Answer: b. comparing the arrival times of the P-and S-waves.
Explanation: The epicenter of earthquakes is the location where the earthquake starts in the earth crust. To calculate the distance to the epicenter of an earthquake, you need to compare the arrival times of the P waves and the S waves. On a seismograph, a line is equivalent to 1 minute but it depends on the scale used.
Determine the magnitude of the earthquake by plotting the distance and amplitude on a chart and joining the two points.
Triangulation is then used to locate the epicenter. Three circles with a radius of the magnitude are drawn around the affected parts. The epicenter is the point where these circles meet.