In order to determine the epicenter of an earthquake, scientists need data from 3 seismograph stations. Seismographs record P waves, S waves, and surface waves.
By looking at the difference in time between the arrival of the P and S waves, scientists can determine how far away the epicenter of the earthquake is from the seismograph station.
Consider this sample data from Tulsa, OK:
The first activity is the P waves. The second activity is the S waves. Each box on the seismograph represents 30 seconds.
The beginning of the P waves and beginning of the S waves are 5 boxes apart, which would be 2 minutes and 30 seconds.
Knowing the S-P wave interval, we can then use this chart to determine how far away the earthquake's epicenter is:
By finding 2 minutes and 30 seconds on the vertical (y axis), we can find where that falls on the S-P wave interval. Then, we can find where that falls on the horizontal (x axis) to determine the distance from the epicenter.
In this case, Tulsa was 1600 km from the epicenter.
