General Info



1. Tsunami Classification

1 April 1946 Aleutian Islands tsunami hitting Hilo, Hawaii. Photo courtesy of Bishop Museum Archives.
1 April 1946 Aleutian Islands tsunami hitting Hilo, Hawaii.
Photo courtesy of Bishop Museum Archives.

 

Characteristics of the Tsunami Phenomena

A tsunami travels outward from the source region as a series of waves. Its speed depends upon the depth of the water, and consequently the waves undergo accelerations or decelerations in passing respectively over an ocean bottom of increasing or decreasing depth. By this process the direction of wave propagation also changes, and the wave energy can become focused or defocused. In the deep ocean, tsunami waves can travel at speeds of 500 to 1,000 kilometres per hour. Near the shore, however, a tsunami slows down to just a few tens of kilometres per hour. The height of a tsunami also depends upon the water depth. A tsunami that is just a metre in height in the deep ocean can grow to tens of metres at the shoreline. Unlike familiar wind-driven ocean waves that are only a disturbance of the sea surface, the tsunami wave energy extends to the ocean bottom. Near the shore, this energy is concentrated in the vertical direction by the reduction in water depth, and in the horizontal direction by a shortening of the wavelength due to the wave slowing down. Tsunamis have periods (the time for a single wave cycle) that may range from just a few minutes to as much as an hour or exceptionally more. At the shore, a tsunami can have a wide variety of expressions depending on the size and period of the waves, the near-shore bathymetry and shape of the coastline, the state of the tide, and other factors. In some cases a tsunami may only induce a relatively benign flooding of low-lying coastal areas, coming onshore similar to a rapidly rising tide. In other cases it can come onshore as a bore - a vertical wall of turbulent water full of debris that can be very destructive. In most cases there is also a drawdown of sea level preceding crests of the tsunami waves that result in a receding of the waterline, sometimes by a kilometre or more. Strong and unusual ocean currents may also accompany even small tsunamis. Damage and destruction from tsunamis is the direct result of three factors: inundation, wave impact on structures, and erosion. Deaths occur by drowning and physical impact or other trauma when people are caught in the turbulent, debris-laden tsunami waves. Strong tsunami-induced currents have led to the erosion of foundations and the collapse of bridges and seawalls. Floatation and drag forces have moved houses and overturned railroad cars. Tsunami associated wave forces have demolished frame buildings and other structures. Considerable damage also is caused by floating debris, including boats, cars, and trees that become dangerous projectiles that may crash into buildings, piers, and other vehicles. Ships and port facilities have been damaged by surge action caused by even weak tsunamis. Fires resulting from oil spills or combustion from damaged ships in port, or from ruptured coastal oil storage and refinery facilities, can cause damage greater than that inflicted directly by the tsunami. Other secondary damage can result from sewage and chemical pollution following the destruction. Damage of intake, discharge, and storage facilities also can present dangerous problems. Of increasing concern is the potential effect of tsunami drawdown, when receding waters uncover cooling water intakes associated with nuclear power plants.

Historical tsunami
A tsunami documented to occur through eyewitness or instrumental observation within the historical record.

Local tsunami
A tsunami from a nearby source for which its destructive effects are confined to coasts within about 100 km, or less than 1 hour tsunami travel time from its source. 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. Over history, 90% of tsunami casualties have been caused by local tsunamis.

Tsunami flow depths exceeding 10 m and flow velocities over 6 m/s overturned and dragged 3-story buildings as much as 50 m during the 11 March 2011 Japan tsunami. Onagawa, Japan.  Photo courtesy of ITIC.

Tsunami flow depths exceeding 10 m and flow velocities over 6 m/s overturned and dragged 3-story buildings as much as 50 m during the 11 March 2011 Japan tsunami. Onagawa, Japan.  Photo courtesy of ITIC.

Maremoto
Spanish term for tsunami.

Damage caused by the 22 May 1960 Chilean tsunami.  Photo courtesy of Ilustre Municipalidad de Maullin, USGS Circular 1187.

Damage caused by the 22 May 1960 Chilean tsunami. Photo courtesy of Ilustre Municipalidad de Maullin, USGS Circular 1187.

Meteorological tsunami (meteotsunami)
Tsunami-like phenomena generated by meteorological or atmospheric disturbances. These waves can be produced by atmospheric gravity waves, pressure jumps, frontal passages, squalls, gales, typhoons, hurricanes and other atmospheric sources. Meteotsunamis have the same temporal and spatial scales as tsunami waves and can similarly devastate coastal areas, especially in bays and inlets with strong amplification and well-defined resonant properties (e.g. Ciutadella Inlet, Baleric Islands; Nagasaki Bay, Japan; Longkou Harbour, China; Vela Luka, Stari Grad and Mali Ston Bays, Croatia). Sometimes referred to as rissaga.

Microtsunami
A tsunami of such small amplitude that it must be observed instrumentally and is not easily detected visually.

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 teletsunami or distant tsunami.

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. In one instance, the research has led to a new concern for the possible future occurrence of great earthquakes and tsunamis along the northwest coast of North America. In another instance, the record of tsunamis in the Kuril-Kamchatka region is being extended much further back in time. As work in this field continues it may provide a significant amount of new information about past tsunamis to aid in the assessment of the tsunami hazard. 

Regional tsunami

A tsunami capable of destruction in a particular geographic region, generally within 1,000 km or 1-3 hours tsunami travel time from its source. Regional tsunamis also occasionally have very limited and localized effects outside the region. Most destructive tsunami can be classified as local or regional. It follows many tsunami related casualties and considerable property damage also comes from these tsunamis. Between 1975 and mid-2012 there were 39 local or regional tsunamis that resulted in 260,000 deaths and billions of dollars in property damage; 26 of these were in the Pacific and adjacent seas. For example, in the Pacific a regional tsunami in 1983 in the Sea of Japan or East Sea, severely damaged coastal areas of Japan, Korea, and Russia, causing more than $800 million in damage, and more than 100 deaths. Then, after nine years with only one event causing one fatality, 10 locally destructive tsunamis occurred in just a seven-year period from 1992 to 1998, resulting in over 2,700 deaths and hundreds of millions of dollars in property damage. In most of these cases, tsunami mitigation efforts in place at the time were unable to prevent significant damage and loss of life. However, losses from future local or regional tsunamis can be reduced if a denser network of warning centres, seismic and water-level reporting stations, and better communications are established to provide a timely warning, and if better programs of tsunami preparedness and education can be put in place.

tsu source pac2012 ngdc wds web


The four images above show confirmed tsunami source locations in the Pacific Ocean, Indian Ocean, Mediterranean Sea, and Caribbean Sea. The symbols indicate cause of the tsunami: Brown Square is a landslide, Red Triangle is a volcanic eruption, Question Mark is an unknown cause, and White Circle is an earthquake and the size of the circle is graduated to indicate the earthquake magnitude. Source: NGDC/WDS-Geophysics.


The four images above show confirmed tsunami source locations in the Pacific Ocean, Indian Ocean, Mediterranean Sea, and Caribbean Sea. The symbols indicate cause of the tsunami: Brown Square is a landslide, Red Triangle is a volcanic eruption, Question Mark is an unknown cause, and White Circle is an earthquake and the size of the circle is graduated to indicate the earthquake magnitude. Source: NGDC/WDS-Geophysics.


The four images above show confirmed tsunami source locations in the Pacific Ocean, Indian Ocean, Mediterranean Sea, and Caribbean Sea. The symbols indicate cause of the tsunami: Brown Square is a landslide, Red Triangle is a volcanic eruption, Question Mark is an unknown cause, and White Circle is an earthquake and the size of the circle is graduated to indicate the earthquake magnitude. Source: NGDC/WDS-Geophysics. 

The four images above show confirmed tsunami source locations in the Pacific Ocean, Indian Ocean, Mediterranean Sea, and Caribbean Sea. The symbols indicate cause of the tsunami: Brown Square is a landslide, Red Triangle is a volcanic eruption, Question Mark is an unknown cause, and White Circle is an earthquake and the size of the circle is graduated to indicate the earthquake magnitude. Source: NGDC/WDS-Geophysics.

 

Regional and Local Tsunamis causing 2,000 or more deaths 


Regional and Local Tsunamis causing deaths since 1975 


Tsunamigenic earthquakes


Earthquakes, volcanic eruptions and landslides.


Teletsunamis.

More than 80% of the world's tsunamis were caused by earthquakes and over 70% of these were observed in the Pacific where large earthquakes occur as tectonic plates are subducted along the Pacific Ring of Fire. Top: Epicentre of all tsunamigenic earthquakes tsunamis have caused damage locally in all ocean basins. Middle: Locations of earthquakes, volcanic eruptions, and landslides generating tsunamis that caused damage or casualties locally. Although the majority of tsunamis that were observed more than 1,000 km away (teletsunamis) were generated by earthquakes in the Pacific, teletsunamis have also caused damage and casualties in the Indian and Atlantic oceans. Bottom: Source locations of teletsunamis generated by earthquakes or volcanic eruptions causing damage or casualties. These data are based on historical records. Source: NGDC/WDS-Geophysics.


Teletsunami or Distant tsunami 
 

A tsunami originating from a far away source, generally more than 1,000 km or more than 3 hours tsunami travel time from its source. 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 kilometres from the source. In the last 200 years, there have been at least 28 destructive ocean-wide tsunamis and 14 have caused fatalities more than 1,000 kilometres from the source. The most destructive Pacific-wide tsunami of recent history was generated by a massive earthquake off the coast of Chile on 22 May 1960. All Chilean coastal towns between the 36th and 44th parallels were either destroyed or heavily damaged by the action of the tsunami and the earthquake. The combined tsunami and earthquake toll included 2,000 killed, 3,000 injured, two million homeless, and $550 million damage. Off the coast of Corral, Chile, the waves were estimated to be 20 metres (67 feet) high. The tsunami caused 61 deaths in Hawaii, 20 in the Philippines, and 139 in Japan. Estimated damages were $50 million in Japan, $24 million in Hawaii and several millions of dollars along the west coast of the United States and Canada. Distant wave heights varied from slight oscillations in some areas to 12 metres (40 feet) at Pitcairn Island, 11 metres (37 feet) at Hilo, Hawaii, and 6 metres (20 feet) at some places in Japan. The worst tsunami catastrophe in history occurred in the Indian Ocean on 26 December 2004, when a M9.3 earthquake off of the northwest coast of Sumatra, Indonesia produced an ocean-wide tsunami that 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. Nearly 228,000 people lost their lives and more than a 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. The event also raised awareness of tsunami hazards globally, and new systems were established in the Caribbean, the Mediterranean and Atlantic.

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 tsunamis causing deaths greater 1000 km from source nov2012 web


The tsunami of 26 December 2004 destroyed the nearby city of Banda Aceh leaving only a few structures standing.  Photo courtesy of Yuichi Nishimura, Hokkaido University.

The tsunami of 26 December 2004 destroyed the nearby city of Banda Aceh leaving only a few structures standing. Photo courtesy of Yuichi Nishimura, Hokkaido University.

Tsunami
Japanese term meaning wave (“nami”) in a harbour (“tsu”). A series of travelling 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 rock falls 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.

Destruction along the waterfront of Hilo, Hawaii from the Pacific-wide tsunami generated off the coast of Unimak Island, Aleutian Island, USA on 1 April 1946.

Destruction along the waterfront of Hilo, Hawaii from the Pacific-wide tsunami generated off the coast of Unimak Island, Aleutian Island, USA on 1 April 1946.

Tsunami generated by 26 May 1983, Japan Sea earthquake approaching Okushiri Island, Japan.  Photo courtesy of Tokai University.

Tsunami generated by 26 May 1983, Japan Sea earthquake approaching Okushiri Island, Japan.  Photo courtesy of Tokai University.

Tsunami earthquake
An earthquake that produces an unusually large tsunami relative to the earthquake magnitude (Kanamori, 1972). Typical characteristics of tsunami earthquakes include long rupture durations for the magnitude of the earthquake, rupture on the very shallow part of the plate interface (inferred from a location near the trench and a low-angle thrust mechanism), and high energy release at low frequencies. They are also slow earthquakes, with slippage along their faults occurring more slowly than would occur in normal earthquakes. The last events of this type were in 1992 (Nicaragua), 1996 (Chimbote, Peru), and in Indonesia in 1994 (Java), 2006 (Java), and 2010 (Mentawai).

Tsunami sediments

Sediments deposited by a tsunami. The finding of tsunami sediment deposits within the stratigraphic soil layers provides information on the occurrence of historical and paleotsunamis. The discovery of similarly-dated deposits at different locations, sometimes across ocean basins and far from the tsunami source, can be used to map and infer the distribution of tsunami inundation and impact.

Sediment layers deposited from successive waves of 26 December 2004 Indian Ocean tsunami, as observed in Banda Aceh, Indonesia. Photo courtesy of Yuichi Nishimura, Hokkaido University.

Sediment layers deposited from successive waves of 26 December 2004 Indian Ocean tsunami, as observed in Banda Aceh, Indonesia. Photo courtesy of Yuichi Nishimura, Hokkaido University.

2. General Tsunami Terms: A... to Tr...

This section contains the general terms used in tsunami mitigation and in tsunami generation and modelling. PART 1: A-TR.

Breaker

A sea-surface wave that has become so steep (wave steepness of 1/7) that the crest outraces the body of the wave and it collapses into a turbulent mass on shore or over a reef. Breaking usually occurs when the water depth is less than 1.28 times the wave height. Roughly, three kinds of breakers can be distinguished, depending primarily on the gradient of the bottom: a) spilling breakers (over nearly flat bottoms) which form a foamy patch at the crest and break gradually over a considerable distance; b) plunging breakers (over fairly steep bottom gradients) which peak up, curl over with a tremendous overhanging mass and then break with a crash; c) surging breakers (over very steep bottom gradients) which do not spill or plunge but surge up the beach face. Waves also break in deep water if they build too high while being generated by the wind, but these are usually short-crested and are termed whitecaps.

Breakerwater

An offshore or onshore structure, such as a wall, water gate, or other in-water wave-dissipating object that is used to protect a harbour or beach from the force of waves.

Sea wall with stairway evacuation route used to protect a coastal town against tsunami inundation in Japan. Photo courtesy of River Bureau, Ministry of  Land, Infrastructure and Transport, Japan.

Sea wall with stairway evacuation route used to protect a coastal town against tsunami inundation in Japan. Photo courtesy of River Bureau, Ministry of  Land, Infrastructure and Transport, Japan.

Water gate used to protect against tsunami waves on Okushiri Island, Japan. The gate begins to automatically close within seconds after earthquake shaking triggers its seismic sensors.  Photo courtesy of ITIC.

Water gate used to protect against tsunami waves on Okushiri Island, Japan. The gate begins to automatically close within seconds after earthquake shaking triggers its seismic sensors. Photo courtesy of ITIC.

Eddy

By analogy with a molecule, a “glob” of fluid within the fluid mass that has a certain integrity and life history of its own; the activities of the bulk fluid being the net result of the motion of the eddies.

Eddies generated by the interactions of tsunami waves as they hit the coast of Sri Lanka, 26 December 2004.  Photo courtesy of Digital Globe.

Eddies generated by the interactions of tsunami waves as they hit the coast of Sri Lanka, 26 December 2004. Photo courtesy of Digital Globe.

Estimated time of arrival (ETA)

Time of tsunami arrival at some fixed location, as estimated from modelling 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). The first wave is not necessarily the largest, but it is usually one of the first five waves.

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. Evacuation routes are sometimes designated to ensure the efficient movement of people out of the evacuation zone to evacuation shelters.

Inundation and Evacuation Map created for the coastal town of Pucusana, Peru.

Inundation and Evacuation Map created for the coastal town of Pucusana, Peru.

Elevated platform used for tsunami evacuation that also serves as a high-elevation scenic vista point for tourist. Okushiri Island, Japan.  Photo courtesy of ITIC.

Elevated platform used for tsunami evacuation that also serves as a high-elevation scenic vista point for tourist. Okushiri Island, Japan. Photo courtesy of ITIC.

 Emergency shelter building that also acts as community centre and Museum for Disaster Prevention. Kisei, Mie Prefecture, Japan. The building is 22-m high, has five floors covering 320 m2 , and holds 500 persons.  Info courtesy of  http://www.webmie.or.jp.

Emergency shelter building that also acts as community centre and Museum for Disaster Prevention. Kisei, Mie Prefecture, Japan. The building is 22-m high, has five floors covering 320 m2, and holds 500 persons. Info courtesy of  http://www.webmie.or.jp.

Historical tsunami data

Historical data are available in many forms and at many locations. These forms include published and unpublished catalogs of tsunami occurrences, personal narratives, marigraphs, tsunami amplitude, runup and inundation zone measurements, field investigation reports, newspaper accounts, film, or video records.

Probabilistic Tsunami Hazard Assessment (PTHA)

An assessment of the probability that a tsunami will reach, or exceed, a given size within a specified interval of time at a particular location. The tsunami size may be measured in various ways, such as: run-up height, flow depth, or tsunami height at the coast. Usually a PTHA would provide probabilities for a range of different time spans, for example from 50 to 2500 years. The assessment may cover a single location, a stretch of coastline, or an area of land (if inundation is included). See also ‘Tsunami Hazard Assessment’ which provides information on some of the techniques that may be used to make a PTHA.

Seiche

A seiche may be initiated by a standing wave oscillating 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 wave

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

Travel time

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

Travel time map

Map showing isochrons or lines of equal tsunami travel time calculated from the source outwards toward terminal points on distant coastlines.

Travel times (in hours) for the 22 May 1960 Chile tsunami crossing the Pacific basin. This tsunami was extremely destructive along the nearby coast of Chile, and the tsunami also caused significant destruction and casualties as far away as Hawaii and Japan. The awareness and concern raised by this Pacific-wide tsunami ultimately led to the formation of the PTWS.

Travel times (in hours) for the 22 May 1960 Chile tsunami crossing the Pacific basin. This tsunami was extremely destructive along the nearby coast of Chile, and the tsunami also caused significant destruction and casualties as far away as Hawaii and Japan. The awareness and concern raised by this Pacific-wide tsunami ultimately led to the formation of the PTWS.

3. Surveys and Measurements

This section contains terms used to measure and describe tsunami waves on mareograph and in the field during a survey, and terms used to describe the size of the tsunami.

Arrival time
Time of the first maximum of the tsunami waves.

Crest length
The length of a wave along its crest. Some times called crest width.

Drop
The downward change or depression in sea level associated with a tsunami, a tide, or some long term climatic effect.

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

Initial rise
Time of the first minimum of the tsunami waves.

Intensity
The measure of strength, force, or energy.

Runup and Inundation.

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

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.  Photo courtesy of Takaaki Uda, Public Works Research Institute, Japan.

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.

Dark area shows inundation area from the 1964 Alaska tsunami.

Dark area shows inundation area from the 1964 Alaska tsunami.  Photo courtesy of NGDC.

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 runup.

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.

Magnitude
A number assigned to the properties of an event such that the event can be compared with other events of the same class.

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

Overflow
A flowing over; inundation.

Post-tsunami survey 
Tsunamis are relatively rare events and most of 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.

After a major tsunami, physical oceanographers, social scientists, and engineers conduct post-tsunami surveys to collect information.

After a major tsunami, physical oceanographers, social scientists and engineers conduct post-tsunami surveys to collect information. These data, including runup and inundation, deformation, scour, building and structural impact, wave arrival descriptions, and social impact, are important for designing better mitigation to reduce the impacts of tsunami on life and property.  Photo courtesy of Philip Liu, Cornell University.

In recent years, following each major destructive tsunami, a post-tsunami reconnaissance survey has been organized to make measurements of runups and inundation limits and to collect associated data from eyewitnesses such as the number of waves, arrival time of waves, and which wave was the largest. The surveys have been organized primarily on an ad-hoc basis by international academic tsunami researchers. A Post-Tsunami Survey Field Guide (http://www.tsunamiwave.info/itic/contents.php?id-28) has been prepared by the PTWS to help with preparations of surveys, to identify measurements and observations to be taken, and to standardize data collections. The Tsunami Bulletin Board e-mail service has also been used for quickly organizing international surveys and for sharing of the observations from impacted areas.

Post-tsunami survey measuring runup along a transect inland from the coast.

Post-tsunami survey measuring runup along a transect inland from the coast.  Courtesy of ICMAM, Chennai, DOD, India.

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

North Shore, Oahu, Hawaii.  09 March 1957 Aleutian Island tsunami.

North Shore, Oahu, Hawaii. During the 9 March 1957 Aleutian Island tsunami, people foolishly explored the exposed reef, unaware that tsunami waves would return in minutes to inundate the shoreline.  Photo by A. Yamauchi, courtesy of Honolulu Star-Bulletin.

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.

Runup
1) Difference between the elevation of maximum tsunami penetration (inundation line) and the sea level at the time of the tsunami. In practical terms, runup is only measured where there is a clear evidence of the inundation limit on the shore.

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 attack, etc., and measured ideally at a point that is a local maximum of the horizontal inundation. Where the elevation is not measured at the maximum of horizontal inundation this is often referred to as the inundation-height.

Runup distribution  Set of tsunami runup values measured or observed along a coastline.

Tsunami stripped forested hills of vegetation leaving clear marker of tsunami runup, Banda Aceh, 26 December 2004 Sumutra tsunami.

Tsunami stripped forested hills of vegetation leaving clear marker of tsunami runup, Banda Aceh, 26 December 2004 Sumutra tsunami. Photo courtesy of Yuichi Nishimura, Hokkaido University.

Banda Aceh, Indonesia, 26 December 2004.

Runup can often be inferred from the vertical extent of dead vegetation, from debris normally found at ground level that are observed stuck on electric wires, in trees, or at other heights, and from water line marks left on building walls.  In extreme cases, cars, boats, and other heavy objects have been lifted and deposited atop buildings. Banda Aceh, Indonesia, 26 December 2004. Photo courtesy of C. Courtney, Tetra Tech EMI.

Sieberg tsunami intensity scale A descriptive tsunami intensity scale which was later modified into the Sieberg-Ambraseys tsunami intensity scale described below (Ambraseys 1962).

Modified Sieberg Sea-wave Intensity Scale
1) Very light. Wave so weak as to be perceptible only on tide-gauge records.

2) Light. Wave noticed by those living along the shore and familiar with the sea. On very flat shores generally noticed.

3) Rather strong. Generally noticed. Flooding of gently sloping coasts. Light sailing vessels or small boats carried away on shore. Slight damage to light structures situated near the coast. In estuaries reversal of the river flow some distance upstream.

4) Strong. Flooding of the shore to some depth.  Light scouring on man-made ground. Embankments and dikes damaged. Light structures near the coasts damaged. Solid structures on the coast injured. Big sailing vessels and small ships carried inland or out to sea. Coasts littered with floating debris.

5) Very strong. General flooding of the shore to some depth. Breakwater walls and solid structures near the sea damaged. Light structures destroyed. Severe scouring of cultivated land and littering of the coast with floating items and sea animals. With the exception of big ships all other type of vessels carried inland or out to sea. Big bores in estuary rivers. Harbour works damaged. People drowned. Wave accompanied by strong roar.

6) Disastrous. Partial or complete destruction of man-made structures for some distance from the shore. Flooding of coasts to great depths. Big ships severely damaged. Trees uprooted or broken.  Many casualties.


Significant wave height
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.

Spreading
When reference is made to tsunami waves, it is the spreading of the wave energy over a wider geographical area as the waves propagate away from the source region. The reason for this geographical spreading and reduction of wave energy with distance traveled, is the sphericity of the earth. The tsunami energy will begin converging again at a distance of 90 degrees from the source. Tsunami waves propagating across a large ocean undergo other changes in configuration primarily due to refraction, but geographical spreading is also very important depending upon the orientation, dimensions and geometry of the tsunami source.

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

The 26 December 2004 earthquake resulted in 1.2 m of land subsidence in the Car Nicobar.

The 26 December 2004 earthquake resulted in 1.2 m of land subsidence in the Car Nicobar, Nicobar Islands, India leaving houses that were once above sea level now permanently submerged.  Photo courtesy of ICMAM, Chennai, DOD, India.

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 an amplitude modified in some way by the tide gauge response.

Mareogram (sea level) record of a tsunami.

Mareogram (sea level) record of a tsunami.

Tsunami intensity
Size of a tsunami based on the macroscopic observation of a tsunami's effect on humans, objects, including various sizes of marine vessels, and buildings. 

The original scale for tsunamis was published by Sieberg (1923), and later modified by Ambraseys (1962) to create a six-category scale.  Papadopoulus and Imamura (2001) proposed a new 12-grade intensity scale which is independent of the need to measure physical parameters like wave amplitude, sensitive to the small differences in tsunami effects, and detailed enough for each grade to cover the many possible types of tsunami impact on the human and natural environment. The scale has 12 categories, similar to the Modified Mercalli Intensity Scale used for macroseismic descriptions of earthquake intensity.

Tsunami magnitude
Size of a tsunami based on the measurement of the tsunami wave on sea level gauges and other instruments.

The scale, originally descriptive and more similar to an intensity, quantifies the size by using measurements of wave height or tsunami runup.  Iida et al. (1972) described the magnitude (m) as dependent in logarithmic base 2 on the maximum wave height measured in the field, and corresponding to a magnitude range from -1 to 4:

                          m = log2 Hmax

Hatori (1979) subsequently extended this so-called Imamura-Iida scale for far-field tsunamis by including distance in the formulation.  Soloviev (1970) suggested that the mean tsunami height may be another good indicator of tsunami size, and the maximum intensity would be that measured nearest to the tsunami source. A variation on this is the Imamura-Soloviev intensity scale I (Soloviev, 1972). Shuto (1993) has suggested the measurement of H as the height where specific types of impact or damage occur, thus proposing a scale which can be used as a predictive quantitative tool for macroscopic effects.

Tsunami magnitudes have also been proposed that are similar in form to those used to calculate earthquake magnitudes. These include the original formula proposed by Abe (1979) for tsunami magnitude, Mt:

                        Mt = logH + B

where H is the maximum single crest or trough amplitude of the tsunami waves (in metres) and B is a constant, and the far-field application proposed by Hatori (1986) which adds a distance factor  into  the  calculation.

Tsunami period
Amount of time that a tsunami wave takes to complete a cycle. Tsunami periods typically range from five minutes to two hours.

Tsunami period (dominant)
Difference between the arrival time of the highest peak and the next one measured on a water level record.

Tsunami wave length
The horizontal distance between similar points on two successive waves measured perpendicular to the crest. The wave length and the tsunami period give information on the tsunami source. For tsunamis generated by earthquakes, the typical wave length ranges from 20 to 300 km. For tsunamis generated by landslides, the wave length is much shorter, ranging from hundreds of metres to tens of kilometres.

Water level (maximum)
Difference between the elevation of the highest local water mark and the elevation 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 crest
1) The highest part of a wave.

2) That part of the wave above still water level.

Water trough
The lowest part of a wave.

 

2. General Tsunami Terms: Ts...

Part 2 of General Terms: Ts...

Tsunami bore

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

Tsunami bore entering Wailua River, Hawaii during the 1946 Aleutian Island tsunami.  Photo courtesy of Pacific Tsunami Museum.

Tsunami bore entering Wailua River, Hawaii during the 1946 Aleutian Island tsunami. Photo courtesy of Pacific Tsunami Museum.

Tsunami damage

Loss or harm caused by a destructive tsunami. More specifically, the damage caused directly by tsunamis can be summarized into the following: 1) Deaths and injuries; 2) houses destroyed, partially destroyed, inundated, flooded, or burned; 3) other property damage and loss; 4) boats washed away, damaged or destroyed; 5) lumber washed away; 6) marine installations destroyed, and; 7) damage to public utilities such as railroads, roads, bridges, power plants, water or fuel storage tanks, or wastewater facilities, etc. Indirect secondary tsunami damage can be: 1) Damage by fire of houses, boats, oil tanks, gas stations, and other facilities; 2) environmental pollution or health hazards caused by drifting materials, oil, and hazardous waste spillages; 3) outbreak of disease of epidemic proportions, which could be serious in densely populated areas.

Tall reinforced concrete buildings served as vertical evacuation refuges during the 11 March 2011 Japan tsunami, saving many lives. Minamisanriku, Japan. Photo courtesy of ITIC.

Tall reinforced concrete buildings served as vertical evacuation refuges during the 11 March 2011 Japan tsunami, saving many lives. Minamisanriku, Japan. Photo courtesy of ITIC.

The 11 March 2011 tsunami leveled the town of Ofunato, Japan. Photo courtesy of ITIC.

The 11 March 2011 tsunami leveled the town of Ofunato, Japan.
Photo courtesy of ITIC.

Tsunami dispersion

Redistribution of tsunami energy, particularly as a function of its period, as it travels across a body of water.

Tsunami edge wave

Wave generated by a tsunami that travels along the coast.

Tsunami forerunner
A series of oscillations of the water level preceding the arrival of the main tsunami waves, mainly due to the resonance in bays and shelves that could occur before the arrival of the main tsunami.

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 wave.

Tsunamis can be generated by submarine landslides, or by subaerial landslides that enter the water.  Courtesy of LDG-France.

Tsunamis can be generated by submarine landslides, or by subaerial landslides that enter the water. Courtesy of LDG, France.

Most tsunamis are generated by large, shallow, thrust earthquakes that occur as a tectonic plate is subducted. Shallow earthquakes also occur along spreading ridges, but these are not large enough to cause tsunamis. Large, shallow earthquakes also occur along transform faults, but there is only minor vertical motion during the faulting so no tsunamis are generated.

Most tsunamis are generated by large, shallow, thrust earthquakes that occur as a tectonic plate is subducted. Shallow earthquakes also occur along spreading ridges, but these are not large enough to cause tsunamis. Large, shallow earthquakes also occur along transform faults, but there is only minor vertical motion during the faulting so no tsunamis are generated.

Tsunamis are most often generated by shallow earthquakes.

Tsunamis are most often generated by shallow earthquakes

Tsunamis can be generated by pyroclastic flows associated with volcanic eruptions. Courtesy of LDG-France.

Tsunamis can be generated by pyroclastic flows associated with volcanic eruptions. Courtesy of LDG, France.


Tsunami generation theory  The theoretical problem of generation of the gravity wave (tsunami) in the layer of elastic liquid (an ocean) occurring on the surface of elastic solid half-space (the crust) in the gravity field can be studied with methods developed in the dynamic theory of elasticity. The source representing an earthquake focus is a discontinuity in the tangent component of the displacement on some element of area within the crust. For conditions representative of the Earth's oceans, the solution of the problem differs very little from the joint solution of two more simple problems: The problem of generation of the displacement field by the given source in the solid elastic half-space with the free boundary (the bottom) considered quasi-static; and the problem of the propagation of gravity wave in the layer of heavy incompressible liquid generated by the known (from the solution of the previous problem) motion of the solid bottom. There is the theoretical dependence of the gravity wave parameters on the source parameters (depth and orientation). One can roughly estimate the quantity of energy transferred to the gravity wave by the source. In general, it corresponds to the estimates obtained with empirical data. Also, tsunamis can be generated by other different mechanisms such as volcanic or nuclear explosions, landslides, rock falls, and submarine slumps.

Tsunami hazard

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

Global tsunami source zones. Tsunami hazards exist in all oceans and basins, but occur most frequently in the Pacific Ocean. Tsunamis can occur anywhere and at any time because earthquakes cannot be accurately predicted. Courtesy of LDG-France.

Global tsunami source zones. Tsunami hazards exist in all oceans and basins, but occur most frequently in the Pacific Ocean. Tsunamis can occur anywhere and at any time because earthquakes cannot be accurately predicted. Courtesy of LDG, France.

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 impact

Although infrequent, tsunamis are among the most terrifying and complex physical phenomena and have been responsible for great loss of life and extensive damage. Because of their destructiveness, tsunamis have important impacts on the human, social, and economic sectors of societies. Over the past 3500 years, there have been 279 fatal tsunamis and more than 600,000 deaths. Tthe worst catastrophe in history was the 26 December 2004 Sumatra, Indonesia tsunami that killed 228,000 people in 12 Indian Ocean countries and caused $10 billion in damage. The Pacific Ocean, however, is where 75% of the world’s tsunamis occur.99% of the deaths were caused by local tsunamis, which are those hit in less than 1 hour tsunami travel time. Since 80% of the tsunamis are generated by shallow great earthquakes, shaking and damage from the earthquake is the 1st hazard to address before the tsunami arrives. 

In Japan, which has one of the most populated coastal regions in the world and a long history of earthquake activity, tsunamis have destroyed entire coastal populations. There is also a history of severe tsunami destruction in Alaska, the Hawaiian Islands, Indonesia, and South America. The last major Pacific-wide tsunami was the 11 March 2011 Japan tsunami which killed more than 18,000 in Japan and 2 persons in the far field.

Estimated tsunami inundation at Iquique, Chile, based on numerical model results.  Courtesy of SHOA-Chile.

Estimated tsunami inundation at Iquique, Chile, based on numerical model results. Courtesy of SHOA, Chile.

Tsunami numerical modelling  

Mathematical descriptions that seek to describe the observed tsunami and its effects. Often the only way to determine the potential runups and inundation from a local or distant tsunami is to use numerical modelling since data from past tsunamis is usually insufficient. Models can be initialized with potential worst case scenarios for the tsunami sources or for the waves just offshore to determine corresponding worst case scenarios for runup and inundation. Models can also be initialized with smaller sources to understand the severity of the hazard for the less extreme but more frequent events. This information is then the basis for creating tsunami evacuation maps and procedures. At present, such modelling has only been carried out for a small fraction of the coastal areas at risk. Sufficiently accurate modelling techniques have only been available in recent years, and these models require training to understand and use correctly, as well as input of detailed bathymetric and topographic data in the area being modelled.

Numerical models have been used in recent years to simulate tsunami propagation and interaction with land masses. Such models usually solve similar equations but often employ different numerical techniques and are applied to different segments of the total problem of tsunami propagation from generation regions to distant areas of runup. For example, several numerical models have been used to simulate the interaction of tsunamis with islands. These models have used finite difference, finite element, and boundary integral methods to solve the linear long wave equations. These models solve these relatively simple equations and provide reasonable simulations of tsunamis for engineering purposes. Tsunami warning centres use numerical models to forecast expected wave arrival times, directions of maximum tsunami energy, strength of near-shore water currents, and coastal wave height. This important information helps emergency response officials to plan and focus relief on where the impact is expected to be the greatest.

Calculated maximum tsunami wave heights for a M9.0 Cascadia subduction zone earthquake. The model was calculated after tsunami deposits found in Japan and elsewhere suggested that a repeat of the 1700 Cascadia great earthquake would generate a destructive teletsunami. Courtesy of Kenji Satake, Geological Survey of Japan.

Calculated maximum tsunami wave heights for a M9.0 Cascadia subduction zone earthquake. The model was calculated after tsunami deposits found in Japan and elsewhere suggested that a repeat of the 1700 Cascadia great earthquake would generate a destructive teletsunami. Courtesy of Kenji Satake, Geological Survey of Japan.

Complex numerical model calculated to match the 1958 Lituya Bay, Alaska landslide-generated local tsunami which caused the largest runup ever recorded (525 m). The complex model matches very closely the detail of the second order eddies and splash effects that laboratory experiments showed. Courtesy of Galen Gisler, Los Alamos National Laboratory.

Complex numerical model calculated to match the 1958 Lituya Bay, Alaska landslide-generated local tsunami which caused the largest runup ever recorded (525 m). The complex model matches very closely the detail of the second order eddies and splash effects that laboratory experiments showed. Courtesy of Galen Gisler, Los Alamos National Laboratory.


Tsunami observation
Notice, observation or measurement of sea level fluctuation at a particular point in time caused by the incidence of a tsunami on a specific point.

1946 Aleutian Islands tsunami rushing ashore in Hilo, Hawaii.  Photo courtesy of Pacific Tsunami Museum.

1946 Aleutian Islands tsunami rushing ashore in Hilo, Hawaii.  Photo courtesy of Pacific Tsunami Museum.

Tsunami preparedness

Readiness of plans, methods, procedures, and actions taken by government officials and the general public for the purpose of minimizing potential risk and mitigating the effects of future tsunamis. The appropriate preparedness for a warning of impending danger from a tsunami requires knowledge of areas that could be flooded (tsunami inundation maps) and knowledge of the warning system to know when to evacuate and when it is safe to return.

Tsunami hazard sign approved by International Standards Organization (ISO) in 2008

Tsunami hazard sign approved by International Standards Organization (ISO) in 2008


Tsunami evacuation route sign, Chile.

Tsunami evacuation route sign, Chile

Tsunami evacuation building  sign in Japan, approved by ISO     Tsunami safe place sign in Japan, approved by ISO

Tsunami evacuation building and safe place signs in Japan, approved by ISO

 

 Tsunami evacuation area signs, Hawaii, USA.   Tsunami evacuation area signs, Hawaii, USA    

Tsunami evacuation area signs, Hawaii, USA

 

Tsunami hazard zone sign, Washington, USA

Tsunami hazard zone sign, Washington, USA

 

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 fracture 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. In the deep and open ocean, they travel at speeds of 500 to 1,000 km per hour (300 to 600 miles per hour). The distance between successive crests can be as much as 500 to 650 km (300 to 400 miles). However, in the open ocean, the height of the waves is generally less than a meter (3 feet) even for the most destructive teletsunamis, and the waves pass unnoticed. 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 features modify both the waveform and rate of advance. Specifically, tsunami waves undergo a process of wave refraction and reflection throughout their travel. 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.

Model of tsunami propagation in the southeast Pacific, nine hours after generation. Source: Antofagasta, Chile (30 July 1995). Courtesy of LDG-France.

Model of tsunami propagation in the southeast Pacific, nine hours after generation. Source: Antofagasta, Chile (30 July 1995). Courtesy of LDG, France.

Tsunami resonance

The continued reflection and interference of tsunami waves from the edge of a harbour or narrow bay that can cause amplification of the wave heights, and extend the duration of wave activity from a tsunami.

Tsunami risk

The probability of a particular coastline being struck by a tsunami multiplied by the likely destructive effects of the tsunami and by the number of potential victims. In general terms, risk is the hazard multiplied by the exposure.

Tsunami simulation

Numerical model of tsunami generation, propagation, and inundation.

Tsunami source

Point or area of tsunami origin, usually the site of an earthquake, volcanic eruption, or landslide that caused large-scale rapid displacement of the water to initiate the tsunami waves.

Tsunami velocity or Shallow water velocity
The velocity of an ocean wave whose length is sufficiently large compared to the water depth (i.e., 25 or more times the depth) can be approximated by the following expression:

c = square root of (gh)

Where:

c: is the wave velocity

g: the acceleration due to gravity

h: the water depth.

Thus, the velocity of shallow-water waves is independent of wavelength L. In water depths between ½ L and 1/25 L it is necessary to use a more precise expression:

c = square root of (gL/2(pi))[tanh(2(pi)h/L)])

Wave height and water depth. In the open ocean, a tsunami is often only a tens of centimetres high, but its wave height grows rapidly in shallow water. Tsunami wave energy extends from the surface to the bottom in the deepest waters. As the tsunami attacks the coastline, the wave energy is compressed into a much shorter distance creating destructive, life-threatening waves.

Wave height and water depth. In the open ocean, a tsunami is often only a tens of centimetres high, but its wave height grows rapidly in shallow water. Tsunami wave energy extends from the surface to the bottom in the deepest waters. As the tsunami attacks the coastline, the wave energy is compressed into a much shorter distance creating destructive, life-threatening waves.

Tsunami zonation (tsunami zoning)

Designation of distinctive zones along coastal areas with varying degrees of tsunami risk and vulnerability for the purpose of disaster preparedness, planning, construction codes, or public evacuation.

Tsunamigenic

Capable of generating a tsunami. For example: a tsunamigenic earthquake, a tsunamigenic landslide.

Destruction of Hilo Harbor, Hawaii, 1 April 1946. The tsunami generated off the coast of Unimak Island, Aleutian Islands, raced across the Pacific, coming ashore in Hawaii less than five hours later. Photo courtesy of NOAA.

Destruction of Hilo Harbor, Hawaii, 1 April 1946. The tsunami generated off the coast of Unimak Island, Aleutian Islands, raced across the Pacific, coming ashore in Hawaii less than five hours later. Photo courtesy of NOAA.

4. Tide, Mareograph, Sea Level

This section contains terms to describe sea level and the instruments used to measure tsunami.

Cable ocean-bottom instrument
An instrument at the ocean bottom connected to the land by a cable that provides power for the measurement and transmission of data from the seafloor to the coast. Cables can extend for ens of kilometers offshore and across oceans. They enable real-time, multi-sensor seafloor observatories to be deployed for long-term monitoring. Examples of sensors on cabled systems are seismometers to measure earthquakes, sensitive pressure gauges to measure tsunamis, geodetic sensors to measure seafloor deformation, and cameras. Japan operates several cable systems.

Schematic diagram of cabled ocean system for monitoring earthquakes and tsunamis.

Schematic diagram of cabled ocean system for monitoring earthquakes and tsunamis.  Courtesy of JMA.

Cotidal
Indicating equality with the tides or a coincidence with the time of high or low tide.

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 as small as one cm, and a moored surface buoy for real-time communications. An acoustic link is used to transmit data from the seafloor to the surface buoy.  The data are then relayed via a satellite link to ground stations, which demodulate the signals for immediate dissemination to the NOAA tsunami warnings centres. The DART data, along with state-of-the-art numerical modelling technology, are part of a tsunami forecasting system package that will provide site-specific predictions of tsunami impact on the coast.

Dart II System.

Low Water
The lowest water level reached during a tide cycle. The accepted popular term is low tide.

Mareogram or Marigram
1) Record made by a mareograph.

2) Any graphic representation of the rise and fall of the sea level, with time as abscissa and height as ordinate, usually used to measure tides, may also show tsunamis.

17 February 1996 Irian Jaya Tsunami

17 February 1996 Irian Jaya Tsunami.

Mareograms of tsunami signals measured by an underwater gauge located 50 km outside the entrance to Tokyo Bay in about 50 m of water (upper trace), and another gauge located at the shore (lower trace). The tsunami is detected on the outside gauge about 40 minutes before it reaches shore (arrows). The offshore gauge was developed by Japan's Port and Harbours Research Institute.

Mareograph
A recording sea level gauge.  Also known as a marigraph or tide gauge.

Mean sea level
The average height of the sea surface, based upon hourly observation of tide height on the open coast or in adjacent waters which have free access to the sea. These observations are to have been made over a “considerable” period of time. In the United States, mean sea level is defined as the average height of the surface of the sea for all stages of the tide over a 19-year period. Selected values of mean sea level serve as the sea level datum for all elevation surveys in the United States. Along with mean high water, mean low water, and mean lower low water, mean sea level is a type of tidal datum.

Probable maximum water level
A hypothetical water level (exclusive of wave runup from normal wind-generated waves) that might result from the most severe combination of hydrometeorological, geoseismic and other geophysical factors that is considered reasonably possible in the region involved, with each of these factors considered as affecting the locality in a maximum manner. This level represents the physical response of a body of water to maximum applied phenomena such as hurricanes, moving squall lines, other cyclonic meteorological events, tsunamis, and astronomical tide combined with maximum probable ambient hydrological conditions such as wave level with virtually no risk of being exceeded.

Reference sea level
The observed elevation differences between geodetic benchmarks are processed through least-squares adjustments to determine orthometric heights referred to a common vertical reference surface, which is the reference sea level. In this way, height values of all benchmarks in the vertical control portion of a surveying agency are made consistent and can be compared directly to determine differences of elevation between benchmarks in a geodetic reference system that may not be directly connected by lines of geodetic leveling. The vertical reference surface in use in the United States, as in most parts of the world, approximates the geoid. The geoid was assumed to be coincident with local mean sea level at 26 tidal stations to obtain the Sea Level Datum of 1929 (SLD 290). National Geodetic Vertical Datum of 1929 (NGVD 29) became a name change only; the same vertical reference system has been in use in the United States since 1929. This important vertical geodetic control system is made possible by a universally accepted, reference sea level.

Refraction diagrams
Models using water depths, direction of wave, separation angle, and ray separation between two adjacent rays as input, produce the path of wave orthogonals, refraction coefficients, wave heights, and travel times.

Sea level
The height of the sea at a given time measured relative to some datum, such as mean sea level.

Sea level station 
A system consisting of a device such as a tide gauge for measuring the height of sea level, a data collection platform (DCP) for acquiring, digitizing, and archiving the sea level information digitally, and often a transmission system for delivering the data from the field station to a central data collection centre.  The specific requirements of data sampling and data transmission are dependent on the application.  The GLOSS programme maintains a core network of sea level stations.  For local tsunami monitoring, one-second sampled data streams available in real time are required.  For distant tsunamis, warning centres may be able to provide adequate warnings using data acquired in near-real time (one-minute sampled data transmitted every 15 minutes).  Sea level stations are also used for sea level rise and climate change studies, where an important requirement is for the very accurate location of the station as acquired through surveying techniques.

Rarotonga sea level station, Avarua Harbor, Cook Islands.

Rarotonga sea level station, Avarua Harbor, Cook Islands.  The fiberglass electronics package (a), antenna (b), solar panel (c) were installed on a pier. Conduit (d) containing cables connecting the sensor, located at a depth of five feet below low-tide water level, to the data collection platform containing the electronics above, was externally attached to the tube containing the sensor (e).

Tidal wave
1) The wave motion of the tides.

2) Often incorrectly used to describe a tsunami, storm surge, or other unusually high and therefore destructive water levels along a shore that are unrelated to the tides.

Tide
The rhythmic, alternate rise and fall of the surface (or water level) of the ocean, and of bodies of water connected with the ocean such as estuaries and gulfs, occurring twice a day over most of the Earth and resulting from the gravitational attraction of the moon (and, in lesser degrees, of the sun) acting unequally on different parts of the rotating Earth.

Tide amplitude
One-half of the difference in height between consecutive high water and low water; hence, half of the tidal range..

Tide gauge
A device for measuring the height (rise and fall) of the tide. Especially an instrument for automatically making a continuous graphic record of tide height versus time.

Tide station
A place where tide observations are obtained.

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.

Port Louis, Mauritius radar.

GLOSS sea level stations employ a number of instruments to measure sea level, including down-looking radars to measure sea level.  Port Louis, Mauritius.  Photo courtesy of University of Hawaii Sea Level Center.

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