How is S-minusP time related to distacefrom epicenter?

Answer 1

S-waves travel more slowly than P-waves, so the further away from the epicentre (or strictly speaking, the focus), the greater the time difference between the first arrival of the P and S-waves.

If you have 3 or more seismic stations, and you know the velocity of the P- and S-waves, you can convert the time difference into distance, and pinpoint the location of the earthquake.

Answer 2

Four types of waves are generated from earthquakes.P,S,L,R.
P wave is the primary wave. Generated from the point called focus. The speed of this wave is from 13 to 17 kilo meters per second. Moves the particles in its direction of propagation. you can assume that this is non destructive wave. The secondary waves comes from the point of focus. The speed is nearly half of the primary wave. This moves the particles perpendicular to the direction of its propagation. Destructive. Do not pass through liquid. These waves are called body waves. generated from earthquakes. But the difference in speed will be 8 K/M per second is almost constant.Therefore arrival time of S wave minus P wave in seconds multiplied by 8 will give you the distance of epicenter in K/M. We need the seismiograph from three stations only to know the location of epicenter or focus point.