Seismic Waves: Body Waves and Surface Waves

Core Idea

Earthquakes generate body waves (P and S waves) and surface waves (Rayleigh and Love) that propagate through and along the Earth. Wave velocities depend on rock composition and density; travel-time differences constrain hypocenter location, focal depth, and Earth's internal structure.

Explainer

From your study of wave properties and elastic wave propagation, you know that waves transmit energy through a medium by displacing particles from their equilibrium positions, and that the speed of propagation depends on the medium's elastic properties and density. When an earthquake ruptures a fault, it converts stored elastic strain energy into seismic waves that radiate outward in all directions. These waves fall into two fundamental categories: body waves that travel through Earth's interior, and surface waves that travel along the boundary between the Earth and the atmosphere.

Body waves come in two types. P waves (primary waves) are compressional — they push and pull particles in the same direction the wave travels, exactly like a sound wave in air or the compression pulse you can send down a Slinky by pushing one end. Because they involve volume changes (compression and expansion), P waves can travel through solids, liquids, and gases, and they are the fastest seismic waves, typically moving at 5–8 km/s in crustal rocks. S waves (secondary waves) are shear waves — they displace particles perpendicular to the direction of propagation, like the sideways wiggle that travels down a rope when you flick one end. Shear deformation requires a material that resists shape change, which liquids do not, so S waves cannot propagate through liquids. This single fact is how we know Earth's outer core is liquid: S waves generated by earthquakes on one side of the planet are absent from seismograms on the other side at specific angular distances, creating an S-wave shadow zone that maps the liquid core's boundary.

Surface waves are generated when body waves interact with Earth's free surface. Rayleigh waves produce an elliptical rolling motion — particles move both vertically and horizontally in the direction of propagation, like ocean waves but in solid rock. Love waves produce purely horizontal shearing motion perpendicular to the direction of travel. Surface waves are slower than body waves but carry more energy and produce larger ground displacements, which is why they cause the most damage during earthquakes. They also have a property called dispersion: longer-wavelength surface waves penetrate deeper into the Earth and travel faster than shorter-wavelength ones, so a surface wave train spreads out as it propagates. Seismologists exploit this dispersion to map how velocity changes with depth in the crust and upper mantle.

The practical power of seismic waves lies in their travel-time differences. Because P waves always arrive before S waves at any given station, and the time gap between them increases with distance from the earthquake, recording the P–S arrival time difference at three or more seismograph stations allows geologists to triangulate the earthquake's location and depth. Beyond locating earthquakes, the way seismic wave velocities change with depth — speeding up in denser rock, slowing in partially molten zones, vanishing (for S waves) in liquid — provides the primary evidence for Earth's layered internal structure: crust, mantle, liquid outer core, and solid inner core. Seismology is essentially using earthquakes as a natural source of illumination to image the deep Earth, much as medical ultrasound uses sound waves to image the body's interior.