1. Locating the Epicenter
The source of an earthquake is called the focus, which is an exact location within Earth were seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of Earth directly above the focus. This is the location that scientist calculate. The focus is directly below, however, scientist can not determine its depth.
To locate the position of an earthquakes epicenter, we need a seismogram reading from at least three different seismograph stations and a travel - time graph which shows the speed of both P- and S-waves.
Reference: Pages

2. Locating the Epicenter
Step 1: 3 Seismogram Records
Find the difference in arrival time between the P-wave and the S-wave.
To do this refer to each of the three seismogram readings and record arrival times for P-wave and S-wave.
One example is shown below:
P-Waves
S-Waves
Arrival time of P-waves (23 Minutes)
Arrival time of S-waves (27 Minutes)

3. Locating the Epicenter
Step 2: Record Difference in Arrival Times
Subtract the arrival time of the S-wave from the arrival time of the P-wave.
27 minutes – 23 minutes = 4 minutes
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the travel-time graph.

4. Locating the Epicenter
Step 2: Record Difference in Arrival Times
Subtract the arrival time of the S-wave from the arrival time of the P-wave.
27 minutes – 23 minutes = 4 minutes
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the travel-time graph.
Slide the red line representing minutes up between the P-wave and S-wave lines until it fits between the two lines.

5. Locating the Epicenter
Step 2: Record Difference in Arrival Times
Subtract the arrival time of the P-wave from the arrival time of the S-wave.
27 minutes – 23 minutes = 4 minutes
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the travel-time graph.
Slide the red line representing minutes up between the P-wave and S-wave lines until it fits between the two lines.
Read off the distance below. Distance = 2500 km.

6. Locating the Epicenter
Step 3: Record Distance to the Epicenter:
The S-P interval tells us the distance to the epicenter from each seismograph station where the earthquake was recorded.
The epicenter of the earthquake is located 2500 kilometers from the first seismograph station.
You need to do the same procedure for two other seismograph stations.
Seismograph station 1 = 2500 km Assume that the other readings are:
3500 km
4500 km
Seismograph station 2 = 3500 km Seismograph station 3 = 4500km

7. Locating the Epicenter
Step 4: Use Triangulation to Pin Point the Epicenter:
The distance to the epicenter from each seismograph station is; Station # d1 = 2500km Station # d2 = 3500km Station # d3 = 4500km
At each station we can draw a circle on a map that has a radius equal to the distance to the epicenter from each seismograph station.
Three such circles will intersect in a point that locates the epicenter of the earthquake.

8. Sample Problem Answer:
The table below provides information collected at seismic stations A, B, and C for the same earthquake.
2000
Answer:
(i) Why are no S - waves recorded at station A?
No S-waves are recorded at station “A” because station “A” is located in a shadow zone, an area 103E - 143E from the earthquake epicenter.

9. Sample Problem Answer:
The table below provides information collected at seismic stations A, B, and C for the same earthquake.
2000
Answer:
(ii) What is the distance from station B to the epicenter?
The difference in arrival time between P-wave and S-wave is 4 min. and 30 sec. (00:04:30). Graph this on the time scale (5 blocks) and find the position between the P- and S-wave lines. Then read off the distance on the x-axis as shown in the diagram.

10. Sample Problem Answer:
The table below provides information collected at seismic stations A, B, and C for the same earthquake.
2000
Answer:
(iii) What is the arrival time of the S - wave for station C?
Locate 2000km on the diagram and draw a line straight up until you intersect the S-wave line, (as indicated by blue arrow). Then draw a line horizontally left to intersect the travel-time (y-axis) axis, and read off the time (as indicated by the red arrow).
Arrival time of the S-wave at 2000km would be 7 minutes.