Japan Disaster: 9.0 Earthquake

Transcription

1 Well thank you all for coming. So I'm here to talk about the earthquake itself, and then we have other speakers who will talk about the tsunami and nuclear power, and radioactive fallout. So what happened on March 11? 1

2 So first of all the ground shook for several minutes, and there were people with cameras all over Japan who caught this. So here is a little bit of footage in one grocery store. Now those of you who have been in earthquakes in California, by now the earthquake would have been over. They don't last this long usually in California, but a 9.0 lasts for minutes. It seems an eternity when you're in it. 2

3 So basically what happened on March 11 is there was slip on a fault that caused that ground shaking, and that caused a tsunami. 3

4 So what in the world is a fault? Well this is a photograph of a small one. It's this fracture right here in the rocks. It's a fracture in the earth's crust. And large faults are needed to cause large earthquakes, and so imagine this is the entire earth's crust, and a fracture going through that. So what I mean by slip on a fault is that the rocks on either side of that fracture slide past each other. So you can see that this spot here used to be right up here, and so there was slip along that fracture. And that might have happened in one earthquake, or maybe several. 4

5 So this was the epicenter of the March 11 earthquake, this is Japan right here. So there has to have been a fault, and there has to have been some slip. And so looking at this diagram, you can kind of see that there's something strange going on east of Japan. There's a deep sea trench here, and the ocean gets extremely deep along there. 5

6 And this is part of a much larger trench called the Japan Trench, and a whole series of other trenches that we have in this part of the world. And if you plot earthquakes happening over a long period of time, you get a pattern like this. 6

7 And so you'll notice that earthquakes do not just occur randomly on earth, they occur along narrow zones, and they're especially common where those deep sea trenches are. 7

8 And so this is because earth's outer shell is broken into pieces that we call plates. And so Japan is at a place where it's particularly complicated. You have four different plates coming together, right about where the earthquake occurred. It happened right in here. And so the two plates that were involved were the Pacific plate, and believe it or not the North American plate. 8

9 There's a little piece of North American plate over there, and there's two plates that are moving toward each other there at 3.3 inches a year. Now for reference, along the San Andreas fault we have two plates moving, and they move at about two inches a year, okay? So this is quite a bit faster, almost twice as fast. Now you might not think three inches a year is very much. But if it's locked up for a century or so, it adds up to quite a bit. So let me show you a little animation of what's happening there. So this would be Japan right here, and this is the Pacific plate. And as it dives down underneath Japan, it doesn't just cause earthquakes whenever we get slip on this fault, but it also causes volcanic eruptions, which you can see. And that is another hazard that the Japanese people have learned to live with. It also makes for beautiful mountains, like Mount Fuji. Alright, so that's what's happening. We have two plates moving toward each other, and one ends up sliding underneath the other one. 9

10 Why would slip on a fault though, cause the ground to shake? Why does it cause so much disturbance? Well, what happens is that whenever there is slip on a fault -- at least whenever there's rapid slip on a fault, a huge amount of energy is released. And what is happening is, if you would imagine this would be sort of like the San Andreas fault. 10

11 So here's one plate here, here's another plate. So there's two plates, and the thing is they move steadily past each other at a couple of inches a year, or in the case of Japan more than three inches a year. But the problem is that they're stuck together. So here's what happens. 11

12 Two plates are moving. 12

13 Two plates are moving. 13

14 So as those two plates move, right where they come together, there's no slip 'cause they're stuck. 14

15 And then an earthquake happens, and all of that motion that happened all those years since the last earthquake happens all at once, right where the two plates come together. 15

16 So I'll show you that again. 16

17 See how these plates are bending? 17

18 Energy is stored in there when they bend. 18

19 And that energy is released when there's slip, and they can unbend. 19

20 So see how these are straight now? So, you know, you've probably experienced something like this if you've ever had a rubber band and you stretch it, okay? While you're stretching that rubber band, that's like the bending of those two plates along the fault, okay? So this rubber band gets stretched, and there's energy stored in this rubber band. And there's also a force that's trying to make this rubber band go back to its normal shape, okay? And that's the force that actually makes the slip happen on the earthquake. So we have gradual motion, gradual motion, gradual motion, and the rocks near the fault keep getting more and more stretched. And then when the earthquake happens, energy is released in the form of kinetic energy, okay? So that's what happens with this fault. Okay, so I have another animation that shows the same kind of thing, but it's more like what happened in Japan. So this would be like the San Andreas, and in Japan though, instead of the two plates sliding past each other, one is diving down underneath the other one. So this is part of a television show about a tsunami, but we're going to skip ahead to this part right here, okay? So this was the Indonesia earthquake, but the same kind of thing was happening in Japan, okay? So notice how as the one plate is sliding underneath the other, they're stuck together, right in here, okay? So they're slipping see how they're stuck together right there? And notice how these rocks are bending more and more? And so at a certain point it can't bend any more and we get slip on that fault. And we cause an offset on the sea floor, and we cause a tsunami, and earthquake waves. But we'll deal with the tsunami in the next talk. 20

21 Why does the ground shake when fault slip occurs? Shaking only happens when fault motion is very rapid. The rocks bounce back and forth before settling into their new shape (like a vibrating string). But let's think about why does the ground shake? Why is this energy released as shaking? Well, that shaking only happens when the fault motion is very rapid. That's when the energy is released all at once. If it's released gradually, then we don't get the shaking. And every once in a while we're lucky, and we have slip on a fault that happens over a day or two, and nobody gets hurt. It's just gradually that energy gets released, and everything's fine. But unfortunately, most of the time when we have slip on a fault, it happens very rapidly, over a few seconds or at most a few minutes, and then the energy is released very rapidly, and we get shaking. So I can demonstrate that with a piece of PVC pipe, okay? So this represents the ground that's near the fault, okay? And this ground gets bent by the fault. So let's imagine that there's another plate over here, and it's moving down, okay? And it's bending this down, and bending it and bending it and bending it. Okay, now when the earthquake happens, it gets unbent really quickly. Whoops, let's do that again. Okay? [ Banging ] And it vibrates, alright? If I do it slowly, nothing happens. But if I do it quickly -- [ Banging ] --then the rocks nearby have to vibrate around their new position before they settle into it. And that's what causes the shaking to happen when there's slip on a fault. And so this is analogous to playing a string instrument. That's how you make the sound on a guitar. You stretch that string, and then you suddenly let it go so that it can go back to its normal shape, and it vibrates before it settles down. So that's what the ground is doing near a fault when there's rapid slip on the fault, it vibrates. 21

22 Okay, but we don't just feel it right where the fault slipped. Sometimes we feel it tens of kilometers, or maybe hundreds of kilometers away. Well, what's going on is that the energy travels through the ground as waves. So that vibration that starts in one spot causes the rocks next to it to vibrate, which cause the rocks next to that to vibrate, and it goes out in these waves. And that's what I'm doing, I'm disturbing the air right near my mouth, which is disturbing the air right near it, and eventually it gets to you so you can hear it. Same kind of thing with seismic waves. 22

23 Okay, so I have an animation of the seismic waves as they left that slip motion in Japan, and traveled through the entire earth. Okay, so there's the March 11 earthquake. Okay. So there is the earthquake, it occurred, you can see the front of the waves that left that epicenter of the earthquake. This is what it looked like on the earth's surface, that's what it's looking underneath the surface. And there's different kinds of waves, which are color coded here. The ones traveling on the surface are yellow, they're pretty slow. The blue and the red ones are much faster. And you can see that these waves don't just go to the other side of the earth and stop. They bounce off of the intersection of the core and the mantle, they bounce all over the place. And we have seismic stations all over the world that record this. And as you might imagine, the way these waves bounce around and bend, it really helps us see what the inside of the earth looks like. So for a seismologist, it's very exciting to have a 9.0 earthquake, because it's like being able to do a sonogram of the earth, okay? We can't purposely do it, but when it naturally happens, then we get to get a picture of what the inside of the earth looks like, and also get all kinds of information about the earthquake itself. So on the top there, you can see various seismic stations on earth recording the waves. 23

24 Now, the action didn't stop with the 9.0 earthquake that was the initial slip on that subjection [phonetic] zone. There were aftershocks, and they went on for quite a while. So this is an animation. I'm linking to another website, so this takes a minute. But this is an animation of the aftershocks happening. So when there's a large earthquake, what happens is those rocks nearby which were bent more and more, and then all of a sudden were straightened, they have to readjust. And so there's lots of small faults nearby that also move, because they get bent even more by the action near the fault. So this is showing each one of these is an aftershock. And this is the graph showing you the aftershocks for I think this is just a week after the earthquake. So this is the unnerving part of these large earthquakes, is that it's not over when it's over. You keep getting more earthquakes. 24

25 So these are some maps showing where all these earthquake epicenters were. This is the main 9.0 earthquake right here, and then every other circle is another earthquake. The bigger the circle, the larger the magnitude. And so this shows you the same thing without the map. This is the plate foundry by the way, right here. 25

26 So here's a graph. We actually had some foreshocks with this earthquake. We had some smaller earthquakes that happened before the big one. The only problem with foreshocks is you never know that that's what they are until the big earthquake happens afterwards. But these were foreshocks. The ground was adjusting already before the big earthquake happened. And here it is, and then here's all these other earthquakes that happened afterwards. 26

27 So I said this was a magnitude 9. So a magnitude 9 earthquake moves a large piece of a fault. So this is the piece of the fault which is along that subjection zone. That large of a piece moved, and it moved as much as a 130 feet, all at once. So that's how we determine magnitude, is by how big a piece of the fault slipped, and how far. 27

28 Formula for Moment Magnitude (M w ) M w = (2/3 log 10 M o )-10.7 M o = AD M o A D Seismic moment : amount of energy released during the earthquake Rigidity of the rocks: how well the rocks resist being bent Area of the part of the fault that ruptured during the earthquake Average amount of slip that occurred along the fault during the earthquake So we have this formula that we use. And so these were the numbers. 28

29 Values for the Magnitude 9.0 Japan Earthquake of 2011 Length of fault rupture Depth of fault rupture Average amount of slip 300 km (186 miles) 150 km (93 miles) 35 meters (115 feet) We had a 300 kilometer long, and 150 kilometer deep piece of the fault that moved, average of about 35 meters. 29

30 Japan 2011 San Francisco 1906 Length of fault rupture 300 km 400 km Depth of fault rupture 150 km 10 km Average amount of slip 35 meters 5 meters So just for reference, the San Francisco earthquake in 1906, which is the biggest earthquake we've had, that we've ever recorded on seismographs in California, is considerably smaller. So a 9 is a lot bigger than a 7.8, which is what the San Francisco earthquake was. 30

31 So, here's the bottom line. If you're in an earthquake, and you're in a building in California, what should you do? 31

32 Yeah, you guys -- you crouch under a desk and hang onto it, okay? Most injuries in earthquakes are caused by objects falling on you and hitting you. So that's why you want to get under the desk -- I have a desk handy here, I'm sorry you don't. But that's the best thing to do. And to hang onto it, because sometimes it'll move. 32

33 Questions Question 1: What about the doorway? Is it really that bad an idea? Answer: The doorway is not any better than anywhere else in a building in California. Because in California we've had strong earthquake codes for 100 years, and so the rest of the building is framed just as well as a doorway is. And doorways have doors in them, which can swing and hit you. So it'd be better to get under a desk. Question 2: Is a 9 about the biggest earthquake you can have? Answer: The biggest we've ever recorded is a

34 So this is the easy way to remember what you should do in an earthquake. You should drop, cover, and hold on. Thank you for coming. 34