Let's get some seismic background on Mexico, which is prone to earthquakes. The country sits atop three of the Earth's largest tectonic plates --- namely the North American Plate, the Cocos Plate, and the Pacific Plate. Quakes occur whenever their crusts grind or butt up against one another. This makes Mexico one of the most seismically active nations in the world. The country is also home to soft soil, which can act as an amplifier for tremors -- making smaller earthquakes feel larger. Tuesday's tremor occurred on the 32nd anniversary of a massive 1985 earthquake that killed thousands in Mexico City. On Tuesday, just hours before the tremblor hit, many Mexicans took part in earthquake drills, held annually on September 19th.