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.
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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.
