Seismic waves are waves of mechanical energy generated by earthquakes and other sudden disturbances within the Earth. They travel through geological materials or along the Earth's surface and transport energy away from their source. Seismic waves are responsible for the shaking experienced during earthquakes. They are commonly divided into body waves, which travel through the Earth's interior, and surface waves, which travel along or near the Earth's surface. The principal body waves are P waves and S waves.
The major surface wave types are Love waves and Rayleigh waves. Each type has different particle motion, propagation speed and sensitivity to the physical properties of the material through which it travels. Seismologists use these differences to determine earthquake locations, investigate Earth's internal structure and characterise ground motion.
P waves are primary waves because they are generally the first seismic waves to arrive at a seismological station. They are compressional waves, meaning that particles in the material oscillate broadly parallel to the direction in which the wave propagates. P waves can travel through solids, liquids and gases because compression can occur in all of these states. Their speed depends on the density and elastic properties of the medium.
As P waves travel through the Earth, changes in velocity and direction occur where material properties change. Their ability to travel through liquids was historically important in establishing that the Earth's outer core is liquid. P waves usually travel faster than S waves in the same material, which makes their arrival time difference useful for locating earthquakes.
S waves are secondary waves because they generally arrive after P waves. They involve shear deformation, with particle motion perpendicular to the direction of propagation. S waves require a material capable of sustaining shear stress and therefore do not propagate through fluids such as liquid water. Their absence from certain regions of the Earth's surface after large earthquakes provided important evidence that the outer core is liquid. S wave velocity depends on density and shear rigidity. Within the solid Earth, S waves can reveal information about variations in rock properties and discontinuities. Their particle motion can produce significant shaking, particularly because horizontal or vertical shear movements can affect structures in different ways depending on building characteristics and ground conditions.
Surface waves travel along the Earth's surface and generally decay in amplitude with increasing depth. Love waves involve predominantly horizontal shear motion, while Rayleigh waves produce a rolling motion involving both vertical and horizontal components. Surface waves usually travel more slowly than many body waves but can produce strong and prolonged ground motion. Their characteristics depend strongly on surface geology and layered structures.
Because their amplitudes generally decrease with depth, surface waves are particularly important in understanding earthquake damage near the ground surface. Different frequencies of surface waves travel at different speeds in many geological settings, a phenomenon called dispersion. Analysis of this behaviour can help scientists infer the properties of subsurface materials and geological layers.
Seismic waves change speed, direction and amplitude as they encounter different materials. Reflection occurs when wave energy is returned from an interface, while refraction involves a change in direction caused by changes in wave velocity. Partial transmission and conversion between wave types can also occur at boundaries. These behaviours allow seismologists to construct models of Earth's internal structure.
Major discontinuities such as the crust mantle boundary, known as the Mohorovicic discontinuity, and boundaries within the mantle and core can be identified through seismic observations. Seismic tomography uses large collections of earthquake wave observations to construct three dimensional models of variations in seismic velocities inside Earth. This technique is analogous in broad principle to medical imaging, although the physical problem and data interpretation are very different.
The velocity of seismic waves depends on material properties such as density, elastic modulus and shear modulus, as well as temperature, pressure, composition and physical state. In general, seismic waves travel faster through many rigid rocks than through less consolidated sediments, although the exact relationship depends on the material. Water saturated sediments can have distinctive seismic properties. Seismic velocities can also change with depth because pressure increases and minerals undergo transformations. The resulting travel times allow scientists to infer underground structures that cannot be directly observed. Seismic waves are therefore not merely a source of earthquake shaking. They are one of the most important tools available for studying the otherwise inaccessible interior of the planet.
Seismometers record ground motion produced by seismic waves. A seismic trace can show the arrival of P waves, S waves and surface waves, with their relative timing and amplitude providing information about the earthquake. The difference between P wave and S wave arrival times can be used to estimate the distance to the source. Multiple stations then allow earthquake location. Waveform amplitude and frequency content can provide information about magnitude, source duration and rupture characteristics. Engineering seismology also studies ground acceleration, velocity and displacement because structures respond differently to different frequencies and durations of shaking. Seismic waves can therefore be analysed both as geological signals and as engineering hazards.
Seismic waves are fundamentally elastic disturbances in the Earth rather than ordinary electromagnetic waves. They require a physical medium for propagation and involve deformation of matter. Their study connects earthquake science with elasticity, wave mechanics, geology and geophysics. Different wave types provide complementary information because P waves, S waves and surface waves respond differently to material properties and boundaries.
Seismic waves can be generated naturally by earthquakes, volcanic processes, landslides and impacts, and artificially by controlled sources used in exploration geophysics. Their propagation patterns provide information about earthquake sources, underground geological structures and the mechanical properties of rocks. Understanding seismic waves is therefore essential both for interpreting earthquakes and for using earthquakes as natural probes of the Earth's interior.
