A Glossary of Tsunami and Seismic-Related Terminology
The glossary is divided into two sections, one of basic terms found in the content of the Web site and the other of technical terminology that may be found in publications and research.
Click on the terms that are linked to see illustrations.
Basic Terms
| A | B | C | D | E | F | G | H | I | L | M | N | O | P | R | S | T | U | V | W |
Aftershock: an earthquake that follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency over time.
Arrival time: time of arrival, usually of the first wave of the tsunami at a particular location.
Bore: a steep, turbulent, rapidly moving wave front that typically occurs in river mouths or estuaries.
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 bottom), which form a foamy patch at the crest and break gradually over a considerable distance; (b) plunging breakers (over fairly steep bottom gradient), 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."
Breakwater: an offshore structure such as a wall that is used to protect a harbor or beach from the force of waves.
Consolidated: tightly packed. Composed of particles that are not easily separated.
Continental Drift: the theory first advanced by Alfred Wegener, that Earth's continents were originally one landmass. Pieces of the landmass split off and migrated to form the continents.
Crust: the thin outer layer of the earth's surface, averaging about 10 kilometers thick under the oceans and up to about 50 kilometers thick on the continents. This is the only layer of the Earth that humans have actually seen.
Community Vulnerability Assessment Methodology (CVAM): CVAM is a methodology that can be used to conduct community-wide risk and vulnerability assessments. CVAM is a GIS-based process that allows emergency managers, planners, building officials, and anyone else interested in reducing the impacts of hazards to analyze physical, social, economic, and environmental vulnerability to hazards at the local level.
Disaster: a crisis event that surpasses the ability of an individual, community, or society to control or recover from its consequences.
Earthquake: shaking of the earth caused by a sudden movement of rock beneath its surface.
Eddy: a current of water moving contrary to the main current, especially in a circular motion.
Epicenter: the point on the earth's surface directly above the location in the earth (focus) where an earthquake originates.
ETA: estimated time of arrival. Computed arrival time of the first tsunami wave at coastal communities after a specific earthquake has occurred.
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.
Exposure: the number, types, qualities, and monetary values of various types of property or infrastructure and life that may be subject to an undesirable or injurious hazard event.
False warning: a warning for a potential tsunami, which proved to be an insignificant tsunami.
Fault: a weak point in the earth's crust and upper mantle where the rock layers have ruptured and slipped. Faults are caused by earthquakes, and earthquakes are likely to reoccur on pre existing faults.
FEMA Regional Offices: the Federal Emergency Management Agency has 10 regional offices and two area offices. Each regional office serves several states. Regional staff work directly with the states to help plan for disasters, develop mitigation programs, and meet needs when major disasters occur. The U.S. West Coast is served by FEMA Regional Offices IX and X. Region IX covers Arizona, California, and Nevada. Region X covers Idaho, Oregon, and Washington.
Focus: that point within the earth where the first motion of an earthquake and its elastic waves originate.
Great earthquake: an earthquake having a magnitude of 8 or greater on the Richter scale.
Hazard: an event or physical condition that has the potential to cause fatalities, injuries, property damage, infrastructure damage, agricultural loss, damage to the environment, interruption of business, or other types of harm or loss.
HAZUS: the FEMA HAZUS loss estimation methodology is a software program that uses mathematical formulas and information about building stock, local geology, and the location and size of potential earthquakes, economic data, and other information to estimate losses from a potential earthquake. HAZUS is capable of using two separate geographic information systems (MapInfo® and ArcView®) to map and display ground shaking, the pattern of building damage, and demographic information about a community. Once the location and size of a hypothetical earthquake is identified, HAZUS will estimate the violence of ground shaking, the number of buildings damaged, the number of casualties, the amount of damage to transportation systems, disruption to the electrical and water utilities, the number of people displaced from their homes, and the estimated cost of repairing projected damage and other effects.
Insignificant tsunami: a tsunami having no measured run-up of a meter or more anywhere in a coastal area.
Intensity: A measure of the effects of an earthquake at a particular place on humans, structures, and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.
Inundation: the depth, relative to a stated reference level, to which a particular location is covered by water; the maximum horizontal distance inland that a tsunami penetrates.
Inundation area: an area that is flooded with water.
Inundation line (limit): the inland limit of wetting measured horizontally from the edge of the coast defined by mean sea level.
Landslide: an abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis.
Liquefaction: the process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress. In other words, solid ground turns to jelly.
Local/regional tsunami: source of a tsunami within 620 miles (1000 kilometers) of the area of interest. A local tsunami, also known as a near-field tsunami, has a very short travel time (30 minutes or less), while mid-field or regional tsunami waves have travel times on the order of 30 minutes to 2 hours. Note: the term local tsunami is sometimes used to refer to a tsunami of landslide origin.
Locally generated tsunami: a tsunami that has been generated by a displacement of the ocean floor adjacent to the coastal area it strikes.
Magnitude: a measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake. There is no beginning or end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.
Microearthquake: an earthquake having a magnitude of 2 or less on the Richter scale.
Microtsunami: a tsunami of such small amplitude that it must be observed instrumentally and is not easily detected visually.
Mitigation: sustained action that reduces or eliminates long-term risk to people and property from natural hazards and their effects (i.e., engineering roads or bridges to withstand earthquakes).
Modified Mercalli Scale of Intensity: a scale used to measure the intensity of earthquakes. See the Modified Mercalli Scale of Intensity in comparison to the Richter Scale
NOAA: National Oceanic and Atmospheric Administration, the federal agency responsible for tsunami warnings and monitoring. Part of the Department of Commerce. Specifically, the National Weather Service (NWS) is the branch of NOAA that operates two tsunami warning centers and disseminates warnings. See "PTWC" and "WC/ATWC".
Normal earthquake: an earthquake caused by a slip along a sloping fault where the rock above the fault moves downwards relative to the rock below.
No warning: a significant tsunami occurred, but no warning was given.
Overflow: a flowing over, inundation.
Pacific Disaster Center (PDC): an information processing center to support emergency managers in the Pacific region. Funded by the U.S. Department of Defense. Pacific Disaster Center (PDC)
Pacific-wide tsunami: a tsunami striking some 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).
Plate: One of the huge sections which make up the earth's crust. The plates are continuously moving.
Plate tectonics: the theory that the Earth's crust and upper mantle (the lithosphere) 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.
PTWC: Pacific Tsunami Warning Center. Originally established in 1948 as the SSWWS, located in Ewa Beach near Honolulu. Responsible for issuing warnings to Hawaii, to U.S. interests in the Pacific other than the west coast and Alaska, and to countries located throughout the Pacific. PTWC is a part of NOAA's National Weather Service.
Recovery: the process of restoring normal public or utility services following a disaster, perhaps starting during but extending beyond the emergency period to that point when the vast majority of such services, including electricity, water, communications, and public transportation, have resumed normal operations.
Short-term recovery does not include the reconstruction of the built environment, although reconstruction may commence during this period. Long-term recovery is the process of returning the community, to the extent possible, to the conditions that existed prior to the event, preferably while taking advantage of opportunities to mitigate against future disasters.
Regional tsunami: a tsunami capable of destruction in a particular geographic region, generally within about 1000 kilometers of its source. Regional tsunamis occasionally have very limited and localized effects outside the region.
Regional Tsunami Warning/Watch Bulletin: a message issued initially by the Pacific Tsunami Warning Center, which is based only on seismic information, to alert all participants of the possibility of a tsunami and advise them that a tsunami investigation is under way. Those areas that are within 0 to 3 hours from the estimated time of arrival of the first wave are placed in a Tsunami Warning status. Those areas within 3 to 6 hours are placed in a Tsunami Watch status. It will be followed by additional bulletins until it is either upgraded to a Pacific-wide Tsunami Warning or until it is cancelled.
Response: actions and activities that support state and local government efforts to save lives and to protect public health, safety, and property.
Richter scale: The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device.
Risk: the potential for losses associated with a hazard, defined in terms of expected probability and frequency, causative factors, and locations/areas affected.
Seismic: of or having to do with earthquakes.
Seismic belt: an elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60 percent of the world's earthquakes occur in the circum-Pacific seismic belt. Seismicity Earthquake activity.
Seismographs: an instrument that records the motions of the Earth, especially earthquakes.
Seismogram: a written record of an earthquake, recorded by a seismograph.
Seismologist: a scientist who studies earthquakes.
Significant tsunami: a tsunami having a measured runup of one meter or more.
Subduction: the process in which one lithospheric plate collides with and is forced down under another plate and is drawn back into the earth's mantle.
Subduction zone: an area of convergence between a sinking plate and an overriding plate.
Tidal wave: common term for tsunami used in older literature, historical descriptions, and popular accounts. Tides, caused by the gravitational attractions of the sun and moon, may increase or decrease the impact of a tsunami, but have nothing to do with their generation or propagation. However, most tsunamis (initially) give the appearance of a fast-rising tide or fast-ebbing as they approach shore and only rarely as a near-vertical wall of water.
Tide: the periodic rise and fall of the sea surface 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 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.
TIME: the Center for the Tsunami Inundation Mapping Effort, to assist the Pacific states in developing tsunami inundation maps; this Center is a part of National Oceanic and Atmospheric Administration's, Pacific Marine Environmental Laboratory.
Tsunami: Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides. They are one or a series of huge sea waves caused by earthquakes, submarine earthquakes, volcanic eruptions, landslides, or other large-scale disturbances of the ocean floor. The word tsunami is Japanese, meaning "harbor wave."
Tsunami damage: loss or harm caused by a destructive tsunami. More specifically, the damage caused directly by tsunamis can be summarized by the following: 1) deaths and injuries; 2) houses destroyed, partly 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, electric power plants, water supply installations, etc. Indirect secondary tsunami damage can be 1) damage by fire of houses, boats, oil tanks, gas stations, and other facilities; 2) environmental pollution caused by drifting materials, oil, or other substances; 3) outbreak of disease of epidemic proportions which could be serious in densely populated areas.
Tsunami earthquake: a tsunamigenic earthquake that produces a much larger tsunami than expected for its magnitude.
Tsunami effect: the result, consequence, or outcome in the aftermath of a tsunami disaster.
Tsunami generation: tsunamis are generated primarily by tectonic dislocations under the sea, which are caused by shallow focus earthquakes along areas of subduction.
Tsunami hazard: danger from a tsunami to life and property.
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 destructiveness, tsunamis have important impacts on the human, social, and economic sectors of societies. The last major Pacific-wide tsunami occurred in 1960. Many other local and regional destructive tsunamis have occurred with more localized effects.
Tsunami Information Bulletin: a message issued to advise participants of the occurrence of a major earthquake in the Pacific or near-Pacific area, with the evaluation that a potentially destructive Pacific-wide tsunami was not generated.
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.
Tsunami risk: exposure to loss or injury caused by tsunamis.
Tsunami Warning Bulletin: a warning message issued throughout the Pacific based on confirming that a tsunami has been generated that poses a threat to the population in part or all of the Pacific. A Tsunami Warning will be followed by additional bulletins with updated information until it is cancelled.
TWS: Tsunami Warning System. This is an organization of 26 Pacific Member States which coordinates international monitoring and warning dissemination. Operates through ICG/ITSU.
Unconsolidated: loosely arranged, not cemented together, so particles separate easily.
Vulnerability: the level of exposure of human life, property, and resources to damage from natural hazards.
WC/ATWC: West Coast/ Alaska Tsunami Warning Center, established in 1967 originally to issue warnings to Alaska
about local tsunami events. WC/ATWC is now responsible for issuing warnings for
any event likely to impact either Alaska, the west coast of the U.S., or the
Pacific coast of Canada. This Center is a part of National Oceanic Atmospheric Administration's National Weather Service.
Technical Terminology
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AAir-coupled tsunami or atmospheric tsunami: tsunami-like waves generated by a rapidly moving atmospheric pressure front moving over a shallow sea at about the same speed as the waves, allowing them to couple.
Amplitude: the rise above or drop below the ambient water level as read on a tide gage.
Atmospheric tsunami or air-coupled tsunami: tsunami-like waves generated by a rapidly moving atmospheric pressure front moving over a shallow sea at about the same speed as the waves, allowing them to couple.
B
Body wave: a seismic wave that can travel through the interior of the earth. Primary (P-waves) and secondary (S-waves) are body waves.
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 bottom), which form a foamy patch at the crest and break gradually over a considerable distance; (b) plunging breakers (over fairly steep bottom gradient), 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.
Breakwater: an offshore structure such as a wall that is used to protect a harbor or beach from the force of waves.
C
Communications Plan for the Tsunami Warning System in the Pacific: an operations manual for the Tsunami Warning System in the Pacific. The plan lists the tidal and seismological stations that participate in the warning system, preferred methods of communications between the stations and the Pacific Tsunami Warning Center (PTWC), and criteria for reporting. The plan also lists recipients of tsunami watch and warning messages and methods by which the messages are sent, and provides a general overview of the operational procedures of the Tsunami Warning System and of the nature of tsunamis.
Core: the innermost layers of the earth. The inner core is solid and has a radius of about 1,300 kilometers. (The radius of the earth is about 6371 kilometers.) The outer core is fluid and is about 2,300 kilometers thick. S-waves cannot travel through the outer core.
CREST: Consolidated Reporting of Earthquakes and Tsunamis, a project funded through the Tsunami Hazard Mitigation Federal/State Working Group to upgrade regional seismic networks in Arkansas, Washington, Oregon, California, and Hawaii and provide real-time seismic information from these networks and the U.S. National Seismic Network to tsunami warning centers.
Crest length: the length of a wave along its crest. Sometimes called crest width.
Dominant tsunami period: difference between the arrival time of the highest peak and the next one measured on a water level record.
Drop: the downward change or depression in sea level associated with a tsunami, a tide, or some long-term climatic effect.
Earthquake swarm: A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.
Elastic wave: A wave that is propagated by some kind of elastic deformation, that is, a change in shape that disappears when the forces are removed. A seismic wave is a type of elastic wave.
First motion: the initial motion of the first wave, a rise in the water level is denoted by R and a fall by F.
Foreshock: a small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake
Free field offshore profile: a profile of the wave measured far enough offshore so that it is unaffected by interference from harbor and shoreline effects.
Harbor resonance: the continued reflection and interference of waves from the edge of a harbor or narrow bay which can cause amplification of the wave heights, and extend the duration of wave activity from a tsunami.
Horizontal inundation distance: the distance that a tsunami wave penetrates onto the shore, measured horizontally from the mean sea level position of the water's edge. Usually measured as the maximum distance for a particular segment of the coast.
ICG/ITSU: the International Coordination Group for the Tsunami Warning System in the Pacific. This group was established in 1965 as a subsidiary body of the Intergovernmental Oceanographic Commission (IOC) of the United Nations Educational, Scientific, and Cultural Organization (UNESCO), and is composed of national representatives from member states in the Pacific region. This group promotes cooperation and coordination of tsunami mitigation activities. It meets every two years to review progress and coordinate activities resulting in improvements to the Tsunami Warning System. There are currently 25 member states: Australia, Canada, Chile, China, Colombia, Cook Islands, Costa Rica, Democratic People's Republic of Korea, Ecuador, Fiji, France, Guatemala, Indonesia, Japan, Mexico, New Zealand, Nicaragua, Peru, Philippines, Republic of Korea, Singapore, Thailand, Federation of Russia, U.S., and Western Samoa.
Internal tsunami: a tsunami wave manifested as an internal wave and traveling along a thermocline.
ITIC: International Tsunami Information Center established in 1968 by the Intergovernmental Oceanographic Commission (IOC) of the United Nations Educational, Scientific, and Cultural Organization (UNESCO). This center, located in Honolulu, Hawaii, monitors international tsunami warning activities in the Pacific and assists with many of the activities of the International Coordination Group (ICG/ITSU) for tsunami warning in the Pacific.
Leading-depression wave: initial tsunami wave is a trough, causing a draw down of water level.
Leading-positive wave: initial tsunami wave is a crest, causing a rise in water level. Also called a leading-elevation wave.
Leading wave: first arriving wave of a tsunami. In some cases, the leading wave produces an initial depression or drop in water level, and in some cases an elevation or rise in water level.
Lg Wave: a surface wave which travels through the continental crust.
Love wave: a major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it.
Lower low water: refers to the lowest point reached by the lowest low tide in areas having mixed tides.
Major earthquake: an earthquake having a magnitude of 7 to 7.99 on the Richter scale.
Mantle: The layer of rock that lies between the crust and the outer core of the earth. It is approximately 2,900 kilometers thick and is the largest of the earth's major layers.
Maremoto: Spanish term for tsunami.
Marigram: tide gage recording showing wave height as a function of time.
Marigraph: the instrument which records wave height.
Maximum inundation: maximum horizontal penetration of the tsunami from the shoreline. A maximum inundation is measured for each different coast or harbor affected by the tsunami.
Maximum run-up: difference between the elevation of the maximum tsunami penetration and the elevation of the shoreline at the time the tsunami occurs, corrected for the difference in shoreline elevation between the time of measurement and the time of tsunami attack.
Maximum tsunami amplitude: usually measured on a water level record, it is half the value of the maximum peak-to-trough excursion, corrected for the change of tide between that peak and trough.
Maximum water level: difference between the elevation of the highest local water mark and the elevation of the shoreline at the time the tsunami occurs.
Mean height: the average height of a tsunami measured from the lowest trough to the greatest height after subtracting the change in tidal level variation.
Mean Lower Low Water (MLLW): the average low tide water elevation often used as a reference to measure run-up.
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 that have free access to the sea. Along with mean high water, mean low water, and mean lower low water, mean sea level is a type of tidal datum.
Ms: Surface Wave Magnitude: the magnitude of an earthquake as measured from the amplitude of seismic surface waves. Often referred to by the media as "Richter" magnitude.
Mw: Moment Magnitude: the magnitude based on the size and characteristics of the fault rupture, and determined from long-period seismic waves. It is a better measure of earthquake size than surface wave magnitude, especially for very large earthquakes. Calibrated to agree on average with surface wave magnitudes for earthquakes less than magnitude 7.5.
Near-field or local tsunami: a tsunami from a nearby source, generally less than 124 miles (200 kilometers) away. See also local/regional tsunami.
Period: the length of time between two successive peaks or troughs. May vary due to complex interference of waves. Tsunami periods generally range from 5 to 60 minutes.
Probable maximum water level: a hypothetical water level (exclusive of wave run-up from normal wind-generated waves) that might result from the most severe combination of hydrometeorological, geoseismic, and other geophysical factors that has virtually no risk of being exceeded. 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 tides combined with maximum probable ambient hydrological conditions such as wave level.
Reference sea level: The observed elevation differences between geodetic bench marks are processed through least-squares adjustments to determine orthometric heights referred to a common vertical reference surface, which is the reference sea level. The National Geodetic Vertical Datum of 1929 is the vertical reference system (geoid) that has been in use in the U.S. since 1929. This important vertical geodetic control system is made possible by a universally accepted, reference sea level.
Rise: the upward change or elevation in sea level associated with a tsunami, a hurricane, a tide, or some long-term climatic effect.
Run-up: the maximum height of the water onshore observed above a reference sea level. Usually measured at the horizontal inundation limit, run-up is also referred to as the amplitude or height of the tsunami. 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 occurrence, etc., and measured ideally at a point that is a local maximum of the horizontal inundation. In practical terms, run-up is only measured where there is a clear evidence of the inundation limit on the shore.
Rupture zone: the area of the earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.
Run-up distribution: set of tsunami run up values measured or observed along a coastline.
S
Sea level: the height of the sea averaged over a long time compared to tidal fluctuations. Sea level can change over the years. The level of the surface of the sea, especially its position midway between mean high and low water.
Sieberg tsunami intensity scale: a descriptive tsunami intensity scale, which was later modified into the Sieberg-Ambraseys tsunami intensity scale.
Seiche: a standing wave oscillating in a partially or fully enclosed body of water. May be initiated by long period seismic waves, wind and water waves, or a tsunami.
Seismic sea wave: a tsunami generated by an undersea earthquake.
Seismic zone: a region in which earthquakes are known to occur.
Seismograph station: a site at which one or more seismographs are set up and routinely monitored.
Significant wave height: the average height of the one-third highest waves of a given wave group.
Spreading: when reference is made to tsunami waves, it is the scattering and dispersion of the wave energy over a wider geographical area as the waves propagate away from the source region. Tsunami waves propagating across a large ocean undergo other changes in energy configuration primarily due to refraction. Geographical spreading also is very important, depending on the tsunami source region, the orientation of the tsunami generating area, the source dimensions, and the geometry of the system.
SSWWS: Seismic Sea Wave Warning System, the original tsunami warning center established in 1948 after the April 1, 1946, tsunami killed 159 people in Hawaii.
Strike-slip earthquake: an earthquake caused by a horizontal slip along a fault.
T
Teletsunami: source of the tsunami more than 620 miles (1000 kilometers) away from a land contact point. Also called a distant-source or far-field tsunami.
THRUST: the project for Tsunami Hazard Reduction Using System Technology, sponsored by the Office for U.S. Foreign Disaster Assistance/Agency for International Development. A comprehensive program to mitigate tsunami hazards in developing countries.
Thrust earthquake: an earthquake caused by slip along a gently sloping fault where the rock above the fault is pushed upwards relative to the rock below. The most common type of earthquake source of damaging tsunamis.
Tide amplitude: one-half of the difference in height between consecutive high water and low water; hence, half of the tidal range.
travel time: time (usually measured in hours and tenths of hours) that it took the tsunami to propagate from its source to a given point on a coastline.
Travel time map: a map showing isochrons of calculated tsunami travel time from the source outwards toward terminal points on distant coastlines.
Tsunami amplitude: usually measured on a water level record, it is the absolute value of either the difference between a particular peak or trough of the tsunami, and the undisturbed water level at the time.
Tsunami detectability: discovery or determination of the existence, presence, or fact of a tsunami.
Tsunami dispersion: scattering of tsunami energy, particularly as a function of its period, as it travels across a body of water.
Tsunami incident: differing tsunami incidents consist of differing possible combinations of the types of tsunamis (significant or insignificant), source areas (local or Pacific-wide), and warnings (valid, false, or no warning).
Tsunami magnitude: a number that characterizes the strength of a tsunami based on the tsunami wave amplitudes. Several different tsunami magnitude determination methods have been proposed.
Tsunami numerical modeling: 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 run-up.
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 on a coast.
Tsunami period: amount of time that a tsunami wave takes to complete a cycle. Tsunami periods typically range from 5 minutes to 2 hours.
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 300 to 600 miles per hour (500 to 1,000 kilometers per hour). The distance between successive crests can be as much as 300 to 400 miles (500 to 650 kilometers); however, in the open ocean, the height of the waves may be no more than 1 or 2 feet (30 to 60 centimeters), and the waves pass unnoticed. Tsunamis are unique in that the waveform 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 transmitted by a tsunami.
Tsunami simulation: tsunami numerical modeling.
Tsunami source: a point or area of tsunami origin, usually the site of an earthquake, volcanic eruption, or landslide, that has affected the sea floor or a body of water.
Tsunami velocity or shallow water velocity: the velocity of an ocean wave whose length is sufficiently large compared to the water depth.
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.
Tsunamic: having features analogous to those of a tsunami or descriptive of a tsunami.
Tsunamigenic: having the demonstrated or potential capability to generate a tsunami: a tsunamigenic earthquake or a tsunamigenic landslide.
Tsunamigenic earthquake: any earthquake which produces a measurable tsunami. An earthquake that produces an unusually large tsunami relative to the earthquake magnitude. Tsunami earthquakes are characterized by a very shallow focus, fault dislocations greater than several meters, and fault surfaces smaller than for normal earthquakes. They are also slow earthquakes, with slippage along their faults occurring more slowly than would occur in normal earthquakes.
TWSP: refers to the Tsunami Warning System in the Pacific.
USNSN: United States National Seismic Network, operated by the U.S. Geological Survey. Monitors, in real-time, magnitude (M)>5 earthquake activity worldwide and M>3 in conterminous U.S.; UTC Universal Coordinated Time, international common time system, also known as Greenwich Mean Time (GMT) or Zulu Time.
Valid warning: a warning for a potential tsunami, which proved to be a significant tsunami.
Water elevation: a height that water reaches.
Wave crest: the highest part of a wave. That part of the wave above the still water level.
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 measured perpendicularly to the crest.
WCM: Warning Coordination Meteorologist, regional weather service person responsible for providing information on the tsunami warning system to local agencies.
Western States Seismic Policy Council (WSSPC): a multistate consortium of state geologists and emergency managers that are dedicated to providing a forum to advance earthquake hazard reduction programs throughout the western region, and to develop, recommend, and present seismic policies and programs through information exchange, research, and education. The primary goals of the organization are to improve public understanding of seismic risk, improve earthquake preparedness, and provide a cooperative forum to enhance the transfer of mitigation technologies at the local, state, interstate, and national levels. WSSPC is a nonprofit membership organization. Its membership is comprised of two groups: member states and affiliate members. The member states are represented by their geoscience and the emergency management agencies. These members include 13 states (Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming), three U.S. territories (American Samoa, Guam, and Northern Mariana Islands), one Canadian Province (British Columbia), and one Canadian Territory (Yukon).
Affiliate members are private corporations, or private citizens residing in a member state, province, or territory. The Affiliate Members Program is open to members of the seismic hazards reduction community. In 1996 the council's board of directors approved the formation of a Tsunami Hazard Mitigation Committee. The member states are currently Hawaii, Alaska, British Columbia, Washington, Oregon, California, and Guam. The emergency management representatives from Oregon and British Columbia cochair.
Acknowledgments: This glossary was developed in concert with terms and definitions employed by NOAA's National Ocean Service, Oceanic and Atmospheric Research Service, and National Weather Service; the U.S. Geological Survey - National Earthquake Information Center; the Pacific Tsunami Museum in Hilo Hawaii; and the International Tsunami Information Center.
Many of the terms were adapted specifically from Web sites maintained by these organizations, including: 1) NOAA's Oceanic and Atmospheric Research Service; Pacific Marine Environmental Laboratory; 2) the Pacific Tsunami Museum; 3)the U.S. Geological Survey National Earthquake Information Center; 4) the International Tsunami Information Center; and the Federal Emergency Management Agency (FEMA).
Web Bibliography
Cook, Benjamin. 1995. Tsunami. University of Washington, Department of Geophysics. Seattle, Washington: [15 March 2000].
Diaz, Henry F., and Pulwarty, Roger S. (Eds.). 1997. Hurricanes: Climate and Socioeconomic Impacts. Germany: Springer-Verlag Berlin Heidelberg.
Federal Emergency Management Agency (FEMA). 1997. Multi Hazards Identification and Risk Assessment: A Cornerstone of the National Mitigation Strategy. Washington, D.C.: Federal Emergency Management Agency.
International Tsunami Information Center. 1999, October 29.Tsunami Glossary. Honolulu, Hawaii: [14 March 2000]
Pacific Tsunami Museum. 1999, November 23. Glossary of Tsunami Terms. Hilo, Hawaii: [15 March 2000]
United States. Department of Commerce. National Oceanic and Atmospheric Administration. Oceanic and Atmospheric Research Service. Pacific Marine Environmental Laboratory. 2000, March 13. Tsunami Terminology. Seattle, Washington: [16 March 2000]
United States. Department of the Interior. United States Geological Survey. National Earthquake Information Center. 2000, January 18. Glossary of Some Common Terms in Seismology. Golden, Colorado: [16 March 2000]