10.2 Seismic waves and their propagation

Seismic waves are vibrations that travel through and across the Earth after an earthquake or explosion. By studying how these waves behave, geologists can figure out what the planet looks like on the inside, even though no one has ever been there. This topic covers the different wave types, how they move, and what they tell us about Earth's structure.

Types and Characteristics of Seismic Waves

Types of seismic waves

Seismic waves fall into two broad categories: body waves that travel through Earth's interior, and surface waves that travel along the surface. Surface waves generally cause more damage during earthquakes because their energy is concentrated near the ground.

Body waves:

• P-waves (Primary waves) are compressional waves. Particles move back and forth in the same direction the wave travels, like a slinky being pushed and pulled. P-waves are the fastest seismic waves and can travel through solids, liquids, and gases. That's why they arrive at seismometers first.
• S-waves (Secondary waves) are shear waves. Particles move perpendicular to the direction the wave travels, like shaking a rope side to side. S-waves only travel through solids. This one fact turns out to be hugely important for understanding Earth's interior.

Surface waves:

• Love waves produce horizontal, side-to-side shaking. They're the faster of the two surface wave types.
• Rayleigh waves produce an elliptical rolling motion, combining both vertical and horizontal movement. Think of how ocean waves move water in circles. Rayleigh waves are slightly slower than Love waves.

Characteristics of seismic waves

Each wave type has a distinct speed range and behavior as it moves through Earth:

• P-waves: ~6–13 km/s. Velocity increases with depth because deeper rock is denser and has greater elastic stiffness. P-waves travel through the mantle and core.
• S-waves: ~3–7 km/s. Velocity also increases with depth in the mantle, but S-waves cannot pass through liquids. They disappear entirely in Earth's liquid outer core.
• Love waves: ~2–4.4 km/s. Confined to the surface and shallow depths. Their speed depends on the shear modulus and density of near-surface materials.
• Rayleigh waves: ~2–4.2 km/s. Also confined to the surface. Their amplitude decreases with depth, and they travel slightly slower than Love waves.

Seismic waves for Earth study

Seismic waves act as natural probes of Earth's interior. When waves encounter a boundary between materials with different properties, two things can happen:

• Refraction: The wave bends as it crosses into material with a different velocity. This is similar to how light bends when it passes from air into water.
• Reflection: The wave bounces back off a sharp boundary, like the core-mantle boundary.

These behaviors create shadow zones, which are regions on Earth's surface where certain waves don't arrive after an earthquake:

• The P-wave shadow zone (roughly 104°–140° from the earthquake) exists because P-waves refract when they hit the liquid outer core, bending them away from that zone. This told geologists the outer core must be liquid.
• The S-wave shadow zone is even larger (beyond ~104°) because S-waves can't travel through liquid at all. No S-waves reach the far side of the planet, confirming the outer core is liquid.

Seismic tomography takes this further. By collecting travel-time data from thousands of earthquakes recorded at stations worldwide, scientists build 3D images of Earth's interior. Regions where waves travel faster than expected are typically cooler and denser (like subducting slabs), while slower regions are typically hotter (like mantle plumes beneath hotspots).

Factors affecting seismic waves

Factors affecting velocity:

1. Density: Higher-density materials generally produce higher wave velocities. For example, the iron-rich core has different velocities than the silicate mantle.
2. Elastic properties: The bulk modulus (resistance to compression) controls P-wave speed, while the shear modulus (resistance to shearing) controls S-wave speed. The liquid outer core has zero shear modulus, which is exactly why S-waves can't pass through it.
3. Temperature: Higher temperatures generally decrease wave velocities because rock becomes slightly less rigid. Hot mantle plumes show up as slow-velocity zones in tomographic images.
4. Pressure: Higher pressures generally increase wave velocities by compressing rock into a stiffer state. This is why velocities tend to increase with depth.

Factors affecting attenuation (energy loss):

As seismic waves travel, they lose energy. This happens through several mechanisms:

• Geometrical spreading: Wave energy spreads over a larger area as it moves outward. Body wave amplitude decreases as $\frac{1}{r}$ (where $r$ is distance from the source), while surface wave amplitude decreases as $\frac{1}{\sqrt{r}}$. This is why surface waves carry more energy over longer distances and cause more damage far from the epicenter.
• Intrinsic attenuation: Energy converts to heat through internal friction within the rock.
• Scattering attenuation: Small-scale variations in rock composition or structure scatter wave energy in different directions, weakening the main wave.
• Absorption: Seismic energy gradually converts to heat as the wave passes through imperfectly elastic (anelastic) material.