World-Wide Tsunamis: 2000 B.C. - to the present
- Introduction
- Worldwide Occurrence of Tsunamis
- Tsunamis Characteristics
- Data Reliability:
- Caveat (from Introduction to Catalog of Significant Earthquakes)
- Uncertainties in the Significant Earthquake and Tsunami Databases
Introduction
The NOAA/WDS tsunami database is a listing of historical tsunami source events and runup locations throughout the world that range in date from 2000 B.C. to the present. The events were gathered from scientific and scholarly sources, regional and worldwide catalogs, tide gauge data, deep ocean sensor data, individual event reports, and unpublished works. As of 2019, there over 2,200 source events in the database with event validities >0 (-1 = erroneous entry, 0 = seiche). The global distribution of these events is 70% Pacific Ocean, 15% Mediterranean Sea, 9% Caribbean Sea and Atlantic Ocean, and 6% Indian Ocean. There are over 27,000 runup locations where tsunami effects were observed.
Tsunamis, commonly called seismic sea waves--or incorrectly, tidal waves--have been responsible for over 500,000 fatalities throughout the world (227,899 were from the 2004 Indian Ocean earthquake and tsunami). These events are somewhat rare. Major tsunamis occur in the Pacific Ocean region only about once per decade. Therefore, it is important to learn as much as possible from the relatively short history available.
"Tsunami" is a Japanese word meaning "harbor wave." It is a water wave or a series of waves generated by an impulsive vertical displacement of the surface of the ocean or other body of water. Other terms for "tsunami" found in the literature include: seismic sea wave, Flutwellen, vloedgolven, raz de mare, vagues sismique, maremoto, and, incorrectly, tidal wave. The term "tidal wave" is frequently used in the older literature and in popular accounts, but is now considered incorrect. Tides are produced by the gravitational attraction of the sun and moon and occur predictably with twelve hour periods. The effects of a tsunami may be increased or decreased depending on the level of the tide, but otherwise the two phenomena are independent.
Worldwide Occurrence of Tsunamis
Tsunamis have been reported since ancient times. They have been documented extensively, especially in Japan and the Mediterranean areas. The first recorded tsunami occurred off the coast of Syria in 2000 B.C. Since 1900 (the beginning of instrumentally located earthquakes), most tsunamis have been generated in Japan, Peru, Chile, New Guinea and the Solomon Islands. However, the only regions that have generated remote-source tsunamis affecting the entire Pacific Basin are the Kamchatka Peninsula, the Aleutian Islands, the Gulf of Alaska, and the coast of South America. Hawaii, because of its location in the center of the Pacific Basin, has experienced tsunamis generated in all parts of the Pacific.
The Mediterranean and Caribbean Seas both have small subduction zones, and have histories of locally destructive tsunamis. Only a few tsunamis have been generated in the Atlantic Ocean. In the Atlantic Ocean, there are no subduction zones at the edges of plate boundaries to spawn such waves except small subduction zones under the Caribbean and Scotia arcs.
In the Indian Ocean, the Indo-Australian plate is being subducted beneath the Eurasian plate at its east margin. On December 26, 2004, an earthquake off the coast of northern Sumatra generated a tsunami that was recorded nearly world-wide and killed more people than any other tsunami in recorded history. More than 227,899 people were either killed or listed as missing and presumed dead and 1,126,900 were displaced by the earthquake and subsequent tsunami. The estimated economic losses exceed $10 billion. The devastating megathrust earthquake of December 26th, 2004 occurred on the interface of the India and Burma plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate begins its descent into the mantle at the Sunda trench which lies to the west of the earthquake's epicenter. The trench is the surface expression of the plate interface between the Australia and India plates, situated to the southwest of the trench, and the Burma and Sunda plates, situated to the northeast.
Tsunami Characteristics
Most tsunamis are caused by a rapid vertical movement along a break in the Earth's crust (i.e., their origin is tectonic). A tsunami is generated when a large mass of earth on the bottom of the ocean drops or rises, thereby displacing the column of water directly above it. This type of displacement commonly occurs in large subduction zones, where the collision of two tectonic plates causes the oceanic plate to dip beneath the continental plate to form deep ocean trenches. Most subduction occurs along most of the island arcs and coastal areas of the Pacific, the notable exception being the west coast of the United States and Canada. Movement along the faults there is largely strike-slip, having little vertical displacement, and the movement produces few local tsunamis.
Volcanoes have generated significant tsunamis with death tolls as large as 30,000 people from a single event. Roughly one fourth of the deaths occurring during volcanic eruptions where tsunamis were generated, were the result of the tsunami rather than the volcano. A tsunami is an effective transmitter of energy to areas outside the reach of the volcanic eruption itself. The most efficient methods of tsunami generation by volcanoes include disruption of a body of water by the collapse of all or part of the volcanic edifice, subsidence, an accompanying or preceding the eruption. Roughly one-half of all volcanic tsunamis are generated at calderas or at cones within calderas. Submarine eruptions may also cause minor tsunamis.
Locally destructive tsunamis may be generated by subaerial and submarine landslides into bays or lakes. Lituya Bay, Alaska, has been the site of several landslide-generated tsunamis, including one in 1958 that produced a splash wave that removed trees to a height of 525 m. It also caused a tsunami of at least 50 m in the bay. The 1964 Prince William Sound earthquake triggered at least four submarine landslides, which accounted for 71 to 82 of the 106 fatalities in Alaska for the 1964 event. However, it is tectonic earthquake-generated tsunamis (those produced by a major deformation of Earth's crust) that may affect the entire Pacific Basin.
Other possible but less efficient methods of tsunami generation include: strong oscillations of the bottom of the ocean, or transmission of energy to a column of water from a seismic impulse (e.g., a deep-focus earthquake that has no surface rupture); transmission of energy from a horizontal seismic impulse to the water column through a vertical or inclined wall such as a bathymetric ridge; strong turbidity currents; underwater and above-water explosions. Several mechanisms commonly are involved in the generation of a tsunami (e.g., vertical movement of the crust by a seismic impulse or an earthquake, and a submarine landslide).
Our knowledge of tsunami generation is incomplete, because the generation phenomena has not been observed nor measured directly. However, studies of tsunami data suggest that the size of a tsunami is directly related to: the shape of the rupture zone, the rate of displacement and sense of motion of the ocean-floor in the source (epicentral) area, the amount of displacement of the rupture zone, and the depth of the water in the source area.
It is also observed that long-period tsunamis are generated by large-magnitude earthquakes associated with seafloor deformation of the continental shelf; while, shorter period tsunamis are generated by smaller magnitude earthquakes associated with seafloor deformation in deeper water beyond the continental shelf.
Once the energy from an undersea disturbance has been transmitted to the column of water, the wave can propagate outward from the source at a speed of more than 1,000 km per hour depending on the depth of the water. Because the height of the long-period waves in the open ocean is commonly 1 m or less and their wavelength is hundreds of kilometers, they pass unnoticed by observers in ships or planes in the velocity of its waves is reduced, and the height of each wave increases. The waves pile up on shore especially in the region of the earthquake source, producing a "local tsunami." Some dramatic examples of such local tsunamis include those generated by landslides or by volcanic eruptions, which have caused "runup" heights of 30 to 50 m in some coastal areas.
"Runup" is the maximum height of the water observed above a reference sea level. Two other terms may be determined from the runup value: (1) tsunami magnitude, which is defined (Iida and others, 1967) as
- m = log2H
and (2) tsunami intensity, which is defined (Soloviev and Go, 1974) as
- I = log2(21/2 * H),
where H in both equations is the maximum runup height of the wave.
If the energy produced by the generating disturbance is sufficiently large, such as that released by a major deformation of the crust in a trench area, the resulting tsunami wave may cross the open ocean and emerge as a destructive wave many thousands of kilometers from its source. The severity of a tsunami of this type--called a "remote-observed and perhaps cause damage throughout the Pacific Ocean Basin (e.g., the Chile tsunami of May 1960).
Radiation of a remote-source tsunami from the focus of an earthquake is directional, depending on the geometry of the seafloor in the source region. The source region for major tectonic earthquakes is usually elliptical, and the major axis is as much as 600 km long and corresponds to the activated part of the fault. The major part of the tsunami energy is transmitted at right angles to the direction of the major axis, both toward the near shore and along a great circle path toward the shore on the opposite side of the ocean. Thus, tsunamis in Chile have severe impact on Japan; and those in the Gulf of Alaska on the west coast of North America. Hawaii, which lies in the central Pacific Basin, is vulnerable to remote-source tsunamis generated both in the North Pacific and along the coast of South America.
The velocity (V) of a tsunami in the open ocean is expressed as the product of the square root of the depth of the water (d) and the acceleration of the force of gravity (g).
- V = (dg)1/2
Because the speed of the tsunami depends on the depth of the ocean basin, the waves decrease in speed as they reach shallower water. The wavelength is shortened, the energy within each wave is crowded into progressively less water, increasing the height of the wave. The tsunami may increase in height from 1 m in the open ocean to more than 20 m during runup. Also, if underwater ridges are present, they may act as collecting lenses and further intensify the tsunami.
If the tsunami encounters a coastal scarp, the height of its waves increases. Because the long-period wave can bend around obstacles, the tsunami can enter bays and gulfs having the most intricate shapes. Experience has shown that wave heights increase in bays that narrow from the entrance to the head, but decrease in bays that have narrow entrances. Shores of islands protected by coral reefs commonly receive less energy than unprotected coastlines lying in the direct path of an approaching tsunami. Islands in a group may "shadow" one another reducing the tsunami effect. Small islands may experience reduced runup as the tsunami waves may refract around them.
A tsunami wave may break on the beach, appear as flooding, or form a "bore"(violent rush of water with an abrupt front) as it moves up a river or stream. When the trough of the wave arrives first, the water level drops rapidly. Where this occurs the harbor or offshore area may be drained of its water, exposing sea life and ocean bottom. This phenomenon may be the only warning to residents that a large tsunami is approaching. Fatalities have occurred where people have tried to take advantage of this situation to gather fish or explore the strange landscape. The wave returns to cover the exposed coastline faster than the people can run. Although there may be an interval of minutes--or perhaps an hour--between the arrival of waves, the second, third, or later waves can be more destructive than the first. Residents returning too soon to the waterfront, assuming that the worst has past, represent another kind of preventable fatalities.
Caveat
(from Introduction to Catalog of Significant Earthquake) Erroneous statistical conclusions can be drawn from the numbers of earthquakes taken from Catalog of Significant Earthquakes, 2150 B.C. to the present. The reporting of large or destructive earthquakes is not homogeneous in space or time, particularly for periods prior to the 1900s. Because this publication mainly lists those earthquakes that have caused death or damage, the number of earthquake reports is dependent on the written history available for a particular region, as well as on the rate of development of population centers and related structures. Therefore, it is misleading to use the numbers of significant earthquakes in that publication to suggest statistically that there has been an increase in worldwide seismic activity since 1900 or for any time period. that "apparent" increase in activity:
Instrumental seismology is a young science. The first calibrated instruments to measure seismic waves traveling through the earth did not appear until the late 1800s. At that time, seismologists became aware of the vast numbers of earthquakes occurring throughout the world, but because of the insensitivity of their instruments they were able to locate only the large magnitude events.
The 1960s saw two major advances. First, a network of seismological observatories, the Worldwide Standardized Seismograph Network (WWSSN), was installed by the United States Government, principally to monitor underground nuclear tests. These sensitive instruments could detect and identify earthquakes anywhere in the world from about magnitude 4.5.
Second computers became available in the late 1960s. Computers allowed seismologists to leave inaccurate and cumbersome graphical methods of locating earthquakes, and to process the increasing volume of new network data more rapidly than ever before. Prior to 1962, only hundreds of earthquake epicenters were determined each year by Government and academic institutions, but the number increased to the thousands using computerized location methods. In some special local studies, more than 100,000 earthquakes per year were identified and located.
In summary, using the data in Catalog of Significant Earthquakes, 2150 B.C. to the present to suggest that there has been an increase in worldwide earthquake activity is misleading and erroneous. The above observations and reporting factors must also be considered when making statistical studies based on that historical data report.
Uncertainties in the Significant Earthquake and Tsunami Databases
(from Introduction to Catalog of Significant Earthquake) The same problems that can lead to erroneous statistics discussed in the Caveat, also introduce uncertainties in the earthquake and tsunami databases for events prior to the late 1800s, and in some areas for events prior to the installation of the WWSSN in 1962. Before the invention of seismographs in the late 1800s magnitudes, times and locations of earthquakes and tsunami sources were determined from descriptions of earthquake damage and tsunami effects. Earthquake epicenters and tsunami source locations may have been assigned to the wrong places when the names of localitieis were incorrectly transcribed or when some localities had identical or very similar names. Errors may have also been introduced when the local times of earthquakes and tsunami reports were incorrectly converted to Universal Coordinated Time by catalogers. Earthquake and tsunami events can also be listed twice when the dates are recorded in different sources according to the Julian or the Gregorian calendars. As a result, the earthquake and tsunami databases may include listings of events on different dates that are actually descriptions of the same event. For a more complete discussion of these errors please see Historical Seismograms and Earthquakes of the World, edited by W.H.K. Lee, H. Meyers and K. Shimzaki, 1988, Academic Press, Inc., San Diego, California, 513 p.
The tsunami database may also include errors that are unique to that database. One of the most important measurements associated with a tsunami event is the maximum runup height or water height reached above sea level in meters. Unfortunately, it is not always clear which reference level was used. The tsunami database also includes locations where the tsunami was observed, called runup locations. The same problem that occurs when identifying earthquake epicenters can occur when assigning runup locations, where the names of localities were incorrectly transcribed or where some localities had identical or very similar names. In addition, names of locations can change over time adding to the possibility of errors. If tsunami arrival and travel times are available for specific runup locations, they are included in the database. These data are valuable in verifying tsunami travel time models. The definition used in this database is the arrival or travel time of the first wave that arrives at a runup location. The first wave may not have been the largest wave, therefore the travel time reported in the original source may have been the second or third wave.