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

Rescuers searched for survivors through the night after Tuesday’s powerful earthquake shook Mexico City and surrounding states, killing scores and leaving many trapped under collapsed buildings.

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.

Aftershocks?

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.

Photos: Deadly earthquake rocks central Mexico

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Relatives of missing people wait for news in front of a collapsed building in Mexico City on Friday, September 22. A magnitude 7.1 quake hit central Mexico three days earlier.

Photos: Deadly earthquake rocks central Mexico

Natacha Pisarenko/AP

Rescue workers search for survivors Thursday, September 21, at a collapsed apartment building in Mexico City.

Photos: Deadly earthquake rocks central Mexico

Eduardo Verdugo/AP

A religious statue is salvaged from a former convent that was heavily damaged in Tlayacapan, Mexico. This was the second earthquake to hit Mexico in two weeks. A magnitude 8.1 quake struck off the country's southern coast on September 8.

Photos: Deadly earthquake rocks central Mexico

Eduardo Verdugo/AP

A wall is damaged at a home in Tlayacapan on Wednesday, September 20.

Photos: Deadly earthquake rocks central Mexico

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Volunteers organize donations in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

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Rescuers in Mexico City work to save a child trapped inside the Enrique Rebsamen elementary school on September 20. Rescue workers said they believed they'd made contact with a girl trapped in the rubble at the school. But by the next afternoon, navy official Angel Enrique Sarmiento said all the school's children had been accounted for and there was no student in the rubble. He apologized for the confusion.

Photos: Deadly earthquake rocks central Mexico

Rebecca Blackwell/AP

Men carry beams of wood to offer help in Mexico City's Roma neighborhood on September 20.

Photos: Deadly earthquake rocks central Mexico

Rebecca Blackwell/AP

Families prepare to sleep under tarps outside their quake-damaged building in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

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Rescuers and firefighters lower a corpse from a house in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

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Volunteers and rescue workers search for people trapped inside the Enrique Rebsamen school on September 20.

Photos: Deadly earthquake rocks central Mexico

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A volunteer in Mexico City asks for silence as a flattened building is searched for survivors on September 20.

Photos: Deadly earthquake rocks central Mexico

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Members of the Mexican Army nap September 20 after assisting in search-and-rescue missions in Mexico City.

Photos: Deadly earthquake rocks central Mexico

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People look for survivors in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

Eduardo Verdugo/AP

A man walks his bike past a partially collapsed building in Jojutla on September 20.

Photos: Deadly earthquake rocks central Mexico

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Volunteers arrange food and other donated supplies at a distribution point in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

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Children's toys are seen in a damaged building in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

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A survivor is pulled out of rubble in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

Eduardo Verdugo/AP

People sleep on the street next to damaged homes in Jojutla on September 20.

Photos: Deadly earthquake rocks central Mexico

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Soldiers remove debris from a collapsed building in Mexico City on September 20.

Photos: Deadly earthquake rocks central Mexico

Miguel Tovar/AP

An injured person is carried away after being rescued in Mexico City on Tuesday, September 19. The earthquake happened on the anniversary of a 1985 quake that killed an estimated 9,500 people in and around Mexico City.

Photos: Deadly earthquake rocks central Mexico

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Rescue workers remove rubble from a Mexico City building on September 19.

Photos: Deadly earthquake rocks central Mexico

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Rescue workers in Mexico City search for people trapped inside the collapsed Enrique Rebsamen school on September 19.

Photos: Deadly earthquake rocks central Mexico

Rebecca Blackwell/AP

A man comforts a student outside a school in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

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A police officer runs toward the site where a building collapsed in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Eduardo Verdugo/AP

Rescue workers and volunteers search a collapsed building in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Pablo Ramos/AP

A man is rescued under rubble in Mexico City's Condesa area on September 19.

Photos: Deadly earthquake rocks central Mexico

Photos: Deadly earthquake rocks central Mexico

Marco Ugarte/AP

A woman's crushed body hangs from a collapsed building in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Courtesy Lior Sperandeo/CNN Exclusive

Recovery efforts take place at the collapse of a residential building in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Carlos Rodriguez/AP

The quake damaged the Jojutla Municipal Palace.

Photos: Deadly earthquake rocks central Mexico

Miguel Tovar/AP

A search goes on at the scene of a collapsed building in Mexico City's Del Valle neighborhood on September 19.

Photos: Deadly earthquake rocks central Mexico

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Emergency workers remove debris as they search for survivors in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

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People react in Mexico City just after the quake hit.

Photos: Deadly earthquake rocks central Mexico

Rebecca Blackwell/AP

People gather on a Mexico City street after office buildings were evacuated because of the quake.

Photos: Deadly earthquake rocks central Mexico

Rebecca Blackwell/AP

A building is damaged in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Marco Ugarte/AP

A woman in Mexico City cries as she tries to reach people on her cell phone after the quake.

Photos: Deadly earthquake rocks central Mexico

Jose Mendez/EPA

Patients from a Mexico City hospital receive treatment outside after the hospital was evacuated on September 19.

Photos: Deadly earthquake rocks central Mexico

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A woman in Mexico City reacts after the quake.

Photos: Deadly earthquake rocks central Mexico

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People remove debris off a building that collapsed in Mexico City.

Photos: Deadly earthquake rocks central Mexico

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A car is crushed by debris in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

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A woman receives medical assistance after she was injured in Mexico City on September 19.

Photos: Deadly earthquake rocks central Mexico

Ronaldo Schemidt/AFP/Getty Images

People stand inside a Mexico City building that collapsed in the quake.

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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.