On Saturday, a portion of the thrust fault underneath central Nepal ruptured, causing an earthquake that killed at least 5,200 people, injured more than 10,000 and destroyed centuries-old temples, towers and buildings.
Nepal rests on the most dangerous portion of the Himalayan collision zone, a set of earthquake-prone faults stretching 1,200 miles from Afghanistan to Myanmar. Within the last century, this strip of land has hosted two earthquakes classified as “great” by the U.S. Geological Survey — with magnitudes exceeding 8.0. The most recent great earthquake occurred in 1950, when a magnitude 8.6 event struck the eastern portions of the Tibetan plateau near Rima.
To better understand this seismic region, a team of scientists search for evidence of ancient earthquakes in dried-up riverbeds, relying on a database of Medieval texts and archaeological records to guide the way. And by parsing the region into smaller bits, geologist Paul Tapponnier of the Earth Observatory of Singapore and his colleagues have found that individual segments of Nepal’s fault host great earthquakes on a surprisingly regular schedule.
Two weeks ago, these geologists approached the Nepal Geologic Society, warning that a major earthquake in the precise region where Saturday’s earthquake struck was relatively imminent.
“In the talk, we looked at historical descriptions in the Himalayas going back to 1,200 A.D,” Tapponnier said. “Reports of destructive earthquakes in certain areas and not others, along with surviving architecture, can give an idea of a prior earthquake’s location and size.”
Summer monsoons flood these waterways, but during the dry seasons, scientists can walk along the sediment. Ruptures in the seismic fault are hard to spot with modern techniques – like satellite imagery – so walking in dry mudbanks remains one of the best ways to catch signs of a former earthquake.
Evidence of these old quakes can be seen in Z-shaped rips embedded in the sides of the riverbeds. The zagging pattern is the signature of a thrust fault – the geologic phenomenon responsible for the devastation in Kathmandu and other severe earthquakes, such as those near Los Angeles.
“Imagine you’re trying to move a carpet on a floor by pushing on the edge,” said geologist Roger Bilham of the University of Colorado Boulder who also studies tectonics in the Himalayas. “Rather than budge, it rumples at the edge that you’re pushing from.”
The same happens with thrust faults. In the case of the Main Central Thrust, the fault that lies under Nepal, where the Indian subcontinent smashed into the tectonic plate holding the Tibetan Plateau some 40 million to 55 million years ago, neither plate wanted to move, so the ground was forced upward over time, eventually forming the towering Himalayan Mountains.
When the team sees a Z-shaped deformation in the riverbed, they dig a shallow trench in search of charcoal. Using radiocarbon dating, they can pinpoint the dates that earthquakes occurred.
“Putting these things together, we found a site in eastern Nepal – 30 miles east of Kathmandu — that had seen a repeat of two large earthquakes: one in 1255 and the other in 1934,” Tapponnier said. “These two seemed to have ruptured the same length of the main frontal thrust.” In 2013, his group reported these findings in Nature Geosciences. Follow-up excavations have since unearthed evidence for four more earthquakes prior to 1255.
And each of these earthquakes, the team found, has occurred at roughly the same interval of 670 years.
Recently, the search expanded into central Nepal between Kathmandu and Pokhara, where Saturday’s event originated. There, they found evidence for a previous earthquake in 1344, approximately 670 years before Saturday’s quake.
The pattern suggests great earthquakes are unloading in an east-to-west fashion in Nepal. The last great earthquake west of Pokhara happened in 1505, meaning the next will likely occur 670 years later — two centuries from now.
Saturday’s earthquake relieved some, but not all, of the strain built over those long centuries, moving the ground by about 10 feet in 30 seconds, according to the USGS.
“But there are [26 to 30 feet] left, Tapponnier said. “That’s pretty ominous.”