What are the P and S waves an earthquake sends out?

An earthquake does not send out just one wave. It sends out several kinds, and they race each other through the Earth at different speeds. Two matter most, the P wave and the S wave.

The P wave, short for primary, is a compression wave. It squeezes and stretches the rock in the direction it is travelling, a bit like sound squeezing air. It is the fastest wave and it passes through any material, solid or liquid. It reaches a seismograph first, which is how it got its name.

The S wave, short for secondary, is a shear wave. It shakes the ground sideways, at right angles to its direction of travel, like flicking a rope. It is slower than the P wave, typically around 60 percent of its speed, and it can travel only through solids, since liquids and gases cannot be sheared.

That timing gap is a gift. Think of counting seconds between lightning and thunder. A seismologist counts the seconds between the first P wave and the first S wave. For an earthquake less than 200 km away, multiply that number of seconds by 8 and you have the distance in kilometres. For distant quakes, three or more stations can pin down where it happened.

Then come surface waves, which trundle along the ground after the body waves. They are the slowest, but they move the ground the most, so they cause most of the damage. The P wave arrives first, then the S wave, then the surface waves.