Seismic waves are Earth's vibrations, revealing its hidden structure. They come in two main types: body waves that travel through the planet's interior, and surface waves that ripple along its surface. Each type has unique characteristics and speeds.

These waves are crucial for understanding Earth's composition and properties. By studying how they move and change, scientists can create 3D images of the planet's insides, uncovering features like mantle plumes and subducting slabs. Factors like density and temperature affect their behavior.

Types and Characteristics of Seismic Waves

Types of seismic waves

• Body waves propagate through the Earth's interior
  • P-waves (Primary waves) are compressional waves that travel through solid, liquid, and gas, and are the fastest seismic waves (earthquakes, explosions)
  • S-waves (Secondary waves) are shear waves that travel only through solid materials and are slower than P-waves (earthquakes)
• Surface waves travel along the Earth's surface
  • Love waves are horizontal shear waves that cause side-to-side motion (ground shaking during earthquakes)
  • Rayleigh waves combine compressional and vertical shear motion, causing elliptical particle motion (rolling motion during earthquakes)

Characteristics of seismic waves

• P-waves have particle motion parallel to the direction of wave propagation and can travel through the Earth's mantle and core, with velocity increasing with depth due to increasing density and elastic properties (6-13 km/s)
• S-waves have particle motion perpendicular to the direction of wave propagation, cannot travel through liquids like the Earth's outer core, and have velocity that increases with depth in the mantle but disappears in the outer core (3-7 km/s)
• Love waves are confined to the Earth's surface and shallow depths, with velocity depending on the shear modulus and density of near-surface materials (2-4.4 km/s)
• Rayleigh waves are confined to the Earth's surface and shallow depths, with velocity slightly slower than Love waves and amplitude decreasing with depth (2-4.2 km/s)

Seismic waves for Earth study

• Seismic wave velocities indicate variations in Earth's composition and physical properties, with velocity increasing with depth in the mantle due to increasing pressure and temperature
• Refraction occurs when waves encounter boundaries between materials with different properties, while reflection occurs when waves encounter sharp boundaries like the core-mantle boundary
• P-wave shadow zone indicates the presence of the Earth's liquid outer core, while S-wave shadow zone confirms the liquid nature of the outer core
• Seismic tomography uses seismic wave travel times to create 3D images of the Earth's interior, revealing variations in temperature, composition, and density (mantle plumes, subducting slabs)

Factors affecting seismic waves

• Factors affecting velocity:
  1. Density: Higher density materials generally have higher seismic wave velocities (iron-rich core vs. silicate mantle)
  2. Elastic properties: Bulk modulus (incompressibility) and shear modulus affect P-wave and S-wave velocities, respectively (solid mantle vs. liquid outer core)
  3. Temperature: Higher temperatures generally decrease seismic wave velocities (hot mantle plumes vs. cold subducting slabs)
  4. Pressure: Higher pressures generally increase seismic wave velocities (deep mantle vs. shallow crust)
• Factors affecting attenuation:
  • Intrinsic attenuation: Energy loss due to internal friction and heat generation (viscoelastic relaxation)
  • Scattering attenuation: Energy loss due to heterogeneities and irregularities in the Earth's interior (small-scale variations in composition or structure)
  • Geometrical spreading: Amplitude decrease due to the spreading of wave energy as it travels away from the source (1/r for body waves, 1/√r for surface waves)
  • Absorption: Energy loss due to the conversion of seismic energy into heat (anelastic attenuation)