Tsunamis
Introduction
A tsunami (“a wave in port” in Japanese) are the marine gravity waves that are extremely large in length and height, and are the result of the up or down shift of ocean bottom areas with strong underwater and coastal earthquakes, occasionally, though, are the result of volcanic eruptions and other tectonic processes. Tsunamis are among those few natural disasters that are capable of causing huge numbers. First chapter of this research is dedicated to the causes of tsunami, wave formation mechanism, and waves’ various characteristics. The topic of the conducted research is highly relevant to our area if you pay close attention to two highly destructive tsunamis that occurred during the last decade. It is a matter of knowledge, recognition, and correct measures towards the threat of a tsunami that can help to save thousands of lives in the future, as only two tsunamis in Southeast Asia in 2004, and in Japan in 2011 killed nearly 250 thousand people. These tsunamis also demonstrated the dangers of man-made disasters that follow the natural disasters. Even though, such large and devastating tsunamis are rare, a failure to follow basic measures in the face of danger usually results in high death toll, as shown by experience. And the level of readiness for such disaster is strongly dependant on the country’s development level. Quite often developing or not-developed appear to be not prepared to deal with such a powerful disaster that leads to high death toll. So, the main purpose of this essay is to examine the processes causing tsunamis, and the needed measures, and behavior in case of a tsunami threat.
Causes of tsunamis
Phenomenon that we call a tsunami is a series of propagating waves in the ocean that can be formed due to multiple reasons. Common causes of tsunamis include: underwater earthquakes, landslides, and volcanic eruptions. Let us consider those in detail.
Underwater earthquakes cause up to about 85 % of all tsunamis. When an underwater earthquake occurs, a vertical motion of the ocean bottom is generated, which means that part of the bottom goes down, and some part of it rises. After this the water surface gets into the vertical oscillatory motion, struggling to return to its initial condition (mean sea level), which generates a series of waves; but not every underwater earthquake is accompanied by a tsunami.
Tsunamis caused by landslides occur frequently, being the major reason for tsunamis worldwide. Pretty often an earthquake causes a shift that also generates a wave. A standard procedure involves a certain mass of sliding down, or falling into the water, thus, producing waves of great height that reach the opposite shore of the bay. Even though, such cause is considered to be standard, it is very rare; therefore, it is much more likely for underwater to occur in the river deltas. An earthquake may result in a landslide tsunami that is dangerous, and with regular occurrence causing local waves of over 65 feet in height; landslides cannot cause tsunami waves of oceanic proportions.
Volcanic eruptions take about 5 % of all tsunamis that occur. During strong volcanic explosions waves from the explosion can be generated, with water filling the volcanic cavities producing a long wave. However, a formation of a powerful tsunami during the eruption of an underwater volcano is hardly possible. A classic example is the eruption of Krakatau that took 36 thousand lives. But this case is not considered standard as the volcanic eruption of Krakatau submerged several neighboring islands into the water as a direct result of the part of the crust dropping. It can be said that most likely such a drop of one section of the crust and an uplift of the other was the cause for a tsunami.
Apart from the mentioned above, there are other possible causes of tsunamis that include: human activity, falling of celestial objects, and meteorological causes.
Human activity should be considered as one of the causes for tsunamis in our age of nuclear energy, when power of destruction, previously available only to nature appeared in the human hands. In 1946, the U.S. produced a marine lagoon 197 feet in by an underwater nuclear explosion that is equivalent to 20 tons in TNT equivalent. A wave caused by the explosion was 94 feet in height at a distance of 984 feet from the explosion, and at the distance of 21.325 feet was still 6 feet in height. As of this moment any underwater testing of nuclear weapons is prohibited by a series of international treaties.
Falling of large celestial objects can cause huge tsunamis as they colossal drop speed; these objects also have a tremendous kinetic energy, and their weight can reach several tons (hundreds), which all will be passed into the water transforming into giant waves. Wind is among the meteorological causes for big waves, but such waves cannot be considered to be tsunami, as those do not cause flooding of the shore. Such a wind can be the direct result of a sudden change in pressure, or a rapid movement of atmospheric pressure.
Formation of tsunami waves and their characteristics
As a result of the above mentioned reasons, the suppression of water and the process of deformation of the ocean bottom lead to the formation of the water column that shifts on the water surface. Such formations turn into oscillatory motion of the water that appear to be tsunami waves. Tsunami moves in the deep ocean at a speed from 50 to 1000 km/h that is proportional to the square root of the sea area, the distance between successive crests (troughs) waves can vary from 16 thousand feet to 4.921.259 feet, with the height of waves in their occurrence ranging from 16 feet at sea, 32 feet when they reach the shore, and up to 160 feet in adverse terrain areas (bays, valleys, etc.), so the high seas they are not felt by people on board the ships. According to a general classification tsunami waves relate to long waves, with their length reaching several hundred kilometers, and the amplitude over the deepest parts of the ocean usually being 3.2 feet, which makes it difficult to allocate them from the air or from the ship.
It can be concluded that in deep water tsunami waves are long and flat, which makes them not dangerous in the sea, but once they reach the continental shelf, tsunami waves slow down their movement, with their height increasing. It is in these shallow areas tsunami becomes dangerous to life and property as in these areas its height can be more than 100-150 feet, with the destructive force of the waves being huge.
Especially dangerous tsunami proves to be for settlements and structures that are located at the top of bays and coves, wide open to the ocean and a wedge tapering towards the land. Here, as the funnel tsunami catches large mass of water, at the end of the bay, it spills out onto the shore, and floods the mouth of the river valley 7 thousand to 10 thousand feet from the sea. The approaching tsunami to a shore is sometimes accompanied by a tint, which is accompanied by water level fluctuations of small amplitude that are considered to be predictors of tsunamis. This outflow can last from a few minutes an hour. The more the ocean recedes from the coast after the earthquake, the more powerful tsunami will be on dry land. The nature of tsunami propagation into the coast land is defined by topography, with waves usually propagating inside the shore at a distance of 3 thousand feet and extremely rare at a distance of 6 to 9 thousand feet, and more.
Tsunami is a rare phenomenon, with Pacific coast tsunami with the maximum water level rise above 75 feet occurring once every 100-200 years, with the rise of 26 to 75 feet once every 50-100 years, with the rise of 10 to 26 feet once every 20-30 years, elevation of 3 to 10 feet once every 10 years.
Just as there is a scale of earthquake intensity, there is a tsunami intensity scale that is estimated in a 6 point scale:
- very light tsunami. Wave can be recorded only by special equipment.
- light tsunami. Can flood only the flat coast, and is noticed by specialists, or coast habitats.
- average tsunami. Can flood flat, or slightly sloped coast, small vessels can be thrown out to the shore, with port facilities are subjected to mild damage.
- strong tsunami. Coast get flooded, with coastal buildings being damaged. Large and small sailing boats are being thrown on land, and again washed away into the sea. The coast is littered with sand, silt, fragments of rocks, trees, garbage, with a possibility for some casualties.
- very strong tsunami. Coastal areas are completely flooded, with quay walls, coastal structures being harshly damaged, and light structures being completely destroyed. Huge amounts of cultivated land and litter present on the coast, with small ships and vessels being thrown off shore, or carried away into the sea. Harbors are being damaged with such tsunami, people drowned, or missing. A tsunami is accompanied with a strong sound of the oncoming water.
- disastrous tsunami. It brings a complete devastation to the region, with shore being flooded for a considerable distance into the sea. Manmade structures get partially or completely damaged at great distance from the coast, big vessels are severely damaged; and great number of people in casualties or missing.
Signs of approaching tsunami and precautionary measures
But people can notice the possibility of a tsunami by themselves. The natural signal for tsunami is earthquake, even though the epicenter of the earthquake is usually deep in the ocean. On the shore the earthquake is usually much weaker, but still, once it is felt, it is better to go on shore and even climb a hill to prepare for a coming wave. Before the tsunami occurs the water usually recedes away from the shore, exposing of the sea floor hundreds or even thousands of feet into the sea, which can last from a few minutes to a whole. But the wave can also approach without the water actually departing. The movement of the waves can be accompanied by thunder sounds that can be heard meaning the approaching tsunami waves. Sometimes before the tsunami wave comes small flooding of the coastal occurs. As the indication of the coming wave a change in the usual behavior of animals can be noticed, as they experience fear, and, thus, seek some higher grounds.
Any earthquake that took place on the sea shore can cause local tsunamis. If you have experienced such an earthquake, immediately leave the shore. Great number of people gets killed by tsunamis, even though everyone in the area can feel the earthquake and are constantly getting a warning. Noticeable decline or rise in sea level along the coastline should also be considered as a warning. Therefore, you must be vigilant about tsunami warnings in case of distant earthquakes. A tsunami in Hawaii in 1960 killed 61 people and injured several hundred, although the tsunami warning was issued 10 hours before the arrival of the first wave. Therefore, as soon as the danger signal is issued needed services, and people should respond immediately. But if those signals are missed, and when the tsunami comes you find yourself in the water, free from wet shoes and clothes, try to get some any floating objects in the water. Be careful, as the wave is able to carry around large objects and their fragments. After the arrival of the first wave, prepare to meet the second one, because tsunami is usually a whole series of several waves. Therefore, you must stay away from the danger zone until the wave is gone, and if possible, try to leave the dangerous area quickly, as tsunami hazard may exist for several hours.
Conclusion
Tsunami is a quite rare phenomenon under which we understand a series of propagating waves in the ocean that vary in size, and height, with Pacific coast tsunami occurring once every decade, depending on the elevation height of the wave. Tsunamis are among those few natural disasters that are capable of causing huge numbers, therefore, the alarm signals, and precautionary measures should be taken seriously, who live in the coastal area.
Works Cited
Joseph, Antony. Tsunamis: Detection, Monitoring, and Early-Warning Technologies. MA: Academic Press, 2011. Print.
Levin, Boris, and Mikhail Nosov. Physics of Tsunamis. NY: Springer, 2008. Print.
