Like a slow-motion car crash, the Indian subcontinent is colliding with Eurasia. This impact, along a fault known as the Main Himalayan Thrust, is the force driving the rise of the Himalayas. In April 2015 it triggered Nepal’s magnitude 7.8 Gorkha earthquake, which destroyed villages and parts of Kathmandu, killing thousands.
Nepal is no stranger to such temblors—but in the western part of the country, no significant earthquakes have been recorded since 1505. Such a “seismic gap” could be bad news: if the region’s faults are not releasing their pent-up stress every so often, one or more very large and potentially catastrophic earthquakes could result. “With more than 500 years of waiting, the stored energy due to the convergence of India [and Eurasia] could be considerable,” says Zakaria Ghazoui, a geologist at the Institute of Earth Sciences in Grenoble, France. The sudden release of this energy could devastate nearby places such as Pokhara, one of Nepal’s largest cities.
Credit: Mapping Specialists; Source: “Bimodal Seismicity in the Himalaya Controlled by Fault Friction and Geometry,” by Luca Dal Zilio et al., in Nature Communications, Vol. 10, No. 48; January 3, 2019
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To determine whether this seismic gap actually exists, Ghazoui and a team of researchers ventured onto Rara Lake in western Nepal’s Himalayas and retrieved cores of sedimentary layers from the bottom. They suspected the cores might contain records of when any past earthquakes occurred because quakes can cause underwater avalanches that leave behind layers of what Ghazoui calls “disorganized” sediment.
The team found evidence of at least eight avalanches since 1505, corresponding to moderate to large quakes. “We were hoping to find the trace of the 1505 earthquake, but the discovery of the other earthquake-triggered avalanches was a real surprise,” he says. This means that the area’s faults may not be storing as much stress as has been thought. But it also highlights the “almost permanent risk” that the area faces when it comes to earthquake hazards in general, explains Ghazoui, who led the new research, which was published in May in Nature Communications.*
This is the first time that researchers have used lake sediment records to peer into the Himalayas’ earthquake history, according to Roger Bilham, a seismologist at the University of Colorado Boulder, who was not involved in the research—which, he says, “is just a teaser of what can be done.”
*Editor's Note (5/22/19): This story has been updated to include the month of the study’s publication and the journal it appeared in.