1. Roy G Biv Locating Epicenter Lab Helper
Please view as a slideshow (click slideshow->from beginning). Go ahead… do it now.
The purpose of this slide show is just to familiarize you with the steps of locating an epicenter.
If you’d like extra practice, complete the time subtracting and epicenter distance tasks on a separate sheet of paper. (slides 4-8). Note that this is extra practice.
Only Chicago, Tampa and Wink data are necessary for our lab procedure.
READY? Take out your ESRT, a red pencil, a pencil, and a scrap paper, if you happen to have a safety compass, try it out! If not, don’t worry. We have plenty in the classroom and we’ll have plenty of time to practice with them!

2. Finding Epicenters

3. S wave is always the bigger # so it always goes on top
29 m
+60
S wave is always the bigger # so it always goes on top X X
S Wave m 00s -
P Wave m 20s
7m s
When subtracting time, be sure to borrow (if necessary) 60 seconds as shown!
Show every ridiculous step.
Try the time subtraction problems on the following slides. Did you get the accepted answer?

4. Make sure you know how to read a seismogram!
Notice, the time is recorded when the wave begins!
2:33:00
2:35:30
2:35:30 – 2:33:00 =
00:02:30

5. 2:35:10
2:39:20
2:39:20 – 2:35:10 =
00:04:10

6. 2:32:10
2:34:00
2:34:00 – 2:32:10 =
00:01:50

7. Want to try a few extra practice?
(These are data from another earthquake)

8. How to use the P & S
Wave Chart
Back to Chicago, Tampa and Wink ;)

9. We don’t know when the EQ started.
But we know how much time there was between the P&S wave arrivals.
Let’s say 0:04:00
C’mon let’s say it!

10. We have to find a spot on the graph where P&S (careful
We have to find a spot on the graph where P&S (careful!) are separated by 4:00.

11. We use scrap paper for this
We use scrap paper for this. Be sure to mark off the minutes so that you don’t mix your stations up!
Scrap paper
0min
4min

12. Then drop straight down to see the distance to the epicenter.
2,600km

13. We do this by moving the scrap paper over until each line touches
0min
4min
Notice the slanty line for the zero? This is so that you can “ride” the p wave line up until the S line matches. km

14. Now that we know the EQ was 2,600km away, when did it start?
To travel 2,600km, a P-wave…
needs 5:00 minutes

15. Back to the lab…
The difference in arrival time for Chicago is 0:02:30.
That means that Chicago is ______ km from the epicenter.
1,400

16. Now that we know the EQ was 1,400km away, when did it start?
To travel 1,400km, a P-wave…
needs 3:00 minutes

17. If the P-wave was detected at 2:33:00 and needed 3:00 minutes to travel it must have started at…
-3:00
2:30:00
This is the “origin time”
See pg 70 in the review book for more help with origin time.

18. Let’s get acquainted with our new tool: The SAFETY COMPASS 
Commonly referred to by Ms. Susinno as the “bippy”, this dot goes over the zero on the scale and then the city’s dot
Unless there is a (non-fitting) emergency, we use the first pencil hole. This goes on the epicenter distance value on the scale.
This is how we hold the compass to draw a circle. (Of course no Earth Science student in their right mind would use a PEN on a diagram like this kid. And Look!!! He/she already broke the first pencil hole rule! )
Don’t worry, I fixed it 

19. 1,400km

20. Place the “bippy” on the zero on your distance scale and the first pencil hole on the 1400 km line (or whatever your city’s epicenter distance was).

21. Next. The bippy goes on the city dot
The first pencil hole stays at 1400km as you measured on the scale. Our compasses lock 
Next. The bippy goes on the city dot
Then you use the compass to draw the circle around the city!

22. The EQ happened somewhere on this circle

24. x Does your lab look like this?
The Epicenter is where all 3 circles intersect!
Ta Da!
1400 km x
1000 km
2600 km
The CENTER of your X should cover the point of intersection. MAKE SURE the rest of your X is visible!

25. Your circles do not intersect at one exact point!
But sometimes….
Your circles do not intersect at one exact point!
Don’t worry!
You can still find the epicenter
(You little Earthquakeologist, you)

26. Notice, the center of the X is NOT on the line
Notice, the center of the X is NOT on the line. It is in the center of the triangle where ALL 3 CIRCLES INTERSECT! X

27. How to triangulate with an imperfect epicenterX

28. How to triangulate with an imperfect epicenterX

29. How to triangulate with an imperfect epicenterX

30. If you get a nice epicenter, you get to do a VICTORY DANCE!
We’ll have plenty of real practice in the classroom!
The End