OAXACA IN SOUTHERN MEXICO HIT BY QUAKE
The quake, which hit at 0222 local time (0722 GMT), also rattled buildings in Mexico City, some 355km (220 miles) away, sending people on to the streets.
Civil protection officials in the Mexican capital said they had not had any initial reports of casualties.
The quake's epicentre was 120km (75 miles) west-south-west of the city of Oaxaca, the USGS said.
The earthquake hit when many people were asleep.
“I felt it like I almost always do. People came running out of the building,” Pedro Salazar, a security guard in Mexico City, told Reuters news agency.
“We don't have reports of damaged buildings or people hurt,” a civil protection official told the news agency.
In Oaxaca itself, a spokesman from the civil defence department told the Associated Press that they were assessing the situation but did not yet have reports of damage or injuries.