Department of Earth Sciences Manipur University Imphal 795003 India
The Car Nicobar Islands was one of the several islands in Indian Ocean which were severally damaged by the Great Tsunami on 26 December 2004. The great tsunami event caused the devastation, loss of life and property in south and south East Asia. The objectives of the study are to assess the changes in the coastal features of the Car Nicobar Island using various indicators such as the tsunami height, run up elevation, water flow direction, erosion, sand deposition, and coastal subsidence. Andaman & Nicobar Islands are summit of the submarine mountain range lying on the great tectonic suture zone extending from Eastern Himalaya along the Myanmar border to Arakan and finally Sumatra and Lesser Sunda. The IRS P-6 satellite data has been found to be very useful in this regard. The digital analysis of the digital satellite data was done by using the image processing software (ERDAS IMAGINE 8.4) and DTM to quantify the tsunami height (15m), run up elevation (2-19 m), inundation (295-1203 m), and coastal responses. The settlement and coconut plantation has been severely damaged on the island, which is assessed by using GIS analysis of the pre and post satellite data.
The Car Nicobar Islands was one of the several islands which were severally damaged by the Great Tsunami on 26 December 2004. The origin of tsunami was series on undersea earthquakes, the largest being measured 9.3 M. The direct consequence of Great Earthquake, that ruptured the sea floor up to 100 km in places, was displacement of a huge volume of water that translated into tsunami of colossal proportion. The great tsunami event caused the devastation and a loss of life in south and south East Asia including the Andaman and Nicobar Islands. The geological formations represent a period of sedimentation from Cretaceous to Sub Recent period. The Nicobar Islands are part of a great island arc created by the collision of the Indo- Australian Plate with Eurasia. The collision lifted the Himalaya and most of the Indonesian islands, and created a long arc of highlands and islands, which includes the Arakan Yoma range of Myanmar, the Andaman and Nicobar islands, and the islands off the west coast of Sumatra, including the Banyak Islands and Mentawai Islands . htm). These islands are the summits of a submarine mountain range lying on the great tectonic suture zone extending from the Eastern Himalayas along the Myanmar border to the Arakan and finally Sumatra and Lesser Sundas. The geological formations represent a period of sedimentation from Cretaceous to Sub- Recent period. The surface deposits of gravel beds and raised soil covers that are of recent origin seem to be of Holocene age (> 10,000 yrs). The present configuration of these islands took shape only about 26 million years ago. These islands have a tropical climate and temperature ranges from 22º C to 28ºC. Rainfall is heavy due to annual monsoons and measures around 3000 to 3800 mm each year. The physiography of these islands is characterised by undulating topography and intervening valleys. There is no major perennial fresh water river in the islands. There are several rain fed streams. The coastal lines of these islands are wavy with large number of bays and lagoons. Soil cover is thin, varying from 2m to 5m. The coastal flats have an admixture of sand, silty and clay with fine fragments of coral lime. The vegetation of the Nicobars is typically divided into the coastal mangrove forests and the interior evergreen and deciduous Tropical and subtropical moist broadleaf forests. The present studies are made in Car Nicobar islands to assess the damages of tsunami and find out the geological evidences for tsunami characterization using RS data and GIS software. The objectives of the study are to assess the changes in the various coastal features of the island using various indicators such as the tsunami height, run up elevation, water flow direction, erosion, sand deposition, and coastal subsidence.
2 GREAT EVENTS OF 26 DECEMBER 2004:
The Great Earthquake of December 26 2004 that occurred off the west coast of Northern Sumatra took place at the interface between the India and Burma plates, where Burma plate has been referred by Andaman/Nicobar Ridge that acts as a small tectonic plate (Curray et.al., 1982). The event was considered as a mega thrust earthquake referring to the large cracking of the plate boundary. The mega thrust earthquakes often generate large tsunamis that can cause damage over a much wider area than is directly affected by ground shaking near the earthquake’s rupture. Tsunami was generated in the fast slip area (first 650 km at taut length) and the waves propagated in all directions. As a result, strongest waves hit the coasts of Indonesia, Thailand and other nearby areas (Andaman & Nicobar Islands) which are closely located on the east of the epicenter. The devastation in the Nicobar Islands was due to a 10-15 m high tsunami following the 2004 Indian Ocean earthquake. At least 3000 people were believed to have been killed on the disaster. Historical data does not provide a very realistic estimate of earlier tsunami events in the island, however, 31.12.1881 tsunami which was caused by an earthquake of magnitude 7.9 Ms has been recorded by the National Geophysical Data center, NOAA, USA (www.ngdc.noaa.gov/nmdc/servlet/showdatadatasets). The other referred earthquakes and volcanic eruptions of 1847(M?), 1883 (Volcanic eruption) and 1941(M 7.7 Mw) might have caused the tsunami hazards or not in the region is still unrevealed. The possible clues can be explored by trenching the beaches, study their profiles and date them by TL/OSL dating technique to ascertained the historical and pre historical tsunamis in the region.3 MATERIALS AND METHODOLOGY
3.1 Materials Used:
Software — ArcView, ERDAS IMAGINE etc. Data — Survey of India (SOI) topographic Maps, Satellite imageries- IRS-IC LISS III (24 Feb. 1999), IRS-P6 (16 Feb. 2005 & 01 Feb. 2005), GPS-Garmin etrex & Garmin vista, Sony Handycam, Sony Digital still camera, High precision Oregon scientific Altimeter, Measuring tapes, Staff etc.
The IRS P-6 satellite has been found to be very sensitive system in this regard. The tilting of camera to see the affected areas from an easterly path of the satellite has been proven to be of a high practical utility combined with wide viewing of these images, the information may play a significant role in mapping the tsunami efforts for sustainable development in the region. The digital analysis of the P6 digital satellite data was done by using the image processing software (ERDAS IMAGINE 8.4). The various steps have been followed in order to find out the best results in deciphering the precise delineation of the various tsunami affected areas as proposed objectives. It is observed that the image enhancement techniques with brightness contrast and break points have became very successful in delineating the inundation of tsunami water on the islands by acquiring the actual reflectance values from the satellite data. The natural colour simulation was done to map coastal features precisely. The coastal mapping was made by on screen digitization (Fig 1). The wave height, run up elevation, coastal erosion delineation and impact of damages was made by using the DEM/DTM. On the basis of the elevation data of the surface a Digital Terrain Model (DTM) creates topography by geometric surface in a computer environment. This method provides best approach (Alpar B et.al 2004) to a 3D terrain surface using elevation points which were defined on a horizontal plane, from various data sources such as measured data, topographic maps, bathymetric data and images. In the present study the DTM was produced from topographic maps and SRTM data and has a cell size of 22 meters. The field data of the various locations that were collected with the help of handheld GPS (Garmin etrex & vista) were used as overlay on the Digital terrain model to verify the tsunami wave height as well as the distance from the sea.
4 RESULT & DISCUSSIONS:
There are number of observations by various workers on the characteristics of past tsunami hazards such as Yalciner et.al. (1999, 2000 and 2001), Altinok et.al. 1999; Musaoglu 2000; and International Coral Reef Initiative/International Society for Reef Studies (2005) that has provided a rapid assessment and monitoring are worth mentioning. In the present studies tsunami hazards are delineated and assessed by the various observations during investigations and the results have been finalized in the following manner:
4.1 Tsunami Wave Heights
The tsunami wave height are measured based on the satellite imageries and the DEM generated using SOI contour and 1m SRTM data with a vertical resolution of +/-1m.On the basis of the elevation data of the surface a Digital Terrain Model (DTM) creates topography by geometric surface in a computer environment. This method provides best approach (Alpar B et.al 2004) to a 3D terrain surface using elevation points which were defined on a horizontal plane, from various data sources such as measured data, topographic maps, bathymetric data and images. In the present study the DTM was produced from topographic maps and SRTM data and has a cell size of 22 meters. The field data of the various locations that were collected with the help of handheld GPS (Garmin etrex & vista) were used as overlay on the Digital terrain model to verify the tsunami wave height as well as the distance from the sea. In order to assess the tsunami height on all the sides of island, four strategic locations were selected viz. North eastern (NE), North western (NW), South eastern (SE) and South western (SW) part. A maximum tsunami wave height was 15m on the south east and minimum is 0.7 m with a distance from the shoreline of 400 m and 368.58 m respectively (Fig 45-48 and Table 3-6). The photograph taken from a low altitude aircraft also depicts the evidences of tsunami wave heights and the distance from the shore on the SE part of island (Fig 49).
The various indicators such as wave heights, water stains on the buildings, salt burnt trees and broken branches of the trees and debris on the trees are used in the field survey. The manual methods are used to assess the heights. The south-east facing coastlines near Malacca area were struck by the highest waves, some more than 15 m high. Waves that hit the north facing coastline of Mus and Passa were lower, about 8-10 m high, but the area’s low-lying land allowed those waves to penetrate far inland. It is observed during the survey that many two storied buildings are inundated by tsunami waves and trees have the debris carried by tsunami water (Fig 2). The hypothesis of tsunami height was made by measuring wave heights at intermediate points along the coastline and to collect additional data on sediment-deposit profiles (USGS, https://walrus.wr.usgs.gov/tsunami/sumatra05 /heights.html).Effort s were focused mainly around the very south-east end of island around Malacca, Kakana, Chukchucha, Lapathy and Kimus to collect data. Wave heights of 15 m (50 ft) at those sites suggest that the tsunami waves may have been 10 to 15 m high along the entire stretch of coast from Malacca to Kimus.
4.2 Run Up Elevation
In general, the extent of vertical run-up of seawater during tsunamis depends on earthquake parameters, geographical location, velocity of tsunami waves and their frequency, near shore bathymetry, beach profile and land topography. Due to these parametric variations in various coastal areas including islands, the run-up levels and landward penetration characteristics of seawater were the location specific and varied within a location and even in an island. In the case of Nicobar Island the run up levels varied from 2 m to 19m and the distance of penetration from the coast ranged from 295.87 to 1202.57m (Fig 3b). Bilham et.al. (2005) have drawn the preliminary conclusions on the slip pattern of 26 Dec 2004 earthquake that due to high rate of slip in the southern 650 km of the 1300 km North –South rupture zone of 2004 Andaman-Sumatra earthquake, the principal tsunami was generated in the Sumatra area. Time lag between earthquake and land subsidence in Car Nicobar on 26 Dec 2004 which is estimated to be 15-20 min has been interpreted as the high rate of slip was in the Nicobar region resulting generation of near-field high tsunami (19m run up level) in this zone. The above parameters have caused the land subsidence at Nicobar Islands due to the earthquake. The run up levels recorded at Andaman groups of Islands were different due to the low sip rate, caused by the earthquake. Similar types of diversified results were observed in 26th December 2004 Tsunami affected coastal areas in Thailand, Indonesia and Sri Lanka. Run up levels varying from 0.3 to 32 m were recorded in Indonesia and from 2.5 to 10 m in Sri Lanka (Yalciner, et.al, 2005) and Seychelles (2.5m) . It is observed that the run up level was affected by the earthquake mechanism and resulted to different run up levels at different coastal areas. In case of Nicobar Island, the run levels were varied from place to place as shown in the Fig3.
Inundation distances in the island were so large that they were most easily measured from satellite images, where sediment deposited by the waves and vegetation killed by the saltwater are clearly visible (Fig 3). Satellite images show that such waves that struck many such villages Malacca, Kakana, Kimus and Mus located SE, SW and NE parts of the islands. Items broken and bent by the tsunami waves were used to determine flow directions. The team found that the large tsunami waves flowed around natural barriers, flooding lowlying areas behind them. The inundation is marked on Kakana beach and Kimus beach (Fig 4).
4.4 Tsunami Flow Direction
The Car Nicobar Island has been affected by the water inundation. The Team found that the large tsunami waves flowed around natural barrier, flooding low lying areas behind them. Flow direction has been marked by number of indicators such as water mark on the walls of the buildings, trees, DEM, satellite imageries, items broken and bent by the tsunami waves were used to determine the flow direction. It is observed that Nicobar Island, being very near to the earthquake source, the tsunami water inundated from all the sides and flown towards the inland. The flow direction of the tsunami water was from all sides of the island (Fig 5).
4.5 Sediment Deposits
The team surveyed beach profiles to document erosion (common near the coast) and deposition (common inland) by the tsunami. Sand eroded from beaches probably provided much of the sand that was deposited inland. The survey team dug trenches in the tsunami deposits to measure their thickness and to examine other characteristics that can shed light on how high the waves were and how fast the water was flowing. Data from the sediment deposits will not only reveal about the recent tsunami but also help them recognize and interpret the deposits of ancient tsunamis, which, in turn, will help them better understand an area’s tsunami history and its likely tsunami risk. The Kakana beach and Aukchung beach are selected for the study. The sand deposit is 2.5 m at Aukchung beach and 1 m at Arong beach (Fig 6).
4.6 Coastal Subsidence
The subsidence observed at beaches near Malacca Jetty is 0.75 m and a temple on Malacca beach which is submerged by 1-1.25 m approximately (Fig 7). It is inferred from the observation that the subsidence may be a result of an earthquake (mega thrust caused the rise of sea floor and subsidence at beach). It is also predicted by the model used by USGS survey of Sumatra that the type of earthquake that caused the tsunami—a megathrust—will raise the sea floor above the fault rupture and cause subsidence near the coast. The pre and post tsunami images have been studied for the observation of subsidence.
4.7 Erosion and Coastal Response
The Car Nicobar Island underwent significant modification by the tsunami. The shoreline eroded, beach sand was carried inland, and the coastal plain was flooded. Compared to the erosion and deposition of sediment by the tsunami that occurred relatively quickly, within hours of the initial tsunami impact, coastal subsidence resulted in additional erosion and shoreline retreat during the weeks and months following the tsunami. The observations from the field studies indicate coastal erosion at Aukchung, Kimus and Malacca villages where the road is washed off, Mangroves and bridges (Fig 8) are damaged and underwater cracks extended inward where the high tide water is filled up. In addition, some parts of beaches have new sand deposition, located near Aukchung and at me places near Kakana where the beaches started rebuilding as soon as a few weeks after the tsunami, probably adding sand from nearby offshore. These reformed beaches were migrating landward through over-wash processes. Some beaches were still migrating landward, impacting roads and redevelopment plans for coastal villages. It is expected that the future shoreline retreat may continue to impact redevelopment in some areas.
4.8. Assessment of the Damages
In order to assess the damages caused by the Great tsunami in the Car Nicobar Island the pre and post tsunami satellite data are critically analysed in the GIS domain. For this a base map is generated from the SOI topographic map of the region. The Base Map is later on updated from the field data and information obtained from the Nicobar Island Administration office. Based on this map the coastal area of the two scenes (pre and post) have been classified and vectorised using the ERDAS vector and ArcView software (Fig 1). Overlay analysis of these classified vector data is performed to find out the changes in the coastal corridor of the Island and ultimate the assessment of the damages is done form this analysis. The result of the assessment is tabulated as below (Table 1).
Table 1 Damage Assessment of Tsunami impact
The Great Indonesian Earthquake of 26 December 2004 and a subsequent Great Tsunami event has led to the wide spread devastation in the Indian Ocean coast and all islands in its vicinity. These events have created ways and means to explore the nature’s mystery of natural processes/hazards in the Indian Ocean. The damages caused by these hazards are being accounted to the losses of millions of dollars and loss of property and life of people and many more homeless. The application of High resolution Remote Sensing data and GIS techniques are used to assess the tsunami hazards in the Car Nicobar Island. Maximum tsunami wave height was 15 meters of the SE pats of the island and minimum of 0.7 meters with a distance from the shore line of 400 meters and 368.58 metes respectively. The run up levels varied from 2-19 m and penetration distance from the coast, ranges from 295.87-1202.57 m on the inland. The tsunami water flowed from all the directions to the island. A considerable part of all the existing beaches hosting mostly infrastructural, commercial and residential complexes have been fully damaged. The sand deposits at Aukchung and Arong beaches are 2.5 and 1 meter respectively. The coastal subsidence near Malacca Jetty is 0.75 m and a temple on Malacca beach is 1-1.25 meters. The car Nicobar Island underwent significant coastal modification and the over all coastal area affected by the tsunami is 2075.26 ha. The study indicates considerable coastal erosion at Aukchung, Kimus and Malacca villages.
The financial assistance provided under the DST grant No. SR/S4/ES-135/3.5/2005 is thankfully acknowledged. The authors are grateful to A&N Administration for their help during the field work.