Big Aftershocks In Nepal Could Persist For Years
Aftershocks following Saturday's magnitude-7.8 quake in Nepal are jangling nerves and complicating rescue operations. So far, there have been more than a dozen quakes of magnitude 5 or higher, and another two dozen between magnitude 4.5 and 5.
That rate of aftershocks is in line with a forecast by Andrew Michael and colleagues at the U.S. Geological Survey in Menlo Park, Calif. The forecast is based on the location of the initial quake — along a deep boundary between colliding continental plates. In this case it feels like a large number because they're striking a heavily populated region, Michael says, "so every single one of these small aftershocks is being felt."
"Unfortunately this is simply what earthquakes do," says Ross Stein, scientist emeritus at the USGS and consulting professor of geophysics at Stanford University.
The initial earthquake in Nepal released stresses underground that had been building up for 150 years. It ruptured a network of faults in a region 150 miles long and 100 miles wide, along the boundary of two colliding tectonic plates.
That reshuffling of stress underground, Stein says, is now triggering smaller earthquakes near the epicenter and on nearby faults — aftershocks.
"What's happening, particularly for these more remote aftershocks, is they are striking on the neighboring faults," Stein says, "and these neighboring faults could rupture in subsequent large earthquakes."
In fact, there's a 1 to 2 percent chance in the next year or two that an aftershock in this area could be even bigger than the original quake.
There is also a record of two large quakes in quick succession in this region, so a twin quake now "wouldn't be unprecedented here either," Stein says.
That means it's very challenging for people in Nepal to figure out when it's safe enough to go back into buildings. Some more recent buildings are relatively safe, because they have been built with reinforcing steel — rebar — which increases earthquake resilience.
"Unfortunately, you can't tell when you're in that building if it has rebar, and if it's been properly built," Stein says. "If concrete were translucent, the world would be a safer place."
And you can't simply wait it out until the hazard has passed. "It's kind of a cruel part of aftershocks that we cannot depend on them getting smaller," Stein says. "They just get less and less frequent with time."
The USGS forecast warns there is still a better than 50-50 chance of another aftershock that's magnitude 6 or larger in the next week, and also in the next month — and also over the course of the next year.
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Transcript
AUDIE CORNISH, HOST:
Aftershocks following Saturday' 7.8 magnitude quake in Nepal are jangling nerves and complicating rescue operations. So far there have been more than a dozen quakes of magnitude 5 or higher, and NPR's Richard Harris reports that aftershocks are likely to persist for years.
RICHARD HARRIS, BYLINE: You may think of an earthquake as a singular event, but in fact, the first jolt is the beginning of a massive reshuffling of stresses underground. Roth Stein, scientist emeritus at the U.S. Geological Survey, says there's nothing unusual about the pattern of aftershocks in Nepal.
ROB STEIN, BYLINE: Unfortunately, this is simply what earthquakes do.
HARRIS: The pattern so far is matching a forecast by Andrew Michael and colleagues at the USGS. He says the reason it seems like there are so many is that they're striking a heavily populated region.
ANDREW MICHAEL: And so every single one of these small aftershocks is being felt.
HARRIS: Aftershocks result because the original earthquake released stresses that had been building up for 150 years. It's ruptured a network of faults in a region more than a hundred miles long along the boundary of two colliding tectonic plates. Stein says that reshuffling of stress underground is now triggering smaller earthquakes near the epicenter and on nearby faults.
STEIN: What's happening, particularly for these more remote aftershocks, is they are striking on the neighboring faults, and the neighboring faults could rupture in subsequent large earthquakes.
HARRIS: In fact, there's a 1 or 2 percent chance in the next year or so that an aftershock in this area could be even bigger than the original quake.
STEIN: And historically, because there have been a millennium of great earthquakes along this boundary, a number of those very large earthquakes have been couples - were twins. So that wouldn't be unprecedented here either.
HARRIS: Stein says people are acting quite rationally when they decide to sleep outside rather than risk going back into buildings. Some more recent buildings are relatively safe because some of them have been built with reinforcing steel, Rebar, which increases earthquake resilience.
STEIN: Unfortunately, you can't tell when you're in that building if it has Rebar and if it's been properly built. So if concrete were translucent, the world would be a safer place.
HARRIS: Stein says unfortunately there's no clear time when geologists could say that the risk of more damaging earthquakes has passed.
STEIN: It's kind of a cruel part of aftershocks - is that we cannot depend on them getting smaller. They just get less and less frequent with time.
HARRIS: The USGS warns that there's still a 50-50 chance of another magnitude 6 or larger aftershock in the next week and also in the next month and also over the course of the next year. Richard Harris, NPR News. Transcript provided by NPR, Copyright NPR.
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