Hi everybody and welcome to this episode of Class Captures. This is your host Jasper Sr. Today I'm going to talk about how to determine P and S wave travel times and how to use page 11 in your reference table. So it would definitely help to have your reference table out while you're watching this video. So the first thing I want to go over is the different axes.
The horizontal axis is the axis for distance and you'll see on the bottom it's labeled epicenter distance and it has a number times 10 to the third. So the first thing I want to do is I want to actually go through scientific notation because the whole numbers one to 10 on the bottom aren't really just whole numbers. There's something else.
All right. So what exactly is 10 to the third? Well, 10 to the third is 1000. And the way you get to that is 10 times 10 times 10. So 10 7 times 10 is 100. And 100 times 10 is 1,000.
All right. So if we just take one of these, like 8, for example, 8 times 10 to the third is 8 times 1,000 equals 8,000. All right. So that's how you figure out part of the distance scale at the bottom here. Now, each box equals 200 kilometers.
And the way you calculate this is very similar to how you calculate Contour intervals when you're reading a topographic map. So you take that 1,000 difference that's in between each of those whole numbers there. And you divide it by the five spaces. So you get 1,000 divided by five.
And that's 200. So 200 kilometers. So each little box here is 200 kilometers. So if we looked at another just a random point here, the red dot with the X in it, that would be 1,200 kilometers.
Now let's look at the vertical axis. This is a scale of time and this is minutes. So the minutes are divided into 60 seconds.
And so again, it's similar the way we found out what the little boxes were for the horizontal axis. We take 60 seconds and we divide them by the number of spaces, which is three. So 60 divided by three is 20. So each little box is 20 seconds. So if we just picked a spot like this one.
So it'd be three minutes and 40 seconds. So let's start looking at the chart itself. Now, this isn't necessarily designed to be as much of a content lesson as it is more just looking at this chart and reading it.
So one thing that you do need to understand though, is that an earthquake has P waves and S waves, two types of energy that it releases and they go. One, P waves go faster than the other type, which is S waves. And a good way, when I'm first teaching people about earthquakes, to explain it is like lightning and thunder.
Lightning, you see the lightning and you have to wait a little while to hear the thunder. And that's because light and sound go much different speeds. Just like P waves and S waves.
So P waves are like the lightning. They happen first. They arrive first at your location.
They're traveling faster. They travel as compression waves, which are sort of like, if you could picture almost like dominoes falling. The molecules hit into each other in a linear pattern. And because they're compression waves, they travel through liquids and solids. All right.
So to use this to find the travel times, you go, we'll just pick 3000, for example. We draw a line up to the P wave. And then we draw a line. over to the time axes. And we see there 3,000 kilometers for a P wave to go 3,000 kilometers, it would take five minutes and 40 seconds using those scales that I just, just explained.
Now using the S waves is exactly the same. I just want to make sure that you use the correct line. All right. So it's the other line it's labeled. S.
If you do have a printed paper copy of your reference table, you might want to use a highlighter to highlight them different colors or something like that to help you remember. S waves are pretty easy to remember because there's a lot of at the letter S. So they're secondary waves. That's what they're also known as. They arrive second.
There's an S in second. They travel slower. There's an S in slower.
And they're shear waves, side to side waves. and that's if you took like if you took a jump rope and you um kind of wiggled it back and forth really fast and it would wiggle like a snake down and transmit energy down to the other end those are shear waves all right and the energy actually is traveling at 90 degrees to the direction the the wave is going so this is the reason it can only travel through solids so that there's another s only solids all right so there's a lot of s's to help you remember s waves But we figure out their... Their travel time is exactly the same. I picked 4000 for this example.
Alright, we go up. We again make sure we use the correct line and then we go over. So for an S wave to go 4000 kilometers, it would take 12 minutes and 40 seconds. So this is how you use the chart on page 11 of your reference table to determine P and S waves. Thanks for watching and stay tuned for more episodes of Class Captures.