Because P-waves can travel in anything. Let's think about how those P-waves would travel. Well, they would just go in straight lines. There's nothing that would refract the P-waves. They would just go in straight lines radially outward from where the earthquake occurred. Now, at a first approximation, we know that as we go deeper and deeper into Earth there's more and more rock above that. The weight of that rock is kind of compressing the rock below it.
So you get higher and higher pressures and higher and higher densities. So this is a uniform Earth. But let's imagine an Earth that's made up of uniform material, that's all solid, a completely solid Earth, but one where the density is constantly increasing as you go down. So let's just think about it before we go into the continuous case. Because we're talking about the density as you go deeper, it's just getting continuously more dense, let's think about the discrete case, where we have the least dense layer.
So let me draw it right over here. So let's say this is the surface of the Earth. This is least dense. Then let's say you have another layer over here that is more dense.
So this is more dense. Let's say you have another layer that's even more dense. So you have another layer over here that's even more dense. And then let's do one more layer. Let's do this layer here, this is the densest layer. So in general, your P-wave, your seismic wave, is going to travel faster in denser material. So it's going to travel the fastest here, then here, then here. It's going to travel the slowest in this least dense material.
So if you're coming in at an angle let's think about what's going to happen. So let's say you have your P-wave coming in at an angle like this. So it's going straight through the least dense material. Let me do a slightly shallower angle. So let's say it's like that. What's going to happen when it goes into the more dense material? So once again, let's imagine our little car. So this tire's going to be able to go faster before the tires on the other side.
So the car is going to be deflected to the left, to the down left. So now it's going to travel like this. So it's now going to travel something like this. Now, what's going to happen at this boundary? Once again, imagine the car. This tire right here is going to be able to travel faster before the other tire, so it'll be deflected even more in that direction.
Then we go, and we go to the densest material. Once again, the tires on kind of the bottom side when we look at this way are going to be able to move faster before the other tire. So we're going to get deflected even more. So you see, as you go from least dense material to more dense material you're kind of curving outward.
So if this was continuous, if you had a kind of a continuous structure, where as you go down it just gets more and more dense as you go. So this is less dense, and then it just continuously gets more dense. So this is the most dense down here. The two principal types of seismic waves are P-waves pressure; goes through liquid and solid and S-waves shear or secondary; goes only through solid - not through liquid. The travel velocity of these two wave types is not the same P-waves are faster than S-waves.
Thus, if there is an earthquake somewhere, the first waves that arrive are P-waves. In essence, the gap in P-wave and S-wave arrival gives a first estimate of the distance to the earthquake. Ask them what kind of earthquake wave motion this resembles.
The answer is shear waves. Remind them that in shear waves particles of material move back and forth perpendicular to the direction in which the wave itself moves. S-waves S stands for secondary are shear earthquake waves that pass through the interior of the Earth. S-waves don't change the volume of the material through which they propagate, they shear it.
Note: The motion of the rope due to shear waves is much easier to observe than the compression waves, but the shear waves travel more slowly than compression waves.
In an earthquake, scientists can observe the arrival of compression waves before the arrival of shear waves using seismographs. You may choose to show a close-up of a record seismogram for a single earthquake event, and ask your students to point out different seismic waves.
In addition, shear waves cause much more damage to structures since it is easier to shake surface rocks than it is to compress them. Encourage your students to critically evaluate their slinky and rope setup.
Ask them if they see any limitations associated with their setup. Ask them to compare and contrast their simple setup with actual vibrations caused by seismic waves traveling through the Earth or along its surface. For instance, seismic waves carry energy from the source of the shaking outward in all directions not in one direction only as the setup shows. Optional Both primary and secondary waves are body waves pass through the interior of the Earth.
Surface waves travel along the Earth's surface. Two examples of surface waves are Rayleigh waves and Love waves. Explain to your students that Rayleigh waves cause the ground to ripple up and down like water waves in the ocean before they break at the surf line whereas the Love waves cause the ground to ripple back and forth like the movement of a snake.
Ask the students to recall how scientists use seismic wave observations to investigate the interior structure of the Earth. This is similar to checking the ripeness of a melon by tapping on it. To understand how scientists see into the Earth using vibrations, one needs to understand how waves or vibrations interact with the rocks that make up the Earth.
Introduce to your students the two simplest types of wave interactions with rocks: reflection and refraction. Ask students to define reflection. They should be able to give simple examples like echoes or reflection in a mirror. Explain to your students that echoes are reflected sound waves, and that students' reflections in a mirror are composed of reflected light waves. Tell students that a seismic reflection happens when a wave impinges on a change in rock type.
Part of the energy carried by the wave is transmitted through the material refracted wave and part is reflected back into the medium that contained the wave. Refraction can be demonstrated by dropping a coin in a bottle filled with water.
The coin changes direction when it hits the water's surface and won't sink to the bottom vertically. In other words, the path of the coin refracts changes direction when moving from the air into the water. Explain to students that seismic waves travel fast, on the order of kilometers per second. The speed of a seismic wave depends on many factors. Ask students to think about a few factors that can change the speed of a seismic wave e. Students should be able to answer these questions based on the knowledge they acquired throughout this lesson.
Seismic waves travel faster in denser rocks; temperature tends to lower the speed of seismic waves; and pressure tends to increases the speed. Caution: The speed of a seismic wave generally increases with depth, despite the fact that the increase of temperature with depth works to lower the wave velocity.
Aa Aa Aa. Lesson 6: Seismic Waves. The earliest scientists first observed the waves that earthquakes produce before they could accurately describe the nature of earthquakes or their fundamental causes, as discussed in Lessons 1—5. Therefore, the earliest solid scientific advances in seismology concerned earthquake waves.
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