The reason Venezuela suffered a double earthquake in less than a minute

Northern Venezuela on the evening of June 24 was shaken by two earthquakes, of which the first one with a magnitude of 7.2 occurred at 6:04 p.m. with the epicenter at a depth of 22 km, near the capital San Felipe of Yaracuy state. The second earthquake with a magnitude of 7.5 took place 39 seconds later, the epicenter was at a depth of 10 km and about 5-10 km from the first epicenter.

This is the strongest earthquake recorded in Venezuela in more than a century. Venezuela’s interim President Delcy Rodriguez said on June 25 that at least 164 people were killed and 971 injured, of which La Guaira state north of the capital Caracas suffered heavy damage.

Mark Quigley, associate professor of Earthquake Science at the University of Melbourne in Australia, said that a double earthquake occurs when two epicenters appear relatively close to each other in space and time, but are essentially two separate seismic events.

“The intensity of individual seismic shocks in a double earthquake is often similar. This phenomenon is different from the usual sequence, in which a large earthquake often causes aftershocks of significantly smaller intensity,” he said.

Seismic waves in double earthquakes also have different occurrence times and origins. The epicenters of the two earthquakes in Venezuela were only a few kilometers apart, but seismic wave data released by the US Geological Survey (USGS) showed that they likely originated from different fault lines.

This information is consistent with previously created maps of active fault lines in the area. They show the existence of large strike-slip faults, where rocks slide past each other in an east-west direction, linked to smaller faults in different directions.

According to Associate Professor Quigley, it is very likely that the first earthquake triggered the second one. “The displacement of the Earth’s crust at the fault line that caused the first earthquake may have increased the stress on the fault line leading to the second one. In addition, the propagation of seismic waves from the first event likely shook nearby fragile fault lines,” he said.

Mark Allen, professor of Earth Sciences at Durham University in the UK, has a similar view. He said it was unusual for two major earthquakes to occur in less than a minute. “It is possible that the 7.2-magnitude event ruptured a section of the fault line and transferred stress to another fault line, causing it to also rupture and leading to a 7.5-magnitude earthquake,” he said.

Adam Pascale, an expert at the Institute of Seismology in Australia, noted that the second earthquake was actually twice as strong as the first.

“The number 7.5 degrees may not seem much stronger than 7.2 degrees, but the actual intensity is more than double because the scale uses logarithms. That means the shaking will last a lot longer. The first one caused a tremor and then 40 seconds later there was a larger event,” he said.

Expert Allen said that the two earthquakes appeared to occur at the boundary between the Caribbean and South American tectonic plates. “Tectonic plates move across each other in this area, similar to the San Andreas fault line in the US state of California,” he said.

In northern Venezuela, the Caribbean and South American tectonic plates slide past each other at a rate of about 20 mm per year, with the Caribbean plate moving east relative to the South American plate. This created major strike-slip faults, including the Bocono, San Sebastian and El Pilar.

Earthquakes with shallow epicenters frequently occur at the boundary between the Caribbean and South American plates, and some can cause damage. The area has suffered several major earthquakes in the past, including a magnitude 7.7 earthquake in 1900 and a magnitude 6.5 earthquake in 1967.

“In the western area of ​​the double earthquake on June 24, the boundary between the two tectonic plates became wider and more complex, easily causing widespread seismic activity with many earthquakes having epicenters at shallow to medium depths,” Associate Professor Quigley said.

Expert Allen warned of the possibility of additional aftershocks in the Caracas area. “Venezuelan’s capital is an earthquake-prone area. Fault lines there may have accumulated additional stress after the events of June 24,” he said.