Inside Colombia's big quake – and why more could be coming

On August 10th, a 7.4 magnitude earthquake struck western Colombia, collapsing buildings and killing hundreds of people. But Rodrigo Alejandro Leon Loya, an earthquake geologist at the Universidad de Los Andes in Bogotá, didn't panic. He ran to his computer. "I want to know what happened or what we happen, more like a state of alert like, okay this is huge, where did they happen, what would be the damage," he recalls. "I run to the computer to start to see signals and the reports." His reaction highlights a central puzzle in seismology: after a big shake, the natural question is when the next one will hit—and scientists cannot give a precise answer.

The tectonic machine beneath Colombia

The ground beneath Colombia is unusually complex because three major tectonic plates interact there. The Nazca plate, an oceanic slab, moves from the Pacific Ocean and smashes into the South American plate. To the north, the Caribbean plate slides past rather than colliding, similar to the San Andreas fault system in California. Wedged between these three big plates is a microplate called the North Andean block. "This triple junction imposes a huge amount of stress into the North Andean block, and this stress is expressed in fault systems that are hosted in the crust," Rodrigo explains. This arrangement produces three distinct kinds of earthquakes in Colombia: shallow crustal quakes from lo