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What is an Earthquake & Why do They Happen?

An earthquake is perhaps the most fearful natural phenomenon that takes place in human life. It is more so because it is unpredictable and arrives without notice or without announcing its vigor and strength. Scientists are constantly in search of the unknown factors surrounding earthquakes. The major aspects of earthquakes come to the fore on reviewing some of the major catastrophes. Minute observations and records have been able to pin-point the focus or the epicenter of these earthquakes in the historical past. These studies could reveal two great seismic belts in the entire globe.

This article is a response to the curiosity of Internet hunters. Therefore, it should start with the very basic question-

What is an Earthquake?

Apparently the answer is simple - numerous tremors, both powerful and weak which result from the disturbances within the body of the earth are called an Earthquake.

Why & How Earthquakes Happen? Reasons could be many; pinpointing the reason is possible after the event. They may be caused by various activities at the earth's surface such as ebb and the flow of the tides, the rush of traffic in the city streets, the tumbling of streams over high falls, magmatic outbursts from within the interior of the earth or explosion of high power nuclear or atomic bomb; so on and so forth.

We should have first-hand information about the two great seismic belts in the globe.


What are They? Most of the catastrophic events of earthquakes are associated with any of these two belts. Seismically active region in these two belts are categorized as follows -

1. Western Coasts of North and South America, the Aleutian Islands and the island groups along the eastern Coast of Asia such as Japan and the Philippines, and thus borders the Pacific Ocean on the east, north and the west.

2. It includes the Mediterranean, the Alps, the Caucasus and the Himalayas and continues into the East Indies, where it intersects the first belt. Uttaranchal- Assam and Andaman- Nicobar island chains fall within the second belt, and thus face frequent earthquakes of devastating nature.

In simpler terms, the earth consists of different rock layers of decreasing densities right from the center, towards its surface. Deep inside at the center, the earth is hot and molten. Because of Earth's rotation and other energy factors, different shells or the rock layers constantly move or slid past each other. As a result, the different continental mass fragments of lesser densities float and move, overriding the denser rock layers or the plates, either towards or away from each other. Such a movement has been taking pace for a long time in the geological past. This movement acts like a conveyor belt and during the journey, the plates meet each other or get an obstacle by other denser plates; the rock layers start descending. We call it a Subduction Zone.

Naturally, in the adjacent sides of the subduction zone, the rock layers get upheaved resulting in folding, thrusting and faulting. The second belt in parts of India i.e. Andaman- Great Nicobar- Java- Sumatra zone is actually a sub ducting zone for which the long island chains have emerged. This zone, in fact, is very dynamic and active, giving rise to deep - intermediate earthquake foci. The sub ducting force in this part, is directed from west to east while the same in Uttaranchal - Assam region is from south to north. This entire zone is susceptible to tremors of higher magnitudes. Significantly, most of the earthquakes in this belt are associated with volcanic activities. The violent outbursts of Kraktao in 1883 were accompanied by severe shocks and about 35,000 people died instantly. The whole village was displaced beneath the bottom of the ocean.

With intermittent volcanic activity and earth tremors, this region (on 26th December 2004) was subjected to catastrophic earthquake of 9 magnitudes in Richter scale, which again recurred on 28th March 2005. The previous one led to tsunami causing severe damage to both life and property. The other parts of India were considered to be earthquake resistant areas or the shield areas. Oldest rocks of more than 3000 m years form the foundations of the continental mass compared to much younger rock sequence in the seismically active belt just described.

Therefore, earthquakes causing loss of life & property around Latur or Koyna raised a serious concern among the geoscientists. One important aspect in this regard should always be kept in mind - the tremors had mostly hit the coastal points excepting a few areas in the hinterland. Recently, a light earthquake (m.l=2.8) struck the coastal Kunnakulam region in the northern Kerala on 20.12.2006 at 19:19 hours local time. The adjoining districts of Mallapuram, Pallakad and Trissur also experienced the same tremors. People became panicky particularly when the famous festival of Trissur Pooram was at its peak.

Why Earthquakes Happen? It is known that the west Coast of India, as a whole, had been affected by numerous fault sets in the recent geological past around 80,000 to 1 million years ago. Kathiawar coast with milliolite limestone was raised high from beneath the seabed. These faults are sharp N-S or E-W trending. The overall E-W trending Palghat Gap is well known. It lies across the Western Ghats in Kerala forming a major break in the continuity of the hills and connects Western Coastal Plain with the rest of the southern States. This landform or the Pass is bounded by steeply rising Nilgiri hills to the North and Anai Malai- Palni hills to the south. In the offshore, about 90 km west of Ponnani there lies a topographic high known as Ponnani mount. It emerges at the continental slope adjoining the shelf edge. ENE-WSW striking broad valley with steep northern wall has been observed about 25 km SE of the Ponnani Mount in the offshore. This valley, about 8 km wide falls in the same line with the Palghat Gap present in the NE of the area.

The other studies like magnetic observations confirm that the Palghat Gap is a faulted graben and continues beyond the land and towards the seabed in the offshore. Similar fault planes criss-cross this domain of Western Coast. Although these fault planes are inactive at present, it can reactivate to any extraneous force of considerable magnitude like impounding of sea waves or tides or even high power explosion & rush of heavy traffic. Incidentally, the area around Palghat Gap forms a weak zone with fault traces and contacts of younger rock formations like Vakrala sandstone with the oldest gneissic rock of more than 3000 m.years. Who knows, the famous festival of Trissur Pooram accompanying huge explosion of fire works and thousands of processions might aggravate the weak planes and causes tremor.

Contributing Writer: Mr. Asimendu Bandopadhyay uses his free time to write. He has developed a writing style revealing the bondage of nature and life. He wants to share & communicate those events with readers for their valuable feelings and interactions. The author was working as a Director in Geological Survey of India in the last phase of his service career. In his younger days, he worked in various field of geology with background of foreign training in United Kingdom under United Nations Development Programme Scheme in Marine Exploration. He gathered vast experience both in land and Ocean. Active participation as Chief Scientist in as many as 15 different geological cruises in the Bay of Bengal are to his credit. He took part in search of Fe-Mn nodules cruising 45 days at a stretch in sea in the Indian Ocean in Skandy Surveyor, a Norwegian Research Vessel. Published scientific papers in national and international journals. Besides scientific milieu, came across many human-inhuman experiences. Vast field of Rajasthan, the place of desert and his initial placement in the job ended in the deep sea through majestic happenings covering human feelings and scientific search. [email protected]





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