Physical Geology the 'Indian Ocean

Physical Geology The 'Indian Ocean tsunami' which happened on December 26, 2004 was one of the worst natural disasters to have taken place in recent times. This disaster which ravaged the coastlines of the Indian Ocean resulted in a staggering number of victims, both dead and missing. Indonesia, which was the worst affected, recorded around 160,000 victims followed by Sri Lanka, India and Thailand with 35,000, 16,000 and 8,300 victims respectively. (Shiki; Yamazaki, 14); (Satake) Other regions which bore the brunt of this great tsunami include Myanmar, the Maldives, Seychelles, Kenya and Somalia.

The primary reason for this great tsunami was an earthquake which happened at 00:58:53 GMT at 07:58:53 local time measuring 9.3 on the Richter scale whose epicenter was under the seabed off the coast of the Sumatran Island of Indonesia in the eastern region of the Indian Ocean. The location of the epicenter was 3.31°N and 95.95°E and positioned around 250 kilometers south-southeast from Banda Aceh in Sumatra.

This earthquake was gauged to be the largest earthquake in the last 40 years and its reach in terms of the aftershock zone extended for more than 1000 kilometers right up to the Indian islands of Andaman and Nicobar. Studies of the aftershock distribution revealed a main fault rupture zone which was ninety kilometers width wise and extended up to the Andaman Islands along the 1200 kilometers rupture line. Total fault movement near Sumatra was gauged to be approximately fifteen meters with reducing replacement towards the north.

The rupture, the longest ever brought about by an earthquake, took place at comparatively shallow depth -- focal depth of ten kilometers to thirty kilometers below the seabed along the subduction zone. (Satake); (Richmond; Jaffe; Gelfenbaum; Morton, 237); (Fehr); (Chester, 213) This event took place in two phases spanning across several minutes. The first phase took place in the southern section and involved a 100 kilometers wide by 400 kilometers long rupture which traveled in a northwest direction at the rate of several thousand kilometers per hour.

The second phase took place in the north and traveled relatively slowly, the fault type gradually changing from subduction to strike-slip. In the days following the earthquake hundreds of aftershocks took place, one of which triggered another localized tsunami but on a much smaller scale. According to researchers like Lay and colleagues, the "seismic moment of this single event" is comparable to the "cumulative moment" of all the worldwide earthquakes that took place in the ten years prior to this event.

(Satake); (Richmond; Jaffe; Gelfenbaum; Morton, 237); (Fehr); (Chester, 213) The earthquake released total energy which was approximately calculated to be equivalent to the energy released by two-hundred billion tons of explosives. However, the total energy of the tsunami was estimated to be only around 2% of the total energy of the earthquake that generated it. (Bernard; Robinson, 12) The resultant tsunami destroyed Banda Aceh and several other places on the Sumatran coast within minutes of the earthquake. The tsunami reached the coasts of Thailand, Sri Lanka and India within two hours following the earthquake.

It traveled further and reached the east African coast within a few hours and resulted in several deaths in Somalia. The reach of the tsunami stretched as far as the Arctic Ocean through the Atlantic and Pacific pathways. Focusing of waves due to the East Pacific rise led to local peak-to-peak amplitudes measuring approximately one meter on the western coast of Mexico. The tsunami waves measured on an average of twenty meters and reached a maximum of thirty meters in Banda Aceh and other adjacent regions of Sumatra.

In the rest of the region, the waves varied from five meters to fifteen meters in height with Myanmar recording only three meter -- high waves. The damage and casualty figures were in direct proportion to the tsunami height distribution and suggested that the tsunami source was mainly centered in the 700 kilometers wide region in the southern part of the aftershock zone.

(Satake); (Bilham; Engdahl; Feldl; Satyabala) Several field studies have been conducted since then by the scientific community to study the geological causes and impacts of this major geological event. This is imperative not only to understand the paleo-tsunami deposits but also for better tsunami prediction in the future in order to prevent large scale destruction of life. To understand the causes and effects of the megatsunami, it is essential to understand the geology of the region. Sumatra and the adjoining region are tectonically active.

It has a number of faults and young volcanoes lending the region its geologically complex characteristic. The western part of Sumatra consists of a linear ridge having several emergent islands which are separated from the main island by a number of marine basins which are approximately 1 -- 2 kilometers in depth and are fringed by the five kilometers approximate deep Sunda Trench on the southwest. The island of Sumatra lies in the region where two major obliquely subducting tectonic plates occur at the southeast Eurasian margin.

(Richmond; Jaffe; Gelfenbaum; Morton, 240) The geological reasons that led to the earthquake -- triggered megatsunami can be attributed to the subducting of the Indian plate beneath the Burma microplate in the region where the epicenter of the Sumatra -- Andaman earthquake took place. This subduction, which takes place at a rate of five to six centimeters yearly, leads to the dragging and deformation of the upper plate. On crossing a certain limit, the strain causes the two plates to rebound resulting in an interplate earthquake.

This single displacement resulting from the earthquake corresponded to accumulated plate motion of approximately 250 years. (Satake); (Fehr, Irene; et. al) The epicenter of this earthquake is situated extremely close to the quadruple plate junction formed by the Sunda, Burma, India and Australia plates. This implies that the variations in the subduction style of the Australia-Sunda and India-Burma plates helped by the emerging growth of the diffuse India-Australia boundary may be blamed for the earthquake which triggered the megatsunami.

Data suggests that the drag force exerted on the adjacent Australian plate by the violent plunging of the Indian plate beneath the Burmese plate released stress locked up for a long time at the boundary of the Sunda and Australian plates and triggered another earthquake almost as big as the earlier one on 28th March 2005.

(Kundu, 288) The comparative studies of the relative motions of the four tectonic plates in this region have aided in understanding the tectonic situation of the earthquake, the frequency of occurrence and the degree of the rupture. Combination of various studies have resulted in an Euler vector, the rotation pole of which reveals that net convergence between Burma and India has a northwest-southeast orientation. The perpendicular component of the trench is estimated to be released in such earthquakes.

Since this component is around fifteen to twenty-five mm every year, based on the pole position, location and rate all along the trench, it is expected that similar earthquakes may occur on an average of once in every 400 years. (Stein; Okal) The immediate geological impacts of the 2004 Indian Ocean tsunami, which was a "basin-wide event," were recorded by various scientists. The impact was wide-spectrum and affected several geologic settings apart from physiographic provinces and coastal environments. The shape of the seabed changed and also rose by a few meters.

This activity, which can be described as a watershed event in the geological history of our planet, displaced around 30 km3 of seawater. A high-resolution survey conducted by HMS Scott, a Royal Navy vessel in February 2005 on the region surrounding the earthquake epicenter, found that the impact of the earthquake on the seabed's topography was huge. Several thrust ridges, some of them as high as 1.5 kilometers had crumpled triggering landslides that extended up to several kilometers. In Sumatra, landslides and subsidence were observed.

Thailand witnessed widespread effects including morphological changes to beaches, lagoons, rivers and navigation channels. (Richmond; Jaffe; Gelfenbaum; Morton, 242); (Strand; Masek, 35); (Bernard; Robinson, 21); (Chester, 205) Damages were also observed to inland tsunami deposits and coral reefs. The seawater also contaminated groundwater and inland fresh water reserves and sinkholes were formed in the karst regions. Chunks of coral from offshore coral reefs were torn away and borne to the Kao Lak beach in Thailand by the gigantic waves.

Thai beaches were also found to contain a thin greenish -- tinted layer of tsunami deposits. These deposits consisting of fine sand to clay or silt are different in characteristics from the ones deposited by storm waves and are considered to be very valuable tools for the study of paleo -- seismicity. Other important geoscience observations in the aftermath of the 2004 tsunami were gas emissions observed in Arakan, Myanmar and the renewed activity of a mud volcano near Baratang in the Andaman Islands in India.

(Richmond; Jaffe; Gelfenbaum; Morton, 242); (Strand; Masek, 35); (Bernard; Robinson, 21); (Chester, 205) The ground shaking resulting from the fault-rupture induced Sumatra-Andaman Islands earthquake did not cause any long-term geological damage to the South Indian Peninsula -- SIP. 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.