Podcast: Monitoring earthquakes
14 April 2009 by Richard Hollingham
Earthquakes are a deadly natural hazard. Every week, somewhere around the world, an earthquake occurs that's big enough to cause serious damage to people, roads and buildings. Seismologists are unlikely to be able to predict when or even where an earthquake will strike next, but they can reduce the hazards they create.
Earthquake experts Dr Brian Baptie and Dr Susanne Sargeant from the British Geological Survey (BGS) talk to science writer and broadcaster Richard Hollingham about how the BGS monitors earthquakes all over the world - including the UK.
There are anywhere between 100 and 200 detectable earthquakes every year in the British Isles and a network of UK-based seismometers set up by the BGS can pick up ruptures in the Earth's crust not just in the UK, but from all over the world. Every time there's an earthquake in the UK, people can go to the BGS website to record their experiences.
To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.
Female voice: The Planet Earth podcast, presented by Richard Hollingham.
Richard Hollingham: This really is a stunning view out over the city of Edinburgh, from Blackford Hill, I'm inside the Royal Observatory of Edinburgh with Brian Baptie, who's a seismologist with the British Geological Survey. Now Brian we're in a lecture theatre, surrounded by chairs and tables, but curiously there's a door in the floor here.
Brian Baptie: Well this trap door leads down into an underground vault and inside the vault we've got one of our seismometers, which is a device that we use to record earthquakes from all around the world. And it's deep down, you have to go down some stairs, through this trap door in this unlikely looking place, through another couple of doors, and once we're in there we'll be inside the vault and you'll be able to see the equipment.
Richard Hollingham: Okay, so let's move the tables and chairs out of the way first, let's get those shifted.
Brian Baptie: That's the hatch open.
Richard Hollingham: So we've just opened up the hinge of the large trap door in the floor to reveal - it's like Harry Potter this - a staircase that goes down beneath the lecture theatre, so let's head down the stairs.
Brian Baptie: That's the first door... [door opening] second door... [door opening] and finally this is the entrance to the vault.
Richard Hollingham: Okay, so we've come into what appears to be a concrete-lined room, with a bit of water dripping from the ceiling, and there are three hefty concrete blocks on the floor, like enormous pieces of Lego.
Brian Baptie: Well these are what we call 'piers', and they're built directly onto the rock of the hill. So this hill is an old volcano and these piers are built directly onto it, and if you look around you'll see that the floor is actually suspended, so that we, when we're walking around on the floor in here, we don't actually come into contact with the piers, so our vibrations in here...
Richard Hollingham: Oh I see, so this is a hollow, a hollow floor...
Brian Baptie: So it's a suspension floor, if you like. And on the piers, on each of the piers, there's a seismometer, which looks like a very large tin can, but they're actually incredibly sensitive devices, they can pick up vibrations that are around thousandths of a millimetre.
Richard Hollingham: And what are you picking up then, on these seismometers?
Brian Baptie: Well we pick up earthquakes from all around the world. So large, or even moderate sized, earthquakes from the other side of the world - places like Indonesia, Japan, Tonga. We pick up earthquakes from all these places on a fairly routine basis. We also pick up vibrations that are rather closer to home and we actually detect between 100 and 200 earthquakes that come from the British Isles and immediate offshore area every year.
Richard Hollingham: So 200 a year in the UK?
Brian Baptie: Up to 200 a year.
Richard Hollingham: Now this is part of network isn't it? This isn't the only one. There are, what, secret vaults around the country?
Brian Baptie: Yeah, we have about 100 instruments all around the country. Not all the vaults are as big as this one, some of them are in underground tunnels or old bunkers. Some of them are just buried in holes in the ground. And the data from all those instruments is transferred automatically using the internet back to our offices in Edinburgh, and then we can analyse that data and use it to work out exactly where the earthquake was, when it occurred and how big it was.
Richard Hollingham: Well we made it out of the vault and into the offices of the British Geological Survey. Now Brian this office is where you used to analyse the sounds of earthquakes.
Brian Baptie: That's right, this office is what was called the replay lab and, although nowadays we record all our data digitally and analyse it using computers, about ten years ago all our data used to be recorded on analogue magnetic tapes and our analysts would listen to these tapes - speed it up about 50 or 100 times - and then they would use their ears to tell the difference between different types of seismic events.
Richard Hollingham: And you've got some of the recordings of earthquakes, some of the things you can hear.
Brian Baptie: So that was a magnitude 5.4 earthquake from North Wales, recorded somewhere in Britain. It was actually the largest earthquake that we have a recording of in our archive. You can tell when you listen to it, it's actually a really quite a distinctive double sound, and that's because there are two seismic waves that propagate when there's an earthquake, through the Earth, and those travel at different speeds and arrive at the receiver at different times.
The next one is an underwater explosion. So, typically we pick up quite a few of these underwater explosions every year, it's when the Navy quite often they find old World War 2 mines, or perhaps the Navy are doing some kind of naval exercise, where they're setting off explosions in the water, and we can pick them up.
Richard Hollingham: Sounds like a shot being fired!
Brian Baptie: That's right. So, much, much higher pitch, completely different type of signal, we didn't have the distinctive double bang, we just had this relatively high pitched pop, if you like, which you can think about in terms of something going pop in the water and creating a huge bubble.
Richard Hollingham: And the final one.
Brian Baptie: So the final one is a magnitude 6.9 earthquake from Japan. So the vibrations from this earthquake have travelled all the way through the Earth and been recorded here.
[Deep, long rumble, like thunder]
Richard Hollingham: So, let's get this straight, that was a recording made here of an earthquake in Japan, on the other side of the world?
Brian Baptie: That's right, when you have really large earthquakes, a huge amount of energy is released and that energy travels all the way through the Earth and we can pick up the vibrations from that earthquake on the other side of the world. Although nobody would be able to feel, I mean these are still tiny vibrations, I mean they are of the order of thousandths of millimetres, something like that. We can pick them up and then we can put the signal together, speed it up and listen to it, and that's what it sounds like. So it's quite a lot deeper than the other two signals we listened to, it's like a long, low rumbling that lasts for several seconds.
Richard Hollingham: Now there is another way of measuring the effects of earthquakes. And with me here as well is Susanne Sargeant, who has a tea trolley, you have a tea trolley with cups and saucers, and there's a little jug on here as well. And you use the sounds that these make to monitor earthquakes?
Susanne Sargeant: It's certainly one of the things that you might expect to hear during an earthquake. If you listen to this you can hear, as I roll the trolley, you can hear the clatter of the crockery [clattering crockery noise] and we can use the effects of ground shaking on everyday objects like cups and saucers, furniture to look at the strength of shaking. So these everyday objects act just like instruments and they give us another view on how an earthquake shakes the ground.
Richard Hollingham: How do you actually do that, every time there's an earthquake, say in the UK, you're going round asking people what they felt and the things that happened?
Susanne Sargeant: Now with our website we have an online questionnaire that the public can fill in, so after an earthquake they are invited to come onto our internet page and fill in the questions. So we ask questions like: were they sitting or standing up, how did the shaking feel, was it rolling or swaying, jerky, side to side, were they frightened, were the animals around them frightened, did people notice it, did people run outside? And, as the ground shaking becomes stronger, these effects on people, animals, everyday objects, and even buildings, become stronger.
Richard Hollingham: And how do you use this information, whether you've gathered the seismic results or you're listening to them or you've got the surveys of your clattering cups - how is this useful?
Susanne Sargeant: Well we use this information in seismic hazard assessment, which the engineering community in the UK then use to design buildings, like nuclear power stations and dams - ones which are sensitive structures. Generally, it's not needed for house building in the UK, but it is needed for these larger structures.
Richard Hollingham: Brian, if we can just come back to you, we've been to the vault, we've heard the sounds of earthquakes, we've even rattled tea cups, how much do you now understand about earthquakes and their impacts as a result of these sorts of... these sorts of studies?
Brian Baptie: Well earthquakes are probably one of the deadliest nature hazards that we know about, every week somewhere around the world there's an earthquake of a size that's big enough to cause some serious damage. And, although we really have improved our understanding of earthquakes over the last 20 or 30 years, there's still a lot of unanswered questions. For instance, we can't predict earthquakes, and that's the holy grail if you like, we'd really like to be able to do that, whether we will or not, we don't know.
What our work here is really based on is trying to improve our understanding of the hazards that are posed by earthquakes, and by combining the type of studies that Susanne talked about, looking at what people feel during an earthquake, and also combining the information that we record on our seismometers, in this country and all around the world, we can use that to try and build up a better picture of how earthquakes behave. And then try and use that to minimise the risk that earthquakes pose to people all around the world.
Female voice: You've been listening to Richard Hollingham talking to scientists funded by the Natural Environment Research Council.