Overview
This lecture explains how seismic waves from earthquakes travel through the Earth, the types of waves involved, their effects, and how they help us understand Earth's internal structure.
Earthquake Origins
- Earthquakes release energy that travels as seismic waves through the Earth.
- The epicenter is the surface location directly above the earthquake's point of energy release.
- The focus (or hypocenter) is the exact location within the Earth where energy is released.
Types of Seismic Waves
- Seismic waves are divided into body waves (travel through Earth's interior) and surface waves (travel along Earth's surface).
- Body waves include P-waves (primary or pressure waves) and S-waves (secondary or shear waves).
- Surface waves include Rayleigh waves (rolling motion) and Love waves (side-to-side motion).
Body Waves
- P-waves travel fastest, compressing and stretching material in the direction of movement.
- S-waves travel slower, moving material perpendicular to their direction (side-to-side).
- P-waves arrive first, followed by S-waves; the time gap increases with distance from the focus.
- P-waves cause little damage; S-waves cause slightly more.
Surface Waves and Destruction
- Surface waves arrive after body waves and cause most earthquake damage.
- Rayleigh waves make the ground roll like ocean waves, while Love waves move the ground side-to-side and are most destructive.
Properties of Seismic Waves
- Refraction is the bending of waves as they pass through materials of different densities, causing waves to curve toward lower density.
- Reflection is the bouncing of waves off boundaries like the Mohorovicic discontinuity (Moho), increasing local shaking.
- Different wave paths due to reflection and refraction can focus energy and cause severe damage far from the epicenter.
Wave Transmission through Materials
- P-waves and Rayleigh waves can travel through fluids (liquids or gases).
- S-waves and Love waves cannot travel through fluids, as fluids cannot support side-to-side motion.
- The presence or absence of certain waves helps scientists detect liquid layers within Earth.
Key Terms & Definitions
- Epicenter — point on Earth's surface above the earthquake focus.
- Focus (Hypocenter) — location within Earth where an earthquake's energy is first released.
- P-wave (Primary/Pressure wave) — fastest seismic body wave, compresses and stretches material.
- S-wave (Secondary/Shear wave) — slower seismic body wave, moves material side-to-side.
- Rayleigh wave — surface wave causing rolling ground motion.
- Love wave — surface wave causing side-to-side ground motion, most destructive.
- Refraction — bending of a wave as it passes between different materials.
- Reflection — bouncing of a wave off a boundary between materials.
- Mohorovicic Discontinuity (Moho) — boundary between Earth's crust and mantle.
Action Items / Next Steps
- Review examples of seismic wave effects from recent earthquakes.
- Prepare for next lecture on how wave behaviors reveal Earth's interior structure.