Seismic Waves
When an earthquake strikes, it releases enormous amounts of energy that travel outward through the Earth as seismic waves — a form of mechanical wave. Like all mechanical waves, seismic waves require a medium (solid or liquid rock) to propagate; they cannot travel through a vacuum. Scientists who study these waves are called seismologists, and the instruments they use — seismometers — can detect ground motion caused by earthquakes thousands of kilometres away. By analysing seismic records, we can probe the deep interior of our planet in much the same way a doctor uses ultrasound to image the human body. There are two primary body waves — waves that travel through the bulk of the Earth — called P-waves (primary or pressure waves) and S-waves (secondary or shear waves). P-waves are longitudinal: the rock particles oscillate parallel to the direction of wave travel, alternately compressing and stretching the medium, just like sound waves in air. S-waves are transverse: particles oscillate perpendicular to the direction of travel, similar to waves on a string. Because these two wave types travel at different speeds and respond differently to Earth's materials, together they reveal the layered structure of our planet. The fact that S-waves cannot propagate through liquids — liquids have no shear strength — was pivotal in the discovery that Earth's outer core is liquid. When seismologists noticed that S-waves produced a 'shadow zone' on the far side of earthquakes, they deduced that a liquid layer must be blocking them. This single observation, combined with P-wave data, gave us our modern picture of Earth: a solid inner core, liquid outer core, mantle, and crust.
- P-waves: Longitudinal Pressure Waves
- S-waves: Transverse Shear Waves
- Wave Speed and Earth's Material Properties
- Locating Earthquakes with the Time Difference
- Surface Waves and Damage
P-waves: Longitudinal Pressure Waves
P-waves are the fastest seismic waves, typically travelling at 5–8 km/s through crustal rock and up to 13 km/s near the core-mantle boundary. Like sound, they are longitudinal: the medium alternately compresses and rarefies in the direction of propagation. Because both solids and liquids resist compression (they have a bulk modulus), P-waves travel through every layer of Earth, including the liquid outer core. They arrive first at a seismometer after an earthquake — hence the name 'primary'.
S-waves: Transverse Shear Waves
S-waves travel at roughly 60% of P-wave speed in the same material — typically 3–5 km/s in crust. They are transverse waves, meaning particle motion is perpendicular to propagation direction. Crucially, shear waves require a medium with a non-zero shear modulus. Fluids (liquids and gases) have zero shear modulus, so S-waves cannot propagate through them. This property means S-waves are entirely blocked by Earth's liquid outer core, creating a large shadow zone between about 103° and 143° of arc from an earthquake's epicentre.
Wave Speed and Earth's Material Properties
The speed of a seismic wave depends on the elastic properties and density of the medium. For P-waves, speed increases with bulk modulus K and shear modulus G, and decreases with density ρ. For S-waves, speed depends only on the shear modulus and density. As depth increases, pressure compresses rock, raising K and G faster than density increases, so wave speeds generally increase with depth. This causes seismic rays to bend (refract) along curved paths — a process described by Snell's Law — allowing waves to return to the surface far from their origin.
Locating Earthquakes with the Time Difference
Because P-waves outpace S-waves, the time gap between their arrivals at a seismometer grows with distance from the earthquake's focus (hypocentre). By measuring this S-P time lag at three or more stations and using known wave speeds, seismologists triangulate the epicentre — the point on the surface directly above the focus. This method is the practical foundation of earthquake location and is routinely used by national geological surveys worldwide.
Surface Waves and Damage
In addition to body waves, earthquakes generate surface waves (Love and Rayleigh waves) that travel along Earth's surface. Surface waves are slower but carry the most energy to the surface and cause the greatest structural damage. Love waves shake the ground sideways; Rayleigh waves produce an elliptical rolling motion. Although not the focus here, surface waves complement body-wave data in building a complete picture of Earth's structure and earthquake hazard.
The Key Equation
- v_P — P-wave speed (m s⁻¹): Speed of the longitudinal compressional wave through the medium
- v_S — S-wave speed (m s⁻¹): Speed of the transverse shear wave through the medium
- K — Bulk modulus (Pa): Resistance of the material to uniform compression; zero for an ideal fluid in shear context
- G — Shear modulus (Pa): Resistance to shear deformation; zero for liquids and gases, explaining why S-waves cannot pass through them
- \rho — Density (kg m⁻³): Mass per unit volume of the medium; higher density reduces wave speed
Worked Example
Problem. A seismometer records a P-wave arrival at 09:14:32 and an S-wave arrival at 09:15:12. The average P-wave speed in the region is 6.0 km/s and the average S-wave speed is 3.5 km/s. Estimate the distance from the seismometer to the earthquake's epicentre.
Answer. The epicentre is approximately 336 km from the seismometer.
Origins · 1 minInge Lehmann and how this was discovered
Inge Lehmann was a Danish seismologist who, in 1936, discovered Earth's solid inner core. By carefully analysing anomalous P-wave arrivals inside the S-wave shadow zone — arrivals that could not be explained if the entire core were liquid — she deduced that a solid inner core must reflect and refract waves to produce the observed signals. This discovery, published in a paper simply titled 'P'', fundamentally revised our model of Earth's internal structure and remains one of the great contributions to geophysics. Lehmann lived to the exceptional age of 104, continuing scientific correspondence almost until her death. She once remarked that her meticulous data-filing habits — learned partly from a progressive co-educational school she attended as a child — were key to spotting the faint seismic signals that revealed the inner core.
Related Concepts
- Transverse and Longitudinal Waves — P-waves are longitudinal and S-waves are transverse; understanding both wave types is essential to grasping how seismic waves behave and differ from one another.
- Wave Speed — The contrasting speeds of P-waves and S-waves, and how wave speed depends on medium properties, underpin the method of locating earthquake epicentres.
- Reflection and Refraction of Waves — Seismic waves refract as they pass through layers of different density and elasticity, and reflect at boundaries — processes explained by Snell's Law and the general principles of wave refraction.
- Simple Harmonic Motion — Each particle of rock displaced by a seismic wave undergoes approximately simple harmonic motion, linking oscillation theory directly to wave propagation.