This September, two devastating earthquakes struck Mexico in rapid succession. The first, a Sept. 7 magnitude-8.2 temblor near the southern state of Chiapas, killed an estimated 96 people and sent aftershocks shivering through the region. Just 12 days later and a few hundred miles away, a magnitude-7.1 quake shook central Mexico; hundreds more people died, including 19 children who were crushed when their elementary school collapsed.
The chance of the two tragedies occurring in such a short span of time by sheer coincidence was just one in 200, said seismologist Ross Stein. But there is no evidence to suggest that the quakes were connected.
However, Stein's research suggests that the combination of temblors added to the stress along faults in the earthquake-prone country, possibly doubling the likelihood that another earthquake of magnitude 7 or greater will strike next year.
“Another shoe could drop,” said Stein, a longtime U.S. Geological Survey scientist who is now chief executive of the earthquake awareness company Temblor, during a presentation of his findings this week at the annual meeting of the American Geophysical Union.
Earthquake prediction is notoriously difficult: Despite years of installing seismometers and sifting through data, scientists have found no reliable warning signals. Forecasting quakes is mostly a matter of weighing probabilities — and trying to convince the public to get prepared.
Scientists in Mexico had no warning for either of the two September earthquakes, and the second struck far away from the zone where aftershocks from the first still rocked the earth.
That second quake happened on the 32nd anniversary of a 1985 temblor that killed thousands, and just two hours after Mexico City's annual earthquake drill. The chance of this extraordinary confluence of events was 1 in 900,000, according to Stein, “and we all know that's a coincidence, right?” he said. “We have to remember coincidences do happen.”
Xyoli Perez-Campos, a seismologist at the National Autonomous University of Mexico, said she agreed with Stein's conclusion that the two quakes were unconnected.
She also showed how the region remains shaky: As of this week, the Sept. 7 quake had triggered nearly 13,000 aftershocks, including a magnitude-6.1 quake near Oaxaca.
“Earthquakes tend to beget earthquakes,” Stein said. The September temblors relieved the stress on the faults that slipped, but they added to the stress on other faults, pushing them closer to failure.
Mexico sits beside a spot where two tectonic plates meet, making it prone to seismic activity. In 2018, the country had a 25 percent chance of experiencing a quake nearly as bad or worse than magnitude 7. But considering the stresses resulting from this year's quakes, that chance is now as high as 55 percent, Stein said.
It's just a probability, but Stein hopes it will spur people to prepare. Most earthquake fatalities are typically tied not to the magnitude of the quake, but the quality of the infrastructure where the quake hit. The Los Angeles Times reported that the school where 19 children died appeared to be built of brittle concrete with insufficient reinforcing steel.
“If we can build right, earthquakes will take no lives and do no damage,” Stein said. “What we have to do as earth scientists is figure out where the risk is high, let people know, and drive the effort.”