Slashdot: Seismologist Explains Mexico's Back-To-Back Earthquakes

Seismologist Explains Mexico’s Back-To-Back Earthquakes
Published on September 20, 2017 at 03:20PM
An anonymous reader quotes a report from The Verge: 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 200 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 U.S. 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.