1.3 Basic principles of seismic wave generation and propagation

Seismic waves are the Earth's messengers, carrying vital information about its structure and composition. These waves come in different types, each with unique properties that determine how they move through the planet's layers.

Understanding seismic wave behavior is key to unraveling Earth's mysteries. From their generation during earthquakes to their complex interactions with various materials, these waves provide crucial insights into our planet's inner workings.

Wave Types

Primary and Secondary Waves

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• P-waves propagate through solids and liquids as compressional waves
  • Travel fastest among seismic waves
  • Alternating compression and rarefaction of material
  • Particle motion parallel to wave propagation direction
• S-waves move through solids only as shear waves
  • Second to arrive after P-waves
  • Oscillate perpendicular to wave propagation direction
  • Cannot travel through liquids or gases

Surface Waves

• Love waves move horizontally in a side-to-side motion
  • Confined to the surface of the Earth's crust
  • Cause horizontal shifting of the ground
• Rayleigh waves roll along the ground like ocean waves
  • Combine compressional and shear motions
  • Decrease in amplitude with depth
  • Produce both vertical and horizontal ground motion

Wave Properties

Fundamental Wave Characteristics

• Wave velocity measures the speed at which a wave travels through a medium
  • Depends on the elastic properties and density of the material
  • P-waves travel faster than S-waves in the same medium
• Amplitude represents the maximum displacement of particles from their equilibrium position
  • Relates to the energy carried by the wave
  • Larger amplitudes indicate stronger ground shaking (earthquakes)

Wave Measurements

• Frequency denotes the number of wave cycles passing a fixed point per unit time
  • Measured in Hertz (Hz)
  • Inversely related to wavelength
• Wavelength spans the distance between two consecutive wave crests or troughs
  • Measured in meters (m)
  • Calculated using the formula: $\lambda = v/f$ (wavelength = velocity / frequency)

Wave Behavior

Wave Interactions with Media

• Attenuation describes the decrease in wave amplitude as it travels through a medium
  • Caused by geometric spreading and absorption
  • Affects seismic wave energy over distance
• Reflection occurs when waves encounter a boundary between different materials
  • Portion of wave energy bounces back into the original medium
  • Angle of incidence equals angle of reflection
• Refraction happens when waves change direction upon entering a new medium
  • Results from velocity differences between materials
  • Follows Snell's Law: $\frac{\sin \theta_1}{\sin \theta_2} = \frac{v_1}{v_2}$ (ratio of sines of angles equals ratio of velocities)

Complex Wave Phenomena

• Diffraction allows waves to bend around obstacles or spread through openings
  • Explains how seismic waves can "wrap around" the Earth's core
  • Produces wave interference patterns
• Mode conversion transforms one wave type into another at interfaces
  • P-waves can convert to S-waves and vice versa
  • Occurs at boundaries between different rock layers or the core-mantle boundary