Quake, this time in Bengal

G,S,Mudur, TT, New Delhi, April 27: Saturday's 7.9-magnitude temblor in Nepal may have triggered the earthquake near Mirik in Darjeeling district this evening, but scientists say the implications of the Himalayan quakes and aftershocks of the past 48 hours remain unclear.

A 150km slice of the Indian crust slipping beneath the Himalaya lurched forward, marking Saturday's primary quake and releasing energy that has been showing up as aftershocks or more quakes like the 5.1-magnitude event in Mirik at 6.05pm.

"The primary event may cause displacements elsewhere in the region, even at a distance. The Mirik event today could be one such consequence," said Malay Mukul, professor of earth sciences at the Indian Institute of Technology, Bombay.

Scientists are debating the implications of Saturday's event for future earthquakes in the Himalaya.

Some geophysicists believe it released enough accumulated tectonic energy to reduce the probability of a great earthquake of magnitude 8 occurring here again over the next several decades.

But others caution that the length of Saturday's rupture is too short to rule out an even more powerful earthquake somewhere along a stretch between eastern Himachal Pradesh and northern Bihar.

Their disagreement reflects the uncertainties still embedded in earthquake science.

"This 7.9-magnitude event is likely to have released a significant enough amount of energy to push back the return of a similar earthquake," said Ajay Paul, senior seismologist at the Wadia Institute of Himalayan Geology, Dehradun.

"Earthquakes cannot be predicted but this event makes it unlikely that this region will see an earthquake of magnitude 8 or so for, conservatively, perhaps another 50 years."

A US geologist who has long predicted that the region should anticipate a great earthquake of magnitude 8 or so said that while the potential for a correspondingly larger earthquake in the region remained, a greater earthquake in the near future seemed unlikely.

"We have no precedent in the Himalayas for a magnitude 7.8 earthquake triggering a magnitude 8 earthquake, so our best guess is that the recent earthquake will not be soon followed by a larger event," said Roger Bilham, professor of geological sciences at the University of Colorado.

A longer return period would be significant from the perspective of protecting buildings and other structures from future earthquakes. Civil engineers have been arguing for retrofitting existing buildings and investing in earthquake-proofing of future buildings.

The scale of construction makes the task overwhelming but, Paul said, a longer return period --- essentially, several decades of quiescence --- would allow time for northern and eastern India to protect future structures.

But not all scientists think the region can be declared safe from a magnitude 8 temblor for now.

"The energy released appears insufficient to preclude a future earthquake," said Kusala Rajendran, a scientist at the Centre for Earth Sciences at the Indian Institute of Science in Bangalore.

"Early seismic data analysis of Saturday's event suggests the rupture zone is about 150km long. That leaves a stretch of nearly 450km along this segment where there is still strain just waiting to be released."

Some features of the earthquake are puzzling scientists. The Himalaya is older northward, and the epicentre of the primary earthquake lay on a fault about 80km northwest of Kathmandu, which is at a distance from faults in the southern zone of the Himalaya.

"The younger faults in the southern zone are expected to be more active and more susceptible for rupture than the faults in the north," Rajendran said. "We still need to understand why this northern fault ruptured."