The Telegraph
Since 1st March, 1999
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The highrise hammer

New Delhi, Oct. 8: The earthquake of magnitude 7.6 that struck the western Himalayas today was the strongest in the region in decades, according to scientists who are now scrambling to fathom its tectonic triggers.

The epicentre was 90 km northeast of Islamabad in a region sandwiched between two continental plate collision zones that geologists have named the “main Karakoram thrust” and “the main mantle thrust”.

Seismologists at the Indian Meteorological Department said initial observations indicate that the depth of the earthquake was about 30 km, a bit deeper than that of the Gujarat earthquake of 2001 which had a depth of about 20 km.

“Besides the magnitude, the depth of an earthquake also determines the area where it is felt and its potential to cause damage,” said Atindra Kumar Shukla, director of the earthquake risk evaluation centre in New Delhi.

This region on the edge of the western Himalayas has experienced dozens of earthquakes in the past but the strongest recorded so far was a 6.7 magnitude tremor in 1943. Earthquakes in this zone are typically the result of the collision of the Indian plate with the Eurasian plate and related crustal movements.

The specific crustal movement that triggered this earthquake is yet to be identified, Shukla said. The epicentre is close to three geological features ' the main mantle thrust, the main boundary thrust and the Tarbela fault ' each of which are zones where sections of the crust collide or rub against each other. “The cause may be at any of these three boundaries.”

Seismologists said the deeper an earthquake, the greater is the potential danger that it poses to highrises. The earthquake today toppled two apartment towers and damaged several others in Islamabad.

Earthquakes give rise to ground vibrations in a range of frequencies ' low and high frequencies.

As they move towards the surface, the high frequencies weaken faster than the low frequencies. “So low frequencies tend to dominate in deep earthquakes,” said Sushil Gupta, a senior engineering seismologist. “These low frequencies pose a risk to very tall structures such as highrise buildings,” Gupta said.

Seismologists in New Delhi who recorded the event through a network of stations began to release information within 20 minutes, but there were discrepancies between their calculations and those by seismologists elsewhere.

The US Geological Survey's earthquake centre classified the event as 7.6 magnitude. But Indian seismologists initially announced a figure of 6.8 and then fixed it at 7.4. The US Geological Survey has listed the depth of the earthquake as 10 km.

IMD seismologists said such discrepancies are not unusual because the calculations depend on the network of stations used for observations.

Indian seismologists are handicapped because existing government policies do not allow them to share or receive real-time seismic data from all over the world.

“It’s important for us to have a large network to get detailed earthquake data in real time,” said science and technology secretary V.S. Ramamurthy. “We’re telling the government to change this policy,” he said.

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