Mexico had two major earthquakes this month. Here’s why

Story highlights

The two earthquakes struck hundreds of miles apart

But they have some similarities

CNN  — 

For the second time in two weeks, a powerful earthquake struck Mexico, toppling buildings, cracking highways and killing hundreds of people.

Just like the deadly earthquake earlier this month, Tuesday’s temblor caused heavy and prolonged rattling in the capital. Although the two earthquakes struck hundreds of miles apart, they have some similarities, experts say.

The 7.1-magnitude earthquake Tuesday was about 650 kilometers from the epicenter of the 8.1-magnitude earthquake that hit September 8, said Jana Pursley, a geophysicist with the US Geological Survey.

Both earthquakes seem to be 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.

Two major earthquakes in the same country within a short span of time may sound rare, but it’s not a surprise in such a seismically active region.

“It is not very unusual to get earthquakes and aftershocks occurring in sequence,” Fatahi said. “When fault lines rupture, they can induce further ruptures as a chain effect in other parts of the same fault or nearby fault lines.”

Mexico City especially vulnerable

The Mexican capital is especially at risk of major earthquakes because of its location.

“The downtown of Mexico City is notoriously vulnerable to earthquakes because of the very soft and wet ground underneath. Its soil amplifies shaking like Jell-O on a plate, and is prone to liquefaction, which is the ability to transform dirt into a dense liquid when sufficiently churned,” wrote John Vidale, a seismologist and director of the Southern California Earthquake Center.

CNN Meteorologist Pedram Javaheri said this has been the case for hundreds of years.

“Mexico City was built on what is now a dry lakebed,” Javaheri said. “This region about 700 years ago was a very shallow lakebed. The city, one of the most densely populated in the world, is situated directly on top of it. This plays a large role in the intensity and how everything plays out when it comes to shaking.”

Tuesday’s earthquake struck at a depth of about 33 miles (51 km).

“Anything below 70 kilometers is considered a shallow quake,” CNN meteorologist Allison Chinchar said. “That’s important, because shallow earthquakes often cause the most damage, compared to the ones that are deeper, regardless of the strength. But this also was a relatively strong earthquake.”

Pacific Ring of Fire

Both quakes occurred on the so-called Pacific Ring of Fire, a 25,000-mile area shaped like a horse shoe that stretches from the boundary of the Pacific plate and the smaller plates such as the Philippine Sea plate to the Cocos and Nazca plates that line the edge of the Pacific Ocean.

It is one of the most seismically active zones on the planet, and about 80% of all earthquakes strike there, said Hongfeng Yang, a seismologist at the Chinese University of Hong Kong.

Some of the deadliest earthquakes occurred around the Ring of Fire, including a 9.5-magnitude quake that hit Chile in the 1960s and is considered the strongest recorded.

“In highly seismic areas such as Ring of Fire, it is possible that two large earthquake occur almost one after another in a matter of days,” Fatahi said.

Five tectonic plates – Cocos, Pacific, Caribbean, Panama and North American – collide in central and southern Mexico, making the region one of the most unstable, he added.


Fatahi said aftershocks can happen minutes, days or months after the main one hits. But in the case of Tuesday’s earthquake, it’s probably not an aftershock to the massive one earlier this month, because aftershocks don’t happen hundreds of kilometers from the original earthquake weeks later, he said.

The latest earthquake will probably be followed by aftershocks in the days and weeks ahead, he said.

“However, since two major earthquakes of magnitude 7 and above have occurred in a matter of 10 days in the region toward southern part of North American tectonic plate, significant stored energy has been released from the ground, which means that the likelihood of much larger earthquakes in the region has reduced now. “

The earthquake struck on the anniversary of a 1985 Mexico quake that killed nearly 10,000 people. Fatahi said that date is just a coincidence – unlike hurricanes and other disasters, earthquakes can strike at any time without warning.

“Is not yet possible to predict earthquakes well in advance to avoid casualties,” Fatahi said.

Mexico City has a system that warns of strong shaking off of the country’s coast, according to the USGS. The system consists of sensors that detect shaking from a large earthquake and rapidly determine the location and magnitude.

Since Mexico City is hundreds of miles from the main plate boundary, it gets about a minute or more of warning of the impending shaking.

Earthquakes start deep in the ground and move so fast – 50 times faster than a Category 5 hurricane – so it’s almost impossible to issue a warning well in advance, according to Fatahi.

But there are other ways to save lives.

“Earthquakes do not kill; but inadequately designed and poorly constructed buildings, infrastructure and lifeline systems can,” Fatahi said.

The best way to save lives is to design structures that can withstand earthquakes and retrofit old buildings to fit the code, he said.

Concern in California

The quake that affected Mexico City set off jitters in California.

Julie Dutton, a geophysicist at the US Geological Survey’s National Earthquake Information Center, said the office had a large volume of calls from concerned Californians.

A small magnitude-3.6 earthquake rattled Los Angeles on Monday night. California’s infamous San Andreas Fault stretches more than 800 miles and is about 10 miles deep, according to the US Geological Survey.

But Dutton said there is no correlation and no additional cause for concern for California.

“We see earthquakes all the time and it’s not an alarmist situation for us,” Dutton said. “It’s a good reminder that California is also a highly seismic active zone.”

Professor Andrew Newman, with the School of Earth and Atmospheric Sciences at Georgia Tech, agrees. “Globally, we don’t see any direct connections between earthquakes,” Newman said. “Whenever we have a large earthquake, aftershocks happen only locally.”

Seismologist Lucy Jones sent a barrage of tweets following the central Mexico earthquake.

“(Magnitude 3) quakes happen somewhere in the world every few mins. 6 in SoCaL last week. No connection to Mexico,” she said.

Correction: A previous version of a map above incorrectly identified the location of the Ring of Fire. It has been corrected.