Introduction
Every earthquake releases energy that radiates outward as seismic waves — elastic disturbances in rock that travel at kilometres per second through Earth's interior. Seismologists divide these waves into two families. Body waves travel through Earth's interior along curved paths dictated by velocity gradients and density contrasts; they arrive first at distant seismographs. Surface waves are trapped near Earth's surface and travel more slowly, but their long wavelengths mean they shake the ground for far longer and at lower frequencies — making them the dominant cause of structural damage in large earthquakes. Understanding the four main wave types is the foundation of seismology, earthquake engineering, and our ability to image Earth's deep interior.
The two body wave types differ in how they deform the rock they pass through. P-waves (primary waves, or compressional waves) compress and extend the rock in the direction of wave propagation — like sound waves in air, the rock alternately squeezes together and pulls apart along the direction of travel. P-wave velocity in continental crust is typically 5–7 km/s, rising to ~8 km/s in the uppermost mantle. Because P-waves involve volumetric compression, they can travel through any medium — solid rock, liquid outer core, water, and air (the sonic booms sometimes heard before shaking arrives are P-waves that entered the atmosphere). S-waves (secondary waves, or shear waves) distort rock perpendicular to the direction of travel — like waving a rope side to side or up and down; S-wave particle motion can occur in any plane perpendicular to propagation, giving horizontally polarised S-waves (S_h) and vertically polarised S-waves (S_v). S-wave velocity in continental crust is typically 3–4 km/s, roughly 1/√3 ≈ 0.577 times the P-wave velocity in the same material. Crucially, shear motion requires the material to have rigidity; liquids and gases have no shear modulus, so S-waves cannot travel through them. This property proved Earth has a liquid outer core: no S-waves arrive in the shadow zone on the far side of the planet.
P-wave velocity is given by v_P = √((K + 4μ/3)/ρ) where K is the bulk modulus (resistance to compression), μ is the shear modulus (resistance to shearing), and ρ is density. S-wave velocity is v_S = √(μ/ρ). The ratio v_P/v_S = √((K/μ + 4/3)) is always greater than √(4/3) ≈ 1.16, which is why P-waves always arrive before S-waves. In typical crustal rock, v_P/v_S ≈ 1.73. The time difference between P and S arrivals at a seismograph station — the S-P time — directly indicates the distance to the earthquake: distance ≈ S-P interval (in seconds) × ~8 km (5.0 mi). The physical insight matters more than the maths: stiffer materials transmit P-waves faster, which is why wave speeds increase with depth as rock becomes more compressed.
Surface waves arise from the interaction of body waves with Earth's free surface. Love waves are shear-type surface waves: they shake the ground horizontally, transverse to the propagation direction, with no vertical motion. They are generally the fastest surface waves. Rayleigh waves combine vertical and horizontal (along-propagation) motion in a retrograde elliptical path — like a wave rolling backward — and are slightly slower than Love waves but typically carry the most energy of any wave type. Surface wave amplitudes decay less rapidly with distance than body waves (1/r rather than 1/r²), so they dominate the seismogram at large distances.
The period of surface waves relevant to building damage is 1–10 seconds, matching the natural resonance period of multi-storey buildings, which is why large distant earthquakes can topple skyscrapers in sediment-filled basins far from the epicentre (as happened in Mexico City in 1985, 350 km (217 mi) from the M8.0 Michoacán earthquake).
Key Terms
A compressional (primary) body wave in which particle motion is parallel to the direction of wave propagation — the rock alternately compresses and extends along the travel direction, like sound waves in air. P-waves travel at 5–8 km/s in continental crust and can propagate through solids, liquids, and gases, including Earth's liquid outer core. They are always the first seismic wave type to arrive at a seismograph after an earthquake.
A shear (secondary) body wave in which particle motion is perpendicular to the direction of wave propagation — the rock is distorted in any plane transverse to the travel direction, like waving a rope side to side or up and down. S-waves carry two independent polarisation components: horizontally polarised S-waves (S_h) and vertically polarised S-waves (S_v), both recorded on three-component seismometers. S-waves travel at 3–5 km/s in continental crust and can only propagate through materials with a non-zero shear modulus (μ > 0). Because liquids have μ = 0, S-waves cannot enter Earth's liquid outer core, creating the S-wave shadow zone and proving the outer core is molten.
A surface wave in which particle motion is purely horizontal shear, directed transverse to the propagation direction, with no vertical component. Love waves are generally the fastest-travelling surface wave type and are particularly destructive to structures and foundations susceptible to horizontal shear forces. They are guided by a low-velocity layer near Earth's surface and were named after A.E.H. Love, who derived the mathematical theory of their existence in 1911.
A surface wave in which particle motion follows a retrograde elliptical path combining vertical and horizontal (in-plane) motion — the ground surface rolls backward like an ocean wave. Rayleigh waves travel slightly slower than Love waves but typically carry the most energy at large epicentral distances. Their periods of 1–10 seconds match the natural resonance periods of multi-storey buildings, making them the dominant cause of structural collapse in large distant earthquakes. Named after Lord Rayleigh, who predicted their existence mathematically in 1885.
The time interval between the arrival of the S-wave and the arrival of the P-wave at a seismograph station. Because P-waves travel faster than S-waves, the S-P time grows proportionally with the distance from the earthquake to the station: distance (km) ≈ S-P time (seconds) × ~8 km/s. By measuring the S-P time at three or more stations and drawing circles of the corresponding distances, seismologists triangulate the earthquake's epicentre and locate the hypocenter.