P-wave

That was the P-wave just now. The S-wave is coming.

This automated alarm is triggered by the initial elastic P-wave of a tremor, which immediately precedes the slower but more destructive secondary wave, which then shakes the ground back and forth perpendicular to the direction it is moving.

An earthquake shakes the ground with two kinds of waves. The new system uses information from a low-energy wave that moves quickly but causes little damage. Experts call this the P-wave. The slower but stronger S-wave is the main threat to life and property.

P-waves are the first to arrive; that is, they travel fastest. These are ‘push-pull’ waves analogous to sound waves, in which each particle moves back and forth as the waves propagate.