The devastating earthquake that struck Nepal in April released only a fraction of the energy still trapped in the underlying fault, meaning the area has the potential to host another large earthquake in the future, researchers say.
In April, a magnitude-7.8 earthquake struck 50 miles (80 kilometers) northwest of Nepal's capital of Kathmandu, killing more than 9,000 people and flattening entire villages. Geologists thought this quake originated on the Main Himalayan Thrust.
"The Main Himalayan Thrust is a fault that has produced large earthquakes every century or so," said study lead author Jean-Philippe Avouac, a geophysicist at the University of Cambridge in England. "Nepal lost two kings to these quakes, one in 1255, another in 1344. The last large earthquake to hit Nepal, a magnitude-8.2 earthquake in 1934, destroyed Kathmandu, as did a magnitude-7.6 earthquake in 183." [Nepal Earthquake Photos: Odd Effects of Kathmandu Temblor]
The researchers found the quake spread eastward at speeds of about 6,700 mph (10,800 km/h), traveling a distance of about 87 miles (140 km), "unzipping the lower edge of the locked portion of the Main Himalayan Thrust fault over which the Himalayas were built," said Avouac, who, along with colleagues, detailed the findings online today (Aug. 6) in the journal Nature Geoscience.
This earthquake "was actually relatively small," Avouac said. "Although it was certainly a tragedy, with close to 10,000 people killed, it's not in the family of the very large earthquakes this area can see."
Indeed, the April quake unlocked only a small fraction of the fault. "A strip of the fault about 120 kilometers wide (75 miles) is fully locked from one end of the Himalaya to the other over a distance of 2,000 kilometers (1,240 miles)," Avouac said.
This long, fully locked western part of the fault "has not ripped since 1505," he added, and he expects that it could release a much more powerful quake than the April temblor. "At some point there will be an earthquake there, and it will be quite scary — there is more energy to release, since energy has built up there since the last earthquake."
Avouac suggested it was pure chance that the April earthquake traveled eastward instead of westward. "If the earthquake had propagated westward instead of eastward, it could have really been a disastrous earthquake, because there are a lot of people living there in front of the Himalayas now," he said. "That didn't used to be true — before the '50s and '60s, few people lived there, since it was mostly jungle, and there was a lot of malaria. But now the jungle has been cleared."