We’re Shook! What Causes Earthquakes Like the Ones That Hit Mexico City

Two powerful earthquakes collapsed buildings in Mexico City recently. On Sept. 19, the anniversary of a 1985 quake that devastated the city, a 7.1 magnitude quake killed more than 200. That quake came just two weeks after an 8.2 magnitude quake slammed the country.

Several earthquakes also shuddered along the West Coast this week, albeit on a much smaller scale. One struck on Sept. 15 in Alaska, another hit Los Angeles on Sept. 18, and an aftershock shuddered in a rural area near San Diego about two hours later. While no damage came as a result of these quakes, climate scientists say they can use the data.

Mexico City shattered by recent quake

Mexico City earthquake rescuers

Rescuers, firefighters, policemen, soldiers, and volunteers remove debris after a powerful quake in Mexico City. | Ronaldo Schemidt/AFP/Getty Images

The Washington Post reported the Sept. 19 earthquake in Mexico weighed in at 7.1 magnitude. Its center rested near the Puebla state town of Raboso, about 76 miles southeast of Mexico City. Earlier that day, workplaces across Mexico city enacted drills on the anniversary of the 1985 quake, a magnitude 8.1 shake. That one killed thousands of people and decimated the city.

On Sept. 7, an 8.2 magnitude quake killed at least 90 people, destroyed thousands of homes, and affected tens of millions of people in Mexico and in Guatemala, The New York Times reported. President Enrique Peña Nieto flew to the region to assess the damage. Several leaders in Latin America and elsewhere offered assistance to Mexico, including the presidents of Colombia, Chile, Ecuador, Bolivia, Venezuela, and Spain. Residents in Mexico City said the more recent tremors felt worse, and as of press time, damage was still being assessed.

Both earthquakes came as a result of the rupture of fault lines within the North American tectonic plate, according to Behzad Fatahi, associate professor of geotechnical and earthquake engineering at the University of Technology Sydney. While surprising, series of earthquakes do happen. Several U.S. cities joined Mexico on the seismic scale, this week, to varying degrees of severity.

Next: Mexico City wasn’t the only one hit by tremors.

Who else felt the ground shake this week? Surprisingly, many of us

Students drill for earthquakes

Residents in Alaska, LA, and British Columbia also felt mild tremors this week. | David McNew/Getty Images

According to an Associated Press story in the Seattle Times, the Alaska earthquake struck on Sept. 15 and weighed in at a 5.1 magnitude. It quivered about 135 miles northwest of Juneau, the state’s capital. A string of larger earthquakes struck earlier this year along the same Eastern Denali Fault. That sequence topped out with a 6.3 magnitude quake, 10 times stronger than the one on Sept. 15.

Just before midnight on Sept. 18, a 3.6 magnitude earthquake shook Los Angeles, according to Newsweek. The U.S. Geological Survey said some parts of northwest Los Angeles experienced moderate shaking, but most of the tremors registered as “light” or “ weak.”

About two hours later, a 2.6-magnitude quake hit just outside Ocotillo Wells, about 100 miles east of San Diego. That quake also originated on the San Andreas fault line. Scientists warn of the impossibility of predicting exactly when and where catastrophic quakes will hit, but these smaller earthquakes may help foretell disasters like Mexico City’s.

Director of the Southern California Earthquake Center Thomas H. Jordan hopes scientists can create an app to track the chances of an earthquake. But how realistic is that technology?

Next: How we can actually predict earthquakes before they hit

Predicting earthquake risk? There’s an app for that (soon)

San Andreas heat map

Scientists work on new technology to help predict earthquakes. | David McNew/Getty Images

“We can’t make the kind of detailed predictions that meteorologists can make,” Jordan told the LA Times. “But it’s not like we know nothing. We do know something … In my view, you want the public to know everything the seismologists know, and we can basically give you a forecast on a weekly basis or a daily basis.”

Smaller earthquakes or tremors precede about half of all large earthquakes, he explained. Scientists pay attention to when moderate earthquakes hit particularly sensitive spots for clues. For example, when an earthquake occurs along the San Andreas fault, that can foreshadow larger quakes further along the fault line.

A study recently published in the journal Seismological Research Letters detailed a new statistical model on how one earthquake triggers another. Twenty leading scientists co-authored it, including those from the U.S. Geological Survey, the Southern California Earthquake Center, and the California Geological Survey. They plan to discuss it at the annual meeting of the Southern California Earthquake Center later this month.

No other model like this one exists around the world, Jordan told the Times. That’s because no other region has such a complete database of known faults and how they might react if a moderate quake struck very close to a large, sensitive fault that could produce a catastrophic event. But how can we tell whether one of these events is coming? Read on.

Next: These tremors could be just the first of many.

Like bad luck, earthquakes often come in groups

earthquake rescue efforts in Mexico City

Two quakes hit Mexico City in the space of two weeks, complicating relief efforts. | Estrella Alfredo/AFP/Getty Images

Responses to the recent LA earthquake came as a collective shrug, the LA Times reported. “We get these size earthquakes fairly frequently,” said U.S. Geological Survey seismologist Zachary Reeves, of LA’s recent tremor. “Any severe damage would be pretty unlikely.”

That said, “Big earthquakes can come in clusters. And one can trigger the other,” said U.S. Geological Survey seismologist Ned Field. The most famous example of these incidents happened in 1857, the last time Southern California saw a 7.8 magnitude earthquake.

It came on Jan. 8, 1857 and about nine hours before the big quake hit, smaller earthquakes started in the northernmost point of the fault. Just before dawn, a 6.1 magnitude earthquake struck, followed by a 5.6 at sunrise. At 8:24 a.m., the Big One headed south from Monterey County and shuddered into LA in about two minutes.

“We used to think aftershocks were just little earthquakes that followed the big ones, and who cares about something smaller?” Field explained. In reality though, “aftershocks can be larger.” He noted that a cluster of small earthquakes might mean “increased stress in that area. If you have more little things popping off, you have more opportunities for one of those to grow into something big,” he said.

Not all small and moderate earthquakes are made equal, scientists noted. Most of the time, small and moderate earthquakes are not a big deal, Jordan said, “unless that small earthquake is occurring near a major fault like the San Andreas.” In 2016, he announced that the southern part of the San Andreas Fault “looks like it’s locked, loaded and ready to go.”

Next: So what exactly does that mean for the California fault? We’ve got the answer.

Under pressure: San Andreas gets ready to blow

a film on San Andreas fault lines

A film produced by the United States Geological Survey shows how shock waves from a catastrophic 7.8 magnitude earthquake on the San Andreas Fault would fan out across Southern California. | David McNew/Getty Images

“The fact is that there has not been a major release of stresses in the southern portion of the San Andreas fault system since 1857,” Expert Matthew Blackett wrote in The Conversation. In 1906, some of those seismic stresses resulted in the San Francisco Bay area earthquake, a 7.8 magnitude event. It happened again, in northern California, during the 6.9 magnitude 1989 Loma Prieta earthquake.

“Events of those magnitudes have not occurred along the San Andreas fault in the south of the state, leading to the suggestion that one is imminent and, given the amount of stress that might actually have accumulated, when it arrives it will be the ‘Big One,’” Blackett explained.

A recurrence would most likely fall between 7.0 and 8.0, and could unleash a “great amount of destruction.”
The movie San Andreas dramatized the destruction, but how far off is it? Next, Blackett explains the movie’s accuracy.

Next: How Hollywood might have actually seen this coming already

Did Hollywood foretell San Andreas’ fury?

the San Andreas fault area

The San Andreas movie fictionalized earthquakes, but how real was it? | David McNew/Getty Images

In the film, the fault produces an earthquake at a 9.0 magnitude. Blackett said the fault’s capabilities likely top out around 8.0. That said, studies show a 7% chance of an 8.0 magnitude quake, but a 75% chance of a 7.0 magnitude quake over the next 30 years.

“While magnitudes of 7.0, 8.0, and 9.0 might sound negligibly different, the energy that such events would unleash varies significantly, with a magnitude 9.0 event releasing 32 times more energy than a magnitude 8.0 and 1,000 times more energy than a magnitude 7.0,” the scientist explained. He called damage “inevitable” but the film’s sequence of events “unlikely.” For example, a tsunami occurs in the movie, but since the San Andreas fault does not rest under the ocean, it won’t displace that amount of water.

“The opening up of a massive chasm is also from the land of fantasy, as the plates are sliding relative to each other, not away from each other,” Blackett added. So what would be affected? Read on to find out.

Next: Earthquakes cost a whole lot of money.

Scientists predict astronomical earthquake economic costs

Mexico City building rubble

Death and economic tolls continue to rise after Mexico quakes. | Jonathan Utz/AFP/Getty Images

In The Conversation, Blackett explained damage from a San Andreas quake centers on constructions straddling the fault. Thanks to the 1972 Alquist-Priolo Earthquake Fault Zoning Act, buildings on the fault are rare. That said, some 966 roads, 90 fibre optic cables, 39 gas pipes, and 141 power lines do cross the fault zone. Experts estimate projected damage at $33 billion, as well as fires and water breaks causing more damage than the shaking itself. The overall death toll likely reaches 1,800.

For scale, consider the latest Mexico City quake. That 7.1 magnitude quake killed more than 200 people and as of press time, experts expected many more. The U.S. Geological Survey predicts many more fatalities and economic losses in the hundreds of millions.

Nieto called the situation “an emergency,” and urged residents not to return to their homes until they were deemed safe. Buildings may yet fall from quake-related instability. More than 3,400 Mexican soldiers deployed to affected areas, and Mexico City Mayor Angel Mancera told Reuters many people remain trapped inside collapsed buildings.

While officials could not predict the Mexico City tragedy, they may be able to use smaller quakes in LA, British Columbia, and Alaska to learn more before the next disaster strikes. We do not currently have the ability to 100% prevent earthquake fatalities, but as research continues, science is on our side.