Shear particles at right angle to direction of propagation - S waves. At seismic recording station away from earthquake. Travel speeds vary by density of material. By Knowing the speed at which different earthquake waves travel, seismologists can locate the distance away from their station the earthquake occurred. Several stations do this and where circles cross, earthquake is located.
L waves are even slower and travel all the way along the surface.
So, by knowing the spread, you can interpret the distance from you. If 3 stations do this, they locate the earthquake. About 6000 earthquakes are measured each year. Most are small and not noticed. Mercali scale is used.