Centre For Remote Sensing Bharathidasan University
Tiruchirappalli, Tamil Nadu
C J Kumanan
Centre for Remote Sensing, Bharathidasan University,
Tiruchirappalli,Tamil Nadu, India
The recent Sumatra tsunami which has caused a greater devastation in Sumatra, Andaman – Nicobar islands, Sri Lanka, East coast of India and even the far away region like Somalia coast has opened up a new chapter in tsunami disaster in Indian ocean. The paper provides an overview on the triggering mechanism and the susceptibility of Indian coast
Tsunamis are the series of tidal waves or “tidal train” generated due to the vertical displacement of water column caused by the earthquakes, landslides, volcanic eruptions, impact of meteorites etc., in the ocean. These tidal waves have extremely long periods from 2 minutes to over an hour and more, with a wavelength of even 100 kms. The height of the tsunamis varies from 1 meter to 100 meters, though the higher magnitude ones are rare, but typically produced when major chunks of islands collapse or the giant meteorites fall into the ocean.
The historical tsunamis record shows that these are quiet common in Pacific ocean, less in Indian ocean and least in Atlantic ocean. These tsunamis have been predominantly triggered by the violent earthquakes and rarely by the movements of huge ice sheets into the ocean, volcanic eruption and massive landslides.
However, these tsunamis have caused greater devastation in the islands and the continents occurring in and in close proximities to the source of triggering events and at times even the far away continental margins and the coastal zones depending upon the magnitude of the triggering earthquakes and the dynamism of such tsunamis. However, the disaster vulnerability equally depends on the geological setting, geomorphology and the geometry of the coasts which stand exposed to such tsunamis. In this context, the recent Sumatra – The Sumatra-Andman tsunami that occurred on 26th December 2004, has opened up a big chapter in the tsunamis vulnerability in south Asia/Indian ocean.
GLOBAL PLATE TECTONICS AND TSUNAMIS
All the continents made up of independent plates remained as one piece of land called “PANGAEA” around 200 million years ago and later got drifted and moved in different directions due to the mantle convection currents. For example, India has moved towards north northeasterly to the tune of 6000 kms almost during the last 60 million years, the north and south America towards westerly, Africa towards northerly, Antarctica towards southerly and Australia towards easterly so on and so forth. These continents so moved in different directions occupy the present geographical positions, along with their respective plate boundaries (Fig.1). However, these major plates have a number of minor plates within them. These major plate boundaries either occur as divergent (ridges) or as convergent (subduction zones) plate margins in divergent plate margins. As the forces act in opposite directions, pushing the plates away from each other, long and linear ridges are formed.
As, such divergent plate margins are predominantly positioned in ocean (Fig.1) such ridges formed are called as oceanic ridges (Eg: Mid Atlantic ridge, Carlsberg ridge, South East Asia ridge etc., Fig.1). In contrast, as the forces converge in convergent plate boundaries, these forces push the plates, occurring on either sides, towards each other thereby causing collisions between the two such plates and in that process one of the plates goes underneath (subducts) the other plate. Because of such subduction, the over riding plate gets deformed into huge folded mountain belts, at many times with volcanic eruptions from beneath.
For example, because of the north northeasterly compression and the resultant collision and subduction of the Indian plate beneath the Eurasian plate, the Himalayan mountains were formed. This subduction zone along Himalaya – Burma – Andaman – Sumatra is called as Indo – Burma – Sumatra subduction zone or Indo – Burmise arc (Fig.1&2). These divergent and convergent plate boundaries are hence tectonically and seismically active and so prone for earthquakes, and in between the two, the subduction zones are more prone for higher magnitude earthquakes as the plates collide there.