Transcript for:
Understanding Earthquakes and Seismic Waves

(Opening Music) Welcome back geology fans! The shaking of earthquakes is a violent demonstration of energy moving through the Earth. Just as the energy waves and ripples move through water, seismic waves that make earthquakes move through the Earth. Another analogy is a ringing bell. When you strike a bell it rings as energy waves move both within the metal of the bell, and trapped at its surface. When an earthquake releases energy into the Earth, waves go out from that point making waves that both move within the Earth, and then also get trapped on its surface. Sitting on the surface of the Earth as most of us are, we are interested in the surface location directly above where this point source of energy starts; the place where the energy will express itself the most violently. We call this location the epicenter of the earthquake, the point on the surface above where the earthquake energy is released. Many earthquakes have sources many kilometers below the surface, meaning the place energy is released can be kilometres below the epicenter. This location of exact energy release within the Earth is called the earthquake focus. It is from this focus that the energy waves are released, and is analogous to the place where a pebble is thrown into the water to generate ripple energy in a pond. Of course the ripples of an earthquake move out in all directions from the focus in three dimensions. And it is best we don't think of the focus as being a point on a fault, but rather an area along that fault that slips all at once. Even with an area slipping, not just a point, it is restricted to a place in the Earth, and is thus defined as the focus of the earthquake. From that focal disturbance, the seismic waves travel out in all directions, and eventually some of their energy is trapped at the surface of the Earth, just as our ringing bell has energy waves moving both within the metal and upon the surface of the bell. But seismic waves come in a variety of flavors. The variety of seismic waves are broken first into the body waves and the surface waves. But each of those is further split in two. Let's start with the waves inside the Earth, the body waves. The two types of body waves are easily demonstrated with a good spring like a slinky toy. Take a slinky and stretch it out with a bit of tention creating elastic deformation, then give one end of the slinky push and watch an energy wave travel from the disturbed end to the other, and then reflect back again. But there are two ways we could push one end of the slinky. We could push the slinky in the same direction as the length of the slinky and see a wave that alternately compresses and stretches to transmit the energy wave. Alternatively, we could push the slinky end perpendicular, to the side, relative to the length of the slinky, and you get this other waveform which kicks the slinky side-to-side to transmit the wave. The first wave is called a P-wave, and the second wave is an S-wave. The P-wave is also known as a pressure wave, and the S-wave is also called a shear wave, and you might think P stands for pressure and S for shear, and if it helps you remember it then go ahead and make that connection. But there's another original meaning for P and S; primary and secondary. The fastest seismic wave is the P-wave and the second-fastest is the S-wave. If two cars leave the same spot heading towards you, and they're going different speeds, the fastest car will reach you first and the slower car reaches you second. So it goes with P and S-waves. They leave the focus point to reach you at the surface with the faster P-wave reaching you first and thus being primary ,and the S-wave coming in secondary. The closer you are to the focus, the shorter the lag between this P and S wave arrival time, the separation time increasing with increasing distance from the focus. But here is some good news with the earthquakes. The P-wave rarely does much damage, and the S-wave only a bit more. The slowest waves are the surface waves and they do the most damage in the end. So the good news here is that you get a less destructive warning with the P-wave that much worse is quickly approaching. The two types of surface waves are Rayleigh waves and Love waves, named after respective scientists. Rayleigh waves are slightly less destructive on average than Love waves, but can't be as easily demonstrated with a slinky. A better analogy are those water ripples or waves in the ocean. As water waves pass by, the water itself doesn't move, just the energy wave going through it. What happens to the water or a boat on the water bobbing on waves, is a circular rolling motion. As Rayleigh waves pass by, the ground rolls like waves in the ocean. This can topple many buildings, but most destruction has been attributed to the other surface wave. Poetically, the most destructive is the surface Love wave. This is like an S-wave if we trapped it at the surface. Imagine the surface of the Earth kicking side to side as these waves pass, and you will see why this causes more buildings to fall than any other wave. To recap, seismic waves leave a focus with P waves going through the body of the Earth and arriving first, the S waves also within the Earth arrive second, and when these waves hit the surface a lot of this energy gets trapped right along the skin of the Earth creating Rayleigh and Love waves, which in time usually come close on the tail of the S waves. Watching this scene from the earthquake of Kobe, Japan, possibly the most expensive earthquake in history, we see these two stop when they feel the P-wave hit, and slight S-waves began right before the major shaking of the surface waves. You can imagine if these people were out in the middle of an open field that it is much less likely for them to be injured. You get a real feel from watching earthquake videos why we say earthquakes don't kill people, buildings kill people. To fully understand what is going on in an earthquake we need to understand some of the other properties of these waves. Refraction is the term used for the bending of waves, and as the reason we see a mirage glimmering in the distance. Light waves leave heated air to less heated air, and bend as they go from less to more dense air. In a sense, the waves bend toward lower density, so it goes with seismic waves which also bend toward lower density material as they move. Thus many waves which started going down into the Earth curve back up until they hit the surface. For this reason, the P-wave often hits us as if traveling from more directly below us, and thus kicks us up and down. So if you feel an upward jolt, stay alert for increasing shakes. Besides refraction, the bending of waves, there is also reflection, the bouncing of waves. The bottom of the crust, also known as the Mohorovicic discontinuity, or Moho for short, was discovered as it was a reflective surface within the Earth. P and S-waves both can bounce off the Moho, and then hit the surface with a bit more concentrated shaking. All this reflection and refraction is the reason you can be a little closer to the epicenter over the focus, and still have less shaking than an area further away. This is why San Francisco was so badly shaken during the World Series in 1989, interrupting the baseball game and tragically collapsing the Bay Bridge. The convergence of reflected and refracted waves concentrated, unfortunately, on San Francisco, and thus increased the shaking and damage. Another interesting property of these waves that helps distinguish them is that all of them go through the solid earth, but only some can go through fluids like liquids or gases. We know Rayleigh waves can go through fluids thanks to our analogy to water waves. Water is a fluid that can roll in Rayleigh waves. That explains the waves at water's surface, just as it explains Rayleigh waves at the Earth's surface. But you can also hear underwater, and when you hear my voice now through air it is a type of pressure wave when my vocal cords kick the air like a slinky P-wave. P-waves can go through fluids whether liquid or gas. That leaves the S-waves and Love waves; they can't go through fluids. S-waves are like Love waves, kicking side-to-side perpendicular from the direction of movement. Think about trying to do this in a fluid. When a fluid is kicked side to side like this, it isn't elastic and snap back into place. It swirls off and dissipates the energy into random motion, heat. If Love waves on land encounter a lake or ocean, they can't cross, while their cousin Rayleigh waves can. Inside the Earth, if there were any liquid down there we could tell because P-waves would pass through, maybe refracting and reflecting on the way, while the S-waves would be absorbed and fade out. When we come back next time, we will find what all this bending, bouncing, and blending out does to help us see within our planet. We may live on the Earth's skin where the worst shaking takes place, but at least earthquakes give us body waves that act like x-rays on the body of our planet. When we come back next time, we look within and find our fundamental structure, here on Earth Explorations.