Hey everybody, welcome back to Explanation 5, the podcast where we take the questions you always want to ask and talk about them in a way that's easy to understand. We are your hosts, I'm Tim. And I'm Kevin. So Kevin, today we're talking about earthquakes.
Earthquakes are cool. And the first question we'd like to start with is, if an earthquake is a giant moving plate, then why is it that we always talk about an epicenter as a single point where an earthquake starts and not like the entire fault line? Right, right. So, earthquake, you want to think about it as basically the buildup and sudden release of energy caused by two tectonic plates interacting with each other.
And the boundaries between tectonic plates are not actually a clean, uniform slice like you would make it a cake. They're more like what you see if you drop the dinner plate and you crack it in half. So the plates are elastic and jagged at the boundaries. When they move across each other, the pointy shape of the edge means they kind of get caught up at certain spots.
And the slow motion of the plates causes tension to build up at these spots. So when one sign finally kind of gives away, there's a release of energy from that single relatively small point, which is the epicenter. And it's that release of energy that causes shockwaves to travel outward from that point of release, kind of like ripples in a pond. Those waves are the actual earthquake.
Now, there are some pretty cool technology these days that help protect structures like skyscrapers against earthquakes, right? Of course. It's really important, especially in certain states like California. For large skyscraper-type buildings, the very top of it will actually be some kind of atrium with a large concrete ball hanging from the top.
So as the building moves, the ball will move in the opposite direction, keeping the building in the same place. It's actually called a tuned mass damper. That tuned part of it is an important detail, isn't it? Of course.
It's tuned to oppose the frequency of the building so that it directly negates any resonance built up by wind. And one of the coolest ones is actually in the Taipei 101 building. They plated the damper in gold and turned it into a tourist attraction. It doesn't stop moving completely, but it reduces it.
It also more rapidly decays resonance if it begins, so the movement slows down sooner. Interesting. So how about the rest of modern buildings? They don't all have a tuned mass damper at the top of them.
How are they designed to be earthquake-resistant? Also a good question. So this is a basic method of design. For more complicated structures such as high-rises and long bridges, engineers would use special techniques such as other types of dampers like sloshing water tanks at the top of a high-rise or actually base isolation, basically special bearings at the base of the structure to allow it to partly move with the earthquake rather than try to withstand the force from trying to stay in place.
How do they know how big an earthquake to plan for? So first of all, the engineers, they have to first pick a level of earthquake to design for, actually. Generally, the building codes specify an earthquake that has a 1 in 2,500 year chance of occurring. So, you know, smaller earthquakes will happen more often because of this.
This means that for a 50-year structure life, you get something like a 2% chance of an earthquake being bigger than what the structure was designed for. This doesn't necessarily mean that the structure will fail, right? Got it. So it's all in the probabilities.
Exactly. Exactly. They generally allow for a certain amount of damage to occur in the design of the earthquake, but for it to only occur in controlled locations probably, right? So like a more progressive, so no progressive collapse or ductile failure.
We allow steel to yield, like permanently to form a certain amount, and maybe reinforce concrete to crack. Engineers, they do this structural analysis to basically see how the structure behaves in an earthquake. And just like in the Type A101, the most important thing is the period or frequency of vibration of the structure. You get resonance and higher forces on the building if it vibrates at a similar frequency to the earthquake shaking.
But for simpler buildings, you can do this actually on paper by hand. For more complicated structures, engineers will use a 3D computer model. And after the whole structural analysis is done, the engineers will have an idea of the kind of forces on the structure due to a local earthquake. They will basically start designing selected critical parts of the structure to take that force and detail it to ensure that the overall structure doesn't fail even if it does become a little damage.
So I'm not sure if this is related, but in the U.S. you find a lot more houses are built with wood than, say, concrete or steel. Why is that? Right.
Yeah, yeah. I mean, in the context of earthquakes, wood flexes better than steel and concrete without fancy midi-game technology like we were just talking about. But actually, this isn't really the reason. It's essentially because wood is cheap, abundant, and easy to transport and easy to work with.
You know, in North America, there is plenty of lumber that makes very good construction materials. Europe was heavily deported pretty early on. And actually, part of the drive for colonization, at least for England and France, was that good timber for your naval fleets. In areas of the world where construction grade lumber is more scarce, you'll actually see more buildings and homes built out of stone and concrete.
Now, if we take aside structures, you also have full landmasses that are affected by earthquakes, and it's always been an interesting question when an earthquake occurs. I think there was one in Greece in recent years where they said the whole island shifted by three feet. How did people measure that? Yeah.
Well, that's when people really start paying attention to what those surveyors actually do all day in your city. You may have seen a survey mark on your city sidewalk or just somewhere. You know, it's usually a small disk that kind of has like a plus random, you know. So they call points like this control, essentially the coordinates for a very specific place on the Earth's surface.
So basically when they say something like the island shifted by three feet, they really mean that that island moved on the Earth's surface three feet from where it used to be. And I remember we also did an episode a little while ago about Mount Everest, and there was a question about when the height changed due to an earthquake. I'm guessing this was also measured in a similar way. Yes, yes.
The surveyors do that, and in that example, it moved by about three centimeters as a result of an earthquake a few years ago. Well, Mount Everest has three centimeters to give. I'm sure it doesn't make too big a difference. Did you learn something new?
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