The country's seismologic service initially gave a magnitude of 8.4, which if confirmed would be the most powerful ever recorded in this quake-prone country.
The quake shook a large swath of the country and was felt as far north as Mexico City - 600 miles from the quake epicenter - where people ran out of their homes in their pajamas as buildings trembled and swayed.
A tsunami warning and the prospect of aftershocks kept the nation on alert.
"Based on all available data ... widespread hazardous tsunami waves are forecast for some coasts," the Pacific Tsunami Warning Center said.
The quake struck at a depth of 21 miles, the USGS said.
It is the strongest to hit quake-prone Mexico at least since 1985, when an 8.1-magnitude earthquake in Mexico City killed more than 10,000 people.
Tsunami alert.
Tsunami waves have been measured off Mexico's Pacific coast.
The Pacific Tsunami Warning Center says waves of 1 metre (3.3 feet) above the tide level were measured off Salina Cruz. Smaller tsunami waves were observed on the coast or measured by ocean gauges in several other places.
The center's forecast said Ecuador, El Salvador and Guatemala could see waves of a metre or less.
No threat was posed to Hawaii and the western and South Pacific.