primary wave

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Cross-sectional diagram of Earth showing how S-waves and P-waves from an earthquake travel through the planet's layers, with S-waves stopping at the liquid outer core and P-waves bending as they pass through the solid inner core, creating shadow zones.

Figure 16: Cross section with shading proportional to the velocities of compressional (P) waves through the Earth.

The types of seismic waves explained

How P waves travel

primary wave

seismology

Also known as: P wave, compressional wave

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July 13, 2026

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Cross-sectional diagram of Earth showing how S-waves and P-waves from an earthquake travel through the planet's layers, with S-waves stopping at the liquid outer core and P-waves bending as they pass through the solid inner core, creating shadow zones. — earthquake: P waves and S waves P and S waves travel through the planet Earth after an earthquake. Scientists studying the waves produced by earthquakes learned that Earth's core has separate liquid and solid layers. S waves do not travel through liquid, but P waves do. A simplified diagram shows the S waves on the left and the P waves on the right, but waves of both types would actually radiate in all directions.

In seismology, seismic waves are vibrations generated by events like earthquakes. The primary wave, or P wave, is a type of body wave that travels through the Earth's interior. It is a longitudinal wave, meaning particles in the medium vibrate back and forth in the same direction as the wave's travel, similar to sound waves. P waves are the fastest seismic waves and can pass through both solids and liquids. They are the first to reach a seismic recording station, originating from the earthquake's focus and traveling towards the epicenter on the surface. These waves are crucial for understanding Earth's internal structure, as their speed and behavior change as they pass through different layers, such as the solid mantle and the liquid outer core.123

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earthquake
Principal types of seismic waves
The P seismic waves travel as elastic motions at the highest speeds. They are longitudinal waves that can be transmitted by both solid and liquid materials in the Earth’s interior. With P waves, the particles of the medium vibrate in a manner similar to sound waves—the…
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The P seismic waves travel as elastic motions at the highest speeds. They are longitudinal waves that can be transmitted by both solid and liquid materials in the Earth’s interior. With P waves, the particles of the medium vibrate in a manner similar to sound waves—the…
Read More

infrasonics
…a transverse body wave; the P-wave, a longitudinal body wave; and the L-wave, which propagates along the boundary of stratified mediums. L-waves, which are of great importance in earthquake engineering, propagate in a similar way to water waves, at low velocities that are dependent on frequency. S-waves are transverse body…
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…a transverse body wave; the P-wave, a longitudinal body wave; and the L-wave, which propagates along the boundary of stratified mediums. L-waves, which are of great importance in earthquake engineering, propagate in a similar way to water waves, at low velocities that are dependent on frequency. S-waves are transverse body…
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longitudinal wave
The P (primary) seismic waves are also longitudinal. In a longitudinal wave, each particle of matter vibrates about its normal rest position and along the axis of propagation, and all particles participating in the wave motion behave in the same manner, except that there is a…
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The P (primary) seismic waves are also longitudinal. In a longitudinal wave, each particle of matter vibrates about its normal rest position and along the axis of propagation, and all particles participating in the wave motion behave in the same manner, except that there is a…
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Today in History—March 29
A Woman Discovers Earth’s Core
Lehmann focused on P (primary, or pressure) and S (secondary, or shear) waves. P waves pass through all states of matter; S waves travel only through solids.
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Lehmann focused on P (primary, or pressure) and S (secondary, or shear) waves. P waves pass through all states of matter; S waves travel only through solids.
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Earth exploration
Seismic refraction methods
…can travel through a body: P waves (primary) and S waves (secondary). P waves are compressional waves and travel at the highest velocity; hence, they arrive first. S waves are shear waves that travel at a slower rate and are not able to pass through liquids that do not possess…
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…can travel through a body: P waves (primary) and S waves (secondary). P waves are compressional waves and travel at the highest velocity; hence, they arrive first. S waves are shear waves that travel at a slower rate and are not able to pass through liquids that do not possess…
Read More