When an earthquake hit Nepal on April 25, known as the Gorkha earthquake, entire villages were flattened, large buildings toppled in the capital Kathmandu, and over 9,000 people lost their lives. Geologist Jean-Philippe Avouac has been studying the region for over two decades, looking at how mountains are formed and earthquakes triggered. When he heard that a magnitude 7.8 earthquake had hit the country, he expected the death toll to be in the hundreds of thousands. It turns out that while the quake was big, it didn’t release all of the energy stored in the area, hence the lower-than-expected loss of life.
“At first when I saw the news trickling in from Kathmandu, I thought that there was a problem of communication, that we weren't hearing the full extent of the damage,” explained Avouac, a professor of geology at the California Institute of Technology (Caltech) and lead author of a new paper on the earthquake. “As it turns out, there was little damage to the regular dwellings, and thankfully, as a result, there were far fewer deaths than I originally anticipated.”
The new study in Nature Geoscience has shown that the Nepal quake occurred on what’s called the Main Himalayan Thrust (MHT), the fault line along which the Indian plate is pushing under the Eurasian. As the Indian plate moves under at a rate of about 2 centimeters (0.8 inches) a year, it pulls the Eurasian plate down with it, building up stress in these “locked” sections. Eventually the upper plate breaks free, releasing the energy and causing an earthquake. This is what caused the one in Nepal, but the researchers found that it ruptured only small sections of this locked zone.
“The Gorkha earthquake didn't do the job of transferring deformation all the way to the front of the Himalaya,” said Avouac. “So the Himalaya could certainly generate larger earthquakes in the future, but we have no idea when.” The last major earthquake to have shaken the region occurred in 1505, and so the stresses have been building up for centuries.
In a second paper published in Science Express, a separate team from Caltech also looked at why comparatively few buildings were destroyed in Kathmandu. They found that for such a large-magnitude quake, the high-frequency shaking – which is normally the main cause for low level buildings to collapse – was actually very mild in the capital. The high-frequency waves from the quake manifested on a deeper edge of the rupture, away from Kathmandu.
Both teams were able collect the data using GPS stations, accelerometers and seismological stations. Interestingly, they were even able to plot how the earthquake spread using stations not only in Nepal, but on the other side of the world in the U.S. and Europe. The research represents the first complete account of what physically happened on April 25, and the researchers hope that it will help them to better predict how and when future earthquakes might hit.