On April 25, the Gorkha earthquake hit Nepal, killing over 9,000 people. There were entire villages that were flattened, and large buildings in the capital Kathmandu collapsed.
Geologist Jean-Philippe Avouac has been in the region for over 20 years studying how mountains are formed and earthquakes triggered. When Avouac heard that an earthquake with a magnitude of 7.8 had hit, he expected a death toll of hundreds of thousands. It turns out that it didn’t release all of the energy that was stored in the area, making the death toll lower than he had expected.
Avouac, who is a professor of geology at the California Institute of Technology (Caltech) and lead author of a new paper on the earthquake, explains: “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. 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, which was published in Nature Geoscience, showed that the Gorkha earthquake had occurred on the bottom of the locked portion of the Main Himalayan Thrust fault (MHT), the fault line along which the Indian plate is pushing under the Eurasian.
According to IFL Science, the Indian plate moves under at a rate of about 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 researchers found it ruptured only small sections of this locked zone.
Earthquake in Nepal:
Avouac said in a statement: “The Gorkha earthquake didn’t do the job of transferring deformation all the way to the front of the Himalaya, so the Himalaya could certainly generate larger earthquakes in the future, but we have no idea when.”
Jean Paul Ampuero, assistant professor of seismology at Caltech and co-author on the Nature Geoscience paper, said: “With the geological context in Nepal, this is a place where we expect big earthquakes. We also knew, based on GPS measurements of the way the plates have moved over the last two decades, how ‘stuck’ this particular fault was, so this earthquake was not a surprise, but with every earthquake there are always surprises.”
In another paper that was published in Science Express, a separate team also from Caltech had also looked at why there were so few buildings destroyed in Kathmandu. They found that the high-frequency waves that came from the quake had manifested on a deeper edge of the rupture, which was farther away from Kathmandu, making the high-frequency shaking very mild. It is this high-frequency shaking that is the main reason for low level buildings to collapse.
Domniki Asimaki, a professor of mechanical and civil engineering at Caltech, said: “The buildings in downtown Los Angeles are much taller than those in Kathmandu, and therefore resonate with a much lower frequency. So if the same shaking had happened in L.A., a lot of the really tall buildings would have been challenged.”
The two teams collected data using GPS stations, accelerometers, and seismological stations. Having been able to see how the earthquake spread, with not only stations in Nepal but in the U.S. and Europe also, it has become the first real complete account of what happened on April 25. With all the data, the researchers are hoping to get a better understanding and to be able to predict how and when future earthquakes may occur.