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Big quakes may hit Delhi, other northern Indian cities

Parts of the Indian capital and many of the country's principal cities could be hit by high-intensity earthquakes in future, destroying life and property, a new Indo-US study has said.

A study on the Black Mango Fault (BMF), a seismic fault that passes through Haryana, has shown that the Indo-Gangetic plain, which covers parts of Delhi, Uttar Pradesh, Bihar, West Bengal and Assam, may be hit by big quakes in future, a team of Indian and American researchers reported in Friday's issue of the international journal Science.

Neighbouring Bangladesh could also be hit.

The study was undertaken by a team comprising Senthil Kumar, a geology postgraduate from Madras University doing his PhD in Nevada, S G Wesnouskyat, also from the University of Nevada in Reno, T D Rockwell and D Ragonaat from the San Diego State University, G G Seitzat from the Lawrence Livermore National Laboratory in California and V C Thakur from the Wadia Institute of Himalayan Geology in Dehradun.

One or more quakes could strike the Indo-Gangetic plains, the fertile tract through which the country's major river Ganges and its tributaries flow, as the region lies on the southernmost and the most active among three big Himalayan seismic faults, the study revealed.

The BMF is the most active among the three big Himalayan seismic faults.

A fault is a fracture or a zone of fractures between two blocks of rock that may be a few millimetres to thousands of kilometres long. Faults allow the blocks to move relative to each other, which may occur rapidly in the form of a quake or slowly, in the form of creep. Most faults produce repeated displacements over geologic time.

Three major faults occur along the length of the Himalayan arc. The northernmost is the Main Central Thrust (MCT), which emerges along the northern edge of the high Himalayas and is generally considered inactive.

The next, called the Main Boundary Thrust (MBT), marks the southern edge of the lesser Himalayas and causes local activity.

The southernmost is the 2,500-km-long Himalayan Frontal Thrust (HFT), which is now considered to be the most active of the three.

HFT delineates the Indian plate, which continuously moves towards the Himalayas. The BMF lies at the end of this fault and crosses it 80 km north of Dehradun, in Uttaranchal.

The region of most severe activity associated with major historical quakes along the Himalayan arc is generally bounded by the MCT to the north and the HFT to the south. Modern day seismicity along the HFT is concentrated at depths of 10-20 km below the MCT.

Studies on a trench excavated across the BMF in the Indian side of the Himalayas provide the 'first direct observations' about the size, displacement and frequency of past quakes that have produced ground ruptures along the HFT, Senthil Kumar told IANS.

Earlier, it was not possible to establish the repeat time of quakes based on historical records alone.

"The trench displays evidence of two large surface rupture quakes during the past 650 years, subsequent to 1294 AD and 1423 AD, and possibly another at around 260 AD causing large ruptures," Kumar said.

The first two quakes resulted in ground displacement of about five metres and 3-4 metres respectively, which is comparable to the displacements observed in the Chi-Chi earthquake, measuring 7.6 on the Richter Scale, that rocked Taiwan on September 20, 1999. The impact of the third was still larger, he said.

"Our results clearly show that very large surface rupturing quakes have and will continue to occur along this densely populated corridor adjacent to the HFT," according to Kumar, who said the impact would be devastating keeping in view the shaking amplification potential in the region.

The BMF was studied because expressions of fault scarps (sedimental slopes caused by quakes) are very clear and unambiguous and it lies close to one of those plate-boundary faults worldwide where paleo-seismic investigation is possible as most other areas are beneath the sea, the scientists said.

The findings also indicate that major historical quakes were initiated at a point beneath the MCT and propagated southward along the HFT, they write.

The ongoing collision of the Indian plate with Eurasia has resulted in four major quakes along the HFT in the last 100 years. The last of these was the 1950 Arunachal Pradesh quake measuring 8.6. The other three occurred in Shillong (1897), Kangra (1905) and Bihar/Nepal (1934).

In January, some 20,000 people were killed and thousands rendered homeless in an earthquake that hit Gujarat.

The study endorses with fresh evidence a similar finding reported in the same journal in August this year by another Indo-American team that said the Himalayas may be hit by one or more earthquakes of high intensity in the future, threatening about 50 million people in the Indo-Gangetic plains.

Indo-Asian News Service

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