The deadly earthquakes that have struck Venezuela’s northern coast were an event known as a “doublet.” The U.S. Geological Survey says doublet earthquakes happen when a pair of similar-sized quakes hit close in location and time. On Wednesday, a 7.2 magnitude quake hit first, followed by a magnitude 7.5 just 39 seconds later. Doublets are not as common as a typical earthquake where a main shock is followed by much smaller aftershocks. But the director of the USGS earthquake science center in California says doublets can happen anywhere in the world. Doublets indicate a complex fault structure, like the one in Venezuela that’s known as the Bocono fault.