Tsunamis are powerful ocean waves triggered by underwater disturbances like earthquakes, landslides, or volcanic eruptions. They can travel across entire oceans at high speeds, growing in height as they approach land. These massive waves pose a serious threat to coastal areas.

When tsunamis hit shore, they can cause devastating flooding, erosion, and destruction of buildings and infrastructure. Understanding how tsunamis form, move, and impact coasts is crucial for developing early warning systems and protecting vulnerable communities from these geological hazards.

Tsunami Generation Mechanisms

Earthquakes as the Primary Cause

• Earthquakes are the most common cause of tsunamis, particularly those with a magnitude greater than 7.0 and shallow focal depths (less than 70 km)
• Tsunamigenic earthquakes usually involve vertical motion along a thrust or normal fault, causing sudden uplift or subsidence of the seafloor
  • The 2004 Indian Ocean tsunami was generated by a magnitude 9.1 earthquake along the Sunda Trench off the coast of Sumatra, Indonesia
  • The 2011 Tōhoku earthquake (magnitude 9.0) in Japan caused a massive tsunami that devastated the country's northeastern coast
• The rapid displacement of water creates a series of waves radiating outward from the source area

Other Tsunami Triggers: Landslides and Volcanic Eruptions

• Submarine or coastal landslides can generate tsunamis when large volumes of rock and sediment suddenly slide downslope and displace water
  • Landslide-generated tsunamis are typically more localized compared to earthquake-generated tsunamis but can still be highly destructive in nearby coastal areas
  • The 1958 Lituya Bay tsunami in Alaska was caused by a massive landslide triggered by an earthquake, generating a wave with a maximum height of 524 meters
• Volcanic eruptions can generate tsunamis through various mechanisms, including underwater explosions, caldera collapses, pyroclastic flows entering the water, or flank collapses
  • The 1883 eruption of Krakatoa in Indonesia generated a destructive tsunami that claimed over 36,000 lives
  • The 1792 collapse of Mount Mayuyama in Japan triggered a tsunami that killed an estimated 15,000 people in the nearby city of Shimabara

Tsunami Wave Characteristics

Wave Properties in the Deep Ocean

• Tsunami waves in the deep ocean have long wavelengths (up to hundreds of kilometers), low amplitudes (typically less than 1 meter), and travel at high speeds (up to 800 km/h)
• Tsunamis are shallow-water waves, meaning their wavelength is much greater than the water depth
  • This allows them to propagate at a speed that depends on water depth: $c = √(gd)$, where $c$ is wave speed, $g$ is gravitational acceleration, and $d$ is water depth
• As tsunami waves propagate across the ocean, their speed decreases when they reach shallower water near coastal areas, while their amplitude increases, leading to higher wave heights

Wave Train and Dispersion

• Tsunami wave trains can consist of multiple waves, with the subsequent waves sometimes being larger than the initial wave
  • The 2004 Indian Ocean tsunami had multiple waves, with the third wave being the largest in some locations
• Dispersion, the process by which waves of different wavelengths travel at different speeds, can cause tsunami waves to spread out and separate as they propagate across the ocean
  • This can result in a series of waves arriving at the coast over an extended period, complicating evacuation and response efforts

Tsunami Height and Power

Influence of Bathymetry and Coastal Configuration

• Bathymetry, or underwater topography, significantly influences tsunami wave height and impact
  • Shallow continental shelves and narrow bays or inlets can amplify tsunami heights, while deeper waters and gentle slopes may reduce wave heights
  • The presence of underwater ridges or canyons can focus or disperse tsunami energy, affecting wave heights along the coast
• Coastal configuration, such as the shape and orientation of the coastline, can focus or disperse tsunami energy
  • Headlands may experience higher wave heights due to wave refraction, while bays may experience amplified wave heights due to funneling effects
  • The V-shaped bay of Rikuzentakata, Japan, experienced wave heights of up to 13 meters during the 2011 Tōhoku tsunami

Nearshore Processes and Tidal Influence

• Nearshore hydrodynamic processes, such as shoaling, refraction, and diffraction, transform tsunami waves as they approach the coast, affecting their height, direction, and energy distribution
  • Shoaling causes tsunami waves to increase in height as they enter shallower water, while refraction can cause waves to bend and converge on headlands or diverge in bays
• Tidal stage at the time of tsunami arrival can influence the extent of inundation and damage
  • Tsunamis arriving at high tide will have a greater chance of overtopping coastal defenses and causing more extensive flooding
  • The 2011 Tōhoku tsunami arrived during high tide in some areas, exacerbating the extent of inundation and damage

Role of Coastal Vegetation

• Coastal vegetation, such as mangrove forests or coastal wetlands, can help dissipate tsunami energy and reduce the extent of inundation, although their effectiveness depends on factors such as vegetation density and tsunami intensity
  • Mangrove forests in the Andaman and Nicobar Islands helped reduce the impact of the 2004 Indian Ocean tsunami in some coastal areas
  • Dense coastal forests in Sendai, Japan, helped reduce the extent of inundation during the 2011 Tōhoku tsunami

Coastal Impacts of Tsunamis

Inundation and Erosion

• Inundation, or flooding of low-lying coastal areas, is one of the most destructive impacts of tsunamis
  • The extent of inundation depends on factors such as wave height, coastal elevation, and the presence of natural or artificial barriers
  • The 2011 Tōhoku tsunami caused inundation up to 10 km inland in some areas of Japan
• Erosion caused by tsunami waves can significantly alter coastal landscapes, removing sediment, reshaping shorelines, and undermining the stability of coastal structures
  • The 2004 Indian Ocean tsunami caused extensive erosion along the coasts of Indonesia, Thailand, and Sri Lanka, damaging beaches, coastal forests, and coral reefs

Infrastructure Damage and Secondary Hazards

• Strong tsunami currents can damage or destroy ports, harbors, and coastal infrastructure, such as breakwaters, jetties, and seawalls, disrupting maritime activities and coastal protection
  • The 2011 Tōhoku tsunami destroyed or damaged over 300 ports and harbors in Japan, causing significant economic losses and disrupting supply chains
• Tsunami waves can cause extensive damage to buildings, roads, bridges, and other coastal infrastructure, particularly those not designed to withstand the force of the waves and the impact of debris
  • The 2004 Indian Ocean tsunami destroyed or damaged over 570,000 houses in Indonesia alone
• Tsunami deposits, such as sand, silt, and debris, can be carried inland by the waves and left behind after the water recedes, burying or damaging structures and agricultural land
  • Tsunami deposits from the 2011 Tōhoku tsunami were found up to 4.5 km inland in some areas of Japan
• Secondary hazards, such as fires resulting from damaged electrical or gas lines, hazardous material spills, or contamination of water supplies, can exacerbate the impacts of tsunamis on coastal communities
  • The 2011 Tōhoku tsunami caused a nuclear disaster at the Fukushima Daiichi Nuclear Power Plant, resulting in the release of radioactive materials and the evacuation of over 100,000 people