Earthquake, Tsunami and Geomorphic change

15 May Earthquake, Tsunami and Geomorphic change

Earthquake, Tsunami and Geomorphic change

Prof. Thrivikramji.K.P.,

thrivikramji@gmail.com 

Dept. Of Geology, University of Kerala, Krishnavilas, Sankar Lane, Sasthamangalam, Trivandrum 695 010

Abstract

Earthquake (EQ) is a phenomenon that occurs not only in the planet earth, but also in others like moon and mars. Accumulation of large volumes of data pertaining to bottom topography, geological and tectonic features of the oceanic basins, especially in the post 2^nd WW period, gave a better picture of the interior make up of the planet earth.

Progress gained in the mapping of seabed topography, provided a well refined picture of the morphology of the seabed like, chain of submarine mountains or the mid-oceanic ridges (MOR), guyots or sub-sea volcanic cones with planed off apices and deep oceanic trenches (DOT) along which subduction of lithospheric plates take place.

The distribution of the MOR, subduction phenomenon, island arcs, and  the fold mountain belts (like Himalayan-Alpine belt) in the continents afforded a basis for division of the crust into several oceanic or continental lithospheric plates and further insight on the architecture of the crust.

Alongside, construction of new tools for drilling and sampling the seabed and the design development of very sensitive magnetometers for submarine geomagnetic studies and further fine tuning of radiometric dating, opened up the new vista of submarine rock-magnetic studies. The results thereof strongly suggested a procession of magnetic reversals through time. This new format of the seabed formed the foundation of the concept of seafloor spreading.

New plate, created by eruption of basaltic lava (accretion) in the MOR, moves  laterally, for which room is provided by consumption (destruction) of the plate along a converging plate boundary by the process of subduction like in a  DOT – a steady state system. The subduction zones are areas of intense EQ activity. Thus, this revolutionary concept of plate tectonics offered a support for the continental drift theory and zonation of EQs in the oceans and continents.

Plate margins are of different types, viz., converging, diverging and strike slip types. For example along a converging plate margin can result in a fold mountain belt (like Himalayas) between the Indian and the Asian plates.

The EQs characterizing such zones in the ocean basins are a major cause of Tsunamis, while others are submarine landslides particularly on huge deltas and non-MOR submarine volcanic activity. Continued accumulation of data, led geologists to infer that majority of earthquakes, large and small, are a result of release of stress accumulating along the plate margins due to plate migration away from the MOR but toward a converging plate boundary.

At a converging plate boundary, one of the opposing plates will buckle under the other. The friction, between these oppositely moving lithospheric plates, prevents any movement between the plates until such time, when the friction is overwhelmed by the accumulated stress – very much comparable to arm wrestling. So, this stress release sets off EQ waves viz., P or push waves, S or shear waves and the L waves that travel over the surface of the crust. Of these the P and S waves are most destructive.

EQs originating from stress release at converging plate margins in a subduction zone or collision of continental plates (Indian plate and Asian plate) where one plate rides over the other, elevation changes of the order of several meters to 10s of 100s  or even a few thousands meters (Pamir Plateau) are commonplace, like in the submarine EQ of Dec. 26, 2004.

In the latter case, the Burmeese lithospheric plate of 7.0 or 8.0 Km thickness, rode over the Indian sub-ducted plate, causing upwelling of millions of tons of seawater setting off the Asian Tsunami – which unleashed a massive destruction in parts of the shoreline of northern Indian Ocean.

Table. 1. Worst EQs (Source Newsweek, Jan., 2005)

With modern towable side scan sonar, the 3D imaging of gross features of seabed topography was recently completed by a British Scientific Team. The maps showed displacement of large slivers of lithosphere (and the overlying sediment).  Such features compellingly prove that landform modifications are very much part of the EQs even under submarine conditions.

Mountains, plateaus and rift valleys are the tectonic land forms resulting from huge magmatic movements related to plate tectonic activity. These occur on the continents and some in the oceanic basins.

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