howdy everyone and welcome to our next video of module four where we will explore seismic waves the actual waves that are produced during these earthquakes okay so let's recap very quickly why do we feel an earthquake the shaking or vibrations from an earthquake travel outwards in all directions from the focus think of them like the waves that travel outward when you throw a rock in the lake these waves released from an earthquake are called seismic waves now seismic waves again these are the waves of energy that are moving outwards in all directions three dimensions from the earthquake focus remember the focus is where the earthquake originates within the earth's surface and the earthquake originates where the rock breaks where we have that um stress being exceeded and movement occurring the rock is breaking this is where our earthquake originates this is where our seismic waves are being generated again traveling 360° in three dimensions right at that point now there are two main categories of seismic waves we have body waves which includes P and S waves and we have surface waves we'll talk about each of these types here in more detail let's start with body waves primary waves or better known as Pwaves are the fastest waves they are the first waves to arrive in an earthquake event these are sometimes called pressure waves or compressional waves which is why they can also be referred to as P waves p standing for primary or pressure because they are compressional waves they move particles within the medium they are traveling through in the same direction it is moving producing a pushpull kind of wave like this slinky shown here and they also have the ability to travel through all mediums including solids liquids and gases let's compare that to our other type of body wave secondary waves or swaves s waves are slower than P waves so they are usually the second to arrive in an earthquake event hence the name secondary they are also shear waves which S could technically stand for secondary or shear wave with shear waves the particles of the medium it travels through move perpendicular at right angles to the direction of movement just like the rope shown there and because we cannot actually shear liquids and gases S waves can only travel through solids remember way back when we talked about the Earth's interior I mentioned we know the layers of the Earth because of these S&P waves where the S-waves were unable to travel through the liquid outer core well here I'm explaining that phenomenon a little bit further now here is a figure showing the different motions of the two body waves p waves are shown on the left with the slinky where they're where they compress and expand through the material as they pass and S waves are shown on the right there with the rope that oscillates perpendicular to the direction of wave motion also remember that during an earthquake the P waves arrive first because they're faster followed by the S waves now surface waves are the most dangerous during an earthquake and they are responsible for most of the damage to structures surface waves are the slowest wave so they are the last to arrive in an earthquake but again they cause the most intense shaking and therefore have the largest amplitudes on a seismog which we'll see in the next video now there are two main types of surface waves that you don't need to memorize called love waves or L waves and rally waves or R waves now when L waves pass the ground surface moves back and forth like a slithering snake shown on the bottom figure raves make the ground surface roll up and down both types of waves die out with increasing depth but they're most destructive at the surface now here's a nice figure that shows all three waves during an earthquake here at the top of the earth we have our hypoenter or focus and at this point seismic waves begin propagating body waves travel 360° from the rupture on the earth's surface and within the interior and those body waves are in the form of those P and S waves whereas the surface waves are only traveling 360 degrees on the Earth's surface here I have three seismographs shown which we'll get into later but each seismograph records these waves as they pass by the Pwaves are the fastest so they arrive first but they're usually the smallest in amplitude because they're compressional waves moving in the same direction um as the ground so that the wave is traveling through then we see the S-waves which arrive second and they are slightly larger than Pwaves because they're shearing the ground perpendicular to the direction the wave is traveling then finally we see the surface waves which arrive last because they're the slowest but as you can see they're the largest wave and they cause the most destruction rolling the ground up and down and shaking it side to side now the farther away these seismograph stations are from the focus the longer the gap is between each of these three waves and the smaller the amplitudes all right so can we use these seismoggrams to locate an earthquake we'll talk about that next