Friday, September 15, 2017

Unusual Mexico earthquake may have relieved stress in seismic gap

When Vlad Manea heard about the deadly magnitude-8.2 earthquake that struck the coast of Mexico’s Chiapas state on 7 September, he was stunned, but not altogether surprised. A geophysicist at the National Autonomous University of Mexico (UNAM) in Juriquilla, Manea is one of only a handful of earth scientists who study seismic activity in the region. For more than a century, there had been little activity to study—precisely why Manea thought the area could be due for a big one.
The epicenter of the quake, which struck just before midnight local time, was just southeast of the Tehuantepec gap, a 125-kilometer-long stretch of Mexico’s Pacific coast that has been seismically silent since record-keeping began more than a century ago. All along that coast, the ocean’s tectonic plates meet the continental North American plate and are forced underneath it. Violent earthquakes mark the release of built-up pressure between the grinding plates. But the ruptures have somehow avoided the Tehuantepec gap and the Guerrero gap, more than 500 kilometers to the northwest.For decades, scientists have monitored the Guerrero gap because of its proximity to Mexico City. A rupture there could devastate the capital, which is built on a drained lakebed that amplifies seismic waves. In 1985, a magnitude-8.1 quake near the Guerrero gap killed thousands, spurring the city to install a seismic alert system and tighten building codes. Those measures seemed to help last week: The capital sustained little damage in spite of considerable shaking.
The Tehuantepec gap has received far less attention. “It was considered the little brother,” says Manea, who began studying it in the early 2000s along with his wife, UNAM geophysicist Marina Manea. Their first priority now is to figure out how much of the Tehuantepec gap slipped in last week’s quake, which killed more than 90 people and destroyed or severely damaged the homes of 2.3 million more, mostly in the states of Chiapas and Oaxaca. Although the epicenter was just outside the gap, more than 1000 aftershocks have been recorded, many in the gap itself. Vlad Manea says some of them may have been strong enough to release stored pressure and close the gap—which would make future quakes in the region less likely.

On shaky ground

Last week’s temblor may have relieved pressure in one of two “seismic gaps” in the subduction zone off Mexico’s coast, where tectonic plates grind past one another.
Credits: (Graphic) G. Grullón/Science; (Data) V. Kostoglodov; Mexico National Seismological Service
He concedes, though, that the quake’s effect on the gap is hard to judge, because of its unusual origin. Most big Mexican earthquakes occur right along the interface between the colliding Cocos and North American plates. But this rupture began 70 kilometers down, within the Cocos plate itself, and rose up before stopping at about 40 kilometers’ depth, likely at the plate interface. “It’s not the same fault that they’re expecting [to close] the Tehuantepec gap,” says Joann Stock, a seismologist at the California Institute of Technology in Pasadena.
That leaves the future risk of the Tehuantepec gap unclear. In fact, Stock says, last week’s quake might have even added stress at the gap and increased chances for future slipping. But, she adds, the depth of the shaking had at least one benefit: The rupture didn’t break through all the way to the ocean floor, which dampened tsunamis. The resulting waves in Chiapas and Oaxaca were only 2 to 3 meters high.
Vladimir Kostoglodov, a seismologist at UNAM in Mexico City, says he is fielding requests for data from researchers around the world who want to investigate this “extremely strange” earthquake and its aftermath. “It’s worth making a big effort to learn what’s happening,” he says. “This might happen in other subduction zones in other parts of the world.”
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