What is the Tsunami?

On Sunday, December 26, 2004, Southeast Asia was hit by one of the worst earthquakes in history, triggering a tsunami. The magnitude was at least 9.0. 

This earthquake, whose epicenter was about 150 kilometers west of the Indonesian island of Sumatra, triggered a tidal wave. This tsunami led to massive flooding in coastal areas along the Indian Ocean: from Sumatra and Thailand to Sri Lanka, India and even Somalia. About 300,000 people were killed and millions were injured and/or homeless. 

In this article we explain how a tsunami is formed, how it propagates in the ocean and why a tsunami can have such disastrous consequences for a coastal area. Finally, we consider the (im)possibilities of limiting the consequences of such a natural phenomenon.

Tsunamis: definition

The tsunami is an exceptional sea wave triggered by a sudden displacement of the ocean floor.

It is sometimes attributed ambiguous names: seismic sea wave because earthquakes are the primary cause of their triggering, tidal wave , even if tsunamis have nothing to do with the tide. Moreover tsunami comes from Japanese which means “wave caused by the tide” …

They are found in all oceans, especially in the Pacific Ocean which is notably delimited by the Ring of Fire, and in certain seas such as the Mediterranean.

Origin of tsunamis

To generate a tsunami, a mechanism is needed that moves a large amount of water in a short time.

An example of such a mechanism is an earthquake that occurs in the Earth’s crust under water. Not every underwater earthquake causes a tsunami. Two plates sliding horizontally past each other can cause an earthquake, but hardly any water will be displaced. 

There must be a vertical movement of the seabed. In addition, the earthquake must be quite strong (magnitude > 7.0) and not take place too deep under the seabed.

If a tsunami is generated, its height also depends on the depth of the water under which the earthquake occurs. If the earthquake occurs under a shallow sea or ocean, the difference in water depth between the place of origin and the coastal area is small. 

As a result, the difference in propagation speed of the tsunami is also small and the height of the wave will increase little. The earthquake of December 26, 2004 occurred under a sea area more than one kilometer deep.

The Earth’s crust consists of several plates that move relative to each other. Off the west coast of Sumatra, the Indo-Australian Plate is gradually sliding northeast under the Eurasian Plate. 

Due to the shear resistance between the two plates, the Eurasian plate is pulled downwards and the plate bends up. During the earthquake, the Eurasian plate ‘shot loose’ and moved about 20 meters along the fault surface. 

The fracture plane makes an angle of 13 degrees with the horizontal. This results in a vertical displacement of approximately 4.5 meters. Due to the relaxation, the bent part is stretched again. This loosening and stretching causes a seafloor rise on the ocean side and a fall on the continental side.

Hydro-dynamic characteristics of tsunamis

Tsunami in the open sea

In the case of ocean waves, the terms used are essentially the same as those that apply to other types of waves:

• wavelength : distance between 2 successive peaks;
• frequency or period : time interval between 2 successive peaks;
• height : difference in level between ridge and hollow;
• amplitude : difference in level between the ridge and the mean sea level. Attention! For most waves, the amplitude is half the height;
• run-up . The amplitude of the tsunami in its contact with the coast (surge) is called run-up by the Anglo-Saxons, it is about the maximum altitude of the zone flooded by the tsunami;
• speed ;

Tsunamis are classified according to their magnitude , which is the total energy released by the tsunami. Several magnitude scales are used, one of the most practical being that of Imamura Iida, where the magnitude is equal to the logarithm (in base 2) of the maximum height of the main wave along the coast:

m = logH max

Its operation being identical to the Richter scale for earthquakes, it is easy to compare the magnitude of a tsunami and that of the earthquake that generated it. However, this formula does not take into account the geographic extent of tsunamis.

Characteristics of tsunamis

The wavelength of ocean waves is on average 100 m, while that of tsunamis can exceed 200 km.

Their speed of propagation in the open sea is several hundred km / h (180 km / h for the tsunami of September 2, 1992 in Nicaragua), and can be ten times faster than that of normal waves (around 90 km / h). because the speed increases with the depth (no influence of the roughness of the bottom).

Example: 500 km / h for the tsunami of 12/26/2004 in the Indian Ocean and 800 km / h for the tsunami of April 1, 1946 born in Alaska and which devastated the city of Hilo in Hawaii 4.5 hours more late (18 m high, 150 victims).

• In the case of a series of tsunamis, the frequency between 2 peaks is high, up to one hour, despite their rapid speed, because the frequency depends above all on their wavelength which is very large.
• In the open sea, the amplitude of tsunamis generally remains low, less than 1 m, but can reach several meters in some cases.

The causes of tsunamis

Each event that causes a significant displacement of the ocean floor also causes the displacement of an equivalent volume of water, which can give rise to a tsunami. Most tsunamis result from earthquakes, but others can also result from volcanic eruptions, landslides or human activities (nuclear tests).

The earthquakes

Most often, tsunamis are the result of earthquakes occurring near the coast. Any earthquake that generates a tsunami is called a Tsunamigenic Earthquake . One of the most famous is the one that hit the coast of Portugal in 1755. It produced a series of 5 m high tsunamis that claimed 60,000 deaths in Lisbon, or 1/4 of the city’s population. This example is significant because it is often thought that tsunamis are the exclusive domain of the Pacific, and even the French coasts are not spared. The Sumatran 8.9 earthquake in December 2004 also produced numerous tsunamis with several waves of ten meters high which were devastating.

The magnitude of tsunamis is generally related to that of the earthquakes that initiated them. Thus, a large earthquake risks generating a large tsunami.

For example, the tsunami of March 28, 1964 which partially destroyed Hawaii was triggered in Alaska by the Good Friday earthquake of magnitude 9 (the strongest known with that of Chile in 1960).

However, this correlation is far from being as simple because tsunamis result mainly from vertical deformations of the crust, even of small magnitude as evidenced by the famous earthquake which destroyed San Francisco in 1906 without producing a tsunami despite its magnitude of 8.3 on the Richter scale (450 victims, 28,000 houses destroyed). The cause lies in the release of less than one meter, despite more than 6 m of sliding along the partially submerged San Andréas fault.

On the other hand, earthquakes causing conformal or inverse faults to play or replay are able to generate tsunamis, even for limited releases.

However, even along vertical faults, large earthquakes sometimes produce only modest or no magnitude tsunamis. Conversely, small earthquakes can trigger tsunamis of exceptional magnitude. This last specific category of tsunamigenic earthquake is called by Japanese experts tsunami earthquake ( Tsunami Earthquake).

The 2 most famous examples of tsunami earthquakes are those of Sanriku (Honshu) on June 15, 1896 (24 m high, 26,000 victims) and April 1, 1946 off the island of Unimak (Aleutians, Alaska), which reaches Hawaii with an amplitude of 18 m in Hilo.

Tsunami earthquakes in most cases originate along an active plate margin characterized by a deep oceanic trench (subduction zones). There are two main reasons that moderate earthquakes produce large magnitude tsunamis:

1. The sliding of sediments in an accretion margin. The very large sediments that make up the accretion prism, in unstable equilibrium, can slide along the Benioff plane and cause an exceptional tsunami (probably the case of the Sanriku 1896 and Unimak tsunamis in 1946).
2. In subduction zones without an accretion prism, the main trigger is the creation of a new rupture plane, a new vertical fault.

E.g .: earthquake of September 2, 1992 in Nicaragua, magnitude 7 (moderate), 60 km from the coast (contact Cocos and Caribbean plates) which triggered a tsunami 8 to 15 m high which affected the entire coast west of the country.

For risk prevention, seismologists no longer only use the Richter scale, but the “seismic moment”, a measure which takes into account the elastic properties of the crust and the average area of ​​the area where dislocations occur. crust occur during an earthquake.

Volcanic eruptions

The frequency of tsunamis caused by an eruption is much lower than that of the previous ones: only 2% in the Mediterranean, mainly in Italy, especially by Vesuvius (11 times, eg in 79 BC and especially in 1631). And Only 6 of the 109 regional tsunamis triggered in the Kurils-Kamchatka region from 1737 to 1990.

On the other hand, the magnitude of tsunamis of volcanic origin can be much greater than that of tsunamis of seismic origin. The two most catastrophic tsunamis in history were triggered by the eruption of an explosive-type island volcano: Santorini 1600 BC and Krakatoa 1883. In these two cases, the formation of several successive tsunamis was linked to a Plinian eruption. followed by the formation of a caldera which lowered the ocean floor by several hundred meters.

In other cases, tsunamis can result from: a flank collapse of a volcano, generating an avalanche of debris; the arrival in the sea of ​​pyroclastic flows (fiery clouds – see Krakatoa) or debris flows (lahars). In the latter cases, the greater the volume of material entering the sea, the larger the tsunami.

Landslides

Tsunamogenic landslides are often associated with earthquakes or volcanic eruptions, but not always. Most often, tsunamis are triggered along the walls of submarine canyons, the sides of which crumble from time to time. This is particularly the case along the west coast of the United States. The 1964 Alaska earthquake (Good Friday) generated at least 20 landslides. That of Lituya Bay (Alaska) of July 9, 1958 of magnitude 7 caused a landslide which pushed the sea up to 60 m altitude on the opposite shore, devastating the forest.

The mini-tsunami which affected Nice airport was also caused by a landslide along an underwater canyon.

Landslides and landslides can generate, in the worst case, mega tsunamis characterized by powerful waves up to 300 meters high with a propagation speed of more than 900 km / h … This risk exists still presently.

Anthropogenic factors

We are of course thinking of the nuclear tests that triggered tsunamis:

• at Bikini Atoll in the Marshall Islands in the 1940s and 1950s
• in Mururoa

There are also other factors that cause mini-tsunamis in lakes, for example, but these are not tsunamis in the true sense of the word.

Prevent or limit damage cause by Tsunami?

Preventing a tsunami is impossible, because the earthquake that causes the tsunami cannot be prevented. However, it is possible to think about how the damage can be limited. This includes the following:

• Provide information to the local population about the occurrence of tsunamis and the actions to be taken. For example, making people aware of the fact that the retreat of the sea could be an omen of a tsunami and letting people know where to go at that moment (at least not to the sea!).
• Install a tsunami warning system. Important aspects here are the questions of how the entire population can be reached – also in remote areas – and how evacuation should be arranged. Incidentally, a tsunami warning system for Banda Aceh would have hardly made any sense: there was less than half an hour [7] between the occurrence of the earthquake and the moment when the tsunami reached this city of more than 300,000 inhabitants.
• Making rules for land use in the coastal zone. For example, no longer allowing people or companies to settle close to the coast. This will prove very difficult because in many cases the coast is the main source of income for the population.
• Take measures to ensure that the tsunami loses its energy before it reaches built-up areas. It has been suggested to plant mangrove forests. However, it is expected that the strip of mangrove must be unrealistically wide (several to hundreds of kilometers) to slow down the tsunami.
• Realize adapted buildings. For example, multi-story buildings with an open construction on the ground floor: only columns or load-bearing walls parallel to the expected flow direction. 

How to prevent the risks associated with tsunamis

There is no technical means of protection, only prevention is possible, and it is of 2 types.

Short-term prevention: setting up warning systems

International

International warning system in the Pacific. Very focused on earthquake monitoring, especially tsunami earthquakes. Based in Honolulu and managed by NOAA (National Oceanic and Atmospheric Administration). Equipped with around thirty seismic stations and 78 tide gauges. It gives the alert 1 hour before the arrival of a tsunami. This device remains effective only for populations living more than 750 km from the source.

Regional

This is why we had to set up many regional warning systems, as in Tahiti, for distances of 100 to 750 km from the epicenter of an earthquake. In this case, the alert is given approximately 10-12 minutes after the earthquake.

In Japan, the OBS (Ocean Bottom Seismograph) system can detect earthquakes in the open sea using seismographs and instruments that measure the pressure exerted by the water. Two systems at 2200 m and 4000 m depth. Data is transferred every 20 s. by cable to surface stations, then by telephone to the Tsunami Warning Center of the JMA (Japan Meteorological Agency) in Tokyo.

Filters of different frequencies are used in order to partially erase the signals generated by the tides (LF) or other parasitic signals which modify the water pressure, especially those induced by changes in temperature but also those resulting from data transmission in the event that the device itself receives the tremors of the earthquake.

Local

The population is alerted less than 10 minutes before the arrival of a tsunami (less than 100 km away). Ex .: THRUST (Tsunami Hazards Reduction Utilizing Systems Technology) in Valparaiso (Chile).

Long term prevention

Establishment of comprehensive databases

Ex: Russian database for the Kurils-Kamtchatka region. Nearly 8,000 earthquakes and 124 tsunamis recorded from 1737 to 1990, including 109 regional and 15 transpacific.

Same in Japan: over 1300 years of data, 332 tsunamis up to 1984.

Simulation

For several years, seismologists have used the technique of seismic inversion, which consists of analyzing seismic waves in detail to determine the origin of earthquakes. This technique is also applied to tsunamis: it consists in analyzing for each known tsunami the times of arrival at the coast and their amplitude in several sites. Then we reconstitute the shape of the wave and its speed of propagation, in order to go back to the trigger mechanism.

The propagation of tsunamis is relatively easy to model because the factors which intervene in the speed of the waves are better known than those which control the speed of seismic waves.

Several models have been developed, including those by Mansinha-Smylie (1971) which takes into account the displacement of the ocean floor on either side of a fault. Model criticized because it gives an initial wave profile different from that observed. Other models were developed in the 1990s, such as those of Satake et al., Abe et al., Yoshida et al. or Imamura and Shuto. All use the transoceanic propagation model based on the Linear Long Wave theory applicable only in deep water. There is also a French model, developed at CEA.

In the end, these models still remain imperfect.