ACAPULCO, Mexico —
The quake occurred at a depth of 15 miles (23 kilometers) and its epicenter was about 40 miles (66 kilometers) from that of the April 18 quake that shook central and southern Mexico.
The earlier quake occurred in a section of the Pacific Coast known as the Guerrero Seismic Gap, which is a 125-mile (200-kilometer) section where tectonic plates meet and have been locked, causing huge amounts of energy to be stored up with potentially devastating effects, the USGS said. It said a magnitude-7.6 temblor struck in the section in 1911.
The U.S. agency said Thursday's quake was an aftershock of the April 18 temblor.
"The earthquake is indeed within the Guerrero Seismic Gap," USGS research geophysicist William Barnhart wrote in an email to The Associated Press. "But since it is consistent with being an aftershock of the magnitude-7.2, it is neither an abnormal event, nor does it significantly reduce the remaining stored stress in the seismic gap."
The USGS says the Guerrero Gap has the potential to produce a quake as strong as magnitude 8.4, potentially much more powerful than the magnitude-8.1 quake that killed 9,500 people and devastated large sections of Mexico City in 1985. The 1985 quake was centered 250 miles (400 kilometers) from the capital on the Pacific Coast.
Mexico City is vulnerable to distant earthquakes because much of it sits atop the muddy sediments of drained lake beds. They jiggle like jelly when the quake waves hit.
Associated Press writer Mark Stevenson in Mexico City contributed to this report.