1. Locating the Epicenter
The method of triangulation

2. This presentation should be viewed as a slideshow
Throughout the slide show, press the SPACE BAR when the appears in the lower left hand corner

3. The method of triangulation
by knowing the distance to the epicenter of at least three seismic stations we can determine, or triangulate, its location.
3 seismic stations
St. Louis
Ellensburg
San Diego
The relative distance
of the quake from each station
The intersection of the three circles gives the location of the epicenter

4. How do we go from this….
…to that?
St. Louis
Ellensburg
San Diego

5. We will exploit the fact that
S-Waves travel a little slower than P-Waves P S

6. between the P and S waves
We will exploit the fact that
S-Waves travel a little slower than P-Waves
With time the distance
between the P and S waves
increases!
Start
Time A
Time B
The greater the distance between the
P and S waves the farther we are from
the epicenter! P S P S P S S

7. The time between the arrival of the P-Wave and the S-Wave
The Distance between the P and S Wave
The time between the arrival
of the P-Wave and the S-Wave

8. Three seismograms of the same quake
Station 1
P-Wave
S-Wave
Time in seconds
9.2 sec
Station 2
Step 1: Identify P and S-Wave
Step 2: Measure the time between their arrivals
Step 3: Do this for all three station seismograms
Station 3

9. Three seismograms of the same quake
Station 1
P-Wave
S-Wave
Time in minutes
9.2 seconds
Station 2
P-Wave
S-Wave
16.5 seconds
Station 3
P-Wave
S-Wave
21.2 seconds

10. The Travel Time Graph
Helps us convert the time interval between arrival times of P and S waves into distances from the epicenter for each station
S Wave
P Wave
2,000
4,000
6,000
8,000
10,000
Distance between epicenter and seismograph (km) 10 20 30 40 50
Time (seconds)
Station #1
9.2 seconds
3,000 km
6,500 km
8,750 km
Station #2
16.5 seconds
Station #3
21.2 seconds
_,___ km
_,___ km
_,___ km

11. Finding the epicenter using triangulation
This circle was drawn using a compass set at a radius scaled to 3000km using the map scale.
Station #1: (Seattle)
9.2 seconds
3,000 km
The earthquake occurred
along the Alaskan
Aleutian Island Chain –
an active subduction zone!
Station #2 (Sudbury)
16.5 seconds
6,500 km
Notice the scale bar!
Station #3 (St. John’s)
21.2 seconds
8,750 km
1,000 km

12. Determining the magnitude of the quake
We need to have two pieces of data:
1) The distance to the earthquake
2) The amplitude of the biggest recorded wave
20 mm
Station 1: Seattle
P-Wave
S-Wave
Highest Amplitude
R & L-Waves

13. The Distance-Amplitude Graph
Helps us determine the magnitude of an earthquake based on the distance to the epicenter and the amplitude of the highest wave
Distance (km)
Magnitude (M)
Amplitude
(mm) 50
200
400
600
1000
2000
3000
4000
5000 1 2 3 4 5 6
0.1
0.2
0.5 10 20
100
Station #1: (Seattle)
9 min, 20 sec
3,000 km
Magnitude 5.2
20 mm
Here we used only one seismogram to find magnitude – you must plot all three in your assignment!