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Background information on disasters and natural hazards
From the Dealing with Disasters
online resource
Earthquakes
Floods
Hurricanes
Famine
Earthquakes
An earthquake is a sudden shaking or movement of the earth’s
crust.
Why do earthquakes happen?
The ground beneath our feet seems to be stable and fixed in one
place, but in fact it is constantly moving. The earth’s crust
is made of massive interlocking blocks of rock (tectonic plates),
rather like a three-dimensional jigsaw. The crust floats on a layer
of semi-liquid rock called the mantle. This semi-liquid rock flows
from place to place, which causes the blocks of rock in the crust
to move against each other. Most of these movements are so small
that they can only be detected with special scientific instruments.
However, sometimes there is a sudden release of energy in one place,
and the crust moves very violently. This can cause cracks in the
ground (fissures), and shaking of the earth’s crust (tremors).
Some earthquakes are caused by human activity. When very large
reservoirs (deeper than about 100m) are filled with water, the weight
of the water can disturb the rocks below and cause immediate shallow
earthquakes. Over a number of years, the water can spread deep into
the rocks, and destabilise the whole area.
In 1965, five years after it was first filled, the Koyna Reservoir
in India caused an earthquake of magnitude 6.5 on the Richter scale,
which killed 177 people, injured 2,200 and left thousands of people
homeless.
Where do earthquakes happen?
Earthquakes can happen anywhere in the world, but some places are
more at risk than others. Nearly all earthquakes (about 95 per cent)
happen at the point where mobile plates in the earth’s crust
join each other (faults). Parts of China, Japan, the Philippines,
South America, and the USA are situated on major faults. Earthquakes
can start deep in the crust (up to 270km deep), but the most dangerous
earthquakes start closer to the surface, less than 30km deep. Tremors
spread outwards from the epicentre, and a single earthquake can
affect an area of many hundreds of square kilometres.
How do we measure earthquakes?
There are different ways of measuring earthquakes. The Richter
scale (which is open-ended) is a method of measuring the energy,
or intensity of an earthquake (how strong it is). The Modified Mercalli
scale (which has twelve classes) is a way of measuring the effects
of an earthquake (how damaging it is). An earthquake that is small
in magnitude (low on the Richter scale) can still cause a lot of
damage (high on the Modified Mercalli scale). Different countries
may use different scales of intensity: Japan uses the Japan Meteorological
Scale (JMS), which has seven classes of earthquake intensity; the
Modified Mercalli scale (MM) is used in the Americas.
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In 1988, an earthquake of magnitude 6.9 on the Richter scale
struck an area of Armenia where there were several large towns
and cities. At least 25,000 people were killed, and 130,000
people were injured. The following year, an earthquake of
similar magnitude, 7.1 on the Richter scale, struck California
in the USA. Only 67 people were killed.
The Armenian earthquake happened at night, close to major
cities where houses were made from heavy concrete, which are
likely to collapse under stress. In the city of Leninakan,
80 per cent of buildings were destroyed. About 14 buildings
were hardly damaged at all. These buildings were the only
ones in the city which had been designed to withstand earthquakes.
The earthqake in the USA happened in an area of California
where fewer people live. Houses in this area are usually designed
to resist earthquakes. Although major roads were destroyed,
fewer people than normal were out driving, because many people
were watching an important baseball game in the area.
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Why do we need to measure earthquakes?
Measuring earthquakes helps us to learn more about them. Scales
which measure the effects of earthquakes, like the Modified Mercalli
scale, help us to understand how earthquakes affect people. This
means that we can understand more about how earthquakes cause damage,
injury, and loss of life. This can help communities to prepare better,
and minimise their risk.
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The Modified Mercalli scale
The Mercalli scale was developed in 1902 by Giuseppe Mercalli.
Since then, the scale has been changed to include the effects
of earthquakes on more recent inventions such as cars and
skyscrapers – things which didn’t exist in Mercalli’s
day! This scale has twelve classes. Some of the classes are
summarised below.
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Class I
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Not felt, except by a very few people.
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Class V
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Felt by nearly everyone. Some windows broken. Unstable objects
fall over. Pendulum clocks may stop.
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Class VII
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Everybody runs outdoors. Buildings designed to resist earthquakes
and made of strong materials are hardly damaged at all; ordinary
buildings are moderately damaged; buildings not designed to
resist earthquakes, or made from weaker materials, are severely
damaged. Earthquake is noticed by people driving cars.
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Class XII
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Total devastation. Earthquake waves are seen on the surface
of the ground. Objects are thrown upwards into the air.
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Can we tell when an earthquake is going to happen?
It is very difficult to predict an earthquake, and most earthquakes
happen with little or no warning. This is because scientists still
do not fully understand the causes of earthquakes. Predicting earthquakes
can be expensive, because lots of special scientific equipment is
needed. Poorer countries and regions will not be able to afford
such equipment.
If a prediction is wrong, then there can be serious consequences.
Many people cannot afford to stop working, even for a few days.
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In 1986 56,000 people were evacuated from the towns of Lucca
and Modena in Italy. Shops and businesses closed for two days.
No earthquake happened, and local people were angry about
the loss of business, and the inconvenience of the evacuation.
The mayor was forced to resign.
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One way to predict future earthquakes is to look at how many earthquakes
have happened in a particular area in the past. Studies suggest
that an earthquake of magnitude 8 or more on the Richter scale will
hit California on average once every 160 years.
If an earthquake is detected as soon as it starts, then better
precautions can be taken: for example, power stations can be shut
down, hospital generators started, and the emergency services can
be alerted.
| In February 1975, the
Chinese authorities successfully predicted a major earthquake
in the city of Haicheng-Yingkou, and many people were evacuated.
Although more than one thousand people were killed, this
was far fewer than might have been expected. Successful
predictions such as this one, which save lives, are very
rare.
In Japan, special instruments called accelerometers, which
detect earthquakes as soon as they start, are located every
20km along the Tokaido railway line. When an earthquake
is detected, electricity is automatically shut down, and
the train’s brakes come on. The track is then inspected
for damage. Over the last twenty years, trains have been
stopped about 100 times by this system. |
How are people affected by earthquakes?
The effects of earthquakes are very variable. People are more likely
to die or be injured where large numbers of people live close together,
and where local buildings are not designed to resist earthquakes.
About 95 per cent of people killed in an earthquake are killed by
falling buildings.
An earthquake can cause other dangerous events, such as landslides,
floods, fires, and huge ocean waves called tsunamis.
When an earthquake happens it is important to help survivors quickly,
and most of the first-aid and rescue work will be carried out by
people already in the region. After an earthquake, the first thing
people do is to search for their family and friends. People can
be helped better when there are good transport systems, and trained
local emergency workers such as nurses and paramedics.
Why are some people more at risk than others?
| Factbox
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There are more than 3000
perceptible earthquakes (ones that people can feel)
worldwide every year.
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The first known earthquake
detector was invented in China in 132 A.D. by Zhang
Heng. It used a pendulum suspended inside a bronze
jar.
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About 360 earthquakes
in Britain are detected by scientists every year.
Only about 20 of these can be felt by people.
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Earthquakes
are most dangerous when they happen at night. This
is because people may sleep through the first tremors
(foreshocks), and so have less time to prepare. Also,
lying flat in bed means that you are more likely
to be hit by falling objects than if you are standing
up.
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Floods
A flood is when water flows or rises above and beyond its normal
place. The danger this causes to people and buildings is called
the flood hazard.
What is a flood?
The most common kind of flood happens when a river overflows its
banks, and water spreads on to the surrounding land (a riverine
flood). This is caused by rainwater or melting snow draining into
the river faster than the river can discharge water into the sea.
The amount of water that a river can hold before a flood starts
is known as channel capacity.
Other causes of flooding are strong tides, storms at sea, hurricanes,
and tsunamis.
Where do floods happen?
Floods are a normal part of the life of a river. Most rivers have
areas nearby known as floodplains which fill with water when the
river becomes too full. Floodplains are usually on low ground, often
near the coast.
The chances of a flood happening in any one place are related to
the shape of the river and the land around it, the local and regional
weather, and the ways in which people live on and use the surrounding
land.
Low-lying coastal regions can also be at risk from flooding, especially
when there are severe storms at sea.
Living in floodplains
From the dawn of human history, people have chosen to live in floodplains.
Regular flooding improves the fertility of the soil, so people can
grow more food; living beside a river gives people and their animals
and crops easy access to life-giving water; rivers are important
for transport and communication; and many industries, both traditional
and modern, require a regular supply of water.
In 1955 10 million people lived in the floodplains of the USA.
By 1985 the number had doubled to 20 million people.
| Living with flooding - Bangladesh
Bangladesh is the largest river delta in the world, and the
country is used to dealing with regular flooding. On average,
20--30 per cent of the land is covered by the floods each
year.
The season of floods in Bangladesh is known as kharif, and
the season of drought is known as rabi.
Most people in Bangladesh work on the land, and farmers have
developed ways to use the floodwaters and the natural fertility
that they bring. The crops grown in Bangladesh, such as rice
and jute, thrive in wet conditions. Rice is the main food
crop in Bangladesh, and farmers grow different kinds of rice
in different seasons of the year, depending on the amount
of water standing on the land.
The soil in Bangladesh is one of the most fertile in the
world. Flooding can improve soil fertility by encouraging
the growth of algae, causing plants killed by the water to
decompose, and by starting chemical reactions in the soil.
All of these processes release vital minerals which help crops
to grow.
Bangladesh has about 250 rivers. Many of its major rivers
start in other countries – 90 per cent of the water in
Bangladeshi rivers comes from outside the country. The three
main rivers in Bangladesh are the Padma (or Ganges), the Jamuna
(or Brahmaputra), and the Meghna.
| One river, many
countries
The Brahmaputra begins in the Chemayungdung Mountains,
which are partly in Nepal. In Tibet, it is called the
Yarlung Yangbo. It passes through Lhasa on its way to
China, where it is called simply the Yangbo. It enters
India in the north-east state of Arunachal Pradesh,
where it is called the Dihang. Meandering through Assam,
it is known as the Brahmaputra, the name by which it
is known internationally. In Bangladesh it is called
the Jamuna, and it keeps this name until it joins the
Ganges, and eventually ends up in the Bay of Bengal. |
Flooding in Bangladesh happens every year, and the Bangladeshi
people are usually well prepared. Problems can occur when
the floods are much greater than usual, or happen with little
or no warning. Over the last decade or so, Bangladesh has
suffered a number of very severe floods, most recently in
1998. The reasons for this are not clear, and different theories
have been put forward.
It has been suggested that deforestation in the Himalayas
has increased the amount of water and sediment in Bangladeshi
rivers, which in turn increases the flooding. Another theory
blames the construction of the Farakka Barrage. This was built
by India just beyond the border with Bangladesh to regulate
the amount of water in the River Hoogly, which flows out of
the Ganges. Overall, the flow of water in the Hoogly has been
reduced, but the flow of water into Bangladesh via the Ganges
has increased. Others say that the recent major floods are
part of normal statistical variation. Whatever the precise
combination of reasons for the increased flood hazard, it
is clear that any solution must be an international one. |
| Living
with flooding -- the USA
Large parts of the USA are regularly at risk from flooding.
Estimates show that between 50 and 90 people are killed in
floods each year.
In the 1930s, dams were built on many of the large rivers
in the USA to control the flow of water, but it was found
that dams alone were not enough to solve the problem. People
still moved into vulnerable areas, and dams themselves can
cause severe flooding if they fail.
Nowadays, a combination of approaches is used in addition
to dams and other engineering solutions. A national flood-insurance
scheme encourages vulnerable communities to insure against
flood damage. At the same time, flood-warning systems and
flood-proofing measures have been improved. In some areas,
local governments have bought land in floodplains, to make
sure that these areas are kept clear of excessive housing
development.
The experience of the USA shows that even countries with
many resources at their disposal must still search for the
right solutions -- there is never a simple answer. |
Why are some people more at risk than others?
Hurricanes
What is a hurricane?
A hurricane is a tropical storm with strong winds, which starts
at sea and can travel long distances. This kind of storm has different
names in different parts of the world. In the Atlantic Ocean, they
are called hurricanes; in the western Pacific Ocean, they are called
typhoons; in the Indian Ocean and Australasia, they are called tropical
cyclones.
In the northern hemisphere the winds travel anti-clockwise, and
in the southern hemisphere they travel clockwise.
A tornado, or whirlwind is a smaller, more compact version of a
hurricane which often forms over land. Tornadoes are common in the
USA, but can also happen in Australia, Japan, and Central Asia.
They occasionally happen in Europe.
Why do hurricanes happen?
Hurricanes are caused by water evaporating from warm seas (the
sea temperature must be at least 26ºC). The water vapour rises in
the air before it condenses in the upper atmosphere. Condensation
releases heat, which causes the air to rise more quickly. As the
air above the region of evaporation rises, more air is drawn in,
blowing across the surface of the sea. In turn, this increases the
rate of evaporation, which fuels the process even more – and
a hurricane is formed.
Where are the effects of hurricanes felt?
Hurricanes out at sea are not necessarily dangerous, except to
shipping. Most damage occurs when a hurricane hits the land, and
coastal regions are especially at risk.
How are hurricanes measured?
Wind speeds can be measured on the Beaufort Scale, which runs from
0 to 12. As well as measuring how powerful a hurricane is, it is
important to know how damaging it might be. This is measured using
an intensity scale, which shows what happens, or might happen when
the hurricane hits land.
| The Saffir-Simpson scale
This intensity scale estimates how damaging a hurricane might
be. It runs from 1 to 5. Some of the classes are summarised
below. |
| Class 1 |
Winds 120—150 kilometres per hour (km/hr);
damage to trees and shrubs; some damage to signs; some low-lying
coastal roads flooded. |
| Class 3 |
Winds 175—210 km/hr; there may be a storm
surge 2.5—3.6m above normal sea level; many trees blown
down; lots of damage to roofs, doors, and windows; serious
coastal flooding; evacuation of people living near the coast
might be necessary. |
| Class 5 |
Winds more than 250 km/hr; there may be a
storm surge more than 5.5m above normal sea level; many buildings
blown down; evacuation of people up to 16km from the coast
might be necessary. |
Can we tell when a hurricane is going to happen?
It is often hard to tell where hurricanes will start. However,
once a hurricane has formed, then scientists can often track its
path across the ocean and predict where it will hit land.
Hurricanes have been given names since the 1940s, to help in studying
individual hurricanes and in warning people of their impact. When
wind speeds reach 40 mph, then a tropical storm is declared, and
a name is given. A storm is officially considered to be a hurricane
when wind speeds reach 75 mph.
How are people affected by hurricanes?
The main danger in a hurricane is the storm surge. As the hurricane
approaches land, sea levels rise and coastal regions may experience
severe flooding. High winds are an obvious danger, and people are
at risk from falling buildings and from objects blown around by
the wind. Sometimes hurricanes cause torrential rain, creating flash
floods.
Many people may be drowned in the floods. Those who have lost their
homes will need temporary shelter, and help to rebuild. If local
communities are destroyed, and people are forced to move away from
the area, it can be hard for people to find work and support themselves
and their families. When crops and stores of grain and food are
destroyed, recovery takes much longer, and food has to be provided
from outside the affected area.
| Hurricanes
in Jamaica – Hurricane Gilbert, 1988
Hurricane Gilbert hit Jamaica on 12 September 1988. Because
Jamaica already had good warning and sheltering systems, only
45 people were killed, including 11 people who were shot by
the police as suspected looters.
However, many buildings were damaged. In Jamaica, locally-made
aluminium sheeting is a common and cheap roofing material.
In high winds, the sheets of metal tore away from the houses.
Damage to hospitals reduced medical services at a time when
they were needed most, and expensive medical equipment was
ruined.
The main telephone exchange lost part of its roof, and all
international phone lines were cut off. Crops, food stores,
stables and barns were damaged. Jamaica went from being self-sufficient
in food to being dependent on imports. Even though relatively
few people were killed, this example shows that damage to
buildings can have far-reaching consequences.
At the time of hurricane Gilbert, about 40 per cent of the
damage was covered by insurance policies, which meant that
reconstruction could start quickly. New building codes suitable
for local conditions were introduced, including reinforcement
of walls and the use of hurricane straps to hold roofs down.
Jamaica is increasingly prepared for future emergencies, although
much still remains to be done. |
Why are some people more at risk than others?
Famine
What is famine?
A famine is when large numbers of people in a region are unable
to get enough food to keep themselves alive, a situation which leads
to a sharp increase in deaths from starvation. Famine is difficult
to define, because it is not an absolute measure. Instead, a number
of indicators may be used to judge where and when there is
a risk of famine, and when a famine is considered to have started.
Indicators include child mortality figures, and the presence of
certain illnesses caused by starvation.
With any attempt to define an actual famine, it must be remembered
that many, many people in the world today exist in a state of chronic
malnutrition. The World Heath Organisation believes that more than
100 million children are moderately to severely malnourished. Oxfam
estimates that 18 million people worldwide die of hunger every year.
When people are surviving on the absolute minimum of food, then
any change in the food supply, however small, can have devastating
consequences.
Why do famines happen?
Famines can happen for many reasons, and actual shortage of food
is only one possible cause of famine. In fact, in today’s era
of mass transport there is very rarely an absolute shortage of food.
People with money can often buy food, or travel to places where
food is more readily available. At present, enough food is produced
to feed everyone on the earth. The problem is getting the right
food in the right place.
Famine is caused when people do not have access (sometimes called
‘entitlement’) to food. In areas where people depend on
the food that they grow, a period of drought, or other severe weather,
can lead to crop failure. If there is no way for local people to
earn money to buy food, and reserve stocks of food are low, then
there are no alternatives -- a famine is threatened.
In many areas of the world today, famine has been triggered by
war. Wars disrupt local economies, often for long periods, and destruction
of crops is frequently used as a weapon of war; large numbers of
refugees gathering in a small area can put great strain on the food
supply. Up to 110 million land-mines lie in fields and roads around
the world – good land remains uncultivated while people go
hungry.
In areas prone to spells of drought, communities have developed
strategies for coping with periodic food shortages. These might
include building up communal stores of grain, a system of sharing
food between families and communities, foraging for wild foods,
and moving temporarily to another place. The declaration of a famine
by international governments and other agencies can overshadow the
fact that people have almost always been coping without assistance
for very long periods.
Where do famines happen?
Famine is not dependent on geography, and most countries have experienced
famine at some time in their history. The reason why countries of
the North no longer have famines is not primarily because the amount
of available food has increased (although this is one factor), but
because supply and distribution are more efficient, and because
people in Northern countries usually have the resources to buy what
they need. In times of local shortages, food can be imported from
other countries.
Can we tell when a famine is going to happen?
One of the reasons why it can be hard to predict a famine is that
people do not always agree on definitions. Another difficulty is
that accurate information about nutrition and health in a particular
area can be hard to gather.
| "It is
important to keep records of rainfall, the price of food at
market, and annual harvest yields. I also look out for people
who are collecting wild fruits or who are surviving on leaves
or plant roots – famine foods. This kind of information
gives us early warnings about coming food shortages. [We]
can then convince the aid agencies of our need before things
get much worse and become a disaster . . . with things as
they are, these facts and figures can mean the difference
between life and death." Guitano Erib, who works for
the Sudan Relief and Rehabilitation Association. Quoted in
Global Express, no. 11, October 1998. |
It is clear, however, that many people live with hunger every day
of their lives. In such circumstances it is important to look for
long-term solutions, instead of simply responding to each crisis
as it happens. There is evidence that the right kind of support
can significantly increase people’s ability to avoid famine,
and, if it does happen, to recover more quickly.
How are people affected by famine?
Before a famine is declared, many people in the area will have
been living with chronic malnutrition for some time. However, even
in conditions of extreme famine, not everybody is affected equally.
Children, the elderly, pregnant and breastfeeding women, and those
already suffering from disease will be worst hit.
Those who have transport, or are strong enough to walk, may leave
the area. Some people never return, and communities cannot be rebuilt.
In other areas, an influx of refugees will put pressure on scarce
resources, spreading the problem more widely.
Farming suffers as animals die, and people become too weak to work
the land. When grain stores have been used for food, there is no
seed for next season’s planting, and there will be no harvest.
Although food aid is an important short-term solution to famine,
it is not always effective, and can sometimes even make things worse.
It is hard to make sure that food gets to those who need it most,
and in the long term food aid can disrupt the local economy and
make it harder for the region to get back on its feet. If governments
or charities flood the area with large amounts of free, or very
cheap imported food, then local farmers cannot get a good price
for the crops they grow.
Why are some people more at risk than others?
Factbox
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Supermarkets
and shops in the UK throw away more than £350 million
worth of food every year. |
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Even
during times of famine, many Southern countries continue
to export food to the North. This is because they need
to earn money to pay the huge debts that they owe to organisations
like the World Bank. |
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The
average person in the USA consumes 3,650 calories per
day. The average person in Bangladesh consumes 1,925 calories
per day. An adult needs between 2000 and 3000 calories
per day to stay healthy. |
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Farm
animals consume nearly half the world’s grains.
An acre of cereals can produce five times more protein
than
an acre devoted to meat production. |
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