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Global Distribution of tectonic
Tectonics hazards include earthquakes, volcanoes and
Most of the world’s earthquakes occur in clearly defined linear
patterns, generally following plate boundaries.
These linear chains can be in broad belts or narrow belts and
generally are found on the ‘Pacific ring of fire’.
Broad belts of earthquakes are associated with subduction
zones (where one plate is forced under the other) and collision
Narrow belts of earthquakes however are associated with
constructive margins, where new material is formed and plates
are moving apart.
What is an earthquake?
An Earthquake is a series of
vibrations or seismic waves which
originate from the focus.
Focus: The focus is the point at
which the plates release their
tension or compression suddenly.
Epicentre: Marks the point of the
surface of the Earth immediately
above the focus of the earthquake.
A large earthquake can be
preceded by smaller tremors known
as foreshocks and followed by
• Aftershocks can be devastating
because they damage buildings that
have already been damaged by the
• Seismic waves are able to travel along
the surface of the Earth and through
the body of the earth.
Following an earthquake two types of body waves (waves within the Earth’s interior) occur.
The first kind are P-Waves (primary or pressure waves) which travel by compression and
expansion. These waves are able to pass through all matter be it solid, liquid or gas.
The transverse S-Waves or Shear waves move in a series of oscillations at right-angles to
the direction of movement. S-waves arrive after P-waves as they travel slower. This is
because they have no rigidity to support sideways motion and therefore they cannot pass
through liquids and gases – only solid material.
SURFACE WAVES unlike body waves travel only through the crust, surface waves are of a
lower frequency than body waves, and are easily distinguished on a seismogram as a result.
Though they arrive after body waves, it is surface waves that are almost entirely responsible
for the damage and destruction associated with earthquakes. This damage and the strength
of the surface waves are reduced in deeper earthquakes.
Mohorovičić discontinuity, often called Moho, is the
boundary between Earth's crust and the mantle. It was
discovered by Yugoslavian geophysicist Andrija Mohoroviči
when analysing an earthquake in Croatia in 1909.
Mohorovicic realised that the velocity of a seismic waves is
related to the density of the material that it is moving
through and that slower waves were travelling from the
focus of the earthquake through the upper layer of the
crust, whereas faster waves had accerlearated due to a
higher density material being present at depth. He
suggested that a change in density from 2.9g/cm3 to
3.3g/cm3 marks the boundary between the Earth’s crust.
Khan Academy video lecture on
Later geologists found a shadow zone, an area between 105° and 142°
from the source of the earthquake, within which they could not detect shock
The explanation was that shock waves had passed from a solid to a liquid.
Thus S-Waves would stop and P-Waves would be refracted. The geologists
concluded that there was a change in density from 5.5g/cm3 at 2900km to
a density of 10g/cm3. This was effectively the boundary between the
mantle and the core. Within the earth there is an inner core of very dense
solid material – the densit of the inner core goes up to as much as
13.6/cm3 at the centre of the Earth.
The nature of rock and sediment beneath the ground influences the
patterns of shocks and vibrations during an earthquake. Unconsolidated
sediments such as sand shake in a less predicable way than solid rock.
Hence the damage is far greater to foundations of buildings. P waves from
earthquakes can turn solid sediments into fluids like quicksand disrupting
sub surface water conditions. This is known as liquefaction or fluidisation
and can wreck foundations of large buildings and other structures.
Comparison of scales
Richter Scale Mercalli Scale
Measures: The energy released by the
The effects caused by the
Measuring Tool: Seismograph Observation
Calculation: Base-10 logarithmic scale
obtained by calculating logarithm
of the amplitude of waves.
Quantified from observation of
effect on earth’s surface, human,
objects and man-made structures
Scale: From 2.0 to 10.0+ (never
recorded). A 3.0 earthquake is 10
times stronger than a 2.0
earthquake and 100x stronger
than a 1.0 quake.
Scale of I-XII, I being not felt, XII
Consistency: Varies at different distances from
the epicenter, but one value is
given for the earthquake as a
Varies depending on distance from
Problems: Doesn't’t take damage into
Strength and depth of earthquake + Number of aftershocks:
the stronger the earthquake the more damage it can do. For
example, an earthquake of 6.0 on the Richter scale is 100x
more powerful than one of 4.0; the more aftershocks there
are the greater the damage that Is done. Earthquakes that
occur close to the surface (shallow-focus earthquakes)
potentially should do more damage than deep focus
earthquakes as more of the energy of the later is absorbed
by overlying rocks.
Population density: if an earthquake that hits an area of high
population density such as the Tokyo region of Japan, could
inflict far more damage than one that hits an area of low
population and building density.
Type of buildings: MEDCs generally have better quality
buildings, more emergency services and funds to recover
from disasters. People in MEDCs are more likely also not
have insurance cover than those in LEDCs and are better
educated as to how to respond in such events.
Time of day: An earthquake during a busy time, such as
rush hour, may cause more deaths than one at a quiet time.
Industrial and commercial areas have fewer people on them
on Sundays, homes have more people in them at night.
Distance from the epicentre: The closer a place is to the
epicentre of the earthquake the greater the damage that
Type of rocks and sediments: loose materials may act
like liquid when shaken, a process known as liquefaction;
solid rock is much safer and buildings should be built on
flat areas formed of solid rock.
Secondary hazards: an earthquake may cause mudslides
and tsunamis and fires; also contaminated water,
disease, hunger and hypothermia.
Economic development: this affects the level of
preparedness and effectiveness of emergency response
services, access to technology and quality of health
Resultant hazards of
Most earthquakes occur with little if any advance warning. Some places, such as California and
Tokyo which have considerable experience of earthquakes and have developed earthquake action
plans and information programmes to increase public awareness about what to do in an earthquake.
Most problems are associated with damage to buildings, structures and transport systems. The
collapse of building structures is the direct cause of many injuries and deaths, but it also reduces the
effect of the emergency services. In some cases more damage in caused by the aftershocks that
follow the main earthquake, as they shake the already weakened structures. Aftershocks are more
subdued but long lasting and more frequent than the main tremor. Buildings partly damaged during
the earthquake may be completely destroyed by the aftershocks.
Some earthquakes involve surface displacement, generally along fault lines. This may lead to the
fracture of gas pipes, as well as causing damage to lines of communication. The cost of repairing
such fractures is considerable.
Earthquakes may cause other geomorphological hazards such as landslides, liquefaction (the
conversion of unconsolidated sediments into materials that act like liquids) and tsunamis.
There are both primary affects and secondary affects. Primary effects are the immediate damage
caused by the quake such as collapsing of infrastructure, whereas secondary effects are after-affects
of a earthquake such as tsunamis, landslides and spread of disease.
Earthquakes and human
Human activities can trigger earthquakes, or
alter the magnitude and frequency of
earthquakes in three main ways…
1. Through underground disposal of liquid
2. By underground nuclear testing and
3. By increasing crustal loading
Disposal of liquid waste
Water that is salty or polluted by chemicals needs to be disposed of in a manner that
prevents it from contaminating freshwater sources. It is usually most economical to
isolate and inject it into deep underground wells, below any aquifers that provide drinking
water. This process is known as wastewater injection.
Most wastewater currently disposed of is saltwater found in the same rock formations as
oil and gas. This saltwater comes up as a by-product during the oil and gas production
process. It is often in large volumes and is too salty or contains minerals and other
chemicals that economically preclude it from being cleaned and released at the surface
In Denver, Colorado, wastewater was injected into underlying rocks during the 1960s.
Water was contaminated by chemical warfare agents, and the toxic wastes were too
costly to transport off-site for disposal. Thus it was decided to dispose of it down a well
over 3500m deep. Disposal began in March 1962 and was followed soon afterwards by
a series of minor earthquakes, in an area previously free of earthquake activity. Between
1962-65 over 700 minor earthquakes are monitored in the region.
The injection of the liquid waste into the bedrock lubricated and reactivated a series of
deep underground faults which had been inactive for a long time. The more wastewater
was put down the well, the larger the number or minor earthquakes.
testing has triggered
earthquakes in a number of
places. In 1968 testing of a
series of 1200 tonne bombs
in Nevada set off over 30
minor earthquakes in the
area over the following
three days. The explosion
from nuclear testing results
in a simultaneous release of
plate tension which results
in minor earthquakes.
Increased crustal loading
Earthquakes can be caused by adding increased
loads on previously stable land surfaces. For
example, the weight of water behind large reservoirs
can trigger earthquakes.
What should people do
People deal with earthquakes in a
number of ways. These include:
1. Do nothing and accept the hazard
2. Adjust to living in a hazardous
environment – strengthen your
3. Leave the Area.
The main ways for preparing for
1. Better forecasting and warning
2. Improve building design/location
3. Establish emergency procedures
There are a number of ways of predicting
and monitoring earthquakes, which involve
the measurement of…
1. Small-scale ground surface changes
2. Small-scale uplift or subsidence
3. Ground tilt
4. Changes in rock stress
5. Micro-earthquake activity (clusters of
6. Anomalies in the Earth’s magnetic field
7. Changes in electrical resistivity of
A volcano is an opening
in the Earth’s crust
where magma – a
mixture of red-hot liquid
rock, mineral crystals,
rock fragments and
dissolved gases from
inside the planet erupts
onto the surface.
Types of Volcanoes
There are two main types of Volcanoes:
The shape of a volcano depends on the type of lava it contains.
Very hot, runny lava produces gently sloping shield volcanoes
Thick material produces cone-shaped volcanoes (Plinian type).
These may be the result of many volcanic eruptions over a long
period of time.
The shape of the volcano also depends on the amount of change
there has been since the volcanic eruption.
Cone volcanoes are associated with destructive plate boundaries,
whereas shield volcanoes are characteristic of constructive
boundaries and hotspots.
Types of Volcanoes:
Shield volcanoes are low with gently
sloping sides and are formed from layers of
Eruptions are typically non-explosive but
Shield volcanoes produce fast flowing fluid
[lava] that can flow for many miles.
Shield volcanoes are usually found at
constructive boundaries and sometimes at
volcanic hotspots. Examples of the largest
shield volcano is Mauna loa on Hawaii. It is
also one of the Earth’s most active
volcanoes and is carefully monitored. The
most recent eruption was in 1984.
Types of volcanoes: Cone
Acid lava that flows from cone or
dome volcanoes is much more
viscous than lava which flows from
Dome volcanoes have much
steeper sides than shield
volcanoes. This is because the
lava is thick and sticky. It cannot
flow very far before it cools and
An example is Puy de Dome in
the Auvergne region of France
which last erupted over 1 million
Volcanoes are classified in a number of ways.
These include the type of flow, type of eruption
and level of activity….
Aa flow is a few metres thick. It consists
of two distinct zones – an upper rubbly
part, and a lower part of solid lava which
cools slowly. Aa surfaces are a loose
jumble or irregularly shaped cindery
blocks with sharp sides.
By contrast, pahoehoe flow is the least
viscous of all lavas; rates of advance can
be slow. It has a cool surface, with flow
underneath the surface. Pahoehoe
surfaces can be smooth and glossy but
may also have cavities; surfaces may
also be crumpled with channels
The amount of silica makes the difference
between volcanoes that erupt
continuously, such as those on Iceland
and Hawaii and those where eruptions
are infrequent but violent, such as Japan
Lava is released where the oceans meet the
continents absorbs silica-rich sediments; this
causes the lava to become less viscous and
block the vents until enough pressure has built
up to break them open. Each year about 20km2
of land is covered by lava flows. These may
initially reach temperatures of over 1000°c,
resulting in severe social and economic
disruption. However, cooled lava flows are very
fertile and therefore attract dense population
settlement and intense agriculture production.
There are a number of ways of reducing lava
flows. These include spraying them with water,
bombing them and seeding the lava with
Volcanoes are found in three states – extinct,
dormant and active. An extinct volcano will
never erupt again. A dormant volcano has not
erupted in 2000 years. An active volcano has
erupted recently and is likely to erupt again.
Types of volcanic
Volcanic eruptions are often thought of as cataclysmic explosions that
produce vast quantities of lava, ash and other volcanic materials.
However, volcanoes can actually erupt in a range of different ways. A
volcano can erupt in a range of different ways during different eruptions
and even during different stages in the same eruption.
These different types of volcanic eruptions include…
1. Plinian eruptions
2. Hawaiian eruptions.
These are the most explosive and violent of volcanic eruptions. They
produce huge plumes of ash and gas that typically takes the shape of
a huge mushroom cloud.
In Plinian Eruptions the magma has high silica content. They are
highly explosive and the AD79 eruption that buried Pompeii and
Herculaneum was one of these.
Plinian eruptions are started by highly viscous magma that has high
gas content. As the magma emerges it depressurizes and this allows
the gas to expand, propelling pyroclastic material as high as 45 km in
the air, at hundreds of feet per second, up and out of the
Troposphere. These eruptions can last for days and create a
sustained and tall eruption plume, which drops huge amount of
tephra, fallen volcanic material, on surrounding areas. Additionally, a
Plinian eruption can produce extremely fast moving lava flows that
destroy everything in their path.
In Hawaiian Eruptions the lava is more basic and
basaltic, with low gas pressures and low silica
content. This means the lava is very runny.
These eruptions are generally not explosive or
destructive and do not throw huge amounts of
Tephra or pyroclastic material in the air.
Instead they produce low-viscosity, low-gas-
content lava that flows over large areas producing
gently sloping shield volcanoes and lava
Eruptions can form fire fountains, Lava thrust up
to 50m in the air for many hours. The general
eruption style is a steady lava flow from a central
vent, which can produce wide lava lakes, ponds
of lava forming in craters or other depressions.
Strombolian eruptions are named after Stromboli in
The effects are impressive but not particularly
They eject short bursts of lava 15 to 90 meters in the
air. The lava has a fairly high viscosity (it’s quite thick
due to its high silica content), so gas pressure builds
up before material can be ejected from the volcano.
These regular explosions can produce impressive
booming sounds, however the eruptions are relatively
Lava flows from Strombolian eruptions are not
common though they do throw out large quantities of
Eruptions are commonly marked by a white cloud of
steam emitted from the crater
Types of eruptions
Icelandic lava eruptions
Characterised by persistent
Large quantities of basaltic
lava build up vast horizontal
Violent gas explosions blast out
plugs of sticky or cooled lava.
Fragments build up into cones of
ash and pumice.
Vulcanian eruptions occur when
there is very viscious lava which
solidifies rapidly after an
Often the eruption clears a
blocked vent and spews large
quantities of volcanic ash into
Characterised by very
powerful blasts of gas
pushing ash clouds high into
More violent than Vulcanian
Volcanic hazards can be divided into six main
1. Lava flows
2. Ballistics and Tephra louds
3. Pyroclastic flows
4. Gases and acid rain
5. Lahars (mudflows)
6. Glacier bursts
As and debris falls steadily from the volcanic cloud, blanketing the ground with a deposit known as
a pyroclastic fall. These can be very dangerous, especially as the fine ash particles can damage
people’s lungs. Also ash is fairly heavy – a small layer only a few centimetres thick can be enough
to cause a building to collapse. Dust and fine particles also cause havoc with global climate
A pyroclastic flow is a fast-moving current of hot gas and rock (collectively known as tephra), which
reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach
temperatures of about 1,000 °C. Pyroclastic flows normally hug the ground and travel downhill, or
spread laterally under gravity. Their speed depends upon the density of the current, the volcanic
output rate, and the gradient of the slope. They are a common and devastating result of certain
explosive volcanic eruptions.
Lahars are another hazard associated with volcanoes. A combination of heavy rain and unstable
ash increase the hazard of lahars. Lahars are extremely dangerous especially to those living in
valley areas near a volcano. Lahars can undercut banks and cause houses on those banks to be
destroyed. Lahars can bury and destroy manmade structures including roads and bridges. At
Nevado del Ruiz, lahars destroyed an entire city; filling the first floor of a hospital with mud,
breaking windows, floating cars, and leaving debris in the tops of trees
The hazards associated with volcanic eruption vary spatially. Close the volcano people are at risk
of large fragments of debris, ash falls and poisonous gases. Whereas, further away pyroclastic
flows may prove hazardous and mudflows and debris flows may have an impact on more distant
Primary Hazards Secondary Hazards Socio-Economic
• Pyroclastic flows
• Volcanic bombs
• Lava flows
• Ash fallout
• Volcanic gases
• Atmospheric ash
• Acid rainfall
• Destruction of
• Loss of life
• Loss of farmland
• Destruction of
roads, airstrips and
• Disruption of
The strength of a volcano is measured by the
Volcanic Explosive index (VEI). This is based on the
amount of material ejected in the explosion, the
height of the cloud it creates, and the amount of
damage caused. Any explosion above level 5 is
considered to be very large and violent. A VEI 8
refers to a super volcano.
Scientists are increasingly successful in predicting volcanoes. Since 1980 they have
correctly predicted 19 of Mt St Helens, 22 eruptions and Alaska's redoubt volcano in
1989. There have been false alarms too: in 1976 72000 residents of Guadeloupe were
forced in leave their homes, and in 1980 Mammoth Lake in California suffered from a
reduction in tourist numbers owing to mounting concern regarding volcanic activity.
Volcanoes are easier to predict than earthquakes since there are certain signs. The main
ways of predicting volcanoes include monitoring using…
1. Seismometers to record swarms of tiny earthquakes that occur as the magma rises
2. Chemical sensors to measure increased sulphur levels
3. Lasers to detect the physical swelling of the volcano
4. Ultrasound to monitor low-frequency waves in the magma, resulting from the surge of gas
and molten rock
However, it is not always possible to state exactly when a volcanic eruption will happen.
Living with a volcano
People often choose to live in volcanic areas because they are useful in a
variety of ways…
Some countries, such as Iceland and the Philippines, were created by volcanic
Some volcanic soils are rich, deep and fertile and allow intensive agriculture, for
example in Java. However, in other areas such as Sumatra and Iceland the soils
are poor due to leaching or climate.
Some volcanoes are culturally symbolic and are apart of the national identity such
as Mt Fuji in Japan.
People live close to volcanoes because Geothermal energy can be harnessed by
using the steam from underground which has been heated by the Earth's magma.
This steam is used to drive turbines in geothermal power stations to produce
electricity for domestic and industrial use. Countries such as Iceland and New
Zealand use this method of generating electricity.
Volcanoes attract millions of visitors around the world every year. Apart from the
volcano itself, hot springs and geysers can also bring in the tourists. This creates
many jobs for people in the tourism industry. This includes work in hotels,
restaurants and gift shops. Often locals are also employed as tour guides.
One hazard that is closely associated with volcanic activity is the
Rain brings soot and ash back to ground and this becomes a
heavily saturated mudflow
Heat from volcanoes melts snow and ice – the resulting flow picks
up sediment and turns it into a destructive lahar.
Example: Nevado del Ruiz, Colombia and Mt. Ruapehu New
The term tsunami is
Japanese for ‘harbour
90% of tsunamis occur in
the Pacific basin.
They are generally caused
by earthquakes but can be
caused by volcanoes and
Tsunamis have the
potential to cause
widespread disaster, as
was the case in the South
Asian tsunami in 2004, and
Japan tsunami in 2011.
Tsunami warning systems:
At present it is impossible to predict precisely where and
when a tsunami will happen. In most cases it is only
possible to raise alarm once a tsunami has started.
In the cases of submarine volcanoes it is possible to
monitor these to predict the risk of tsunami.
The first effective tsunami warning system was
developed in 1948 in the Pacific, following the 1946
tsunami. The system consisted of over 50 tidal stations
and 31 seismographic stations, spread between Alaska,
Hong Kong and Cape Horn. When water passes a
critical threshold a warning is automatically sent to
Honolulu. In addition, the earthquake epicentre is plotted
and magnitude investigated. Its effectiveness has been
improved by the use of satellites.
In theory there is time to issue warnings. However, the
impacts will vary with shoreline morphology.
Many LEDCs lack early warning systems, and as a
result tsunamis can have dire consequences in such
Factors affecting the
perception of risk
At an individual level there are three important influences
upon an individuals response to any hazardous event:
1. Experience: the more experience a person has of
environmental hazards the greater the adjustment to the
2. Material well-being: those who are better off have more
3. Personality: is the person a leader or a follower, a risk-
taker or risk-minimiser?
Ultimately there are just three choices:
1. Do nothing and accept the hazard
2. Adjust to the situation of living in a hazardous
3. Leave the area.
The level of adjustment will
depend, in part, upon the
risks caused by the
1. Identification of the
2. Estimation of the risk
3. Evaluation of the cost
caused by hazard.
from mass movements
What is mass movement?
Mass movement can be defined as the large scale
movement of weathered material in response to gravity.
Essentially, it’s when a cliff or other structure that is not
horizontally orientated has been weathered to the point at
which it starts to collapse.
Mass movements can occur in many ways…
As slow movements such as soil creep
As fast movements, such as avalanches
As dry movements, such as rock falls
As fluid movements such as mudflows.
Factors affecting mass
Slope of the land surface: When the slope gradient is steep, mass movement is rapid.
Water content: Water acts as a lubricant, which makes it easier for material to move.
Following a period of heavy rain, regolith is heavier and flows more quickly.
Human activity: Hills are often cut into during the construction of roads. This makes
the hill unstable, which can lead to mass movement. The vibrations from machinery
and traffic can also result in mass movement, as can deforestation.
Tectonic activity: volcanic activity can lead to the movement of material downslope.
Mudflows (lahars), occur when the snow or ice covering a crater melts during an
eruption. This mater mixes with soil to form mud. Earthquakes can also cause areas
of hills or mountains to move downslope.
Plants and trees: the tree/plant roots bind soil in place and prevent mass movement.
Slope material: Mass movement is also affected my slope material, whether it is
consolidated (such as rocks) or unconsolidated.
Geological structure: Rocks with faults, and potential weaknesses within it are more
vulnerable to weathering and less resistant to downslope movement.
Shear stress and
The likelihood of a slope failing depends on the
relative strength and resistance of the slope,
compared with the force that is trying to move it.
Mass movements occur when the shear stress on a
slope exceeds the shear resistance – therefore the
slope can not overcome the forces of gravity and a
mass movement begins.
Gravity has two effects, it can act to move the
material downslope (slide component) or it can act
to stick the particles to the slide (stick component).
Classification of mass
Mass movements are classified in
a number of ways, including speed
of movement and the amount of
water present. In addition it is
possible to distinguish between
different types of movement, such
as falls, flows, slides and slumps.
Slow movements: soil creep,
Fast movements: earth flow,
Rapid movements: slides, rock
falls, slumps, avalanches
Check out AS level notes for description of each
mass movement type.
Avalanches are mass movements of snow and ice. Newly fallen snow may
fall off older snow, especially in winter, while in spring martially thawed
snow moves, often triggered by skiing.
Avalanches occur frequently on steep slopes over 22°, especially on north-
facing slopes where the lack of sun inhibits the stabilisation of the snow.
They are also very fast. Average speeds in an avalanche are 40-60km per
hour, but speeds of up to 200 per hour have been recorded in Japan.
Avalanches are classified in a number of ways…
The type of breakaway – from a point formed with loose snow, or from an
area formed of a slab
Position of the sliding surface – the whole snow cover or just the surface
Water content – dry or wet avalanches
The form of the avalanche – whether it is channelled in cross section of
Although avalanches cannot be prevented, it is possible to reduce their impact through fences,
rakes, wedges, afforestation etc.
So why do avalanches occur?
The underlying processes in an avalanche are similar to those in a landslide. Snow gets its strength
from the interlocking of snow crystals and cohesion caused by electrostatic bonding of snow crystals.
The snow snow remains in place as long as its strength is greater than the stress exerted by its
weight and the slope angle.
The process is complicated by the way in which snow crystals constantly change. Changes in
overlying pressure, compaction by freshly fallen snow, temperature changes an the movement of
melt water through the snow cause the crystal structure of the snow to change which may make it
unstable causing an avalanche.
Loose avalanches, comprising fresh snow, usually occur soon after a snowfall. By contrast slab
avalanches occur at a later date, when the snow has developed some cohesion. The latter are much
bigger than loose avalanches and cause more destruction, often started by a sudden rise in
temperature which causes melting. The melt water lubricates the slab and makes it unstable.
Many of the avalanches occur in spring when the snowpack is large and temperatures are rising.
Tropical storms (cyclones)
Tropical storms bring intense rainfall and very high winds, which may in turn cause storm surges and
coastal flooding, and other hazards such as flooding and mudslides.
Tropical storms are also characterised by enormous quantities of water. This is due to their origin
over tropical seas. High-intensity rainfall, as well as large totals – up to 500 mm in 24 hours
invariably cause flooding.
Their path is erratic, so it is not always possible to give more than 12 hours notice of their position.
This is insufficient for proper evacuation measures.
Tropical storms develop as intense low pressure systems move over tropical oceans. Winds spiral
rapidly around a calm central area known as the eye.
The diameter of the whole storm may be as much as 800km although the very strong winds that
cause most of the damage are found in a narrower belt up to 300km. In a mature tropical storm
pressure may fall to as low as 880mb, this and the strong contrast in pressure between the eye and
outer part of the storm leads to strong winds.
Tropical storms move excess heat from low latitudes to higher latitudes. They normally develop in
the west-war flowing air just north of the equator. They begin life as a small-scale tropical
depression, a localised area of low pressure that causes warm air to rise. This causes thunderstorms
which persist for at least 24 hours and may develop into tropical storms, which have greater wind
speeds of up to 117km/hour. However, only 10% of these tropical disturbances ever become tropical
wind speeds above 118km/h
Conditions needed for
tropical storm formation
Sea temperatures must be over 27° to a depth of 60m as
the warm water gives of large qualities of heat when it is
condensed – the driving force behind the tropical storm.
The low pressure area has to be far enough away from the
equator so that the Coriolis force creates rotation in the
rising air mass – if it is too close to the equator there is
insufficient rotation and the tropical storm will not develop
Conditions must be unstable – some tropical low pressure
systems develop into tropical storms but scientists are
unsure why some do and others do not.
How are tropical storms
Tropical storms are the most violent, damaging and frequent
hazard to affect the tropical regions. They are measured on
the Saffir-Simpson Scale, which is a 1-5 rating based on the
tropical storm’s intensity. It is used to give and estimate of
the potential property damage and flooding expected along
the coast from a tropical storm landfall.
Wind speed is the determining factor in the scale, as storm
surge values are highly dependent on the slop of the
continental shelf and the shape of the coastline in the
landfall region. Tropical storms can also cause considerable
loss of life.
Factors affecting impact of
Tropical storm paths are unpredictable, which makes effective management
of the threat difficult.
The distribution of the population will depend whether the risk associated
with tropical storm is increased or reduced. Places like the Caribbean
islands, for e.g. where much of the population lives in coastal settlements
and is exposed to increase sea levels and flooding therefore will experience
far greater impacts then that of a sparsely populated in land area.
Hazard migration depends upon the effectiveness of the human response
to natural events – including urban planning laws, emergency planning,
evacuation measures and relief operations such as re-housing schemes
and distribution of food aid and clean water.
LEDCs continue to lose more lives to natural hazards, due to inadequate
planning and preparation.
Information regarding tropical storms is received
from a number of sources including…
1. Satellite images
2. Aircraft that fly into the eye of the tropical storm to
record weather information
3. Weather stations at ground levels
4. Radars that monitor areas of intense rainfall.
Preparing for tropical
Know where your
emergency shelters are
Have disaster supplies on
Protect your windows
Trim back branches from
Check home and car
Make arrangements for
pets and livestock
Listen to the radio or tv for
Make sure your car is full
Bring in outdoor objects
and anchor objects that
cannot be brought inside
Secure buildings by
closing and boarding up
Remove outside antennas
and satellite dishes
Assist in search and
Seek medical attention for
Clean up debris and
effect temporary repairs
Report damage to utilities
Watch out for secondary
Reducing vulnerability of
New buildings should be designed to be wind and water
resistant. Design standards are usually incorporated into
Communication and utility lines should be located away from
the coastal area or installed underground.
Areas of building should be improved by raising the ground
level to protect against flood and storm surges
Protective river embankments, levees and coastal dikes should
be regularly inspected for breaches due to erosion and
opportunities should be taken to plant mangrove trees to
reduce breaking wave energy.
Tornadoes are small and short-lived but highly
destructive storms. Because of their severe nature and
small size, comparatively little is known about them.
Measurement and observation within them are difficult.
A few low-lying, armoured probes called ‘turtles’ have
been placed successfully in tornadoes.
Tornadoes consist of elongated funnels of cloud which
descend from the base of a well-developed
cumulonimbus cloud, eventually making contact with the
In order for a vortex to be classified as a tornado, it
must be in contact with the ground and the cloud base.
Within tornadoes are rotating violent winds, perhaps
Pressure gradients in a tornado can reach an estimated
25 mb per 100m.
How tornadoes form
Moisture, instability, lift and wind shear are the four key ingredients in
Most tornadoes rotate cyclonically (anticlockwise in the northern
hemisphere and clockwise in the southern hemisphere)
The standard explanation is that warm moist air meets cold dry air to form a
Many thunderstorms form under these conditions, which never even come
close to producing tornadoes. Even when the large-scale environment is
extremely favourable for tornado-type thunderstorms, not every
thunderstorm spawns a tornado.
The most destructive and deadly tornadoes develop from super cells (which
are rotation thunderstorms with a well-defined low pressure system called a
Tornadoes can last from several seconds to more than an hour.
Step by step formation
Step 1: Like all winds and storms, tornadoes begin when the sun heats up the surface
of the land. As the warm, less heavy air begins to rise, it meets the colder, heavier air
above it. Note that wind shears make it even easier to set them off. A wind shear is
when two winds at different levels and speeds above the ground blow together in a
location. Step 2: The faster moving air begins to spin and roll over the slower wind.
As it rolls on, it gathers pace and grow in size. Step 3: At this stage, it is an invisible,
horizontal wind spinning and rolling like a cylinder. As the winds continue to build up,
stronger and more powerful warm air forces the spinning winds vertically upward,
causing an updraft. Step 4: With more warm air rising, the spinning air encounters
more updraft. The winds spin faster, vertically upwards, and gains more momentum.
Step 5: At this stage, the spinning winds, creates a vortex and the wind has enough
energy to fuel itself. Step 6: The tornado is fully formed now and moving in the
direction of the thunderstorm winds. When the pointed part of the tornado touched the
ground from the cloud, it is often referred to as 'touch down' As it moves it rips off
things along its patch.
Ted Talk: How tornadoes
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button will show.
Click this to watch…
About a thousand tornadoes hit the USA each year with an average of 60 people dying per year from them. A
tornado’s impact as a hazard is extreme with three damaging factors at work…
1. Winds are often so strong that objects in the tornado’s path are simply removed or very severely damaged
2. Strong rotational movement tends to twist objects from their fixings, and strong uplift can carry some debris upwards
into the cloud.
3. The very low atmospheric pressure near the vortex centre is a major source of damage. When a tornado
approaches a building, external pressure is rapidly reduced, and unless there is a nearly simultaneous and
equivalent decrease in internal pressure, the walls and roof may explode outwards in the process of equalising the
Most tornado damage is due to multiple-vortex tornadoes or very small, intense single-vortex tornadoes. The wind in
most multiple-vortex tornadoes may only be strong enough to do minor damage to a particular house. But one of the
smaller sub vortexes, may strike the house next door with winds over 300m/hr causing complete destruction. Also
there are great differences, in construction from one building to the next so whilst one building may be flattened the
other may be barely touched.
Although winds in the strongest tornadoes may far exceed those in the strongest tropical storms, tropical storms
typically cause much more damage. Economically tornadoes cause about 1/10 as much damage per year, on
average, as tropical storms.
Tropical storms tend to cause much more overall destruction due to their much larger size, longer duration and
variety of ways they damage property. Whilst tropical storms tend to be tens or hundreds of km across lasting many
hours, tornadoes tend to be a few hundred yards in diameter and last only for a matter of minutes.
The main problem with anything that could realistically stand a
chance of affecting a tornado (for e.g. an atomic bomb) is that it
would be even more deadly and destructive than the tornado itself.
Lesser things like huge piles of dry ice would be too hard to deploy in
the right place fast enough, and would probably not have a significant
effect on the tornado
Nor is there any proof that seeding can or cannot change tornado
potential in a thunderstorm. This is because there is no way of
knowing that the things a thunderstorm does after it has been seeded
would not have happened anyway.
This includes any presence or lack of rain, hail, wind gusts or
tornadoes. Because the effects of seeding are impossible to prove or
disprove, there is a great deal of controversy among meteorologists
about whether it works, and if so, under what conditions and to what
Assessing and mitigating
damaging effects of mass
Landslides and other forms of mass movement are widespread and cause extensive damage and loss of life each
year. With careful analysis and planning, together with appropriate stabilisation techniques, the impacts of mass
movements can be reduced or eliminated.
Assessment of the hazards posed by potential mass movement events are based partly on past events, to evaluate
their magnitude and frequency. In addition mapping and testing of soil and rock properties determines their
susceptibility to destabilising processes. Maps showing areas that could be affected by mass movement processes
are important tools for land use planners.
Eliminating or restricting human activities in areas where slides are likely may be the best way to reduce damage
and loss of life. Land that is susceptible to mild failures may be suitable for some forms of development (recreation
or parkland) but not others (such as residential or industrial).
Early warning systems can provide forecasts of intense rain. High-risk areas can be monitored and remedial action
In addition to assessment, prediction and early warning, some engineering schemes can be applied to reduce the
damage of mass wasting. These include retaining devices, drainage pipes, grading of slope and diversion walls.
Concrete blocks or gabions may be used to strengthen slopes. Slopes subject to creep can be stabilised by draining
or pumping water from saturated sediment.
Over steepened slopes can be made gentler by regrading. However, not all communities can afford such measures
and so may opt for low-cost sustainable forms of management.
Hazard management, risk
Hazard management includes a body of theory
which includes: risk, prediction, prevention, event
Learning to live with
Most places with a history of earthquakes have
developed plans that enable people to deal with them.
The aim is to reduce the effect of the earthquakes and
thus save lives, buildings and money.
The ways of reducing earthquake impact include…
and public information.
Earthquakes killed about 1.5 million people in the 20th century and the number
of earthquakes appears to be rising. Most of the deaths were caused by the
collapse of unsuitable and poorly designed buildings. More than a third of the
world’s largest and fastest growing cities are located in regions of high
earthquake risks, so the problems are likely to intensify.
It is difficult to stop an earthquake from happening, so prevention normally
involves minimising the prospect of death, injury or damage by controlling
building in high-risk areas and using aseismic designs. In addition warning
systems can be used to warn people of an imminent earthquake and inform
them of what to do when it does happen.
Insurance schemes are another form of preparation by sharing the costs
between a wide group of people.
The seismic gap theory states that that over a prolonged period of time all parts
of a plate boundary must move by almost the same amount. Thus if one part of
the plate boundary has not moved and others have, then the part that has not
moved is likely to move next. This theory has been used successfully to
suggest that an earthquake was likely in the Loma Preita segment of the San
2. Building design
Increasingly, as the availability of building land is reduced, more and more people are
living in seismic areas. This increases the potential impact of an earthquake. However,
buildings can be designed to withstand the ground-shaking that occurs in an
Single story buildings are more suitable than multi-storey structures, because this
reduces the number of people at risk, and the threat of collapse over roads and vacation
Some tall buildings are built with a soft storey at the bottom, such as a car park raised
on pillars. This collapses in an earthquake, so that the upper floors sink down onto it and
this cushions the impact.
Mounting the foundations of a building on rubber mounts which allow the ground to
move under the building is widely used. This isolates the building from the tremors.
Building reinforcement strategies include building on foundations built deep into
underlying bedrock, and the use of steel-constructed frames that can withstand shaking
Land use planning is another important way to reducing earthquake risk.
In theory, by altering the fluid pressure deep
underground at the point of greatest stress
in the fault line, a series of small and less
damaging earthquake events may be
triggered which could release the energy
that would otherwise build up and create a
Additionally, a series of controlled
underground nuclear explosions might
relieve stress before it reaches critical