It is clear that the earthquake that hit Nepal this past April was devastating, with over 9,000 casualties, the destruction of valuable historic artifacts, and a trail of aftershocks that included avalanches on Mount Everest and the Langtang Valley. But why, exactly, did it happen? A new study offers an in-depth exploration of the situation underground, helping to better predict what the next rumble will look like in this earthquake-prone region and how locals can prepare for the future. Here are five takeaways from the study.
The earthquake was actually relatively small.
"This is not one of the large Himalayan earthquakes; it's relatively small since it only unlocked a very small friction of the fault line," says Jean-Philippe Avouac, the lead author on the study. According to his research, Avouac says the thrust unzipped only the lower edge of the fault — just a small portion of the fault line — before stopping in Kathmandu. The damage, of course, was still disastrous, owing not just to the thrust but also because the area's architecture and land has weakened from a history of ravaging natural disasters.
And it could easily have been much worse.
Had the thrust been larger and continued west of Kathmandu, the destruction might have been greater and the situation "quite different", says Avouac. Certain areas around the Himalayas are gold mines for damage because of quakes they've endured in the past, and the west — where a particularly bad earthquake hit in 1505 — is incredibly vulnerable. "[The quake] stopped in the area of the 1974 earthquake, where there was not a lot of strain," says Avouac. "You have a long, large area where the fault is locked and where we haven't had earthquakes since 1505, so there is lots of elastic strain." That earthquake might have been an 8+ and much worse than the 1505 quake. "This Gorkha quake is a reminder that at some point we'll have a large earthquake in the area that's been locked."
The quake wasn't the kind that is particularly damaging to buildings.
This study was one of the first times that researchers have been able to see the shape of the plate slip — that little rupture underground that causes a collision and damage to occur. According to Avouac's GPS findings, the plate slip was smooth and short — which is better news than brittle and long, which would have meant more rumbling and a bigger loss of life. "It turns out that the earthquake didn't generate a lot of ground shaking that is destructive to regular dwellings," he says. If the rupture had been rougher, he adds, "we would have higher frequencies and more damage to regular buildings which were poorly built to begin with."
But it was bad for tall structures.
Many historic buildings in the area — including the 60 meter-tall Dharahara tower that even survived the much more brutal 1934 earthquake (an 8.4) — crumbled to the ground, and Avouac says that's because this type of quake's resonance, rather than it's initial thrust, targets tall buildings rather than regular dwellings. "A lot of the very tall buildings, because of their inertia, are much more vulnerable at these frequencies, so it bent back and forth," Avouac says. "It did that maybe 10 times over the 20 seconds of resonance."
The next quake might be gentler than previously predicted.
Since this rupture was smooth, short and soft, Avouac says hopefully it's an indication that future Himalayan quakes could be so, too. "We expected a brittle slip, and that's not what we observed, so it's good news," says Avouac. "Large megathrust Himalayan quakes may have mechanical properties such that they do not generate high frequencies and become less destructive than we think."