The plates of the earth's crust are moving, and that movement causes earthquakes, explains Warner Marzocchi, a seismologist at the National Institute of Geophysics and Volcanology in Rome.

For Mexico City, being built on an area of sediment in a valley amplifies the effect of the movement, he says.

After the 1985 quake which killed 10,000 people, the average quality of buildings in the city has "increased significantly", he said.

But his institute will try to work out why individual buildings collapsed once their data is collected, he said.

And they will try to ascertain whether the earthquake on 7 September is connected to this one. They are close in date, and this one is is a magnitude 7.1 following a magnitude 8.1 - a not unusual pattern.

But the two tremors were geographically distant from each other within Mexico, Mr Marzocchi says.

Mexico City's transport system has praised the work of the volunteers and rescue workers by tweeting a photo of a tired emergency worker travelling on the city's metro.

City residents have been urged to use public transport so as to leave the streets empty for ambulances and other emergency vehicles.

Image source, MetroCDMX

The Mexican Interior Minister says the armed forces and police "will continue working without rest until all possibilities of finding people alive are exhausted".

He also urged people whose homes had been damaged not to risk spending the night there.

Image source, Getty Images

Force India driver Sergio Pérez has donated three million pesos ($170,000; £124,500) to victims of the earthquake in his native country.

The Mexican GP is set to take place in the capital from 27-29 October.

"I'm deeply concerned for what my country is living," said Pérez, 27. "It's time to be united."

Los Angeles Times correspondent Kate Linthicum says of the fear felt by people when the quake hit the city:

Volunteers from World Vision have gone to the quake-affected region to help rescuers, Alejandro Cota tells the BBC.

They are carrying emergency kits of water, hygiene equipment and food along. And making "friendly spaces" for children, with games and support for their emotional and physical needs.

Mexican daily El Universal describes how one building in Mexico City completely collapsed in the space of 50 seconds.

Mariana, one of the residents, described the scene:

“It was like an explosion, first the windows, then the building, it's the most destructive and fast thing I've ever seen," she said.

Mariana managed to escape along with her mother but she says not everyone was so lucky.

She told the paper that one woman died because she held onto a column when the building started to shake instead of running away.

Image source, Getty Images

Mexicans have told of the spirit of solidarity among those affected by the earthquake.

Alex, a Mexico City resident, said: "What I like is the fact that people are helping each other".

Hanna Monsivais, who works for the charity Save the Children in the capital, said people had shown "enormous solidarity".

The UK Foreign Office has updated its advice to people travelling to Mexico:

Mexico's Federal Electricity Commission says 70% of the electricity supply which had gone down has now been restored.

The commission says that more than 3.4 million homes and businesses now have power again.

Image source, Reuters

Pope Francis spoke about the earthquake at the weekly general audience at the Vatican on Wednesday morning.

The Pope also blessed an image of the Virgin of Guadalupe which a Mexican choir had brought along.

Mexico had barely begun to deal with the aftermath of a quake earlier in September when this one struck. But are they related? BBC science correspondent Jonathan Amos explains.

Image source, AFP

Image source, Reuters

The army has been deployed to help the rescue effort and soldiers are working alongside emergency crews and civilians to try to free people from the rubble.

Pictures from scenes across the affected are here.

The quake struck near the capital, Mexico City. and close to the border of Morelos and Puebla states.

Our map shows the Enrique Rébsamen school, where two adults and 22 children were found dead, while another 12 adults and 30 children are missing.

The town nearest the quake's epicentre is Axochiapan in Morelos state.

One of the buildings damaged by the quake is the church in the town centre.

One person died when its dome collapsed, Mexican daily Reforma reports.

It is still before dawn in Mexico City, where rescue workers are at the site of collapsed buildings, trying to rescue anyone trapped inside and assessing the damage.

Image source, Reuters

Image source, Reuters

Image source, Reuters

The epicentre of the quake was in the state of Puebla, where 43 people are reported to have died.

The governor of Puebla, Tony Gali, tweeted that Puebla was "still standing, [full of] solidarity and strong".

On the face of it, Tuesday's earthquake seems to be unconnected to the one that hit on 7 September, BBC science correspondent Jonathan Amos writes.

"The 7 September, magnitude 8.1 event certainly struck a similar region of the tectonic boundary (where the earth's plates meet), but the hypocentre - the point of origin within the Earth - was some 650km from Tuesday’s magnitude 7.1 quake.

"But what is related, for sure, is the driver," he says.

Local media is reporting that part of the Enrique Rebsamen school collapsed while rescuers were trying to reach survivors.

Soldiers at the site reportedly also administered oxygen to a trapped child through a tube.

Image source, Getty Images

Manuel Mavroleon Aguayo is a reservist in the Mexican army. He was out from 14:00 local time clearing rubble and helping survivors.

"I was part of a team who pulled out seven people from the rubble of a building. Two were alive, the other five did not make it," Mr Mavroleon says.

"I was tasked with speaking to the wife of one of them. I've never experienced anything like this before.

"I am incredibly tired and in a few hours I will be up again to help with the rescue effort.”

Image source, AFP