USGS
Two almost simultaneous earthquakes in northern Venezuela would have resulted from distinct faults, in a rare case of a “seismic double” — which worsened their impact, in a region already vulnerable to destructive shocks.
On Wednesday night, shortly after 6pm local time, they violently shook the north of Venezuela.
The first occurred near San Felipe, capital of the state of Yaracuy. Just 39 seconds lateranother earthquake hit the area close to the town of Yumare, 5 to 10 km from the first.
The strong shock was felt throughout the regionincluding in the Venezuelan capital, Caracas, about 150 km east of the epicenters of the earthquakes. Several buildings collapsed and authorities admit that the number of victims may amount to tens of thousands.
In addition to the strong ground shaking, it is likely that phenomena of terrain instabilityincluding landslides and liquefaction, throughout the region.
The earthquakes occurred in a mountainous area country, where slope failures are common. Furthermore, the type of sediment beneath the capital, Caracas, amplifies seismic waves and worsens the damage caused by earthquakes.
According to the US Geological Survey (USGS), the earthquakes constituted a “seismic duplex“: a precursor shock of , followed, 39 seconds later, by the main earthquakethis one with magnitude 7.5.
What is a “seismic doublet”?
As explained by , a seismic duplex is a pair of earthquakes that occur close in time and space.
Unlike a typical seismic sequencein which a larger earthquake is followed by significantly weaker aftershocks, doublets are earthquakes of similar magnitude, linked by a causal relationship, but seismologically distinct.
This means that the seismic waves from each shock are separated by an interval of time and/or originate from different sources.
Although the epicenters of the earthquakes in Venezuela were only separated by a few kilometersthe seismic wave report released by the USGS suggests that they had different fault origins, with different types of rupture.
This interpretation is compatible with active faults in this region, which show major shutdown failuresin which rock blocks slide laterally relative to each other in an east-west direction, connected to sets of smaller faults with different orientations.
It is likely that the first earthquake triggered the second. This may have happened because the displacement of the Earth’s crust on the fault from the first earthquake increased the stress on the fault behind the second.
Furthermore, the passage of seismic waves from the first shock may have jolted nearby faults that were already prone to rupture, causing them to give way.
Seismic doublets are uncommon, but they do happen. In 2023, a doublet, with magnitudes of 7.8 and 7.7. These earthquakes occurred just 95 km and nine hours away each other, caused the deaths of 16 thousand people and widespread destruction.
In 1988, a “seismic triplet“, a series of three earthquakes separated by just half an hour, occurred in Tennant Creek, Australia.
