Tsunamis Lecture Notes

  • Discussion on the impact of tsunamis, comparing wave characteristics, causes, and effects on society.

Waves

  • Different types of waves discussed, with emphasis on:
      - Wind waves: Result from energy transfer due to wind blowing across the sea surface.
      - Tsunami waves: Result from the displacement of a mass of water.

Observation 1

  • Peted By Himen Lorens's exploration into characteristic observations.

Observation 2

  • Measurement focusing on wave characteristics, specifically the numerical observation 7841.

Tsunami vs. Wind Wave Motion

Cause of Tsunamis

  • Tsunamis are initiated when a significant mass displaces water.
      - Elements involved:
        - Push from the displaced mass.
        - The equilibrium level of water.
        - The time taken for wave propagation influenced by gravitational pull.

Wave Characteristics

  • Characteristics of waves are determined by:
      - Area of mass displacement
      - Amount of displacement
      - Velocity of movement

Wave Physics

  • Factors affecting wave physics include:
      - Distance from the tsunami source
      - Volume of water displaced
      - Shape and velocity of the mass causing displacement
      - Refraction, reflection, and interference of waves.

Wave Height

  • Higher wave heights as a result of interacting variables, including coastal topography, and the methods waves propagate.

Refraction and Interference

  • Objects, such as islands, can cause waves to bend and interfere.

Tsunami Wave Animation

  • Presentation of a 29-hour, 43-minute elapsed time animation of tsunami wave amplitude (in meters) with measurements and data from the NOAA Pacific Tsunami Warning Center.

Example Tsunamis

Example #1

  • An earthquake that displaces a large ocean floor area can create:
      - Wavelengths between 200-500 km and periods ranging from 10 minutes to 2 hours.

Example #2

  • A local landslide that displaces a small area of water can create:
      - Wavelengths of a few hundred meters with wave periods in minutes.

Tsunami Wave Behavior in Harbors

  • Tsunamis tend to grow in height as they enter harbors or narrow spaces,
      - Example: An 8 m (26 ft) wave can amplify to a 30 m (98 ft) wave under such circumstances.

Tsunami Waves Coming Ashore

Open Ocean Dynamics

  • In open waters, tsunamis have a wave velocity of 700 km/h (435 mph) and a wavelength of 200 km (124 mi).

  • Behavior illustrated with graphical representations of wave velocities at different depths.

Shallow Ocean Dynamics

  • As tsunamis approach shallow areas, they slow down significantly, increasing height:
      - Continental shelf wave speed decreases to 160 km/h (100 mph).
      - At the beach, wave speed drops to 40 km/h (25 mph).

Mechanisms of Wave Height Growth

  • Faster waves catch up to slower waves; with a decreasing wavelength, the height increases, akin to seismic wave amplification.

Key Terms

  • Inundation distance: How far inland the tsunami travels.

  • Run-up elevation: Maximum elevation reached during inundation.

  • Drawback: Water being pulled away from the coast before a tsunami strikes.

Coastal Topography and Tsunami Behavior

  • Coastal shape significantly influences how far and where water flows inland when a tsunami approaches, depicted through animations.

Coastal Bathymetry

  • Bathymetry directly affects wave height; shallower areas result in higher waves.

Tsunami Amplification**

  • Narrow bays and inlets can amplify wave height as they concentrate energy.

Wave Trains

  • Tsunami wave behavior is characterized by wave trains consisting of:
      - 6-25 waves arriving over 3-6 hours; with the first wave not always being the highest.

Storm Waves vs. Tsunamis

Differences and Comparisons

  • Storm Waves:
      - Exhibits limited drawback and shorter wave periods (5-20 seconds).
      - Smaller volume of water with shallow wave base.

  • Tsunamis:
      - Significant drawback with extended inundation distance.
      - Longer wave periods (10 minutes to 2 hours) and larger volume of water with deep wave bases.

Tsunami Behavioral Phases in Coastal Regions

  • Illustration over time of tsunami wave approach, including drawback, height increase, and land submersion details.

Hollywood Depictions of Tsunamis

Hollywood 1

  • Examination of accuracy in tsunami depictions, such as in the 2015 movie "San Andreas."
      - Highlights inaccuracies, including portrayal of wave characteristics.

Hollywood 2

  • Examination of the 2012 movie "The Impossible."

Causes and Cases of Tsunamis

Tsunami Causes

  • Notable causes identified include:
      - Earthquakes
      - Landslides
      - Volcanic eruptions
      - Meteorite impacts

Detailed Analysis of Earthquake Tsunamis

  • Vertical fault movement, either upward or downward, can displace water into powerful tsunamis.

Megathrusts

  • Megathrust earthquakes produce the largest tsunamis due to significant displacements.

Notable Tsunami Events

Chilean Tsunami (1960)

  • Magnitude 9.5 earthquake: the most powerful ever recorded, resulting in significant casualties (1,000 deaths) and tsunami wave heights.

Indonesian Tsunami (2004)

  • Magnitude 9.3 earthquake, resulting in a catastrophic loss of life (230,000 deaths).

Other Significant Tsunami Events

  • Banda Aceh’s experience with the 2004 tsunami.

  • Historical landslide in Lituya Bay, Alaska (1958) produced a tsunami with a surge of 524 m (1,722 ft).

  • Newfoundland, 1929, undersea landslide causing widespread communication disruptions.

  • Krakatau eruption in 1883 that killed over 36,000 people due to tsunami impacts.

Society's Response to Tsunamis

Relief and Recovery Challenges

  • Post-tsunami challenges include damaged infrastructures such as roads and communication systems.

Tsunami Detection and Hazard Management

  • Use of DART (Deep-ocean Assessment and Reporting of Tsunamis) stations for tsunami detection via wave pressure.

  • Development of hazard maps to outline tsunami inundation zones and evacuation plans.

Seawall Infrastructure Post-2011

  • Japan's initiative to construct a 400 km (249 mi) network of seawalls for tsunami defense.

Critical Thinking Exercises

  • Suggestions on appropriate actions during a tsunami threat.
      - Evade to tall structures, flee, or seek natural cover like trees.

Historical Community Wisdom

  • An example of community preparation is highlighted with residents of Simeule Island, who successfully fled elevations in response to immediate earthquake shaking and avoided disaster due to traditional oral histories emphasizing safety practices.