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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