3. Other names for S- waves are secondary waves, shear waves, and sometimes elastic S-waves. Other names for P-waves are primary waves and compressional waves. 4. You need 3 stations, because scientists find the difference between the arrival times of the primary and the secondary waves at each of the 3 stations, then the time difference is used to determine the distance of the epicentre from each station. The greater the difference in time, the further away the epicentre is. A circle is drawn around each station, with a radius corresponding to the epicentre’s distance from that station. The point where the three circles meet is the epicentre. If you only had two stations, you could only predict the epicentre, as the point where all three circles meet wouldn’t be complete, you’d have to try and estimate where the third one would intercept. This would greaten the chance of error and isn’t as accurate.
