3. Surveys and Measurements

This section contains terms used to measure and describe tsunami waves on mareograph and in the field during a survey, and terms used to describe the size of the tsunami.

Arrival time
Time of the first maximum of the tsunami waves.

Crest length
The length of a wave along its crest. Some times called crest width.

The downward change or depression in sea level associated with a tsunami, a tide, or some long term climatic effect.

Elapsed time
Time between the maximum level arrival time and the arrival time of the first wave.

Initial rise
Time of the first minimum of the tsunami waves.

The measure of strength, force, or energy.

Runup and Inundation.

Inundation or Inundation-distance
The horizontal distance inland that a tsunami penetrates, generally measured perpendicularly to the shoreline.

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.

Tsunami inundation generated by the earthquake of 26  May 1983, at Oga aquarium  in Japan.  Photo courtesy of Takaaki Uda, Public Works Research Institute, Japan.

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.

Dark area shows inundation area from the 1964 Alaska tsunami.

Dark area shows inundation area from the 1964 Alaska tsunami.  Photo courtesy of NGDC.

Inundation line
Inland limit of wetting, measured horizontally from the mean sea level (MSL) line. The line between living and dead vegetation is sometimes used as a reference.  In tsunami science, the landward limit of tsunami runup.

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.

A number assigned to the properties of an event such that the event can be compared with other events of the same class.

Mean height
Average height of a tsunami measured ftom the trough to the crest after removing the tidal variation.

A flowing over; inundation.

Post-tsunami survey 
Tsunamis are relatively rare events and most of their evidence is perishable. Therefore, it is very important that reconnaissance surveys be organized and carried out quickly and thoroughly after each tsunami occurs, to collect detailed data valuable for hazard assessment, model validation, and other aspects of tsunami mitigation.

After a major tsunami, physical oceanographers, social scientists, and engineers conduct post-tsunami surveys to collect information.

After a major tsunami, physical oceanographers, social scientists and engineers conduct post-tsunami surveys to collect information. These data, including runup and inundation, deformation, scour, building and structural impact, wave arrival descriptions, and social impact, are important for designing better mitigation to reduce the impacts of tsunami on life and property.  Photo courtesy of Philip Liu, Cornell University.

In recent years, following each major destructive tsunami, a post-tsunami reconnaissance survey has been organized to make measurements of runups and inundation limits and to collect associated data from eyewitnesses such as the number of waves, arrival time of waves, and which wave was the largest. The surveys have been organized primarily on an ad-hoc basis by international academic tsunami researchers. A Post-Tsunami Survey Field Guide (http://www.tsunamiwave.info/itic/contents.php?id-28) has been prepared by the PTWS to help with preparations of surveys, to identify measurements and observations to be taken, and to standardize data collections. The Tsunami Bulletin Board e-mail service has also been used for quickly organizing international surveys and for sharing of the observations from impacted areas.

Post-tsunami survey measuring runup along a transect inland from the coast.

Post-tsunami survey measuring runup along a transect inland from the coast.  Courtesy of ICMAM, Chennai, DOD, India.

Drawdown of sea level prior to tsunami flooding. The shoreline moves seaward, sometimes by a kilometre or more, exposing the sea bottom, rocks, and fish.  The recession of the sea is a natural warning sign that a tsunami is approaching.

North Shore, Oahu, Hawaii.  09 March 1957 Aleutian Island tsunami.

North Shore, Oahu, Hawaii. During the 9 March 1957 Aleutian Island tsunami, people foolishly explored the exposed reef, unaware that tsunami waves would return in minutes to inundate the shoreline.  Photo by A. Yamauchi, courtesy of Honolulu Star-Bulletin.

The upward change or elevation in sea level associated with a tsunami, a tropical cyclone, storm surge, the tide, or other long term climatic effect.

1) Difference between the elevation of maximum tsunami penetration (inundation line) and the sea level at the time of the tsunami. In practical terms, runup is only measured where there is a clear evidence of the inundation limit on the shore.

2) 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 attack, 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.

Runup distribution  Set of tsunami runup values measured or observed along a coastline.

Tsunami stripped forested hills of vegetation leaving clear marker of tsunami runup, Banda Aceh, 26 December 2004 Sumutra tsunami.

Tsunami stripped forested hills of vegetation leaving clear marker of tsunami runup, Banda Aceh, 26 December 2004 Sumutra tsunami. Photo courtesy of Yuichi Nishimura, Hokkaido University.

Banda Aceh, Indonesia, 26 December 2004.

Runup can often be inferred from the vertical extent of dead vegetation, from debris normally found at ground level that are observed stuck on electric wires, in trees, or at other heights, and from water line marks left on building walls.  In extreme cases, cars, boats, and other heavy objects have been lifted and deposited atop buildings. Banda Aceh, Indonesia, 26 December 2004. Photo courtesy of C. Courtney, Tetra Tech EMI.

Sieberg tsunami intensity scale A descriptive tsunami intensity scale which was later modified into the Sieberg-Ambraseys tsunami intensity scale described below (Ambraseys 1962).

Modified Sieberg Sea-wave Intensity Scale
1) Very light. Wave so weak as to be perceptible only on tide-gauge records.

2) Light. Wave noticed by those living along the shore and familiar with the sea. On very flat shores generally noticed.

3) Rather strong. Generally noticed. Flooding of gently sloping coasts. Light sailing vessels or small boats carried away on shore. Slight damage to light structures situated near the coast. In estuaries reversal of the river flow some distance upstream.

4) Strong. Flooding of the shore to some depth.  Light scouring on man-made ground. Embankments and dikes damaged. Light structures near the coasts damaged. Solid structures on the coast injured. Big sailing vessels and small ships carried inland or out to sea. Coasts littered with floating debris.

5) Very strong. General flooding of the shore to some depth. Breakwater walls and solid structures near the sea damaged. Light structures destroyed. Severe scouring of cultivated land and littering of the coast with floating items and sea animals. With the exception of big ships all other type of vessels carried inland or out to sea. Big bores in estuary rivers. Harbour works damaged. People drowned. Wave accompanied by strong roar.

6) Disastrous. Partial or complete destruction of man-made structures for some distance from the shore. Flooding of coasts to great depths. Big ships severely damaged. Trees uprooted or broken.  Many casualties.

Significant wave height
The average height of the one-third highest waves of a given wave group. Note that the composition of the highest waves depends upon the extent to which the lower waves are considered. In wave record analysis, the average height of the highest one-third of a selected number of waves, this number being determined by dividing the time of record by the significant period. Also called characteristic wave height.

When reference is made to tsunami waves, it is the spreading of the wave energy over a wider geographical area as the waves propagate away from the source region. The reason for this geographical spreading and reduction of wave energy with distance traveled, is the sphericity of the earth. The tsunami energy will begin converging again at a distance of 90 degrees from the source. Tsunami waves propagating across a large ocean undergo other changes in configuration primarily due to refraction, but geographical spreading is also very important depending upon the orientation, dimensions and geometry of the tsunami source.

Subsidence (uplift) 
The permanent movement of land down (subsidence) or up (uplift) due to geologic processes, such as during an earthquake.

The 26 December 2004 earthquake resulted in 1.2 m of land subsidence in the Car Nicobar.

The 26 December 2004 earthquake resulted in 1.2 m of land subsidence in the Car Nicobar, Nicobar Islands, India leaving houses that were once above sea level now permanently submerged.  Photo courtesy of ICMAM, Chennai, DOD, India.

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.

Mareogram (sea level) record of a tsunami.

Mareogram (sea level) record of a tsunami.

Tsunami intensity
Size of a tsunami based on the macroscopic observation of a tsunami's effect on humans, objects, including various sizes of marine vessels, and buildings. 

The original scale for tsunamis was published by Sieberg (1923), and later modified by Ambraseys (1962) to create a six-category scale.  Papadopoulus and Imamura (2001) proposed a new 12-grade intensity scale which is independent of the need to measure physical parameters like wave amplitude, sensitive to the small differences in tsunami effects, and detailed enough for each grade to cover the many possible types of tsunami impact on the human and natural environment. The scale has 12 categories, similar to the Modified Mercalli Intensity Scale used for macroseismic descriptions of earthquake intensity.

Tsunami magnitude
Size of a tsunami based on the measurement of the tsunami wave on sea level gauges and other instruments.

The scale, originally descriptive and more similar to an intensity, quantifies the size by using measurements of wave height or tsunami runup.  Iida et al. (1972) described the magnitude (m) as dependent in logarithmic base 2 on the maximum wave height measured in the field, and corresponding to a magnitude range from -1 to 4:

                          m = log2 Hmax

Hatori (1979) subsequently extended this so-called Imamura-Iida scale for far-field tsunamis by including distance in the formulation.  Soloviev (1970) suggested that the mean tsunami height may be another good indicator of tsunami size, and the maximum intensity would be that measured nearest to the tsunami source. A variation on this is the Imamura-Soloviev intensity scale I (Soloviev, 1972). Shuto (1993) has suggested the measurement of H as the height where specific types of impact or damage occur, thus proposing a scale which can be used as a predictive quantitative tool for macroscopic effects.

Tsunami magnitudes have also been proposed that are similar in form to those used to calculate earthquake magnitudes. These include the original formula proposed by Abe (1979) for tsunami magnitude, Mt:

                        Mt = logH + B

where H is the maximum single crest or trough amplitude of the tsunami waves (in metres) and B is a constant, and the far-field application proposed by Hatori (1986) which adds a distance factor  into  the  calculation.

Tsunami period
Amount of time that a tsunami wave takes to complete a cycle. Tsunami periods typically range from five minutes to two hours.

Tsunami period (dominant)
Difference between the arrival time of the highest peak and the next one measured on a water level record.

Tsunami wave length
The horizontal distance between similar points on two successive waves measured perpendicular to the crest. The wave length and the tsunami period give information on the tsunami source. For tsunamis generated by earthquakes, the typical wave length ranges from 20 to 300 km. For tsunamis generated by landslides, the wave length is much shorter, ranging from hundreds of metres to tens of kilometres.

Water level (maximum)
Difference between the elevation of the highest local water mark and the elevation of the sea-level at the time of the tsunami. This is different from maximum run-up because the water mark is often not observed at the inundation line, but maybe halfway up the side of a building or on a tree trunk.

Wave crest
1) The highest part of a wave.

2) That part of the wave above still water level.

Water trough
The lowest part of a wave.



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