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.