12:01 p.m. update:
Tecpan, near the epicenter, shook ferociously, causing a "wave of panic" and some roofs to cave in, said Mayor Crisoforo Otero Heredia. But there were no injuries.
In Chilpancingo, the capital of Guerrero state, a wall collapsed and in Acapulco civil protection crews had found nothing so far except scared citizens who were forced to take refuge in the heavy rain that was hitting the region.
In Mexico City, elegantly dressed businesswoman Carmen Lopez was leaving a downtown office building when the ground began to shake. She dashed across the street to a leafy median as light poles swayed violently above her.
"That was just too scary," said Lopez, as she quickly started dialing her cellphone to alert friends and family.
Behind her, thousands of people poured out from neighbouring office buildings, following pre-planned evacuation routes to areas considered safe from any potential of falling glass.
The quake had a depth of 15 miles (23 kilometres). The USGS downgraded the magnitude from 6.8.
A 7.2-magnitude quake with an epicenter about 40 miles (66 kilometres) from Thursday's quake shook central and southern Mexico on April 18.
A strong earthquake hit Mexico City Thursday morning. The force of the quake sent office workers streaming into the streets.
The 6.8 earthquake hit 13 km west of Tecpan de Galeana at a depth of 10 km.
There were no immediate reports of injuries or damage.
39km W of Atoyac de Alvarez, Mexico
60km SE of Petatlan, Mexico
80km WNW of Coyuca de Benitez, Mexico
303km SW of Mexico City, Mexico
Located atop three of the large tectonic plates, Mexico is one of the world's most seismically active regions. The relative motion of these crustal plates causes frequent earthquakes and occasional volcanic eruptions.