seismic activity is also one of the most important sources of information about the interior of the earth In this video we're going to read about the earthquakes So what is an earthquake it simply means shaking of the earth It is caused due to release of energy which generates waves that travel in all directions Now the question is why does the earth shake to understand that you'll have to look at this picture This is a fault A fault is a sharp break in the crustal rocks Rocks near a fault tend to move in opposite directions that creates friction But at some point of time their movements overcome the friction As a result they slide past one another This causes a release of energy and the energy waves travel in all directions The point where the energy is released is called the focus of an earthquake or there is also a term given to it called the hypoenter The energy waves traveling in different direction reach the surface The point on the surface nearest to the focus is called epicenter This place is the first one to experience the waves All natural earthquakes take place in the lithosphere The lithosphere refers to the portion of depth up to 200 km from the surface of the earth An instrument called seismograph records the wave reaching the surface The waves look something like this The curve shows three distinct sections each representing different types of wave patterns We'll get back to this pattern in a moment Earthquake waves are basically of two types Body waves and surface waves I need you to understand the term body waves Sounds a little larger within the body right so the body waves are generated due to the release of energy at the focus that is the epicenter and moves in all direction traveling through the body of the earth You know it travels internally within the earth's surface and moves the interior of the earth That's why it is referred to as body waves because it is traveling through the body of the earth like a tremor Now it is obvious that when these body waves move towards the surface they will come in contact with the surface rocks and generate new set of waves These are called surface waves Now the velocity of waves changes as they travel through materials with different densities And we know that the materials with higher density recside downwards Therefore the denser the material the higher is the velocity The direction also changes as they reflect or refract when coming across materials with different densities That's why when the seismic waves reach towards the surface of earth there the materials are of lower density and that slows down the intensity of the waves leaving behind a fault Now going back to the picture that I showed you So if you look at this picture there are two types of body waves They are called PNS waves So the letter PNS stands for primary and secondary Now you can relate when I say primary they are the first one that gets recorded on a seismograph The instrument and then comes the secondary wave You can look at this illustration to understand how these wave looks The Pwave that is the primary or pressure wave is a pulse of energy that travels quickly through the earth's solid materials as well as liquids It forces the ground to move backward and forward as it gets compressed and expanded The S-wave that is the secondary or sheer wave follows more slowly with a swaying rolling motion that shakes the ground back and forth perpendicular to the direction of the wave If at all the concept of PNS wave isn't clear to you I recommend that you watch this video by Dr Keith Miller where he physically demonstrates PNS seismic waves The video by him is extremely helpful I link the video in the description and you can go and have a look at it Now going back to this picture after body wave that is PNS waves comes the surface waves The surface waves are the last to report on seismograph They look something like this This wave spreads across the surface of the earth These waves are also more destructive and damaging They cause displacement of rocks and hence the collapse of structures occurs If you want to see more of such educational content make sure you're subscribed By doing so you'll get an alert when my next video comes Thanks for watching and I'll see you in the next one