Overview
This lecture explains the causes of earthquakes, how seismic waves travel through the Earth, and the types of seismic waves detected during an earthquake.
Causes of Earthquakes
- Earthquakes are the shaking of the Earth due to the release of energy in the crust.
- Energy release occurs along faults, which are sharp breaks in crustal rocks.
- Movement along a fault overcomes friction, causing rocks to slip and release energy as seismic waves.
- The focus (hypocenter) is the point inside the Earth where energy is released.
- The epicenter is the point on the Earth's surface directly above the focus.
- All natural earthquakes occur in the lithosphere (up to 200 km deep).
Seismic Waves
- Seismographs are instruments that record seismic waves reaching the Earth's surface.
- Seismic waves are mainly two types: body waves and surface waves.
- Body waves travel through the interior of the Earth.
- Surface waves travel along the Earth's surface after body waves reach surface rocks.
- Wave velocity increases with the density of the material they travel through.
- Waves change direction via reflection or refraction at materials of different densities.
Types of Body Waves
- Body waves are divided into P-waves (Primary) and S-waves (Secondary).
- P-waves are the fastest, arrive first, and travel through both solids and liquids by compressing and expanding the ground.
- S-waves arrive after P-waves, move more slowly, and shake the ground perpendicular to wave direction (side-to-side motion).
Surface Waves
- Surface waves arrive last on seismographs and travel only along the Earth's surface.
- Surface waves cause the most damage by displacing rocks and collapsing structures.
Key Terms & Definitions
- Fault — a break in Earth's crust where movement occurs.
- Focus (Hypocenter) — the point inside Earth where an earthquake originates.
- Epicenter — the surface point vertically above the focus.
- Lithosphere — Earth's outer layer up to 200 km deep where earthquakes occur.
- Seismograph — instrument that records seismic waves.
- Body Waves — seismic waves traveling through Earth's interior (includes P- and S-waves).
- P-wave (Primary/Pressure wave) — fastest body wave, compresses and expands ground, travels through solids and liquids.
- S-wave (Secondary/Shear wave) — slower body wave, moves ground perpendicular to direction, travels only through solids.
- Surface Waves — seismic waves traveling along the Earth's surface, causing most destruction.
Action Items / Next Steps
- Watch the recommended video demonstration on P- and S-waves by Dr. Keith Miller for further understanding.