7.2 Seismic waves: P-waves, S-waves, and surface waves

Earthquakes release energy in the form of seismic waves. These waves come in different types, each with unique properties that help scientists understand Earth's structure. P-waves, S-waves, and surface waves travel at different speeds and interact with Earth's layers in distinct ways.

Seismic waves provide crucial information about our planet's interior. By studying how these waves move through Earth, scientists can map out its layers, from the crust to the inner core. This knowledge is essential for understanding plate tectonics and earthquake behavior.

Seismic Wave Types

Body Waves and Surface Waves

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• Seismic waves divide into body waves and surface waves
  • Body waves include P-waves and S-waves
  • Surface waves include Love waves and Rayleigh waves
• P-waves propagate through solid and liquid media as compressional waves
  • Particles oscillate parallel to wave propagation direction
  • Move fastest among all seismic waves
• S-waves propagate only through solid media as shear waves
  • Particles oscillate perpendicular to wave propagation direction
  • Travel slower than P-waves but faster than surface waves
• Surface waves travel along Earth's surface
  • Love waves cause horizontal shearing of ground surface
  • Rayleigh waves produce elliptical particle motion
• Velocity hierarchy follows P-waves > S-waves > Surface waves

Wave Propagation Through Earth

• P-waves travel through all Earth layers including the core
• S-waves cannot propagate through liquid outer core
  • Creates "shadow zone" on opposite side of Earth
• Surface waves confined to outermost Earth layers
• Body waves refract and reflect at layer boundaries
  • Due to changes in density and elastic properties
• Body wave amplitude decreases with distance from source
  • Results from geometric spreading and attenuation
• Surface waves attenuate more slowly with distance
  • Makes them more destructive in earthquakes

Seismic Wave Characteristics

Wave Velocities and Material Properties

• Seismic velocities relate directly to elastic properties and density of propagation medium
• P-wave velocity (Vp) determined by:
  • Bulk modulus
  • Shear modulus
  • Density of material
• S-wave velocity (Vs) determined by:
  • Shear modulus
  • Density of material
• Vp/Vs ratio provides information about:
  • Composition of Earth materials
  • Physical state of Earth materials
• Velocity discontinuities indicate composition or property changes
  • Examples include Mohorovičić discontinuity and core-mantle boundary
• Seismic velocities generally increase with depth
  • Due to increasing pressure and density
  • Can decrease in areas of partial melting or liquid outer core

Wave Behavior and Attenuation

• Body waves refract and reflect at layer boundaries
  • Caused by changes in density and elastic properties
• P-wave and S-wave amplitude decreases with distance from source
  • Due to geometric spreading as waves propagate outward
  • Attenuation from energy loss in the medium
• Surface waves attenuate more slowly compared to body waves
  • Contributes to their destructive potential in earthquakes
• Love waves cause horizontal shearing motion of ground surface
• Rayleigh waves produce elliptical particle motion at surface

Wave Velocities and Earth's Layers

Velocity Structure of Earth's Interior

• Seismic velocities generally increase with depth
  • Result of increasing pressure and density
• Velocity discontinuities mark significant layer boundaries
  • Mohorovičić discontinuity between crust and mantle
  • Core-mantle boundary between mantle and outer core
• Velocity decreases can occur in specific regions
  • Partial melting zones in upper mantle
  • Liquid outer core
• P-wave velocities higher than S-wave velocities in all solid layers
• S-waves cannot propagate through liquid outer core
  • Creates S-wave shadow zone on opposite side of Earth
• Surface wave velocities vary with frequency (dispersion)
  • Provides information about shallow Earth structure

Relationship to Earth Properties

• Vp/Vs ratio indicates material properties
  • Higher ratios suggest more mafic composition
  • Lower ratios indicate more felsic composition
• Velocity changes reflect variations in:
  • Temperature
  • Pressure
  • Composition
  • Physical state (solid vs liquid)
• Low velocity zones may indicate:
  • Partial melting
  • Change in mineral structure
• High velocity zones often represent:
  • Colder, denser material
  • Subducted slabs in mantle

Seismic Waves for Earth's Structure

Seismic Imaging Techniques

• Seismic tomography creates 3D images of Earth's interior
  • Uses travel times and amplitudes of seismic waves
  • Reveals velocity variations in crust, mantle, and core
• P-wave and S-wave arrival time analysis determines:
  • Earthquake locations
  • Velocity models of Earth's interior
• Seismic reflection and refraction methods study:
  • Detailed structure of crust
  • Upper mantle structure
• Surface wave dispersion analysis constrains:
  • Shear wave velocity structure of crust
  • Upper mantle velocity structure
• Receiver function analysis uses converted seismic waves
  • Provides information on sharp velocity contrasts
  • Helps image crust-mantle boundary (Moho)

Insights into Earth Structure

• S-wave shadow zones provide evidence for liquid outer core
• Body wave travel times reveal:
  • Overall radial structure of Earth
  • Major discontinuities between layers
• Surface wave dispersion shows:
  • Variations in crustal thickness
  • Upper mantle structure
• Seismic anisotropy indicates:
  • Mantle flow patterns
  • Preferred mineral orientations
• ScS phases (S waves reflected off core) constrain:
  • Core-mantle boundary structure
  • D" layer properties
• PKP phases (P waves traveling through core) reveal:
  • Inner core structure
  • Inner core anisotropy