All books

Professional

Apps About Coach Log in Start reading

Earth and Space Sciences · Glossary

What is P wave; S wave?

Also known as: P wave · S wave

Definition 11.1 High School Earth and Space Sciences · Chapter 11 — Seismic Waves Reveal the Interior

A P wave is a seismic wave that alternately compresses and stretches the rock: the rock moves back and forth along the direction in which the wave travels. It crosses solids, liquids and gases, and it is the fastest seismic wave, so it arrives first. An S wave shears the rock: the rock moves at right angles to the direction in which the wave travels. It crosses only solids, and it arrives after the P wave.

Seismologists of the Puerto Rico Seismic Network and of the USGS install a temporary seismometer in south-western Puerto Rico, on 10 January 2020. Photograph: Thomas Pratt, USGS (public domain).
Seismologists of the Puerto Rico Seismic Network and of the USGS install a temporary seismometer in south-western Puerto Rico, on 10 January 2020. Photograph: Thomas Pratt, USGS (public domain).

Examples

Example 11.4 (An earthquake 60∘60{}^{\circ} away)

Observed travel times are 608 s608\,\mathrm{s} for P and 1101 s1101\,\mathrm{s} for S at 60∘60{}^{\circ}, 536 s536\,\mathrm{s} and 967 s967\,\mathrm{s} at 50∘50{}^{\circ}. A station that records an S wave 493 s493\,\mathrm{s} (8 minutes 13 seconds) after the P wave therefore lies 60∘60{}^{\circ} from the epicentre: about 60×111.2≈6670 km60 \times 111.2 \approx 6670\,\mathrm{km}. At 50∘50{}^{\circ} the delay would only be 431 s431\,\mathrm{s}.

Read in context →