Physical Geology the 'Indian Ocean Thesis

Excerpt from Thesis :

In fact, the entire damage was caused by the tsunami itself along with other factors like the geology and geography of the region. The damage that the tsunami caused to mainland India, a seismically quiescent region, was concentrated mainly on the eastern coastline but some damage due to diffraction also occurred on the southernmost tip of the western coastline. A factor which played a major role in the scale of devastation that occurred in this region is the bathymetry of the Bay of Bengal. The Bay of Bengal is quite shallow with an average depth of 2600 meters. The shallowness can be attributed to the huge amount of sediment that the Ganges River deposits in it. (Strand; Masek, 40)

After the earthquake, the bay was sealed off by the deep Sunda Trench and the tsunami waves were reflected back towards the Indian Ocean striking the eastern Indian coastline within two hours. As the waves traveled across the Cocos Basin, they gradually got amplified. The long-term geological impacts were more visible in the Andaman and Nicobar Islands. The peaks of a submarine mountain range constitute the Indian islands of Andaman and Nicobar. These mountains lie over a major tectonic fault zone. In the Andaman and Nicobar islands, the earthquake on December 24, 2004 resulted in mud volcano eruptions, ground deformations, liquefaction, tilting of the islands and tectonic subsidence apart from the immense damage to life and property by the tsunami. The eastern coasts of the islands were inundated whereas the coral beds on the western sides were exposed. The central regions of the Trinkat and Teressa Islands went under water, cutting both the islands into two parts. (Strand; Masek, 40)

Since the orientation of the subduction zone earthquake was in a north-south direction, the general path of the tsunami waves was towards the east and the west. It has been observed that coastlines that have a mass of land between them and the point of source of a tsunami usually escape the tsunami's fury. It is for this reason that Bangladesh, situated at the northern part of the Bay of Bengal, had very few fatalities in spite of the fact that it is a low-lying country and situated relatively close to the rupture zone. However, steep bathymetric features and diffraction around landmasses can propagate tsunami waves. Thus, Kerala, an Indian state on the western coast and the western coast of Sri Lanka also suffered damages due to the tsunami. In fact, Colombo or Sri Lanka witnessed a larger tsunami reflected from the Maldives Islands which reached the Sri Lankan coast around two and a half hours after the first tsunami came. Again, distance does not ensure safety since the tsunami caused more devastation in Somalia, which is much farther away than nearby Bangladesh. (Bernard; Robinson, 43)

In Sri Lanka, coral reefs were found to be lightly damaged and sediment was eroded from the beaches and shoreface and traveled as far as 400 meters inwards to be deposited in pockets and sheets with a thickness ranging from five centimeters to thirty-seven centimeters. (Richmond; Jaffe; Gelfenbaum; Morton, 247) In Maldives, the impact of the tsunami ranged from complete overwashing of the islands to inundation of the regions at the margins of the islands. Islands which were situated near the eastern reef rim standing in the path of the tsunami faced greater changes to coastal morphology. There was widespread beach erosion typified by the formation of 0.3 -- 0.5 meters high erosional scarps. Human activity enhanced the force of the tsunami in many areas including the Maldives. The reef areas revealed signs of dredging of gravel and sand which accelerated the shoreline erosion problem resulting in severe damage to several coastal structures. (UNEP Asian Tsunami Disaster Task Force, 7-8)

The tsunami waves resulted in the deposition of gravel-sized reef debris on the eastern beaches in the form of 10-20 centimeters deep and 10-20 meter wide sheets. In the case of island overwashing, the western sandy beaches developed prominent scarps leading to sediment deposition on the adjoining western lagoon and reef flat areas. The island interiors, however, displayed the deposition of thin and patchy sediments. It has been observed that wider reef flats result in more dissipation of wave energy and thus have a more coastal protection role. This factor may have led to the local variability observed in the impact of the tsunami. Again, fringing coral reefs usually provide protection against normal waves but in the case of tsunami waves, the shorelines sheltered by such fringing reefs suffered major coastline and terrestrial damage. In such locations, damage was concentrated close to the deeper channels that let the waves break closer to the shore. Thus the two basic benefits that the fringing reefs afford to coastlines, viz. The combination of ocean access and shelter which have permitted coastal development to take place just over the high tide line, have also become the primary reasons for their more vulnerability to tsunami damage. (UNEP Asian Tsunami Disaster Task Force, 8)

Thus to conclude, it may be said that the extent and scale of damage that the great earthquake and the resultant megatsunami caused on 26th December, 2004 represents geologically important seismic event which has captured the attention of geologists, sociologists, scientists and political leaders worldwide and several studies in various disciplines have been conducted based on this event. Geologically, this event is not only significant in order to study and reconstruct ancient tsunamis that may have had an impact on various aspects of various geological features but it is also important in order to better construct early tsunami or earthquake warning systems in the future in order to minimize the scale of damage.


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