One method, the moment magnitude, is determined from frequency measurements. Geologists can also learn about an earthquake by measuring the velocity of its waves. Velocity measures how fast the wave front is traveling. To distinguish velocity from frequency, think of a snake moving across the ground. Wave velocity can be used to help determine an earthquake's epicenter, or point of origin.
P waves progagate in the same direction that the wave is traveling.
One example is when two kids hold either end of a slinky, and one pushes an end toward the other, then pulls back to the original position. The resulting wave is a compression wave, or P wave. But S waves propagate perpendicular to the direction they're traveling. Since P waves travel faster, they always appear first on a seismogram, followed by S waves. Geologists measure how far apart, in seconds, these two waves appear at the measuring station.