Online media for this item is not available.
Description
Michael Pentz introduces the unit. Russell Stannard introduces his discussion on P and S waves in earthquakes with a brief definition of the two kinds of waves (compressional and transverse waves).
... Ian Gass discusses the effect of P and S waves on the earth.
| 1 | 00:00:01,000 | 00:00:04,000 | Russell Stannard: Well a physicist is interested in many different types of wave - |
| 2 | 00:00:05,000 | 00:00:10,000 | sound waves, waves on the surface of water, waves going down springs… |
| 3 | 00:00:11,000 | 00:00:15,000 | ..and waves on ropes, and all these waves have in common the fact that their… |
| 4 | 00:00:16,000 | 00:00:19,000 | ..velocity depends upon the nature of the medium through which they’re travelling. |
| 5 | 00:00:20,000 | 00:00:22,000 | Now, we can divide these waves into two main classes, |
| 6 | 00:00:23,000 | 00:00:26,000 | there are the compressional waves: and these are waves in which… |
| 7 | 00:00:27,000 | 00:00:34,000 | ..the medium moves along the direction of the wave motion itself; |
| 8 | 00:00:35,000 | 00:00:36,000 | then there are the transverse waves which move at… |
| 9 | 00:00:37,000 | 00:00:39,000 | ..right angles to the direction of the wave motion. |
| 10 | 00:00:40,000 | 00:00:43,000 | Well, let’s take a look at these two waves and fix the difference in our minds. |
| 11 | 00:00:46,000 | 00:00:50,000 | (Russell Stannard) Here we have a row of dots representing a row of atoms. |
| 12 | 00:00:51,000 | 00:00:55,000 | If we give them a nudge you see the pulse is passed on from one to the other. |
| 13 | 00:00:57,000 | 00:00:59,000 | And so on down the line. |
| 14 | 00:01:04,000 | 00:01:08,000 | If we repeat this motion many times over then we get a series of pulses… |
| 15 | 00:01:09,000 | 00:01:14,000 | ..or a wave train, and the distance between these arrows is a wavelength. |
| 16 | 00:01:15,000 | 00:01:19,000 | Now if you fix your attention on just one of those particles you’ll see… |
| 17 | 00:01:20,000 | 00:01:23,000 | ..that it’s not in fact moving from left to right down the line, |
| 18 | 00:01:24,000 | 00:01:26,000 | all it is doing is just oscillating about its mean position. |
| 19 | 00:01:29,000 | 00:01:32,000 | And if we have an extended solid like the Earth, |
| 20 | 00:01:33,000 | 00:01:36,000 | then of course we have several of these rows of atoms. |
| 21 | 00:01:37,000 | 00:01:40,000 | And so this is the effect that gets produced. |
| 22 | 00:01:45,000 | 00:01:48,000 | Now this is the other type of wave, the transverse wave, |
| 23 | 00:01:49,000 | 00:01:52,000 | here the humps and troughs are moving from left to right. |
| 24 | 00:01:53,000 | 00:01:56,000 | But if you notice the particles, they are not moving in that direction at all, |
| 25 | 00:01:57,000 | 00:02:01,000 | they’re just moving up and down at right angles to the motion of the wave. |
| 26 | 00:02:14,000 | 00:02:18,000 | And here in the Earth we have a great many of these rows of atoms, |
| 27 | 00:02:19,000 | 00:02:23,000 | we can imagine them now in layers moving up and down relative to each other. |
| 28 | 00:02:25,000 | 00:02:28,000 | The disturbance still going from left to right. |
| 29 | 00:02:35,000 | 00:02:39,000 | Russell Stannard: Right, well the geophysicists are interested in… |
| 30 | 00:02:40,000 | 00:02:45,000 | ..both of those types of wave. The first they call P waves and the second S waves. |
| 31 | 00:02:46,000 | 00:02:50,000 | And these waves are created in earthquakes and we have here… |
| 32 | 00:02:51,000 | 00:02:53,000 | a famous earthquake region, it’s the San Andreas Fault, |
| 33 | 00:02:54,000 | 00:02:58,000 | we can see that in fact the picture is divided into two parts. |
| 34 | 00:02:59,000 | 00:03:01,000 | You’ve got a line running along here, and this is in fact… |
| 35 | 00:03:02,000 | 00:03:07,000 | ..the line of the fault and over here we have the direction of a river which… |
| 36 | 00:03:08,000 | 00:03:12,000 | ..comes down to this point here and then suddenly jumps discontinuously over there. |
| 37 | 00:03:13,000 | 00:03:16,000 | Well, it’s quite clear that what has happened is that part of the Earth’s crust… |
| 38 | 00:03:17,000 | 00:03:21,000 | ..there has moved along the fault line relative to the other part. |
| 39 | 00:03:24,000 | 00:03:27,000 | (Russell Stannard) Now one can demonstrate what is happening here… |
| 40 | 00:03:28,000 | 00:03:34,000 | ..with the aid of this model. I’ve got here two blocks with springs attached… |
| 41 | 00:03:35,000 | 00:03:39,000 | ..to them and with the help of this model we can in fact put the Earth’s surface… |
| 42 | 00:03:40,000 | 00:03:43,000 | ..features back to where they were before the earthquake occurred, |
| 43 | 00:03:44,000 | 00:03:45,000 | in other words, let’s move it along like this so as… |
| 44 | 00:03:46,000 | 00:03:48,000 | ..to join up the rivers there and there. |
| 45 | 00:03:49,000 | 00:03:53,000 | And what happened in the past then was that stresses built up over the… |
| 46 | 00:03:54,000 | 00:03:58,000 | ..whole of this region and suddenly they gave way, you got a sudden slip… |
| 47 | 00:03:59,000 | 00:04:05,000 | ..and of course when you get this mass of earth moving this generates all kinds of waves… |
| 48 | 00:04:06,000 | 00:04:08,000 | ..and this is why we’ve got these springs here, with the help of these waves… |
| 49 | 00:04:09,000 | 00:04:14,000 | ..we can investigate the types of wave that we get in these various directions. |
| 50 | 00:04:15,000 | 00:04:18,000 | Well, let’s first of all take a look at a direction which is… |
Available online
