1. Tsunami Classification

01 April 1946, 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 (km) 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 results in a receding of the waterline, sometimes by a kilometer 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 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 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.

Maremoto
Spanish term for tsunami.

22 May 1960 Chilean tsunami.

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. 

Numerical Modelling

Numerical modelling snapshots of the water surface 10 minutes after a pyroclastic flow on the southeastern part of Monserrat Island led to a submarine landslide and the generation of a tsunami.  Courtesy of LDG-France.

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, meaning their destructive effects are confined to coasts within a hundred km, or up to a thousand km, respectively, of the source -- usually an earthquake. It follows many tsunami related casualties and considerable property damage also comes from these tsunamis. Between 1975 and 2007 there were 34 local or regional tsunami in the Pacific and adjacent seas that resulted in deaths and property damage.

For example, 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 without an event, 11 locally destructive tsunamis occurred in just a seven-year period from 1992 to 1998, resulting in over 5,300 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 programmes of tsunami preparedness and education can be put in place.

Pacific Ocean


Indian Ocean


Mediterranean Sea


Caribbean Sea 

The four images above show 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:  National Geophysical Data Center / World Data Center.

 

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 60% 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 causing damage or casualties. These data are based on historical records. Source: National Geophysical Data Center / World Data Center.


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 26 destructive ocean-wide tsunamis and 9 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, 2,000,000 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 138 in Japan. Estimated damages were US $50 million in Japan, US $24 million in Hawaii and several millions 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 at Hilo, Hawaii, and 6 metres 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 a 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 1 million people were displaced, losing their homes, property, and their livelihoods.  The magnitude of death and destructiveness caused immediate response by the world's leaders and led to the development of the Indian Ocean tsunami warning and mitigation system in 2005.  The event also raised awareness of tsunami hazards globally, and new systems were established in the Caribbean, the Mediterranean and Atlantic.

 


26 December 2004 tsunami.

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 traveling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below or near the ocean floor. (Also called seismic sea wave and, incorrectly, tidal wave). Volcanic eruptions, submarine landslides, and coastal rockfalls can also generate tsunami, 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.

01 April 1946, Aleutian Island tsunami.

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 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), 1994 (Java), 1996 (Chimbote, Peru) and 2006 (Java).

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 26 December 2006 Indian Ocean tsunami.

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.

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