The velocity of a P wave relies on density and compressibility, or the resistance to compression. S wave velocity depends on density and rigidity, or resistance to distortion or shearing. As a result, P waves travel 1.7 times faster than S waves. L waves are the last to occur and have the longest periods. They are on or near the surface because they continuously reflect from the top to the bottom of surface layers.
Microseisms interfere with the recording of primary, secondary, and long waves.
P, S, and L waves are important to show the sizes of earthquakes while microseisms also had important uses. Surprisingly, by using microseisms to predict weather, the reports were fairly accurate, considering the lack of technology of the time. Two theories arose about their origins. The first theory is known as the coast theory and states that microseisms originate from the surf and any other action on the steep coasts. The second is the cyclone theory that relates to cyclones over deep water.