Notes on Tsunamis and Megatsunamis

Course: ERTH-1070
Institution: UCONN College of Liberal Arts and Sciences, Department of Earth Sciences
Lecture Topic: Tsunamis

Definition of a Wave: Waves are caused by the transfer of energy from their source to the water.
Nature of Tsunamis: Tsunamis are significantly faster and larger than wind-driven waves.
Formation of Tsunamis:
- Primarily caused by earthquakes.
- Other causes include volcanic eruptions, landslides, and meteorite impacts.
Preceding Events: Earthquakes typically precede tsunamis, often followed by an unusually low tide.
Impact on Coastlines: Tsunamis erode typical coastal barriers and deposit material in areas that are normally low in deposition.
Megatsunamis: Can be triggered by massive landslides or meteorite impacts.
Risk Mitigation: Tsunami risk can be reduced through awareness, evacuation protocols, and elevating structures off the ground.

A popularized image of tsunamis depicts them incorrectly. Tsunamis should not be visualized as massive breaking waves, but rather as a rapidly rising and fiercely pushing tide.

Types of Waves:
- Wind Waves:
- Formed by winds that blow across the near-surface layer of the ocean.
- Characterized by shorter wavelengths (300-600 feet) and period (5-20 seconds).
- Tsunamis:
- Have energy distributed through the entire water column, from the surface to the ocean floor.
- Much longer wavelengths (300-600 miles) and periods (5 minutes - 2 hours).
- Can travel in deep water at speeds of 500-600 mph (or 800-1,000 km/h).

Standing Waves: These involve orbital motion, meaning individual water particles do not move in the direction of the wave.
- The height of such waves is approximately 0.5 times their wavelength.
Breaking Waves: Break when depth is less than 0.5 times the wavelength, with the top moving faster than the bottom when depth is less than 0.05 times the wavelength.

Primary Causes:
- Earthquakes: Major driver of tsunami formation.
- Landslides: Both on land and submarine landslides contribute to tsunami generation.
- Volcanic Activity: Submarine volcanic eruptions can also instigate tsunamis.
- Meteorite Impact: Rarely causes tsunamis but can lead to significant mega effects.

Offshore Earthquake: A key indicator.
Warning Systems: Improved significantly since the Boxing Day 2004 tsunami.
Visible Signs:
- Retreat of the sea (unusually low tide).
- Rapid tide recession.
- Observing drag marks along the coast indicating water withdrawal.
- Immediate action is advisable: move to higher ground.

Geomorphological Impact:
- Tsunamis can transfer high energy to otherwise low-energy environments, leading to significant landscape alteration.
- They can destroy environments that are generally capable of withstanding strong forces, disrupting both biotic and sedimentary environments.
Post-Damage Environment:
- Severe erosion of coastal barriers, beaches, reefs, and dunes.
- Deposits can be recognized geologically, indicating previous tsunami events.

Tsunami deposits are often found in calm environments onshore (sediment deposits) or offshore (sediment dragged out to sea).
Example: Recurring tsunami deposits in Japan illustrate the transition from normal soil formation to sand and coarse debris layers due to past tsunami events.

Definition: A tsunami resulted from a large and sudden displacement of material, such as rocks or sediment, into a body of water.
Example: The Lituya Bay megatsunami of 1958 reached heights of 524 meters (1,700 feet) following a landslide in the fjord.

A significant landslide resulting from the abrupt collapse of an entire fissure could trigger a megatsunami.
Models suggest that such an event could achieve approximate heights of 10 meters (30 feet) upon reaching the east coast, but this scenario is now considered unlikely.
More likely, predictions suggest a series of smaller landslides would occur, with maximum wave height reaching 2 meters in the US.