Colombia earthquake: What makes some quakes so deadly?

As Colombia reels from one of the strongest earthquakes in its recorded history, which killed at least 250, just weeks after more than 6,000 deaths in twin quakes in neighbouring Venezuela, many turned to scientists to learn why this series of tremors was so deadly.

Moving plates

Colombia and Venezuela sit on the boundaries of several moving tectonic plates, whose slow, constant friction creates a build-up of energy that is periodically released, causing an earthquake.

German Prieto, a geologist at the National University of Colombia (UNAL), told Reuters that Colombia’s quake involved the Nazca plate sliding beneath the South American plate far beneath the Earth’s crust, which is just about 40km deep in the Andean region.

The US Geological Survey estimated the magnitude of the Aug 10 quake at 7.4, striking at a depth of 100km, versus the 7.2 and 7.5 magnitudes of Venezuela’s late June quakes, at a shallower depth of less than 20km.

The depth of the epicenter in Colombia’s quake sent tremors as far as Panama, but gave energy more room to dissipate, averting a far worse disaster.

“Colombia is a seismic country and we have to be prepared for these natural disasters,” Prieto said. “You can’t predict or prevent them, but their effects can be mitigated.”

Motion and magnitude

An earthquake’s magnitude is measured by a logarithmic scale that gets bigger with every unit, so that a quake of magnitude 7 is more than 30 times stronger than one of magnitude 6.

Colombia’s strongest earthquake, one of 8.8 magnitude that struck some 50km off the northern Ecuadorean coast in 1906, unleashing a powerful tsunami that killed hundreds, released 126 times more energy than Colombia’s latest quake.

Another factor is motion. The events in Colombia and Venezuela were strike-slip earthquakes, set off by the horizontal movements of Earth’s crust, leading to a type of shaking that is far more dangerous for buildings.

Major earthquakes can often also cause aftershocks, a grave risk for those returning to structures damaged by the first impact. Colombia had registered more than 100 aftershocks by the afternoon of Aug 11.

Both of Venezuela’s June earthquakes occurred along a separate fault, initial data shows. The second was larger, leading the US Geological Survey to classify the initial 7.2-magnitude quake as a “foreshock”.

Terrain and infrastructure

Countries particularly prone to earthquakes, such as Chile and Japan, enforce regulations to ensure buildings can withstand fierce shaking, with use of strong yet flexible designs that let them sway without snapping.

Informal housing or poor investment in engineering, however, can be devastating when quakes hit densely populated cities. The 2010 Haiti earthquake is estimated to have killed more than 300,000 at a cost equivalent to 120 per cent of the prior year’s gross domestic product.

Colombia began designing earthquake-resistant construction in the 1970s and adopted its first building code in 1984 after a destructive quake in Popayan. Its current code was established in 2010.

Prieto said location, materials and building code compliance are key to making earthquake-resistant cities, adding that he believed this week’s destruction should be studied to improve existing building regulations.

Another key factor is soft terrain. Mexico City, built on a dried-up lake bed, is vulnerable even to faraway earthquakes because of the amplifying effect of the soil structure.

Venezuela’s coastal area of La Guaira, built on soft, loose soil, was hard hit in June, as were Colombia’s western valleys this week.

Forward planning

Imposing costly and hard-to-implement building codes in areas rarely hit by serious quakes can be difficult, while region-specific risks are also tough to guard against, said Suzan van der Lee, a seismologist at Northwestern University.

“Our earthquake record is long, but not so long that we’ve seen every possible type of shaking,” she told Reuters. “We’re often surprised when earthquakes happen in ways we haven’t seen before.”

Building networks of seismic stations and sharing data across borders is key to help lower risks, van der Lee added.

“Global sharing of data is very, very important because most earthquakes occur on tectonic plate boundaries that are always near political and continental boundaries.” REUTERS