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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.