Glossary

11.4 Tsunami and coastal hazard assessment

4 min read•august 14, 2024

Tsunamis are powerful ocean waves triggered by underwater disturbances like earthquakes or landslides. They can travel vast distances at high speeds, growing in height as they approach land. Understanding their behavior is crucial for coastal communities to prepare for and mitigate potential devastation.

Assessing tsunami hazards involves studying past events, using numerical models, and implementing early warning systems. By combining historical data, paleotsunamis, and advanced simulations, scientists can create hazard maps and evacuation plans to protect vulnerable coastal areas.

Tsunami Generation and Propagation

Mechanisms of Tsunami Generation

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  • Tsunamis are typically generated by large, sudden displacements of the seafloor due to earthquakes ( earthquakes), landslides (submarine or coastal), or volcanic eruptions (underwater explosions or caldera collapses)
  • The initial of a tsunami is determined by the amount of vertical seafloor displacement, while the wavelength is related to the size of the source area
  • Other factors influencing tsunami generation include the water depth at the source, the efficiency of energy transfer from the seafloor to the water column, and the directivity of the source mechanism

Characteristics of Tsunami Propagation

  • Tsunamis are shallow-water waves, meaning their wavelength is much larger than the water depth, allowing them to propagate at high speeds across ocean basins with minimal energy loss
  • As a tsunami approaches shallower water near the coast, wave shoaling causes the wave height to increase and the wavelength to decrease, leading to potentially destructive inundation
  • Tsunami propagation is influenced by bathymetry, with wave refraction (bending of wave fronts due to varying water depths) and diffraction (spreading of wave energy around obstacles) affecting the distribution of wave energy along the coastline
  • Resonance effects in bays and harbors can amplify tsunami wave heights, increasing the risk of damage in these areas (Crescent City, California during the 1964 Alaska tsunami)

Paleotsunamis and Historical Records for Hazard Assessment

Paleotsunami Evidence and Analysis Techniques

  • Paleotsunami deposits, such as sand layers in coastal sediments or boulder accumulations, provide evidence of past tsunami events and can help estimate recurrence intervals and maximum inundation distances
  • Techniques for identifying paleotsunami deposits include sedimentological analysis (grain size, sorting, and structure), geochemical markers (saltwater indicators or marine microfossils), and microfossil assemblages (diatoms, foraminifera)
  • Dating methods, such as radiocarbon dating or optically stimulated luminescence (OSL), are used to determine the age of paleotsunami deposits and reconstruct the timeline of past events

Integration of Historical Records and Modern Modeling

  • Historical records, such as written accounts, photographs, and tide gauge measurements, can provide valuable information on the extent and impacts of recent tsunami events (1755 Lisbon tsunami, 1960 Chile tsunami)
  • Combining paleotsunami and historical data with modern modeling techniques allows for a more comprehensive assessment of tsunami hazards and the development of probabilistic tsunami hazard maps
  • Probabilistic hazard assessment considers the likelihood and severity of tsunami events based on the available evidence, helping to inform risk management decisions and land-use planning

Numerical Modeling for Tsunami Inundation and Hazard Mapping

Numerical Models and Governing Equations

  • Numerical models simulate tsunami generation, propagation, and inundation based on physical equations and boundary conditions
  • Shallow-water equations, such as the nonlinear shallow-water equations (NLSWE) or Boussinesq equations, are commonly used in tsunami modeling due to their ability to capture essential wave dynamics while remaining computationally efficient
  • Model inputs include bathymetry, topography, and initial conditions (e.g., earthquake source parameters or landslide volume and location)

Model Outputs and Applications

  • Model outputs include wave heights, flow velocities, and inundation depths, which can be used to create hazard maps and inform and evacuation planning
  • Nested grid approaches, with increasing resolution towards the coast, are often employed to balance computational efficiency and accuracy (global, regional, and local scales)
  • Model validation using historical tsunami data and benchmarking against analytical solutions or laboratory experiments is essential for ensuring the reliability of tsunami hazard assessments
  • Hazard maps derived from can guide land-use planning, building codes, and insurance rates in tsunami-prone areas

Tsunami Early Warning Systems and Community Preparedness

Components and Effectiveness of Early Warning Systems

  • Tsunami early warning systems rely on a network of seismic and sea-level monitoring stations to detect potential tsunami-generating events and issue timely warnings to at-risk communities
  • The effectiveness of early warning systems depends on factors such as the density and distribution of monitoring stations, the speed and accuracy of data processing and analysis, and the reliability of communication channels
  • False alarms or missed events can undermine public trust in early warning systems, highlighting the importance of robust detection algorithms and clear communication protocols (2018 Sulawesi tsunami)

Community Preparedness and Mitigation Strategies

  • Community preparedness is critical for reducing the impact of tsunamis, and involves elements such as public education, evacuation planning, and the development of resilient infrastructure
  • Tsunami hazard maps and evacuation routes should be widely disseminated and regularly updated based on the latest scientific information and modeling results
  • Regular tsunami drills and exercises help to maintain public awareness and ensure the smooth implementation of evacuation procedures during an actual event (annual tsunami drills in Japan)
  • Vertical evacuation structures, such as reinforced buildings or dedicated evacuation towers, can provide safe refuge in areas with limited horizontal evacuation options
  • Post-event surveys and assessments can provide valuable insights into the effectiveness of early warning systems and community preparedness, highlighting areas for improvement and informing future mitigation strategies (2011 Tohoku tsunami)

Key Terms to Review (18)

Amplification: Amplification refers to the process by which the intensity of seismic waves or tsunami waves is increased, often due to the interaction with geographical features or human-made structures. In the context of natural disasters, such as tsunamis, amplification can significantly impact how waves behave as they approach coastlines, affecting the extent of flooding and destruction in coastal areas.
Continental shelf: The continental shelf is a submerged landmass that extends from a continent's coastline to the edge of the continental slope, typically characterized by shallow waters and rich marine resources. It plays a crucial role in coastal hazard assessments, especially during events like tsunamis, where the shelf can influence wave energy, height, and propagation.
Early warning system: An early warning system is a set of tools and processes designed to detect potential disasters, such as tsunamis, and provide timely alerts to communities at risk. This system aims to minimize loss of life and property by facilitating rapid responses and effective evacuation plans in coastal areas prone to such hazards.
Evacuation plan: An evacuation plan is a strategic outline that provides specific instructions for safely moving people from a location to a predetermined safe area in the event of an emergency. It is critical in scenarios such as natural disasters, including tsunamis, where rapid action can save lives. These plans often involve designated evacuation routes, assembly points, and communication strategies to ensure an organized response during a crisis.
Field surveys: Field surveys are systematic methods of collecting data directly from the environment, often using various geophysical techniques to analyze geological features, soil conditions, and other physical characteristics. These surveys are crucial for understanding potential hazards like tsunamis and coastal erosion, as they provide real-world insights that can inform safety measures and disaster preparedness.
Gordon Fulford: Gordon Fulford is a notable figure recognized for his contributions to the study of tsunami and coastal hazard assessment. His work has significantly advanced the understanding of how tsunamis impact coastal areas, including their physical effects on landscapes and infrastructure. Through research and modeling, Fulford has provided valuable insights into risk assessment and mitigation strategies for coastal communities vulnerable to tsunami threats.
Land use planning: Land use planning is the process of evaluating and determining the best ways to utilize land resources effectively, considering social, economic, and environmental factors. It involves making decisions on how land should be developed or preserved to balance growth with sustainability. This process is crucial for managing natural hazards and ensuring that development aligns with safety and environmental regulations.
Numerical modeling: Numerical modeling is a computational technique used to simulate physical phenomena by solving mathematical equations through numerical approximations. It allows researchers and scientists to analyze complex systems and predict behaviors by creating virtual representations, particularly important in understanding events like tsunamis and assessing coastal hazards.
Risk assessment: Risk assessment is the systematic process of identifying, evaluating, and prioritizing risks associated with potential hazards. This involves understanding the likelihood of an event occurring, the potential impacts it may have, and determining the best strategies to manage those risks. In contexts such as tsunami and coastal hazards, risk assessment plays a critical role in developing effective response plans and enhancing community resilience against natural disasters.
Subduction Zone: A subduction zone is a region of the Earth's crust where one tectonic plate moves under another and sinks into the mantle. This process leads to various geological phenomena, including the formation of deep ocean trenches, volcanic arcs, and intense seismic activity. The interactions at subduction zones are crucial for understanding gravity anomalies, tsunami generation, and the mechanics of mantle convection related to plate tectonics.
Submarine landslide: A submarine landslide is a geological event where a large mass of sediment and rock collapses and moves down the continental slope into the ocean. These slides can occur due to various triggers like earthquakes, volcanic activity, or human activities and can result in significant displacement of water, potentially leading to tsunamis. Understanding submarine landslides is crucial for assessing coastal hazards and the risks they pose to nearby communities.
Submergence: Submergence refers to the process in which land is gradually submerged below sea level due to various geological and environmental factors. This phenomenon is often a result of tectonic activity, rising sea levels, or the impact of natural disasters like tsunamis, which can lead to significant coastal hazards.
Volcanic tsunami: A volcanic tsunami is a type of tsunami generated by volcanic activity, such as an explosive eruption, landslide, or the collapse of a volcanic island. These tsunamis can occur suddenly and with little warning, posing significant risks to coastal communities located near active volcanoes. Understanding volcanic tsunamis is crucial for hazard assessment and developing effective warning systems to mitigate their impacts.
Vulnerability analysis: Vulnerability analysis is the process of identifying, quantifying, and prioritizing the vulnerabilities within a system or community, particularly in relation to natural hazards. This approach helps in understanding how susceptible an area is to threats, allowing for better preparedness and response strategies. By assessing both physical and socio-economic factors, vulnerability analysis informs risk management practices aimed at reducing the impact of disasters like tsunamis and volcanic eruptions.
Walter Munk: Walter Munk was a renowned oceanographer and geophysicist, known for his groundbreaking work on ocean waves and their interactions with the Earth's crust. He made significant contributions to the understanding of tsunami behavior, which has crucial implications for coastal hazard assessments. His research laid the foundation for predicting the impact of tsunamis on coastal communities, influencing disaster preparedness and response strategies globally.
Wave height: Wave height is defined as the vertical distance between the crest of a wave and the trough, essentially measuring the size or intensity of the wave. In the context of coastal hazards, wave height plays a critical role in assessing potential impacts on shorelines and human structures, particularly during extreme weather events or tsunamis. Understanding wave height is vital for predicting flooding, erosion, and damage to coastal ecosystems and communities.
Wave period: Wave period is the time it takes for one complete wave cycle to pass a fixed point, usually measured in seconds. This concept is crucial in understanding the behavior of waves, including tsunamis, as it relates to wave speed, wavelength, and the energy carried by the wave. A longer wave period often indicates a more powerful wave, which is essential when assessing potential coastal hazards.
Zoning Regulations: Zoning regulations are laws and rules set by local governments that dictate how land can be used within specific areas. These regulations help in organizing urban development, protecting public health, safety, and welfare, and mitigating risks associated with natural disasters, including tsunamis and coastal hazards.
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