A seismic wave is an elastic wave that propagates through the Earth, carrying the energy released by an earthquake or other seismic source. Seismic waves are classified as body waves (P and S) that travel through the Earth's interior, or surface waves (Love and Rayleigh) that travel along its surface.
A seismic wave is an elastic disturbance that propagates through the Earth's interior or along its surface, transferring energy from a seismic source to distant locations. Seismic waves are generated by earthquakes, explosions, volcanic activity, and anthropogenic sources such as traffic and construction. They are the primary signal recorded by seismographs and the basis for everything from earthquake location to global imaging of the Earth's interior.
Seismic waves are classified into two broad families. Body waves travel through the interior of the Earth and include P-waves (primary, compressional) and S-waves (secondary, shear). P-waves are the fastest seismic waves, arriving first at a station, and involve particle motion parallel to the direction of propagation. S-waves are slower, involve particle motion perpendicular to propagation, and β critically β cannot propagate through fluids. This is why S-waves do not pass through the Earth's outer core, a key observation that led to the discovery of the core's liquid state. Surface waves travel along the Earth's surface and are further divided into Love waves (horizontal shear, no vertical motion) and Rayleigh waves (retrograde elliptical motion in the vertical plane). Surface waves are slower than body waves but carry more energy and cause the strongest shaking at long distances from the source.
The velocity and attenuation of seismic waves depend on the elastic properties and density of the material they pass through β specifically, the bulk modulus, shear modulus, and density. This dependence is what makes seismic waves such powerful probes of the Earth's interior. By measuring travel times from many sources to many receivers, seismologists construct tomographic images of the mantle, core, and crust. At the engineering scale, shear wave velocity (Vs) β the speed at which S-waves travel through soil and rock β is one of the most important parameters for site characterization and ground motion prediction. The frequency content of seismic waves is also critical: high-frequency waves attenuate more rapidly with distance, so near-source motions are rich in high frequencies while distant motions are dominated by low frequencies. This has direct implications for structural response, since the frequency of the ground motion interacts with the natural frequency of the structure β the phenomenon of resonance.