This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites. I'm Oliver Strimpel. Unless you live in a place like Japan or California, most of us have never experienced an earthquake, but we certainly hear about them regularly, especially when the damage they wreak is newsworthy. This podcast focuses on earthquakes, the geological faults on which they occur, and their impact on humans. James Jackson is professor of active tectonics at Cambridge University and an expert on the geological processes, including earthquakes, that are shaping our continents today. James Jackson, welcome to Geology Bites. What's going on, geologically speaking, when an earthquake strikes?

James Jackson: What happens is rocks break and slide past each other on surfaces called faults. You could think of a knife cut in the Earth, and the rocks are sliding past each other on that knife cut. Now, normally it's held together by friction, and you have to overcome that friction. So imagine you have a brick on a table and you push the brick. You have to push a certain amount before it actually jumps forward, and that's what's happening in the earthquake: friction which is being overcome. And it's the shaking as the rocks grind past each other, doing that movement, which causes the damage. The earthquake people feel is the shaking, but the cause of it is this breaking of rocks on faults.

Oliver Strimpel: So do we know where all these faults are?

James Jackson: We've got a pretty good handle on that, because what happens is that they move again and again in earthquakes. The same fault will move again and again. It only moves a few meters, even in the biggest earthquakes, but given the huge time we have on Earth, after thousands of these earthquakes, you actually make the landscape. So the landscape is often a clue to where the faults are, and so reading the landscape is a bit like learning to read a new alphabet. Once you speak the language, you have a head start on where these things are.

Oliver Strimpel: Does that mean that if there are faults away from landscape features, we don't know where they are?

James Jackson: Well, most of the faults have some effect on the landscape. The faults can either move up and down or sideways. Moving up and down makes mountains, or it makes basins and lakes, depending which side of the fault you are on. And so that's a help in places where this is going on. The other faults move sideways, as they do, for example, on the San Andreas Fault in California. They still end up as features in the landscape, because as the rocks grind past each other, the faults get broken up, and they get easily eroded by streams and rivers and so on. So they quite often make linear valleys, or scarps, or features in the landscape, which are quite often visible from the air, for example in air photographs or satellite photographs, and they're often followed by roads and railways. So people exploit these things also.

Oliver Strimpel: So when an earthquake strikes, how much movement might occur across a fault?

James Jackson: Well, that is basically what determines the size of the earthquake, or the magnitude, or the Richter magnitude, as the press refer to it. But even in the really, really biggest earthquakes, the movement is 10 or 20 meters. The biggest we know of in modern times was the one in Japan in 2011, which moved 50 meters, which was extraordinary, but that really was unusual. Most of the small earthquakes will move just a few millimeters or so, to meters. So that's really what determines the size of the earthquake. So for a magnitude 9 earthquake, you're talking about maybe 10 meters or more of slip; for a magnitude 5 earthquake, you're talking about a centimeter or a few centimeters. That's really what determines the size of the earthquake.

Oliver Strimpel: And how could we measure this slip?

James Jackson: Well, there are various ways of doing it. Sometimes it breaks the surface and you can just see it, because actually the magnitude also determines how long the faulting is. So the great earthquake in Sumatra in 2004, which produced the tsunami, for example, was 1,200 kilometers long. That's from here, where I'm speaking, Cambridge, near London, to Rome, right? That's a great length, and it moved 20 meters or so, a huge amount. So that kind of thing is easy to spot. The smaller ones are often underground and don't have any motion visible at the surface at all. But we have very sophisticated ways of seeing what's going on. We can tell from the sound, or shaking, that the earthquakes generate how much slip there's been. That's the science of seismology. We can see how much the ground surface round about moves by looking from space with radar images or optical images. And we also now use GPS to do the same thing. GPS is the technology that is used for your satnav in the car, for example, which, if you process it carefully enough, will show you how things move to within a few millimeters. You don't need that accuracy for your car, but we can do it when we use it for our purposes.

Oliver Strimpel: So did we do that, for example, for the big 2011 earthquake in Japan?

James Jackson: Yes, you do. Because actually, and this is quite interesting, the analogy I gave you of pushing a brick across the surface of the table isn't quite right. Imagine you had a sponge and you push the sponge across the surface. The sponge will actually compress quite a bit before it suddenly jerks forward. And that's what happens to the rocks. So you can see the rocks actually bending and bulging before earthquakes, and you can see after earthquakes how they relax again and they change shape. So after the great Japan earthquake in 2011, the whole of Japan unbent and stretched; it relaxed about 3 meters. So Japan was 3 meters wider after the earthquake than it was before.

Oliver Strimpel: And do we measure that by having GPS sensors on each coast?

James Jackson: Well, yes. In fact, there are hundreds or thousands of GPS sites measuring all the time, particularly in places like Japan, and that's increasingly true throughout the world. And part of the reason is to monitor earthquakes and so on. These sites will tell where they are relative to each other to within a few millimeters, so if they move several meters, that's very, very easy to spot. But actually, to be honest, the main reason they're being installed everywhere around the world is that this is now how you define property boundaries on maps. You no longer say the boundary of my house is where that tree is, or where this river is; you just give it a latitude and longitude from the GPS, and that's the basis now of modern surveying. So that's why it is now rapidly expanding as a resource.

The other thing you can do, and this is another modern trick, which is very interesting, you can do this with radar images from space or optical images from space, is you take a photograph of a large area and you divide it up into little pixels, little patches on the ground, maybe 50 centimeters across, and then you take another picture after the earthquake. Round the edges of your picture, the pixels haven't moved around. So you simply superimpose one on the other, and round the edges of your picture, where the pixels haven't moved, they all overlap perfectly. And where the fault is, they've all moved slightly, and you can measure how much they've moved, and this will again give you the movement of the ground, to within a few centimeters. It's not as accurate as GPS, but you cover a much wider area, instead of just a single point. And these things are very powerful when they're all used together. So if you use seismology and the space imaging and the GPS all at the same time, you really can nail what happened in extraordinary detail, which we could only have dreamt about 20 years ago.

Oliver Strimpel: That's amazing. So with these satellite image maps, can we sort of actually see this building moved so many meters over that way, and river move that way?

James Jackson: Yes, indeed, you can, easily. And actually, this is a bit of a pain in the neck for land surveys in countries like New Zealand. Their national grid is moving around all the time. It has to change with time, because this is happening throughout the country, and the same is true in Japan. The downside of measuring things to within a millimeter is that most of them are moving by a few millimeters a year.

Oliver Strimpel: So although most of the earthquakes we hear about happen in areas that we know are high risk, like, you mentioned, Sumatra and Japan, and I suppose that applies to the whole western rim of the Pacific, as well as places like California, earthquakes actually do also happen elsewhere, but much less often. Why is that?

James Jackson: Yes, this is an interesting feature of our modern Earth today: the really, really big earthquakes happen around the rim of the oceans, in the Pacific, and in places like Indonesia. But that is not actually where most people die in earthquakes. Many more people die in earthquakes in the middle of continents, and particularly in Asia. So the great earthquake belt stretches all the way through the Mediterranean, the Middle East and into Asia, all the way to China, through places like north India, Tibet and Central Asia. That area, which is a huge, huge area, has smaller earthquakes, but they kill many more people. And there are multiple reasons for that, but the reason why it's so different is because the continents behave in a different way from the oceans. The oceans are really the story of plate tectonics, which everybody learns about now, even in primary school: that the Earth is split up into these spherical caps which move around relative to each other, and the earthquakes happen where these caps interact with each other. Round the edges of the oceans, in places like Japan, South America, Central America or Alaska, for example, what happens is the ocean slides underneath the adjacent continent, rather like a conveyor belt. Imagine a conveyor belt, or an escalator, the staircase going back into the ground. That's what's happening to the ocean as it slides under the continent, and that interface, where the ocean slides under the continent, that's where the earthquakes happen. So there's essentially one fault which is doing the job, and it has to move pretty often, because it's moving quite fast, typically a few centimeters per year.

Now, the continents are quite different, because the continents are not really part of that story. The continents are like floating on these plates; they're like passengers. And they don't cause any trouble if they're just an island in the middle of the Pacific or something. You get a problem where one continent bashes into another continent. And this is the problem with India, for example, which belonged adjacent to Madagascar and Antarctica and Australia in the Southern Ocean. It broke away and crossed as an island in the Indian Ocean until it bashed into Asia. And you have a situation where, because the continents are made of different material from the ocean, much less dense (the continents are like a life jacket), they simply won't go down the hole like the escalator or the conveyor belt, and they just bash into the next continent, and the whole thing crunches up. And so what's happened in the case of India is India has smashed up Asia all the way from the edge of the Himalayas to Mongolia, which is 3,000 kilometers further north. The whole place is smashed up. So there are many, many earthquakes over a much wider region, instead of essentially a single big fault and big earthquakes, as you have around the edges of the oceans.

Oliver Strimpel: So what happens over millions and millions of years, as movement happens repeatedly over either a single fault, as you mentioned, at the plate boundaries, or this massive network of faults that you may have in the interior of a continent?

James Jackson: Well, India collided with Asia about 50 million years ago, and it's going at about 4 centimeters per year. It's not like a car crash; it's not stopped. It's still plowing into Asia at that rate, 4 centimeters a year, about the rate your fingernails grow. And over 50 million years, India has plowed 2,000 kilometers into Asia and crushed it up. And that's why there are hundreds and even thousands of faults in that part of Central Asia. And because each of them is contributing to that job of allowing India to move north, each particular one doesn't have to move very often, whereas the faults offshore Japan or South America, for example, have to do all the work. They're moving frequently because they have to do all the work; there's nobody else to do it. So you'd expect earthquakes to repeat every few tens or maybe a hundred years or so in Japan and South America. In Central Asia, where there are thousands of faults, each fault may only repeat its movements every few thousand years, often way beyond human memory or recorded history. And so they tend to catch people unawares. People are unaware that they're living in a dangerous place, because there are earthquakes all over the place; there's nothing particularly that distinguishes where they are.

Oliver Strimpel: Let's come down to the fatal attraction between cities and earthquakes. Why do we build cities in areas that are prone to earthquakes?

James Jackson: Yeah, this is an interesting feature, again, of Central Asia, really, or this great Alpine-Himalayan earthquake belt from the Mediterranean to China: earthquakes occur and seem quite often to target places where people live. And you think, is it just bad luck, or is nature being vindictive? And the reason for that is that the earthquakes and the faults are responsible for making the mountains, and in particular responsible for the edges of the mountains, where the mountains meet deserts or high plateaus like Tibet. And that's where people live. People like living on the edges of mountains, because that's where the water supply is; it's where agriculture is possible. It's usually where the trade routes run, on the edge of the mountains, and of course it's strategically important for access to those trade routes as well. So what happens is that settlements grow up along those trade routes on the edges of mountains, and then they develop, and they become bigger and bigger. And so the populations concentrate in these dangerous places. It would be fine to live in the middle of the flat desert in Iran or Central Asia, but there's no livelihood there. There's no water, no agriculture possible. You can't live there. And so you live where life is possible in these desert countries, and that is almost inevitably in places where life is possible because the faulting has created the circumstances that make agriculture and water supply possible.

Oliver Strimpel: So you've described how that can happen at the edge of mountain ranges, such as the Himalayas or the Alps, where you have, well, in the Himalayan case, India plowing into Asia. Does that also apply on coastlines? Because we love to build cities on coastlines, in places where there are earthquakes.

James Jackson: Yes. Just as tectonic plates crunch together, they can also pull apart and drift apart, and that's, for example, why the coastline is where it is between Greece and Turkey, or between Norway and Scotland. Although Norway and Scotland is no longer active; it did it all about 100 million years ago. But now it's active in places like Greece, the central Aegean Sea, the Sea of Japan, Greece and Turkey, and indeed that is why the coastline is where it is. That's where most of the action is; that's where the earthquakes are. And so if you live in countries where there are earthquakes, it's a good idea to ask why the landscape looks like it does. And that's nearly always because of faulting associated with earthquakes, in places like California or the Mediterranean, the Middle East, Central Asia, New Zealand, East Africa. The mountains are there because of the activity on faults in earthquakes.

Oliver Strimpel: So what impact have earthquakes had over history on these cities that have succumbed to this attraction to earthquakes?

James Jackson: Well, this is very interesting: a number of modern cities have grown into huge population centers along, for example, the old Silk Routes, which connected China to the Mediterranean. A lot of cities along those routes have been destroyed multiple times in the past. But in the past, they tended to be rather small; they tended to be stopovers on trade routes. Now many of them are even capital cities of countries in Central Asia. They've been destroyed in the past, but whereas in the past, say, if they were unlucky, they lost 10,000 people or so, now they are cities of a million or more, and they're very much more vulnerable. They're still in the dangerous place, but they've increased the population dramatically without, I'm sorry to say, in most cases, increasing their resilience to the earthquakes that are bound to happen again.

Oliver Strimpel: Can we help people in these cities prepare for the next earthquake by predicting when it'll happen, or roughly when it'll happen? For example, I know that we can detect bulging movement on volcanoes that forewarn us of eruptions on volcanoes. Does that also apply on the tectonic scale, to continents?

James Jackson: Well, now you've touched on a very interesting question, and we have to be careful how we use words in this area. Short-term prediction means saying there's going to be an earthquake sometime within, you know, let's say next Wednesday, plus or minus a day, in some particular place, of such-and-such a size. And we can't do that. The science isn't there. People have looked very hard for precursors, as they're called, some sort of warning sign beforehand. There's nothing which has been found to be reliable, and it's not something which has been successful. We contrast that with forecasting. Long-term forecasting is fine. Long-term forecasting is simply saying, where there's been an earthquake in the past, we know it will happen again in the future sometime. You can't say when, but we can say quite a lot, with a bit of work, about how big that earthquake will be, what its characteristics will be, how much the likely shaking will be, how long it'll last, how much the fault might move. And all these things are of interest to architects and engineers and planners, because if they know what the threat is, what they're up against, they can actually design things for it. And that's been very successful, again, in countries around the Pacific Rim, for example. It's much harder to do in Central Asia, because there are more faults, most of which we don't know about, and there are generally other priorities.

And this is the problem: in many cities, in places where earthquakes are less frequent but more dangerous, people have other priorities. So if I was to say to you that the last earthquake in your city was 500 years ago, you would say, "Well, every day it takes me two hours to get to work. There's congestion, the air quality is rubbish, there are problems of water supply, traffic, and you're talking about an earthquake 500 years ago? That's not my problem." But actually it is your problem, eventually, but it's not highly prioritized. By contrast, if you lived in Chile, for example, you feel earthquakes all the time, and you know that you'll feel a magnitude 5 earthquake every year, you'll feel a 7 probably every 10 years, and there'll probably be a magnitude 8 earthquake in your lifetime. You know this is not a theoretical threat; it will affect you, your children and your grandchildren, and people are quite happy to prioritize it. And so in fact they're helped around the Pacific Rim by having many, many more earthquakes in the same place, repeating as it were, compared with Central Asia, where they're more distributed and less frequent in any one particular place. And so we can help them, but they need to put in the work to know what the hazard is that they're actually facing. There's no real shortcut to that, but it's just, on the whole, not prioritized. Places like Japan, New Zealand, California and Chile have put in decades of really hard scientific effort to find out what they're up against, so they can plan and prepare, and they've been superbly successful at it.

Oliver Strimpel: When you say what they're up against, does that mean mapping the faults in detail, so we have a better idea of where the next one might be that's going to rupture?

James Jackson: Yes, certainly that's part of it. But there are two things which concern you. If you're an engineer building a dam, for example, in a river gorge, you want to know why that gorge is there. And if the gorge is there because there's a fault and the river's flowing down it, that affects how you build your dam, which might get physically ripped apart. So there's an interest in knowing where the fault itself is, for sure, for particular facilities and threats. But also, in many places, for planning purposes, statistics are good enough. You just want to know, in this particular area, what is the largest likely ground shaking, the duration and frequency of the shaking, in order to allow people to plan how to build buildings and so on. So there's a difference between being physically ripped apart by being astride the fault and being shaken because you're next to the fault.

Oliver Strimpel: How good are the historical records, speaking of knowing what the impact has been in history in these high-risk areas?

James Jackson: Ironically, in the high-risk areas, which are certainly in Central Asia and the Alpine-Himalayan region, the historical record is much better than around the Pacific Rim. The historical record in the right places, at least between the Mediterranean and China, goes back 2,000 years or so, whereas around the Pacific Rim, they've been inhabited by people writing records or keeping records for not very long, a few hundred years, except for Japan. But in Asia there's an enormous amount of information in the records of empires. So the Byzantine Empire, the Ottoman Empire, the various Persian empires, and of course China were quite bureaucratic. They kept very good records of these things, which have been an enormous gold mine. People go back and look at these accounts kept by the bureaucracies of those times. It's a very, very scholarly activity. You have to be a fantastic linguist and historian just to get to first base, to be able to look at this stuff, and then you have to know what's being talked about in terms of the geology, the seismology, the engineering, to know how to interpret it. These records are very often exaggerated. They have particular styles of describing these calamities, and they're not interested in things like ground motion; they're interested in disruption to life and property and trade and so on. But you can interpret these things, and they are extremely helpful. You have to get at that sort of information in these other countries, which have been settled for less long, in a more forensic, detective kind of way, by looking at the geological evidence, which is also possible, but hard work.

Oliver Strimpel: That's fascinating, that the work that needs to be done is both very much historical and sociological and really human in nature, going back these thousands of years, as well as, on the other end, very cutting-edge, measuring precisely where faults are and using all the battery of techniques that we have today to figure out the present situation.

James Jackson: It's very interesting, and there's a very good relationship between these groups. The historical work is an immense, long-term work of scholarship. It's not the sort of thing you achieve in a three-year research grant. It's something where people spend their lifetime researching the history of earthquakes in Iran or something like that, and they come up with these historical earthquakes. And then it's the job of the modern scientists to figure out, OK, where was the fault which was responsible for that earthquake? Can we actually find on the ground what did it, and the evidence that it really did do it, and put the whole thing together? So there's a very close working relationship between these different groups.

Oliver Strimpel: So you live in Cambridge, near London, and I live in Boston. Are we in a safe area from an historical earthquake point of view?

James Jackson: In the sense that most of the earthquakes in the United States are on the West Coast, and most of the earthquakes in Europe are by the Mediterranean, heading eastward through Italy, Greece and Turkey and so on. It doesn't mean we don't have any. We have some, and most continents have smaller earthquakes. We have them even in Cambridge. I remember, from the last 10 years, a couple of earthquakes which woke people up there. They're very, very small by the standards of these proper earthquake countries, things people wouldn't get out of bed for, but they're not negligible; you have to watch out slightly. So for example, if you're building something which is very sensitive, like a nuclear power station, you really need to know what's going on. So you will have some assessment of what you think the likely largest historical earthquake was, or might be in the future, and you will overdesign to be safe against that. And so although people in England really don't think about it, the electricity generating board, who run the nuclear power stations, certainly do, and they've designed them, in order to be conservative, for what they think is the largest historical earthquake.

And the same is true, for example, of the East Coast of the United States. And that's more interesting, because actually, as you probably know, the Atlantic opened in the Jurassic, 150 million years ago or something, and of course the Appalachians essentially belonged up against Ireland and Scotland, and you'd think it's just the other half of the situation in the North Sea. Now, in the UK and northern Europe, we have a long historical record, mainly based on monastic records, monastic chronicles, which go back nearly 2,000 years. And there really has never been a really big earthquake in the UK and the North Sea in those last 2,000 years, nothing above about magnitude 5 and a half, which is really not going to cause trouble. And you go to the United States, and the same is not true. It's been settled for a few hundred years, at least, with a good historical record for the last few hundred years. And in that time there have been several earthquakes quite a lot bigger than anything in Europe in the last 2,000 years. The most famous was in Charleston, South Carolina, in 1886. There's one at Cape Ann, which is near where you live in Massachusetts. I think that was about 1755. But these were magnitude 6-plus, and certainly bigger than anything in northwest Europe. Why there should be that contrast between what are, geologically, if you like, two halves of the same mirror, like the slab you just broke and split into two, why the American side should behave so differently from the European side, no one really understands, but they do. And so you have to design for certainly bigger earthquakes than people are contemplating in northwest Europe.

Oliver Strimpel: Did the Charleston earthquake do a lot of damage in South Carolina in 1886?

James Jackson: It was a big deal, and it was very well investigated, and it was written up, and there's a lot of detail about it. It didn't do too much damage, because of course in those days in South Carolina people mostly lived in wooden houses, which were quite well built, and they behave very well. Wood is the best building material you can have in earthquakes, but in northwest Europe wood is an expensive building material, so it's not that common.

Oliver Strimpel: I'm going to ask you, for my last question, to be a little bit of a forecaster. I know you don't like to do that. But what I'm wondering is that, even though the earthquakes that have happened in these dangerous belts in Central Asia and elsewhere are on timescales longer than an individual human life, so there isn't a personal memory that perhaps makes people aware and attuned to the risk, one thing that has happened on a shorter timescale is this enormous growth in our scientific knowledge and understanding, which you've alluded to in this talk. Do you have a sense as to whether this knowledge can be brought to bear to make this connection between cities and faults a little less fatal in the next few years and decades, while development proceeds in these countries?

James Jackson: I hope so. That is an excellent question, and that has occupied me for the last 10 years: how do you actually turn the scientific knowledge into some increased public safety? And it's presumptuous to think the problem is just scientific. Of course it's not. It's to do with wealth and sociological conditions, cultural conditions, economics, all those kinds of things as well. In the long run, you know, the only thing which works is education. People need to know what they're up against. They need to know it's not hopeless, right? The great iniquity, I think, is short-term earthquake prediction, which is like the promise of a panacea, a silver bullet. If you allow the public to believe in it, it also allows them to think it's not their problem: the scientists will sort it out one day; they'll say the earthquake's going to be on Wednesday at 9:30, and we don't have to do anything. That is fatal. That attitude kills lots of people. And that's one of the roles of the scientists, to say, actually, that isn't going to happen.

On the other hand, there are things you can do to make life safer, big time. There are lots of very switched-on, good people in those areas, in governments and NGOs, and civic leaders who want to do the right thing and who care about this, and they need help. The scientists can help them by helping them be realistic about what the public is going to face. And there are a number of such studies, in countries like China and Nepal, where you do a detailed study. There was one, incidentally, in Kathmandu, which had the big earthquake in 2015. Before that earthquake, an NGO had put together a study of what would happen if the previous earthquake, which was in 1934, in Kathmandu, repeated. What would happen if that 1934 earthquake repeated today, given the changed circumstances in terms of greater population, population density, the quality of buildings, the lifelines, by which they mean things like roads and water supply and power supply? What would happen today? And they did that study, and it wasn't some arm-wavy publicity stunt. It was two years of really hard scientific evaluation of what that earthquake was in 1934: what caused it, where the fault was that moved, what the ground shaking was, and what that ground shaking would do today.

And they published it as a story, almost as a cartoon picture book, illustrating the consequences of such an earthquake for a family in Kathmandu. So the father would be working on the other side of the city; the bridges over the river would have collapsed, so it would take him two days to get home. What do people do? They're going to camp on the football field. They'll have 23 days without water, and so on and so on. And this was a story to get people to concentrate their minds on what would actually happen. And then in 2015, the earthquake did happen, and you could read this storybook and more or less turn over the page and say, this is what's going to happen tomorrow, and it more or less did. They had a lucky escape, because the earthquake wasn't quite as big as 1934. So instead of losing 100,000 people, they lost about 10,000 people.

Oliver Strimpel: James Jackson, thank you very much.