MEXICO CITY.- A powerful 7.1 earthquake shook central Mexico on Tuesday, collapsing homes and bridges across hundreds of miles, killing at least 138 people and sending thousands more fleeing into the streets screaming in a country still reeling from a deadly temblor that struck less than two weeks ago.

In Mexico City alone, at least 36 people died and 44 buildings were severely damaged Tuesday, said Carlos Valdes, director of Mexico’s National Center for the Prevention of Disasters. Ten other people were reported killed in the surrounding state of Mexico, 29 in the state of Puebla and 64 across the state of Morelos.

The earthquake struck on the anniversary of a 1985 temblor, which killed thousands of people and devastated large parts of Mexico City — a tragedy that President Enrique Peña Nieto had commemorated earlier in the day.

Like many in the city, Edgar Diaz, a 20-year-old architecture student at a university in Condesa, took part in an earthquake drill at 11 a.m. Hours later, the real one hit.

“We all went running. The staircases were swaying,” he said.

After establishing that his friends were all right, he joined the line of people helping clear rubble from the collapsed apartment building.

“What’s important right now is saving lives,” he said

Mexico City is prone to major damage in earthquakes because it sits on an old lake bed that amplifies the shaking.

The U.S. Geological Survey calculated the preliminary magnitude of Tuesday’s temblor at 7.1. The epicenter was about 80 miles southeast of Mexico City in the state of Puebla.

Susan Hough, a USGS seismologist, said the quake was probably related to the one that struck off the coast of Mexico’s Oaxaca state on Sept. 7, which the government calculated as a magnitude 8.2 and her agency as an 8.1.

“An 8.1 is big enough that having an aftershock this big and this distant — it isn’t too surprising,” Hough said. “It’s unusual, but it fits in with the picture that we’ve grown to understand.”

Read more at the San Diego Union-Tribune.

Written by: Kate Linthicum, Rong-Gong Lin II and Alexandra Zavis