Tsunamis

After the tsunami disaster in Asia the world has become more aware of this natural phenomena. It is not a new theme for those in the research field in areas where tsunamis can occur, and Livan Hernandez of CECAT has done some investigation in this regard. EcoSouth is pleased to share his article that explains how Tsunamis develop and provides information about Tsunamis in recent historical periods.

Livan Hernandez

 

These impressive waves of seismic origin are caused by strong and sudden movements at the seabed, generally due to earthquakes. In Latin American they pose a significant threat on the east coast of South America, where each tremor of more than 7.5 magnitude within the ocean can generate tsunamis. In the Caribbean they are less frequent and cause less damage, which makes it difficult to economically justify means of mitigation. Even in the zones where tsunamis present a significant threat, the means of mitigation are only feasible in highly urbanized centers.

 

The principle mechanisms generated by tsunamis are:

  1. Dislocation in the ocean floor produced by an earthquake of a magnitude more than 6.5 on the Richter scale.1, which provokes sudden rising or sinking of the crust, with the resulting displacement of the column of water; the zones with greatest propensity of this phenomena can been seen in figure 2. Tectonic movements cause 96% of the observed tsunamis.
  2. Submarine volcanic eruptions are responsible for 3% of the tsunamis.

3.       Sliding of the continental slope contributes to 0.8% of tsunamis.

 

Other natural mechanisms that generate tsunamis are: flow of currents of turbulence or lava toward the ocean; loosening of glaciers; and through artificial means are nuclear detonations at the surface or bottom of the ocean. All these phenomena are less common but of great importance because of their local effects.

 

There is little probability that earthquakes with shallow epicentres (less than 60 km), and with magnitudes less than 6.4 on the Richter scale would generate a tsunami. However, those with magnitudes greater than 7.75 could cause tsunamis of high risk, as shown in figure 1. 

 

 


Figure 1. Variation of intensity of earthquakes Richter vs Profundity of the center.

 

 

1-) Richter: Surname of the scientist that conceived the scale to measure the magnitude of earthquakes.

 

 

It is necessary to point out that shallow earthquakes constitute some 75% of the total seismic energy freed annually, as well as the most relative frequency in the world, reaching more than 72%.

 

Given their origins, tsunamis are very frequent in the Pacific Ocean; during the period considered between 1900 and 1986, 247 tsunamis were observed in the Pacific, of which 29% were spawned near Japan. Figure 2, shows the coasts where plates overlap and where earthquakes can occur and, consequently, tsunamis. Note the precise coincidence with the borders (union) that unite the continental plates.

 


Figure 2. Coasts where plates overlap


It is possible to draw up maps of the propagation of tsunamis, as has been done with the maps of waves; the difference is that the tsunamis are refracted in all parts according to the variations in depth, while the waves occur only at the coast (figure 3).

 


Figure3. Map of development of the tsunami wave of Papua New Guinea, July 1998. The wave map show at intervals of 30 minutes, the speed at which the wave advances.


These characteristic differ notably from waves generated by winds. All waves have an orbital effect that reaches a profundity equal to the half of its wave length; thus a wave generated by wind can reach some 300 meters of wave length only in great storms, which indicates that they would have an effect up to 150 meters of profundity.

 

Tsunamis normally have wave lengths more than 50 kilometers and can reach up to 1000 kilometers, in which case the orbital effect is constant and vigorous at any part of the marine base.

 

1.1-) Impact of a Tsunami.

 

The magnitude of the effects of a tsunami in coastal areas will depend upon a series of physical factors and of the existence or not of inhabited sites. We describe the scale of intensities of tsunamis, their destructive power, their effects on the coast and damage caused.

 

Table 2. Comparison of range between common waves and tsunamis.

CHARACTERISTIC

TSUNAMI

COMMON WAVE

Length of wave

From 100 to 150 km

90 m Atlantic Ocean
300 m Pacific Ocean

Maximum speed

900 km/h and more

< 100 km/h

Period

From 10 to 90 min.

< 15 sec.

Height or amplitude

       a-) High sea

       b-) COSAT

a-) Few centímeters.

b-) 1-30 m

a-) < 13 m
b-) 6 m

Influence on the seabed

Totally perturbs the seabed

None, only on the beach

 

Source; modified from Ramírez, 1986

1.2-) Scales of intensity of  tsunamis

 

Table 1. Scale of grades of tsunami according to Wiegel.

Grade of tsunami
(m)

Height of wave H
(m)

Maximum height above sea level of inundation R (m)

Description
of damages

0

1 - 2

1 - 1.5

No damages produced.

1

2 - 5

2 - 3

Houses inundated and destroyed boats swept away.

2

5 - 10

4 - 6

Humans, boats and house swept away.

3

10 - 20

8 - 12

Extensive damages along 400 m length of coast.

4

> 30

16 - 24

Extensive damages along more than 500 km along the coastline.

Source: Monge, 1993

 

The height of wave H corresponds to the difference of level between the crest and valley. On the other hand, the maximum elevation above sea level of inundation R, corresponding to the coast where the effects of the tsunami are maximum.

 

Figure 4. Description of the action of the tsunami at the coast.

 

 

1.3-) ¿What to do when confronted with a Tsunami?

       

In 1985, UNESCO formally validated an offer of the United States to amplify its existing tsunami alert center in Honolulu to constitute the Pacific Tsunami Warning Center (PTWC). It also established the International Coordination Group (ICG/ITSU) and the International Tsunami Information Center (ITIC) to review the activities of the International Tsunami Warning System for the Pacific (ITWS). The Tsunami alert system in the Pacific has converted into a nucleo of a real international system. Twenty-eight nations are members of ICG/ITSU: Canada, Chile, China, Columbia, Cook Islands, Ecuador, Fiji, France, Guatemala, Indonesia, Japan, Korea, México, New Zealand, Peru, Philippines, Singapore, Thailand, Hong Kong, United States, Russia y West Samoa, among others.

 

Means suggested.

1.If you live at the coast and sense an earthquake sufficiently strong to crack walls, it is possible that within the following twenty minutes a seaquake or tsunami could occur.

2.If you are alerted about the proximity of a seaquake or tsunami, locate to a higher area at least 30m above sea level, above the natural level of the sea in natural ground.

3.            About half the tsunamis first present themselves as a withdrawal of the sea that leaves grand extensions of the seabed dry. Run to an elevated area, do not hesitate, as a tsunami can reach a velocity of more than 100 km/h.

4.            If you are in a boat, rapidly direct it out to sea. A tsunami is destructive only near the coast. Actually, 5,600 m out to sea or at a profundity greater than 150 m, you can consider yourself secure.

5.            Always remember that a tsunami can penetrate rivers, ravines or salt marshes several kilometers land inward, for which reason you should move away from such areas.

6.            A tsunami can have ten or more destructive waves within 12 hours; attempt to have at hand warm clothing, especially for the children.

7.            Instruct your family about the escape route and posterior meeting place.

 


1.4-) Principal Tsunamis in recent history.

 

Year

Place of origin of the Tsunami

Damages registered

1946

Aleutian Islands, Alaska

In Hawaii, 4.5m waves, $26 million damages y 96 people died.

1952

Kamchatka Península, Russia

In Hawaii, 4m waves and losses of $US1 million, no loss of human life.

1960

Valdivia, Chile.

(Earthquake of 9.5 on Richter scale, 2,000 people died, 4,000 to 5.000 in the entire region, 3,000 injured. 2.000.000 lost their homes.

Rivers changed course.

 New lakes formed. Mountains moved. The geography, as never before seen, was markedly modified.

In the minutes after, a Tsunami swept Hawai the little that remained standing.

The ocean withdrew for several minutes and afterwards a large wave rose, destroying in its path houses, animals, bridges, boats and, of course, many human lives.

Some ships were moved several kilometers from the sea, upriver.

As a consequence of the earthquake, Tsunamis originated that swept the coasts of:

- Japan (138 deaths and damages of  $US 50 million),

- Hawaii (61 deaths and damages of $US 75 million)

-Philippines (32 deaths and disappearances. 

- The east coast of the United Status also registered a tsunami that caused damages for more than $US 500.,000.

 

1964

Prince Williams Sound, Alaska

In Whittier 4.3m waves, 122 people dead and damages of $US 106 millIon.

Julio 1998

Papua, New Guinea

7m waves, 6,000 deaths and damages of hundreds of millions of dollars.

26/12/2004

Island of Sumatra

Affected all of southeast Asia, 600,000 people among dead and disappeared. Economic losses of thousands of millions of dollars.

 

 

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