Geology for Engineers - Earthquake Study Notes

Introduction to Earthquakes

  • Definition: An earthquake is the vibration caused by the sudden movement of rock bodies along faults.
  • Trigger: Earthquakes are caused when differential stress overcomes frictional forces, leading to elastic rebound.

Mechanism of Earthquakes

  • Elastic Rebound Theory:
    • Developed by Harry Fielding Reid in the early 1900s.
    • Rocks accumulate energy as they deform when subjected to forces.
    • Once the internal strength is exceeded, a sudden movement occurs, releasing energy.
    • This results in a snap back to the original un-deformed shape (rebound).

Key Terms

  • Hypocenter (Focus): The point within the Earth where slippage occurs.
  • Epicenter: The point on the Earth's surface directly above the hypocenter.

Earthquake Precursors

  • Foreshocks: Smaller earthquakes occurring before a major earthquake, not always predictive.
  • Aftershocks: Lesser magnitude earthquakes that follow the main shock, helpful in assessing slippage size and area.

Plate Tectonics and Earthquakes

  • Convergent Boundaries:
    • Compression Forces: As plates collide (e.g., 2015 Nepal earthquake), energy is built up along thrust faults.
  • Transform Boundaries:
    • Horizontal movement (e.g., San Andreas Fault) can generate large earthquakes.

Fault Dynamics

  • Fault Rupture & Propagation:
    • Displacement along faults can vary, causing slow fault creep or sharp movements.
    • Locked segments can build energy over hundreds of years, leading to significant earthquakes.

Seismology Basics

  • Seismology: The study of earthquake waves.
  • Seismographs: Instruments that detect and record seismic waves.
    • Developed by Zhang Heng over 1,900 years ago.

Types of Seismic Waves

  1. Body Waves: Travel through the Earth’s interior.
    • P Waves (Primary Waves): Push-pull waves that compress and expand rock in the direction of travel.
    • S Waves (Secondary Waves): Move perpendicular to the direction of travel.
  2. Surface Waves: Travel along the Earth's surface; cause the most damage.
    • Responsible for building destruction due to their longer duration and amplitude.

Wave Speed and Size

  • Wave Speeds:
    • P waves travel 1.7x faster than S waves; surface waves are the slowest.
    • Example speeds: P waves in granite ≈ 6 km/s, S waves ≈ 3.6 km/s.

Locating Earthquake Epicenters

  • Use P-S wave arrival time differences to calculate distance from seismograph stations.
  • Triangulation: This method utilizes multiple seismograph readings to determine the epicenter's location.

Measuring Earthquake Size

  • Magnitude: The energy released by an earthquake, usually assessed by seismograph data.
  • Intensity: The level of ground shaking at a specific location, can vary based on damage.
    • Modified Mercalli Intensity Scale: Ranges from levels I (not felt) to XII (total destruction).

Moment Magnitude Scale

  • Estimates total energy released based on:
    • Displacement on fault plane.
    • Area of fault rupture.
    • Strength of rocks that failed.
  • A gradient of 32-fold increase in energy corresponds to a 1-point increase in magnitude.

Earthquake Destruction Factors

  • Factors Influencing Damage:
    • Magnitude, duration of shaking, local geology, distance from epicenter, and infrastructure quality.
  • Seismic Vibrations: Can trigger liquefaction, tsunamis, fires, and landslides.

Liquefaction and Tsunamis

  • Liquefaction: Saturated ground behaves like liquid, risking structural stability.
  • Tsunamis: Waves generated by underwater earthquakes; can travel thousands of kilometers and significantly impact coastal areas.
    • Early warning systems are crucial for minimizing destruction.

Major Earthquake Zones

  • Circum-Pacific Belt: Known as the Ring of Fire; prone to significant seismic activity due to convergent boundaries.
  • Alpine-Himalayan Belt: Resulting from collisions between major tectonic plates.

Earthquake Prediction and Mitigation

  • Short-Range Prediction: Monitoring precursors is often challenging.
  • Long-Range Prediction: Determines cyclic occurrences of earthquakes based on historical data.
  • Building Design: Constructing earthquake-resistant structures and retrofitting older buildings to reduce hazards.