P Waves are primary waves

Introduction
P waves are produced by all earthquakes. They are compression waves that form when rocks break due to pressure in the Earth. S waves are secondary waves that are also created during an earthquake. They travel at a slower speed than the p-waves.

Slinky showing P waves, Photo by Myrna Martin

Slinky demonstrating how primary waves
travel through the Earth
, Myrna Martin

Primary waves
Earthquake waves travel through the Earth in all directions from the focus. The waves moving away from the point where the rocks ruptured cause rock particles to expand and contract like the slinky in the picture. This type of earthquake wave causes rocks to compress and expand as the waves move through all parts of the Earth. 

Recording seismic waves
Primary waves, are the first to arrive at recording stations. S waves, also produced by all earthquakes arrive a short time after at recording stations. Scientists use the travel time of the two waves to determine the distance to the focus and epicenter of an earthquake from their reporting station. Scientists must use three stations to calculate the exact point where the rocks ruptured and was the focus of the earthquake.

Compression waves
Compression waves expand rock particles ahead of the waves and compress them as they travel away. They can also compress and expand air. When they reach the Earth's surface they create a noise often associated with earthquakes. Sometimes people hear a sharp thud while others think the sound is loud and like wind blowing through leaves in a tree.

Primary wave travel
P waves move through solids, liquids and gases while S waves only travel through solids. Compression waves travel through all parts of the Earth including the solid inner core and liquid outer core of the Earth.

Discovering Earth's inner core
Inge Lehmann in 1936 used the travel times of these waves through the center of the Earth to prove the existence of a solid inner core of the Earth.

More Earthquake Links

Love Waves Find out more about Love waves that can topple tall buildings during large earthquakes!

The Mercalli Scale Did you know there can be twelve different intensities of an earthquake but only one magnitude? Find out why!

Normal Fault Find out why miners often find valuable ore in sites where normal faults have occurred.

Megathrust Earthquake Find out why megathrust earthquakes will occur off the coasts of Alaska, Washington, Oregon and California in the future.


Earthquake Zones Find out where and why earthquakes occur and why there are earthquake zones.

P waves Find out more about seismic waves that are produced by large and small earthquakes.

What Causes Earthquakes. Find out what causes earthquakes and other fascinating facts about when our planet shakes, rattles, and rolls.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.