The powerful earthquake that devastated Nepal late in the morning on 25 April, causing at least 3200 deaths, could be a fuse that ignites other powerful quakes in a region of the Himalayas that had been seismically quiet for centuries, experts say.
The 7.9-magnitude earthquake was long overdue: The fault segment that ruptured hadn’t seen an earthquake since 1344 C.E., according to Laurent Bollinger, a geologist from the French Alternative Energies and Atomic Energy Commission. This temblor originated 15 kilometers underground, where the Indian plate slides under southern Tibet at a rate of about 20 millimeters per year along the Main Himalayan Thrust fault. The plates snag against each other, building up pressure until the crustal rock gives out. The locked plates under Nepal have been close to the breaking point for centuries, says Vinod Gaur, a geophysicist at Bangalore’s CSIR Fourth Paradigm Institute who co-authored a Science article in 2001 warning of the possibility of highly destructive earthquakes in the Himalayas.
The Kathmandu temblor seems to have released a portion of the strain building up in the central seismic gap (CSG), a 600-kilometer-long region south of Nepal straddling a major fault that has been eerily quiet for at least 500 years. While the CSG’s earthquake history is disputed—some geologists say a large quake in 1505 C.E. ruptured the gap, while others argue that the 1505 quake wasn’t large enough to do so—specialists concur that the CSG is overdue for a megaquake measuring greater than magnitude 8.
The 25 April earthquake wasn’t large enough to release all the CSG’s pent-up strain, according to Bollinger, Gaur, and other geologists studying the gap, but it did relieve strain at the eastern end. To release more strain, “earthquakes are now needed further west of the gap,” Bollinger says. The 25 April earthquake may well herald gap-filling quakes, Gaur says. When a portion of a lengthy fault ruptures, he says, it is like making a tiny nick in a piece of cloth and stretching it. This builds pressure along the tear’s edges and makes it susceptible to further rips. While that could happen farther along the CSG, the timing is impossible to predict. “It may rupture tomorrow, or it can rupture 75 years from now,” Gaur says.
In Nepal, meanwhile, the death toll is bound to rise—thanks in part to the region’s geology. Situated on an ancient lakebed, the Kathmandu Valley’s soil is soft and liquefies easily. “The ground motion gets amplified, and people there can feel [earthquakes] very vigorously,” says Vineet Kumar Gahalaut, a geologist at the National Geophysical Research Institute in Hyderabad, India.