Not an aftershock but a quake: Professor

G.S.Mudur, TT, New Delhi, April 26: The earthquake that convulsed Nepal yesterday triggered dozens of aftershocks and a fresh quake today, implying what some scientists said is an eastward-moving subterranean instability whose pattern and outcome cannot be predicted.

Indian government seismologists documented more than 45 aftershocks today after yesterday's 7.9-magnitude earthquake, as well as a 6.9-magnitude earthquake 65km east of Kathmandu a little past noon. Yesterday's earthquake had its epicentre about 77km northwest of Kathmandu.

"Most (of the) aftershocks had magnitudes between 4 and 6," said Laxman Singh Rathore, director-general of the India Meteorological Department.

"They may continue for some time, with smaller magnitudes. We cannot predict how long they may last --- weeks, months or even years."

Since the primary earthquake, a 6.6-magnitude aftershock yesterday and the 6.9 earthquake today shook buildings and structures across several states in northern and eastern India.

Today's tremors from the 6.9 earthquake, which Calcutta felt eight seconds past 12.38pm, were over in 10 seconds; yesterday's shake from the primary quake had lasted a minute and a half.

Preliminary observations of the focal mechanism of yesterday's primary earthquake suggest that it occurred in what some scientists consider a "dangerous zone".

Its epicentre, about 15km under the ground, lies within what geologists call the Main Himalayan Thrust, the wedge-shaped section of the Indian plate that has slipped beneath the Tibetan plate as the Indian continental plate thrusts under the Eurasian plate at about 45mm per year.

"A primary event on the Main Himalayan Thrust is something that worries me," Sridevi Jade, a senior scientist at the Council of Scientific and Industrial Research Centre for Fourth Paradigm Institute who has conducted extensive research in the Himalaya, told The Telegraph .

Today's earthquake, Jade said, suggests an eastward-moving instability.

Geophysicists are concerned that the primary earthquake, which released energy equivalent to 712 Hiroshima-class atomic bombs, may have transferred some of its energy elsewhere and destabilised the Main Himalayan Thrust, giving rise to the earthquake east of Kathmandu today.

"The 6.9 quake today is certainly not an aftershock," said Shyam Sunder Rai, professor of earth sciences at the Indian Institute of Science Education and Research, Pune.

"Yesterday's main event and today's quake occurred in two completely different domains."

For earthquakes to be labelled "aftershocks", Rai said, they need to occur in the close vicinity of the primary epicentre and be linked to the same geological fault system.

But a US-based geologist cautioned that it might still be too early to say whether today's earthquake was an aftershock or a new event.

"The strict definition is that an aftershock should occur within the fault area that ruptured in the original earthquake," said Larry Ruff, a seismologist at the University of Michigan who has studied large earthquakes around the world.

"But we need to know the exact limits of the section of the original fault that broke --- and we don't know that for the Saturday earthquake yet."

Ruff is now trying to get the best estimate for the length of the rupture using seismic waves data.

According to Ruff, energy propagation after an earthquake is a phenomenon even mechanical engineers studying cracks in materials are familiar with. Stresses can build at the edge of the crack and such energy transfer may load tectonic strain at other sites along a fault.

The North Anatolian fault in Turkey is an example where an earthquake in the mid-20th century propagated energy elsewhere along the fault,