Ep. 293: Earthquakes

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We always say that the Universe is trying to kill you, but actually, the Earth isn’t so fond of you either. Certain parts of planet Earth are prone to earthquakes, where the planet’s shifting plates can cause the ground to shake violently. We’ve had a few devastating earthquakes in recent years, but do they also happen on other worlds?

Fraser: Astronomy Cast episode 293 for Monday, February 11, 2013 – Earthquakes
Fraser: Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
Fraser: My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director or cosmoquest.org.
Fraser: Hi Pamela, how are you doing?
Pamela: I’m doing well, how are you Fraser?
Fraser: It’s going really well. Unfortunately through the madness of space and time we are recording this before we leave for South by Southwest but by the time it gets into your feed it will be after so I hope we had fun.
Pamela: Yes
Fraser: I hope you got a chance to meet us and see us and say hi to us. That’s very confusing. It’s wibbly-wobbly timey-wimey right?
Pamela: I’m not that kind of a doctor.
Fraser: (Laughs) No you aren’t that kind of a doctor but you are an astrophysicist but pretty close though. Alright well lets get back to the show then.
Fraser: We always say that the Universe is trying to kill you, but actually, the Earth isn’t so fond of you either. Certain parts of planet Earth are prone to earthquakes, where the planet’s shifting plates can cause the ground to shake violently. We’ve had a few devastating earthquakes in recent years, but do they also happen on other worlds? Have you ever been in an earthquake Pamela?
Pamela: I’ve been in earthquakes of sizes that make you consider whether or not you need to stand in your driveway but that haven’t caused damage more significant than cracks in the walls. I’ve experienced a steady stream of earthquakes in all sorts of different places on the planet.
Fraser: So I live on Vancouver Island which is one of the most earthquake prone places on earth.
Pamela: Really?
Fraser: Yeah, we share the same fault, pretty much, as California so we’re in the ring of fire and we get pretty bad earthquakes here. I’ve been in a few already . One was pretty nerve wracking: I was in a tall office building in Vancouver and the earthquake ended up being a 5.5 I think. The whole building was creaking; you could hear the girders inside the building creaking back in forth. (Simulates creaking sound) It was nerve wracking. Then there was another one that just happened a couple months ago; I didn’t feel it but it was like a 7.9 that was a few hundred kilometers from where I live on the Queen Charlotte islands. Yeah so that was big but fortunately it hit a pretty unpopulated area so nobody was hurt. We’re expecting the big one so we have tsunami warning systems all over the place. I had to write a letter to my children about how if there was an earthquake it would be like: “Mommy and Daddy love you very much and we’re going to try and find you but help other people”. You have to write a sad letter to your children.
Pamela: And this is like on file with your school system?
Fraser: Yeah they have it ready to go for the kids in case of an earthquake and we can’t reach each other. We live in earthquake central, earthquake country here on Big Cooper island… and cougar country but that’s a whole different story.
Pamela: We have both those things here as well but we are missing the tsunami part. That’s one of the things that people have realized that you don’t have to be on one of the big plate boundaries that everyone thinks of to experience earthquakes. Here in Illinois we have the San Madras fault which is responsible for destroying the city of St Louis back in the 1800’s. It periodically lets off some moderate sized earthquakes and they are waiting for the next one that will re-flatten St Louis but, obviously, we don’t know when that’s going to happen. We have the cougars. My university mascot is actually the cougar and ironically the first year that I lived here my husband went on a business trip and prior to leaving he said he hoped he didn’t experience an earthquake while out there. I woke up in the middle of the night and went to kick the dog for scratching on the bed and making the bed shake and realized the dog was not on the bed and it was an earthquake. I had this moment of “Well I grew up in California so I know I’m supposed to go stand in a doorway but I’m in a 120 year old house on top of two nice temper-pedic mattresses and box spring sets. If the house is going to collapse I’m probably better where I am on top of the foamy stuff.” So I stayed in place.
Fraser: So lets go back and get to the geology lesson part of this actual show… the science that everybody is waiting for
Pamela: (Laughs)
Fraser: So what is an earthquake?
Pamela: At the end of the day as the planet earth, or whatever world it’s occurring on, releases pent-up elastic energy that as the plates of the planet move they build up stress in a variety of different ways. Sometimes this stress is magma welling up from underneath and it cracks the surface. Sometimes it’s two plates moving aside one another and friction stops them from moving but they’re trying to move. If you’ve ever tried to bust open a stuck nut you know that you keep pushing on the wrench and pushing on the wrench and then all of a sudden it goes.
Fraser: Then you make you’re knuckles bloody as you…
Pamela: Exactly. The planet does that too and sometimes the planet makes more than just your knuckles bloody. That’s just one of those unfortunate things about releasing this amount of energy.
Fraser: Now you say that is a release of elastic energy but I wouldn’t imagine rock to be elastic.
Pamela: I think it’s because we’re used to thinking of rocks as this nice little thing that we use to build walls and to bash things when we’re too lazy to go get a hammer when we’re small children. The reality is that they’re just another substance that is no different from a cement wall or a piece of granite that you can press and press and press and it will only give so much before it breaks. Even granite will give a little bit before it breaks and it’s that amount of giving that is the elastic energy getting stored up in the system.
Fraser: I always do this experiment with children to show them how experiments work. You push your hands together really hard and then you try and move your hands one way or the other. Your hands move in these, kind of, fits and starts; that’s that elastic energy that is being released in these quick jolts.
Pamela: The majority of the energy doesn’t go into the earthquake; the majority of the energy gets released in the heating up of the systems in things that are less destructive in some ways. What’s really terrifying is the bulk of the energy isn’t released in the earthquake if you start to think about it.
Fraser: Right, I know that with the Indonesian earthquake and the recent one in Chile…
Pamela: Haiti
Fraser: …in Haiti, there was a tremendous amount of energy that was released. They would describe it in gigatons of nuclear weaponry and then they calculate how much the planet Earth’s rotation changes.
Pamela: It’s a change in the moment of inertia as the planet. This is one of those things that we forget to realize is that we are essentially a liquid filled ball. One neat experiment that you can do is take 3 cans of soup, empty one of the cans out and eat it, keep the other one nice and friendly and liquid and freeze the other one. Make sure it’s something like chili that won’t cause the can to deform because of the water expanding. You now have three objects: one solid, one liquid and one empty. Roll them down a slope and they’re going to roll at different rates because they have a different moment of inertia This is the same effect as an ice skater goes faster than when they pull their arms in and slower as they put their arms out. Those are different moments of inertia and this means that the mass is distributed in a different way. If you physically move where the solid rocky stuff floating on top of the magma is on the surface of the planet, that’s the equivalent of moving how your hands holding, in this case, weight are located held away from your center. This changes how the planet rotates just like freezing and thawing that can of soup changes how it rolls down the inclined plane.
Fraser: I know that there are different kinds of plate interactions that can lead to these different kind of earthquakes right? Then there are other events like vulcanism and even just some random events. Let’s talk about what are the underlying causes that will get you an earthquake.
Pamela: So the things that will get you an earthquake basically boil down to having plates that are slipping beside one another. One is going up or it’s going down; those are the basic geometries You have subduction going in, you have things being created then they well apart. We don’t experience this on land very much but in the center of the ocean the plates will move apart and even moving apart with the magma welling up it’s still keeps things moving and creating tectonic energy. It’s things moving and as they move they either have to go up, go sideways or go under. Those are basically the choices you’ve got.
Fraser: And you’ve got this situation, right, where they’re not moving just smoothly; the plates aren’t sliding against each other very smoothly. They’re really just going in fits and starts and the longer it’s taking to move then often the worse the earthquake is going to be.
Pamela: Right and depending on how the waves get to you, you will also experience them in different waves. Anyone who has been through multiple earthquakes will may have started to notice that sometimes it’s like everything just goes “kaboom” and you’re not sure if it was a sonic boom or an earthquake. Sometimes you’ll sit there and feel like a really semi truck or train just keeps going and going and going and going past where you are. If you watched any videos from the Japanese earthquake, they actually had a continuous rumbling motion.
Fraser: That’s what ours feel like.
Pamela: Yeah and what you experience depends on the geology of the land that you were on top of and where you are relative to the originating source of the earthquake. When these events happen they send waves propagating through the planet. One form of these waves, the pressure waves, the P-waves, they work just like sound waves moving through the atmosphere, water or ground. It basically is a compression wave that moves through the ground and jolt things as it goes. This is a very fast moving wave; this is actually the wave that your dog starts going off prior to the earthquake starting to make things shake in a noticeable way. What your dog is keying in on is that P-wave that is moving fast and getting to you first. The P-waves can go through all different types of materials: they can propagate through water, they can propagate through land and they’re going to get you no matter where you are. That’s one kind of wave. Now in addition to that we have S-waves; these are up and down waves. The way to think of these two is if you were to take a slinky, and I should have found a slinky prior to this episode, if you send a wave that makes the rings in the slinky closer and further away as it moves through the slinky, that’s a P-wave. An S-wave is where you move the ends up and down and you end up with the slinky forming a series of side-to-side S’s turned on their side that are all interconnected, this is your sine wave essentially moving through. This requires a rocky body; this requires soil; this requires something solid that the S-wave is going to move through. We can actually map planets using earthquakes by looking to see when the P-wave arrives or if the S-wave arrives and it’s by combining this information that we’re able to figure out where inside the planets you have the liquids and the solids that will allow the S-waves to transmit as well.
Fraser: So in an earthquake will we feel both of those waves? Will we feel the P-wave or the S-wave?
Pamela: You may not feel the P-wave but you’ll definitely feel the S-wave. There’s actually one additional type of wave; it’s the surface wave that causes the massive damage. You have the P-waves and the S-waves that are traveling underground and then the surface waves are kind of the planetary equivalent of taking a sheet and shaking the edge and seeing this single wave go rippling across the top of the surface. There’s a lot of particle movement that is lateral that causes this to happen in addition to the up and down that gets experienced. This is where all of the damage ends up occurring You also end up with particle motions where you end up underneath an eddy with these rolling motions of the particles underneath. Everything essentially gets flipped upside down and rearranged.
Fraser: You mentioned briefly about how earthquakes are not only a dangerous natural disaster but scientists use them to probe the interior of the earth. What’s actually going on there?
Pamela: It’s not just the earth that we’ve done this with, we’ve actually actively tried to do this with the moon. What’s happening is we have a series of seismic detectors all over the surface of our world. In some cases these are detectors that are set up to monitor earthquake zones because earthquake waves do, as many of us know, move slower than the speed of Twitter. These move at fairly reasonable speeds such that, if there is an earthquake in Los Angeles that goes off, you have time to contact people who are tens to hundreds of miles away and say “earthquake”. We have monitoring stations set up; these are also used to sound tsunami warning alerts. Japan has a series of networks that will actually send off alarms when an earthquake is sensed somewhere so that people have that chance to get under their desk before the earthquake hits where they are. We also have detectors that used to monitor things more mundane and, in some ways, more dangerous such as a nuclear explosion. A nuclear explosion under the ground will trigger a characteristic set of seismic waves to move through the planet. We watch and monitor nations like North Korea using the artificially generated seismic activity that they create when we test weapons; we also test volcanoes this way. With all of this data together you are able to map the inside of the planet.
Fraser: Right and they guess specifically right? You get this situation where the waves move at different speeds or different refraction angles through the different layers in the earth and that’s how you can probe where they are.
Pamela: Right, so the P-waves are going to move through everything at a set speed. Just like we can use the speed of light to figure out when this event went off, off of the surface planet based on Boston seeing it first and Cape town seeing it later, that means that the light had to travel. Lets assume that it’s not actually light but something like neutrinos that will travel through the plane Lets say you are detecting neutrinos from a gamma ray burst; if you detect them in Boston first and then Cape town later, you’ll know which one of those two places are physically closer to the source. We can do this same thing with seismic detectors. Who ever detects it first is closer and by having these spread all across the surface of the planet you can measure all the different travel times and pinpoint the location of the epicenter of the earthquake.
Fraser: So now as I mentioned at the beginning of the show how we’re going to talk about how this is actually a common feature on many objects in the solar system. What are some of the other places in the solar system where scientists have detected some kind of motion?
Pamela: The primary place that we’ve detected these things is the moon. We dumped some seismic detectors on the moon; they aren’t working anymore. We weren’t so much detecting tectonic motion on the moon or vulcanism… the moon is dead. We were detecting when the moon got clobbered by small rocks falling from the sky.
Fraser: Oh really? So a moon quake is the moon getting hit by some object somewhere on its surface?
Pamela: Yes and we were able to detect the meteor explosion over Russia from seismic detectors as well. You hit the planet or the moon and they ring like a bell. Just like you can determine the geometry of the bell from how it rings, you can determine the geology of a moon or a planet from how it rings.
Fraser: Is there any earthquakes on Mars? Mars-quakes?
Pamela: So far as we know Mars is also seismically dead. In the past, clearly, it had all sorts of interesting activity: Look at the volcanoes that it has on it. Nowadays whenever we need to set up a seismic system, as far as we know- and this may not be entirely accurate because we’re always learning new things- Mars is for the most part seismically dead and we wouldn’t expect to detect anything except for impact events.
Fraser: I know that scientists are not even sure when Olympus Mons shut down and so you could have a situation that could even have another eruption so we don’t really know how long it’s been. There could be quakes near Olympus Mons if there is something moving underneath.
Pamela: More than likely Olympus Mons is part of the volcano chain that is from a series of different hot spots that had the volcanoes erupt above them and create islands just like the islands of Hawaii. As the plate moves the hot spot wells up new islands. What we’re seeing is on one side of the Hawaiian islands are new islands that are bright and sharp and poking up out of the ocean. On the other side ones that are more weathered and starting their decay back into the ocean. On Mars there is a chance that we could get, if it is still active, a new crack in the surface that out-gasses and oozes however it will. It’s unlikely given all of the information that we have on it that we are going to see a new giant “Mons”, new giant volcano, form. There is always a potential for out-gassing which might explain some of the methane detections that we have had on this little world.
Fraser: Now when I think of the poster child of vulcanism I think of Io. Would there be earthquakes on Io?
Pamela: I think they’d be called Io-quakes. These are going to be magmatic eruptions, which is always fun to try and say, caused by this constant up-welling of lava from inside this very molten, very torqued planet that is currently undergoing tidal forces that are ever changing, squishing its system like a stress ball. As that magma comes up it is not a smooth and easy process. Anyone who lives in Iceland has experienced this for themselves; if you watch the seismic monitors in Iceland you see a whole variety of different activities that vary from normal tectonic activities like when the plate that is split in the middle of Iceland moves apart. You also see lots of magmatic activities like different chambers filled and new chambers filled and it’s that magmatic activity that you’d also see on Io.
Fraser: There isn’t a lot of solid ground, there isn’t a lot of places you would want to stand on Io without getting rock raining back down on top of you from these volcanoes It would be a pretty dangerous place; I think earthquakes would be one of the last things you’d be worried about.
Pamela: It’s probably a lot like Hawaii. Luckily lava is not that fast moving with shield volcanoes that aren’t in an explosive eruption so if you’re not at the very top you generally have a chance to watch the lava flow around you now. This doesn’t mean there haven’t been lots of tourists in Hawaii that haven’t done things like go where they shouldn’t and realize they’re on the last piece of land that isn’t lava and they are thus cut-off and have to get rescued with helicopters It is the type of process that an alert scientist could observe an eruption without death. (Laughs)
Fraser: It’s rock on earth and magma and such on Io but what about other places that have cryovolcanism like a place like Enseladus or even like Europa has these what looks like plate tectonics but it’s ice right?
Pamela: It’s hydraulic activity much like what is experienced with glaciers, just like you can detect glacier movement in seismographs. You’d expect on Europa that it’s even more fundamental to have the geology of the system changes where you have this hydraulically driven motion with up-welling of hot water that actually circulates the surface at a certain level as well as the gravitational tidal forces that the system is undergoing. All of this acts to cause neat banding cracks and all sorts of interesting spiral formations. It’s very complicated; people are still working to try and figure this out. You definitely get the stress and the strain in the system that leads to slope faulting a lot like you see here on earth.
Fraser: You can almost imagine that it would be equivalent to crossing glaciers here on earth. If you weren’t careful chasms would open up and as these movements happen you would get dangerous conditions. It would be a pretty scary place to hang out on the surface of.
Pamela: In general, time scales of things happening are low enough that you’re good as long as you’re not standing right at the place that the sides, at any given moment, would move. Those high stress places are usually identifiable
Fraser: I know that Venus is very similar to Earth in size, mass and composition. Do you think Venus experiences any earthquakes? I’m just going to keep calling them earthquakes, I’m not going to call them Venus-quakes. They’re earthquakes. If it’s the ground it’s an earthquake.
Pamela: Quakes. Seismic activity.
Fraser: Right, seismic activity.
Pamela: So Venus is one of these worlds that we’re still trying to figure out and it likes to perplex us because it’s under this thick cloud layer of deadly toxic stuff. These opaque toxic fumes that cover the planet in clouds make it very hard for us to get the same level of geologic understand that we get from looking at Mars and the moon. We have done radar imagine of the surface of Venus; the Magellan Space Probe did this. As far as we can tell it has volcanoes It has a very young surface; it doesn’t have the amount of cratering that we would expect from a non-evolving surface. This has lead some to speculate that every once in a while all of the heat that is built up within Venus so we didn’t see any reason to believe that there are plates on Venus. The idea is that Venus, every once in a while, has this massive, essentially, upwelling across it’s entire surface and the heat keeps building up and then the entire surface basically gets resurfaced in a massive outburst of volcanic activity and tectonic activity.
Fraser: So it just turns itself inside-out?
Pamela: Yes
Fraser: In what kind of a time frame?
Pamela: Unclear. It’s not short. It would be at the order of magnitude million time year.
Fraser: Still that would be an unpleasant place to be. It was already unpleasant to hang out on. That would really take it to the hell-scape. Right. So one last question for you. Do you think there are more earthquakes now or more earthquakes on the earth in the past?
Pamela: That’s always one of those interesting questions where different people have different ideas. The reason that they have different ideas is because as our planet cools, just like every other cooling body in our solar system, the seismic activity is going to get less and less. It’s unlikely that there is going to be a massive volcanic outburst again like what was experienced in Siberia many many years ago. This is in part because our planet is settling out but this is happening at the millions of years time scale. So do we experience less earthquakes that were probably experienced by plankton that first occupied the oceans and got swooshed onto land in tsunamis? Probably. Do we experience fewer earthquakes than early man? Probably not, it’s just that we’ve gotten lucky in recent history. Anyone who is living in Haiti, Armenia, China, Chile or any of the places that have had these magnitude 7 or 9 like was experienced in Japan. They’re thinking: Lucky? The reality of what happened in Japan was the cities were built and constructed for the earthquake, it was the tsunami that was the issue. Anywhere on the planet we can have a massive slip. Cities like New York have done recent simulations and found that the city will collapse if there is a magnitude 7 earthquake. We really do need to worry about complacency about places that aren’t prepared and are statistically unlucky. We need to be better about building for earthquakes everywhere that we build but that costs money so it doesn’t happen; this is a true concern moving forward.
Fraser: Yeah we have pretty tight building constraints here on Vancouver island and in Vancouver as well. It’s very similar to the Japanese standards and actually a building that I was in got completely rebuilt up to earthquake standards. It was quite impressive with cables and new, sort of, skeletal structure inside the building to protect it. Here where I live for example, they are all wood houses. You wouldn’t buy a house made out of brick; it would be madness.
Pamela: Where I get concerned is when I was an undergraduate at Michigan State we had a moderate earthquake like the type that makes you go “Did I really experience that?”. As a result of it there were cracks in between the wings of the dorm I lived in and the main part of the building. They had problems with the brick outer shell to the building because it started peeling away and they had to do emergency construction to sure-up the ability of the brick to stay attached to the dorm building. That was an earthquake at the level of “I’m not sure I experienced that”. But it was Michigan, we weren’t prepared.
Fraser: We get those every couple of months. “Is that me having a panic attack or is that an earthquake?”
Pamela: (Laughs)
Fraser: “Is that a cat scratching itself or is that an earthquake?” Cool well thank you very much Pamela and we’ll talk to you next week.
Pamela: It’s been my pleasure, thank you.
 
This transcript is not an exact match to the audio file. It has been edited for clarity.