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.

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, 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.

Massive destruction in the town of Aonae on Okushiri Island, Japan caused by the regional tsunami of 12 July 1993.

Massive destruction in the town of Aonae on Okushiri Island, Japan caused by the regional tsunami of 12 July 1993.  Photo  courtesy of Dr. Eddie Bernard, NOAA PMEL

Banda Aceh, Sumatra, Indonesia before 26 December 2004 tsunami.

Banda Aceh, Sumatra, Indonesia after 26 December 2004 tsunami.

Banda Aceh, Sumatra, Indonesia. The tsunami of 26 December 2004 completely razed coastal towns and villages, leaving behind only sand, mud, and water where once there had been thriving communities of homes, offices, and green space.  Photo courtesy of DigitalGlobe.

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 waves.

Tsunamis can be generated by submarine landslides, or by subaerial landslides that enter the water.

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.

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.

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

Global tsunami source zones.

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.

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, 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 destruction to property.  Because of their destructiveness, tsunamis have important impacts on the human, social and economic sectors of societies. Historical records show that enormous destruction of coastal communities throughout the world has taken place and that the socio-economic impact of tsunamis in the past has been enormous. In the Pacific Ocean where the majority of these waves have been generated, the historic record shows tremendous destruction with extensive loss of life and property.

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, and South America, although records for these areas are not as extensive. The last major Pacific-wide tsunami occurred in 1960. Many other local and regional destructive tsunamis have occurred with more localized effects.

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

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

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 modeled.

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.

Historical data are often very limited for most coastlines.  Consequently, numerical modelling may be the only way to estimate potential risk.  Techniques now exist to carry out this assessment.  Computer software and the training necessary to conduct this modelling are available through programmes such as the IOC Tsunami Inundation Modelling Exchange (TIME) Programme.

Calculated maximum tsunami wave heights for a M9.0 Cascadia subduction zone earthquake.

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 tsunmi.

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.

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.

International tsunami hazard sign.


Evacuation Route sign used in Chile.

Tsunami evacuation route sign, Chile.

    

Tsunami evacuation building and safe place signs, Japan.

 

        

Tsunami evacuation area signs, Hawaii, 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.

Tsunami propagation - 30 July 1995.

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 which 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 of gravity h the water depth.
Thus, the velocity of shallow-water waves is independent of wave length L. In water depths between 1/2 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.

Wave height and water depth.  In the open ocean, a tsunami is often only a tens of centimeters 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.

Tsunamic
Having features analogous to those of a tsunami or descriptive of a tsunami.

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

Destruction of Hilo Harbor, Hawaii 01 April 1946.

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.

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