### Magnitude

The magnitude of an earthquake is a number that describes the amount of energy released during an earthquake. It ranks one earthquake vs. another.

The magnitude is a single number assigned to each earthquake event that does not depend on distance or location away from an epicenter. It does not depend on the type of material encountered by radiating earthquake waves. The magnitude is like the volume setting on an MP3 player. It describes loudness at the source, not exactly how loudly you hear it at different distances or through different kinds of walls.

## Observe & Discuss:

### The marathon as an example of dispersion

Runners wear different colors based on how fast they run.
Dispersion is a

• Observe runners that move at different speeds along the course of a marathon.
• Observe and recognize that the runners in different colors moved at different speeds?
• Describe how runners of different speeds separated as the race went on?
• Discuss how the degree of sepration is related and thus a clue to the distance from the start.

### What is the analogy?

• Discuss and recognize how this scenario is an analogy for the behavior of P and S waves.
• Because P and S waves move at a specific and constant speed, the lag time between these two waves is unique for each distance from an epicenter.
• For P and S-waves traveling at 6.5 and 3.5 km/s, at 100 km, the lag time is 14 seconds.
• Thus lag time can be used to determine distance from an epicenter.

## Observe & Discuss:

### The marathon as an example of dispersion

Runners wear different colors based on how fast they run.
Dispersion is a

• Observe runners that move at different speeds along the course of a marathon.
• Observe and recognize that the runners in different colors moved at different speeds?
• Describe how runners of different speeds separated as the race went on?
• Discuss how the degree of sepration is related and thus a clue to the distance from the start.

### What is the analogy?

• Discuss and recognize how this scenario is an analogy for the behavior of P and S waves.
• Because P and S waves move at a specific and constant speed, the lag time between these two waves is unique for each distance from an epicenter.
• For P and S-waves traveling at 6.5 and 3.5 km/s, at 100 km, the lag time is 14 seconds.
• Thus lag time can be used to determine distance from an epicenter.