Quake on ocean bed produces large waves: experts

By S. Shanker

CHENNAI, OCT. 3. The September 25 earthquake, centred in the Bay of Bengal, would have created large troughs and ridges on the ocean bed, according to experts of the Institute of Ocean Management, Anna University.

The Indian Meteorological Department placed the epicentre at 63.5 km east-southeast of Pondicherry, 69 km east of Cuddalore and 143 km southeast of Chennai.

Prof. P. Ramesh, institute head, says that when an earthquake occurs on the ocean floor, it causes shock waves to travel through water and produce large waves, Tsunamis, which pick up momentum in shallow waters. The Tsunamis are known to touch speeds of 700 to 800 km/hour and rise upto 20 metres high. During the 1964 Alaska earthquake, Tsunamis engulfed coastal areas and caused severe damage along the west coast of North America, particularly Crescent City, California.

Referring to reports that Pondicherry did not feel the magnitude of the earthquake as much as Chennai and Cuddalore did, he said the impact depended on ``local surface and sub-surface geological conditions'' apart from the magnitude of the quake. Satellite images showed `fault lines' running through Tamil Nadu and cracks under the plateau.

The country's seismic zoning maps (which grade different regions depending on their vulnerability to earthquake forces) are also being revised. An area underlain with unstable ground (sand or clay) was likely to experience more noticeable effects than an area with firm ground, such as granite, though it may be equi- distant from the epicentre. The Chennai hydrological map does not indicate a large spread of rocky areas in the subsurface.

In addition, the `focal depth' of the earthquake, the distance from the epicentre and design of buildings, other structures and construction, have a decisive bearing on the ``destructiveness'' of quakes.

Citing an example, he said earthquakes east of the Rockies affected larger areas than what was experienced in the west of the mountains.

The common cause of earthquakes was attributed to faulting (breaking of rocks), which resulted from the collision of plates, sending shock waves in all directions. The example being the San Andreas Fault, which is about 960 km long and 32 km deep, with the North American plate sliding southeast against the Pacific plate which is sliding northwest. The plates slide past each other at 5 cm per year, though there are parts which have not moved over years. Scientists anticipate the stationary points as potential sites for future quakes.

Most earthquakes occur at boundaries, where plates meet. In fact locations of earthquakes and the kind of `ruptures' they produce help scientists define plate boundaries, which are of three types-spreading zones, transform faults and subduction zones. Most of the spreading zones are found in the ocean. The spreading zones usually have earthquakes at a shallow depth (within 30 km of the surface).

The UNESCO's Oceanographic Commission, the International Hydrological Programme and the World Meteorological Organisation, are developing a system to predict storm surges triggered by underwater earthquakes and volcanoes to provide warning to risk areas.

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