In Our Time - Seismology

Episode Date: April 7, 2022

Melvyn Bragg and guests discuss the study of earthquakes. A massive earthquake in 1755 devastated Lisbon, and this disaster helped inspire a new science of seismology which intensified after San Francisco in 1906 and advanced even further with the need to monitor nuclear tests around the world from 1945 onwards. While we now know so much more about what lies beneath the surface of the Earth, and how rocks move and crack, it remains impossible to predict when earthquakes will happen. Thanks to seismology, though, we have a clearer idea of where earthquakes will happen and how to make some of them less hazardous to lives and homes.WithRebecca Bell Senior lecturer in Geology and Geophysics at Imperial College LondonZoe Mildon Lecturer in Earth Sciences and Future Leaders Fellow at the University of PlymouthAnd James Hammond Reader in Geophysics at Birkbeck, University of LondonProducer: Simon Tillotson

BBC Sounds, music, radio, podcasts. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoyed the programs. Hello, on the 1st of November, 1755, a massive earthquake and tsunami devastated Lisbon and its people, making it one of the deadliest in history. The disaster inspired a new science of seismology, the study of earthquakes.

which intensified after San Francisco in 1906 and later with the need to monitor nuclear tests. And as a consequence, we now know so much more about what lies beneath the surface of the Earth, how that moves and cracks and snaps, though it remains impossible to predict when earthquakes will happen. When we meet to discuss seismology are Rebecca Bell, lecturer in geology and geophysics at Imperial College London, Zoe Milden, lecturer in earth sciences and future leaders fellow at the University of Plymouth, and James Hammond, reader in Geophysics at Birkbeck University of London. James Hammond, what's an earthquake?

I think when we think about an earthquake, it's worth thinking about the forces that are causing these earthquakes. If we think about plate tectonics, this kind of jigsaw-like assemblage of plates that are on the surface of the earth, these are all moving around relative to each other all the time. sometimes they move away from each other sometimes towards each other sometimes rubbing up against each other

and these cover the entire planet these cover the entire planet right yeah and so these forces are building up stresses along the boundaries and as these stresses it's this energy essentially is building up over hundreds or thousands of years

why is that where does this energy come from the whole thing that drives the whole caboosh is comes from plate tectonics right so essentially the earth has one objective. It has had one objective for four and a half billion years and that's to cool down.

We're a big lump of rock floating in space and it's all this primordial energy so we're going right back here. There's all this primordial energy from the material coming together and it's trying to cool down. What's happening at the moment is the mantle is convecting

and that's driving these big tectonic plates that we stand and walk around on, it's moving that around on the surface. And you can imagine, as these plates come together, that's quite a lot of energy. I mean, it's very slow, about the speed your fingernails are growing. That's how fast they're moving. But huge land masses, there's a lot of energy coming together. Now, we obviously don't see a boundary between these tectonic plates, right? These are solid rock. But essentially, they're moving slowly away apart from each other or coming together and

stress is building up on the rock. And obviously if you build up enough stress, enough force, enough energy on the rock, it will break at some point. And this is why we get these huge amounts of shaking. We can get anything from a magnitude 9 earthquake like we see in Tohoku in 2011. Or we can get lots of little earthquakes,

such as smaller magnitudes. And we may get tens of thousands of magnitude 5s for every one magnitude 9 earthquake that we see. And so it's these huge forces that are happening over hundreds and thousands of years that are driving these earthquakes. And that's why we see primarily we see these earthquakes at the plate boundaries. Like they map out where these tectonic plates are on it. The Lisbon earthquake happened on All Saints Day, so there was concern about it that somebody up there wasn't looking after us.

But did that set off the first notable steps toward understanding what was going on with us? and initiating seismology. It suddenly had a big impact across Europe, but also the wider. I mean, Lisbon was devastated. The rulers of the time decided to survey the parishes of the region and ask them, what happened? How much shaking was there?

How many aftershocks did you feel? Did you observe anything unusual at sea or in the rivers or in the wells, even with your animals? And it was the first time that an attempt to, had been made to quantify what happened during an earthquake. So it was really the start of that science of understanding earthquakes. I think the big step forward in our understanding of what happens during an earthquake was the 1906 earthquake in San Francisco.

And from that came this, the elastic rebound theory that we talk about, where these stresses building up along a fault over hundreds and thousands of years and eventually snap to release this energy. And you, Rebecca, about how do seismometers work? What are they to start with? Yeah, so a seismometer is the name that we give to a device that measures the ground motion, and a seismograph is the instrument

which measures the ground motion and records it. So it's easiest, I think, to understand the technology if we talk about some of the earliest seismographs. So they involve a mass, which is suspended, so basically it's a pendulum. And effectively on that mass is basically a pen. That pendulum is above a drum of rotating paper. These are how the earliest seismographs work.

So the idea is that if an earthquake or any ground motion happens, the frame which this pendulum is on will move. The drum of the rotating paper will move. But the mass with the pen on, because it's heavy, it has inertia, it stays where it is. So if there's a lot of ground motion, we'll get a large line on the paper. If there's a small amount, we'll get a little bit.

So as those earthquake waves come through, that pen will begin to draw wiggles and waves on the paper. The amplitude of those waves will give us an idea of the amount of ground motion that there's been. Of course, we do things now digitally. So our modern seismometers still involve a suspended mass on a spring. They'll be surrounded by a copper coil within a magnet. When the earthquake passes, the casing will move.

that coil within the magnet will move, which produces an electric current, and that's what we measure as our measure of the ground motion, and that's then recorded digitally, and it can produce a seismogram digitally that we can then interpret and analyse. And that began to develop it. You can make your own waves, can't you?

Yes, so James, who you've just heard from, is an example of a passive seismologist, so he sits around and waits for earthquakes to happen to help him understand the earth, Whereas I'm an active source seismologist. I make my own bangs to study the earth. You make your own earthquake? I do.

So it's not as scary as it sounds. We make our own seismic waves, but you can make a seismic wave by dropping a weight on the floor, having a vibrating truck, using compressed air in the water to make an air bubble in a pressure wave. Or you can also use dynamite. All of those things will produce seismic waves, just a lot lower energy than earthquakes.

So in my field, our energy is a lot less than earthquakes so I can't image as deeply as James can for example but we can look at a lot of detail quite close to the surface. Do we now know what happened at Lisbon? Lisbon is a really interesting, a really interesting case. So as James explained, thanks to all of these surveys, scientists now can take all of that information

and begin to build maps of the intensity of the event so they can assess how much damage they're, was how people experience the earthquake and use that to come up with an approximate magnitude because of course in 1755 we didn't have seismometers so we don't know for sure what the magnitude was but thanks to those observations we can get an idea so it's still very hotly debated exactly what the magnitude was there are some suggestions that it might have been up to about an 8.5 so a magnitude 8.5 along the west coast of europe is is quite unusual. Earthquake of that size, we kind of expect them in places like Chile and Japan and Sumatra, we get these

very large earthquakes where we have a process called subduction, which is where we have an oceanic plate colliding with another plate where it gets pushed underneath it, so that that's called subduction, that plate ends up going deeper into the mantle. And these types of tectonic plate boundary produce huge faults. And the general idea is that the bigger the fault, the bigger the earthquake you could have. And it's still a bit of a mystery as to where exactly that fault zone is. So, Milton, we come to the Richter scale. What is it? And how did it come about? The Richter scale is one of these things that if you talk to most general sort of members of the public, about anything to do with earthquakes, they say, oh, yeah, I've heard about the Richter scale.

But it's actually not something that we as seismologists use very routinely today. This Richter scale was developed in the aftermath of the 1906 earthquake, which happened in California. And the way that the Richter scale is calculated is it uses the seismograph records that Becky talked about earlier, and we'll look at how big are the wiggles on those seismograph, which is basically telling you how much the ground has moved. And then convert that into the Richter magnitude, which would be numbers between three to ten, perhaps. The problem though now that we have is that the Richter scale was designed to be used to measure earthquakes in California and to use seismographs that come from a what's called a Wood Anderson seismometer. So it's a very specific type of seismometer you need in a very specific location to be able to measure the Richter magnitude.

The way that we measure seismographs nowadays is that we use digital records. so we can't use the Richter scale on these digital records and also earthquakes don't just happen in California they happen all over the world so the Richter scale really isn't used by seismologists anymore but it still comes up very very frequently kind of in the media when earthquakes happen around the world quite often news outlets will quote a Richter scale

whereas actually what they should be quoting is a magnitude scale but seismology was driving ahead Zoe, and the nuclear tests gave it a spurt because it was very useful there. Could you tell the listeners how? Yes, I think really as seismologists, in some ways we owe a lot to actually that period in history because that was really when seismology became a very quantitative and standardized subject around the world. In the early 60s, as the nuclear test plan treaty was being discussed, the US was initially quite resistant to signing any sort of treaty

because there was no way of actually monitoring that treaty and checking whether it was being followed or not. As a result of that, the worldwide standardized seismograph network was created in the early 1960s, and this was a global network of seismometers, which were all the same type of seismometer distributed across the continents. And that's what then gave us the first global picture of where our natural earthquakes happening around the world,

as well as telling us where were nuclear tests being carried out. Thank you, James. James Hammond, can you give some idea of the different energy waves created in an earthquake? When an earthquake happens, as we mentioned earlier, a huge amount of energy is released into the earth, and this propagates out in all directions in the earth. And it travels as sound waves. It's essentially deforming the rock as it travels through.

And certain types of wave that we call compression waves or P waves. What do you mean deforming the rock? What is that? Well, it's essentially squeezing the rock and shearing the rock as it's traveling through, right? The energy travels soon. The waves that squeeze the rock more are called P waves. They travel the fastest. The things will arrive first on the seismograph.

The waves that shear the rock travel a little bit slower. And we call these body waves. These travel down through the interior of the earth. And then we have a third type of wave, which is called surface waves, rather unimaginatively titled, because they travel along the surface of the Earth. And these are the ones that have big amplitudes. They travel even slower, but they have big amplitudes, and they can be very destructive. And it's the fact that they're deforming the rock,

which allows us as seismologists to use this as a probe to image and understand what the interior of a planet is made of. because if the rock is rigid and it's hard to deform it then these seismic waves travel fast and so if we can measure how long it takes let's say to go from an earthquake to our seismometer well that's telling us about something

about the strength of this rock and that could be related to the composition or the temperature or whether it's partially molten and so if we collect enough data we can build up an image of how the strength of the rocks, how the composition of the rocks, how its temperature may be changing inside the earth.

A lot of people think from school, I think, that the mantle is completely molten, and it's kind of bubbling away and convecting like a liquid. And thanks to earthquakes, we know that's not the case. So the mantle is a solid. It does flow, but very, very slowly. It's molten in some places where we have volcanoes, but those are actually quite rare, those places where the mantle actually melt. very specific locations.

So we need to kind of get away from this idea that the mantle is completely molten. So convection is really important in driving plate tectonics, but so too is the cooling of the crust. One of the key drivers of plate tectonics, we now think is that as the crust cools, it gets denser, so it begins to sink. We've already mentioned these subduction zones

that are really important. So the kind of coldest, oldest, densest oceanic crust begins to sink and that then pulls the rest of the crust or almost specifically what we call the lithosphere which is the earth's crust and the upper part of the mantle that's rigid it pulls that away and that's how we have opening at the oceans so this all starts to form a kind of cycle at seduction zones we've got lithosphere being destroyed we've got new oceanic crust and lithosphere being produced at mid-ocean ridges in the middle of the ocean and that together with convection cells is what kind of drives plate tectonics on earth.

James, I would add to that. I mean, when we look inside the earth, which we can do using seismology, right? And Zoe mentioned these global networks of seismometers that we have. This has allowed us to build a 3D image of the interior of the planet. And it's complicated and it's messy. And we do see these seduction zones. We can use seismology to, much like a doctor uses a CT scan to develop, use x-rays to build an image of your body. We use seismic waves to do the same for the earth.

And we can see these dense, cold slabs sinking all the way down almost to the core. And we can see hot material that the seismic waves travel through slower. We see that rising up and causing volcanoes like Hawaii, for example. Now it's still solid, this material, it's hot, but it's solid, because the seismic waves, certain sizing ways, can't travel through liquids very well. So we know it's solid from seismology. But when it gets close to the surface,

it starts to melt the rocks, and therefore we get these volcanoes. The only way we can look inside the planet is through seismology. James has alluded to this, that sheer waves don't travel through liquids. That's how we know that the mantle is solid, but there are other layers within the surface. the earth if you go even deeper that are liquid and we don't get sheer waves traveling through those

and you can actually do quite a nice little experiment to show this yourself for example if you take a hard boiled egg and a raw egg and spin them on the put them on a table down on a table and then spin them so what you're doing by spinning them is you're putting a sheer stress or a sheer force onto the egg now the hard boiled egg will keep spinning and spinning and spinning and it will spin for quite a while. It can take that sheer stress that you've put on it. But your raw egg, which remember has liquid inside it, you

try to spin it and it will stop spinning almost immediately. The liquid can't maintain that sheer stress and we use the same kind of principles to look at whether the mantle is solid or liquid and we see sheer waves travelling through it and therefore we know that it must

be solid. You're out in Greece Zoe doing your field work can you see around you? in your field, faults on the surface, and what do they look like? We can indeed, and that is, it's one of the reasons why we're in Greece. It's a fantastic place to study faults. Greece as a whole is extending. So what that means is that it's being kind of pulled apart in a north-south, roughly north-south

direction. And that creates what are called extensional faults. So that's where, it's a fault where either side of the fault, which is this line of geological weakness is getting further apart from each other. And the way that we see these in Greece is that these faults form, faults form these incredible flat rocky surfaces on the hillsides, which are inclined at about kind of 45 to 60 degrees. And so if you're looking onto a mountain, you can see this, you can see a fault. The way that that's been created is that the top of the mountain has gone up and the bottom of the mountain to the bit in front of you has gone down.

You can see them across hillsides, not just in Greece, but also through Italy and Turkey and other parts of the world. James, you spend, as I understand it, a lot of your time near volcanoes. Can you tell us about the connection between earthquakes and volcanoes? Volcanoes, I mean, in a fascinating place. I was lucky enough to be in La Palma last year working with a group on that eruption in the Canaries. and, you know, one of the fascinating things about the eruptions on volcanoes is that almost every eruption, in fact, pretty much every eruption, is preceded by some sort of activity. And normally, that would be include earthquakes.

And what's happening is beneath the volcano, as it goes into unrest, magma is moving. So magma might be You know Beneath the volcano there's a network of Magma sitting there Molten Rock distributed from shallow regions all the way down into the mantle Where it's being generated

And some of that magma will move into And collect into bigger regions So it gets to a bigger enough volume That it can break the rock Through the buoyancy forces or whatever And as it breaks the rock, that's an earthquake. And so we can track almost the movement of this magma.

And indeed, in La Palma, that's what they did. You could almost daily see the progression of this magma moving to the surface. And that's enough for them to warn people, move people out of the way. It's not enough to stop the eruption happening, but it's enough to make sure that loss of life doesn't happen. and indeed it didn't happen in that case. So earthquakes are a key part of understanding and monitoring volcanic eruptions.

Rebecca, Rebecca, Mel, not all faults are the same. Perhaps no two of the same. Can you tell us more about that? Of course, yeah. So this again goes back to this concept or this theory of plate tectonics and the way that these plates move. So we know that in some places we have tectonic plates moving apart,

which involves excephased. extension and things being pulled apart. So in that case, we end up with faults, which are exactly as Zoe described in Greece. They're what we call normal faults, and they're related to extension. And what happens is they're quite steep. How are they related to extensions? And what are the extensions? So these are occurring in places where tectonic plates are moving apart.

So as they're moving apart, that means they get stretched. So as the crust gets stretched, it begins to crack and it begins to break. and it begins to break and the cracks that form are what are called normal faults and that's the situation that's happening in the Gulf of Corinth in central Greece where Zoe is right now. So these normal faults

tend to be about 30 kilometres long or so. They can join with other faults and they can become longer zones. But the earthquakes we get on them tend to, they're not like the magnitude 8.5s, like the Lisbon one we started with. They tend to, the earthquakes tend to be a bit smaller. In other places, we've got tectonic plates moving side by side.

This is what's happening along the San Andreas fault, and this produces faults called strike slip faults. So these involve a lot of kind of horizontal motion. They can also involve some up and down motion as well, but primarily it's kind of side to side. And then we have those zones where we have tectonic plates colliding, and we've already mentioned the subduction zones quite a few times now. That's what happens when we have.

at least one oceanic plate involved in the collision. In other places like the Himalayas, for example, we've got two continental plates then involved in the collision. Those continental plates are much lighter and more buoyant. They're less dense than the oceans. That's why they're land and the ocean sinks into the mantle, and that's why we get the oceans. So when those two very buoyant plates collide,

one can't slip under the other, so they end up being pushed up into mountain belts like the Himalayas. And both of those cases where we have that collision produces a type of fault called a reverse fault or a thrust fault. And that's when we end up with one side of the rock being thrust up over the other. All of these different faults, you know, they produce different types of earthquakes at the seduction zones. As we mentioned, this is the location on earth where we get the largest faults on earth. So when you get one of those oceanic plates subducting, the boundary between those two plates is effectively a gigantic fault zone.

And these can be thousands of kilometres long, hundreds of kilometres wide. So these are the locations where you get magnitude nines or greater, like the 2004 Boxing Day, Sumatra earthquake, the 2011 to Hokku earthquake in Japan. And because those thrust faults, those mega thrust faults, as we call them, are underwater, that also means that there are a very big tsunami risk as well. So we've got huge earthquakes and the potential for a very large tsunami. Zoe, Zoe Mulden, what's meant by seismic hazard? What are the relevant factors we need to know about that?

So at its most basic, seismic hazard is the danger to human populations as a result of earthquakes. So if you think about the saying, you know, there's that saying of, you know, if a tree, falls in a forest and no one's there to hear it, does it make a sound? With earthquakes, it's kind of similar in that if you have an earthquake that happens in the middle of nowhere, nobody lives there, what's the danger posed by that earthquake? Probably not very much. Whereas if you have an earthquake that occurs right underneath a major city, then obviously that is going to affect the people living in that city. There are different types of hazards associated with earthquakes. So For example, the ground can actually break and move and you can get steps formed in the landscape due to faulting.

And we would call that a primary hazard. And this actually happened in the 1906 earthquake in California that it was because of ground ruptures or breaks at the earth's surface that broke gas pipelines across the city, which resulted in fires, which actually caused a lot of damage. what we call secondary hazards, things like tsunamis and landslides can be triggered by earthquakes. So if you have an earthquake that happens underwater, and particularly if it's a thrust earthquake or a subduction earthquake like what Becky was just talking about, that can create tsunamis like the 2004 Sumatra or the 2011 Tohoku tsunami. I ask the question that a lot of people, and we want to answer. why can't we predict when Earth's Glac will happen? You have all this instrumentation, you've got this history,

you've got many fine brains working on it all over the world. Why can't you predict it? No, sure. I mean, I'm happy to answer it because I'm not one of the fine brains working on this particular question. But if I go back to what I was saying about the volcano eruption, I mean, we have two advantages with a volcano that do allow us to offer some sort of forecast.

It might just be days or hours, but we can do that. One is we know where a volcano erupt is going to erupt, right? Because there's a big volcano there. So that's one advantage. The other is that we have these precursory signals that I mentioned, these earthquakes or maybe gases being emitted or ground deformation that's linked to magma moving. For an earthquake, we have no clear precursory signals, not that we've observed systematically to date.

So the first thing we know is when a big earthquake happens, right? It's very hard for us to see anything happening before that time. Now, over longer timescales, we can measure the strain that's built up, so how the ground is moving associated with these tectonic forces, and that helps us have a kind of what we call a probabilistic approach. we could say, okay, in this area, there's a high chance that you might get an earthquake at some point in the next 10 years, 20 years, something like that. But that's about as far as we can go. One area seismology is really helping, though.

And this is in places like Japan do this very well. And California is more recently. Is because we have these different types of seismic wave and they travel. at different speeds. So the P wave, which is the wave that arrives first, is small in amplitude and it travels fast. And so if we have a dense network of seismometers deployed within a region,

we can see this P wave arriving quite early on. And then what we can do, and what they do in Japan is they alert everyone immediately that an earthquake, the shaking from an earthquake has happened and shaking is on the way. You mean everyone? Everyone. Everyone's phones, televisions, radios, computers, people will get an alert. And you may get seconds of warning before the big shaking happens, maybe a minute or two. But that's enough for us in the studio here to jump under the tables or to leave the house or to shut down the gas lines or stop the trains running. And, you know, so it's a very effective way of saving lives in an earthquake, as long as as the education of the population, people know what to do when they get this alert.

And so I think we can successfully live in these areas that are prone to earthquakes. And, you know, we have ways of making buildings that are resilient to this shaking. We have education that we can provide to people. I would say it's not necessarily widely distributed. You know, in certain parts of the world, there's a much higher. susceptibility to the hazard, but we can do it as a civilization. From seismology, we can work out the broad areas where earthquakes are going to occur. So, for example, we know, with me here in Greece, we know that Greece is an earthquake-prone country.

There have been earthquakes in the past, which we've recorded both from seismometers, but also from historical records. So we know that more earthquakes are going to happen here in the future. And so there are a couple of different ways that we can try and work out, as James was talking about a probabilistic approach, you know, what's the probability of another earthquake happening soon? The most common approach is that what we would do is look at the historical seismicity and say, where have earthquakes been recorded over the last 50 to 100 years? Okay, that tells us where earthquakes might happen in the next 50 to 100 years. But the problem you get is that those, the faults, like what I'm out here looking at in the field, these faults don't have earthquakes that frequently.

Some of them might only have earthquakes every thousand years, 10,000 years, maybe even longer. And this is a real challenge of seismic hazard, is trying to match up what do we detecting the site from seismometers, and how does that relate to the long-term record of faults and earthquakes that we see geologically? that might connect to Rebecca. I certainly want to ask you that there are some earthquakes that are so slow they're almost imperceptible but important when they happen. So can you tell us about why they're so slow? Absolutely.

So up until about the year 2000, we kind of understood that faults could move in a couple of different ways. We understood this elastic rebound theory, which James opened the show talking about. I always hoped you'd come back to that. It's too good to miss. So elastic rebound. So Elastic Reband theory is also called stick-slip behaviour.

It's almost as good as yesterday. Absolutely. So this is the idea that a fault will stick for a period of time that could be tens, hundreds, thousands of years, and once enough stress, enough pressure is built up to overcome the friction on that fault. It will then suddenly slip in seconds to minutes and produce our earthquake. So we knew that some faults failed. slipped in this way, but we also knew that some faults could slip what we call a seismically, which is where the two blocks of rock either side of the fault move

past each other very slowly at the rate your fingernails grow as we spoke about earlier the rate tectonic plates move without any earthquakes at all so they're just kind of sliding very passively so we kind of had this idea that faults failed in these two end members but around the year 2000 scientists began to to use GPS technology, global positioning system, the same technology that we've all got in our smartphones that we use in our maps to find our way around. GPS stations were put out over countries prone to earthquakes,

particularly in areas where we have these subduction zones, where we've got these huge megathrust faults. And the idea was that they were going to monitor how much the continent deformed and get an idea of what the fault was doing. So when these GPS, networks were first analyzed at the beginning of this century, we could see that there were periods of time where it looked like these big megathrust faults in places like New Zealand, for example,

were locked and the GPS stations were kind of being pushed very, very slowly because the fault was locking and stress was building. But then in the data, we could see evidence that there were times where the GPS stations were moving back the other way. And that's what we'd expect to see if there was an earthquake. And the amount by which they were moving was as much as if up to magnitude 7 in some cases earthquake was happening. But the strange thing is that nobody felt anything. Nobody, no seismometers detected anything either. So this was a completely new type of fault slip style. So it's, you know, quite incredible that even, you know, in the year 2000, we can discover something so new and so fundamental. So these events are now called slow slip events. They've

been detected at lots of places around the world, not just its subduction zones, but in other places too, we see them on the San Andreas fault as well. So their characteristics are that they involve fault slip, which is faster than the tectonic plates move, so it's faster than the rate your fingernails grow, but the movement on the faults is much too slow to produce the energy, to produce seismic waves and shaking. So we detect them thanks to those GPS networks. So a really big question for scientists right now is why do we get slow slip? What is it about the fault zone that means it fails in this way rather than the stick slip behavior? And also what does it mean for seismic hazard? If we have these slow slip events that seemingly are quite benign and safe,

does that mean we are relieving stress on those faults, meaning that the seismic hazard reduces? But these slow slip events, you know, they're not that big. They're happening over parts of the fault a few tens of kilometres in length. So if we have lots of these slow slip events, could that actually be adding stress to the parts of the fault surrounding it? So could these slow slip events actually trigger a large earthquake in the future?

So we don't really know at the moment whether these slow slip events almost are a good thing for seismic hazard or a bad thing. And it also raises the question of, if there is a link between slow slip and large earthquakes, could we use the presence of having slow slip events I don't want to say the P word, but to almost predict

that a large future earthquake might be more likely. But at the moment, the science, you know, it's only 20 years old, so it's still in its infancy. So we've still got a lot to learn about what slow slip means for seismic hazard. James, you're now understanding the faults deep in the ocean with your seismology.

How far have you got with that? Well, the problem with the oceans is it's hard to deploy a seismometer. in the ocean, right? Because it's covered in water. We like to bury our seismometers in the land. And so if we go back to the global seismic networks that Zoe was talking about,

they were deployed all over the globe, but we had to rely on ocean islands, essentially, to fill in those gaps. So we have these huge data to coverage issues. And the problem there is going back to this doctor, the x-ray or the CT scan analogy, if you have parts of your body that you can't fire x-rays through or you don't have a sensor to record that energy

then you'll have fuzzy bits of that image the doctor will look at it and say okay I can see what's going on over here but this bit is a little bit unclear because we've not put any x-rays through that bit of the body we have the same problem with the earth in that the sources are earthquakes that happen along plate boundaries and our receivers are seismometers

that we typically can only deploy on land. So some of the research Becky does, for example, we deploy seismometers in the oceans. You basically push a seismometer off the side of a boat. You let it sink down to the bottom. You wait two years and you keep your fingers crossed and hope it floats back to the surface and you can find it and download the data.

But they're very expensive. And the ocean is very, very big. so it's hard for us to use those to cover the oceans. One of the really exciting developments, I think there's two. One, there's kind of so-called floating seismometers that essentially use set off drifting on currents in the oceans. They can record pee waves, energy that arrives, and they transmit their data every so often,

and that's helping us to fill in some of the ocean gaps. But one of the things I think is really exciting, and this is very new, this is very much in its infancy is people are now using fiber optic cables as seismometers and as a seismic wave travels through

a fiber optic cable it changes the shape of that cable slightly changes its scattering properties and so we can interrogate that we can fire light down the cable and we can reconstruct the seismic signal as it's traveled through the whole of this cable

and of course the ocean is littered with fibre optic cables for our communications and everything, and I'm sure we'll be putting many more down there. And so there's a lot of, a bit of excitement in the seismology community that this might be a way that we can instrument the oceans. We're coming to the end of our time now. Zoe, what for you are the most interesting technical advances at the moment, and what are you looking forward to?

There are lots of areas, and I think my particular interest is in seismic hazards. So how can we use seismology and other observations to better understand the seismic hazard. Because ultimately, people are affected all around the world. And James has already alluded to the Elastic Rebound Theory. And so the idea with that is that if stress is building up at a fault, a known constant rate, at some point it will reach a level that the fault can't sustain anymore and there will be an earthquake.

If it was that simple, all we would need to do is seismologist is measure. how quickly the fault is having stress put on it and how strong the fault is. And we'd be able to predict earthquakes and go home and the three of us would probably be out of jobs. But we know that that Elastic Rebound Theory model is not that simple. Around the world, we observe that earthquakes can be clustered together in time. So for example, where I am here in Greece, there were about 10 large earthquakes up to sort of magnitude 6 and 7s in 100 years. So that's about one earthquake every 10 years-ish. But there haven't been any big earthquakes since 1981.

Now, 1981 is 40 years ago. So why were there so many earthquakes in that previous 100 years and so few earthquakes in the last 40 years? That's a really hard question to answer. But it's a really important question to answer to try and better understand the hazard that people that living in this area and, you know, other areas of the world face.

James. seismology has now become a truly interplanetary discipline. We've only been recording earthquakes on Earth, I think the first teles seismic earthquake, an earthquake that happened in Japan, was recorded in Potsdam at the end of the 19th century. But now... That means an earthquake that happens in one place,

but the river vibrations go right through the earth long, long way. Exactly. And having you affected another. Exactly, right? And we record these all the time. magnitude, anything of them about magnitude five and a half upwards, we can pretty much record that anywhere on the planet using the modern day instrumentation. Now the exciting thing is,

2018, we deployed one of these on Mars. This is the Insight mission to Mars. It landed a seismometer. It even lifted that seismometer off the lander, put it on the surface of Mars. Incredible engineering achievement. And it's been sitting there for about three years, three or four years now, recording Marsquakes, right?

And we recorded hundreds of these Mars quakes, and papers came out just last year telling us what the size of the core was on Mars and that it's still molten, right? And this is remarkable. And, you know, we've had seismometers on the moon. The Apollo astronauts were deployed seismometers on the moon. We've recorded moon quakes.

Future missions are planned to go, I think there's a future mission plan to go to tight. one of Saturn's moons, it will have seismometers. Hopefully soon we'll record Titan quakes. And we can use this to image, understand how all of these other planets and planetary bodies form what their internal structure is. And so I think, you know, seismology is going off into the stars, basically, to explore other planets now, much the way we did on Earth over the last 100 or so years. Well, thank you very much.

Thank you, Zoe Milden. Rebecca Bell and James Hammond and to our studio engineer Emma Hath. Next week we'll be discussing Max Weber's ideas on charismatic authority and why some leaders have it for better or worse. Thanks for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. So it's between this week.

So I'm just going to ask you something with you, what would you like to have said that you didn't find you had time to say and that's the same applies of the other two as well. One of the things I wanted to say, which is actually a very key misconception I find, is when we talk about magnitude 5, we've sort of talked about like a magnitude 5

or a magnitude 7 or a magnitude 9. And that scale, as we've talked about it, is linear. So, you know, it goes 4, 5, 6, 7, 8, magnitude 9, which is kind of the biggest earthquakes we've recorded. But that scale isn't linear. So it's not that a 5 is a bit bigger than a 4, and a six is a bit bigger than a five and so on. Actually, a magnitude five is 30 times bigger than a magnitude four.

And a magnitude six is 30 times bigger than a magnitude five and so on and so forth. So towards the beginning of the programme, James talked about kind of one big earthquakes, so like a magnitude nine in Tohoku or lots of like small magnitude five earthquakes. That's because a magnitude nine is 30 to the power of four, which I'm afraid I can't do in my head, times more energy than a single magnitude 5 earthquake. If anyone has a calculator, please feel free to calculate that. Rebecca, do you want to come in? Yeah, sure.

So I think what I would like to talk about as a bit more about this active source seismology. So we talked quite a lot about how we image the interior of the earth using earthquakes, and we can learn about the core and the liquid outer core and the mantle and it. properties, but using this active source technology, which again is when we make our own earthquakes, our own seismic waves ourselves, we can actually image the upper kind of 15 kilometres of the earth in an incredibly fine detail. So we can image, for example, the upper kind of few tens of meters to the scale of centimetres. And this is really useful for laying pipelines and the fiber optic cables perhaps. But we can also image down to 15 kilometers, which is a good chunk of the

that's crust to resolutions of tens of meters. And I'm not sure everyone kind of appreciates that, just how much information we can get out with seismic waves alone, which are basically just sound waves. So using this technology, we can look for these faults underground, so the types of faults that Zoe is seeing at the surface. You know, sometimes those are completely buried, and we don't have any of evidence of them.

Or we'd like to know how they're dipping, what their slip rates are, if we can map horizons that we can date either side of the fault, we can work out how fast they're slipping. That's really good information for seismic hazard, and we can get all of that from this active source technology. It's probably not surprising that it was a really big technology used by the oil and gas industry to find reservoirs for drilling.

It led to drilling of fewer dry wells, because we can pinpoint exactly the best pace to drill. And now, as we're coming to this energy, transition, I think this technology is going to have a really big role in things like carbon capture and storage to work out reservoirs which are suitable for pumping CO2 in and such that the geology is right that we can be confident that the CO2 doesn't escape. And it will also be useful for things like geothermal energy so we can work out the fault networks and where might be good locations to drill for hot fluids coming up from depth.

I like, can I? Yeah, I was actually, I was going to bring things very back down to Earth. with something else that I, James alluded to in the podcast and I wanted to say in a little bit more detail because James talked about these earthquake early warning systems and he talked about people kind of getting under the table and things like train stopping using these earthquake early warning systems and he talked about the value of education. Now there are places in the world where they do earthquake drills

so in the same way that we in the UK do like fire drills every so often at work or school or wherever. There are places in the world that do earthquake drills. And while we don't get earthquakes in the UK, you know, a lot of places that people might go on holiday are earthquake-prone countries. So Spain, Greece, Turkey, Italy, you know, all of these are pretty close, you know, within Europe.

And all of them can have damaging earthquakes. And if you feel an earthquake, the advice is very, very simple. It is drop, cover, hold. So in other words, drop is get under a table or a bench or a bed or some way of kind of protecting your head, which is covering your head. So, you know, if you are in a building and you're in an earthquake,

let's imagine, you know, a picture falls off the wall and hits you on the head. You know, you're not going to be very happy if that happens. But if you're under a table, then that's actually protecting yourself from those kind of small things falling off the walls or off the ceilings. And hold on. So basically hold on until the earthquake stops and then get out of whatever building you are in. And yeah, James kind of alluded to that by saying get under a table. But it has been proven to save lives. And it can make a really, really big difference to how badly affected you would be.

you will be by an earthquake if you're inside a building and you experience one. Rebecca. That's, yeah, completely agree with that advice. I guess the only caveat is if you were at the coast. So the tagline at the coast in many countries is long, strong, gone. So if you feel an earthquake that goes on for a very long time or it's very strong, those are good indicators that it could be tsunamogenic. And you should immediately go to high ground and get away from the coast.

So, yeah, what was it? Are we drop, drop, cover, hold? Drop cover, hold, long, strong, gone. I don't think we have a tagline for mouse quakes yet, so we don't have to worry too much about that one right now. Well, thank you all very much indeed. That was terrific. In our time with Melvin Bragg is produced by Simon Tillotson.

Death by Conspiracy, a new podcast from BBC Radio 4. Gary Matthews was an artist and photographer, a familiar sight on the streets of his hometown in Shropshire. But in the last few years, he was drawn to conspiracies. And when the pandemic hit, Gary, who was suspicious of experts, ignored the rules. When he died of COVID-19, his distraught family and friends were left searching for answers. I'm Mariana Spring, the BBC's specialist disinformation reporter, and I've been investigating what happened to Gary by delving into the conspirators.

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