1. Finding the Epicenter

3. Simplified version of seismographs

4. The greater the difference in arrival times between P and S waves, the further the station is from the epicenter
The following graph in your reference tables will allow you to determine epicenter distance for various stations receiving P and S waves

5. If the difference in arrival time between the P and S wave is 8 min 20 sec, here is how you find the
distance to the epicenter:
Mark off the time difference
on the “Y” axis. Use a scrap
piece of paper for this.
Slide this difference between
the P and S graphs until the
top touches the S wave curve
and the bottom touches the
P wave curve
Epicenter distance
Is around 6800 km.
Read the epicenter distance on the “X” axis

6. Use your travel time graph and determine
the epicenter distance for each station below

7. Station P arrival S arrival diff. distance
Station A 10h 9m 10h 16m 15s 7m 15s km
Station B 10h 4m 10h 7m 5s 3m 5s km
Station C 10h 7m 10h 12m 30s 5m 30s km

8. Once you have determined the epicenter distance for each station, can other information can be found?
Yes: the exact location of the epicenter
How many different receiving stations does it require
To locate the exact epicenter of the earthquake? 3

9. Where the 3 circles Intersect is the epicenter Draw distance
circles around
each station
with a compass
to locate the
Epicenter
Where the 3 circles
Intersect is the
epicenter

10. A minimum of 3 stations
are required to locate the
epicenter of an earthquake

11. We can also determine the time that the earthquake occurred:
Station P arrival S arrival diff. distance
Station A 10h 9m 10h 16m 15s 6m 15s km
Station B 10h 4m 10h 7m 5s 3m 5s km
Station C 10h 7m 10h 12m 30s 5m 30s km
We can also determine the time that the earthquake occurred:
Let’s use station B to do this. Since station B is 2000 km from
the epicenter, use the travel time graph to
find out how long the P wave traveled to reach
Station B.

12. Station B is 2000km from the epicenter
According to the travel time graph the P wave traveled for 4 minutes
Now simply subtract 4 minutes from the P wave arrival time (10 h 4 m) at
Station B to get the time the quake occurred
10 h 4 m – 4 m = 10h 0m
Arrival time – travel time =
time the quake occurred