1. Finding the Distance to the Epicenter of an Earthquake!
Finding an Epicenter
Finding the Distance to the Epicenter of an Earthquake!

2. 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.

4. 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

5. 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

6. 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

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

8. TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME

9. 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

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

11. Recording Board Difference in arrival times:
San Francisco: 4:10
2,800km
Open your compass to the EXACT distance on the scale.
1,000
2,000
3,000
4,000
5,000

12. . . . Use your carefully set compass to
draw a circle around each city. . . .
You've found
the epicenter!

13. Finding the Earthquake Origin Time
You take a distance to epicenter that you have already determined, or one given to you
Let’s say the epicenter distance is 4400km, and the arrival time is 3:21:15 PM
Well, use your ESRT and do some time math!!!
Let’s try it

14. Do it! Find the distance on the horizontal axis.
P wave travel time = 00:07:40
Find the distance on the horizontal axis.
Go up to the point where you hit the P-wave travel line.
Go over to the vertical axis and read off the travel time for the P-wave.
Epicenter dis = 4400km

15. Now, do the math!
Take the arrival time, and subtract the travel time from it!
3:21:15 – 00:07:40 =
3:13:35
So, the origin time of the Earthquake is 3:13:35 PM.
Now YOU try it for San Francisco!!