Venezuela declared a national emergency following two earthquakes of 7.2 and 7.5 magnitude that occurred just 39 seconds apart on Wednesday. This phenomenon, known as a seismic doublet, is rare.

However, there are documented cases in various regions of the world that continue to be studied to expand our understanding of the mechanisms of stress is transferred within the Earth’s crust and the interaction between geological faults.

The US Geological Survey reported that both earthquakes had their epicenters northwest of the town of Yumare, near the city of Morón and about 160 kilometers (99 miles) west of the Venezuelan capital of Caracas. The quakes occurred at depths of 20.3 and 10 kilometers and were felt across much of northern Venezuela and in several Caribbean countries.

The sequence was classified as a severe seismic doublet, a phenomenon that occurs when two earthquakes of similar magnitude strike almost simultaneously in nearby areas. According to USGS estimates, the probability that an earthquake will be followed by another of greater magnitude in a nearby area within a week is approximately 5 percent.

One of the most significant seismic doublets in recent history occurred about three years ago in Turkey and Syria, when a 7.6-magnitude earthquake was followed by a 7.8-magnitude one. A study suggested that the phenomenon was the result of tectonic stress that had built up over more than two centuries along the fault responsible for the first quake. When it ruptured, it released stress that was transferred to a nearby fault, rapidly triggering the second event.

An apartment building in Catia La Mar, Venezuela, was severely damaged after a double earthquake.

Photograph: Juan BARRETO / AFP via Getty Images

How Does a Seismic Doublet Occur?

Experts believe a similar process may have occurred in Venezuela. Mark Allen, a professor in the Department of Earth Sciences at Durham University, explains in a statement reported by SMC Spain that “it is likely that the first earthquake caused a segment of the fault to rupture and transferred the stress to another fault, which in turn ruptured, triggering the second earthquake.”

Allen also noted that the geological characteristics of the region where the epicenters were located played an important role. “The events appear to have taken place at the plate boundary between South America and the Caribbean. In this region, the plates move laterally relative to one another, similar to what occurs along the San Andreas Fault in California,” he says.

These conditions favor the generation of strike-slip earthquakes, which occur when two blocks of the Earth’s crust move horizontally along a fault and suddenly release the stress accumulated over long periods.