Mark Quigley, The University of Melbourne

On Wednesday evening just after 6pm local time, two earthquakes violently shook northern Venezuela.

The first one struck near San Felipe, the capital of the state of Yaracuy. Just 39 seconds later, another quake struck near the town of Yumare, within 5 to 10km from the first one.

Powerful ground shaking was felt across the region, including in Venezuela’s capital Caracas about 150km east of the earthquake epicentres. Buildings collapsed, and authorities report the casualty toll may be in the thousands.

In addition to strong shaking, ground failure including landslides and liquefaction are anticipated to have occurred throughout the region. The earthquakes happened in a mountainous region where slope failures are common. And the type of sediment beneath Caracas amplifies seismic waves and enhances earthquake damage.

According to the US Geological Survey (USGS), the earthquakes were a “doublet”: a magnitude 7.2 foreshock followed 39 seconds later by a mainshock, this one with a magnitude of 7.5.

What is an earthquake ‘doublet’?

An earthquake doublet is a pair of earthquakes that happen within a short time and distance from each other.

Unlike a typical earthquake sequence, where a larger earthquake is followed by significantly smaller aftershocks, doublets are earthquakes of similar magnitude that are causally linked, but seismologically distinct. This means the seismic waves from each quake are separated by a gap in time, and/or originate from distinct sources.

Although the Venezuelan earthquake epicentres were within mere kilometres of each other, seismic wave information from the USGS suggests they likely originated from different faults with different rupture styles.

This is consistent with previously developed maps of active faults in this region. These show large strike-slip faults, where rocks slide past each other in an east-west direction, linked with arrays of smaller faults in various orientations.

It’s likely the first earthquake triggered the second one. This could have happened because Earth’s crust displacement in the first earthquake fault increased stress on the second earthquake’s source fault. Additionally, the passage of seismic waves from the first earthquake could have rattled nearby faults already prone to a rupture, causing them to fail.

Earthquake doublets are uncommon, but they do happen. In 2023, an earthquake doublet struck Turkey and Syria, measuring at magnitudes of 7.8 and 7.7. These happened just 95 kilometres and nine hours apart, affecting 14 million people and causing widespread damage.

In 1988, a “triplet” – a series of three earthquakes just half an hour apart from each other – occurred in Tennant Creek in Australia.

Why is Venezuela so prone to earthquakes?

The doublets in Venezuela occurred along the diffuse onshore boundary between the Caribbean and South American tectonic plates.

In northern Venezuela, these plates slide past each other at a rate of about 20mm per year as the Caribbean Plate moves east relative to the South American Plate. This produces large strike-slip faults, including the Boconó, San Sebastián, and El Pilar fault systems.

This active plate boundary generates frequent shallow earthquakes, some of which can be damaging.

The region has experienced several significant earthquakes in the past. These include the magnitude 7.7 Caracas earthquake in 1900 and a magnitude 6.5 earthquake in 1967.

West of the recent earthquake, the plate boundary becomes broader and more complex, and is prone to widespread seismic activity, with many shallow to intermediate-depth earthquakes.

Mark Quigley, Associate Professor of Earthquake Science, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article (https://theconversation.com/was-venezuela-struck-by-an-earthquake-doublet-heres-what-we-know-so-far-286136).