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A magnitude 7.4 earthquake struck Colombia early this week, killing hundreds of people. The loss of life is tragic, though so far, the death toll is much lower than in neighboring Venezuela. That country was hit by a similarly sized pair of quakes last month. NPR's Katia Riddle looks at what these two seismic events have in common and what accounts for their differences.
KATIA RIDDLE, BYLINE: The Colombia earthquake is what seismologists call an intermediate-depth earthquake. William Barnhart works for the U.S. Geological Survey, or USGS, and has been tracking this event.
WILLIAM BARNHART: So it's an earthquake that happened about a hundred kilometers down inside the Earth. That's somewhat deep.
RIDDLE: Barnhart says this part of the world has seen its share of these.
BARNHART: These earthquakes are somewhat common in South America. There's been 32 or 33 of these since the early 1900s.
RIDDLE: These earthquakes can be relatively benign unless they reach denser population centers like this one did.
BARNHART: It still was strong enough to cause a fairly significant amount of damage.
RIDDLE: The USGS says it is possible the eventual toll could climb towards 1,000. That's compared to more than 6,000 people killed in Venezuela last month. In terms of overall damage, he says, in Colombia...
BARNHART: It's an order of magnitude smaller than in Venezuela.
RIDDLE: That's in part because the Venezuela earthquake was closer to the surface of the Earth. Harold Tobin is the director of the Pacific Northwest Seismic Network. He says the Colombia quake was deeper and its reach more diffuse.
HAROLD TOBIN: An earthquake like this tends to affect a wider area than a shallow earthquake like the one we saw in Venezuela a few weeks ago.
RIDDLE: Depth and reach are just a couple of the many factors that determine how deadly an earthquake is, says Tobin.
TOBIN: But then how it actually affects people depends on all kinds of things, including, you know, just literally where people live relative to the earthquake, how high the population density is, how good or bad construction practices are in the region.
RIDDLE: Colombia does have earthquake-resistant building standards, which can reduce the risk of collapse, but many communities remained vulnerable. That's because building construction is only part of the equation, says Tobin. Another thing that matters is the way the seismic waves moved through the Earth.
TOBIN: And it turns out, unfortunately, they get amplified by places where we have - instead of bedrock at the surface, we have, you know, layers of sediment, right? Loose, softer sediment shakes more. People, especially in mountainous regions, tend to live in the lowlands because that's where you can farm.
RIDDLE: But in a quake, that sediment can become deadly. Venezuela had even more damage because the earthquake's most severe effects were concentrated in heavily populated areas along the country's northern urban corridor. William Barnhart from USGS says, despite their similarities in magnitude and close geography, it would be a mistake to think that these two events are related. These quakes just happened to occur in relatively close proximity.
BARNHART: It's coincidence. There's a magnitude 7 earthquake on Earth about once a month.
RIDDLE: The natural world produces events like this all the time, says Barnhart. It's when they cause devastating loss of life that they command our attention. Katia Riddle, NPR News. Transcript provided by NPR, Copyright NPR.
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