Ground beneath Kanchenjunga slips south, alarm bells ring for Bengal

19 Sep

New Delhi, Sept. 18: A behemoth chunk of rock about 20km beneath the mountains around Kanchenjunga lurched southward today in the 6.9 magnitude earthquake that scientists say should stir the Bengal and Sikkim governments into a state of high alert.

The first and preliminary analysis of the mechanisms underlying the earthquake near the Sikkim-Nepal border suggests that the western edge of Sikkim had slipped southward relative to Nepal during the earthquake, geophysicists and seismologists said.

The first earthquake was followed by two smaller 4.8 and 4.6 magnitude earthquakes on the eastern side of the main event in a pattern that points to additional build-up of strain in subterranean rocks across Sikkim, three scientists tracking the events said.

“The movement of rock associated with the 6.9 event on the western side of Sikkim is likely to have caused a pile-up of strain towards its east leading to the two subsequent events,” said Supriyo Mitra, a seismologist at the Indian Institute of Science Education and Research, Calcutta. “In a region where strain has accumulated for years, the 6.9 event was a trigger for the next two,” Mitra told The Telegraph.

Geophysicists say the two smaller earthquakes would not have released the enormous strain that has accumulated in the region. “The two (smaller) quakes were like peanuts,” said Vinod Gaur, a geophysicist at the Indian Institute of Astrophysics, Bangalore.

G.C Debnath, director of the Alipore meteorological centre, said: “The first tremor with an intensity of 6.9 on the Richter scale happened at 6.11pm, followed by a 5.7 intensity aftershock at 6.21pm. The third one happened at 6.43pm with 5.3 intensity, while the last one occurred at 7.24pm with intensity of 4.6 at Richter scale.”

“These are not aftershocks because they are not on the same fault as the first event,” said Mitra, the seismologist at the Indian Institute of Science Education and Research, who maintains four seismic monitoring stations in Sikkim.

Scientists say release of the accumulated strain in Sikkim could happen in months or years — and earthquakes cannot be predicted. “I think prudence dictates that Bengal and Sikkim should initiate action — at least across Sikkim and north Bengal — to scrutinise buildings such as schools and hospitals, where a large number of people assemble, for vulnerability and retrofitting using available engineering solutions,” said Gaur.

The Himalayan region marks the zone where India is slipping under Tibet at the rate of about 18mm per year. This tectonic plate movement leads to strain build-up, which is released when subterranean rocks along the slip zone reach their fracture points.

Scientists estimate that the amount of energy released during the 6.9 earthquake was equivalent to 340,000 tonnes of TNT, or about 22 times the energy released by the atomic bomb dropped on Hiroshima.

Geophysicists say earthquakes in the Himalayas are likely to cause significant shaking of structures built along the Gangetic plains across northern and eastern India. “The contrast between the soft sediments lying above the harder rock all along the Gangetic plains is likely to cause amplification of energy,” said Shyam Sunder Rai, a scientist at the National Geophysical Research Institute, Hyderabad.

“Whatever energy has been released by the earthquakes today does not rule out the possibility of an even bigger quake in other areas along the Himalayan region,” said Rai.

Gaur believes moderate earthquakes may mean that more are to come. “Moderate earthquakes in plate boundary environments such as the Himalayas therefore verily presage closeness of the region to a great rupture of magnitude 8 or so.”

The US Geological Survey said this region has experienced relatively moderate seismicity in the past with 18 earthquakes of magnitude 5 or larger over the past 35 years within 100km of the epicentre of today’s event. The largest of these was a magnitude 6.1 earthquake in November 1980 about 75km southeast.

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