Lecture 24

Namaskar. So, last time we talked about seismic waves and considering that, we were talking about the interior of earth. And we had stopped here where we were talking about P-wave shadow zone and S-wave shadow zone. So, now let us see how the other waves, Love and Rayleigh waves behave. And then we will see all four together, like body wave and surface wave. So, in body wave, you have P-wave and S-wave, and in surface wave, you have Love wave and Rayleigh wave. Now, before reaching there, what we talked about last time, that the shadow zone, and this is also very important, because we were talking about how S-wave propagates and how P-wave propagates, and it will depend on which layer of the interior of the earth those waves will pass through and will not pass through. So, like this, our P-waves, all those layers, your mantle, outer core, inner core, whether it is liquid, solid or partially molten, it will pass through. But as I told you last time, its velocity will vary. We are mainly looking at its P-wave and we have already discussed the characteristics of P-wave last time. So, it will pass through all the layers, whether it is partially molten or solid, or liquid, or gaseous, it will pass through, but its velocity will vary. And we had discussed that it will refract in some area here, because it has a boundary and the property of the material is changing, so it will change from there, and because of this, it will have a shadow zone, because its path will deflect, and because of the path deflecting, you will not get it in that area. So, P-wave shadow zone will be formed like that. So, this is a cartoon, a sketch, which tells that if you have P-wave and S-wave, and suppose an earthquake is on this portion, this is the epicenter, if it is visible on the surface, the earthquake must have come on the depth, the hypocenter, we will talk about that, what is hypocenter, epicenter. But if you assume that this is our epicenter, so here we started generating waves from below, so how will your P-wave and S-wave propagate, and let us assume that wherever we make waves here, all the waves will pass through here, these are all the stations, these are all the stations, which we will record. So, now we have understood that where will be the shadow zone of P-wave, we know this, and what will be the shadow zone of S-wave, okay. So, if we see in this way, so here also different colors are given, this is your purple and blue color is given. So, if we see this, so these stations, because S-wave will come from here and will not go through this, S-wave inner core and outer core, because if it does not pass from inner core, then there is no possibility of going to outer core. So, it will leave from here, from this boundary, because these stations will receive from all of these. So, these stations will receive from all of these, but in these stations, S-wave will not receive. So, one is this, and because P-wave will pass through and through, so this will reflect, it will reflect a little, so because of deviation, we will get here and will not get in this portion, this will be the shadow zone. So, this portion will be our P-wave shadow zone, and this whole area from here to here, this whole area will be the S-wave shadow zone. So, in these stations, we will get only and only P-wave. So, these are the things that we will understand, that this area, where P-waves did not come, because earthquake happened in this way, wherever it happened, we will know, but it did not come through it, so because of shadow zone, it happened. So, this was about P-wave and S-wave, we are seeing about other stations. Now, if we see further, then we will see how we find out its magnitude and how we locate its epicenter, we will talk about that also in this lecture. So, let us move forward, S-wave is in a little detail, because I will skip it a little, but we can do some discussion on it, that whatever you will get in the literature, as it is said that PCP, SCS, such symbols will be seen. So, basically it has been shown that from where P-waves have come back after reflecting, they have come from the core boundary, so it will be said that it is bouncing back from the core boundary. So, SCS is like this, which is your secondary wave or shear wave, it has come back. So, the important thing here is that all the waves, although our S-wave is inner core, because the outer core is liquid, so it will not pass through it. So, whatever happens, our S-wave will reflect from here, either it will reflect from here or it will go along with it and go out, it will deflect here and it will reflect and come back. But the P-wave, it has the capability that it will come from here and it will pass through it, the velocity will be low. So, in a way, we will say that it will deflect or reflect. So, it came to us at the other place, it was reflected here, it was also reflected here and a little and it moved here and then it moved out here. So, this is called P-K-I-K-P. So, this is your P-K-I-K-P, it came out of the inner core and passed through both the outer core and inner core. So, it has been given a nomenclature. So, this was something, it bounced back directly from here and it remained within the crust and passed through the mantle. So, this is purely P-wave and some changes will be seen in terms of the velocity. So, this was one and if we look at the P- and S-wave and surface wave, then as I said that mainly this epicenter is measured in this way. We will talk about this later that the amplitude and timing of the first wave which will be received and the second wave which will be received, it is taken to calculate the epicenter and the magnitude is also taken to calculate. So, mostly if the earthquake is happening here, then your seismograph or seismic station is here, so what will it receive first. So, this is given here, but as we are going to talk about this now, so this is your surface wave, in which Rayleigh and Love wave will come and this one is our body wave. And this S-wave is also considered, although its amplitude will not be so much, but it is short of a secondary process, because your S-wave is reflecting and coming back from somewhere. So, that is why its amplification will not be so strong, which we are getting directly. As we can see that S, which is P- and S-wave, we will get it directly, so we will get this, because the signal will not be so strong. So, suppose if your S-wave is coming here, then what will S-wave do, as we have seen, it still partially damages, it will not do so much damage, though it is short of a secondary process. So, it will not be so much damaged though it is short of a secondary process. 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So now you can do one more thing here, which I was showing last time, the map, in which you have multiple lines here, okay, so you can do this exercise too, here it is shown here, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise, so you can do this exercise. So, these are the two main plates, the Pacific plate and the North American plate, the moment is shown here. So, this is the trench zone here, but there are a lot of fault lines here, which have different velocities, or we can say that the slip varies. Now, if we talk about the earthquake, it is a simple process, and some people may know that basically what we are doing, we are applying stress, we are giving a pressure, we are giving a force, and in that we are creating a strain. So, the earth material is not brittle in every portion, but if it goes to the depth, then it will not be in sort of a brittle nature or ductile deformation, it will be elastically deformed. So, basically, if we take any material, earth material, then it is seen that it is an elastic deformation, then it will go to the ductile, and some portion will consume the permanent deformation, and its shape will not change. But the material, the earth material is seen, it is elastic, but the time will come when it will go to the ductile deformation process, and the final failure will occur, and this final failure will fracture, and this fracture will produce an earthquake. So, deformation of rocks under tectonic stresses. So, if you look at it a little more, you will understand it better. So, this is what you have, you have stress, and you are increasing the stress, you are developing strain in it. So, it will be elastically deformed up to some portion, which we call elastic deformation that it is reversible, it will come back. But this does not happen, when will it come back? Suppose you have stopped applying force. So, then it goes to the plastic deformation, which is non-reversible, and finally your brittle. that is your failure, and that is your earthquake actually. So, if you understand this simple process, then you will understand the whole process of earthquake. And if this happens, then your seismic waves will be generated, so you will understand the portion we saw later. So, let us talk about one more example. Suppose you have taken a rock block, or you have taken a cylindrical block, and you have deformed it. So, under the condition represented of this, it is not a simple fracture, it is a Potomac fracture. So, this goes into brittle deformation. So, brittle deformation means that this is in a shallow crust. So, this condition will be in shallow crust, where your surface will also come in the crust. But the other one, this one, this remains in the ductile. It is deformed, but in this prominent fracturing is not done. So, this is a deeper condition, where your rock softens. The rock will not remain tight, it will become soft. So, if you deform the soft rock, then you will be in a different deformation pattern, which we will call ductile deformation. But if it comes to the surface, then it will go into brittle deformation. So, keep these things in mind, then you will understand that if the deformation reaches the surface, then it will be brittle. And if we go to the depth, then it will be ductile. Thank you so much. Namaskar.