LR

Lecture 14: Earthquakes

Earthquakes Overview

  • Definition: Sudden movement along a fault due to built-up stress.

Fault

  • Fault: Fracture in rocks where movement has occurred.

Earthquake Process

  • Step 1: Stress builds up on a fault, accumulating energy.

  • Step 2: Stress reaches a critical level.

  • Step 3: Stress is released as a quake, causing permanent deformation along the fault.

Earthquake Characteristics

  • Moment of stress (energy) release along a fault plane.

  • Results in shock waves (seismic waves) propagating away from the quake.

Hazard Factors

  1. Magnitude: Energy released during a quake; measured on a 1-10 scale.

  2. Depth: Shallow quakes are typically more dangerous.

  3. Population Density: More people at risk increases hazard.

  4. Related Hazards: Includes liquefaction, landslides, tsunamis.

  5. Distance to Epicenter: Closer distance increases risk.

  6. Rock Type: Seismic waves travel faster through hard rock, causing less shaking.

Earthquake Magnitude

  • Each magnitude increment represents a 10x increase in size and 32x more energy.

  • Example differences between magnitudes (7 vs 6, 5, etc.) indicate increases in area and energy released.

Global Seismic Hazard

  • Proximity to active fault systems reflects seismic hazard level.

Seismology Tools

  • Seismometer: Measures seismic wave arrivals.

  • Seismogram: Displays arrival time and magnitude of seismic waves.

Seismic Wave Types

  1. Body Waves:

    • P Waves (Primary): Fastest, can travel through any medium.

    • S Waves (Secondary): Slower, can only travel through solids.

  2. Surface Waves: Move along the Earth's surface, cause the most damage.

    • Include Rayleigh and Love Waves.

Locating Epicenter

  • Requires data from at least 3 seismic stations (triangulation) to determine exact location.

Earth's Interior Understanding

  • Seismic waves reveal physical structure through varied travel speeds in different materials.

  • Refraction indicates changes in wave speed due to density and temperature differences in materials.

Shadow Zones

  • Regions where S waves cannot travel due to liquid outer core, helping to determine its size.