1. Aim: How do we locate Epicenters?

2. An earthquake just occurred somewhere in the United States…
What is the area called where the earthquake occurred?
Name 2 types of waves that come out of this area?
Describe the differences between these 2 types of waves
Epicenter
P-Wave and S-Wave
P-Waves travel faster, arrive first, travel through liquids and solids ; push and pull the ground as they move (compress and expand).
S-Waves travel slower, arrive second, travel only through solids; lift and lower the ground as they move (transverse).

3. I. Locating an Earthquake Epicenter
The difference in arrival times of P and S waves help locate the epicenter.
The greater the difference, the greater the distance to the earthquakes epicenter.
Take a worksheet from the top bin to practice subtracting P and S wave times.

4. II. Distance to Epicenter (working with ONE wave)
Using the ESRT page 11 we can relate the P or S wave’s travel time to the distance we are from the epicenter.
Take out p. 11 of the ESRT
Examples:
A P-wave takes how long to travel 2600 km?
An S-wave travels how far in 4 min?
After 8 min and 20 sec. a P-wave travels?
An S-wave travels 4.0 x 103 km in…
5 min
1000 km
5000 km
00:12:40 min

5. Work on questions 1 - 8 ONLY!
Get an Earthquake P and S wave Travel Time worksheet from the bottom bin.
Work on questions ONLY!

6. III. Distance to Epicenter (with TWO waves)
Where does the difference between the P and S wave exist on this chart?
“Spread Technique”
Ex: S-wave – P-wave = 00:04:45
1) Use the time axis to mark off the difference in time.
2) Match the P and S wave lines with the marks you made.
3) Read down from this intersection to find the equivalent distance.

7. Complete the worksheet

8. IV. P- Wave Travel Time (or s-wave travel time)
Use the distance from the previous step to find the travel time.

9. V. Origin Time The origin is when the earthquake began. (start)
Subtract the travel time from the arrival time
Origin (start) Travel Time Arrival (end)

10. V. Epicenter Location
Data from three or more seismographs must be used to find the exact location of an earthquake epicenter.
Epicenter
- Use the distance to epicenter value and a protractor to draw these circles.

11. Closure:
Looking at the chart on page11 of your ESRT, explain why the difference in travel time between the P and S wave increases as you move further from the epicenter of an earthquake.
- Since the p-wave is faster, it travels farther than the s-wave in the same period of time. This causes the difference in travel time between the two to increase as distance from the epicenter increases.
Review Book: p #1-10