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A magnitude-6.4 earthquake struck the coast of southern Mexico on Thursday about 60 miles northwest of Acapulco, the second big quake in the region in a month, according to the U.S. Geological Survey.

The quake rocked downtown Mexico City, located about 170 miles northeast of the epicenter, in Guerrero state. There were no immediate reports of serious injuries or major damage in the capital.

In Tecpan de Galeana, 9 miles from the epicenter, part of a highway bridge fell, El Universal reported. The bridge was being repaired from flood damage last fall and a strong quake three weeks ago that ruptured 22 miles from the city.

The mayor described Thursday's shaking as ferocious and said it caused a "wave of panic." Some roofs collapsed.

Initial, tweeted reports from a reporter said the quake lasted for as long as 40 seconds in the capital, where office workers streamed into the streets. The report said the city had a 68-second warning.

"It was very scary. Some of my colleagues suffered panic attacks because the buildings moved," Carmen Lira, a secretary in Mexico City, told the Los Angeles Times. "It felt very strong."

The USGS downgraded the magnitude of the quake from an initial 6.8 to 6.4.

The Pacific Tsunami Warning Center said initial indications found no tsunami threat to Hawaii.

On April 18, thousands fled their offices and homes after a magnitude-7.2 quake rattled several states in central and southern Mexico. No major damage or serious injuries were reported.

The country's earthquake detection system likewise gave Mexico City residents time to get out of buildings. The Economist explained afterward how it works and why it's unique:

The epicenter of earthquakes hitting Mexico City, which lies roughly in the centre of the country, is usually to the south, where the North American continental plate rubs up against the Cocos plate. This means that quakes are felt first in states such as Oaxaca, on the Pacific coast. So seismologists installed sensors in the south of the country that detect the first tremors and send a warning to the capital. The seismic wave moves at about 7,000 miles per hour. That sounds fast, but it means that it takes the quake nearly two minutes to travel the 200 miles from Oaxaca to Mexico City. Meanwhile the sensors' signal arrives virtually instantaneously in Mexico City, where alarms are sounded, giving people just enough time to scamper out into the street before the earthquake arrives.

The system was developed after the 1985 quake that devastated Mexico City and killed an estimated 10,000 people.

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