Epicenter and Amplitude Notes

The distance of an earthquake from a seismograph can be determined from the seismogram. The difference between the arrival times of the P wave and the S wave (S-P interval) is related to the distance to the earthquake's focus.

By knowing the speeds of the P and S waves through the Earth, seismologists can create a chart relating the S-P interval time to the foci distance. This chart provides the distance from the seismograph (observer) to the earthquake's epicenter.

sample seismogram

Procedure for Finding the Epicenter

  1. Locate the point on the seismogram where the P wave arrives. This is where the pen first leaves the 'zero' line of the seismogram. Don't use the first peak of the P wave. Read the P wave time of arrival from the X-axis.
  2. Similarly, locate the time of arrival for the S wave.
  3. Subtract the P time from the S time to find the S-P interval.
  4. Locate this S-P interval time on the S-P graph.
  5. Follow this time across the graph until you reach the S-P line. Drop down to the X-axis and read the distance to the epicenter.
s-p graph

Epicenter Example

  1. P wave arrival time= 0 seconds
  2. S wave arrival time= 37 seconds
  3. S-P interval = 37 - 0 seconds = 37 seconds
  4. Follow the 37 second line across the S-P graph until you reach the S-P line. Drop down to the X-axis and read the distance to the epicenter. Distance = 360 km.

Amplitude

The seismogram amplitude of the earthquake is found by measuring the peak value of the S wave traces. In the seismogram above, the amplitude is 180 mm.