### Preconceived notions

* This diagram may be better viewed in full-screen mode.

## Observe & Discuss:

### WHERE do you think the earthquake started?

• What is an epicenter?
• Why is the epicenter close enough to the focus (origin) for most purposes?
• Why do most earthquakes occur close to the Earth's surface, relative to the entire depth of the Earth?/li>

### Could the earthquake have started 100 miles away?

• What was the magnitude of the earthquake, closer to 3 or 7? Can you tell?
• How far away was the earthquake, more or less than 100 miles away? Can you tell?
• Did the earthquake start closer to Mexico, Los Angeles or Kansas? How can you tell?/li>

### Conclusions that need to be reached

• When your home or your seismic equipment is shaken by an earthquake, you can't tell where the earthquake originated even though knowing the location of the epicenter is important.
• Based on past experience, you know that the earthquake must have originated relatively nearby, usually within a few hundred miles radius.
• You don't know the DISTANCE or the DIRECTION of the epicenter.
• Thus you need a method to locate the epicenter.
• If you had an infinite number of seismic stations, you could share the information and figure out the epicenter, but that wouldn't be economical.
• Scientists have devised an economical method that uses the information from just three stations to solve the problem.
• That method determines DISTANCE based on S-P lag time and exact location by triangulation.
• People want to know about epicenters right after an earthquake to know where to check for damage and send emergency assistance.
• Scientists want to know where a new earthquake falls along a pattern of previous earthquakes because it provides clues about the causes of earthquakes and the likelihood of future events.
• People want to know about the likelihood of future events to minimize loss of life and property.