Thursday, September 18, 2008

Kosi floods in Bihar, India may trigger major earthquakes in future.

The Kosi floods in Bihar, India may trigger major earthquakes in future.
By
Dr. Nitish Priyadarshi
The recent Kosi floods and deposition of sediments in the North Bihar Plain may trigger earthquakes in coming future. Deposition of sediments alter the loading of the earth’s crust and tectonic stresses in its interior. Such stresses could reactivate preexisting faults.

As North Bihar is already under seismic zone such types disastrous flooding and sedimentation may trigger earthquakes in future.

New research indicates sediment buildup in seismic zone may play a major role in determining the severity of earthquakes.

Northern Bihar plain falls in the seismic zone of India and is liable to severe earthquakes as on 15th January, 1934 and 20th August, 1988. 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 most damaging and significant of the seismic events was the Bihar –Nepal 1934 earthquake. The Bihar-Nepal event, with an estimated magnitude of 8.1, occurred on 15th January 1934 and was followed by several aftershocks of much lower magnitudes. These continued through 19th January 1934 and with much lower frequency till the 13th April 1934. the main shock was felt over a very large area of 4.92 million sq.km and was recorded by almost all seismic stations in the world. The death toll associated with the event was 7253. this is possibly due to the fact that the shocks occurred during the day and also took some 2.5 minutes to reach their maximum intensity. Severe dislocations were caused by damages to railway, telegraph and other communication lines. Sand and water emissions were noticed from many fissures and from individual vents, a feature common in alluvial tracts.
During the early hours of Sunday, the 21st August,1988 an earthquake rocked severely the entire North Bihar and adjacent areas. The shock was felt almost all over North and Eastern India, from Rajasthan in the west to North Eastern states in the east and from Himalayan foothills in the north to Chhotanagpur plateau in Jharkhand State in the south. Dharbhanga and Madhubani were the worst affected districts, but damages of gradually diminishing intensity extended upto Ranchi and Palamau in Jharkhand State.
The earthquake immediately devastated a large area of Madhubani, Munger, Saharsa and several other districts of North Bihar, killing about 300 people.
Seeing the severity of the earlier earthquakes, the disaster management department of the Bihar State should be kept on high alert so that lives and property may be saved in the event of any major future earthquakes. It cannot be said with accuracy that when the future earthquake may show its presence but seeing the amounts of sediments being brought by the Kosi river, major earthquakes in coming days cannot be ruled out.
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