Tsunami - Definition and Facts
Type
Geohazards
Cluster
Coordinating agency or organisation
- United Nations Educational, Scientific and Cultural Organization (UNESCO)
- Intergovernmental Oceanographic Commission (IOC-UNESCO)
- British Geological Survey (BGS)
Tsunami is the Japanese term meaning wave (‘nami’) in a harbour (‘tsu’). It is 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 (IOC, 2019).
Primary reference(s)
Additional scientific description
A tsunami may also be referred to as a ‘seismic sea wave’ and, incorrectly, a ‘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 of 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 (IOC, 2019).
Tsunami-like phenomena generated by meteorological or atmospheric disturbances are known as meteotsunami (UNESCO and IOC, 2019).
The Intergovernmental Oceanographic Commission (IOC) uses the following terms to assess the scale and impact of a tsunami (IOC, 2019):
- Travel time: Time required for the first tsunami wave to propagate from its source to a given point on a coastline.
- Arrival time: Time of the first maximum of the tsunami waves. Inundation or Inundation-distance: The horizontal distance inland that a tsunami penetrates, generally measured perpendicularly to the shoreline.
- Inundation (maximum): Maximum horizontal penetration of the tsunami from the shoreline. A maximum inundation is measured for each different coast or harbour affected by the tsunami.
- Inundation area: Area flooded with water by the tsunami. Inundation height: Elevation reached by seawater measured relative to a stated datum such as mean sea level or the sea level at the time of tsunami arrival, at a specified inundation distance. Inundation height is the sum of the flow depth and the local topographic height. Sometimes referred to as tsunami height.
- Inundation line: Inland limit of wetting measured horizontally from the mean sea level line. The line between living and dead vegetation is sometimes used as a reference. In tsunami science, the landward limit of tsunami run-up.
- Leading wave: First arriving wave of a tsunami. In some cases, the leading wave produces an initial depression or drop in sea level, and in other cases, an elevation or rise in sea level. When a drop in sea level occurs, sea level recession is observed
- Mean height: Average height of a tsunami measured from the trough to the crest after removing the tidal variation.
Run-up
- Difference between the elevation of maximum tsunami penetration (inundation line) and the sea level at the time of the tsunami. In practical terms, run up is only measured where there is clear evidence of the inundation limit on the shore.
- 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 event, 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.
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.
Tsunami period: Amount of time that a tsunami wave takes to complete a cycle, or one wavelength. Tsunami periods typically range from 5 to 60 minutes. Tsunami period is often measured as the difference between the arrival time of the highest peak and the next one measured on a water level record.
Tsunami wavelength: The horizontal distance between similar points on two successive waves measured perpendicular to the crest. The wavelength and the tsunami period give information on the tsunami source. For tsunamis generated by earthquakes, the typical wavelength ranges from 20 to 300 km. For tsunamis generated by landslides, the wavelength is much shorter, ranging from hundreds of metres to tens of kilometres.
For more terms see IOC (2019).
Metrics and numeric limits
Not available.
Key relevant UN convention / multilateral treaty
Not available.
Related UN Observances
World Tsunami Awareness Day - 5 November
Examples of drivers, outcomes and risk management
Tsunamis are created by an underwater disturbance such as an earthquake, landslide, volcanic eruption, and meteorite or generated by meteorological or atmospheric disturbances.
Primary hazards/damage. 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 (IOC, 2019:6).
Tsunami associated wave forces have demolished frame buildings and other structures. Considerable damage is also 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 (IOC, 2019:6).
Secondary hazards/damage can result from sewage and chemical pollution following the destruction. Damage of intake, discharge, and storage facilities can also present dangers. Of increasing concern is the potential effect of tsunami drawdown, when receding waters uncover cooling water intakes associated with nuclear power plants (IOC, 2019:7).
Risk management for tsunamis includes guidelines on tsunami risk assessment/management. Examples include IOC (2015) and UNDRR (2017).
Regional Coordination and Centres: The IOC is coordinating the implementation of a global tsunami warning system, building upon its experiences in the Pacific to establish regional warning systems for the Indian Ocean (IOTWMS); Caribbean Sea (ICG-CARIBE-EWS); and the North-eastern Atlantic, the Mediterranean and connected seas (ICG-NEAMTWS). The regional systems coordinate international tsunami warning and mitigation activities, including the issuance of timely and understandable tsunami bulletins to IOC Member States.
The Intergovernmental Coordination Group for Tsunamis addresses tsunami risk globally through the following groups:
ICG-PTWS Intergovernmental Coordination Group for the Pacific Tsunami Warning and Mitigation System, formerly ICG/ITSU, was renamed by Resolution EC-XXXIX.8 of the IOC Executive Council in 2006 as proposed by the International Coordination Group for the Tsunami Warning System in the Pacific at its 20th Session in 2005 (Recommendation ITSU-XX.1). There are presently 46 Member States in the ICG-PTWS. ICG/ITSU, the International Coordination Group for the Tsunami Warning System in the Pacific was established by Resolution IV-6 of the 4th Session of the IOC Assembly in 1965. The Pacific Tsunami Warning Center (PTWC) serves as the Tsunami Service Provider (TSP) for the Pacific Ocean. Other TSPs for specific regions of the Pacific Ocean are the North West Pacific Tsunami Advisory Center (NWPTAC) and the South China Sea Tsunami Advisory Center (SCSTAC). The ICG-PTWS presently comprises over 40 Member States and oversees warning system operations and facilitates coordination and cooperation in all international tsunami mitigation activities.
ICG-IOTWMS The Intergovernmental Coordination Group for the Indian Ocean Tsunami Warning and Mitigation System (ICGIOTWMS) was formed in response to the tragic tsunami on December 26th 2004, in which over 230,000 lives were lost around the Indian Ocean region. The ICG-IOTWMS comprises 28 Member States. There are three TSPs in the Indian Ocean, hosted by the governments of Australia, Indian and Indonesia.
ICG-NEAMTWS The Intergovernmental Coordination Group for the Tsunami Early Warning and Mitigation System in the Northeastern Atlantic, the Mediterranean and connected seas (ICG-NEAMTWS) was formed in response to the tragic tsunami on 26 December 2004, in which over 230,000 lives were lost around the Indian Ocean region (Indian Ocean Tsunami Information Centre, no date). The ICG-NEAMTWS consists of Member States bordering the North-eastern Atlantic and those bordering and within the Mediterranean and connected seas. There are currently five accredited Tsunami Service Providers (France, Greece, Italy, Portugal, Turkey) in the NEAM region providing tsunami services and alerts to subscribing Member States.
ICG-CARIBE-EWS The Intergovernmental Coordination Group for the Tsunami and Other Coastal Hazards Warning System for the Caribbean and Adjacent Regions (ICG-CARIBE-EWS) was established in 2005 and currently comprises 32 Member States and 16 Territories in the Caribbean.
Tsunami Service Providers (TSPs) are centres that monitor seismic and sea level activity and issue timely tsunami threat information within an ICG framework to National Tsunami Warning Centres (NTWCs) / Tsunami Warning Focal Points (TWFPs) and other TSPs operating within an ocean basin. The NTWCs / TWFPs may use these products to develop and issue tsunami warnings for their countries. TSPs may also issue public messages for an ocean basin and act as NTWCs providing tsunami warnings for their own countries. Currently there are nine operational TSPs.
National Tsunami Warning Centres (NTWCs) are a centre officially designated by the government to monitor and issue tsunami warnings and other related statements within their country according to established national Standard Operating Procedures.
World Tsunami Awareness Day, 5 November every year: The United Nations, through UN Resolution 70/203 adopted on 22 December 2015, has designated 5 November as World Tsunami Awareness Day (UNDRR, 2020). The day aligns with the International Day for Disaster Reduction (13 October) and the seven targets of the Sendai Framework for Disaster Risk Reduction 2015–2030 (ITIC, 2020). The IOC is a key international partner of the UNDRR on World Tsunami Awareness Day.
Tsunami Ready is a voluntary community recognition programme that promotes tsunami hazard preparedness as an active collaboration among federal, state/territorial and local emergency management agencies, community leaders and the public. The main goal of the programme is to improve public safety before, during and after tsunami emergencies. It aims to do this by establishing guidelines for a standard level of capability to mitigate, prepare for and respond to tsunamis, and working with communities to help them meet the guidelines and ultimately become recognised as ‘tsunami ready’ by the National Weather Service. It was first implemented in the United States .To date, there are 26 IOC-UNESCO Tsunami Ready recognised communities in 18 countries and territories, excluding those implemented in the United States.
Community engagement with evacuation zones and the ‘blue lines’ project In New Zealand, the Wellington Region Emergency Management Office has developed the Blue Line Project in collaboration with communities in Wellington’s southern coastal suburbs. In this project, the local community helps to plan evacuation routes and safe locations based on indicative evacuation zone mapping, and blue lines are painted on the road surface at the maximum estimated tsunami inundation extent. Accompanying evacuation signage is installed. Community members are engaged early in the project, publicising the work and helping to develop blue line locations, evacuation zone maps and information boards. The communities participating in the Blue Line Project can be considered to have a higher degree of public education regarding tsunami evacuation than other communities (Fraser et al., 2016). Other communities around the world have used similar community engagement strategies.
This article was originally published in the United Nations Office for Disaster Risk Reduction (UNDRR) on this link Tsunami | UNDRR (https://www.undrr.org/understanding-disaster-risk/terminology/hips/gh0006)
References
Fraser, S.A., E. Doyle, K.C. Wright, S.H. Potter, J. McClure, D.M. Johnston, G.S. Leonard, M.A. Coomer, J.S. Becker and S. Johal, 2016. Tsunami response behaviour during and following two local source earthquakes in Wellington, New Zealand. International Journal of Disaster Risk Reduction, 16:123-133.
Indian Ocean Tsunami Information Centre, no date. What is IOTWS?. Accessed 21 April 2021.
IOC, 2015. Revised Guidelines on Tsunami Risk Assessment and Mitigation for the Indian Ocean Intergovernmental Oceanographic Commission (IOC). Accessed 14 October 2020.
IOC, 2019. Tsunami Glossary, 2019. Fourth Edition. IOC/2008/TS/85 rev. 4. Intergovernmental Oceanographic Commission (IOC). Accessed 14 October 2020.
ITIC, 2020. World Tsunami Awareness Day on November 5th . International Tsunami Information Centre (ITIC). Accessed 14 October 2020.
Kong, L.S.L., 2013. Pacific Tsunami Warning and Mitigation System (PTWS). In: Bobrowsky P.T. (eds) Encyclopedia of Natural Hazards. Encyclopedia of Earth Sciences Series. Springe. Accessed 21 April 2021
UNDRR, 2017. Words into Action Guidelines: National Disaster Risk Assessment Tsunami hazard and risk assessment. Accessed 21 April 2021.
UNDRR, 2020. World Tsunami Awareness Day. Accessed 21 April 2021.
UNESCO and IOC, 2019. World Tsunami Awareness Day. Accessed 21 April 2021.
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