Seismic waves are waves of energy generated by earthquakes, explosions, volcanic activity, landslides, impacts and other sudden disturbances that propagate through Earth or along its surface. In earthquake science, seismic waves are the principal means by which energy released at the earthquake source travels away from the rupture. They are broadly divided into body waves, which travel through the Earth's interior, and surface waves, which travel along or near the Earth's surface.
The main body waves are P waves and S waves, while important surface waves include Rayleigh waves and Love waves. Seismic waves differ in particle motion, propagation speed, ability to travel through different materials, wavelength, frequency and effects on structures. Their behaviour provides information about both the earthquake source and the internal structure of Earth.
P waves, or primary waves, are compressional body waves in which particles move approximately parallel to the direction in which the wave travels. They are usually the fastest seismic waves generated by an earthquake and therefore arrive first at a seismic station. P waves can travel through solids, liquids and gases because they involve compression and expansion of the material rather than the shear deformation required by S waves.
Their velocity depends on the density and elastic properties of the material through which they travel. When P waves encounter changes in material properties, they can be reflected, refracted and converted into other wave types. The travel paths of P waves through Earth have allowed scientists to infer the structure of the crust, mantle and core.
S waves, or secondary waves, are shear body waves in which particles move perpendicular to the direction of wave propagation. Unlike P waves, S waves require a material capable of supporting shear stress and therefore do not travel through fluids. This property provided important evidence that Earth's outer core is liquid because S waves do not propagate directly through it. S wave velocity depends on the rigidity and density of the material.
They generally travel more slowly than P waves, so their arrival after the P wave provides useful information about earthquake distance. S waves can generate strong horizontal and vertical ground motion and can contribute substantially to structural damage during earthquakes. Their behaviour at boundaries between different materials also helps seismologists investigate Earth's internal composition.
Surface waves travel along the Earth's surface or within a shallow layer near it and generally decay with depth. Rayleigh waves produce a rolling or elliptical particle motion involving both vertical and horizontal components. Love waves produce predominantly horizontal shear motion and require an appropriate layered structure for their propagation. Surface waves are generally slower than P waves and S waves, but they can have large amplitudes and persist for relatively long periods.
Their prolonged ground motion can be particularly damaging to buildings and infrastructure, especially when the wave periods interact strongly with the natural periods of structures. Surface waves are also important in studying near surface geological structure and can be generated by both natural earthquakes and artificial seismic sources.
Seismic waves behave differently when they encounter changes in density, elasticity or composition. Reflection occurs when part of a wave is sent back toward its source, while refraction involves a change in direction caused by changes in wave speed. At interfaces, wave energy can also convert between P and S modes. These behaviours allow seismologists to infer boundaries within Earth. The discovery and interpretation of seismic shadow zones were particularly important in establishing that Earth's core has distinct physical properties.
P wave paths are refracted strongly by the core mantle boundary, while S waves do not travel through the liquid outer core. Modern seismic tomography uses variations in seismic wave travel times and waveforms to construct three dimensional images of Earth's interior, in a manner broadly analogous to medical imaging but applied to geophysical structures.
Seismic waves are characterised by amplitude, frequency, wavelength, period, phase and velocity. Frequency describes how many oscillations occur per unit time, while wavelength is the spatial distance between equivalent points on successive cycles. Wave velocity is related to frequency and wavelength through the relationship velocity equals frequency multiplied by wavelength. Amplitude describes the size of the oscillatory motion and is related to the energy carried by the wave.
Higher frequency waves generally attenuate more rapidly over distance in many geological materials, while longer period waves can travel farther and interact differently with large structures. These physical properties are important for earthquake engineering because buildings respond differently to different frequencies and durations of ground motion.
Seismometers detect seismic waves by measuring ground motion. A typical seismic instrument records variations in ground displacement, velocity or acceleration as a function of time, producing a seismogram. Networks of instruments allow scientists to compare arrival times and waveforms at different locations. The P wave to S wave arrival time difference can estimate the distance to an earthquake, while the relative amplitudes and spectral characteristics of waves provide information about magnitude, source mechanism, attenuation and local geological conditions. Seismic stations also detect waves from distant earthquakes, enabling global monitoring of earthquakes and the Earth's interior. Artificial seismic sources are used in exploration seismology to investigate underground geological structures and locate resources or map subsurface formations.
Seismic waves are therefore both a consequence of earthquakes and a scientific tool for studying Earth. Their propagation reveals the location and size of seismic sources, while their interactions with different materials reveal the structure and properties of the planet. P waves and S waves travel through Earth's interior, whereas Rayleigh and Love waves travel near the surface.
The differences in their speeds, particle motions and responses to liquids and solids allow seismologists to distinguish materials and identify boundaries deep beneath the surface. Seismic waves are also central to earthquake early warning, structural engineering, tsunami research and monitoring of volcanic and induced seismicity. Their scientific importance arises from the fact that wave behaviour carries information about both the disturbance that generated the waves and the medium through which they travel.
