[SOUND] [MUSIC] So now that we've talk about falls. Were now going to introduce couple of other key concepts about earthquakes. We’re going to need for our discussion further long. We're going to introduce the concept of an earthquake focus to start with. Basically, if you think about where the rock breaks and where the energy is generated, that's what geologists or seismologists refer to as the focus. There's another term that you may come across, the word hypocenter. It's a synonym for focus. But both of them just simply refer to the area in which the earthquake energy is being generated. Now, that energy, once it's been generated, will propagate outwards from that point in a series of waves which symbolically we tend to represent on diagrams by a series of concentric circles. So for example, if this is the surface of the Earth and there's a fault down at depth and the fault suddenly slides, we can imagine that there's a point or an area where the earthquake is generating energy and that energy then propagates outwards as a series of rings and when those rings reach the surface, it will be causing the surface to ungulate. And that's the vibration that we see as the earthquake. Now, this point, this area right here, that's the focus. It's the place where the earthquake energy is generated. If we draw a line straight upwards to the surface of the Earth. That point right there is called the epicenter. So if you think about it, you can plot epicenters on a map as a dot. Focus, well that's down below those epicenters. So generally when you see a map showing earthquakes, what you're seeing is a map that's plotting the position of epicenters. Geologists or seismologists actually recognize a variety of different specific kinds of earthquake energy or earthquake waves. The first important distinction that we make is between waves that travel within the earth. In other words, within solid rock of the planet. And those are called body waves meaning that they are travelling in the body of the rock. The second type of waves are those that travel along the surface of the earth and not surprisingly those are called surface waves. So let's first consider the body waves. We distinguish between two types of body waves based on the way in which particles move as the wave passes. I have a slinky here, just a simple spring to represent the atoms within a body of rock. If I push on one end of the spring, you'll notice this vibration, there it goes, passes through the spring, bounces off my hand and goes back, but basically, the back and forth motion as that wave passes is in the same direction that the wave is moving overall. And that's called a P-wave or a Compressional wave. And simplistically, the definition of it is it's a wave in which the vibration direction, the back and forth motion, is in the same direction as the wave propagation direction, the way in which the vibration moves through the material. Second kind of wave, also a body wave is called an S-wave. So let's see what an S-wave looks like with our friendly slinky right here, and again this is a spring. We saw before that a P-wave is a compressional wave, notice that I'm moving my hand back and forth in the direction of propagation. An S-wave is different. I'm moving my hand perpendicular to the direction of the propagation. So when I have a pulse like that, it propagates in that direction, whoops, I can't do three things at once. But my hand motion was perpendicular to that direction. Okay, so both of these are body waves. They're both passing through the solid rock inside the earth. Now, there are two other kinds of waves, and both of them are examples of surface waves. And that means that they are waves that are moving along the surface of the earth. In one, the earth's surface is moving in a sort of, snake-like motion. And in another, it's sort of moving in an undulatory type motion. One of those is called a Love Wave The other is called a Rayleigh Wave. They're named after the scientists that first recognized their characteristics. Okay, so we've now introduced four different kinds of waves. Two kinds of body waves, two kinds of surface waves. The important thing that we'll be seeing when we start talking about how the records of earthquakes look, is to realize that these different kinds of waves do not all travel at the same velocity. P-Waves travel the fastest, in fact that's why they're called P-Waves. P stands for primary because they are the first waves to appear at a distance from the earthquake. S-Waves are the next fastest, they stand for, the S stands for secondary because they're the second waves to appear. Surface waves travel more slowly, and they will appear at sometime after the first two kinds of waves have appeared. So, in sum, the point of this lesson is to introduce earthquakes. We've learned what an earthquake is. We've seen that the vibrations that are earthquakes can be generated by the breaking of rock or by the slip on a pre-existing fault. We've seen that there are different kinds of faults. Now, when an earthquake happens, it happens at a place called the focus. But in general, when you see a map of earthquakes, what you're seeing are points called epicenters, which were are the points on the surface of Earth above where the focus lies. Finally, we've seen that earthquakes don't happen instantaneously and they take several seconds or even a few minutes to happen and we've seen that the earthquake waves travel at different speeds so the arrival of the earthquake waves takes a period of time as well [MUSIC]
