Tuesday, August 4, 2009

Sedimentation by Himalayan Rivers may cause Earthquakes and Land subsidence in Eastern India.

It's not a question of whether the big one is coming, only of when.
by.
Dr. Nitish Priyadarshi
Image of the Ganges River delta and the Bay of Bengal acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS). This image shows the massive amount of sediments delivered to the Bay of Bengal by the Ganges River, sediments that are derived from erosion of the Himalayan mountain range to the north.
Sediments deposited in Bay of Bengal


Sediment loads in Kosi River in Bihar.

The Indian landmass, a floating continent started to collide with the Asian landmass some 20 million years ago (m y). After its separation from South Africa and Madagascar the floating continent must have been like a Noah’s Arc carrying all its fauna and flora on its body. The great collision between the two landmasses led to the formation of the youngest and tallest mountain ranges, the Himalayas.

Once the Himalayas started to rise a southward drainage developed. The Himalayas subsequently controlled the climate of the newly formed continent, and there started the season of monsoon as well. The river system thus developed because of rains and melting snow started to drain south into the fore-deep. The newly formed rivers were like sheets of water flowing towards the fore-deep carrying whatever came in their way. Once the rivers reached the plains their gradients became lesser, their hydraulics changed and they started to dump their load. During monsoons these rivers carried a sediment load which was many times more than their normal load. All the material they carried was dumped enroute their final destination, the Sea.

The sediments are carried from their point of origin to the local stream network commonly by mass weathering processes, typically soil creep, and eventually become part of the stream load. Very fine fragments move quickly along the network as suspended load, but the downstream progress of larger fragments is usually very slow. Thus, the weathering process does not end in the source area but continues to operate during the long process of stream transport.

Sedimentation rates generally cannot be expressed in absolute data because periods of rapid sedimentation alternate with periods of slower deposition, non-sedimentation, or erosion. Nevertheless, it is important to gain some understanding of the average values of net sedimentation in various depositional environments in order to better comprehend the geological and chemical processes that take place on the surface of the earth. An understanding of net sedimentation rates has become increasingly valuable with onset of intensive water pollution studies, because sedimentation is one of the most important processes in the removal of pollutants from natural waters.

Presently sedimentation loads are being considered as one of the possible cause of earthquakes. It works on the theory that deposition of sediments alters the loading of the earth’s crust and tectonic stresses in its interior. Such stresses could reactivate preexisting faults.
Combination of the biological, chemical, geological, and geographical factors that influence sedimentation rates are almost infinite, are different for each depositional environment, and have continuously fluctuated throughout the past.
The most extensive vertical deposition of sediments by Himalayan rivers flowing through Uttar Pradesh, Bihar, Jharkhand, and Bengal States of India, occurs during floods (July to October).

Coleman (1969) investigated channel deposition and erosion patterns of the braided Brahmaputra River in India during flooding and found that as the current velocity decreased, rapid sedimentation occurred, and as much as 3 m. of sediment was deposited along the channel bottom. When a meandering river floods its banks, its velocity is rapidly checked, and sediment deposition occurs adjacent to the banks. The rate of floodplain deposition usually ranges from several mm to several cm/year (Kukal, 1971).
Each year these rivers were flooded leaving behind a fresh layer of sediments. The Indo-Gangetic plains are a product of such floods. Study carried out by Rajiv Sinha, of Geoscience group, IIT Kanpur has brought to light amazing quantity of sediment load carried by the Ganga River in its present hydrodynamic regime. Gangetic Rivers erode bulk of the sediments from upstream areas in the Himalayas and deposit part of it in the alluvial plains and a significant part in the Bay of Bengal. His study reveals that the Ganga river annually erodes around 749 million tonnes of sediments, mostly from the Himalayas, brings about 729 million tonnes at Farrakka and finally dumps 95 million tonnes in the Bay of Bengal. Thus the floodplain of the Ganga gets an annual increment of about 65 million tonnes of sediments.

The quantity of sediments eroded by the river depends upon the gradient, distance from the source area and also the geology and geomorphology of the terrain. Thus Ganga at Haridwar and Yamuna at Allahabad are characterized by low sediment yield of 150-350t/km2/yr, while the eastern tributaries like Kosi and Gandaki carry a much higher sediment load of 1500-2000t/km2/year.

Along the river's traverse, large tributaries enter the Ganga and significantly increase its flow and change its character. The Ganga is joined by the Ram Ganga, Yamuna, Ghaghara, Gomti, Gandak and Kosi tributaries. The rivers of the Ganga basin carry one of the largest sediment loads in the world. Today sediment loads in the Ganga are higher than in the past due to the complete deforestation of the Gangetic plains and the ongoing deforestation of the Himalayan foothills.

Sedimentation in plains of Ganga River and Bay of Bengal.

In the plains Kosi (major tributary of Ganga) River is building up a large delta of its own through which its channels have wandered for centuries. It is believed that the Kosi originally joined the Mahananda, a river coming from the Darjeeling Himalayas. It is known that the Kosi flowed by Purnea (Bihar) 200 years ago, but its present course is about 160 km to the west of that place, having swept over an area of 10,500 sq. km on which it has deposited huge quantities of sand and silt (Krishnan, 1982). It now joins the Ganga 32 km west of Manihari but formerly it used to join that river near Manihari itself. The Kosi is notorious for its frequent and disastrous floods and the vagaries of its channels. In high flood it is said to have a flow of nearly one million cusecs loaded with much gravel, sand and silt (Krishnan, 1982).

The Hooghly River (main channel of the Ganga in West Bengal) estuary is notorious for its sand banks and dangerous shoals of which the James and Mary Sands, 56 km below Calcutta (now Kolkata) and between the mouths of the Damodar and Rupnarain, are well known. New areas are being reclaimed by the sediments brought down by the Ganga. These are known as the Sundarbans.

Compared to the Peninsular rivers, the three main Himalayan river systems are mighty giants. The Indus carries to the sea an average of about a million tons of silt per day, the Ganges a little less and the Brahmaputra a little more (Krishnan, 1982). The Irrawaddy has been estimated to transport about two-third million tons of silt per day. The Himalayan rivers are fed both by rain and snow, by rain during June to September and by snow during the warmer half of the year. In their courses through the mountains they have good gradients and carry much coarse materials including pebbles and boulders, brought in by glaciers and also torn off from the beds and banks. They carry enormous quantities of fine sand and silt derived from the Himalayas as well as from higher peninsular up-lands.

The Ganga and the Brahmaputra have changed their courses in the plains frequently in historic and pre-historic times leaving behind huge sediments in the plains. Deposition of sediments in Bihar, Bengal, and in Bay of Bengal is going on from the geological past. Millions of tons of sediments are being deposited per day by the Himalayan rivers in the Eastern India thrusting pressure over the crust below.

Now it is widely accepted that huge sediment loads may cause mild to high tremors even in the non-seismic zone. This may be due to the great lateral thrust of sediment load contributing to stress imbalances or due to the reactivation of subterranean faults by the newly developed stresses or due to increased pore pressure in the adjoining rocks which lowers their shearing strength, resulting in earthquake occurrence.

An earthquake is generally caused by dislocation in the earth’s crust along pre-existing cracks or faults. The cause of earthquakes is probably the existence of such faults or cracks in the bottom of the depression hidden under alluvium. Moreover, there are well marked reversed faults at the junction of the outer and the inner Himalayas, and when dislocation occurs along these faults, earthquakes result.

An additional factor favoring dislocation along such surface or subterranean faults is the strain which exists between the Himalayas and the Bihar plains. This strain is due to the following facts. The section of the Himalaya north of the Bihar is the highest mountain region of the world. The higher a region, the more it is subjected to erosion. So, vast amount of sediments are being eroded from the Himalayas and carried down to the Bihar plains as in the case of Kosi river which contributes heavy sediment in Bihar plains. The silt yield of the Kosi is about 10 cubic yard /acre/yr, one of the highest in the world. As the mountains are eroded they are deloaded and have a tendency to rise. On the other hand, the plains get loaded by the sediments and have a tendency to subside. These opposed tendencies of movements between the Himalayas and the Bihar plains cause strain in the hinge-zone, i.e. in the southern part of the mountains. Here fault already exists. Dislocation may occur along these faults as a result of the strain and devastating earthquakes may result.

The entire area has undergone downwarping due to Himalayan upheaval resulting in the formation of transverse faults and dislocations in the basement rocks, along pre-existing faults or cracks aided with occasional earthquakes. The foothills of the Himalayas, the Indo-Gangetic plains and the sedimentary basins of Vindhyans are all quake-prone areas of the Bihar state.

Several faults have been identified in the region and some have shown evidence of movement during the Holocene epoch. The West Patna Fault runs in a NE-SW direction from near Arrah in the south to the Nepalese border near Madhubani in the north. Running almost parallel to it is the East Patna Fault which extends from the south-east of Patna in the south to the Nepalese border to the east of Madhubani. Another fault, this one also lying parallel to the previous two, is the Munger-Saharsa Ridge Fault which runs from Biharsharif to near Morang in eastern Nepal. Apart from these there are east-west running tear faults in the region that control the courses of the main rivers.
The Gandak fan is bounded by the courses of the Ghagra and Rapti in the west, the Ganga in the south and the Rohini in the north. The courses of all these streams are along faults (Mohindra and Prakash, 1994).

The Gangetic plains, of which the Kosi megafan forms a part, is bound by E-W faults, which on analogy with the main boundary thrust may be thrust faults. The Kosi megafan is bound on the west by a NE trending prominent sinistral fault causing an offset of some 20 km of the Siwaliks juxtaposed against the Gangetic alluvium. There are several NW trending faults on the eastern fringes of the Kosi megafan (Mahadevan, 2002).

Bengal basin, having an area of 89000 square kilometers and sedimentary fill of 10-15 km, is the northernmost of the east coast basins of India . Indian Shield and Shillong massif form the western and northern limits of Bengal Basin. Eastwards the Basin extends into Bangladesh and is bounded by Arakan Yoma geanticlinal uplift. Southwards Basin plunges into Bay of Bengal beneath the continental shelf. Tectonically the basin can be divided into four structural elements i.e. basin margin fault zone, shelf, hinge zone/slope break and basin deep.

The tectonic history of Bengal Basin indicates that the drainage pattern in the Bengal basin as a whole had been and is greatly controlled by the tectonic features of the basin. Considerable evidence has been recorded of significant tectonic movements within and along the boundary of the basin in late Tertiary and the Quaternary times. Auden (1949) postulated that the western margin of the Bengal basin is faulted and the major tectonic movements have taken place along this zone in the Pleistocene.

Rocks at the depth in crust are subjected to the load pressure of the overlying column of rocks and sediments. This pressure is related to the thickness and mean density of the overlying material or sediments. Several million years under stress, most rocks will exhibit the kind of ductile behaviour familiar to all geologists. The rocks under higher stresses, however, will fracture and generate earthquakes (Park, 1983).

The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, CA and the coast of Northern California at 5:12 A.M. on Wednesday, April 18, 1906. The 1906 San Francisco earthquake was caused by a rupture on the San Andreas Fault, a continental transform fault that forms part of the boundary between the Pacific Plate and the North American Plate. This fault runs the length of California from the Salton Sea in the south to Cape Mendocino to the north, a distance of about 800 miles (1,300 km). The earthquake ruptured the northern third of the fault for a distance of 296 miles (477 km). The maximum observed surface displacement was about 20 feet (6 m); however, geodetic measurements show displacements of up to 28 feet (8.5 m).
It was interpreted that earthquake was caused due to large seasonal sediment loads in coastal bays that overlie faults as a result of the erosion.

Sedimentation also cause land subsidence. Subsidence may result from the accumulation of large volumes of sediment at the earth's surface in what is known as a sediment basin. An obvious setting in which this occurs is at river deltas. Each day, the Mississippi River deposits up to 1.8 million metric tons of sediment at its mouth near New Orleans. The weight of this sediment contributes to a gradual subsidence of the land on which New Orleans resides. Basins between mountains also can subside due to the weight of accumulating sediments.

Wherever sediments accumulate, we can be certain that in some other locality, a source has been relatively elevated with respect to the place where the strata are being deposited.

A delta is a subsidence-prone area because it receives a huge volume of sediments, which can be compressed due to post depositional consolidation, and the load of which can result in detectable isostatic sinking of the earth's crust.

In the year 2008 lots of reports were there regarding development of big cracks on the surface overnight in many parts of Uttar Pradesh state of India. This may be the side effects of land subsidence.

Two prehistoric seismic events dated to have occurred: (i) during 1700 to 5300 years BP and (ii) earlier than 25,000 years BP. From last several years Ganga Basin has not been affected with any major tremors or earthquakes, except of 1833, 1934 and 1988 earthquakes which rocked North Bihar and Nepal. Seeing the load of sediments, possibilities of major earthquakes cannot be ruled out in Eastern India including Bihar, neighbouring Uttar Pradesh and Jharkhand, and Bengal Basin. Most affected areas may be Munger, Dharbanga, Purnia, Bhagalpur, Saharsa, Supaul, Katihar, Patna in Bihar State, Sahibganj, Godda, Pakur etc. of Jharkhand State. It's not a question of whether the big one is coming, only of when.

Reference:

Auden, J.B., 1949. Proc. Ind. Nat. Instt. Sciences.,15.

Coleman, J. M., 1969. Brahmaptura River: Channel processes and sedimentation. Sed. Geol.,3, pp. 129-239.

Dasgupta, S., Pande, P., Ganguly, D., Iqbal, Z, Sanyal, K, Venkatraman, N.V., Dasgupta, S., Sural, B., Harendranath, L., Mazumdar, K., Sanyal, S., Roy, K., Das, L.K., Misra, P.S., Gupta, H. 2000. "Seismotectonic Atlas of India and its Environs", Geological Survey of India.


Krishnan, M.S. 1982. Geology of India and Burma. CBS publishers and distributors, India.

Kukal, Z., 1971. Geology of Recent Sediments. New York: Academic Press (in Czechoslavakia: Prague, Czechoslovak Academy Sci.), 490p.

Mahadevan, T. M. 2002. Geology of Bihar and Jharkhand. Geological society of India, Bangalore.

Mathur, S.M., "Physical Geology of India", National Book Trust of India, 1998.


Mohindra, R. and Prakash, B. 1994. Geomorphology and neotectonic activity of the Gandak mega-fan and adjoining areas, middle Gengetic Plains. Jour. Geol. Soc. India, v.43, pp. 149-157.

Park, R.G., 1983. Foundations of Structural geology. Blackie & Son Ltd. Glasgow.

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