Venezuela announced state of emergency in the country after two earthquakes with a magnitude of 7.2 and 7.5 on the Richter scale, which occurred just 39 seconds apart on Wednesday. This phenomenon, called a seismic doublet, is infrequent.
However, there are documented cases in various regions of the world that are still being investigated to expand our understanding of the mechanisms of stress transfer in the Earth’s crust and the interactions between geological faults.
The United States Geological Survey said the epicenters of both earthquakes were northwest of the city of Yumare, near the city of Morón and about 160 kilometers (99 miles) west of the Venezuelan capital, Caracas. The quakes occurred at depths of 20.3 and 10 km and were felt across much of northern Venezuela and several Caribbean countries.
The sequence was classified as a forceful seismic doublet, a phenomenon that occurs in two earthquakes of similar magnitude hit nearby areas almost simultaneously. According to ultrasound estimatesthe probability that another larger earthquake will occur in a nearby area within a week is about 5 percent.
One of the most significant seismic doublets in recent history occurred about three years ago in Turkey and Syria, when a magnitude 7.6 earthquake was followed by a magnitude 7.8 earthquake. Research suggests 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, quickly triggering a second event.
How is a seismic doublet formed?
Experts believe that a similar process may have taken place in Venezuela. Mark Allen, professor at the Department of Geosciences at the University of Durham, explains in a statement provided by SMC Spain that “it is likely that the first earthquake ruptured a section of the fault and transferred stress to another fault, which in turn ruptured, causing the second earthquake.”
Allen also noted that the geological characteristics of the region where the epicenters were located played an significant role. “The events appear to have occurred at the plate boundary between South America and the Caribbean. In this region, the plates move laterally against each other, just as they do along San Andreas bug in California,” he says.
These conditions favor strike-slip earthquakes, which occur when two blocks of the Earth’s crust move horizontally along a fault and suddenly release stresses that have accumulated over long periods of time.