The quake that hit Mexico City today comes less than two weeks after a magnitude 8.1 quake struck the country
The second major earthquake to strike Mexico in less than
two weeks has caused catastrophic damage in the country’s capital. The
magnitude 7.1 temblor started at around 1:15PM — cracking highways, collapsing buildings, and, so far, killing more than 50 people.
Less than two weeks ago on September 7th (local time), a magnitude 8.1 quake struck roughly 400 miles southeast
from today’s. It’s not common to hear of such strong earthquakes
happening back-to-back so close to one another, says John Bellini, a
geophysicist with the US Geological Survey. “Usually you don’t have
large ones in the same general region right away,” Bellini says. “But in
highly [seismically] active regions of the world, it can happen.”
Mexico qualifies as highly active. The country sits at the boundary of three
pieces of the Earth’s crust that fit together like a jigsaw puzzle —
called tectonic plates. Today’s quake originated on a fault within the
Cocos plate, which is on Mexico’s western edge. “Whether or not faults
rupture depends on the kind of stress that builds up,” Bellini says. The
Cocos plate scoots rapidly under the continental crust of the North
American plate, which “builds up the stress and strain at a faster
rate,” Bellini says. “So you’re liable to have more frequent earthquakes
because of that.”
Mexico City is especially prone to severe damage because
of the ground it sits on — an ancient lakebed that quivers like jello,
Bellini says. When earthquake waves pass through it, it jiggles,
magnifying the vibrations. “So the reason that Mexico City seems
susceptible to more damage is because of this amplification effect of
the lake bed,” Bellini says.
At this point, a reasonable person might wonder whether the September 8th quake might have triggered today’s. But the epicenters of the two quakes are about 400 miles apart,
and it’s unusual for such a strong aftershock to appear this long after
a major quake. So it’s unlikely the two are related, Bellini says.
“Usually to have something directly related, they happen within minutes
of each other, whereas this has been a week and a half,” Bellini says.
“But I’m sure somebody will study that to see if there’s any kind of
relationship.”
There’s another coincidence, too: Today’s earthquake falls on the 32nd anniversary of the 1985 Mexico quake that left roughly 10,000 people
dead. But Bellini says not to read into the timing too much. There’s no
such thing as an earthquake season, and earthquakes can happen at any
time: “There’s no particular time of year that earthquakes happen over
another. They’re not weather related.”
Unlike most natural disasters, there’s no way to predict
or forecast earthquakes. That makes preparation way more important,
whether it’s through building codes or earthquake drills like the one Mexico City conducted earlier today.
Though it is tempting to be lulled into complacency during seismically
quiet periods, planning ahead is still the only defense for earthquakes.
https://www.theverge.com/2017/9/19/16336276/mexico-city-back-to-back-earthquakes-september-seismology
