#### 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? (drag left and right to change time)

**P-wave Travel Time = minutes**

How long did it take the S-wave to reach the point?(drag left and right to change time)

**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).

*EPICENTER DISTANCE* =

EARTHQUAKE P-WAVE AND S-WAVE TRAVEL TIME

TRAVEL TIME (mins)

EPICENTER DISTANCE (10^{3}km)