a single seismic station, the seismogram record will yield a measurement of
the S-P time arrival (seconds from when P arrived to when S arrived). The
distance between the station and the event can be calculated. Let’s look at
an example of an earthquake that occurred somewhere in the United States and
determine an approximate location.
We will use 3 stations including Prince Rupert, British Columbia (A), New Orleans, Louisiana (B) and Honolulu, Hawaii (C). The arrival of the P and S waves have already been determined in the graph provided. Subtract the S arrival from the P arrive and determine the "Time Lag" and record the information.
Multiply the seconds of S-P time by 8 km/s for the kilometers of distance and record them on the graph. Use the scale and draw a circle on a map around the station's location using a compass, with a radius equal to the distance. With the S-P time from a second station, the circle around that station will narrow the possible locations down to two points. It is only with a third station's S-P time that you can draw a third circle that should identify which of the two previous possible points is the real one.
Can you determine where the earthquake’s