Tsunami Glossary

Arrival time: Time the first the tsunami waves arrive.

Bathymetry: The measurement of water depths in oceans, seas, and lakes; also information derived from such measurements.

Bathymetric Chart: A topographic map of the bed of the ocean, with depths indicated by contours (isobaths) drawn at regular intervals.

Beach: An accumulation of loose sediment (usually sand or gravel) along the coast.

Beach face: The section of the beach normally exposed to the action of the wave uprush. Also referred to as the foreshore of a beach.

Beach Loss: A loss of sand from the active beach which is calculated based on volume.

Beach Narrowing: A decrease in the useable beach width caused by erosion.

Beach Width: The horizontal dimension of the beach measured normal to the shoreline and landward of the high water mark line on coasts.

Breakwater: A man-made structure protecting a shore area, harbor, anchorage, or basin from waves.

Coast: A strip of land of indefinite width (may be several kilometers) that extends from the shoreline inland to the first major change in terrain features. The part of a country regarded as near the coast.

Coastline: The line that forms the boundary between the coast and the shore. Commonly referred to as the line that forms the boundary between the land and the water (especially the water of a sea or ocean, also called the shoreline).

Coastal Area: The land and sea area bordering the shoreline. An entity of land and water affected by the biological and physical processes of both the sea and land and defined broadly for the purpose of managing the use of natural resources.

Coastal Defense: General term used to encompass both coast protection against erosion and sea defense against flooding.

Coastal Erosion: The wearing away of coastal lands, usually by wave attack, tidal or littoral currents, or wind. Coastal erosion is synonymous with shoreline (vegetation line) retreat.

Coastal Strip: A zone directly adjacent to the waterline, where only coast related activities take place. Usually this is a strip of some 100 m wide. In this strip, coastal defense activities take place. In this strip often there may be restrictions to land use.

Coastal Zone: The transition zone where the land meets water; the region that is directly influenced by marine hydrodynamic processes. Extends offshore to the continental shelf break and onshore to the first major change in topography above the reach of major storm waves.

Coastal Zone Management: The integrated and general development of the coastal zone. Coastal Zone Management is not restricted to coastal defense works, but includes also coastal development in economical, ecological and social terms.

Crest Length: The length of a wave along its crest. Sometimes called crest width.

Deep Ocean Assessment and Reporting of Tsunamis (DARTS): An instrument for the early detection, measurement, and real-time reporting of tsunamis in the open ocean.

Elapsed Time: Time between the maximum level arrival time and the arrival time of the first wave.

Estimated Time of Arrival (ETA): Time of tsunami arrival at some fixed location, as estimated from modeling the speed and refraction of the tsunami waves as they travel from the source. ETA is estimated with very good precision if the bathymetry and source are well known (less than a couple of minutes).

High Water Mark: A reference mark on a structure or natural object, indicating the maximum stage of tide or flood.

Historical Tsunami: A tsunami identified as having occurred through eyewitness or instrumental observation within the historical record.

Initial Rise: Time of the first minimum of the tsunami waves.

Inundation: The horizontal distance inland that a tsunami penetrates, generally measured perpendicularly to the shoreline.

Inundation (Maximum): Maximum horizontal penetration of the tsunami from the shoreline. A maximum inundation is measured for each different coast or harbour affected by the tsunami.

Inundation Area: Area flooded with water by the tsunami.

Inundation Line: Inland limit of wetting, measured horizontally from the mean sea level (MSL) line. The line between living and dead vegetation is sometimes used as a reference. In tsunami science, the landward limit of tsunami run up.

Leading Wave: First arriving wave of a tsunami. In some cases, the leading wave produces an initial depression or drop in sea level, and in other cases, an elevation or rise in sea level. When a drop in sea level occurs, sea level recession is observed.

Local Tsunami: A tsunami from a nearby source for which its destructive effects are confined to coasts within 100 km of the source (or, alternatively, less than 1 hour travel tsunami travel time). A local tsunami is usually generated by an earthquake, but can also be caused by a landslide or a pyroclastic flow from a volcanic eruption.

Mean Height: Average height of a tsunami measured from the trough to the crest after removing the tidal variation.

Mean High Water (MHW): The average height of the high water over a 19-year period. For shorter periods of observations, corrections are applied to eliminate known variations and reduce the results to the equivalent of a mean 19-year value. All high water heights are included in the average where the type of tide is either semi-diurnal or mixed.

Mean Low Water (MLW): The average height of the low water over a 19-year period. For shorter periods of observations, corrections are applied to eliminate known variations and reduce the results to the equivalent of a mean 19-year value. All low water heights are included in the average where the type of tide is either semi-diurnal or mixed.

Mean Sea Level (MSL): The average height of the surface of the sea for all stages of the tide over a 19-year period, usually determined from hourly height readings. Not necessarily equal to mean tide level. It is also the average water level that would exist in the absence of tides.

Mean Tide Level: A plane midway between mean high water and mean low water. Not necessarily equal to mean sea level. Also referred to as half-tide level.

Ocean Wide Tsunami: A tsunami capable of widespread destruction, not only in the immediate region of its generation but across an entire ocean. All ocean-wide tsunamis have been generated by major earthquakes. Synonym for tele-tsunami or distant tsunami.

Overflow: A flowing over; inundation.

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. As work in this field continues it may provide important new information about past tsunamis to aid in the assessment of the tsunami hazard.

Period: The period is the time interval required for one full cycle of a wave.

Post-tsunami survey: Tsunamis are relatively rare events and most of their evidence is phemeral. 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.

Recession: Drop in 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.

Regional Tsunami: A tsunami capable of destruction in a particular geographic region, generally within about 1,000 km of its source. Regional tsunamis also occasionally have very limited and localized effects outside the region. Most of the destructive tsunamis can be classified as local or regional, meaning their destructive effects are confined to coasts within a hundred km, or up to a thousand km, respectively, of the source -- usually an earthquake. It follows that many tsunami related casualties and considerable property damage also comes from such tsunami.

Rise: The upward change or elevation in sea level associated with a tsunami, a tropical cyclone, storm surge, the tide, or other long term climatic effect.

Run up: 1) Difference between the elevation of maximum tsunami penetration (inundation line) and the sea-level at the time of the tsunami. 2) Elevation reached by seawater measured relative to some stated datum such as mean sea level, mean low water, sea level at the time of the tsunami arrival, etc., and measured ideally at a point that is a local maximum of the horizontal inundation.

Run up distribution: Set of tsunami run-up values measured or observed along a coastline.

Sea: A large body of salt water, second in rank to an ocean, more or less landlocked and generally part of, or connected with, an ocean or a larger sea. Examples: Mediterranean Sea; South China Sea and Coral Sea.

Seiche: A seiche is the term used to describe oscillations in a partially or fully enclosed body of water. It may be initiated by long period seismic waves (an earthquake), wind and water waves, or a tsunami.

Seismic sea waves: Tsunamis are sometime referred to as seismic sea waves because they are most often generated by earthquakes.

Shore: The narrow strip of land in immediate contact with the sea, including the zone between high and low water lines. A shore of unconsolidated material is usually called a beach. Also used in a general sense to mean the coastal area (e.g. to live at the shore). Also sometimes known as the littoral.

Shoreline: The intersection of a specified plane of water with the shore or beach (e.g. the high water shoreline would be the intersection of the plane of mean high water with the shore or beach). The line delineating the shoreline on nautical charts and surveys approximates the mean high water line.

Significant Wave Height (tsunami): The average height of the one-third highest waves of a given wave group. Note that the composition of the highest waves depends upon the extent to which the lower waves are considered. In wave record analysis, the average height of the highest one-third of a selected number of waves, this number being determined by dividing the time of record by the significant period. Also called characteristic wave height.

Tele-tsunami or Distant tsunami: A tsunami originating from a far away source, generally more than 1,000 km away. Less frequent, but more hazardous than regional tsunamis, are ocean-wide or distant tsunamis. Usually starting as a local tsunami that causes extensive destruction near the source, these waves continue to travel across an entire ocean basin with sufficient energy to cause additional casualties and destruction on shores more than a 1,000 km from the source. In the last 200 years, there have been at least 21 destructive ocean-wide tsunamis. The worst tsunami catastrophe in history occurred in the Indian Ocean on the 26th December, 2004, when a magnitude 9.1 earthquake off of the northwest coast of Sumatra, Indonesia produced a ocean-wide tsunami that also hit Thailand and Malaysia to the east, and Sri Lanka, India, the Maldives, and Africa to the west as it traversed across the Indian Ocean. Over 225,000 people lost their lives, and more than 1 million people were displaced, losing their homes, property, and their livelihoods. The magnitude of death and destructiveness caused immediate response by the world's leaders and led to the development of the Indian Ocean tsunami warning and mitigation system in 2005.

Travel Time: Time required for the first tsunami wave to propagate from its source to a given point on a coastline.

Tsunami: Japanese term meaning wave (“nami”) in a harbour (“tsu”). A series of traveling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below or near the ocean floor. (Also called seismic sea wave and, incorrectly, tidal wave). Volcanic eruptions, submarine landslides, and coastal rockfalls can also generate tsunamis, as can a large meteorite impacting the ocean. These waves may reach enormous dimensions and travel across entire ocean basins with little loss of energy. They proceed as ordinary gravity waves with a typical period between 10 and 60 minutes. Tsunamis steepen and increase in height on approaching shallow water, inundating low-lying areas, and where local submarine topography causes the waves to steepen, they may break and cause great damage. Tsunamis have no connection with tides; the popular name, tidal wave, is entirely misleading.

Tsunameter: An instrument for the early detection, measurement, and real-time reporting of tsunamis in the open ocean. Also known as a tsunamimeter. The DART system and cable deep-ocean pressure sensor are tsunameters.

Tsunami Impact: Although infrequent, tsunamis are among the most terrifying and complex physical phenomena and have been responsible for great loss of life and extensive destruction to property. Because of their destructive potential, tsunamis have important impacts on the human, social and economic sectors of societies. Historical records show that enormous destruction of coastal communities throughout the world has taken place and that the socio-economic impact of tsunamis in the past has been enormous.

Tsunami Hazard Assessment: Documentation of tsunami hazards for a coastal community is needed to identify populations and assets at risk, and the level of that 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 (historical and paleotsunamis) tsunamis may help quantify these factors. For most communities, however, only very limited or no past data exist. For these coasts, numerical models of tsunami inundation can provide estimates of areas that will be flooded in the event of a local or distant tsunamigenic earthquake or a local landslide.

Tsunami Hazard: The probability that a tsunami of a particular size will strike a particular section of coast.

Tsunami Generation: Tsunamis are most frequently caused by earthquakes, but can also result from landslides, volcanic eruptions, and very infrequently by meteorites or other impacts upon the ocean surface. Tsunamis are generated primarily by tectonic dislocations under the sea which are caused by shallow focus earthquakes along areas of subduction. The upthrusted and downthrusted crustal blocks impart potential energy into the overlying water mass with drastic changes in the sea level over the affected region. The energy imparted into the water mass results in tsunami generation, i.e. energy radiating away from the source region in the form of long period waves.

Tsunami Bore: A steep, turbulent, rapidly moving tsunami wave front, typically occurring in a river mouth or estuary.

Tsunami Amplitude: Usually measured on a sea level record, it is:1) The absolute value of the difference between a particular peak or trough of the tsunami and the undisturbed sea level at the time, 2) Half the difference between an adjacent peak and trough, corrected for the change of tide between that peak and trough. It is intended to represent the true amplitude of the tsunami wave at some point in the ocean. However, it is often amplitude modified in some way by the tide gauge response.

Water Level (Maximum): The difference in elevation between the highest local water mark and that of the sea-level at the time of the tsunami. This is different from maximum run-up because the water mark is often not observed at the inundation line, but maybe halfway up the side of a building or on a tree trunk.

Wave Height: The vertical distance between a crest and the preceding trough.

Wavelength: The horizontal distance between similar points on two successive waves measured perpendicularly to the crest.

Wave Period (Period): The time for a wave crest to traverse a distance equal to one wavelength. The time for two successive wave crests to pass a fixed point.

Wave Trough (Trough): The lowest part of a wave.