A house in Sankhu, Nepal house that was already severely damaged in the April 25th earthquake, shakes as a new 7.4-magnitude earthquake hits on May 12, 2015.
Nepal and the rest of the Himalayan region suffered another major earthquake today—a 7.3 magnitude quake that struck about halfway between Kathmandu and Mt. Everest, near the Chinese border. Early reports estimate that up to 1,000 people are injured, and at least 68 have been killed in both Nepal and India.
The region, especially urban areas like Kathmandu and Chautara, had already been reeling from the devastation wrought by the 7.8 magnitude event that hit the country on April 25 and killed over 8,000 people. Hundreds of aftershocks—some as strong as 6.7 magnitude—have continued to hamper relief efforts and keep residents in a panicked state.
This latest quake, however, is not an aftershock, but a brand new seismic event. According to the United States Geological Survey, today’s earthquake occurred 9.3 miles deep in the earth’s crust—the same depth as the April event. Cities and villages in the area have already felt six aftershocks, and the new quake created a whole new wave of landslides further north in the Himalayan mountains.
Is this just horrible luck, or are we seeing Nepal turn into a haven for earthquakes? The USGS previously estimated a 1-in-200 chance of another event similar to the April earthquake occurring. “It wasn’t a high probability, but it wasn’t unexpected,” says Rich Briggs, a USGS research geologist. Unfortunately, it might not even be the last quake to strike region in the near future—and it certainly won’t be the last seismic event that’s observed there.
The cause of the quakes
“In the big picture of things, you’ve got India slamming into Asia,” says Briggs. The Indian subcontinent has been forcing its way under the edge of Nepal—and beneath the Tibetan Plateau—for thousands of years. The initial crash caused the Himalayas to form, and continues pushing the region up and up even today. That ongoing pressure creates a huge amount of geologic strain thousands of feet below the surface. When that strain reaches a pitch point at which the two plates slip very suddenly, the energy releases in a violent shake. “Imagine the wedge of the Himalaya is like a spring, being pushed down. And what we’re seeing is the release of that energy,” Briggs says.
The result is a mountainous landscape that also suffers as a hotbed for seismic disturbance. Although two quakes occurring within weeks of one-another is uncommon, the Himalayas have experienced volatile geological events for millennia—a long time for humans, but just a blink of an eye from a geological time scale. That won’t change any time soon.
These quakes are reshaping the region
A sudden slip in the fault line also means sudden changes in surface elevation. The southern plate is submerging, but it actually rose by as much as 11 feet. This is because the resistance of the Asian plate—the one containing the Himalayas—is exerting so much reverse force that it caused the Indian plate to buckle upwards. In fact, the region around Kathmandu drifted south by about 10 feet. Conversely, some mountains north of the fault may have sunk some 11 feet. But these massive shifts don’t have as much of an effect on people as the smaller scale movements: landslides, cracks, and slumps in the ground.
Big earthquakes can also cause massive ruptures that reach from the buried fault line all the way to the surface. So far, none have shown up in Nepal. Instead, according to Briggs, a “blind” rupture has caused noticeable deformations of the surface, without having come up to the surface itself. Another phenomenon scientists are looking out for are afterslips—where the ground continues to rise and fall around the main rupture, as they adjust to the new forms of strain.
Nepal is teaching scientists more about earthquakes
Researchers often are stymied by the fact that these plate movements occur in the middle of the ocean (sometimes causing devastating tsunamis). Because Nepal’s quakes are happening above the sea level, Briggs and other researchers can get better data for assessing the aftermath. It’s far too dangerous to go down and assess the landscape in person—especially considering the huge potential for landslides—so for now, earthquake researchers are relying on satellite imagery and GPS data to analyze how the ground has moved.
Briggs and other experts are especially interested to find the ruptures, even if they can’t see them on the surface. Unfortunately, for a geological system this large and unpredictable, it’s not easy to find geological evidence of these ruptures. “These kinds of ruptures stay hidden, deep in the ground, even though we can see the consequences on the surface,” Briggs says.
A man waves the Nepalese flag on Durbar Square, in Kathmandu, on May 7, 2015.
There could be more to come
Although these plate movements are exactly what Briggs and his colleagues expected, he shies from predicting what happens next. “We don’t know exactly how they relate to one-another and how one might set another off,” says Briggs. He says each quake exhibits its own line of aftershocks—and with luck, they will decay soon enough.
But given the region’s long history of big earthquakes (such as the 8.0 magnitude that struck in 1934, ), it would be ridiculous to say the region is shake-free from here on out. “The readjustment in [geological] stress could play out in days, weeks, months or decades—we just don’t know,” says Briggs. “There remains a potential for large earthquakes in the region.”
A 7.3 magnitude earthquake is devastating—especially following on the heels of a 7.8 destroyer. But the region, says Briggs, is capable of experiencing worse.
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