1. Do Now:
According to the seismogram below, What time did the P wave arrive?

2. Learning Objective:
I will be able to determine and locate the epicenter of an earthquake.

3. Success Criteria:
I can calculate the time difference between p and s wave arrival time.
I can use pg 11 of the ESRT to determine the distance to the epicenter.
I can draw the circle using a compass to locate the epicenter.

4. What is an Epicenter? 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.
Epicenter

5. A seismograph records earthquake activity by plotting vibrations on a sheet of paper to create a seismogram. Above are some sample seismograms:

6. Triangulation
Need 3 arrival times from 3 different seismic stations then triangulation can be used to find the location of the epicenter and the time of occurrence of the earthquake.
The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart.

7. Collecting 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

8. Collecting 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

9. Collecting 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

10. Difference in arrival times:
San Francisco, CA: :10´
Denver, CO: :20´
Missoula, MT: :00´

11. I can calculate the time difference between p and s waves.

12. Locating the Epicenter
Finally we plot the P and S wave travel-time curves to find the distance from each station to the earthquake epicenter. We do this by finding the unique epicenter distance where the difference in the P and S wave travel times is exactly equal to the difference you calculated from the seismogram. (we use a time/distance curve plot)

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

14. WE 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

15. EPICENTER DISTANCES San Francisco, CA: 4:10 Denver, Co: 3:20
2,800 km
Denver, Co: 3:20
2,000 km
Missoula, MT :00
1,100 km

16. I can use pg 11 of the ESRT to locate the epicenter distance.

17. Epicenter Distances
Using the map scale, and a drafting compass we set it to the appropriate length for the distance from the first location to the epicenter. Place the compass point at this location and draw an arc using the distance as the radius. Repeat for the other two locations. The intersection of the three arcs identifies the epicenter of the earthquake.

18. Recording Board Difference in arrival times:
San Francisco: 41:0
2,800 km
Line your compass up to the EXACT distance on the scale. Use the metric side (cm) beginning from zero.
1,000
2,000
3,000
4,000
5,000

19. . . Use your carefully set compass to
draw a circle around each seismic station. .
1100 km .
2800 km
2000 km
You've found
a possible epicenter!
Does this seem
like a likely location
for an earthquake to occur?

20. 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

21. Group Activity: Calculate p-s time differene
Calculate epicenter distance.
Draw the circles suing the compass.
Place an X where all 3 intersect.

22. Summary:
Go over analysis questions.