P-wave

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

Source: tatoeba (327790)

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.

Source: tatoeba (11035795)

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.

Source: tatoeba (12195419)

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.

Source: wiktionary