The catastrophic April 25, 2015 earthquake near Kathmandu has relieved some of the strain built up at the zone of colliding Indian and Eurasian plates, but scientists say it does not mean the danger of a mega quake in the Himalayas has receded.

People queue up for food in Kathmandu after the devastating earthquake. Credit: Chhatra Karki

The recent earthquake "is smaller than the maximum magnitude we have calculated for this region," Roger Bilham, a geologist at the University of Colorado and one of the authorities on earthquake hazard in the Himalaya, told Nature India. "In many ways it appears to be similar to an earthquake (magnitude 7.7 on the Richter scale) that occurred in 1833," he said.

More than a decade back Bilham predicted that Kashmir, in the western part of the Himalaya, may suffer earthquakes of magnitude 8.9 or ten times more powerful than the recent one in Nepal.

"That danger is still there though one cannot predict when it will happen," says Vinod Gaur, former director of the National Geophysical Research Institute in Hyderabad. "We were expecting bigger than the present one," said Kusala Rajendran whose group at the Indian Institute of Science in Bengaluru has predicted a massive earthquake in the Himalaya in the future1.

Kathmandu residents jolted by aftershocks take shelter in makeshift tents. Credit: Chhatra Karki

Earthquakes occur all along the Himalaya when the strain energy — accumulated between the base of the Tibetan plateau and the advancing Indian plate which under-thrusts Tibet— is suddenly released. When this occurs, segments of faults underneath the mountains break loose and slip over the Indian plate, creating a great earthquake.

The latest quake ruptured one such segment (the so called Central Himalayan Seismic Gap), says Parmesh Banerjee, Technical Director of Earth Observatory of Singapore who had earlier worked at the Wadia Institute of Himalayan Geology in Dehradun. "This may actually weaken the adjacent segment and advance the time of its rupture," he told Nature India .

According to Gaur, the quake ruptured only about 150 km section of Central Himalayan Seismic Gap. "All segments in Himalayas are ready to produce quakes," Vineet Gahalaut, a senior scientist at NGRI said. "The next one may be bigger." There is absolutely no method presently to predict earthquakes.

Radon measure not foolproof

A new study2 based on analysis of long-term data has shown that concentration of the radioactive element "radon" in ground water, long held as an earthquake precursor, does not provide "any definite indication of an impending earthquake”.

For this latest study, the authors, examined 12 year (2002-2014) data of radon concentration, water level, water temperature and rain-fall data available from six stations around the Longmenshan fault in south-western China. The thrust fault had generated two deadly earthquakes during the last decade — Wenchuan earthquake (magnitude 8) in 2008 and Lushan earthquake (magnitude 7) in 2013.

According to the authors long term trend of water radon concentrations and water level associated with these two deadly earthquakes "do not show any definite changes prior to earthquakes that can be used as an indicator of an impending earthquake."

"Our study shows that efforts by the various organisations in India to measure radon (among other parameters) in the Himalaya will not help predict the next major quake," Ramesh Singh of the School of Earth and Environmental Sciences at Chapman University in California and one of the authors told Nature India . "Rather, it is important to teach people living in vulnerable areas how to build a safe building and the steps to be taken during an earthquake."

Gaur agrees. "Every new construction should follow the earthquake design code and all existing public buildings must be inspected and retrofitted." According to Banerjee this is what is done in Japan, another quake prone nation.

Besides the fatalities caused, the Nepal quake poses other problems to India, warns Arun Bapat, former chief of earthquake research at the Central Water and Power Research Station in Pune. "It has generated large amount of loose geological material," he told Nature India . Because the epicentre is in the catchment area of Kosi river, it would bring heavy sediments into India, he said.

"When the huge amount of ice around the epicentre starts melting in couple of weeks, it would increase the water level of Kosi and other rivulets and could cause flood. The Central Water Commission should keep a watch."