The Indian Ocean Tsunami: The Global Response to a Natural Disaster

5: Impact of the Tsunami on the Coastal Ecosystems of the Andaman Islands, India

113 5 Impact of the Tsunami on the Coastal Ecosystems of theAndaman Islands,India Ramesh Ramchandran, Purvaja Ramachandran, Bojarajan Senthilkumar, and Brigitte Urban Any event that causes a significant displacement of the seafloor also causes the displacement of an equivalent volume of water. This is the basic mechanism governing the generation of tsunamis. Although most tsunamis are produced from earthquakes, they can also be caused by volcanic activity, submarine landslides, slumps, meteor impacts, and occasionally by human activity. The primary cause of wave generation is the release of energy and associated crustal deformation resulting from the earthquake. Thus, any earthquake that produces a tsunami is known as a tsunamigenic earthquake. The magnitude of the earthquake does not dictate whether or not a tsunami will be produced or its size; these issues are decided by the type of fault from which the earthquake is generated. The Mw 9.0 December 2004 earthquake is the largest to have occurred in the Andaman-Sumatra subduction zone. This chapter summarizes the findings of various scientific groups that investigated the immediate impacts of the December 26, 2004, tsunami along the coasts of the Andaman and Nicobar Islands, the Indian state in the Bay of Bengal, from field-based and remote-sensing studies. Wherever available, data prior to the December 2004 tsunami were compared to the changes after the event. Study Area: Andaman Islands The Andaman and Nicobar Islands are a group of about 572 islands and islets located off the eastern cost of India in a junction with the Bay of Bengal and 114 Ramchandran, Ramachandran, Senthilkumar, and Urban the Indian Ocean on one side and the Andaman Sea on the other. The islands lie approximately 1,200 km (720 miles) from mainland India and are in close proximity to Myanmar, Thailand, Malaysia, and Indonesia. The Andaman and Nicobar Islands are located in the Bay of Bengal between 6°–14° N and 92°–94° E. They are part of a submerged mountain range related to the Arakan Yoma range of Myanmar. Andaman islands where inhabitation is found are South Andaman, Middle Andaman, and North Andaman (fig. 5.1). Fringing reef and barrier reefs dominate the coasts of the Andaman Islands (960 km2 [(384 sq. miles]), forming a natural barrier to dissipate the wave energy from the Bay of Bengal. Coral reefs form the dominant ecosystem, creating habitats for sea grasses and mangroves in the lagoons and creeks protected by the reef. In the Nicobar Islands there are twelve large and small islands, of which only the large islands are inhabited. The Andaman–Sunda section of the subduction zone has produced many earthquakes in the past, some of which resulted in destructive tsunami surges. The largest among the historical earthquakes occurred in 1833 (Mw 8.7), 1860 (Mw 8.5), 1881 (Mw 7.9), and 1941 (Mw 7.7). They, though large, ruptured only a few hundreds of kilometers (200–300 km [120–80 miles]) of the plate boundary. The 2004 earthquake ruptured more than a 1,200 km (720 miles) length of the arc, stripping the regions that had ruptured in the past (Gupta 2003). Methodology We made detailed physical surveys in addition to monitoring the changes in surface water characteristics after the tsunami. Soil and sediment cores with lengths ranging from 0.4 to 0.7 m (1.32 to 2.31 feet) were sampled from agricultural, mangrove, and coral reef sites of South Andaman. Coring was achieved using a stainless steel 50 mm (2.2 inches) diameter gauge auger. Sediment samples were extruded in the field, wrapped in plastic foils, and transferred into dry ice for laboratory analysis. The cores from Sippighat (11°36'25.6" N; 92°41'30.5" E) and Wright Myo (11°47'26.1"N; 92°42'30.8" E) were sectioned at 2 cm (0.8 inch), while those from Redskin Island (11°34'17.3" N; 92°39'03.2" E) were sectioned at a 1 cm (0.4 inch) interval. As a preliminary investigation, geochemical and trace metal analyses were performed in the core sections using X-ray fluorescence spectroscopy (XRF). Bulk chemistry carried out on sediment cores using XRF gives the overall distribution of major and trace elements in a sediment profile. Assessments using X-ray fluorescence were critical in characterizing the changes in the Fig.5.1.Map of Andaman and Nicobar Islands. Narcondam Island Barren Island...