 This is a record section from the 2004 Sumatra Andaman earthquake. A record section just means that I've taken a bunch of seismograms and arranged them in order of their distance from the earthquake. So on the x-axis here is time and then on the y-axis is distance and degrees. And each of these seismograms is plotted with the name of its station and its distance away from the earthquake. So what I'm going to do now is just focus on one of these stations of the station SBA here and I'm going to show you just by sketching the path that each of these arriving waves took to get from the earthquake to the station. You can see that these colored bars here are arrival time picks for various waves and this is probably done by an automatic picker that knows about how long it takes each type of wave to get from one point to another on Earth. So if I just make this little sketch right here of the Earth and I'm leaving out the crust here but basically this is a cross section. Here's the mantle and here's the outer core and here's the inner core. And let's just pretend that my earthquake happened right up here at the top. We can kind of pick any spot I guess. And I'm going to choose this station SBA because it's handily almost exactly 90 degrees away. That's easy for me to freehand. Let's just put it at 90 degrees. So we'll draw a little house. That's our seismometer over there. And the first arriving wave that gets from the earthquake to this seismometer is this one that's marked in green right here. That's the direct P wave. The path that the direct P wave takes through the mantle is kind of like that. So there's the direct P wave. Notice how it curves. It doesn't follow a straight line path. And that's because seismic wave speed increases with depth. Now the next arriving wave is this rival marked in red and that's double P. What double P does is it goes through the mantle and bounces once in between the earthquake and the station and then it continues on its way back to the station. So in each of these waves is a P wave in the mantle so we call it PP. The next arriving wave, well actually this orange one and this yellow one come practically on top of each other. The orange one is the direct S wave. And the direct S wave follows the exact same path as the direct P wave. Only shear waves are slower so that's why it takes longer to get from the earthquake to the station. The yellow one is a little more interesting. It's an S wave that bounces off the core mantle boundary and then gets to your station. So it's S and then that little bounce point is called little C and then S. So the entire wave is called SCS. And the next arriving wave is this pink one and the pink one is double S which follows the same path as double P. Only shear waves are slower so it takes longer to get to the station. And those are all the waves that are marked here. Now we have this big high amplitude thing coming in a little bit later on and those are actually surface waves. So they travel along a path like this. It takes them longer to get there because the crust doesn't transmit waves as fast.