2.2 Tsunamis: generation, propagation, and coastal impacts
5 min read•august 14, 2024
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 , , 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
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- 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 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: , where is wave speed, is gravitational acceleration, and 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
Key Terms to Review (18)
2004 Indian Ocean Tsunami: The 2004 Indian Ocean tsunami was a catastrophic natural disaster that occurred on December 26, 2004, triggered by a massive undersea earthquake off the coast of Sumatra, Indonesia. It generated powerful waves that propagated across the Indian Ocean, impacting coastal regions in multiple countries and causing unprecedented destruction and loss of life. The event highlighted the complex dynamics of tsunami generation, propagation, and the devastating coastal impacts they can have on vulnerable communities.
Disaster relief: Disaster relief refers to the organized response and assistance provided to individuals and communities affected by natural disasters, aiming to meet their immediate needs and facilitate recovery. This process involves a range of services including medical aid, food distribution, shelter provision, and psychological support, which are critical in the aftermath of events like earthquakes and tsunamis. Effective disaster relief is essential for minimizing the impacts of disasters on society and the environment and can significantly influence patterns of displacement and migration as affected populations seek safety and stability.
Erosion: Erosion is the process by which soil and rock are removed from one location and transported to another, primarily by natural forces such as water, wind, or ice. This process can significantly alter landscapes and is critical in shaping coastal regions and riverbanks, especially during events like tsunamis and floods, which can accelerate erosion rates and impact ecosystems and human infrastructure.
Evacuation Plans: Evacuation plans are detailed strategies designed to guide individuals and communities in safely exiting an area that may be threatened by natural or man-made disasters. These plans are crucial for ensuring public safety, coordinating rescue efforts, and minimizing chaos during emergencies such as earthquakes, tsunamis, hurricanes, and nuclear incidents.
Flooding: Flooding refers to the overflow of water onto land that is typically dry, often caused by excessive rainfall, storm surges, or the rapid melting of snow. It can lead to significant damage to property, infrastructure, and ecosystems, and can cause loss of life. In certain contexts, flooding can occur as a result of natural disasters such as tsunamis and tropical cyclones, making it crucial to understand its dynamics and impacts.
Hydrodynamic Modeling: Hydrodynamic modeling is a computational method used to simulate and analyze the movement of water and the forces acting on it, particularly in relation to waves, currents, and other hydrological phenomena. This modeling plays a crucial role in understanding the generation, propagation, and impacts of tsunamis by providing insights into how these massive waves travel across oceans and interact with coastlines, helping to predict potential flooding and damage in coastal areas.
Numerical modeling: Numerical modeling is a mathematical technique used to simulate and predict complex phenomena by discretizing equations and solving them computationally. In the context of tsunamis, it helps in understanding their generation, propagation across ocean basins, and eventual impacts on coastal regions by using various physical principles and data inputs to forecast behavior under different scenarios.
Risk Assessment: Risk assessment is the process of identifying, evaluating, and prioritizing risks associated with potential hazards and their impacts on human life, property, and the environment. This process is crucial for understanding the likelihood and consequences of various natural disasters, enabling effective preparedness and response strategies to mitigate their impacts.
Seismic tsunami: A seismic tsunami is a series of ocean waves generated by the sudden displacement of a large volume of water, primarily caused by underwater earthquakes. The intensity and magnitude of the tsunami depend on the size of the seismic event, the depth of the ocean, and the distance from the shoreline. These tsunamis can travel across entire ocean basins, impacting coastal areas with powerful waves that can cause severe destruction and loss of life.
Submarine landslide tsunami: A submarine landslide tsunami is a type of tsunami generated by the sudden movement of sediment and rock down the continental slope or seabed, often triggered by underwater earthquakes or volcanic activity. These tsunamis can travel across ocean basins at high speeds and can lead to significant coastal impacts when they reach shore, causing flooding, erosion, and damage to infrastructure. The generation, propagation, and coastal effects of these tsunamis are critical for understanding how they differ from tsunamis caused by surface events, such as earthquakes on land.
Tectonic plate movement: Tectonic plate movement refers to the continuous shifting of the Earth's lithospheric plates over the semi-fluid asthenosphere beneath them. This movement is driven by forces such as mantle convection, slab pull, and ridge push, leading to significant geological events and phenomena like earthquakes and tsunamis. Understanding tectonic plate movement is crucial for analyzing how these events occur, how they can be measured, and their various effects on coastal regions and ecosystems.
Tōhoku earthquake and tsunami: The tōhoku earthquake and tsunami refers to the massive undersea megathrust earthquake that struck off the coast of Japan on March 11, 2011, registering a magnitude of 9.0. This earthquake generated a powerful tsunami that devastated coastal areas, leading to widespread destruction and loss of life, while also highlighting the dynamics of tsunami generation, propagation, and the subsequent coastal impacts associated with such natural disasters.
Tsunameter: A tsunameter is an advanced instrument designed to detect and measure tsunami waves in the ocean. These devices are crucial for monitoring the generation and propagation of tsunamis, allowing for early warnings and better understanding of their potential impacts on coastal areas. By providing real-time data on wave height and speed, tsunameters help researchers and emergency management teams assess the risk and implement timely evacuations in affected regions.
Tsunami warning system: A tsunami warning system is a network of sensors, monitoring stations, and communication protocols designed to detect and provide alerts for tsunami events, enabling timely evacuation and response measures. This system utilizes seismic data, ocean buoys, and tide gauges to assess the likelihood of a tsunami after an undersea earthquake or volcanic eruption. By delivering early warnings, it aims to minimize loss of life and property along coastal regions that are at risk of being impacted by tsunami waves.
Underwater volcanic eruption: An underwater volcanic eruption occurs when magma from beneath the Earth's crust escapes to the ocean floor, resulting in the release of gas, ash, and lava. These eruptions can create significant geological disturbances and are often linked to the generation of tsunamis, as the sudden displacement of water can lead to large waves that propagate across oceans, impacting coastal areas.
Vulnerability analysis: Vulnerability analysis is a systematic assessment that identifies and evaluates the susceptibility of communities, infrastructures, and ecosystems to the impacts of hazards, such as tsunamis. This process considers various factors, including physical location, socioeconomic conditions, and preparedness measures, to determine how likely a community is to experience damage or disruption from a tsunami event. Understanding these vulnerabilities helps in planning and implementing effective mitigation strategies to reduce risks and enhance resilience against future disasters.
Wave formation: Wave formation refers to the process by which waves, particularly those associated with tsunamis, develop and evolve as they propagate through water. This phenomenon is critical in understanding how tsunamis are generated, how they travel across oceans, and the impacts they have when reaching coastlines, where their energy is released dramatically.
Wave Shoaling: Wave shoaling refers to the phenomenon where waves increase in height and decrease in wavelength as they move from deeper to shallower water. This process occurs because the energy of the wave becomes concentrated in a smaller water column, causing the wave to rise dramatically as it approaches the shore. Understanding wave shoaling is crucial for grasping how tsunamis behave as they propagate across ocean basins and interact with coastal environments.