The 7.4-magnitude earthquake that struck Colombia falls into the category of intermediate-depth earthquakes, which have different characteristics from shallow earthquakes. These earthquakes originate at significant depths within the subducting plate, as part of its subduction beneath another tectonic plate, and are not “unknown” in Greece, with notable examples being the 2006 Kythira earthquake and the 2008 Leonidio earthquake.
“The earthquake that struck Colombia was different from the usual shallow earthquakes, such as those that have recently occurred in Venezuela. Its main characteristic was its great focal depth, approximately 110 kilometers, a fact that classifies it as an intermediate-depth earthquake,” said Kostas Papazachos, professor of Lithospheric Physics, Seismology, and Applied Geophysics in the Department of Geology at Aristotle University of Thessaloniki, in a statement to the Athens-Macedonian News Agency (ANA-MPA).
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As he explained, in the region of Colombia, the Nazca oceanic plate subducts beneath South America, reaching depths of up to 650 kilometers. The seismic activity associated with the subducting plate can occur at significant depths, and this particular earthquake originated within this subducting lithospheric plate, at a focal depth of approximately 110 km. It is, therefore, a different seismotectonic process from that of a typical shallow earthquake.
“The earthquake in Colombia was caused by the subduction of the plate at great depth, similar to the relatively recent earthquakes in the southern Aegean, the 6.8-magnitude quake off Kythira in 2006 and the one off Leonidio in 2008. The earthquake in Colombia was significantly larger. “However, we have such earthquakes in the Greek region with significant impacts on the outer Greek arc—Crete, Rhodes, and the southern Peloponnese,” the professor pointed out.
Intermediate-depth earthquakes, as Mr. Papazachos added, have three distinct characteristics. “The first is that, as a rule, they produce almost no strong aftershocks, so we are not ‘at risk’ of a strong aftershock in the coming days or hours. The second characteristic is that the great depth limits—compared to a corresponding shallow earthquake—the intensity of the motion reaching the surface, so their impact is reduced, because the seismic waves have traveled a greater distance (110 kilometers in this case) before reaching the ground and thus weaken. “Third, they are felt and affect much larger areas,” Mr. Papazachos explained.
He also noted that, because the earthquake in Colombia was of large magnitude, the seismic motions were quite strong, reaching approximately 20% of the acceleration due to gravity, that is, around 0.2G, with the maximum ground motion acceleration corresponding to about one-fifth of the acceleration due to gravity.
Regarding the destructive effects of the earthquake, the professor explained that they do not depend solely on the magnitude and depth of an earthquake, as the strength of the structures plays a decisive role. In areas with a vulnerable building stock or inadequate seismic protection, even an earthquake of intermediate depth can cause significant losses. In Colombia, the combination of the earthquake’s large magnitude, the large area over which it was felt, and the vulnerability of parts of the building stock—buildings with virtually no seismic resistance—contributed to the collapses and serious loss of life.
The situation is entirely different when it comes to earthquakes in Venezuela, where recent strong earthquakes were much more shallow and were linked to fault activity along the boundaries of the Caribbean and South America. Shallow earthquakes, as Mr. Papazachos noted, can cause much stronger ground motion near the surface. “We’re talking about South America, a region where the world’s strongest earthquakes have occurred. Further south and closer to the surface, the Chile earthquake had a magnitude greater than 9. The ‘upper limit’ for powerful earthquakes in the South American region is much higher than that of this particular earthquake,” the professor concluded.
Source: Smaro Avramidou, APE-MPE
