Seismic Waves and Earth's Interior

Introduction

• Seismic waves indicate ground vibrations caused by earthquakes.
• They spread energy from the earthquake's source outward, similar to waves from a stone thrown into water.
• Types of seismic waves: Compressional or P, Transverse or S, Love, and Rayleigh.
• Earthquake radiates P and S waves, which interact with Earth's surface to produce surface waves.
• Near earthquakes: Shaking is large due to shear-waves and short-period surface waves, causing infrastructure damage.
• At a distance: Amplitude of waves decreases, and waves become separated due to different speeds.

Types of Seismic Waves

Body Waves

• P-Waves (Primary Waves):
  • Fastest waves, travel through solids, liquids, and gases.
  • Cause vibrations in the direction of travel.
  • Speeds range from ~1 to ~14 km/s.
• S-Waves (Secondary Waves):
  • Slower than P-waves, travel only through solids.
  • Cause perpendicular vibrations to the direction of travel.
  • Speeds range from 1 to 8 km/s.

Surface Waves

• Love Waves:
  • Transverse waves restricted to horizontal movement.
  • Caused by interaction of S waves with Earth's surface.
  • Speeds between 2 and 6 km/s.
• Rayleigh Waves:
  • Slowest waves, cause elliptical particle motion.
  • Speeds between 1 and 5 km/s.

Wave Travel Times and Speed

• Travel time: Relative time for wave travel from earthquake to seismometer.
• Arrival time: Absolute time when wave is recorded, often in Universal Coordinated Time.
• Seismic wave speed depends on rock composition, temperature (decreases speed), and pressure (increases speed).

Locating Earthquakes Using P and S-waves

• Different travel speeds of P and S waves help locate earthquakes.
• Time interval between P-wave and S-wave helps estimate distance to earthquake epicenter.
• At least three seismometers are needed to triangulate the epicenter.

Seismic Wave Interactions

Refraction

• Change in wave direction when passing through different rock types, described by Snell's Law.
• Velocity generally increases with depth, causing upward curving paths for body waves.

Reflection

• Occurs at boundaries with different material properties.
• Used for studying Earth's internal structure.

Dispersion

• Surface waves are dispersive; different periods travel at different velocities.

Earth's Internal Structure

• Main components: core, mantle, and crust.
• Velocity increases with depth due to pressure effects, despite rising temperatures.
• Transition zone: mineral transformation at 400 to 1000 km depth.

Models of Earth's Heterogeneity

• PREM model: Reference model for Earth's structure, showing velocity and density variations.
• Seismic tomography: Maps lateral variations in wave speed, used to study Earth's crust and mantle.

Key Concepts

• Interaction of waves with Earth's structure provides insight into Earth's interior.
• Seismic wave observations help infer geological features and processes.

Prepared by: Charles J. Ammon

For more detailed information, visit the course materials or the provided URL source.