Overview
This lecture reviews the key concepts and types of seismic waves associated with earthquakes, focusing on their characteristics, propagation, and how they are detected.
Earthquake Basics
- An earthquake is the shaking of the Earth's lithosphere, caused by the release of potential energy.
- The earthquake originates at the focus (hypocenter), below the Earth's surface.
- The epicenter is the point on the Earth's surface directly above the focus.
- A fault is the fracture or separation between blocks or plates in the Earth's crust.
- Seismic waves are the vibrations produced by the earthquake.
Parts and Types of Seismic Waves
- There are four main seismic waves: Primary (P), Secondary (S), Love (L), and Rayleigh (R).
- Seismic waves are categorized as body waves (travel inside the Earth: P and S) and surface waves (travel on the surface: L and R).
Characteristics and Propagation of Seismic Waves
- P (Primary) waves: Body waves that propagate longitudinally (back and forth), travel through solids, liquids, and gases.
- S (Secondary) waves: Body waves that propagate transversely (up and down), travel only through solids.
- Love (L) waves: Surface waves, propagate transversely (side by side), travel only through solids.
- Rayleigh (R) waves: Surface waves, propagate elliptically (combination of longitudinal and transverse), travel only through solids.
Media Traversed by Seismic Waves
- P waves traverse all media (solids, liquids, gases).
- S, Love, and Rayleigh waves travel only through solid media.
Real-Life Application Example
- People on solid ground (fisherman on sand) feel all seismic waves, so they experience stronger shaking.
- People in liquid (swimming tourist) feel only P waves, experiencing weaker shaking.
Review: Identifying Wave Types
- Moves side by side, surface, solids only: Love wave.
- Moves back and forth, travels through all states of matter: Primary (P) wave.
- Moves up and down, beneath Earth’s crust, solids only: Secondary (S) wave.
- Propagates elliptically, surface, solids only: Rayleigh wave.
Key Terms & Definitions
- Earthquake — Sudden shaking of the Earth’s lithosphere from released energy.
- Focus/Hypocenter — Origin point of the earthquake inside the Earth.
- Epicenter — Point on the Earth's surface directly above the focus.
- Fault — Crack or separation where plates move.
- Seismic waves — Vibrations generated by an earthquake.
- Body waves — Seismic waves traveling through Earth's interior (P and S waves).
- Surface waves — Seismic waves traveling on Earth's surface (Love and Rayleigh waves).
Action Items / Next Steps
- Review how to identify P, S, L, and R waves on a seismograph.
- Prepare for tomorrow’s lesson comparing wave speed and destructiveness.