Additional shaking of the damaged structure threatened additional collapse and a potential for loss of life. As a branch chief, he discussed the problem with other members of his staff and came up with an idea - deploy an Early Warning System for aftershocks. The key to the system is the difference in speed that radio waves travel as compared to seismic waves. 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.