seismogram analysis In this video we're going to analyze an earthquake seismogram to learn about that earthquake Let's get started Here's an example of a seismogram So imagine for a moment you're sitting at a seismometer and somewhere on the planet an earthquake occurs and your seismometer records the vibrations from that earthquake in the diagram you see here Believe it or not from this simple diagram we can deter determine a lot of information about this particular earthquake Today I'm going to show you how to find the P and S-wave arrival times the lag time the distance to the epicenter the P and S-wave travel times and the earthquake origin time So let's get started When we look at a seismoggram you'll notice two distinct points on the seismogram The first one is when the vibrations first arrive They tend to be smaller vibrations And the second one happens sometime later on They tend to be larger vibrations Now what we need to be aware of is that the first vibrations are always going to reflect the arrival of the Pwaves Pwaves or primary waves travel faster than S waves And so they're always going to arrive to a seismoggram first So all I need to do is look at the arrival of the Pwave and head down to my time scale and record the time that that Pwave arrived In this example it's about 8:21 If you notice there's another big jump in the earthquake vibrations as shown on the seismogram and that reflects the arrival of the S-wave The S-wave or the secondary wave travels slower than the Pwave and therefore it always arrives at least a little bit later on In this case it appears to have arrived at about 8:30 And so I'm going to record that in my table The next thing we want to be able to calculate is what's called the lag time The lag time is simply the difference between the arrival of the Pwave and the arrival of the S-wave This is a really important number because it can help us to figure out other things like the epicenter distance Just keep in mind that if the lag time is a smaller number that means you're closer to the epicenter but we'll talk more about that in a moment First let's calculate the lag time for this earthquake To do this we're going to take the arrival time for the S-wave and subtract the arrival time for the Pwave So in our example we're going to take 8:30 and subtract 8:21 giving us a lag time or a difference in arrival time of 9 minutes We'll add that to our chart Now once we have that piece of information we can do a little bit of work to figure out how far we are from the earthquakes epicenter which you can imagine is a valuable piece of information To do this we have to keep in mind that we have a lag time of 9 minutes And we're going to go and grab our Pwave and S-wave travel time chart found in our Earth Science reference tables I'm going to quickly walk you through this process which we call the wedge method We're going to grab a piece of scrap paper and we're going to line it up with the vertical axis on the chart And we're going to go ahead and mark off whatever our lag time is In this example our lag time is 9 minutes And so you'll see I'll mark that 9 minutes right on the edge of the scrap paper Now I'm going to wedge it in between the two curves until it fits perfectly And when I get it lined up nice and straight up and down and touching the two curves the Pwave and the S-wave curve I know I'm in the right spot And all I need to do is follow the edge of my scrap paper down to the bottom of the chart like so And that will reveal how far I am from the epicenter In this case it appears to be about 7,600 km We can add that to our table So what I've just established is that when I have a lag time of 9 minutes that means I'm 7,600 km from the earthquake epicenter Now once I've got this I can tell some other interesting things including the Pwave and S-wave travel time Now here's what this means This is going to tell me how many minutes a Pwave or an S-wave traveled for from the earthquake to arrive to me In other words how long does it take a Pwave to go 7600 km so let's go back to our chart and let's find that distance that we just established that 7600 down on the bottom of the chart And we'll go up from that until we hit the P curve And once we hit that P curve we'll head over to the left and we'll see that it takes a Pwave about 11 minutes to travel 7600 kilometers So I'm going to add that to my chart I can do the same thing for the S-wave Back to my travel time chart back to my distance of 7600 but this time I'm going to go all the way up to the S-curve and over And I see that my S travel time is going to be about 20 minutes That's an important thing because in order to find the time that the earthquake happened also known as the origin time I need to know the travel times Now there's actually two ways to calculate the origin time We can do it using our Pwave data or we can do it using our S-wave data Now if we did things properly you should get the same answer both times Here's how it works I want to know what time on the clock that the earthquake actually occurred So what I'm going to do is I'm going to look at my Pwave arrival time This is the time that the Pwave got to my seismogram I'm also going to look at the Pwave travel time which is how long it traveled before getting to me Now if you think about this in terms of being something like let's say the pizza delivery man If the pizza delivery man arrived at my house at 8:21 and it took him 11 minutes to get here can't I figure out what time he left the pizza place it's the same idea here We're going to do a subtraction problem with the P arrival minus the P travel Something like this P the Pwave arrived at 8:21 after traveling for 11 minutes which means it left the earthquake at 8:10 So I know my origin time or the time of the earthquake was 8:10 Now theoretically if I do this with the S arrival and travel I should get the same time So let's say the soda man arrives at my house at 8:30 after traveling for 20 minutes Let's do the subtraction S arrival minus S travel is going to give me an origin time of 810 Notice it's the same as when I did it with the Pwaves So I know that the earthquake happened at 810 And so that's all the information we can find by simply looking at this basic seismoggram diagram It's important to be able to find out all of this information and to comfortably use the chart in your Earth Science reference tables