Just imagine
the fateful day about 20 million years ago when the Indian plate came
drifting, torn from the parent land in the southern hemisphere and began
to collide with the Asian Plate. So massive was the collision that it
gave rise to the mighty Himalayas. The process continues and the
Himalayas continue to rise higher and higher, as if trying to kiss the
sky!
Imagine what would have happened if there was no such collision? The
Himalayas would not have been formed! Without the Himalayas there would
have been no river system like the Ganges or the Indus or the
Brahmaputra. Even the rainfall is controlled by the Himalayas. Thus
without the rains and river system we would have been almost a waterless
land. In other words our Himalayas are the water tower of Asia. They
provide water to nearly one fifth population of the world.
The collision, on one hand gave rise to one of the most potent source of
hydroelectric power and also as already stated a major source of water,
but on the other it has given rise to a fragile eco-system, prone to
tremors. The earthquakes of the Himalayas are in themselves a great
hazard and are also responsible for the greater hazard of seismogenic
landslides.
Geologists endeavor to understand the mechanism or causes of past
earthquakes. That helps them to postulate about possible earthquakes in
future.
One of the ways is to identify the active faults. These are actually a
product of the great collision that took place millions of years ago.
Understanding the past behaviours of active faults provides keys to
evaluation of seismic and a landslide hazard says Professor K.S. Valdiya
an earth scientist of international repute in one of his papers. Valdiya
studied the major thrusts that delimit the boundaries of Himalayan
physiographic terranes at crucial spots in the Kumaon Himalayas.
The present landforms of the Himalayas are a product of past tectonic
activities and of course water, the greatest natural carver had been
always at its best in the Himalayas. It is easy to understand that the
rivers carry sediments to the oceans or the basins. Where the sediments
were deposited layer after layer. The process continues even today.
Naturally the oldest layers are at the base and the youngest at the top.
What is baffling is that ancient rocks of the Precambrian Era (3600
million years old) are seen riding over the very young sediments of the
Quaternary Era (1.75 million years old). How does one explain this
topsy-turvy act of the Nature!
It seems all was not quiet during the Quaternary times. Apart from
thrusting of much older rocks over much younger ones, many of the recent
river terraces were found tilted by him. Many of the streams were
beheaded suddenly and the gradient of some rivers was observed to have
become suddenly steep. In nature nothing is sudden. Everything is slow
and subtle.
The ground movements that had caused the above mentioned changes had
also ponded the rivers upstream of the faults. Giant lakes were formed
at such places. The ground movement was related to uplift of the
mountains. When something is uplifted a steeper gradient is formed. In
other words the slops on such mountains are prone to greater erosion.
Means more material was transported to the artificial lakes created by
the nature. The filled up lakes are now found as flat terrains in the
rugged mountain valleys and are extensively used for cultivation and
habitation.
In the story about Garbayang we have read about
how the lake deposits were used to discern the past ground movements and
also how past climatic scenario was worked out! Likewise such lake
deposits where ever found in the Himalayas are being extensively used
for working out the past tremors and also the past climates.
The great collision of the two continental masses was a very complex
kind of collision. If we see the outline of the Himalayas they form a
southwardly curved arc. The central sector of which includes eastern
Himachal Pradesh, Kumaon Himalaya and western Nepal. The Indian land
mass with more ancient mountain ranges like Aravalis when it collided,
the land mass was sub-ducted under the Tibetan Plate. Geophysical
studies confirm the presence of Aravali ridges under the aforesaid
Himalayan arc.
Imagine a metal rod anchored at one side and being pushed from the
other. Under pressure it would bend. Gradually as the pressure will
increase the rod would come under tremendous strain. The only ways the
strain build up can be reduced are, either the pressure being applied is
stopped or the rod ruptures.
Likewise as the Indian plate continues to forge ahead the pressure being
applied continues. Earthquakes are a strain release mechanism of the
rocks under pressure. There have been some major seismic events in 1803
(M ≥ 7.5), 1816 (M 6.8), 1916 (M 7.5), 1945 (M 6.5), 1958 (M 6.2), 1964
(M 6.2), 1968 (M7.0), 1979 (M 6.5), 1980 (M 6.5), 1991 (M 6.6) and 1999
(M 6.8) took place mainly in north-eastern Kumaon and north-central
Garhwal regions. Yet these earthquakes could not have sufficiently
released the strain generated by the India-Asia convergence at a rate of
58 ± 4 mm/year says Valdiya. The central sector of Kumaon has so far not
suffered a great earthquake of the intensity M ≥ 8. Thus it is a seismic
gap according to Valdiya.
As the India and Asia converge about 30% of the impact is absorbed
across the Himalaya at an average rate of ~ 17.7 ± 2 mm per year says
Valdiya. Working out the rate of slips with the help of available
earthquake data, Valdiya has calculated that underneath Tibet this slip
is at a rate of 20 ± 3 mm per year. Compared to these seismic slip in
the central Himalayan arc is apparently locked. In order to measure the
subterranean movements due to earthquakes, precision surveys are
essential. Unfortunately in the Kumaon Himalayas such studies have not
been carried out. Between 1995-99 western Nepal recorded vertical
velocities of 15 ± 2 mm/year and 12 ± 3 mm/ year at Simlikot-Dopho and
Dopho-Jomosom belts respectively. Considering their vicinity to Kumaon,
Valdiya concludes Kumaon sector is also experiencing a vertical
deformation at the same rate.
We know that the Himalayas have risen as a consequence of the great
continental collision. The Himalayas did not rise in one go. There were
several episodes. Thus the Himalaya is made of four broad tectonic
units. These units are separated by large faults called as the thrusts.
Valdiya says the rate of uplift on the east-west trending faults of the
Main Boundary Thrust Zone in Nepal ahs been 3 to 4 mm/year during the
last 400000 to 50000 years. Imagine the continuous process triggered by
the great collision!
Likewise all the four tectonic units of the Himalayas show evidences of
uplift of varying rate. Though imperceptible to the eyes, these
movements many times cause earthquakes of great intensity. The
continuous of the Himalayas can be compared with continuous growth of a
human body. Such growth is possible only as long as the body is young.
So our Himalayas are young and still growing.
The outcome of the growth is the present water tower of Asia. What
haunts the mind is that each phase of growth is related to a major
tectonic activity. In simple words related to earth movement. Such
movements are referred as earthquakes in our parlance. Thus the water
tower is under stress. The population settled on the Himalayan ridges,
slopes and valleys is more prone to disasters due to earthquakes and
related hazards. The Kumaon sector seems to be in the eye of the storm
as all the seismologists feel that the uneasy calm could be shattered
some day. The stresses are building up and the moment they exceed a
limit this sector can buckle under the pressure.
It is better to be aware than taken unawares specially in the matter of
natural hazards.
December 2,
2007
Image of towering Himalaya
in Kumaon by the author.
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