The epicentre was in the Pacific, about 87km (54 miles) south-west of the town of Pijijiapan and at a depth of 70km, the US Geological Survey said.
A tsunami warning has been issued for Mexico, Guatemala, El Salvador, Costa Rica, Nicaragua, Panama and Honduras.
The quake was felt in Mexico City, with buildings swaying and people running into the street.
The tremors there, about 1,000km from the epicentre, were reported to have lasted up to a minute.
Full story, news and photo source credit: http://www.bbc.com/news/world-latin-america-41197831