May 12, 2015
Today, Nepal has been hit with a magnitude 7.3 earthquake, and aftershocks as strong as magnitude 6.3 are still being felt. It has been just over three weeks since the magnitude 7.8 earthquake hit the country, leaving more than 8,000 dead, scores injured and millions displaced.
Sadly, scientists had predicted that another earthquake was coming—and many more will come in the future in this seismically active region.
The Himalayan region had been overdue an earthquake, since the last one that hit Kathmandu 80 years ago. During the earthquake on April 25, however, not all of the pent-up seismic pressure was released. This left room for more earthquakes in the near future.
That near future, however, could have been days—or years—away. Predicting the precise location and timing of an earthquake is not possible. There are simply no signals from the movement of the Earth’s crust that can definitely point to when a quake is triggered.
And then one occurred today, about 18km below the surface and east of Kathmandu. It is not clear yet if this quake has indeed released all the remaining pent-up pressure.
The movement of the Indian plate, which collided with the Eurasian plate and gave birth to the Himalayas, makes the region a seismic hot zone. On average, the Indian plate moves about 18 millimeters towards the Eurasian plate and slips underneath it. This movement loads up some of its energy into earthquake faults, which extend on a line from east to west, and the process is much like loading energy in a spring. And every so often that energy is released in snap, resulting in an earthquake.
Eric Hand and Priyanka Pulla explain in Science (paywall):
Most of the region’s substantial earthquakes have occurred south of the line, where the plates are locked together and strain builds up. North of this “lock line,” however, the Indian plate dives downward and the character of the rock slab changes. Under higher temperatures and rising pressures, the brittle rocks become more plastic, and they creep past the Tibetan crust without rupturing. Or so researchers had thought.
What happened on April 25 makes things worse for the region. When the fault unloaded its stored energy, it tore through a region underneath Kathmandu that had been previously deemed impervious. “We therefore have the potential for bigger earthquakes than we might have otherwise expected,” Gavin Hayes of the US Geological Survey told Science.
This finding raises the possibility of magnitude 9 earthquakes, which would be as large as the earthquake that happened off the coast of Japan in 2011 and triggered deadly tsunamis. This warning is not just for those in Nepal but also for those in India. The central seismic gap that makes this area so earthquake-prone extends all the way into the northern Indian state of Uttarakhand, where its structure is even more poorly understood.
Roger Bilham, a geologist at the University of Colorado, told the Indian Express, “The Indian government’s attitude to seismic studies is apparently to ‘shoot the messenger’.” He claims to have been expelled by the Indian government in 2012 when he ascertained that Nepal is better prepared than India at handling large earthquakes.
Report written by Akshat Rathi, Madhura Karnik and Manu Balachandran.