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Tsunami Terminology

amplitude: The rise above or drop below the ambient water level as read on a tide gage.

arrival time: Time of arrival, usually of the first wave, of the first wave of the tsunami at a particular location.

bore: Traveling wave with an abrupt vertical front or wall of water. Under certain conditions, the leading edge of a tsunami wave may form a bore as it approaches and runs onshore. A bore may also be formed when a tsunami wave enters a river channel, and may travel upstream penetrating to a greater distance inland than the general inundation.

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 AK, WA, OR, CA, and HI and provide real-time seismic information from these networks and the USNSN to the tsunami warning centers.

ETA: Estimated Time of Arrival. Computed arrival time of the first tsunami wave at coastal communities after a specific earthquake has occurred.

first motion: Initial motion of the first wave, a rise in the water level is denoted by R, a fall by F.

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, a United Nations organization under UNESCO responsible for international tsunami cooperation.

IDNDR: International Decade for Natural Disaster Reduction, a United Nations sponsored program for the 1990's.

inundation: The depth, relative to a stated reference level, to which a particular location is covered by water.

inundation area: An area that is flooded with water.

ITIC: International Tsunami Information Center established in 1965. Monitors international activities of the Pacific Tsunami Warning Center and assists with many of the activities of ICG/ITSU.

inundation: The depth, relative to a stated reference level, to which a particular location is covered by water.

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.

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.

local/regional tsunami: Source of the tsunami within 1000 km of the area of interest. Local or near-field tsunami has a very short travel time (30 minutes or less), mid-field or regional tsunami waves have travel times on the order of 30 minutes to 2 hours. Note: " Local" tsunami is sometimes used to refer to a tsunami of landslide origin.

maremoto: Spanish term for tsunami.

marigram: Tide gage recording showing wave height as a function of time.

marigraph: The instrument which records wave height.

Mean Lower Low Water (MLLW): The average low tide water elevation often used as a reference to measure runup.

Ms: Surface Wave Magnitude. 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. 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.

NOAA: National Oceanic and Atmospheric Administration, the federal agency responsible for tsunami warnings and monitoring. Part of the Department of Commerce.

NWS: National Weather Service, the branch of NOAA which operates the tsunami warning centers and disseminates warnings.

normal earthquake: An earthquake caused by slip along a sloping fault where the rock above the fault moves downwards relative to the rock below.

Pacific Disaster Center (PDC): An information processing center to support emergency managersin the Pacific region. Funded by the U.S. Department of Defense.

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.

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.

runup: Maximum height of the water onshore observed above a reference sea level. Usually measured at the horizontal inundation limit.

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.

strike-slip earthquake: An earthquake caused by horizontal slip along a fault.

SSWWS: Seismic Sea Wave Warning System, the original tsunami warning center established in 1948 after the April 1, 1946 tsunami killed 159 in Hawaii.

teletsunami: Source of the tsunami more than 1000 km away from area of interest. 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.

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.

TIME: The Center for the Tsunami Inundation Mapping Effort, to assist the Pacific states in developing tsunami inundation maps.

travel time: Time (usually measured in hours and tenths of hours) that it took the tsunami to travel from the source to a particular location.

tsunami: A Japanese term derived from the characters "tsu" meaning harbor and "nami" meaning wave. Now generally accepted by the international scientific community to describe a series of travelling waves in water produced by the displacement of the sea floor associated with submarine earthquakes, volcanic eruptions, or landslides.

tsunami earthquake: A tsunamigenic earthquake which produces a much larger tsunami than expected for its magnitude.

tsunamigenic earthquake: Any earthquake which produces a measureable tsunami.

tsunami magnitude: A number which characterizes the strength of a tsunami based on the tsunami wave amplitudes. Several different tsunami magnitude determination methods have been proposed.

TWS: Tsunami Warning System, organization of 26 Pacific Member States which coordinates international monitoring and warning dissemination. Operates through ICG/ITSU

USNSN: United States National Seismic Network, operated by the USGS. Monitors, in real-time, magnitude (M)>5 earthquake activity worldwide and M>3 in conterminous US.UTCUniversal Coordinated Time, international common time system, formerly GMT (Greenwich Mean Time).

UTC: Universal Coordinated Time, international common time system (formerly GMT, Greenwich Mean Time).

WC/ATWC: West Coast/ Alaska Tsunami Warning Center, established in 1967 originally to issue warnings to Alaska of local tsunami events. WC/ATWC is now responsible for issuing warnings for any event likely to impact either Alaska, the west coast of the US, or the Pacific coast of Canada.

WCM: Warning Coordination Meteorologist, regional weather service person responsible for providing information on the tsunami warning system to local agencies.

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