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General Article the Indian Ocean Megatsunami Of J. Natn.Sci.Foundation Sri Lanka 2005 33(2): 69-80 GENERAL ARTICLE THE INDIAN OCEAN MEGATSUNAMI OF DECEMBER 2004: THE SCIENTIFIC BASIS OF THE CATASTROPHE ANANDA GUNATILAKA+ 10, Thumbowila, Piliyandala. Abstract: The Earth's surface is made up of 13 rigid a distance of -17,000 km in 22 hours before it lithospheric plates that move towards or away from each struck Hawaii and Japan, when < 200 people other, much like the pieces of a jigsaw puzzle. On the 261h of perished. Basically there have been four December 2004, three of these plates interacted in the eastern Indian Ocean to cause an earthquake of magnitude 9.3 and catastrophic tsunami in 300 years and the first in generated a tsunami or giant ocean wave that devastated the Indian Ocean. The 1881 and 1883 tsunami the coastlines of nine countries including Sri Lanka, killing that were felt in Sri Lanka were non-entities. In over 250,000 people. This is the greatest natural disaster to 1970 and 1992 respectively, 330,000 and 210,000 strike Sri Lanka in our recorded history. In this article, the people reportedly perished in the famous cyclones geological and seismological basis of this awesome catastrophe is explained in terms of causes and effects and assesses the of the Bay of Bengal and over 300,000 in Calcutta potential tsunamigenic risk to Sri Lanka. It is argued here in 1737. Destructive cyclones are more probable on the basis of available data that a tsunami event of similar than a future tsunami affecting Sri Lanka. magnitude has a low probability of occurring in the "foreseeable future" within the Central Indian Ocean region. It is a misconception to believe that all Key Words: convergent margins, earthquakes, high tsunami are caused by earthquakes. In fact, there magnitude, Indian Ocean, me gat sun am^ underthrusting is a tsunami that is plates. seismic activity and earthquakes. These are the massive submarine sediment slides (or undersea INTRODUCTION avalanches) generated within continental slopes due to slope and sediment instability; massive Tsunamis are frequent events in the Pacific Ocean landslides or avalanches that end in the ocean region, where 80% of the World's seismicity is (called mass-movement events); large meteorite concentrated (Figure 1). The Indian Ocean has impacts and violent volcanic eruptions. Even a been traditionally considered a "tsunami slight tremor can trigger off a submarine slide. backwater", where destructive cyclones are more These have also occurred throughout geologic frequent. While a tsunami is to be expected history. The huge sediment masses in motion will somewhere in the Pacific about every ten years, displace a massive volume of water to generate it is more than a "century scale" event in the other destructive tsunami. In 1958, an avalanche that Oceans. Yet, there have been seven tsunami landed in Lituya Bay, Alaska, created the largest occurrences in the Indian Ocean region during the recorded tsunami (524 metres high), bigger than period 1524 to 1943 (Table I), but none of them the tallest building on Earth. The tsunami had a transregional effect like the one in 2004. generated by the stupendous explosion of The December 2004 tsunami is now the greatest Krakatoa volcano in the straits between Java and ever recorded anywhere in terms of loss of life Sumatra (in 1883) reached 40 metres in height and material destruction. In the last 300 years, and drowned 36,000 people on nearby coasts. Its there have been only three tsunami that caused effect was felt in Sri Lanka within hours of the comparable loss of life (from both the quake and eruption. The Ceylon Observer 0f27~~August1883 tsunami). They were the famous Lisbon, Portugal reported the event. Only one death was reported. tsunami (1755): 8.7111 and 60,000 dead; Messina, Fortunately, Sri Lanka does not appear to be in Italy (1908) 7.2M and 160,000 dead: and Awa, danger from these non-seismic tsunamis. Japan (1703); >100,000 dead. The largest earthquake ever recorded (9.5M) and Alternatively, not every high magnitude accompanying tsunami (1964 in Chile), resulted submarine earthquake will generate tsunami.' in a total loss of life of less than 5000. It travelled Just three months after the December 2004 "Consultan,t,National Science Foundation of Sri Lanka, 4715, Maitland Place, Colombo 07 Ananda Gunatilaka Table 1: Tsunami occurences in the Indian the eastern Indian 0cean.l Today, the two ocean from 1524-1943 continents are moving independently of one another and in slightly different directions (Figure Date Location 2). A moving mass (India) forced into an immovable object (Eurasia) resulted in the 1524 Maharashtra, India buckling of the oceanic lithosphere at the 2ndApril 1762 Arakhan, Myanmar equatorial latitudes. This splitting created the 13th 16thJune 1819 Rann of Katch, Gujarat iithospheric Plate. Geologists discovered a broad 31" October 1847 Great Nicobar Island, India zone extending about 1000 km east to west in the 3lStDecember 1881 Car Nicobar Island, India equatorial Indian Ocean, where the ocean floor (7.9M) was compressed and deformed. Deep Ocean 26thAugust 1883 Krakatoa, Indonesia drilling recovered rock samples, which indicated 26thJune 1941 Andaman Islands, India that the buckling and fracturing started around (7.7M) 8 Ma ago - coeval with the maximum elevation 28thNovember 1943 Makran, Baluchistan attained by the Tibetan Plateau in the collision proces~.~ Tsunami, an 8.7M earthquake (March 2005) occurred again in Sumatra, but. no big tsunami The 9.3M megathrust earthquake (Richter Scale) that generated theDecember 2004 was caused. This earthquake destroyed the island tsunami originated offshore to the west of of Nias with much loss of life. It is well to remember that the earthquake belt in Indonesia northern Sumatra close to the tiny island of Simeulue (the large star in Figure 2) and is also coincides with a parallel volcanic belt along the trench (Figures 2 and 7). The purpose of this probably the second highest ever recorded in the article is to give a general and semi-technical sea. The Sumatra area is known for its recurrent earthquake activity as it is on the boundary where analysis of the science behind the 2004 tsunami catastrophe to a wide segment of readers and four tectonic plates of the Earth intera~t.~This emphasize the importance of science in our boundary called a Subduction Zone is represented country's affairs. Two newspaper articles by the bathymetrically by the curved Sunda Trench, a author on the tsunami for the general public long fault plane and a nearby volcanic arc. Here appeared in the The Island (4thJanuary 2005), the north-east moving sea floor (at -6 cm per year) The Daily News (6" January 2005) and The of the Indian Plate is thrust (or subducted) under Sunday Times (16thJanuary 2005). the adjacent and overriding Burma Micro-Plate at an angle of -lo0. Strain builds up in the THE SEISMIC HISTORY OF THE EASTERN subduction zone, until the "frictional lock" in the INDIAN OCEAN rocks is broken and a rupture occurs. The earthquake did not exactly happen at a point along The break-up of the Indo-Australian plate and the the fault. It ruptured over a length of about 400 earthquake in Sumatra km. This initial rupture propagated northwards at around 2 km sec-I over a section of the long The origins of the present seismicity in the eastern fault, producing rock deformation. The interplate Indian Ocean goes back to well over 50 Ma (Eocene thrust bounda.ry region is in fact a decoupled era) when the Indian subcontinent began its7great thrust fault, where the overriding Burma Plate push northward towards Eurasia, resulting in the in addition has a right lateral slip component collision and uplift of the Tibetan Plateau and (transcurrent fault). Himalayan Mountains. Starting about 8 Ma ago, the accumulated mass of these two continents and It is no coincidence that the epifocus of the accompanying stretching of the ocean floor the earthquake at -3.3ON latitude also lines up became so great that the original Indo-Australian with the Indo-Australian plate boundary at its' Plate buckled and fractured under stress, eastern end and is the location of maximum compression where the three plates interacted resulting in two separate plates - the Indian (which also carries Sri Lanka) and Australian, (Figure 2). Trenches are the areas where sea floor with the plate margin located at - 3ON latitude in is consumed by the Earth's mantle during the Asian Tsunami of 2004 Figure 1: Distribution of the World's seismicity (red dots). Note that -80% of the seismicity is in the Pacific Ocean region Source: Press and Siever, 1994. W.H. Freeman & Co. New York. (as the laws of physics demand). This happens at trench. An earthquake occurs when rocks being the mid-ocean ridges lying to the south and west deformed suddenly break along the fault, of the deformed region in the Indian Ocean (Figure releasing long accumulated stresses. The blocks 3). It has been shown that the outward movement of rock on either side of the fault slip suddenly, of newly created sea floor could be accommodated setting off ground vibrations. Slippage is most only if a distinct plate boundary existed between common along plate boundaries. The distribution separate Australian and Indian plates across the of aftershocks from Sumatra to Andaman Islands equatorial Indian O~ean.~In the kastern part of suggests that the rupture had a maximum length this new plate boundary the Australian Plate is of -1200 km parallel to the Sunda Trench and a moving counter-clockwise to the Indian Plate. On width of over 100 km perpendicular to the the western side (i.e. towards Sri Lanka), the two earthquake source.
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