There are two types of seismic waves associated with every earthquake, called P and S waves. They travel in every direction away from the source of an Earthquake called the epicenter. Because they each travel with different speeds, seismologists can determine the distance from a seismic station to an epicenter. An Earthquake occurred in an area where a P wave travels at 5 km/s and the S wave travels at 3 km/s. A seismic station measures an SP lag time of 16.9 seconds (meaning the P wave was detected 16.9 seconds before the S wave).