This week we're going to be talking about earthquakes. There's a lot to be said about earthquakes, but the stories that we'll be presenting largely deal with the risks associated with earthquakes. So I thought it would be appropriate to introduce this subject by exploring the nature of earthquake risk, and I suppose the first question we might ask is well, what is risk? I think we understand it intuitively. Right? We buy automobile insurance. the reason that we buy automobile insurance is to protect ourselves in case that we get in an accident. So, if we get in an accident and it costs a lot of money to repair our car or to right the damage or injury that we've done, and to protect ourselves from the large financial losses we pay an insurance company a small amount of money each month, for that protection. and as you probably realize, the the amount that we pay depends on the probability of an accident. Whereas, say, an adult as opposed to a teenager our insurance costs would be less than it would be in the case of the teenager. if we have a, an expensive car, the cost of the insurance is greater than if we have a junker, simply because if we get in an accident the cost of repair would be greater. So right away we have a sense that risk depends both on probability and also the consequence, the magnitude of the consequence. And in fact that's exactly what risk is. One of the problems with earthquakes is it's very, very difficult. In most cases, to understand what the consequences would be, and therefore, it's very difficult to understand what the earthquake risks are. And I think that's probably illustrated by the destruction of the Presidential Palace in the January 12th 2010 Haiti earthquake which you see here. As far as the probability is concerned the United States Geological Survey has published probability maps for most or perhaps even all of the United States. I'm not actually sure. And here's an example of one such probability map. This is [COUGH] the the Bay Area, the San Fransisco Bay Area. And, what this map shows is the probability of an earthquake of magnitude seven or greater occurring within the next 50 years and a 50 kilometer radius of any particular point that we've chosen here. So, for example, let's say we're in San Francisco itself. And this is in the red zone. So what this map is telling us is that there will be a 40 to 50% probability of there being an earthquake greater than magnitude seven, or greater than or equal to magnitude seven. Within the next 50 years, within 50 kilometers of that of, of San Francisco. Or let's say we were in Livermore, which is in the in, in the valley. this is in the green area, so [COUGH] the map says that the probability of there being a magnitude seven or greater earthquake was in 50 years. In the next 50 years, within 50 kilometers of Livermore, will be somewhere between 10 and 12%. So, that's what these maps mean. [COUGH] Now, where did the USGS get these probabilities from? Well, first of all the historical record. the historical record of earthquakes in California is pretty extensive. There are a lot of earthquakes there. It extends back to the beginning of the 19th Century and perhaps even earlier. I'm not exactly sure how far back it extends. so there's this historical record of earthquakes you know, how often they occur, how large they are, where they occur. And there's also a geological record. So, what geologists do is you know, find and expose fault and make a trench across that fault and map the displacements in the trench. And if you can find fragments of charcoal or wood to date, then you can develop a chronology of earthquakes. In other words, you can develop a chronology of when the faults and thus the earthquakes occurred. And you can also estimate the sizes of the earthquakes by the amount of displacement. So, those historical records combined with the geological records are the basis for these probability maps. Okay, so while we can meaningfully estimate earthquake probability in the in California that's not the case in many parts of the world, such as the eastern United States, where there there are many fewer earthquakes. And thus there's not nearly as good a historical or geological record to base you know, probabilities on. So you know, here's a probability map of of the eastern US. If you've recalled, there was an earthquake in Virginia, in 2011. It was a fairly large earthquake, a magnitude 5.8. It occurred right here. And before that earthquake occurred, we didn't think there was any, we didn't think there was any serious earthquake risk in that particular area. Because it it it was caused by rupture on an unknown fault. But of course, as soon as that fault ruptured then all of a sudden we understood that, that that area was at, at greater risk of earthquakes than perhaps other areas. This, by the way, is a different kind of a probability map than the one that I showed you before. This is a probability map of that, that reflects the intensity of shaking. And that intensity of shaking is measured by ground acceleration, okay. and ground, and it's expressed as ground acceleration as a proportion of g. Now, g is the acceleration due to gravity, the value of, of g is 9.8 meters per second squared. so this map shows the ground acceleration as a proportion of 9.8 meters per second. So, more specifically, this map is saying that there is a, let's, let's take a specific area to illustrate what, how, how to read this map. Let's look at this area right here and the yellow region within that map. so that yellow region corresponds to a g of 0.20 to 0.26. And what this means is that there is a 2% chance, within the next 50 years, of there being an earthquake that will that will result in a shaking intensity equivalent to an acceleration of 0.2 to 0.26 that of of an acceleration due to gravity. Let's talk about some of the characteristics of earthquake risk. And the first one is one I've already mentioned. Namely, we can reduce earthquake risks because by proper building design and construction. In other words, we have some control over the consequences of earthquakes. And this was this is, was particularly well illustrated in the February 27th, 2010 Chilean earthquake. now Chile has been struck by earthquakes many, many times. the largest recorded earthquake ever occurred in 1960 in Chile. It was a magnitude 9.5. there was another large earthquake since then. I can't remember the date. There have been, I think, thirteen earthquakes of magnitude greater than seven in the last forty years or so in Chile. So, Chile is used to earthquakes, and because of this, this long history and experience with earthquakes, the Chileans have very, very strict building and design standards, and they're, and they're strictly enforced as well. So, when the, [COUGH] 2010 earthquake struck, and by the way, the 2010 earthquake was a magnitude 8.8, so it was a megaquake. I think it was the sixth largest earthquake that's ever been recorded. So, it was a very large earthquake. When that earthquake struck you know, the country was reasonably well prepared. Thousands of buildings were damaged or even destroyed, but as you can see in this image this building you know, is probably had to be torn down, but it didn't collapse. And so even though this is a very large event there were only 570-some people killed in this event. So let's compare that with what happened in Haiti. The 2000. A month before the Chilean earthquake on January 12th. there was a magnitude seven earthquake that was a, a disastrous event. More than 200,000 people were killed. and the reason is that Haiti lacked the design and construction standards of Chile. And this is true throughout much of the developing world. I mean, countries that relatively poor countries have many, many problems. And and the funds might not be available. The funds generally aren't available to to construct to the standards that are necessary. Another feature of earthquake risk is that it increases as population increases, and this is a particular problem in many of the very rapidly growing cities in the developing world. Dhaka being one of them. Dhaka is the capital of Bangladesh and the earthquake risk there is one of the subjects of the stories that we're going to be telling this week. But here's a a, a figure showing how the population has increased over the last three decades. This is actually a projection from 2001 data by the World Bank, but it's a pretty decent projection. And now Dhaka has over 15 million people. In other words a population density of almost 28,000 people per square kilometer. So, obviously the more people you have, the more risk there is, and especially in the case of Dhaka where, you know, the influx of people results in an enormously rapid construction of new buildings with basically little construction standards applied to them. So, so this is a, a major issue. we pointed out the fact that Chile was prepared for a, a megaquake. But there are other parts of the world where the regions are not prepared for megaquakes, but are susceptible to them. And one of them is the Pacific Northwest Oregon, Washington, and British Columbia. And these are, the, this region is effected by the Cascades the Cascadia Subduction Zone. there have been large mega-earthquakes in the past. The most recent one was in 1700. It resulted in an enormous tsunami that that struck Japan. but we didn't know about these megaquakes until just the last couple decades. We didn't know that the Pacific Northwest was susceptible to these large events. So, this means that you know, cities like Seattle are particularly at risk because Seattle you know, the building codes were not designed so that the buildings could withstand megaquakes in general. And in fact a noted structural engineer by the name of Peter Yanev wrote many of the region's iconic tall buildings would probably collapse in a megaquake. Finally, I want to talk about another phenomenon associated with large events, particularly large earthquake events. Mainly the phenomenon of cascading risks. And the Tohoku earthquake illustrates this very well. This was another megaquake a magnitude 9.0. It occurred about 130 kilometers off the coast. It was a, a rather unusual earthquake because the, the shaking occurred for some three minutes or so. Now, Japan is well prepared for earthquakes because, you know, like Chile Japan has frequent experience with earthquakes. But what happened in this particular earthquake is that it displaced a large section of the ocean floor and that resulted in this gigantic tsunami. So, that tsunami struck and that was unexpected. of course, it did huge amounts of damage. And one of the things it did is it destroyed the Fukushima Nuclear Power Plant by flooding of the backup generators that would provide electricity to run the cooling water for the for the reactors. So that stopped and three of the reactors melted down and exploded, and the explosion just dispersed toxic radioactive materials through a wide area and some of those areas had to be evacuated. So that's an example of of cascading consequences. A sequence of consequences that are almost impossible to foresee. So here are the points that I've tried to make. The first is that risk is determined by probability and consequence. the second is that earthquake probability can be meaningfully estimated in seismically active regions, based on the historical and geological records, but it's not all that well known in generally aseismic regions because those records are sparse or perhaps don't even exist. the third seems obvious. Namely that earthquake risk can be significantly reduced by employing appropriate building design and construction techniques, but it should be evident that this is not always possible in many parts of the world. We also saw that earthquake risks increase as population grows, that megaquakes present significant risk to some areas that simply aren't prepared for them. Namely, the Pacific Northwest. And finally that megaquakes can lead to unforeseen cascading consequences. So thank you very much, I hope that this has given you some different perspective on earthquake risks and more generally risks from natural hazards.
