Natural and Human-Made Hazards: Disaster Risk Management Issues

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Chapter 89 Natural and Human-Made Hazards

Disaster Risk Management Issues

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The term hazard is usually applied to rare or extreme events in the natural or human-made environment. Hazards can adversely affect human life or property to the extent of causing a disaster or major disruptive situation. Natural hazards are caused by biologic, geologic, seismic, hydrologic, or meteorologic processes in the natural environment and include drought, flood, earthquake, volcanic eruption, and severe storms. When natural hazards affect vulnerable human settlements, structures, and economic assets, they can be disastrous, disrupting the normal functioning of a society and necessitating extraordinary emergency interventions to save lives and the environment.

Human-made hazards are derived from human interactions with the environment, human relationships and attitudes, and the use of technology. For example, transportation accidents, petrochemical explosions, mine fires, building collapses, oil spills, hazardous waste leaks, and nuclear power plant failures are disasters in which the principal and direct causes are human actions. Many hazards have both natural and human components. Desertification results from arid conditions, erosion, and overgrazing; landslides may occur from poorly planned construction on unstable hillsides, and flooding may be caused by dam failures.

The distinction between many natural causes of hazards and the contributions of humans to disastrous situations is becoming increasingly blurred. As populations grow and expand, pressure on land resources may force settlement in vulnerable areas, where hazards such as volcanic eruptions, earthquakes, or floods can become major disasters. When disasters strike major population areas or where disaster-affected people must gather in camps or other common areas to receive relief services, incidences of disease have the potential of becoming epidemics because of overcrowding. Drought may contribute to famine in areas where food shortages result from combinations of lack of rainfall, displacement of people, and lack of access to food supplies. The recent focus on climate change emanates from studies of the effects of climatic conditions and environmental pollution. Variables in these studies form such complex interactions that even computerized models have difficulty predicting the outcomes. Hazards with a combination of causes result in complex disasters and often in complex emergencies. Whatever their causes, disasters have serious political, economic, social, and environmental implications. In less developed areas, disasters can severely set back or reverse development efforts.

Disaster Risk Reduction and Management

This chapter covers 12 hazards, each with significant geophysical components, and discusses their causes, characteristics, predictability, adverse effects, and risk reduction measures. Hazards are viewed from a perspective of seeking means to reduce risks to vulnerable people and societies. The disaster risk reduction approach highlights causes of vulnerability and relates them closely to risk factors in society, such as poverty and economic development. The socioeconomic forces that make people vulnerable to disasters are likely to result from long-term trends. The study of disaster risk management, which formerly focused on natural hazards, now encompasses a range of slow-onset and rapid-onset disasters and their natural and human causes.

Activities associated with the conceptual framework of disaster risk reduction have gained wide usage by governments since the World Conference on Disaster Reduction in Kobe, Japan, in 2005. Disaster risk reduction aims to reduce the probability of disasters by using methods that are financially, environmentally, and culturally sensitive and by using mitigation methods that are agreed on through public consultation. The practice of disaster risk reduction encompasses all aspects of preventing, planning for, responding to, and recovering from disasters, including predisaster and postdisaster activities. A critical feature is training communities to allow them more direct responsibility for disaster reduction. Another key component is improving on unsustainable predisaster conditions through well-planned disaster recovery programs. The essential components of a disaster risk reduction framework are:

Selection of management options depends on the type of hazard and its characteristics. Box 89-1 lists the elements usually found in a disaster preparedness plan for sudden-onset hazards, such as earthquakes, tsunamis, volcanic eruptions, tropical cyclones, and floods. Preparedness measures for slow-onset disasters, such as drought, include early warning systems that alert authorities to precursory conditions and allow preparations to avert food and water shortages.

Slow-Onset Versus Rapid-Onset Hazards

The distinction between slow- and rapid-onset hazards is useful because the methods to deal with them often differ. Rapid-onset hazards often occur with violent intensity and have profound effects on the surrounding environment, resulting in measurable numbers of casualties and damage. Slow-onset climatic changes brought on by deforestation, drought, desertification, or environmental pollution change the suitability of different parts of the world for human habitation, and they affect agriculture and flora and fauna. The effects of slow-onset disasters are often insidious. Their impact can be measured only through environmental studies and in terms of reduction in quality of life and productivity for the affected population. As discussed later, slow onset hazards rarely are identified as disasters or emergencies, although their impacts may be greater or similarly widespread. Variables include levels of public attention to the hazards and the ability of the government to deal with them. Typically, threats become mitigated by slow movement away from hazards; for example, many of the 11,000 residents of the Pacific island nation of Tuvalu have left the country because the island is being swallowed by sea level rise.

Conservatively, an estimated 1.5 million to 2 million people have been killed in rapid-onset disasters since 1946—an average annual death toll of 35,000 to 50,000. The primary killers are earthquakes, tropical cyclones, and floods. Most deaths are concentrated in a relatively small number of communities, predominantly in poorer nations of Africa, Asia, Latin America, and Oceania. In comparison, North America, Europe, Japan, and Australia have average annual death tolls that rarely exceed a few hundred persons. In terms of total numbers of disasters worldwide, during the period from 1991 to 2005, flood disasters occurred most frequently, followed by hurricanes or cyclones and other windstorms.

Although comprehensive data for economic losses from rapid-onset hazards are difficult to obtain, a few examples illustrate the scale of the problem. Annual worldwide losses from tropical cyclones are estimated at between $6 billion and $7 billion. For landslides, the comparable figure exceeds $1 billion. These figures only hint at the impact of such disasters on the affected human population. The eruption of Colombia’s Nevado del Ruiz volcano in 1985 killed approximately 22,000 people and left 10,000 more homeless. An earthquake in Bam, Iran, in December of 2003 claimed at least 30,000 lives and destroyed 80% of the city. The 2005 Atlantic hurricane season caused record damages of $100 billion. Hurricane Katrina alone, in August of 2005, killed 1417 people in three U.S. states, displaced 1.5 million, and caused $75 billion in damages.

The relative human, economic, and social impacts of rapid-onset disasters are usually greatest in smaller, poorer nations. The 1985 earthquake in Mexico City caused economic losses equivalent to about 3.5% of Mexico’s gross national product (GNP). Hurricane Allen in 1980 caused losses in St. Lucia equivalent to 89% of this island nation’s GNP and destroyed 90% of its banana crop, which normally accounts for 80% of the country’s agricultural output. One of the strongest storms in recent history, Hurricane Mitch in 1998 devastated the economies and infrastructure of Honduras and Nicaragua.

Economic losses from rapid-onset hazards are increasing at a fast pace. In the United States, damage to buildings from earthquakes, tropical cyclones, and floods was estimated to increase from approximately $6 billion in 1978 to more than $11 billion in 2000 without additional loss reduction measures. At this same time, it was estimated that a major earthquake in Tokyo would probably kill more than 30,000 people, cause the collapse of 60,000 houses, and set fire to more than 400,000 homes.

Slow-onset disasters take an even greater toll, but precise figures are difficult to establish. Drought currently affects more people than does any other disaster; worldwide, it is estimated that droughts affected more than 18 million people each year during the 1960s, more than 24 million people during the 1970s, and 101 million between 1980 and 1989. During the early 1980s, drought affected up to 30 million people in Africa alone. In the United States, drought leads in economic impact, causing losses of $6 billion to 8 billion per year. Worldwide, droughts have led to famines, resulting in large numbers of deaths and displacements. Increasing desertification in arid areas may be contributing to droughts. Desertification, or decline in biologic productivity, extends to 70% of total productive arid lands (3.6 billion acres worldwide) and may adversely affect the quality of life for 10% of the world’s population, including urban dwellers.

Possible global warming is predicted to occur over the next 100 years as a result of increased atmospheric carbon dioxide caused by the burning of fossil fuels, deforestation, and generation of methane. If this occurs, sea levels will rise and coastal cities worldwide will be inundated. A rise of 1 m (3.3 feet) in sea level could flood 15% of arable land in Egypt’s Nile Delta and completely submerge the tiny islands of the Maldives, currently inhabited by 200,000. Hundreds of millions of people will also be affected if increased ultraviolet radiation is delivered to the earth’s surface as a result of stratospheric ozone depletion caused by continued release of chlorofluorocarbons.

Although global warming and ozone depletion are threats that may become more evident in the future, other forms of environmental pollution, such as water and air pollution, have immediate effects on life today. Massive oil spills, such as the 2010 leakages in the Gulf of Mexico, make headlines, and adverse health effects are seen from contamination and smog. Deforestation, particularly in the tropical rainforests, is highly significant. In addition to its contribution to possible global warming, the loss of forested land increases vulnerability to droughts, landslides, and floods.

Assessing Vulnerability and Risk

Not all hazards become disasters. Whether or not a disaster occurs depends on the magnitude, intensity, and duration of the event and the vulnerability of the community. For example, a severe earthquake is not a disaster unless it significantly disrupts a community by creating large numbers of casualties and substantial destruction. Effective disaster risk management requires information about the magnitude of the risk faced and how much importance society places on the reduction of that risk. Risks are often quantified in aggregated ways (e.g., a probability of 1 in 23,000 per year of dying in an earthquake in Iran). The importance placed on the risk for a hazard is likely to be influenced by the nature of the risks faced on a daily basis. For instance, in Pakistan, where communities are regularly affected by floods, earthquakes, and landslides, people use their meager resources to protect against what they perceive to be the greater risks, such as disease and irrigation failure. In California, where risk for disease is low, communities choose to initiate programs against natural disasters.

Vulnerability is often measured as the susceptibility of buildings, infrastructure, economy, and natural resources to damage from hazards. Many aspects of vulnerability, however, cannot be described in monetary terms and should not be overlooked. These include personal loss of family, home, and income, along with related human suffering and psychosocial problems. Although communities in developed nations may be as prone to hazards as those living in poorer nations, wealthier communities are often less vulnerable to damage. For example, although both southern California and Managua, Nicaragua, are prone to earthquakes, California is less vulnerable to damage because of strictly enforced building codes, zoning regulations, earthquake preparedness training, and sophisticated communications systems. In 1971, the San Fernando earthquake in California measured 6.4 on the Richter scale but caused minor damage and 58 deaths, whereas an earthquake of similar magnitude that struck Managua, Nicaragua, 2 years later reduced the center of the city to rubble, killing approximately 6000 people. Similarly, in wealthy countries, drought and resulting loss of food production and groundwater are managed by use of food surpluses and treated water, but drought in poor nations often leads to deaths from famine, as well as sickness and death from contaminated water supplies.

Disaster Mitigation Strategies

Mitigation involves not only saving lives and reducing injury and property losses, but also reducing the adverse consequences of hazards to economic activities and social institutions. Where resources are limited, they should be directed toward protecting the most vulnerable elements. Vulnerability also implies a lack of resources for rapid recovery.

For most risks associated with natural geophysical hazards, such as volcanic eruptions, tsunamis, and tropical cyclones, little or no opportunity is available to reduce the hazard itself. In these cases, the emphasis must be placed on reducing the vulnerability of the elements at risk. However, for technologic and human-made hazards or slow-onset hazards, such as environmental pollution and desertification, reducing the hazard is likely to be the most effective mitigation strategy.

Actions by planning authorities to reduce vulnerability can be active, in which desired actions are promoted through incentives, or passive, in which undesired actions are prevented by use of controls and penalties. Discussion of mitigation options follows.


Earthquakes are among the most destructive and feared of natural hazards. They may occur at any time of year, day or night, with sudden impact and little warning. They can destroy buildings in seconds, killing or injuring the inhabitants. Earthquakes not only destroy entire cities but may destabilize the government, economy, and social structure of a country.

Causal Phenomena

The earth’s crust is a rock layer varying in thickness from a depth of about 10 km (6.2 miles) under the oceans to 65 km (40.4 miles) under the continents. The theory of plate tectonics holds that seven major and about six minor crustal plates, varying in size from a few hundred to many thousands of kilometers, “ride” on the earth’s mobile mantle. When the plates contact each other, stresses arise in the crust. Stresses occur along the plate boundaries by pulling away from, sliding alongside, and pushing against one another. All these movements are associated with earthquakes.

Faults are areas of stress at plate boundaries that release accumulated energy by slipping or rupturing. Elastic rebound occurs when the maximum point of supportable strain is reached and a rupture occurs, allowing the rock to rebound until the strain is relieved (Figure 89-1). Usually, the rock rebounds on both sides of the fault in opposite directions. The point of rupture is called the focus and may be located near the surface or deep below it. The point on the surface directly above the focus is termed the epicenter (Figure 89-2).

The energy generated by an earthquake is not always released violently and can be small or gradual. Minor Earth tremors are recorded daily in the United States, but whether these are caused by the same processes that can level a city is not known. Most damaging earthquakes are associated with sudden ruptures of the crust.


The actual rupture process may last from a fraction of a second to a few minutes for a major earthquake. Seismic (from the Greek seismos, meaning “shock” or “earthquake”) waves are generated. These last from less than one-tenth of a second to a few minutes and cause ground shaking. The seismic waves propagate in all directions, causing vibrations that damage vulnerable structures and infrastructure.

There are three types of seismic waves. The body waves (P, or primary, and S, or secondary) penetrate the body of the earth, vibrating quickly (Figure 89-3). P waves travel at about 6 km per second (kps) (3.7 miles per second [mps]) and provide the initial jolt that causes buildings to vibrate up and down. S waves travel about 4 kps (2.5 mps) in a movement similar to the snap of a whip, causing a sharper jolt that vibrates structures from side to side and usually resulting in the most destruction. Surface waves (L waves) vibrate the ground horizontally and vertically and cause swaying of tall buildings, even at great distances from the epicenter.

Earthquake focus depth is an important factor in determining the characteristics of the waves. The focus depth can be deep (from 300 to 700 km [186 to 435 miles]) or shallow (less than 60 km [37 miles]). Shallow-focus earthquakes are extremely damaging because of their proximity to the surface. The earthquake may be preceded by preliminary tremors and followed by aftershocks of decreasing intensity.

Earthquake Scales

Earthquakes can be described by use of two distinctly different scales of measurement demonstrating magnitude and intensity. Earthquake magnitude, or amount of energy released, is determined by use of a seismograph, which records ground vibrations. The Richter scale mathematically adjusts the readings for the distance of the instrument from the epicenter. The Richter scale is logarithmic; an increase of one magnitude signifies a tenfold increase in ground motion, or about 30 times the energy. Thus an earthquake with a Richter magnitude of 7.5 releases 30 times more energy than one with a 6.5 Richter magnitude. The smallest quake to be felt by humans was of magnitude 3. The largest earthquakes that have been recorded under this system are 9.5 (Chile, 1960) and 9.25 (Alaska, 1969).

The moment magnitude scale is a successor to the Richter scale and is most often used to estimate large earthquake magnitudes. Theoretically, all magnitude scales should yield approximately the same value for any given earthquake. However, controversy exists over the measurement of the great Indian Ocean earthquake that occurred on December 26, 2004, generating a tsunami that killed more than 280,000 people. The Pacific Tsunami Warning Centre (PTWC) estimated the magnitude as 8.5 on the Richter scale shortly after the earthquake. The U.S. Geological Survey (USGS), using the moment magnitude scale, increased its estimate from 8.1 to 9.0 Other scientists, using moment magnitude, have revised the estimate to 9.3, and the PTWC has accepted this, but the USGS has so far not changed its estimate of 9.0. The most definitive estimate so far has put the magnitude at 9.15.

The earthquake intensity scale measures the effects of an earthquake where it occurs. The most widely used scale of this type is the modified Mercalli scale, which expresses the intensity of earthquake effects on people, structures, and the earth’s surface in values from I to XII (Table 89-1). Another, more explicit, scale used in Europe is the Medvedev-Sponheuer-Karnik (MSK) scale.

TABLE 89-1 Modified Mercalli Intensity Scale of 1931

Scale Description
I Not felt except by very few persons under especially favorable circumstances.
II Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor vehicles may rock slightly. Vibration similar to passing of truck. Duration estimated.
IV During the day felt indoors by many but outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation resembles heavy truck striking building. Standing motor vehicles rocked noticeably.
V Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken. A few instances of cracked plaster. Unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.
VI Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
VII Everybody runs outdoors. Damage negligible in buildings of good design and construction, slight to moderate in well-built ordinary structures, considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motor vehicles.
VIII Damage slight in specially designed structures, considerable in ordinary substantial buildings with partial collapse, great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor vehicles disturbed.
IX Damage considerable in specially designed structures. Well-designed structures thrown out of plumb, greatly in substantial buildings with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
X Some well-built wooden structures destroyed. Most masonry and frame structures with foundations destroyed; ground severely cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.
XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
XII Damage total. Practically all works of construction are damaged greatly or destroyed. Waves seen on ground surface. Lines of sight and level are distorted. Objects are thrown upward into the air.

Location and Predictability

Most earthquakes (95%) occur in well-defined zones near the boundaries of the tectonic plates. These areas bordering the Pacific Ocean are called the circum-Pacific belt. Areas traversing the East Indies, the Himalayas, Iran, Turkey, and the Balkans are called the Alpide belt. Earthquakes also occur along the ocean trenches, such as those around the Aleutian Islands, Tonga, Japan, and Chile and within the eastern Caribbean. Some earthquakes occur in the middle of the plates, possibly indicating where earlier plate boundaries might have been. These have included the New Madrid earthquake in 1811 and the Charleston earthquake in 1816 in the United States, the Agadir earthquake in 1960 in Morocco, and the Koyna earthquake in 1967 in India.

Earthquake prediction was a constant preoccupation for early astrologers and prophets. Some signs of earthquake noted by observers were buildings gently trembling, animals and birds becoming excited, and well water turning cloudy and smelling bad. Although some modern scientists claim ability to predict earthquakes, the methods are still controversial. For example, the 1995 earthquake in Kobe, Japan, which killed more than 5000 people, was not predicted. However, mechanical observation systems make it possible to issue warnings to nearby populations immediately after detection of an earthquake. Reasonable risk assessments of potential earthquake activity can be made with confidence based on the following:

The island of Hispaniola, shared by Haiti and the Dominican Republic, has a history of destructive earthquakes. In January 2010, a magnitude 7.0 earthquake occurred approximately 25 km (16 miles) west-southwest from the capital city Port-au-Prince at a depth of 13 km (8.1 miles). Two years before this, scientists had detected signs of growing stresses in the fault that forms a boundary between the Gonave microplate and the Caribbean plate to the south, specifically the Enriquillo-Plantain Garden fault system, which includes much of Haiti. They warned Haitian officials that the fault was capable of causing a 7.2 magnitude earthquake, only slightly stronger than the actual 7.0 earthquake that eventually occurred. Unfortunately, 2 years is little time to prepare for such an event in a country like Haiti, which endures widespread poverty and lacks resources for preparedness and mitigation. A legacy of poor building standards has increased vulnerability and cannot be easily remedied.

Earthquake Hazards

Earthquakes produce many direct, and sometimes indirect, effects. Landslides, flooding, and tsunamis are considered secondary hazards and are discussed later in this chapter.

Typical Adverse Effects

Ground shaking can damage human settlements, buildings and infrastructure (particularly bridges), elevated roads, railways, water towers, water treatment facilities, utility lines, pipelines, electricity generating facilities, and transformer stations. Aftershocks can do great damage to already weakened structures. Significant secondary effects include fires, dam failures, and landslides, which may block waterways and cause flooding. Flooding may also be caused by seiches (back-and-forth wave action in bays) or by failures in dams and levees. Damage may occur to facilities that use or manufacture dangerous materials, resulting in chemical spills. Communications facilities may break down. Destruction of property may have a serious impact on shelter needs, economic production, and living standards of the affected community. Depending on their level of vulnerability, many people may be homeless in the aftermath of an earthquake.

The casualty rate is often high, especially when earthquakes occur in areas of high population density, particularly when streets between buildings are narrow, buildings are not earthquake resistant, the ground is sloping and unstable, or adobe or dry stone construction is used, with heavy upper floors and roofs.

Casualty rates may be high when quakes occur at night because the preliminary tremors are not felt during sleep and people are not tuned in to receive media warnings. In the daytime, people are particularly vulnerable in large unsafe structures such as schools and offices. Casualties generally decrease with distance from the epicenter. As a rule of thumb, quakes result in three times as many injured survivors as persons killed. The proportion of dead may be higher with major landslides and other secondary hazards. In areas where houses are of lightweight construction, especially with wood frames, casualties are generally much fewer, and earthquakes may occur regularly with no serious, direct effects on human populations.

The most widespread acute, serious medical problems are broken bones. Other health threats may occur with secondary flooding, when water supplies are disrupted (earthquakes can change levels in the water table) and contaminated water is used or water shortages exist, and when people are living in high-density relief camps, where epidemics may develop or food shortages exist.

In the aftermath of the Colombia earthquake of January 1999, which most heavily affected the city of Armenia, the death toll was 1185, and 160,000 people were left homeless, most in urban areas. In Armenia, where 60% to 70% of homes had been destroyed, movement was restricted by fallen debris and unemployment rose from 12% to 35%. People were living in unsatisfactory shelters made with plastic sheeting. Many migrated from the area to other places that could not absorb them. Although international response to aid Colombia was strong, the overwhelming need continued to pose problems. Five weeks after the earthquake, supplies of food, clean drinking water, and shelter materials were still urgently required. Hygiene and sanitation services and essential medicines were desperately needed. Social services were required to work toward normalizing the lives of victims, especially children. In Gujarat, India, where 30,000 people died in 2001, assistance agencies struggled for years to help rebuild the more than 300,000 houses that were lost. In the Kashmir earthquake of 2005, 3.3 million persons were left homeless in Pakistan, and many of them were at risk of dying from the winter cold and spread of disease.

The January 2010 Haiti earthquake affected an estimated 3 million people. It killed approximately 100,000 persons and injured approximately 300,000, although estimates of casualties widely vary. More than 1 million Haitians were left homeless. Vital infrastructure necessary to respond to the disaster, including air, sea, and land transport facilities and communication systems, was severely damaged or destroyed. Treatment of the injured was hampered by the lack of hospitals and morgue facilities; bodies were left to decay on the streets for many days. International assistance was offered in abundance, but the logistical capabilities in Haiti to receive emergency aid were limited. Doubtless, more lives were lost as a result of this vulnerability (Figures 89-4 and 89-5, online).


FIGURE 89-4 Nurse attending earthquake casualties in Haiti.

(Courtesy Pan America Health Organization.)


FIGURE 89-5 Makeshift hospital in Haiti.

(Courtesy Pan America Health Organization.)

Earthquake Risk Reduction Measures

Earthquake warning systems currently in use warn of an earthquake that has already occurred. Examples include those that notify the high-speed trains in Japan, which if derailed would cause hundreds of deaths. One minute before the 2011 Tohoku earthquake was felt in Tokyo, 1000 seismometers sent out warnings that saved many lives. In California, it is technically feasible to develop a system that could warn Los Angeles up to a minute before the arrival of the seismic waves, allowing certain preventive actions, such as taking cover, to occur. However, because predicting the location, time, and magnitude of earthquakes is still likely many years away, warning systems and earthquake prevention measures are currently not reliable alternatives to preparedness. Preparedness actions include the following:


Tsunami is a Japanese word meaning “harbor wave.” Although tsunamis are sometimes called “tidal waves,” they are unrelated to the tides. The waves originate from undersea or coastal seismic activity, landslides, and volcanic eruptions. They ultimately encroach over land with great destructive power, often affecting distant shores.

Causal Phenomena and Characteristics

The geologic movements that cause a tsunami are produced in three major ways (Figure 89-7). The foremost cause is fault movement on the sea floor, accompanied by an earthquake. The second most common cause is a landslide occurring underwater or originating above the sea and then plunging into the water. The highest tsunamis ever reported were produced by a landslide at Lituya Bay, Alaska, in 1958. A massive rock slide produced a wave that reached a high water mark of 530 m (1740 feet) above the shoreline. A third cause of a tsunami is volcanic activity, which may uplift the flank of the volcano or cause an explosion.

Tsunamis differ from ordinary deep ocean waves, which are produced by wind blowing over water. Normal waves are rarely longer than 300 m (984 feet) from crest to crest. Tsunamis, however, may measure 150 km (90 miles) between successive wave crests. Tsunamis also travel much faster than ordinary waves. Compared with normal wave speed of around 100 km/hr (62 mph), tsunamis in the deep water of the ocean may travel at the speed of a jet airplane—800 km/hr (497 mph). Despite their speed, tsunamis increase the water height only 30 to 45 cm (12 to 18 inches) and often pass unnoticed beneath ships at sea. In 1946, a ship’s captain on a vessel lying offshore near Hilo, Hawaii, claimed he could feel no unusual waves beneath him, although he saw them crashing on the shore.

Contrary to popular belief, a tsunami is not a single giant wave. A tsunami can consist of 10 or more waves, termed a tsunami wave train. The waves follow each other in 5- to 90-minute intervals. As tsunamis approach the shore, they travel progressively slower. The final wave speed depends on the water depth. Waves in 18 m (59 feet) of water travel about 50 km/hr (31 mph). The shape of the near-shore sea floor influences how tsunami waves behave. Where the shore drops off quickly into deep water, the waves are smaller. Areas with long shallow shelves, such as the major Hawaiian Islands, allow formation of very high waves. In the bays and estuaries, seiches, in which the water sloshes back and forth, can amplify waves to some of the greatest heights ever observed.

On shore, the initial sign of a tsunami depends on what part of the wave first reaches land; a wave crest causes a rise in the water level, and a wave trough causes a recession. The rise may not be significant enough to be noticed by the general public. Observers are more likely to notice the withdrawal of water, which may leave fish floundering on the exposed sea floor. A tsunami does not always appear as a vertical wall of water, known as a bore, as typically portrayed in drawings. More often, the effect is that of an incoming tide that floods the land. Normal waves and swells may ride on top of the tsunami wave, or the tsunami may roll across relatively calm inland waters.

The flooding produced by a tsunami may vary greatly from place to place over a short distance, depending on submarine topography, shape of the shoreline, reflected waves, and modification of waves by seiches and tides. The Hilo, Hawaii, tsunami of 1946, originating in the Aleutian Trench, produced 18-m (59-foot) waves in one location and only one-half that height a few miles away. The sequence of the largest wave in the tsunami wave train also varies, and the destructiveness is not always predictable. In 1960 in Hilo, many people returned to their homes after two waves had passed, only to be swallowed up in a giant bore that, in this case, was the third wave.


Tsunamis have occurred in all oceans and in the Mediterranean Sea, but the majority of them occur in the Pacific Ocean. The zones stretching from New Zealand through East Asia, the Aleutians, and the western coasts of the Americas all the way to the South Shetland Islands are characterized by deep ocean trenches, explosive volcanic islands, and dynamic mountain ranges.

Prior to 1946, the recorded effects of tsunamis included only local casualties and significant damage. The Tsunami Warning System (TWS) was developed in Hawaii shortly after the 1946 Hilo tsunami and is headquartered in the Pacific Warning Center in Honolulu. There are 26 member countries in the Pacific basin. The TWS works by monitoring seismic activity from a network of seismic stations. A tsunami is almost always generated by an undersea earthquake of magnitude 7 or greater. Therefore special warning alarms sound when a quake measuring 6.5 or more occurs anywhere near the Pacific. A tsunami watch is declared if the epicenter is close enough to the ocean to be of concern. Government and voluntary agencies are then alerted, and local media are activated to broadcast information. The five nearest tide stations monitor their gauges, and trained observers watch the waves. With positive indicators, a tsunami warning is issued.

The TWS met with general success in saving lives during the tsunamis of 1952 and 1957 in Hawaii. In 1960, however, two major earthquakes occurring a day apart rocked the coast of Chile in South America. The first registered 7.5 on the Richter scale and produced a small but noticeable wave in Hilo Bay. The second registered a stunning 8.5, more than 30 times the energy of the first, and authorities predicted generation of a large, destructive tsunami. When the waves hit Hilo, 15 hours after the earthquake, not all the public had taken the warnings seriously, and 61 people were killed. About 7 hours later the tsunami struck Japan, killing 180. By the time that information of conditions in Chile reached TWS, three giant waves had already destroyed villages along an 805-km (500-mile) stretch of coastal South America, arriving only 15 minutes after the earthquake.

Lack of an effective warning system has been blamed for the extensive loss of life from the tsunami generated in the Indian Ocean in December 2004. Over 290,000 are estimated to have died in 11 countries, and thousands more remain missing. Tsunamis have been relatively rare in the Indian Ocean, and the area has no international warning system. The first tsunami-generated wave crashed into Sumatra only 30 minutes after shaking from the earthquake had subsided. The tsunami ultimately traveled nearly 5000 km (3107 miles) to Africa. In contrast to stronger preparedness levels in the Pacific countries, citizens and tourists were not fully aware of the dangers and many watched from the beach with catastrophic results. In Kobe, Japan, the World Conference on Disaster Reduction in January of 2005 laid the groundwork for the first tsunami warning system in the Indian Ocean, much of which was positioned in recent years. Indonesia has set up costly and sophisticated tsunami warning systems and carried out numerous drills. However, 400 people were killed in an October 2010 tsunami on the Mentawai Islands, indicating that those most at risk are not able to receive warnings through communications systems or cannot flee from a tsunami generated close to shore.

Typical Adverse Effects

The force of water in a bore, with pressures up to 10,000kg/m2, can raze everything in its path. The flooding from a tsunami, however, affects human settlements most, by water damage to homes, businesses, roads, and infrastructure. Withdrawal of the tsunami also causes significant damage. As the water is dragged back toward the sea, bottom sediments are scoured out, collapsing piers and port facilities and sweeping out foundations of buildings. Entire beaches have disappeared, and houses have been carried out to sea. Water levels and currents may change unpredictably, and boats of all sizes may be swamped, sunk, or battered (Figures 89-8 to 89-10; Figures 89-8 and 89-10, online).

Tsunami Risk Reduction Measures

Strategies to reduce vulnerability to tsunamis include:

Volcanic Eruptions

A volcano is a vent or chimney to the earth’s surface from a reservoir of molten rock, called magma, deep in the earth’s crust (see Chapter 15). Approximately 500 volcanoes are active (have erupted in recorded history), and many thousands are dormant (could become active again) or extinct (are not expected to erupt again). On average, about 50 volcanoes erupt every year; only about 150 are routinely monitored. Since 1000 AD, more than 300,000 people have been killed directly or indirectly by volcanic eruptions, and currently about 10% of the world’s population lives on or near potentially dangerous volcanoes.

Volcanology, the study of volcanoes, has experienced a period of intensified interest after five major eruptions in the 1980s and early 1990s: Mt St Helens in Washington state in the United States (1980), El Chichón in Mexico (1982), Galunggung in Indonesia (1982), Nevado del Ruiz in Colombia (1985), and Mt Pinatubo in the Philippines (1991). Although the Mt St Helens eruptions were predicted with remarkable accuracy, predictive capability on a worldwide basis for more explosive eruptions has not been achieved. No recognized immediate precursors to the eruption of El Chichón were known. It caused the worst volcanic disaster in Mexico’s history and killed approximately 2000 people. In Columbia, despite sufficient warnings, ineffective implementation and evacuation measures resulted in more than 22,000 deaths from the eruption of Nevado del Ruiz. Galunggung erupted for 9 months, disrupting the lives of 600,000 people. Despite a major evacuation effort from Mt Pinatubo, 320 people died, mainly from collapse of ash-covered roofs. A study of these eruptions underscores the importance of predisaster geoscience studies, volcanic hazard assessments, volcano monitoring, contingency planning, and enhanced communications between scientists and authorities. The world’s most dangerous volcanoes are in densely populated countries where only limited resources exist to monitor them.


Volcanic eruptions may be described as follows in descending order of intensity (Figure 89-12).


FIGURE 89-12 Eruption types. A, Pelean. B, Plinean. C, Vesuvian. D, Vulcanian. E, Strombolian. F, Hawaiian. G, Icelandic.

(Modified from United Nations Development Program: Introduction to hazards, ed 3, Disaster Management Training Programme, UN Organization for Coordination of Humanitarian Assistance, New York, 1997.)


No international scale exists to measure the size of volcanic eruptions. The volcanic explosivity index (VEI) estimates the energy released in a volcanic eruption, based on measurements of the ejected matter, height of the eruption cloud, and other observations. The VEI scale ranges from 0 to 8. The largest eruption recorded was in Tambora, Indonesia in 1815, which had a VEI of 7.

The primary volcanic hazards are associated with products of the eruption: pyroclastic flows, air-fall tephra, lava flows, and volcanic gases. The most destructive secondary hazards include lahars, landslides, and tsunamis.


Systematic surveillance of volcanoes, begun early in the 20th century at the Hawaiian Volcano Observatory, indicates that most eruptions are preceded by measurable geophysical and geochemical changes. Short-term forecasts of future volcanic activity in hours or months may be made through volcano monitoring techniques that include seismic monitoring, ground deformation studies, and observations and recordings of hydrothermal, geochemical, and geoelectric changes. By carefully monitoring these factors, scientists were able to issue a high confidence forecast of the 1991 Mt Pinatubo eruption, allowing a largely successful evacuation. The best basis for long-term forecasting (a year or longer) of a possible eruption is through geologic studies of the past history of each volcano. Each past eruption has left records in the form of lava beds. Deposits and layers of ash and tephra can be studied to determine the extent of the flows and length of time between eruptions.

Problems in Eruption Forecast and Prediction

Although significant progress has been made in long-term forecasting of volcanic eruptions, monitoring techniques have not progressed to the point of yielding precise predictions. For the purposes of warning the public and avoiding false alarms that create distrust and chaos, ideal predictions should provide precise information concerning the place, time, type, and magnitude of the eruption. The importance of enhanced communications between scientists and authorities is also emphasized. Despite sufficient warning, evacuation orders were not issued by local authorities, which resulted in more than 22,000 deaths from lahars produced by Nevado del Ruiz. The eruption of Mt St Helens was adequately monitored and forecasted, but the main explosion still surprised authorities because the volcano did not exhibit expected signs before eruption and because the blast was lateral rather than vertical; 57 people who remained in the danger area were killed.

The greatest constraint to predictability is lack of baseline monitoring studies, which depict the full range of characteristics of the volcano. Accumulating baseline data may require the study of the volcanic activity over thousands of years. Interpretation of baseline data enables differentiation of the precursory pattern of an actual eruption from other volcanic activity, such as intrusion of magma under the surface, which is sometimes termed aborted eruption. Before the 1982 eruption of El Chichón, virtually nothing was known of its history of frequent and violent eruptions. No monitoring was conducted before or during the brief eruption.

Developing countries suffer the greatest economic losses from volcanic eruptions. More than 99% of eruption-caused deaths since 1900 have been in developing countries. Because of shortages of funds and trained personnel, monitoring is also poorest in these countries.

Typical Adverse Effects

Casualties and Health

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