I’ve studied Colombia for years. Here’s why the earthquake was so destructive
Locals and officials in Colombia have begun a massive search and rescue effort to find survivors of a powerful earthquake that struck near San José del Palmar in Chocó, with major damage observed in the cities of Cali, Manizales and Pereira.
At least 132 people have already been confirmed dead, with that number expected to rise significantly in the coming hours and days. Thousands of homes have also reportedly collapsed or been damaged.
The United States Geological Survey said the magnitude 7.4 earthquake occurred primarily due to strike-slip faulting at a depth of approximately 110 kilometres. In simple terms, a strike-slip earthquake occurs when two blocks of Earth’s crust slide horizontally past one another, rather than moving up or down.
This comes just a month or so after earthquakes in Venezuela, which may have destroyed or damaged more than 58,000 buildings and took the lives of thousands.
It’s far too early to tell whether the earthquake in Colombia is connected to the one in Venezuela. But there’s some evidence from Australia that nearby earthquake faults can “communicate” with one another. A seismic slip on one fault can effectively transfer stress to another nearby fault, causing an earthquake.
I’ve spent years experimentally testing reinforced concrete structures, some specifically designed to Colombian construction practices. My research suggests we shouldn’t be surprised so many buildings collapsed in this disaster – and also that we should be concerned about the many other regions around the world with similar practices.
In buildings, slabs, beams and columns provide resistance to gravity loads. But reinforced concrete walls are needed to resist lateral loads from wind and, in rare but extreme cases, earthquakes.
But in Colombia, many buildings have thin concrete walls, typically 7 to 10 centimetres thick. They only have a single layer of steel reinforcement – commonly electrowelded wire mesh, which is very brittle and ruptures quickly.
The thinness of the walls also means that “confinement”, where hooped steel is placed at the edges of a wall to help prevent concrete crushing, is near impossible to place during construction. Nor is it required in building standards.
Along with colleagues from three Colombian universities, I ran full-scale tests at the Earthquake Engineering and Structural Dynamics Laboratory at the Federal School of Technology in Lausanne, Switzerland, on concrete walls built to match common Colombian construction practice. We wanted to find out how these walls perform when shaken the way an earthquake shakes a building.
The results were concerning even before Monday’s earthquake.
When an earthquake shakes a building, the ideal situation is for many small cracks to form at the base of a steel-reinforced wall. This distributes the stress.
If a single, large crack forms because of the low amount of steel, the stress is concentrated. This causes the reinforcing bars to rupture and the wall to fail.
But the walls on these buildings are so thin, even if there was enough steel reinforcement, they’re still liable to fail during an earthquake. Our experimental tests showed there’s just not enough strength for these walls to survive significant ground shaking.
