Strong, Long 7.9 Quake Shakes Mexico City A strong, long 7.9 earthquake with an epicenter in Guerrero state shook central southern Mexico on Tuesday, swaying buildings in Mexico City and sending frightened workers and residents into the streets. The U.S. Geological Survey set the intensity at 7.9 on the Richter scale. Mexico's National Seismological Survey said the temblor had an epicenter southwest of Ometepec. It was six miles (10 kilometers) underground.