"Your Guide to Earthquake Safety"

P-waves and S-waves

Two types of wave travel through the underground from the hypocenter when the earthquake occurs.
The P-waves are longitudinal waves and the S-waves are transverse waves.

The quakes by the P-waves is called as the preliminary tremors and we feel them first. After them, the principal shock by the S-waves occurs.

The time lag of these waves becomes bigger at the place far from the hypocenter.
We use this property of the waves to the “ Earthquake Early Warning ”.

By the way, the term P-waves is derived from the Latin word: primae, meaning primary, and the S-waves from the Latin word: secundae, meaning secondary.