What Is a Tsunami?

A tsunami is a series of ocean waves generated by a large, rapid displacement of water, typically caused by a submarine earthquake, volcanic eruption, underwater landslide, or, rarely, a meteorite impact. Unlike wind-driven surface waves, tsunami waves involve the entire water column from the seafloor to the surface, giving them enormous energy and propagation speeds that can exceed 800 kilometers per hour in the open ocean. The term comes from the Japanese words for harbor ("tsu") and wave ("nami"), reflecting the long history of these events in the Pacific basin. The study and engineering response to tsunamis draws from seismology, physical oceanography, coastal engineering, and signal processing.

Wave Generation and Propagation

Tsunamis originate when a geological event displaces a large volume of water vertically. Subduction zone earthquakes, in which one tectonic plate slides beneath another, account for the majority of historically destructive events; the 2004 Indian Ocean tsunami, generated by a magnitude 9.1 earthquake off northern Sumatra, demonstrated the global reach of these phenomena. In the open ocean, a tsunami's wave height is typically less than a meter, but its wavelength can span hundreds of kilometers, making it virtually undetectable from a ship at sea. As the wave train approaches shallow coastal water, the leading edge slows while the following water continues to pile up, a process called shoaling, which can amplify wave height by a factor of ten or more. Research on wave dynamics and historical events is documented through the NOAA National Centers for Environmental Information's global historical tsunami database, which catalogs events back several centuries.

Detection and Warning Systems

Early detection depends on two complementary sensor networks. Seismic networks detect and characterize candidate earthquakes within minutes, providing an initial assessment of whether a tsunami-generating event has occurred; the speed of seismic waves, roughly one hundred times faster than the tsunami itself, provides a critical lead time. Ocean-based sensors then confirm whether a wave has actually been generated. NOAA's Deep-ocean Assessment and Reporting of Tsunamis (DART) system, described on the NOAA Pacific Marine Environmental Laboratory DART page, uses pressure sensors anchored to the seafloor and surface buoys to detect and transmit real-time water column height data. When a DART buoy records an anomalous signature, that data feeds directly into forecast models at NOAA's two Tsunami Warning Centers, which issue watches, advisories, and warnings to coastal communities with specific inundation estimates.

Coastal Impact and Inundation Modeling

The destruction caused by a tsunami depends on the bathymetry and topography of the receiving coastline, the orientation of the wave train relative to coastal geometry, and the presence or absence of natural or engineered barriers. Numerical models, most using nonlinear shallow-water equations solved over high-resolution digital elevation models, simulate the inundation of coastal land to produce hazard maps used in evacuation planning and building codes. NOAA's MOST (Method of Splitting Tsunamis) model and similar codes are validated against tide gauge and field survey data from historical events. The NOAA Tsunami Detection and Forecasting program describes the integration of seismic data, DART measurements, and inundation modeling into an operational forecasting chain that targets specific at-risk communities along coastlines.

Applications

Tsunami science and engineering have applications in a wide range of fields, including:

• Coastal civil engineering and land-use planning, where inundation maps set setback requirements for buildings and infrastructure
• Emergency management, where modeled arrival times and run-up heights drive evacuation route design
• Marine geology and seismology, where paleotsunami sediment deposits extend the record of prehistoric events
• Underwater sensor networks, where DART buoy designs inform broader ocean monitoring architectures
• Satellite remote sensing, where altimetry instruments have successfully detected open-ocean tsunami signatures