Inside Colombia's big quake – and why more could be coming

The 7.4 magnitude earthquake that struck Colombia was devastating. Host Regina Barber talks to earthquake geologist Rodrigo Alejandro Leon Loya about the tectonic plates that caused the quake and what it was like 150 miles from the epicenter. Interested in more science in the news? Email us your question at [email protected] mailto:[email protected]. Support public media with NPR+ and enjoy perks for over 25 podcasts like this one. This show’s perks include sponsor-free listening. Learn more at plus.npr.org http://plus.npr.org. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

• 0:00Speaker 00

  This message comes from the Nature Conservancy. For 75 years, the Nature Conservancy has helped protect remarkable natural places to ensure they thrive for generations to come. Learn more at nature.org/75.

• 0:14Speaker 01

  Hey, Short Wavers. It's Regina Barber with a quick ask to follow the show on the NPR app or whichever app you're hearing us from. On Apple, it's a plus sign. You won't miss an episode from us and will grow our community. OK, here's the show. You're listening to Short Wave from NPR. Rodrigo Alejandro Leon Loya is used to earthquakes, but not ones like this.

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  This one just started with a very huge amount of displacement. It was very long because of the magnitude. We could feel around minute and a half of shaking.

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  He's talking about the 7.4 magnitude earthquake that struck western Colombia on August 10. The quake left collapsed buildings and destruction in its path and killed hundreds of people. But Rodrigo doesn't live in western Colombia. He lives in Bogota, which is about 150 miles east of the quake's epicenter. And as an earthquake geologist at the Universidad de los Andes in Bogota, he says right when the earthquake hit, he wasn't scared. He was more curious.

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  In the moment, I want to know what happened or what will happen. More like a state of alert, like, OK, this is huge. Where it will happen, what would be the damages? I run to the computer to start to see signals and the reports.

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  And after a big shake like that, a natural question might be, when will the next one happen? And can we predict it? Unfortunately, in seismology, prediction is tough to do.

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  The word prediction is a huge word that is not in the in the science community. But forecast is it's a really good compromise. So we can forecast the number of earthquakes and the range of magnitude each year.

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  Rodrigo says that scientists forecast there will be 17 to 19 earthquakes above a magnitude of seven in any given year worldwide.

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  In the forecast, I guess there is room for six or seven more earthquakes above seven.

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  And potentially one above an eight.

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  This year, actually, we haven't had the big one.

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  Today on the show, the Colombian earthquake and why the depth of an earthquake makes a big difference in the aftermath. Plus, if scientists can predict them, what can we do to safeguard people in the future?

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  OK, Rodrigo, take us beneath the surface. Like, what is physically happening when an earthquake like this, the one that happened in Colombia, when it starts? What plates under the surface are causing the ground to suddenly move?

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  We need to imagine that the Nazca Plate is going down into the mantle. When it reaches about 100 kilometers, the pressure is very, very high. And the slab can host a brittle rupture, an earthquake. So these are not really frequent, but they are not strange for the seismological community.

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  The depth of the earthquake has implications because the deeper you go, the more attenuated the seismic waves will have. This means that it was a big earthquake, but it was the best scenario that Colombia could have in the light of the damage that we are seeing.

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  Because it could have been even higher up in the mantle and closer to the surface, and that would have been more damaging?

• 5:40Speaker 02

  Yeah, like a Venezuelan-style earthquake, for example.

• 5:43Speaker 01

  Hmm. So because Colombia sits in this specific region, it's more susceptible to earthquakes. Can you tell me about these different tectonic plates that are involved in this region?

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  Yeah. There are three big plates that are interacting. That is the Nazca Plate, that is an oceanic crust, oceanic plate, that is traveling from the Pacific Ocean, and it's smashing into the continent. And the continent is the South American Plate.

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  Hmm.

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  To the north, you have the Caribbean Plate, that is not colliding, but rather sliding through.

• 6:27Speaker 01

  Oh, it's sliding. Oh, wow. OK.

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  Yeah, like the San Andreas Fault System, for example. You have the Pacific and the North American Plate sliding each other.

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  OK.

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  But between these three big plates, there is a microplate that is called the Nor Andean Block. And these three plates impose a huge amount of stress into the Nor Andean Block. And this stress is expressed in fault systems that are hosted in the crust. So in Colombia, you can, for this tectonic setting, you can have shallow earthquakes that will be hosted in crustal faults. You can have the mega-thrust earthquakes that are related to the convergence boundary between Nazca and South America.

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  And then you have deep earthquakes, like the Monday earthquake, that is hosted in the slab that is going beneath the South American Plate.

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  So when you have all of these three plates, right, like, does it make it so that it's more active? Like, is there going to be more earthquakes there?

• 7:39Speaker 02

  Yeah, we have a huge amount of earthquakes. And because of this complexity, we have many seismogenic sources. In other places, you just have to worry about the plate boundary earthquakes. But in Colombia, there is at least three different seismogenic types of earthquakes.

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  What's the difference between the earthquake that happened in Venezuela versus the one that just happened in Colombia?

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  Yeah, the main difference is that the Venezuelan earthquake was a really shallow earthquake.

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  Which is one reason why that earthquake was so much more deadly, right?

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  Yeah. Also, the earthquake in Venezuela is very complex because we call it like a multi-rupture event.

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  A multi-rupture event. OK.

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  This means that more than one fault was activated during the earthquake.

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  Oh.

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  And actually, there's an ongoing debate about, from a seismology point of view, you can say that there were two earthquakes separated by 40 seconds.

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  Oh, wow.

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  But from a geology earthquake point of view, I say it was just one event. Just the rupture process was delayed some seconds and then jumped to another segment.

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  So what are the different kinds of earthquakes? So we talked about the subduction one, where like one plate is going under another. And then we've talked about the fault one, where the earthquake is created because those faults are like kind of sliding against each other. What are other kinds?

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  Well, the Monday earthquake was, it's classified as a half-deep earthquake. Actually, there are deeper earthquakes. For example, just going south in the Peru region, you had earthquakes that happened at 500 kilometers depth.

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  Oh, wow.

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  So they are very, very, very, very, very deep. You can imagine the subduction system, and you have the mega-thrust earthquakes in the surface, the inter seismicity, like the Monday earthquake. And then you have very, very deep earthquakes at 500 kilometers.

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  In general, when one earthquake happens, like, what controls whether there'll be more that day, that week?

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  Huge question. And we can say that the shallower, you can have aftershock sequence with more events. The deeper you go, the thermal structure of the slab inhibits the aftershock sequence.

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  Hmm, OK.

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  You always, always, you will have aftershocks. The difference is how many, how many and for how long. The bigger the earthquake, the longer the sequence will last.

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  Because scientists can't predict the earthquakes, like, what does this mean for public safety if we can't predict? Like, how do we, how do we help people?

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  One part is making good hazard maps, comprehensive hazard maps, that can help the engineers to design good houses, good buildings. The other part is designing early warning systems, but there is a very complicated task. For example, you can think of Mexico City. It's very far from the subduction zone. So you have at least half a minute between the arrivals of the seismic waves to the Mexico City.

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  Hmm, wow, OK, OK. Half a minute warning.

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  So the warning system can help the people to evacuate the buildings.

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  Yeah.

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  But if you think about LA, for example, LA is on the top of the faults. So you won't have that, that time.

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  Wow.

• 11:52Speaker 02

  So it depends in where part of the planet you are that you can design the early warning systems.

• 11:58Speaker 01

  So because where the city is and then also how many seismic detectors you have and like your system, what would you, what advice would you give to people that live in these areas that are just very prone? Because they're next to the faults or they're next to like three plates, like you said, these, these zones, what would, what would be your advice?

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  Being aware that things can get messy really fast. And when the earth starts to move, you need to hold on,

• 12:33Speaker 02

  keep calm and wait to the shaking past. Because many people think that when the earth is moving, that they can just get up and, and get out of the building. And we, we saw the, the videos in Monday that people tried to run and they just failed. So.

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  Yeah.

• 12:57Speaker 02

  Yeah, keep calm. It's very easy to say, hard to, hard to do.

• 13:02Speaker 01

  Rodrigo, thank you so much for talking with us today about earthquakes. And I'm glad you are safe after you experienced that earthquake yourself.

• 13:10Speaker 02

  Well, thank you very much for the invitation. As a seismologist, it's always a mixture of, of feelings speaking about this because I like the phenomena, but I don't like the repercussions it has in the societies.

• 13:32Speaker 01

  This episode was produced by Juan Diego Ramirez. It was edited by Burley McCoy and Tyler Jones checked the facts. The audio engineer was Jimmy Keely. I'm Regina Barber. Thank you so much for listening to Short Wave from NPR.

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