Earthquakes
An earthquake is a movement or shaking of the Earth's crust. Most earthquakes occur along a fault. A fault is a crack or break in the Earth's crust along which there has been some movement.

This picture shows the effect on the surface after the movement along such a fault.

The exact location of the crustal movement is called the focus. Since we are usually concerned about effects on the surface, we often refer to the epicenter, which is the location on the surface directly above the focus.

When an earthquake occurs, several kinds of seismic waves are produced, and travel outward from the focus.

Measuring Earthquakes
There are two different scales that are commonly used to measure the severity of an earthquake.

The Richter Scale measures the amount of energy released by the earthquake. It is a logarithmic scale, meaning that a 6 is 10 times more powerful than a 5.

The Mercalli Scale attempts to measure the severity of the earthquake by observing the damage that it causes. A simplified Mercalli Scale is shown below:

Earthquake Waves
Although earthquakes produce several different types of waves, we will focus (no pun intended) on two. P Waves and S waves. Both waves are produced at the moment an earthquake occurs, but they have several different characteristics. It is important to understand the differences between these two waves.

The two pictures below illustrate the difference between the motion in a P wave (the top), and an S wave (the bottom).

Locating the Epicenter
Since P and S waves travel at different rates, we can use them to calculate our distance to the epicenter. P waves travel faster than S waves, and will always arrive at a seismic station first. How far ahead of the S waves they arrive depends on how far away the earthquake is. The further away the epicenter is, the wider the gap will be between the P and S waves. This is similar to the effect during a thunderstorm, when you can estimate how far away the lightning is by timing how long you have to wait for the thunder.

You have a chart on page 11 of the Earth Science Reference Tables to help with this:

To use it, simply find the time delay between arrival of the P wave and the arrival of the S wave. Let's say the P wave arrives at 1:32, and the S wave arrives at 1:37. There is a 5 minute gap between the P and S waves. You would be able to see this gap on a seismograph like the one to the left. So you need to find the place on the chart where the P and S waves are 5 minutes apart.

To do this, draw a line on a sheet of scrap paper that represents 5 minutes on the graph. Then slide the paper up the curves until the 5 minute gap matches the gap between the lines. When you find the spot where the curves are 5 minutes apart, simply drop vertically down to read the distance. In the example above, the earthquake epicenter is 3,600 km away.

Locating the Epicenter of an Earthquake
Once you determine the distance from the seismic station to the epicenter, you could draw a circle around that station to show the possible epicenter locations.	To locate the epicenter exactly, you need 3 stations to all do the same thing. You will end up with 3 circles that only meet in 1 location: the epicenter.

The Earth's Interior
An interesting side-note to the discussion of seismic waves is that they are our main source of information about the structure of the inside of the Earth. Almost everything we know about the structure of the Earth's interior is based on inferences made from the analysis of seismic waves. From the wave travel times, speeds, and refraction (bending) we can estimate the density and composition of the Earth's internal layers.

Here's what we've learned:

The Earth has several distinct layers, including the crust, mantle, outer core, and inner core.

It is believed that the outer core is liquid, and that the other layers are essentially solid. This inference is based mainly on the fact that S waves can't penetrate the outer core. Since these waves can only travel through solids, the outer core is inferred to be of liquid composition.

The failure of S waves to travel through the outer core, along with the bending of waves due to density differences, gives rise to certain shadow zones when seismic waves travel. These shadow zones are areas on Earth that receive no seismic waves.

The structure of the Earth's interior is summarized on page 10 of the Earth Science Reference Tables:

	Notice that on the upper right hand side, there is important density information.
	Also, there is a graph showing how the pressure changes with depth. This is basically a direct relationship (as depth increases, pressure increases).
	Below this, there is a graph showing how the temperature changes with depth. This is also basically a direct relationship (as depth increases, temperature increases).