Short Wave - Understanding Earthquake Aftershocks

Episode Date: February 22, 2023

Monday another earthquake struck southeastern Turkey, near the Syrian border. This time, the quake registered as a magnitude 6.3 — an order lower than the initial, devastating 7.8 magnitude earthquake and the magnitude 7.5 aftershock that struck the area two weeks ago on Feb. 6. A magnitude 6.3 is still considered strong, according to the United States Geological Survey (USGS). And as NPR previously reported, some locals were inside buildings trying to recover belongings lost in the initial quake when Monday's aftershock hit. It made us wonder: What are aftershocks? And how long will people in Turkey and neighboring countries like Syria have to endure aftershocks while piecing their lives back together? Days? Years?For answers, we turned to earthquake geologist Wendy Bohon, who we've previously spoken to about the limitations of earthquake detection.See 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

You're listening to Shortwave from NPR. Monday, another big earthquake struck southeastern Turkey near the Syrian border. Breaking news coming out of Turkey and Syria. There was a magnitude 6.3 earthquake brought down buildings, sparked panic. This, of course, comes two weeks after that devastating quake that killed more than 47,000 people and left mass destruction across that area. That initial quake from two weeks ago was a devastating 7.8 magnitude. It was followed shortly after by a massive aftershock of similar magnitude. We used to think of aftershocks as earthquakes that occurred along the same fault

around the area of the fault that broke that caused the main shock earthquake. After the 1992 landers earthquake, when we had better seismometers and we could see some of the smaller earthquakes, we observed that there was an increase in the rate of earthquakes about one to two fault links away from where the main shock occurred, not just along the fault where it had happened. We called up earthquake geologist Wendy Bohan to help us make sense of this natural disaster. So there's two things that have to happen for it to be an aftershock.

It has to happen after this bigger event within a particular period of time, which we define as when you go back to the rate of seismicity that there was in the area before the main shock happened, and it has to be within that certain distance from where the earthquake occurred. They're normal and they're expected. And in fact, they're the only earthquakes

that we can actually kind of predict. Millions will resume picking up their lives in the coming days and even years as the ground threatens to continue shaking beneath their feet. The U.S. Geological Survey, USGS, predicts smaller earthquakes. Between M3 and M4 will continue to be felt by people near the epicenters, long after people stop feeling the increasing

smaller magnitude earthquakes. So as we've been observing earthquakes, over the last hundred years or so, we've recognized certain patterns that they follow. One of those is something called Omori's Law. And what Omori's Law has pointed out is that the number of aftershocks die off through time. And so you can expect the most aftershocks to be right after shock or the biggest earthquake. As you move away in time from that event, there will be less and less aftershocks. But that doesn't speak to the size of those aftershocks. So you could be, say, 100 days away from the earthquake and you could still have larger magnitude earthquakes happen.

Today on the show, recovery amid continued devastation. As another aftershock strikes turkey, what they are, how they're measured, and how we can work towards making natural disasters just natural hazards. I'm Regina Barber and you're listening to Shortwave, the Daily Science podcast from NPR. So, Wendy, the USGS says that historically deep earthquakes, a greater, than 30 kilometers are much less likely to be followed by aftershocks than shallow earthquakes. What do we know about how deep these quakes are? And what does that tell us about what we can expect in the coming weeks in the way of aftershocks? When we think about aftershocks, what we're really talking about is, you know, for people that are living nearby, what are they going to

experience in the days and weeks to come? And so if you think about earthquake shaking and what you feel that's influenced by a few different things. The magnitude of the earthquake, the distance from the earthquake, and your local rock and soil conditions. The distance from the earthquake can be horizontal distance, like driving distance, but it can also be vertical distance or distance into the earth. So shallow earthquakes can actually be very damaging to population centers because they're very close to where they are at the surface.

So people that are feeling these shallow earthquakes, they're going to feel more of the aftershocks because they're closer to the aftershocks than, say, if it was a deeper event where you're less likely to feel some of those smaller events. So when we see that an earthquake is shallow, when we know that it has a large magnitude, and when we know that there are population centers nearby, that often spells trouble. Okay. So Wendy, all earthquakes have numbers associated with them, right? Like, growing up, that number was how they rated along the Richter scale,

which is a scale created based on earthquakes in the west coast of the United States. But other regions have different geology. what are the limitations of the Richter scale for earthquakes around the world? So a point that most people don't know is that we actually don't use the Richter scale anymore. We use a better scale that was built off the Richter scale. Now that we have better seismometers and now that we can look at all of the different frequencies of earthquake waves that happen, we have what's called the moment magnitude scale. And that takes into account things like the rigidity of the rocks that break, how far the fault slips,

you know, different parameters of the earthquake itself. And so it's still a logarithmic scale, which makes it really unintuitive to people. We don't tend to think in logarithms ever. And so I talk about it a lot with something I call the spaghetti magnitude scale. So if you imagine breaking one strand of spaghetti, then that's a magnitude 5, say. You would have to break 32 strands of spaghetti to be a magnitude 6. And then you would have to break more than 1,000 strands of spaghetti to be a magnitude 6. seven. So it's not like the difference between a magnitude five and six is the same as the difference

between a six and a seven. And so what we'll see, even though like in earthquake sequences like this, you know, you'll have all of these aftershocks, the majority of the energy that's released is released by that largest main shock. The takeaway from this is that the difference between a magnitude six and a magnitude seven is profound. And the amount of energy that's released by these earthquakes is really quite mind-blowing in a lot of cases. And it can cause significant. significant shaking, and as we've seen, unfortunately, in Syria and Turkey, significant amounts of damage. Okay, the first quake was 7.8. This aftershock Monday was 6.3. And the USGS, while not 100% ruling out other scenarios, says the most likely scenario by far is for subsequent

aftershocks to be smaller. Do aftershocks always shrink in magnitude in time? The answer is no. Generally, aftershocks are going to be smaller than the main shock. And there is a, kind of a rule of thumb that we go by called Bath's Law that says most aftershocks, the largest of those aftershocks usually are about one order of magnitude less than the main shock. So with the magnitude 7.8, most of the time you would get an aftershock, at least one, of around a magnitude 6.8 or a little bit smaller. But, you know, that's just most of the time. There is a probability that you could have an aftershock that's larger than that, which we did see in this aftershock sequence. We had the magnitude 7.8. And then a

magnitude 7.5. And then in a very small percentage of cases, you can sometimes have even a larger aftershock, so an aftershock that's larger than the main shock. And in that case, we get into the semantics of changing the names of things. So then the initial earthquake would be a four shock, and then that larger event would be called the main shock. But that only happens four to five percent of the time. So that's a very unlikely scenario. What's most likely is that the aftershocks will continue to die off through time and that they will be smaller than the main shock. Okay. There are going to be years of repairs and constructions following these quakes and any other aftershocks that follow. What makes some buildings more protected from earthquakes than others?

If we know how likely an area is to experience a certain level of shaking, then we can design buildings to withstand that shaking. And that's what we hope to do. And so building construction techniques, the types of materials that are used and the types of construction can make buildings more resistant to earthquake shaking. But they have to be followed. They have to be enforced. And then, of course, the buildings have to be maintained. So there's not really any such thing as a natural disaster. What we have are natural hazards like earthquakes that occur in areas where we have vulnerable populations, vulnerable structures, people that don't know what to do when they feel earthquake shaking. And so when you have a natural hazard, you can

potentially have a disaster. But it doesn't have to be that way if we take the steps before the earthquake to make sure that we can live with the earthquake. As a seismologist, what's on your mind moving forward after these earthquakes? The science is always interesting, but what weighs on me is my heart. You know, I'm a human being and I'm watching other human beings suffer in profound, devastating ways. And I know that they're going to continue to feel aftershocks for a long time to come. I'd love to be able to say to the people in Syria and Turkey, like, you're done, it's good, it's over, time to rebuild. But we know that the earth works in particular ways, and we know that more aftershocks are likely, and they're going to continue to feel shaking.

And they're already, you know, it's such a traumatizing, devastating situation. That's what I wake up thinking about and going to bed thinking about is what these people are experiencing. And the road that they have ahead of them as they try and rebuild. Wendy, thank you so much for taking your time to talk to us about earthquakes, and we really appreciate. Thank you for coming. I appreciate you having me. Have a great day. If you want to hear more on the science of earthquakes, check out the episode we aired earlier this month about why earthquakes are so hard to predict. We'll link it in our episode notes. Today's episode was produced by Liz Metzger, edited by managing producer Rebecca Ramirez,

and fact-checked by Anil Oza. Audio engineer for this episode was Robert Rodriguez. Brendan Crump is our podcast coordinator. Beth Donovan is our Senior Director of Programming, and Anya Grendman is our Senior Vice President of Programming. I'm Regina Barber. You're listening to Shortwave, the Daily Science Podcast from NPR.