A tsunami can be described as a series of long, powerful ocean waves produced by a disturbance at or close to the ocean. This disturbance can result from earthquakes, volcanic eruptions, submarine slides, meteorites hitting the ocean, or the explosion of nuclear devices near the ocean. The word tsunami, a Japanese term, was adopted by an international scientific conference in 1963. It is made up of two characters: tsu and nami, meaning harbor wave. Tsunami are also known as seismic sea waves as they are mostly caused by seismic activity, or earthquakes.
Earthquakes that impact the ocean floor give rise to tsunami. The quake deforms the floor, causing a displacement of water. This triggers a series of waves that move toward the shore. It is not possible to spot the buildup to a tsunami from outside, and it usually cannot be felt on board a ship or a vessel at sea. Documented records of tsunami have existed for years, but there is a renewed interest in researching this natural disaster in the twenty-first century. This can be attributed to the large-scale devastation caused by the Indian Ocean tsunami in 2004.
Historical Background and Scientific Foundations
Over the years, the world has witnessed several instances of tsunami of varying magnitude. The Pacific Ocean has been the most vulnerable to tsunami. This is because of the earthquake-prone regions along its periphery. However, tsunami have also occurred in the coastal regions along the Indian Ocean, the Mediterranean Sea, the Caribbean region, and even the Atlantic Ocean. Countries that are earthquake prone and have populated coastal regions, such as Japan and Indonesia, have suffered huge losses due to tsunami. History records also show tsunami occurring in Alaska, the Hawaiian Islands, and the coastal regions of northwestern Europe. There is historical evidence indicating that a tsunami hit the Minoan civilization in about the fifteenth century BC. It destroyed a major portion of the coastal Minoan settlements on the Aegean Sea Islands. This tsunami is attributed to a volcanic eruption on the Greek island of Santorini. The oldest recorded tsunami dates back to AD 365 when an earthquake in the sea ravaged the city of Alexandria, Egypt. Casualties mounted to thousands of people.
A series of earthquakes took place in Lisbon, Portugal, on November 1, 1755. Soon after the earth-quake, a 50-ft (15-m) tsunami occurred and took the lives of 60,000 people. Krakatoa, a volcanic island in the Indian Ocean, experienced a massive tsunami on August 26 and 27, 1883, with a death toll of more than 36,000 in Java and Sumatra. Waves measuring up to 115 ft (35 m) wiped out several villages situated along the coast.
Japan is one of the worst hit regions due to this phenomenon. In 1896, a series of tsunami caused the deaths of 27,000 people and destroyed more than 10,000 houses. Sixty eight tsunami have been recorded in Japan between 684 and 1984. In April 1946, Hawaii and Alaska bore the brunt of a tsunami. The Hawaiian Islands saw frequent tsunami in the 1950s and 1960s. Several tsunami occurred in 1952, 1956, 1957, 1960, 1964, and 1975. In 1960, Chile experienced a series of earthquakes that generated tsunami killing 1,500 people. The damage caused by these tsunami extended to Hawaii and Japan.
A powerful tsunami occurred in the Moro Gulf in the Philippines on August 16, 1976, taking more than 8,000 human lives, injuring 10,000 people, and rendering 90,000 people homeless. Another tsunami occurred in August 1977 in the Lesser Sunda Islands, Indonesia. On December 12, 1979, a destructive tsunami occurred
WORDS TO KNOW
BENTHIC: Living on, or associated with, the ocean floor.
EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.
RUN-UP HEIGHT: The vertical distance between the mean-sea-level surface and the maximum point attained on the coast.
SHALLOW-WATER WAVES: Waves that have a greater wavelength and in which the ratio between the depth of water and the wavelength is very small.
SHOALING EFFECT: Transformation that takes place when a wave travels from deep water to shallow water resulting in the decrease in wavelength and increase in the height of the wave.
SUBMARINE SLIDES: Marine landslides that can transport subsurface rock and sediment down the continental slope.
in the southwest region of Columbia killing hundreds of people and severely impacting the economy. During the 1990s, 10 tsunami were recorded in Indonesia resulting in thousands of deaths and property damages running to $1 billion.
A huge tsunami occurred in December 2004 in the Indian Ocean. It hit several coastal regions in Thailand, India, Sri Lanka, Indonesia, Malaysia, Myanmar, Bangladesh, Maldives, Somalia, and other regions. This tsunami was triggered by an earthquake of magnitude 9.0, which displaced the ocean floor off the Indonesian Island of Sumatra. This had a far-reaching impact affecting the eastern coastal regions of India and Sri Lanka and even Africa. It killed almost 200,000 people and destroyed property worth billions of U.S. dollars.
A breakthrough was made in the detection of tsunami when the Tsunami Warning System (TWS) was invented. A TWS can monitor seismological and tidal stations and forecast impending tsunami. It can be used for determining tsunamigenic earthquakes, monitoring tsunami, and alerting people in the coastal area of an impending tsunami. With the help of this system, the geological society can predict the occurrence of any earthquake of high magnitude. Using this technology, the meteorological agencies can evaluate changes in the sea level. Subsequently, the warning center can combine this information with data pertaining to the depth and features of the ocean floor. The output aids in calculating the path, magnitude, and the arrival time of a tsunami. Government agencies can use this time to evacuate the coastal areas and thus lessen the destructive impact of the tsunami.
The first TWS was established in the Pacific Region on April 1, 1946, after the tsunami hit the Hawaiian Islands. From October 1953, this warning information was made available to other regions such as California, Oregon, and Washington. It was the Chilean tsunami of May 1960 that motivated several countries to join the TWS. In 1965, the United Nations Educational Scientific and Cultural Organization’s Intergovernmental Oceanographic Commission expanded its Tsunami Center in Honolulu; thus, the Pacific Tsunami Warning Center (PTWC) was established. Twenty-six countries from the Pacific region are members of the Pacific Tsunami Warning System (PTWS) network established by the PTWC in 1968. The International Coordination Group (ICG/ITSU) and the International Tsunami Information Center (ITIC) subsequently emerged. They were primarily established to evaluate and organize the activities of the International Tsunami Warning System for the Pacific (ITWS). Apart from the ITWS, several Regional Warning Systems have been established in tsunami-prone regions including the Soviet Union, Japan, Alaska, and Hawaii. UNESCO recognized the need for installing Tsunami Warning Systems in other regions particularly in the Indian Ocean after the tsunami of December 2004. Soon after the disaster, work commenced on installing these systems in the Indian Ocean, the Caribbean Sea, the Atlantic Ocean, and the Mediterranean Sea.
Impacts and Issues
Tsunami waves are extremely powerful ocean waves that can travel at a speed of 400 to 500 mph (650 to 800 km/h). The crests of major tsunami waves striking the coast are generally between 33 and 100 ft (10 and 30 m) in height. Owing to its enormity, the impact it has on the coastal regions is immense. Further, a tsunami is a consequence of other destructive phenomena such as earthquakes, volcanic eruptions, or submarine slides.
In general, any disturbance capable of displacing a huge volume of water from its equilibrium position can cause tsunami. As mentioned earlier, earthquakes are the primary cause of tsunami. When an earthquake occurs, there is a sudden deformation of the sea floor. The size of deformation depends on various factors such as the earthquake’s magnitude, depth, and fault characteristics. This movement causes the overlying water to become displaced from its equilibrium state. When the displaced water tries to get back to its initial equilibrium state, gravitational force acts on it, giving rise to waves. The extent of vertical sea floor deformation is the prime determinant of the size of tsunami along the coast at the outset. Apart from this, there are other factors influencing the size of tsunami such as the seashore and bathymetric pattern, the speed of the sea floor deformation, and the depth of water near the earthquake source.
Earthquakes can cause submarine landslides, which in turn can lead to tsunami. In the event of a submarine landslide, the sediment moves along the sea floor. This disturbs the equilibrium position of the sea level. Gravitational forces further act on the sea level giving rise to huge sea waves. Similarly, a powerful volcanic eruption occurring in the sea can displace the water column thereby generating a tsunami. Further, subarial landslides and meteorite impacts on the sea can cause a tsunami. The tsunami produced by nonseismic mechanisms are less powerful than those generated by earthquakes; hence, the impact is less.
Offshore, tsunami have shorter wavelengths and often pass unnoticed. However, as they approach the coast, the waves gain momentum and as they enter the land, the waves become powerful enough to destroy life and exterminate property, flora and fauna, and anything else that comes in their way. Additionally, the receding waves pull people, buildings, trees, and sand into the ocean. Tsunami are shallow-water waves because their wavelengths are long, about 300 mi (500 km). The energy of a wave primarily depends on its wavelength. Because a tsunami has a large wavelength, it loses little energy as it advances. In an ocean, a tsunami can easily cover a huge distance without losing much energy. In fact, the speed of a raging tsunami when it travels in a very deep ocean can easily match that of a jet plane.
As soon as a tsunami approaches the coast, its speed decreases considerably. This is because the speed of a tsunami depends on the depth of the water; the lesser the depth of the water, the lesser the speed of an approaching tsunami. However, there is no change in the overall impact of a tsunami. As the speed of a tsunami decreases in shallow water, the shoaling effect takes place whereby the waves grow in height and assume a gigantic form. A tsunami may assume different forms such as a bore, a swiftly rising or declining tide, or a series of breaking waves. A number of factors can alter a tsunami as it approaches the sea shore—reefs, bays, entrances to rivers, undersea features, and the beach slope.
In recent times, the severity of tsunami impact has been greatly augmented because of human activities that harm the ecosystem. Urban development, aquaculture, and tourism are some of the factors that have severely damaged the coastal ecosystem. The impact of tsunami can be direct as well as indirect. The direct impact primarily depends on the ecosystem. If it is well protected by natural barriers, such as a coral reef, coconut palms, or mangroves, the impact is less. In fact, coastal ecosystems experience the worst impact of tsunami. Along with the physical structure of the coastal regions, it destroys the flora and fauna, the corals, and sea grass, and it affects several marine species. As the tsunami waves advance,
IN CONTEXT: TSUNAMI IMPACTS
Following the 2004 Indian Ocean tsunami, most environmental experts expected that the environmental toll of the event would be devastating. Although the human toll is fundamentally and tragically incalculable, studies following the tsunami show that the environmental damage caused by the tsunami was severe but uneven.
Places that were already suffering from environmental damage were the places most affected by the tsunami. On the other hand, places that had healthy coastal ecosystems sustained substantially less human and environmental damage.
Healthy coastal ecosystems such as mangroves, coral reefs, and vegetated sand dunes acted as buffers to the tsunami, often protecting both structures as well as human life. For example in the Yala and Bundala National Parks in Sri Lanka, where sand dunes were completely vegetated, the tsunami had minimal environmental impact. Conversely, in places where the coral reefs had been mined, damage by the waves was most destructive.
they damage whatever obstructs their way, including bridges, seawalls, and buildings.
The indirect impacts of a tsunami are often more harmful than the tsunami. A noteworthy indirect impact of a tsunami is sedimentation due to tremendous runoff of coastal silt, sand, and organic matter. In addition, the floating debris such as buildings, vehicles, boats, and large electrical appliances cause further damage. They damage power lines that may lead to an outbreak of fire. Fires resulting from ravaged ships, oil storage tanks, or refineries can also lead to severe damage. The floating debris can adversely affect corals and other benthic substrates. Spilled harmful chemicals, including oils, paints, freons, and cleansers, are deposited and affect nearshore marine ecosystems. This can lead to disease in corals, algae, fish, and other invertebrates. However, such damages and their impacts do not become evident immediately; it may take months or even years to substantiate the consequences.
Destruction from a tsunami can be in the form of inundation, the impact of sea waves on structure, and erosion. Apart from physical changes, it also brings about chemical changes such as intrusion of salty ocean water, eutrophication of water due to high runoff, raw sewage, and decay of dead plants and animals. Toxic wastes including plastics and other nonbiodegradable wastes give rise to marine garbage.
Tsunami have caused grave destruction of life, property, and ecosystems. Besides, the survivors of this tragedy also suffer from the aftermath for a long time. They are often devoid of the basic necessities of life—
clean drinking water, food, and shelter. A large number suffer from physical injuries as well as mental trauma. Floodwater resulting from a tsunami is also a cause of major health problems. Contamination of water and food brings about food-borne and waterborne illnesses. Furthermore, there is a noticeable increase in infectious and insect-transmitted diseases. With their homes destroyed by the tsunami, the survivors are exposed to harsh weather and other environmental hazards. The Indian Ocean tsunami of 2004, which greatly affected several regions in Thailand, India, Sri Lanka, Myanmar, Indonesia, Africa, and the Maldives, gave impetus to several communicable diseases such as cholera, malaria, dengue, tuberculosis, influenza, and rabies.
Natural disasters such as earthquakes and tsunami cannot be prevented. However, preventive measures may be taken to reduce their impacts. One of the measures is to protect the coastal ecosystem. Some of the methods are promoting bio-shields; restoration of natural barriers, such as mangroves, casuarinas plantations, sand dunes, and coral reefs; and establishing artificial barriers like sea walls and embankments. Another way is conserving natural coastal habitats, for example, by discouraging aquaculture farms. Coastline development should be done only through proper management and in keeping with the Coastal Regulation Zone (CRZ) norms. Further, human activities should be controlled to protect marine ecosystems, and sustainable use of natural resources should be encouraged.
Several Tsunami Warning Systems (TWS) have been set up at various locations around the globe for detecting potential tsunami. However, the technology is not infallible and does not guarantee security. Predictions through a TWS can be inaccurate, causing false alarms or failure to detect a tsunami. Repeated false alarms could lead people to not respond to a warning. Though a TWS operates in real-time, it cannot always be relied upon to save lives. This is because the ferocity of a tsunami gives people little time to react or escape. The real-time operation also makes it difficult to predict a tsunami run-up resulting from an earthquake. Other hurdles include inadequate data and lack of proper communication. The TWS technology is yet to become widespread, and as a result, several vulnerable locations across the world remain unguarded.
Bryant, Edward. Tsunami: The Underrated Hazard. Cambridge: Cambridge University Press, 2001.
Kusky, Timothy M. Geological Hazards: A Sourcebook. Westport, CT: Greenwood Publishing Group, 2003.
Centers for Disease Control and Prevention (CDC). “Emergency Preparedness & Response.”http://www.bt.cdc.gov/disasters/tsunamis/healtheff.asp (accessed February 29, 2008).
Indian Concrete Journal. “An Introduction to Tsunami in the Indian Context.” http://www.nicee.org/ICJ_TsunamiSpecialReortl.pdf (accessed February 29, 2008).
International Tsunami Information Center (ITIC). “Tsunamis on the Move.” http://ioc3.unesco.org/itic/contents.php?id=205 (accessed February 29, 2008).
Office of Climate, Weather, and Water Services. “Tsunami: The Great Waves.” http://www.weather.gov/om/brochures/tsunami.htm (accessed February 29, 2008).
UN Atlas of the Oceans. “Impact of Tsunami on Ecosystems.” http://www.oceansatlas.com/servlet/CDSServlet?status=ND03MTY4NyZjdG5faW5mb192aWV3X3NpemU9Y3RuX2luZm9fdmlld19mdWxsJjY9ZW4mMzM9KiYzNzlrb3M~ (accessed February 29, 2008).
UNESCO. “Tsunami: The Great Waves.” http://ioc3.unesco.org/itic/files/great_waves_en_2006_small.pdf (accessed February 29, 2008).
University of Wollongong. “Introduction to Tsunami.” http://www.uow.edu.au/science/eesc/research/UOW002909.html (accessed February 29, 2008).
WWF-India & Wetlands International-South Asia. “ Post Tsunami Conversation Issues and Challenges Consultative Meeting for Coordinated Action.” http://assets.wwfindia.org/downloads/final_report_may2005.doc (accessed February 29, 2008).