Glossary

bore phenomena

A steep, turbulent, rapidly moving wave front that typically occurs in river mouths or estuaries; bottom friction slows the advancing wave front, and water piles up behind to produce a nearly vertical bore face.

deep-ocean assessment and reporting of tsunamis (DART)

An instrument for the early detection, measurement, and real-time reporting of tsunamis in the open ocean. Developed by the US NOAA Pacific Marine Environmental laboratory, the DART system consists of a seafloor bottom pressure recording system capable of detecting tsunamis, and a moored surface buoy for real-time communications. An acoustic link transmits data from the seafloor to the surface buoy. The data are then relayed via a satellite link to ground stations. The DART data, along with numerical modelling technology, are part of a tsunami forecasting system that provides site-specific predictions of tsunami impact on the coast.

epicenter

The point on the earth’s surface directly above the location in the earth (focus) where an earthquake originates.

evacuation map

A drawing or representation that outlines danger zones and designates limits beyond which people must be evacuated to avoid harm from tsunami waves.

inundation

The maximum horizontal distance inland that a tsunami penetrates.

locally generated tsunami

A tsunami that originates close to home, ie., from a nearby source for which its destructive effects occur to coasts within about 100 km (or, alternatively, less than 1 hour tsunami travel time).

mean sea level

The average height of the sea-surface.

Pacific-wide tsunami

A tsunami striking coastal areas of the Hawaiian Islands, generated by displacement of the ocean floor along the margins of the Pacific (e.g., Japan, Kamchatka, Aleutians, Chile).

paleotsunami

Tsunami occurring prior to the historical record or for which there are no written observations. Paleotsunami research is based primarily on the identification, mapping, and dating of tsunami deposits found in coastal areas, and their correlation with similar sediments found elsewhere locally, regionally, or across ocean basins.

plate tectonics

The theory that the earth’s surface is divided into a number of rigid plates that are either converging, diverging, or slipping past one another; these motions generate volcanism and earthquakes along the plate boundaries.

post-tsunami survey

Tsunamis are relatively rare events and most their evidence is perishable. Therefore, it is very important that reconnaissance surveys be organized and carried out quickly and thoroughly after each tsunami occurs, to collect detailed data valuable for hazard assessment, model validation, and other aspects of tsunami mitigation. This data includes measurements of runups and inundation limits and collection of associated data from eyewitnesses such as the number of waves, arrival time of waves, and which wave was the largest.

recession

Drawdown of sea level prior to tsunami flooding. The shoreline moves seaward, sometimes by a kilometer or more, exposing the sea bottom, rocks and fish. The recession of the sea is a natural warning sign that a tsunami is approaching.

rogue waves

Unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local area; many theorize that a rogue wave is the coincidence of several wave trains, the crest of one train superimposed and amplifying others.

runup

The maximum vertical height of the sea surface reached during a tsunami with reference to mean sea-level; runup is also referred to as the amplitude or height of the tsunami.

seiche

The back and forth oscillation of a standing wave; typically occurring in enclosed or partially enclosed bodies of water, the oscillations continue pendulum fashion after the generating force ceases.

seismographs

Instruments used for measuring earthquake size and the position on the earth’s surface over which they occur.

shallow water wave

A wave occurring in water with a depth less than one-twentieth of the average wavelength.

subduction zone

An area of convergence between a sinking plate and an overriding plate.

subsidence (uplift)

The permanent movement of land down (subsidence) or up (uplift) due to geologic processes, such as during an earthquake.

teletsunami or distant tsunami

A tsunami originating from a distant source, generally more than 1,000 km or more than three hours tsunami travel time from its source.

travel time

The time it takes the tsunami to travel from the source to an area where it has an effect.

tsunami (seismic sea wave)

A series of ocean waves characterized by having long periods and wavelengths that can travel with speeds greater than 500 miles per hour. Tsunami waves can be generated by sudden displacements in the sea floor, landslides, or volcanic activity. In the deep ocean their height may only be a few inches, but as they encounter shallow water their heights increase drastically as the wavelengths shorten and bunch up, resulting in a sudden increase in sea level, thereby flooding low-lying coastal areas.

tsunami generation

Tsunamis are most frequently caused by earthquakes, but can also result from landslides, volcanic eruption, submarine seamount collapse, or infrequently by meteorite impact. Tsunamis are generated primarily by tectonic dislocations under the sea that are caused by shallow focus earthquakes along areas of subduction. The upthrusted and downthrusted crustal blocks impart potential energy into the overlying water mass, resullting in energy radiation away from the source region in the form of long-period tsunami waves.

tsunami hazard assessment

Documentation of tsunami hazards for a coastal community is needed to identify populations and assets at risk. This assessment requires knowledge of probable tsunami sources (such as earthquakes, landslides, and volcanic eruptions), their likelihood of occurrence, and the characteristics of tsunamis from those sources at different places along the coast. For those communities, data of earlier tsunamis may help quantify these factors. Historical data is often very limited. Consequently, numerical modelling of tsunami inundation can also provide estimates of areas that will be flooded. Numerical models are mathematical descriptions that seek to describe tsunami phenomena. Numerical models have been used in recent years to simulate tsunami propagation and interaction with land masses.

tsunami preparedness

Readiness of plans, methods, procedures and actions taken by government officicials and the general public for the purpose of minimizing potential risk and mitigating the effects of future tsunamis.

tsunami propagation

Tsunamis travel outward in all directions from the generating area, with the direction of the main energy propagation generally being orthogonal to the direction of the earthquake farcture zone. Their speed depends on the depth of water, so that the waves undergo accelerations and decelerations in passing over an ocean bottom of varying depth. Variations in tsunami propagation result when the propagation impulse is stronger in one direction than in others because of the orientation or dimensions of the generating area and where regional bathymetric and topographic featues modify both the waveform and rate of advance. Tsunamis are unique in that the energy extends through the entire water column from sea surface to the ocean bottom. It is this characteristic that accounts for the great amount of energy propagated by a tsunami.

wave crest

The highest part of a wave.

wave period

The time required for two successive wave crests or troughs to pass a point in space.

wave trough

The lowest part of a wave.

wavelength

The horizontal distance between two successive wave crests or troughs.

Some definitions excerpted from UNESCO IOC “Tsunami Glossary”