What is a seismic wave?

Seismic waves are vibrations produced by earthquakes, explosions, or other high-energy sources that travel within the Earth or along its surface. Seismographs measure their amplitude and frequency, providing insights into Earth's structure. Artificially generated seismic waves are used in oil and gas exploration and engineering surveys. Earthquakes produce P (primary) and S (secondary) body waves, which travel through the Earth, and Love and Rayleigh surface waves, which travel along the Earth's surface. P-waves are compressional, while S-waves are shear waves that cannot travel through liquids. Love waves displace the medium perpendicular to their path, and Rayleigh waves create an elliptical motion on the surface. When seismic waves encounter boundaries between different rock types, they undergo reflection and refraction, and can also convert between wave types.

How are seismic waves created?

Seismic waves are created by the sudden release of energy in the Earth's crust, mainly from tectonic movements. The Earth's crust comprises tectonic plates that float on the asthenosphere. These plates interact at their boundaries, colliding, pulling apart, or sliding past each other. This interaction occurs along faults, where stress builds up until it exceeds the strength of the rocks, causing them to fracture and slip. The fracturing releases energy in the form of seismic waves, which then propagate through the Earth, causing the ground to shake.

What types of seismic waves are there?

Seismic waves are vibrations produced by earthquakes, explosions, or other high-energy sources that travel within the Earth or along its surface. There are four primary types: P (primary) waves, S (secondary) waves, Love waves, and Rayleigh waves. P waves and S waves are body waves that propagate within the Earth. P waves are faster and can travel through solids, liquids, and gases, involving compression and expansion. S waves are slower and only travel through solids, causing a shearing motion. Love waves and Rayleigh waves are surface waves, traveling along the Earth's surface. Love waves are faster, with horizontal motion perpendicular to the direction of propagation. Rayleigh waves combine longitudinal compression and dilation, resulting in an elliptical motion on the surface.

How do seismic waves move through the Earth?

Seismic waves, vibrations from earthquakes or explosions, travel through the Earth as body waves and surface waves. Body waves include P (primary) and S (secondary) waves. P-waves are compressional, like sound waves, and can travel through solids, liquids, and gases. S-waves are shear waves, moving perpendicularly to their path, and can only travel through solids. Surface waves, including Love and Rayleigh waves, travel along the Earth's surface. Love waves cause horizontal motion, while Rayleigh waves produce elliptical motion. Analysis of these waves provides insights into Earth's internal structure.

How do scientists use seismic waves to study earthquakes?

Seismic waves are crucial for studying earthquakes. Scientists analyze the time it takes for different waves to arrive at various points on the Earth's surface to determine an earthquake's epicenter. The interval between the arrival of primary (P) and secondary (S) waves helps estimate the distance to the epicenter. By examining the directions and amplitudes of initial wave movements recorded on seismographs , scientists can infer the nature of faulting during an earthquake. The patterns of compressional and dilatational onsets reveal the orientation of the fault plane. This analysis helps in understanding the mechanisms behind earthquakes and the structure of the Earth's interior.

What have seismic waves taught us about Earth's inner structure?

Seismic waves, vibrations from earthquakes or explosions, provide insights into Earth's inner structure. Analyzing their speed and path reveals distinct layers: the crust, mantle, outer core, and inner core. P-waves are compressional and travel through solids and liquids, while S-waves are shear waves that only travel through solids. The absence of S-waves in the outer core indicates its liquid state. Changes in wave velocity at boundaries like the Mohorovičić discontinuity (crust-mantle boundary) and the Gutenberg discontinuity (mantle-core boundary) mark transitions in density and composition. Seismic tomography, similar to a CT scan, creates 3D images of Earth's interior by analyzing seismic wave velocities. This technique helps identify hot regions (slow waves) and cold regions (fast waves) in the mantle, and has revolutionized the imaging and definition of mantle plumes and subducting lithospheric plates.

seismic wave

Introduction References & Edit History Related Topics

Images & Videos

The video thumbnail image shows a woman trying to break a bundle of pasta strands as part of a scientific demonstration.

Diagram showing four main types of seismic waves: P wave with alternating compressions and expansions, S wave with vertical undulations, Love wave with horizontal shearing, and Rayleigh wave with rolling elliptical motion.

Figure 18: Profiles of the quality factor (Q; see Table 2), viscosity, and electrical conductivity as functions of depth. The quality factor is determined for shear waves at frequencies of one to 100 hertz (periods of one to 0.01 second).

Graph showing travel-time curves for seismic waves, with time after initial rupture on the vertical axis and distance from earthquake epicenter on the horizontal axis. Three lines represent primary (P) waves arriving fastest, secondary (S) waves arriving next, and Rayleigh and Love waves arriving slowest.

Quick Facts

Key People:: Inge Lehmann; Sir Harold Jeffreys; Beno Gutenberg

Related Topics:: body wave; surface wave; earthquake magnitude; seismic ray; seismic discontinuity

On the Web:: Minnesota State Pressbook - Environmental Geology - Seismic Waves (Nov. 02, 2026)

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Top Questions

What is a seismic wave?
Seismic waves are vibrations produced by earthquakes, explosions, or other high-energy sources that travel within the Earth or along its surface. Seismographs measure their amplitude and frequency, providing insights into Earth's structure. Artificially generated seismic waves are used in oil and gas exploration and engineering surveys.

Earthquakes produce P (primary) and S (secondary) body waves, which travel through the Earth, and Love and Rayleigh surface waves, which travel along the Earth's surface. P-waves are compressional, while S-waves are shear waves that cannot travel through liquids. Love waves displace the medium perpendicular to their path, and Rayleigh waves create an elliptical motion on the surface. When seismic waves encounter boundaries between different rock types, they undergo reflection and refraction, and can also convert between wave types.
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How are seismic waves created?
Seismic waves are created by the sudden release of energy in the Earth's crust, mainly from tectonic movements. The Earth's crust comprises tectonic plates that float on the asthenosphere. These plates interact at their boundaries, colliding, pulling apart, or sliding past each other. This interaction occurs along faults, where stress builds up until it exceeds the strength of the rocks, causing them to fracture and slip. The fracturing releases energy in the form of seismic waves, which then propagate through the Earth, causing the ground to shake.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What types of seismic waves are there?
Seismic waves are vibrations produced by earthquakes, explosions, or other high-energy sources that travel within the Earth or along its surface. There are four primary types: P (primary) waves, S (secondary) waves, Love waves, and Rayleigh waves.

P waves and S waves are body waves that propagate within the Earth. P waves are faster and can travel through solids, liquids, and gases, involving compression and expansion. S waves are slower and only travel through solids, causing a shearing motion.

Love waves and Rayleigh waves are surface waves, traveling along the Earth's surface. Love waves are faster, with horizontal motion perpendicular to the direction of propagation. Rayleigh waves combine longitudinal compression and dilation, resulting in an elliptical motion on the surface.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do seismic waves move through the Earth?
Seismic waves, vibrations from earthquakes or explosions, travel through the Earth as body waves and surface waves. Body waves include P (primary) and S (secondary) waves. P-waves are compressional, like sound waves, and can travel through solids, liquids, and gases. S-waves are shear waves, moving perpendicularly to their path, and can only travel through solids. Surface waves, including Love and Rayleigh waves, travel along the Earth's surface. Love waves cause horizontal motion, while Rayleigh waves produce elliptical motion. Analysis of these waves provides insights into Earth's internal structure.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do scientists use seismic waves to study earthquakes?
Seismic waves are crucial for studying earthquakes. Scientists analyze the time it takes for different waves to arrive at various points on the Earth's surface to determine an earthquake's epicenter. The interval between the arrival of primary (P) and secondary (S) waves helps estimate the distance to the epicenter.

By examining the directions and amplitudes of initial wave movements recorded on seismographs, scientists can infer the nature of faulting during an earthquake. The patterns of compressional and dilatational onsets reveal the orientation of the fault plane. This analysis helps in understanding the mechanisms behind earthquakes and the structure of the Earth's interior.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What have seismic waves taught us about Earth's inner structure?
Seismic waves, vibrations from earthquakes or explosions, provide insights into Earth's inner structure. Analyzing their speed and path reveals distinct layers: the crust, mantle, outer core, and inner core.

P-waves are compressional and travel through solids and liquids, while S-waves are shear waves that only travel through solids. The absence of S-waves in the outer core indicates its liquid state. Changes in wave velocity at boundaries like the Mohorovičić discontinuity (crust-mantle boundary) and the Gutenberg discontinuity (mantle-core boundary) mark transitions in density and composition.

Seismic tomography, similar to a CT scan, creates 3D images of Earth's interior by analyzing seismic wave velocities. This technique helps identify hot regions (slow waves) and cold regions (fast waves) in the mantle, and has revolutionized the imaging and definition of mantle plumes and subducting lithospheric plates.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

seismic wave, vibration generated by an earthquake, explosion, or similar energetic source and propagated within the Earth or along its surface. Earthquakes generate four principal types of elastic waves; two, known as body waves, travel within the Earth, whereas the other two, called surface waves, travel along its surface. Seismographs record the amplitude and frequency of seismic waves and yield information about the Earth and its subsurface structure. Artificially generated seismic waves recorded during seismic surveys are used to collect data in oil and gas prospecting and engineering.

Of the body waves, the primary, or P, wave has the higher speed of propagation and so reaches a seismic recording station faster than the secondary, or S, wave. P waves, also called compressional or longitudinal waves, give the transmitting medium—whether liquid, solid, or gas—a back-and-forth motion in the direction of the path of propagation, thus stretching or compressing the medium as the wave passes any one point in a manner similar to that of sound waves in air. In the Earth, P waves travel at speeds from about 6 km (3.7 miles) per second in surface rock to about 10.4 km (6.5 miles) per second near the Earth’s core some 2,900 km (1,800 miles) below the surface. As the waves enter the core, the velocity drops to about 8 km (5 miles) per second. It increases to about 11 km (6.8 miles) per second near the centre of the Earth. The speed increase with depth results from increased hydrostatic pressure as well as from changes in rock composition; in general, the increase causes P waves to travel in curved paths that are concave upward.

S waves, also called shear or transverse waves, cause points of solid media to move back and forth perpendicular to the direction of propagation; as the wave passes, the medium is sheared first in one direction and then in another. In the Earth the speed of S waves increases from about 3.4 km (2.1 miles) per second at the surface to 7.2 km (4.5 miles) per second near the boundary of the core, which, being liquid, cannot transmit them; indeed, their observed absence is a compelling argument for the liquid nature of the outer core. Like P waves, S waves travel in curved paths that are concave upward.

Cross section of Earth showing the core, mantle, and crust

Britannica Quiz

The Solid Earth Quiz

Of the two surface seismic waves, Love waves—named after the British seismologist A.E.H. Love, who first predicted their existence—travel faster. They are propagated when the solid medium near the surface has varying vertical elastic properties. Displacement of the medium by the wave is entirely perpendicular to the direction of propagation and has no vertical or longitudinal components. The energy of Love waves, like that of other surface waves, spreads from the source in two directions rather than in three, and so these waves produce a strong record at seismic stations even when originating from distant earthquakes.

The other principal surface waves are called Rayleigh waves after the British physicist Lord Rayleigh, who first mathematically demonstrated their existence. Rayleigh waves travel along the free surface of an elastic solid such as the Earth. Their motion is a combination of longitudinal compression and dilation that results in an elliptical motion of points on the surface. Of all seismic waves, Rayleigh waves spread out most in time, producing a long wave duration on seismographs.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Meg Matthias.

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