What
is an Earthquake & Why do They Happen?
Perhaps the
Earthquake is the most fearful natural phenomenon in the human
life. It is more so, because it is unpredictable and arrives without
notice or without announcing it's vigor and strength. Scientists are
constantly in search of this unknown. The major events of the earthquakes
from the human viewpoint are realized on review of some major
catastrophes. Minute observations and records at least 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
in fact, is just a browsing in the memory in 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 are the
results of disturbances within the body of the earth itself logically are
called Earthquake.
Why
& How Earthquakes Happen? Reasons
could be many; pinpointing is always 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, magma tic outbursts from
within the interior of the earth or explosion of high power nuclear
or atomic bomb and so and so forth.
We should have first hand information about the two great seismic belts
in the globe.
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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.
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In a simpler way it can be narrated that the earth as such consists of
different rock layers of decreasing densities right from the center
towards it's 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. This is happening since long geological past.
This movement acts like a conveyor belt and during the journey the plates
meet each other or gets obstacle by other denser plate and the rock layers
start descending when we call a subduction Zone.
Naturally in the adjacent
sides of the subduction zone the rock layers get up heaved resulting
folding, thrusting and faulting. The second belt in parts of India
i.e. Andaman- Great Nicobar- Java- Sumatra zone is actually is a
sub ducting zone for which the long island chains have emerged.
This zone, in fact, is very dynamic and active giving rise deep
to 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. Entire this zone is susceptible to
tremors of higher magnitude. 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 and the whole village was displaced
beneath the ocean bottom.
With
intermittent volcanic activity and earth tremors, this region recently, 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 tsunami causing severe damage to both life and property.
The other parts of India, was used to be considered as earthquake
resistant areas or the shield areas. Oldest rocks of more than 3000 m
years form the foundations of the continental mass compare 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 that the tremors had
hit mostly 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 the 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 before. 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 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 fall 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 traffics. 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 tremors of
human concern.
Contributing Writer: Mr. Asimendu Bandopadhyay
uses his free time in writing. He has developed a writing style revealing
the bondage of nature and the life. He wants to share & communicate those
events of his own with the 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 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 is in 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.
asimendubandopadhyay@yahoo.co.in
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