the receding wave, whereas those in the west would observe sudden rise in water. Such a feature of tsunami propagation was reproduced by computer simulation (Fig. 4). It shows that the water depression, or receding wave, propagate toward Thailand, whereas to the west, say toward Sri Lanka, high water is traveling. In fact, the observed and recorded tsunami on tide gauges in Thailand clearly showed initial depression wave, while those in Sri Lanka or India shows sudden rise of sea level. The computer simulation also shows that tsunami heights are larger to the east and west of the source, in directions perpendicular to the fault. Fig. 3. Cross-section of seafloor deformation for the 2004 Sumatra-Andaman earthquake. 4. Snapshots of computer simulation for tsunami propagation. Fig. Draft Copy Not to be Quoted Without Permission from the Author 5

6 4. Tsunami Warning Systems Although tsunami travels very fast as an ocean wave, the speed is still much slower than seismic waves. The longer the distance from the earthquake source, the longer the time difference between seismic and tsunami wave arrivals. Depending on the distance, tsunami warning systems can be classified into two types: distant tsunami warning and local or regional tsunami warning systems. In the Pacific Ocean, three tsunami warning centers, Pacific Tsunami Warning Center in Hawaii, West Coast/Alaska Tsunami Warning Center in Alaska, and the Northwest Pacific Tsunami Advisory Center in Tokyo, monitor seismic activity and issue tsunami warning. All the centers share information and coordinate message content before issuing warning or advisory messages. Because there are hours before tsunami arrival, it is very important to actually confirm the tsunami generation. For this purpose, sea level monitoring systems, located on coasts and offshore, are necessary. If tsunami is actually measured, the tsunami warning message can be updated. If no tsunami is detected, or when the generated tsunami becomes smaller after some time, the tsunami warning must be canceled. At the time of December 26, 2004 tsunami, Pacific Tsunami Warning Center issued the first information bulletin at 1:14 GMT, only 15 minutes after the earthquake. An earthquake was located off the west coast of Northern Sumatra, but the earthquake size (magnitude) was estimated as 8.0. The second bulletin was issued at 2:08 GMT, 69 minutes after the earthquake yet before the tsunami arrivals at Thai, Sri Lankan or Indian coast. The earthquake size was upgraded to 8.5 and a possibility of local tsunami was mentioned in the bulletin. For local tsunamis, time is more critical. Japan Meteorological Agency (JMA) is responsible to issue tsunami warnings in Japan. Currently, six Regional Tsunami Warning Centers are under operation for 24 hours a day and seven days a week. JMA uses data from hundreds of seismic stations to detect tsunamigenic earthquakes, and data from sea level monitoring stations to confirm the tsunami generation. JMA has constantly improved the tsunami warning system. In the 1950s, it took about 20 minutes to issue tsunami warnings, but now they can issue warning message only 2 to 5 minutes after a large earthquake. To forecast the tsunami arrival times and heights, database with the results of 100,000 pre-made simulations are used. To disseminate tsunami warning message to coastal people, infrastructure is needed in each country. Coastal residents usually receive warning messages through TV, radio, or emergency broadcast systems. Use of modern technology such as cellular phones or satellite broadcast system is being developed. After the 2004 tsunami, various efforts have been made to construct tsunami warning and mitigation systems in the Indian Ocean, because lack of such systems was considered to contribute to the severity of the 2004 Indian Ocean tsunami. The IOC (Intergovernmental Oceanographic Commission) under UNESCO (United Nations Educational, Scientific and Cultural Organization) decided to organize Intergovernmental Coordination Groups in the Indian Ocean with about 30 member countries, as well as North-eastern Atlantic Ocean and the Mediterranean Sea, and Caribbean Seas (Fig. 5). Such an ICG was formed in the Pacific after the 1960 Chilean tsunami, and has been active in the last 40 years to exchange information and coordinate international activities. The International Tsunami Information Center in Hawaii coordinates international activities on tsunami warning systems, helps countries to establish national warning and mitigation systems, collects publications and other materials on tsunami events, and develops educational and awareness materials. Draft Copy Not to be Quoted Without Permission from the Author 6

7 Fig. 5. International coordination for tsunami warning and mitigation systems (left) and operational tsunami warning centers in the Pacific (right). 5. Tsunami Hazard Mitigation Once the coastal residents receive tsunami warning message, they need to know what it means, and where to evacuate. An effective tool is a hazard map showing the tsunami risk zones. Possible flooding zones can be estimated either from historical data of past tsunamis or by computer simulations. Safe evacuation places such as tsunami shelters should be also shown in the hazard maps. Tsunami hazard map will help coastal communities prepare for tsunami hazards. One of the lessons of the 2004 Indian Ocean tsunami is that not only coastal residents but also foreign tourists need to be informed about tsunami hazard, because about a half of tsunami casualties in Thailand was foreign tourists. In the Hawaiian Islands, tsunami hazard maps are prepared and published in the local phone books that are available at every hotel room. Those in highrise hotel buildings are advised to move vertically to the third or higher floors, rather than moving out of the possible flooding area. Fig. 6. Tsunami warning system (center), hazard assessment (left) and education systems (right) to reduce tsunami hazards. Draft Copy Not to be Quoted Without Permission from the Author 7

8 6. Education and Awareness The last, but not the least, important component of tsunami mitigation system is education and public awareness. Before 2004, few people around the Indian Ocean knew the word tsunami. After the 2004 tsunami, numerous books and videos have been published and used for education. In Japan, a famous story, called Inamura-no-hi (fire of rice sheaves), has been used for tsunami education. After a strong earthquake was felt at a coastal village in 1854, the village chief put fire on his just-harvested rice crops to guide villagers to high ground and to save their lives. This story has been translated into nine different languages and cultures for tsunami education. Periodic practice and drills are very important to keep the tsunami warning and mitigation system functional. In May 2006, the first Pacific-wide exercise was held on issuing, disseminating and responding to tsunami warning messages. In Indonesia, tsunami evacuation drills were carried out in several communities including Banda Aceh, Padang and Bali in the last years. 7. Conclusions (1) The 2004 Sumatra-Andaman earthquake, the largest event in the last 40 years, caused the worst tsunami disaster. The 1960 Chilean earthquake also caused trans- Pacific tsunami damage. (2) Large earthquakes repeat at the plate boundaries. Seismological developments since 1960 made it possible to analyze seismic data in real time to estimate earthquake size, type and tsunami potential for the purpose of tsunami warning. Past tsunamis can be studied by historic and geologic data. Data from such seismological studies can be used for future earthquakes and tsunamis. (3) Tsunami propagation depends only on water depth. Tsunami generation and propagation can be numerically simulated on actual bathymetry. Numerical simulations are used for tsunami research, warning system and hazard assessments. (4) Current scientific knowledge and technology make it possible to set up tsunami warning system, which can issue message before actual arrival of tsunamis. International coordination has been made not only in the Indian Ocean but also other oceans in the world. (5) Hazard assessments using hazard maps and education and awareness efforts are equally important to reduce tsunami damage. Draft Copy Not to be Quoted Without Permission from the Author 8

9 References Geist, E.L., V.V. Titov, C.E. Synolakis, Tsunami: wave of change, Scientific American, 294, (2006). International Federation of Red Cross and Red Crescent Societies, World Disaster Report, Kumarin Press, 246 pp. (2005). Lay, T., H. Kanamori, C. J. Ammon, M. Nettles, S. N. Ward, R. C. Aster, S. L. Beck, S. L. Bilek, M. R. Brudzinski, R. Butler, H. R. DeShon, G. Ekström, K. Satake and S. Sipkin, The great Sumatra-Andaman earthquake of 26 December 2004, Science, 308, (2005). Nishenko, S.P., Circum-Pacific Seismic Potential, Pure and Applied Geophysics, 135, (2001). Satake, K. in W. H. K. Lee, H. Kanamori, P. C. Jennings and C. Kisslinger (eds.) International Handbook of Earthquake and Engineering Seismology, 81A (2002). Draft Copy Not to be Quoted Without Permission from the Author 9