On September 19, a massive 7.1 magnitude earthquake rocked Mexico, toppling dozens of buildings. As of press time, the quake had reportedly killed more than 300 people. It struck on the 32nd anniversary of another earthquake 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 had struck in Mexico. Although that quake was stronger, it caused fewer deaths because its epicenter, or point of origin, was off the country’s coast. The September 19 quake originated 120 kilometers (75 miles) outside Mexico City, the country’s capital and home to about 9 million people.