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
- P wave arrival time= 0 seconds
- S wave arrival time= 37 seconds
- S-P interval = 37 - 0 seconds = 37 seconds
- 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.
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.