What Are Seismic Waves?

Seismic waves are elastic waves that propagate through the Earth, carrying energy released by earthquakes, volcanic activity, impacts, or artificial sources such as explosions and vibroseis trucks. They deform the rock and soil through which they travel, and the characteristics of that deformation depend on the mechanical properties of the medium: density, bulk modulus, and shear modulus. Because different Earth materials transmit seismic waves at different speeds and with different attenuation, the wave arrivals recorded at seismometers can be inverted to image the interior of the Earth.

Seismic waves share their governing equations with acoustic waves in the low-amplitude limit. Both are solutions to the elastic wave equation derived from Newton's second law and Hooke's law, and the discipline of elastodynamics provides the mathematical framework for analyzing their propagation, reflection, refraction, and scattering. The distinction from acoustic waves is that solid materials support shear stress, producing wave types with no acoustic analog. The Pacific Northwest Seismic Network's educational materials on earthquake waves provide an accessible entry into how the different wave types are generated and identified on seismograms.

Body Waves

Body waves travel through the interior of the Earth. There are two types. P-waves (primary or compressional waves) propagate by alternately compressing and dilating the material in the direction of travel, with particle motion parallel to propagation. They are the fastest seismic wave type, traveling at roughly 6 kilometers per second through crustal rock, and they can pass through solid, liquid, and gaseous material. S-waves (secondary or shear waves) propagate by displacing material perpendicular to the direction of travel. They are slower than P-waves and cannot propagate through liquids, a property that was used to demonstrate the existence of Earth's liquid outer core. The arrival-time difference between P and S waves at a recording station is the primary observable used to estimate the distance to an earthquake epicenter.

Surface Waves

Surface waves are guided along the Earth's surface and are evanescent with depth. They arrive after the body waves and generally carry more energy at long periods. Two main types exist. Love waves are horizontally polarized shear waves trapped near the surface by a velocity contrast between a softer surficial layer and stiffer rock beneath. Rayleigh waves produce retrograde elliptical particle motion in the vertical plane containing the propagation direction, analogous in form to water surface waves. The IRIS/EarthScope seismic wave animations illustrate these motion patterns clearly. Surface waves dominate the long-period seismogram at teleseismic distances and are the primary agent of shaking damage in large earthquakes, where their amplitudes can exceed those of body waves by an order of magnitude.

Elastodynamics and Wave Propagation

The elastodynamic wave equation governs how seismic disturbances propagate through a medium characterized by its Lamé parameters and density. In a homogeneous isotropic medium, P-wave velocity equals the square root of (λ + 2μ)/ρ and S-wave velocity equals the square root of μ/ρ, where λ and μ are the Lamé constants and ρ is density. In practice, the Earth is heterogeneous, anisotropic, and attenuating, so numerical methods including finite difference, spectral element, and finite element schemes are used to model realistic wave propagation. Seismic tomography uses the residuals between observed and predicted travel times to iteratively update three-dimensional models of Earth's velocity structure. The USGS sediment transport and geophysical research program relies on seismic wave characterization to understand subsurface sediment properties in river and coastal environments.

Applications

Seismic waves have applications in a wide range of disciplines, including:

• Earthquake location and focal mechanism determination
• Seismic reflection and refraction surveys for hydrocarbon and mineral exploration
• Tomographic imaging of Earth's mantle, crust, and core structure
• Structural health monitoring using ambient seismic noise cross-correlation
• Tsunami early warning, where seafloor earthquake parameters drive wave models
• Non-destructive testing of concrete, soil, and rock using ultrasonic P-wave velocity