# 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. ## 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. ## 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.