PHOTOS: 7.4 quake destroys hundreds of homes in Mexico

A powerful earthquake measuring 7.4 on the Richter scale set buildings swaying in southern Mexico on Wednesday.

The quake was followed by a strong 5.1 magnitude aftershock.

Though no deaths were reported, hundreds of homes were destroyed by the temblor, which was centred near the border between the southern states of Oaxaca and Guerrero.

Oaxaca Governor Gabino Cue reported cracks and broken windows in several schools and minor damage to a number of Oaxaca City's iconic monuments and colonial-era buildings. 

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The earthquake's centre was located about 12.4 miles (20 km) underground, according to a USGS update.

The quake shook office buildings and apartment houses and sent hundreds of thousands of people into the streets.

A pedestrian bridge on the outskirts of the city was wrenched from its girders and crushed a passenger bus below.

The bus was empty save for the driver, who escaped with minor injuries.

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Emergency services said 800 houses were damaged in Guerrero state, many of them in Ometepec, near the epicentre of the quake.

Officials in Guerrero, which is home of popular Pacific beach resort Acapulco, could not say if buildings had collapsed.

In the country's capital, Mexico City frightened workers and residents poured into the streets.

Telephone service was down in the city and throughout the area where the quake was felt and some neighboirhoods were without power, Mexico City Mayor Marcelo Ebrard said.

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President Obama's older daughter, Malia, was vacationing in Oaxaca with school friends but was not hurt, White House officials said.

Malia, 13, "is safe and was never in danger," said First Lady Michelle Obama's spokeswoman, Kristina Schake.

Reports that Malia, 13, was travelling in Mexico for spring break with friends first surfaced Monday. As always, the White House asked journalists not to report on the Obama children and several media outlets later scrubbed stories they had posted Monday about the first daughter's vacation.

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Government helicopters were deployed to survey the damage from the air and officials were getting shelters ready for displaced residents.

Mexican President Felipe Calderon said the country's healthcare facilities were intact and "all of the power generation facilities are unaffected and functioning."

According to the Christian Science Monitor, seismologists have traced the quake to a release of strain associated with a subduction zone that runs up the west coast of Central America and Mexico and stops short of the southern tip of Baja California.

Here, the oceanic crust of the Cocos plate is sliding northeastward under the more buoyant rock of the North American plate -- at a pace of some 60 millimeters a year.

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Though the magnitude of the earthquake was extremely high, there was relatively little severe damage and no known fatalities. 

One of the main reasons why is that the earthquake struck very deep into the ground. 

It hit 12.4 miles (20 km) below the surface, which helped to mute the tremor before it got to ground and reverberated among buildings. 

It also helped that the epicentre was far enough away from major cities. Because it was closest to Oaxaca (the country's 10th biggest state by population) the damage was lessened due to a lack of closely-knit buildings.

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Historically, there have been several significant earthquakes along the southern coast of Mexico.

In 1932, a magnitude 8.4 thrust earthquake struck in the region of Jalisco, several hundred kilometers to the northwest of Wednesday's event. On October 9, 1995 a magnitude 8.0 earthquake struck in the Colima-Jalisco region, killing at least 49 people and leaving 1,000 homeless.

The deadliest nearby earthquake occurred in the Michoacan region 470 km to the northwest of Wednesday's event, on September 19, 1985. This magnitude 8.0 earthquake killed at least 9,500 people, injured about 30,000, and left 100,000 people homeless.

More recently, a 2003 magnitude 7.6 Colima, Mexico earthquake 640 km to the northwest of Wednesday's event killed 29 people, destroyed more than 2,000 homes and left more than 10,000 homeless.