Types of seismic waves

The two types of waves you need to know are P-waves and S-waves. Their behavior is necessary to calculate the epicenter (location) of an earthquake.

P-waves (Primary waves)

• Travel faster than S-waves, and arrive at seismic stations first.
• "Push-pull", or compression waves.
• Travel through solids, liquids, and gases.

S-waves (Secondary waves)

• Travel slower than P-waves, and arrive at seismic stations second.
• Side-to-side, or shear waves.
• Travel only through solids.

Visualizing waves

The P-wave compresses matter, moving the sensor side to side.

The S-wave moves perpindicularly, moving the sensor up and down.

Move the slider to the right to simulate the travel of the wave.

Or to put it another way....

You're a sensor just sitting in the ground, waiting for an earthquake like

then a P-wave comes along and you're like

now you know there's an S-wave coming up next so you're all

then the S-wave hits you like

okay, fun's over

Locating an Earthquake Epicenter

Below is an interactive example of how to calculate the epicenter distance based on P-wave and S-wave arrival times.

Drag the underlined minute values to change the time it takes for each wave to reach the point.

How long did it take the P-wave to reach the point?
P-wave Travel Time = minutes

How long did it take the S-wave to reach the point?
S-wave Travel Time = minutes

This means the difference in time was Difference = minutes

(note that as you adjust the values, the slider on the graph changes length)

Drag the slider at the top of the graph to determine the distance at which the time difference betweent the two waves equals minutes (the line between the wave graphs will turn green).