There are several different types of ground waves that are generated when portions of the earth's crust break during earthquakes. The speed of the waves depends upon the density and rigidity of the surrounding rocks. P waves are compressional or push-pull type waves. They are the first to arrive locally. In the San Francisco Bay Area, earthquakes that occur between 5 and 15 kilometers below the surface, typically produce P waves that travel about 6.2 miles per second.
S waves are the second to arrive.
They usually cause most of the damage due to the severe shaking that is produced by the high amplitude waveform. S waves generated by the Loma Prieta earthquake traveled about 2.5 miles per second. The central processor evaluates the data supplied from the epicentral ground motion sensors and determines the magnitude of the aftershock. The system is designed to transmit an alert on all aftershocks with a magnitude greater than 3.7 on the Richter Scale. The alert consists of two, dual tone, multi-frequency signals that activate alarms at the remote receiving locations.