Tsunamis are series of long wavelength water waves generated by the rapid displacement of a large volume of water. The most common cause is sudden vertical movement of the seafloor during a large undersea earthquake, particularly at a subduction zone, but Tsunamis can also result from submarine landslides, volcanic eruptions, volcanic flank collapses, meteorite impacts and other rapid disturbances.
They differ fundamentally from ordinary wind generated waves because their energy extends through a much greater depth of the water column and their wavelengths can reach hundreds of kilometres. In deep water, a tsunami can travel at very high speed while maintaining a relatively small wave height, making it difficult for ships in the open ocean to notice. As the waves enter shallower coastal water, they slow down and their energy is compressed into a smaller horizontal distance, often producing substantial increases in wave height and strong currents.
The generation of Tsunamis depends primarily on the speed, area and magnitude of the initial water displacement. A large earthquake does not automatically generate a major tsunami because the earthquake must produce suitable seafloor deformation. Shallow earthquakes beneath or near the ocean are particularly important when they cause substantial vertical movement of the seabed. Subduction zones are especially significant because one tectonic plate descends beneath another and the plate boundary can store elastic strain over long periods.
During a large earthquake, the seafloor may suddenly rise or fall, displacing the overlying water. The resulting gravitational adjustment creates waves that propagate outward from the disturbed region. Landslide generated tsunamis can behave differently because a large mass of sediment or rock moves rapidly into or beneath the water. Volcanic processes can also generate tsunamis through explosions, caldera collapse, flank failure or pyroclastic material entering water.
Tsunami propagation is governed by shallow water wave physics even when the ocean is deep relative to many ordinary coastal waves. In deep water, the wave speed depends strongly on water depth, approximately following the square root relationship between gravitational acceleration and depth. Because deep ocean depth is large, a tsunami can travel at several hundred kilometres per hour. Its wavelength can be extremely long, so the wave passes through the water column rather than behaving like a surface ripple.
As the wave approaches a continental shelf, decreasing depth reduces propagation speed. Conservation of energy and changes in wave geometry can cause the wave height to increase. Refraction and diffraction around islands and coastal features can alter the distribution of energy. Local bathymetry therefore plays a major role in determining which coastal areas experience the greatest inundation and currents.
A Tsunami approaching a coastline can produce several different forms of hazardous behaviour. The first significant disturbance may be a rise in sea level, or in some cases a rapid withdrawal of water that exposes the seabed. A sudden withdrawal is a recognised natural warning sign, but it does not occur before every tsunami and people should not wait for it before moving to safety if an official warning or strong nearby earthquake indicates risk.
The incoming wave may arrive as a turbulent surge rather than a conventional curling breaker. Multiple waves can occur, with later waves sometimes being larger than the first. Strong currents can continue for hours after the initial arrival, making apparently calm conditions deceptive. Inundation can penetrate far inland where coastal topography is low and flat, while narrow bays, river mouths and harbours can amplify currents or alter wave behaviour.
Tsunami hazards are strongly influenced by coastal geography and human exposure. Low elevation coastal plains, deltas and densely developed shorelines can be particularly vulnerable to inundation. Buildings, roads, bridges, ports, electrical infrastructure and freshwater supplies can be damaged by high water, debris impact and strong currents. The retreating water can carry people, vehicles and structures back towards the sea, while floating debris becomes a major source of secondary injury. Saltwater contamination can damage agricultural land and drinking water resources.
Landslides triggered by earthquakes can create additional local hazards. Tsunami risk therefore depends not only on the magnitude of the generating event but also on bathymetry, topography, distance from the source, warning time, building design, evacuation capacity and population awareness. A distant tsunami may allow hours for warning and evacuation, whereas a locally generated tsunami can reach nearby shores within minutes.
Tsunami warning systems combine seismology, ocean observations, numerical modelling and communication infrastructure. Seismic networks detect earthquakes and estimate their location, depth and magnitude. Ocean pressure sensors and tide gauges can detect changes in sea level and provide direct confirmation of tsunami waves. Deep ocean monitoring systems can detect small pressure changes associated with long wavelength waves before they become large near coastlines. Forecast models use earthquake information, ocean bathymetry and observations to estimate arrival times and potential coastal effects.
Warning centres communicate alerts through official channels, while local authorities may activate sirens and evacuation procedures. Technology cannot eliminate uncertainty, particularly immediately after an earthquake when source parameters are still being refined. For this reason, public education and the ability to respond quickly to strong natural warning signs remain important parts of tsunami preparedness.
Historical and geological records provide evidence of Tsunamis that occurred before modern instruments existed. Sediment layers containing marine sand, shells or chemical signatures can sometimes reveal ancient inundation events inland from the present shoreline. Coral records, historical documents and changes in coastal landscapes can provide additional evidence. Paleotsunami research helps scientists estimate recurrence patterns and identify regions where destructive tsunamis may have occurred repeatedly.
However, recurrence intervals are not perfectly regular, and the absence of a preserved deposit does not prove that no tsunami occurred. Historical observations also vary in quality. Combining geological evidence with earthquake histories and modern modelling allows researchers to develop more realistic hazard assessments. This is particularly important in regions where the instrumental record covers only a small fraction of the time over which large tectonic earthquakes occur.
Tsunami preparedness centres on rapid movement to safer ground or higher elevation after a credible warning or strong local natural warning. In areas close to active subduction zones, people may have only minutes to act after a major earthquake, so evacuation routes must be understood in advance. Vertical evacuation structures can provide additional protection where high ground is unavailable. Coastal planning can use inundation maps to guide land use, evacuation zones and infrastructure design.
Buildings can be designed with consideration of wave forces and debris, although no structure should be treated as universally tsunami proof. Public drills and clear communication can improve response. Tsunamis demonstrate the importance of understanding hazard chains because the initial earthquake may be only the first stage, followed by ocean displacement, wave propagation, inundation, debris movement, infrastructure failure and environmental contamination.
