What is the SP time interval on the seismograph?

The SP time interval on a seismograph refers to the time difference between the arrival of the primary (P) waves and the secondary (S) waves from an earthquake. This interval is crucial for determining the distance to the earthquake's epicenter, as P waves travel faster than S waves. By measuring the SP interval, seismologists can estimate how far away the seismic event occurred. The longer the SP interval, the greater the distance to the source of the earthquake.