1. SEISMIC WAVE BASICS

2. Finding an Epicenter

3. What is an Epicenter?
A point on the surface of the Earth directly above the FOCUS of the earthquake.
The point within the Earth from which earthquake waves originate.

5. Use the data from the recording stations:
Station A: San Francisco, California
P-Wave arrival 3:02:20 S-Wave arrival 3:06:30
What is the time difference between P and S wave arrivals?
4:10

6. Use the data from the recording stations:
Station B: Denver, Colorado
P-Wave arrival 3:01:40 S-Wave arrival 3:05:00
What is the time difference between P and S wave arrivals?
3:20

7. Use the data from the recording stations:
Station C: Missoula, Montana
P-Wave arrival 3:01:00 S-Wave arrival 3:03:00
What is the time difference between P and S wave arrivals?
2:00

8. Difference in arrival times:
San Francisco: 4:10
Denver, Colorado: 3:20
Missoula, Montana: 2:00

9. TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME
4:10
2800Km

10. MOVE THE PAPER UNTIL THE TWO TICK MARKS LINE UP WITH THE P AND S CURVES
WHEN TICK MARKS LINE UP, GO STRAIGHT DOWN AND READ THE EPICENTER DISTANCE
EPICENTER DISTANCE OF 2800 KM

11. EPICENTER DISTANCES San Francisco: 4:10 Denver, Colorado: 3:20
2,800km
Denver, Colorado: 3:20
2,000km
Missoula, Montana 2:00
1100km

12. Recording Board Difference in arrival times:
San Francisco: 41:0
2,800km
Open your compass to the EXACT distance on the scale.
1,000
2,000
3,000
4,000
5,000

13. . . . Use your carefully set compass to
draw a circle around each seismic station. . . .
You've found
a possible epicenter!
Does this seem
like a likely location
for an earthquake to occur?

14. Compare your answer to the historical record

15. Need a closer view to be sure. Go to http://earthquake. usgs
Need a closer view to be sure? Go to and retrieve the seismicity map (like the one on the left) for the state containing your possible solution.
Based on this map, earthquakes have previously occurred in the region where our epicenter fell. Thus our answer seems likely
USGS

16. What if our epicenter had fallen here instead
What if our epicenter had fallen here instead? Does this mean our solution is wrong or less likely? What could help us find out?
Check our work to see if we made a mistake
Additional circles created from other stations e.g. the more stations, the more accurate the location.
A longer historical seismicity record. This record is from
Perhaps of map of historic faults in the region.

17. Credits: This file was originally created by Robert M
Credits: This file was originally created by Robert M. DeMarco of Marlboro Central High School Edits have been added by:
Chris Visco - Sachem High School
Eric Cohen- Westhampton Beach High School
Michael Hubenthal - IRIS Consortium