Chapter 4: Tsunamis

Understand the process of tsunami formation and development.
Understand the effects of tsunamis and the hazards they pose to coastal regions.
Identify geographic regions at risk for tsunamis.
Recognize linkages between tsunamis and other natural hazards.
Understand actions that nations, communities, and individuals can take to minimize the tsunami hazard.

Tsunami: Japanese for "large harbor wave".
Causes: Result from a sudden vertical displacement of ocean water.
- Typically consists of more than one wave.
- Wave sizes range from centimeters to over 100 feet.
- Time between waves can be up to an hour.

Runup: Maximum height that water reaches above sea level.
Inundation: Total horizontal area flooded by water.
- Both vary based on:
- Shape of seafloor offshore.
- Type of vegetation.
- Land topography.
Edge Waves: Generated and travel parallel to the shore, potentially causing wave amplification when interacting with tsunami waves (can cause the second or third waves to amplify).

Earthquakes: Most common trigger, can uplift or subside the seafloor displacing water.
Landslides: Underwater or rock landslides displacing water.
Volcano Flank Collapse: Sudden collapse of volcanic structure causing tsunamis.
Submarine Volcanic Explosion: Release of energy from underwater volcanoes.
Asteroid or Comet Impact: Large impacts can cause mega-tsunamis.

1755: Earthquake (M 9) in Lisbon, Portugal - 20,000 people killed; wave heights of 23 ft in the West Indies.
1883: Volcano collapse - 36,000+ people killed; waves reached 116 ft.
1946: Earthquake (M 8.1) in Aleutians - over 160 killed in Hawaiian Islands.
1960: Earthquake (M 9+) in Chile - over 61 killed in Hawaii.
1964: Alaskan Earthquake (M 8.3) - 130 killed in Alaska and California.
1993: Earthquake (M 7.8) in the Sea of Japan - 120 killed on Okushiri Island, Japan.
1998: Submarine landslide triggered by earthquake in Papua New Guinea (M 7.1) - over 2100 killed.
2004: Sumatran earthquake (M 9.1+) - over 230,000 killed.
2011: Japanese (Tohoku) earthquake (M 9.0) - over 16,000 killed; waves reached heights up to 133 ft.

Stage 1: Ocean floor is uplifted or downshifts suddenly during an earthquake.
- A dome of water forms above the fault.
- The dome collapses generating tsunami waves.
- Aftershocks may produce more waves that radiate outward like a pebble in a pond.
Stage 2: Tsunami moves rapidly in the deep ocean.
- Wave spacing can exceed 60 miles.
- Speed can exceed 450 mph (close to jet plane speed).
- Amplitude (height) of waves generally less than 3 ft - boats in the open ocean do not notice.
Stage 3: Tsunami approaches land.
- Speed decreases as ocean depth decreases (slowing to below 30 mph).
- Amplitude increases as water compresses, causing waves to grow taller.
- Distance between waves decreases.
Stage 4: Tsunami reaches shore and moves inland.
- First wave may range from meters to tens of meters high.
- The trough (lowest part of the wave) can arrive first, exposing the seafloor.
- Arrives as a rapid increase in sea level vs. defined waves; a defined wave may appear as a vertical wall of turbulent water.
- More waves can occur, often larger than the first, striking the coast for several hours before receding back into the ocean in a turbulent flow.

Distant Tsunami:
- Travels to deep ocean at high speed.
- Can travel long distances with little energy loss.
- Can strike remote shorelines.
Local Tsunami:
- Approaches land very quickly.
- Little time for people to react.
Each wave type generally has a height of approximately half that of the original dome of water.

Geologic records provide clues for ancient events.
- Example: Alaskan logs and soil buried below tsunami deposits dated radiocarbon to sometime after 1600.
- Subsidence and matching historical accounts of a Japanese tsunami in 1700.
- Run-up heights estimated between 3-16 ft in Japan.

Most are triggered by earthquakes.
Submarine landslides: Underwater landslides that displace water and generate tsunamis.
On land, rock avalanches can cause displacement, generating tsunamis in nearby water bodies.
Example from 1998: A 7.1 Mw earthquake triggered a submarine landslide in Papua New Guinea, resulting in waves over 50 ft high and significant loss of life.

Human actions can inadvertently exacerbate tsunami impacts.
Increased coastal population leads to higher risk as more people occupy vulnerable areas.
Infrastructure failures during tsunami events can result in severe consequences (e.g., the 2011 Fukushima disaster).