welcome to real civil YouTube channel in this video we are going to cover the sismic waves in last lecture we have covered the elastic rebound Theory now we are going to cover the sismic waves we will see the theory as well as some animations so that our concept get clear so basically sismic waves are classified as Body Waves and surface waves there are two main classifications Body Waves the waves travel through the Earth in all Direction and to all depth that means suppose this is our Earth okay so they travel deep within the Earth okay that's why we called it as a Body Waves because inside the body of the Earth and those travels on the surface are called as surface waves so basic classification is Body Waves and surface waves body waves are again classified into two types of waves first one is the primary waves also called as p waves and secondary waves also called as S waves so let's study primary waves in detail we know that primary waves means P Wes so there are many different names to this waves first one is first name is the primary waves this is also called as pushpull wave it is also called as longitudinal waves also called as compressional waves so these are all same primary waves Push Pull waves longitudinal waves and compress waves so let's see the how it propagates so these wave propagate by longitudinal longitudinal means suppose this in this direction the wave is propagating then there will be a compressive action which mean that the ground is alternately compressed and dilated in the direction of the propagation means what if the wave is going through this then there will be compression then there will be dilation then there will be compression then there will be dilation like that we'll see the animation right now just focus on this P wve here you can see it is getting compressed then dilation then compressed dilation compressed dilation and this is the direction of propagation of the wave so this is how the p waves travel don't look at S waves right now now there are some properties of the p waves p waves means primary waves these are the first wave to hit the Earth surface when they come from the interior they are the first to hit the Earth surface their speed is generally 8 to 13 km/ second therefore when an earthquake occur these are the first waves to reach any sismic station and hence first to be recorded so whenever earthquake occur these are the P these waves means primary waves as they have higher speed so that's why they reach to any earthquake station first that's why they are recorded first these waves travel in a linear Direction and they can travel through solid liquid as well as the gas there is one formula to calculate the velocity of the primary waves so this is the formula where here e is Young's modulus B is poci ratio generally considered as 25 and row is the density so this formula is generally used to calculate the velocity of the P WS again uh don't look at this figure just focus on this figure this is what they make compression dilation compression dilation and this is the direction of the propag of the wave now let's quickly move to the secondary waves again there are many different names Shear waves also called as secondary waves also called as transverse waves okay now let's see their properties these waves arrive after p waves that's why they name is secondary views they are arriving late that means their speed is slow so they are 1.7 times slower than the PVS we have seen one name that is transverse waves the reason is these waves travel in transverse Direction means like this this is suppose longitudinal then this is the transverse so like that the mement will be there okay and they can travel through solid only because they need sheer strength to travel solids have sheer strength but liquid does not have Shear strength that's why these waves cannot propagate through liquid transverse or Shear waves which means that the ground is displaced perpendicular to the direction of the propagation so here you can see the direction is this but the ground is displaced in this direction as you can see in this picture that's why called as transverse waves you can see how the mement is there how ground is moving direction of the wave is like this but the ground is moving like this uh on this slide there is a formula of calculation of velocity of the secondary waves you can just go through uh the meanings of the terms It's s similar to the P Wes again here you can see S waves they move like this the particles moving up and down and the velocity or the direction is forward now next classification first part we have seen Body Waves second part is surface waves so when the vibratory wave energy is propagating near the surface of the Earth rather than the deep in the interior so they are associated with the surface only so there are two types of the surface waves first one is the really wave and second one is the love wave these are called surface waves because their journey is confined to the surface layers of the earth only surface waves travel through the Earth's crust which is the topmost part and they do not propagate into the Earth's interior like P and S waves do they are traveling in the body of the earth and they are RWS or lwes are traveling near the surface they are the slowest surface waves are the slowest as they are slowest that's why they are last to be recorded on the earthquake station generally their speed is 4 to 5 kilom per second now let's see the effect made by the love views so they cause horizontal shearing of the ground their movement is like they move in a much the same way as a snake slithering across the ground like this we know that snake move like this okay so they move the ground horizontally in s waves uh we are seeing like through the elevation point of view and uh for love waves we are like a watching like a plan view okay so from the top view we are watching and yes waves like we are watching from the front so they move like a snake slithering okay so they vibrate in a perpendicular Direction compared to to the wave motion we know that wave motion is like this but the ground is displaced like this this is through the plan view from the top view and they are most destructive now if you talk about really waves we'll see the picture really waves love waves how they move in the next slide so surface waves that cause both horizontal and vertical movement with the ground means side by side as well as a pan down so we can say that motion as a rolling motion we can call it as a rolling motion and this is most destructive so here you can see the p waves compression dilation compression dilation S waves they are going up down up down like this but s Wes they are moving the ground horizontally like that here you can see like this okay whereas here there is a rolling motion of the r views you can pause and you can see again and again so that it gets clear how these waves moves okay thank you for watching the video please like share subscribe the channel if you have any doubt you can comment me we have covered different points in the earthquake engineering like today we have covered sismic waves in last lecture we have covered the elastic rebound Theory before that uh plate tectonic theory and continental drift theory then internal structure of the earth just go through this videos if you have any doubt you can comment me thank you so much