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
& 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.
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
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.
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.
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.
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.