Tsunami Notes

Tsunami Overview

  • Definition: Tsunami is a Japanese term meaning "harbor wave." It is often mistakenly called a tidal wave.

  • Causes:

    • Earthquakes: Most common trigger, especially those of magnitude M ≥ 7.5.
    • Underwater Landslides: Displaces water vertically, generating tsunamis.
    • Volcanic Activity: Flank collapse or explosive eruptions can cause tsunamis.
    • Asteroid Impacts: Rare but catastrophic.

  • Notable Art: "The Great Wave off Kanagawa" (Hokusai, 1829-1833).

Historical Tsunamis

  • 2011 Japan Tsunami:

    • Caused by a magnitude 9.0 earthquake.
    • Pacific Ocean bottom was displaced up to 9 meters (30 feet).
    • Nuclear plant flooded, leading to cooling system failure.

  • Some Historic Tsunamis:

    • 1755: Lisbon Earthquake, ~20,000 deaths, waves ~23 ft.
    • 1883: Krakatoa Eruption, ~36,000 deaths, waves ~116 ft.
    • 2004: Indian Ocean earthquake, ~230,000 deaths.

Mechanism of Tsunami Generation

  • Earthquake Mechanism:

    • Most tsunamis result from seafloor displacement due to earthquakes.
    • Uplift or downshift of seafloor initiates a wave generation process.

  • Four-Stage Process:

    1. Initiation: Seafloor displacement creates a dome of water.
    2. Propagation: Tsunami travels rapidly across deep ocean (up to 720 km/h or 450 mph).
    3. Amplification: As the tsunami approaches the shore, it slows (to 45 km/h) and increases in height.
    4. Inundation: The tsunami moves inland, potential height ranging from meters to tens of meters, devastating structures.

Types of Tsunamis

  • Local Tsunami: Affects nearby coastal areas with very little warning time.
  • Distant Tsunami: Can travel long distances across oceans with minimal energy loss.

Tsunami Risk Areas

  • Most tsunamis occur in the Pacific Ocean, particularly along the "Ring of Fire" where tectonic plate boundaries meet, notably subduction zones.

  • Global Hazard Analysis:

    • 80% of tsunamis are associated with the Pacific.
    • Coastal areas near subduction zones are at the highest risk.

Effects of Tsunamis

  • Primary Effects:

    • Inundation leading to flooding, erosion, and loss of life.
    • Structural damage (e.g., rebuilding coastlines, damage to infrastructure).

  • Secondary Effects:

    • Fires from ruptured gas lines.
    • Contaminated water supplies.
    • Potential outbreak of diseases due to water contamination.

Tsunami Detection and Warning Systems

  • Methods for Detection:

    • Seismographs: To detect underlying earthquakes.
    • Automated tidal gauges: For monitoring sea level changes.
    • Buoy sensors equipped with tsunameters: Detect minute pressure changes in the ocean.

  • Communication Systems: Technology used to relay detection data to authorities is vital for timely warnings.