( By A Working Guide (WHO - OMS, 1989) )

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Chapter 5.Some information on natural disasters

Earthquakes result from continuous geological transformations in our planet. According to the latest theories, the tectonic plates of which the world’s surface consists are in constant movement. It is near the sites of friction between these plates that earthquakes occur.

A particularly dangerous phenomenon is the tsunami (maremoto), a great wave as much as several metres high which crashes down on the coasts as the result of an earthquake on the ocean bed. Sometimes it engulfs people who have fled towards the beaches. A tsunami may cross the ocean and crash down on to beaches thousands of kilometres away.

Table I. Media for information and communication at local level1

- arrangements for listening to national radio or television broadcasts

- local broadcasting, news film, radio forum or newspaper column

- newspapers, weekly magazines

- audiovisual media: photographs, transparencies, audiotapes, audiocassettes, 8-mm films, videocassettes, documentary films

- exhibits and displays, information racks, travelling exhibits, wall posters, designs

- speeches, letters to the newspapers, questions raised in the local political assembly, community discussion groups

- internal communications to administrators and employees in disaster-related jobs (circulars, bulletin boards, publications)

- leaflets, books, manuals, strip cartoons

- wall-sheets, posters, wall-newspapers

- inserts and enclosures in other forms of communication, such as magazines, newspapers, books

- meetings, seminars, conferences, working groups

- eye-witness accounts, discussions in schools, vocational training centres, clubs, public places

- telephone answering service with recorded messages

- public address systems

- special events: disaster simulations, exchanges of visits between communities on aspects of prevention, visiting days at early warning centres, civil protection centres, seismological observatories, dams, etc.

- folk media: story-telling, dance, song, poetry, puppet shows, music, street entertainment, posters

1Adapted from: OFFICE OF THE UNITED NATIONS DISASTER RELIEF COORDINATOR, GENEVA. Disaster prevention and mitigation: a compendium of current knowledge. Vol. 10, Information aspects. New York, United Nations, 1979.

The effects of earthquakes are simply and effectively epitomized in scales of intensity,1

1 See Annex 5.

It is very difficult to foretell the date and intensity of an earthquake but a few recent successes in that regard provide some hope2 At the present time, however, no reliable and generally accepted method of forecasting is available. An earthquake takes place after gradual accumulation of energy connected with subterranean stresses accompanied by important geological changes that may be noted over a period of a few weeks, months or even years before the actual quake. Some phenomena can be observed by the public:

2 On 4 February 1975 at Haicheng in China 400 000 people were successfully evacuated 5 ½ hours before shocks of magnitude 7.3 on the Richter scale destroyed 90% of the dwellings. In May 1977 in Yunnan Province, China, two earthquakes of magnitude 7.6 and 7.5 respectively were predicted, enabling the alarm to be given 8 minutes before the first shock.

· The water level in wells is subject to sudden fluctuations and there are variations in the temperature, level and turbidity of deep underground water.

· Premonitory shocks (foreshocks) may precede the main shock by anything from a few minutes to hundreds of days.


Other premonitory signs can be detected by means of scientific instruments.

Among disasters, floods are by far the most serious in terms of human lives and property. Plains liable to flooding always attract settlement: ease of tillage, water supply, transport and waste disposal. It is because of the concentration of the population on alluvial plains that floods are one of the most deadly natural phenomena. The Yangtse floods of 1931 killed over 3 million people by drowning or famine. WHO was called upon more recently still - in 1982 - to intervene in the same region. The causes of floods are:

· rises in stream levels resulting from abundant rainfall or the melting of the snows,

· ice barrages (the piling-up of large masses of ice coming from upstream) that cause stream levels to rise, and the sudden break-up of the masses of ice, which is responsible for flood waves,

· flash floods caused by intense rainfall and sometimes tornadoes,

· tidal bores,

· storm waves, caused by a combination of lunar tides and very high winds.

Hydrologists and meteorologists can forecast floods with a high degree of accuracy. In every zone exposed to flood risks it is possible to know in general terms the time of year, frequency, rate of flow, duration and depth of a rise in the water level. More specifically a flood can be predicted from a few hours to a few weeks in advance. Various methods of observation make it possible to give warning in various ways: radio, television, newspapers, telephone messages, megaphones, sirens, flags. Continuity of information and keeping the public in areas at risk constantly aware of the danger are very important.

Volcanic activity may range from fumaroles or moderate lava flows up to violent explosions which project various types of material to a great height. The nature of the activity depends on the viscosity of the magma (molten rock) which reaches the surface and on the volume of gases involved:

· Streams of lava vary greatly in volume, spread, thickness and speed of progression. Their path depends on the topography; while they are very impressive, they represent very little risk.

· Explosions of volcanic domes eject volcanic materials: volcanic bombs, blocks, lapilli, ash and scoria.

· Ignimbrite flows, consisting of a mixture of lava, ash and gas, form a cloud that moves at ground level at great speed.

· Nu�es ardentes or hot avalanches: mixtures of volcanic materials and gases hurtle down the slopes at over 100 km/h. A nu�e ardente killed some 30 000 people at Saint-Pierre in Martinique during the eruption of Mont Pel�e.

· Mud streams, a mixture of debris and water, arising, for example, from the sudden melting of glaciers (23 000 dead in Colombia in 1985) or bursting of the banks of artificial lakes in the crater, flow down the slopes at speeds of up to 100 km/h and may cover stretches of hundreds of kilometres; they are very deadly.

· Clouds of volcanic gases, (sulfuric, carbonic or fluoric acid) may contaminate water and crops, inflict burns and suffocate human beings and animals.

Volcanic eruptions leave a trail of destruction and death on the path of lava and mud streams and nu�es ardentes and in areas on which volcanic matter falls. Fires break out; roofs collapse under the weight of the ash; water and plants are contaminated. Sometimes eruptions are preceded or accompanied by earthquakes.


To envisage the type and intensity of a future eruption, the best approach is to proceed by analogy with previous eruptions of the same volcano. A map of the volcano may make it possible to predict the paths of lava flows. The previous periodicity of the eruptions of a volcano, if any, can serve as a very general guide for predicting a new eruption. Some eruptions are preceded by changes in behaviour of the fumaroles or hot springs on the mountain: the appearance of new features, an increase in temperature, changes in the composition of the gases. In other cases there are magnetic changes before an eruption. Some agitation can often be seen among animals. Scientific monitoring of the deformations (upswellings) of the ground and the shocks that accompany volcanic activity is very important. By combining these observations the specialist can predict eruptions, sometimes with astonishing accuracy (the eruption of Mauna Loa in Hawaii in 1942).

Tropical cyclones or hurricanes show a regular seasonal tendency. Every year they claim numerous victims and cause great damage. For example, in November 1970 a cyclone laid waste Eastern Pakistan (now Bangladesh), with a death toll of over 300 000. Cyclones originate over the sea in the tropics, particularly towards the end of summer. A cyclone has a central zone, its ‘eye’, with a diameter varying from 20 to 150 km. Around this calm centre the violent winds move clockwise in the southern hemisphere and anticlockwise in the northern.

The winds generated and accelerated by the difference in pressure between the centre and the periphery may blow at up to 300 km/h. The destructive power of cyclones is due to the force of the winds, to the intense and prolonged rainfall, which may also cause watercourses to flood, and to the tidal waves driven along by the winds to hurl themselves on to the coasts. Cyclones move westwards and die out when they reach land or colder sea surfaces.

Cyclone detection is based on weather radar, satellite data and even messages from airliners. Meteorologists can predict their intensity and path, often with a high degree of accuracy. However, allowance must be made for the possibility of error, since cyclones may follow a very irregular path. In countries liable to cyclones, warning is given by the authorities, generally in radio or television broadcasts. The warnings are followed by bulletins confirming, refining or cancelling them.

Drought and its fearsome consequences, desertification and famine, result from a combination of several factors:

· a reduction in rainfall causing a shortage of water,
· a reduction in vegetation, erosion of the soil, surface evaporation,
· an increase in human and animal populations,
· political and technological decisions at national and international levels.

In rural communities, economic factors (type of agriculture) and social elements (nomadism, semi-nomadism, drift of population towards the towns, etc.) affect the health and survival of families and moreover have an impact on the desertification process.

It is generally accepted that the struggle against desertification must be waged in two complementary ways: on the one hand appropriate political and technical measures must be taken at national and international level and on the other there must be a continuous process of information, education and organization in the local communities. As part of this process the role of the local health personnel is to develop programmes of prevention and adopt methods of health action based on community participation and self-organization. Voluntary workers and the Red Cross can make an important contribution here.


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