Earthquakes and Seismic Waves

Key Terms and Definitions

  • Fault: A crack or a break in the ground where two pieces of land try to slide past each other.

  • Focus: The point where the earthquake originates on the fault line.

  • Epicenter: The point directly above the focus on the surface of the Earth.

  • Fault Scarp: The visible manifestation of a fault on the surface, often resulting from movement during an earthquake.

Earthquake Mechanics

  • Primary Effects: Direct results of an earthquake, such as fault scarps.

  • Secondary Effects: Indirect consequences, including shaking ground, collapsing buildings, fires, landslides, and tsunamis.

  • Stress Buildup: As two land masses push against each other, stress builds until a sudden slip occurs, causing an earthquake.

Secondary Earthquakes

  • Occur after the main earthquake due to stress redistribution along the fault, sometimes leading to more intense shaking.

Types of Seismic Waves

  • Body Waves: Move through the Earth's interior.

    • P Waves (Primary Waves): Fastest, compressional waves, traveling like a slinky.

    • S Waves (Secondary Waves): Slower, shear waves moving perpendicular to the wave direction, can only travel through solids.

  • Surface Waves: Travel along the Earth's surface and are the most destructive.

    • Raleigh Waves: Up and down motion.

    • Love Waves: Side to side motion, more damaging to structures.

Earthquake Measurement

  • Seismographs: Instruments that record seismic waves, differentiating between P Waves and S Waves.

  • Epicenter Location: Requires data from three seismic stations for triangulation to pinpoint the earthquake's surface location.

  • Magnitude Measurement: Determined by the amplitude of seismic waves on a seismograph, although actual power release varies logarithmically (e.g., a 6.0 earthquake releases three times more energy than a 5.0).

Historical Context and Patterns

  • Historical earthquake patterns inform predictions:

    • San Francisco's 1906 quake (8.3) was followed by a quiet period until the 1970s indicating ongoing tectonic stress.

Tsunamis

  • Triggered by undersea earthquakes or land displacement.

  • The tsunami wave moves outward from the source due to the water being displaced, with waves increasing in height as they approach shore due to decreasing water depth.

  • Debris Hazard: Tsunami waves carry debris, making survival difficult, as seen in the 2011 Japan earthquake.

Conclusion

  • Understanding earthquake mechanics through faults, seismic waves, and the Puget Sound region's risk is vital for disaster preparedness. Recognizing primary and secondary effects along with mitigation strategies (e.g., proper building materials, location selection) can significantly reduce impact.

  1. Relationship of Faults, Foci, Epicenters, and Fault Scarps:

    • Faults are cracks in the ground where land pieces move past each other.

    • Focus is the origin point of an earthquake on the fault line under the earth's surface.

    • The epicenter is the point directly above the focus on the Earth's surface.

    • A fault scarp is the visible sign of a fault on the surface resulting from earthquake movement.

  2. What is an Earthquake?

    • An earthquake is a sudden release of energy in the Earth’s crust that creates seismic waves, caused by tectonic stress.

  3. Comparison of Secondary Features Caused by Earthquakes:

    • Secondary effects include collapsing buildings, landslides, fires, tsunamis, and ground shaking, which differ in impact based on proximity to the epicenter and geological conditions.

  4. P Waves vs. S Waves and Epicenter Determination:

    • P Waves (Primary Waves): Fastest, compressional wave type that travels through both solid and liquid.

    • S Waves (Secondary Waves): Slower, shear waves that move perpendicular and can only travel through solids.

    • The epicenter is determined through triangulation from data recorded at three seismic stations.

  5. Determining Rock Layer Densities Using Seismic Waves:

    • Seismic waves travel at different speeds through rock layers depending on their densities, with faster speeds indicating denser materials.

  6. Cause of Shadow Discontinuity:

    • The shadow zone is caused by the inability of S waves to travel through the liquid outer core, leading to areas where seismic waves do not arrive.

  7. Comparison of Rayleigh and Love Waves:

    • Rayleigh Waves: Move in an up-and-down motion, causing vertical displacement.

    • Love Waves: Move side to side, causing horizontal displacement, generally resulting in more damage.

  8. How Seismographs Work:

    • Seismographs detect and record seismic waves by measuring the motion of the ground, distinguishing between P and S waves due to their different speeds.

  9. Seismic Wave Amplitude and Earthquake Magnitude:

    • Magnitude is measured by the amplitude of seismic waves on a seismograph, with energy release increasing logarithmically (e.g., a 6.0 quake releases three times more energy than a 5.0).

  10. Seismic Response on Different Types of Surface Rock:

    • The type of surface rock affects seismic response, with softer rocks amplifying vibrations while harder rocks may transmit waves more efficiently but with less amplification.

  11. How Liquefaction Occurs:

    • Liquefaction happens when saturated soil temporarily loses strength and stiffness due to earthquake shaking, causing it to behave like a liquid.

  12. Predicting Earthquakes:

    • Techniques include monitoring seismic activity, historical patterns, and utilizing advanced technology to identify stress patterns in the Earth's crust.

  13. Earthquakes Forming Tsunamis Process:

    • Undersea earthquakes or substantial land displacement trigger tsunamis by displacing water, generating waves that travel outward and increase in height as they approach land.