JIM MCMAHON/MAPMAN®

On September 19, a massive 7.1 magnitude earthquake rocked Mexico, toppling dozens of buildings. The quake has reportedly killed more than 250 people. The death toll is expected to climb as more rubble is cleared. The earthquake struck on the 32nd anniversary of another quake in Mexico that killed nearly 10,000 people in 1985.

Mexico is one of the most seismically active countries in the world, says Julie Dutton, a geophysicist at the United States Geological Survey in Colorado. That’s because the nation sits atop three tectonic plates. These giant slabs of rock make up Earth’s crust, or outer layer, and can cause earthquakes as they move. The most recent quake occurred near a subduction zone, where one plate dives beneath another.  

Just a week and a half earlier, an even larger 8.1 magnitude earthquake struck in Mexico. Although that quake was stronger, it caused fewer deaths because its epicenter, or source, was off the country’s coast. The most recent quake originated 120 kilometers (75 miles) outside Mexico City, the country’s capital and home to 
more than 11 million people.