Talk of it being upgraded to 7.8 already. USGS details and map here, copy here:
Distances here.
136 km (84 miles) SSW of Huajuapan de Leon, Oaxaca, Mexico
162 km (100 miles) WSW of Oaxaca, Oaxaca, Mexico
170 km (105 miles) SE of Chilpancingo, Guerrero, Mexico
322 km (200 miles) SSE of MEXICO CITY, D.F., Mexico
update: some reports now stating epicentre in Guerrero State.
Update 2: Epicentre now nailed down to South of a place called Omotepec, coastal and on the frontier between the States of Oaxaca and Guerrero. The municipality of Omotepec has a total population of 20,800 people, according to the 2005 census. Hope they’re ok, but anything that starts with a 7 is not something you’d wish on your worst enemy.
Update 3: Holy crap, it went on for 55 seconds. I’ve personally experienced a 6mag of 10 second duration and had to be convinced by hard evidence later it was that short, cos it felt like minutes at the time. But 55 seconds is an eternity and there must be some very upset people up that way today. We send our sincere best wishes.