Here's why:
* P waves (primary waves): These are compressional waves that travel faster through the Earth's interior.
* S waves (secondary waves): These are shear waves that travel slower than P waves.
The difference in arrival time between the P wave and the S wave (known as the S-P time interval) increases with distance from the earthquake epicenter. This is because the waves travel at different speeds and the further they have to travel, the greater the time difference between their arrivals.
By measuring the S-P time interval, geologists can consult a time-distance graph (also known as a seismogram) to determine the distance to the epicenter.
To locate the epicenter, seismologists need data from at least three seismograph stations. By plotting circles on a map, with radii equal to the calculated distances from each station, the point where the circles intersect is the epicenter of the earthquake.
