NATURAL HAZARDS – EARTHQUAKES, VOLCANOES AND TSUNAMIS Lower six – Ms Shalto.
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NATURAL HAZARDS – EARTHQUAKES, VOLCANOES AND TSUNAMIS
Lower six – Ms Shalto
What are natural hazards?What are natural hazards?
A hazard is an event that threatens or causes destruction to human ________ and ________.
A hazard is an event that threatens or causes destruction to human ________ and ________.
Hazards caused by nature are ______________.Hazards caused by nature are ______________.
lives property
natural hazards
Types of natural hazards Types of natural hazards Types Climatic Geological Biological Tectonic
Caused by
Examples
Climatic extremes
Changes of slope
Living things
Plate movements
BlizzardsFrostDroughts
FloodsTornadoes
Avalanches Landslides
Mudflows
Pests Earthquakes Poisoning
Bacteria and diseases
Volcanic eruptions
Tsunamis
Can you give some examples of common natural hazards found
in Hong Kong?
Can you give some examples of common natural hazards found
in Hong Kong? Typhoons, floods, landslides, etc.
Which types of hazards do they belong to?
Which types of hazards do they belong to?
Climatic and geological.
Typhoons
What are natural hazards?What are natural hazards?
Are all hazards natural?
Are all hazards natural?
NO!
Sometimes, hazards are intensified or caused by human activities.Sometimes, hazards are intensified or caused by human activities.
Example
Deforestation in China and Bangladesh Deforestation in China and Bangladesh Flooding
Can you give one more example of
man-made hazards?
Can you give one more example of
man-made hazards?
台灣台灣日本日本San FranciscoSan Francisco
1. Earthquake1. Earthquake
Earthquake, volcanic Earthquake, volcanic
eruption and tsunami are eruption and tsunami are
the major the major tectonic hazards tectonic hazards resulted from endogenetic resulted from endogenetic
forcesforcesMost of the these natural Most of the these natural
hazards are found along hazards are found along
plate boundariesplate boundariesDescribe the distribution of Describe the distribution of
tectonic hazardstectonic hazards
What are natural hazards?What are natural hazards?
Sometimes, one hazard may trigger another.
Can you give an example?
Sometimes, one hazard may trigger another.
Can you give an example?
Earthquakes Volcanic eruptions
Tsunamis
Which type of hazard do these three hazards belong to?
Tectonic hazard.
1. Structure of the 1. Structure of the Earth Earth
The earth can be
divided into
different layers.
and the crust.
They include the core,
the mantle
Hence, the earth
is likean egg. The yolk is
the core. The egg white is the mantle
and the shell is the crust.
Outer core
Inner core
There are continental plates which
make up continents and oceanic plates
which make up ocean floors.
Continent
Continental plateOceanic plate
Mantle
Ocean
How do plate move?
They are dragged by convection currents of magma in the mantle.
Eurasian plate
What are plate boundaryPlate boundaries are where two plates meet.
Pacific plate
African plate
IndoAustralian plate
N. American plate
Arabian platePhilippine
plate
Antarctic plate
Nazca plate
Cocos plate
S. American
plate
Where and why do earthquakes occur?
Where do earthquakes frequently occur?
Where and why do earthquakes occur?Earthquakes are found along . plate boundaries
Plate movement
Friction
Pressure builds up in the rocks
Great pressure
Earthquakes
Energy stored is released
Rock breaks
Strength of the rocks
WHY?WHY?
>>
An An earthquakeearthquake is the is the sudden shaking sudden shaking of of the ground due to the the ground due to the movement of movement of tectonic platestectonic plates..
The convection currents in the mantle The convection currents in the mantle
causes the plates to causes the plates to move away or move away or
towards each othertowards each other, or , or slide past each slide past each
otherother. . Frictions occurFrictions occur along plate boundaries along plate boundaries
and and build up pressurebuild up pressure in the rocks. in the rocks.
When the pressure When the pressure exceeds the strength of exceeds the strength of the rocks, the rocks the rocks, the rocks breaks, releasing the breaks, releasing the stored energy through stored energy through seismic waves seismic waves travellingtravelling
This sudden release of This sudden release of stress is called an stress is called an earthquake. earthquake.
The place where fracture suddenly occurs and releases energy waves is called the focusfocus Seismic waves
Epicentre
Focus Fault
FaultThe point vertically above the focus on the earth’s surface is known as the epicentreepicentre.
The energy released is propagated in the form of seismic wavesseismic waves
Focus vs. magnitude
Shallow earthquake focus at 0-70km beneath groundMedium 71-300Deep >300
Relationship between…Relationship between… The deeper theThe deeper the focus focus, the greater the , the greater the amount of energy stored, the higher amount of energy stored, the higher the the magnitudemagnitude. So earthquakes with a . So earthquakes with a deep focus result in greater deep focus result in greater magnitude. magnitude. (Richter Scale(Richter Scale 1-9)1-9)
Epicenter vs. intensity
The nearer to the epicenter, the greater the intensity is. So greater damage can be found in areas near to the epicenter.
Describe the relationship between focus and magnitude epicenter and intensity
Damage of earthquakes If two earthquakes are of the same If two earthquakes are of the same
magnitude, the one with a magnitude, the one with a shallow shallow focusfocus is more is more more destructivemore destructive as as the same amount of seismic waves the same amount of seismic waves are spread over a larger areaare spread over a larger area
Magnitude vs Intensity
Magnitude is the amount of energy released during an
earthquake. It is measured on the Richter Scale.
Intensity Is the level of damage in terms of property
and human life caused by an earthquake. It is measured on the Modified Mercalli Scale
They are not positively correlated: greater
magnitude does not necessarily means greater intensity.
What is the type of earthquake for A and B respectively? Which one is of a higher magnitude?
A150km
B25 km
If both A and B are of Richter Scale 5, which one is more destructive?
A150km
B25 km
Where and why do earthquakes occur?
First occasion when earthquakes occur
Force involved:
Compressional
Type of plate boundary:
Destructive
Example:
Kobe earthquake, 1995
Plates involved:
Philippine Plate and Eurasian Plate
Along destructive plate boundaries
Earthquakes of all foci occur along the subduction zone where one oceanic crust collides with one continental crust
Where and why do earthquakes occur?Second occasion when earthquakes occur
Force involved:
Compressional
Type of plate boundary:
Destructive
Examples:
Earthquakes in the Himalayas, India and Nepal
Plates involved:
Indo-Australian and Eurasian plates
Along destructive plate boundaries Shallow earthquakes occur along
the destructive plate boundaries where two continental crusts collide with each other
Where and why do earthquakes occur?
Third occasion when earthquakes occur
Force involved:
Tensional
Type of plate boundary:
Constructive
Examples:
Earthquakes in the mid-Atlantic Ocean
Plates involved:
North American and Eurasian plates
Along constructive plate boundaries
Shallow earthquakes occur along the edge of the mid-oceanic ridge where two oceanic crusts move away from each other
Where and why do earthquakes occur?
Fourth occasion when earthquakes occur
Force involved:
Lateral
Type of plate boundary:
Conservative
Example:
San Francisco earthquake, 1906
Plates involved:
North American and Pacific (San Andreas Fault) plates
Along conservative plate boundaries
Shallow earthquakes occur along the fault where two crusts slide past each other
Where and why do earthquakes occur?
Do earthquakes happen at all types of plate boundaries?
Yes, they do.Yes, they do.
Are earthquakes at different types of plate boundaries of similar
strength and frequency?
Earthquakes occur at all types of plate boundaries.NO!NO!
As pressure builds up more rapidly and easilyunder and forces, earthquakes are stronger and more frequent at and plate boundaries.
compressional lateral
destructive conservative
Major belts for earthquakes and Major belts for earthquakes and volcanoesvolcanoes
•Circum-Pacific BeltCircum-Pacific Belt
•Alpine-Himalayan BeltAlpine-Himalayan Belt
2. Volcanic 2. Volcanic eruption eruption A volcano is a
mountain on the earth’s crust through which magma and gases beneath the crust pour out and cool on the earth’s surface as lava.
2. Volcanic 2. Volcanic eruption eruption
When a volcano erupts, gases, lava and pyroclastic materials are ejected.
Sometimes the eruption is so violent that the top of the volcano is blown off to form a caldera.
Where and why do volcanic eruptions occur?
Is there any relationship between the spatial patterns of active volcanoes and earthquakes?
Volcanic eruptions are to earthquakes. closely related
Why are volcanic eruptions closely related
to earthquakes?
High temperature
Weight of the plate above
Pressure builds up in the magma
Volcanic
eruptions
Earthquakes
Produce/extend cracks in the
plates
Cracks reach
the magma chambers
++When there is enough pressure in the magma and the presence of lines of weakness (e.g. faults), volcanic eruptions can occur evenearthquakes are absent.
Do you know at which type of plate boundary will eruptions be generally
more active and violent?
Destructive plate boundaryWHY?WHY?
Along destructiveplate boundaries
• Converging magma builds great stress • Melting of the subducted plate intensifies the pressure• Earthquakes here are more frequent, resulting in more lines of weakness through which the magma rises
Along constructive boundary
• Diverging magma reduces the pressure of magma• Water pressure at the sea bottom suppresses volcanic activities
How Volcanoes are formed at plate margins
3. Tsunami 3. Tsunami TsunamiTsunami is a Japanese word which is a Japanese word which means ‘harbour wave’. means ‘harbour wave’.
The majority of tsunamis are caused The majority of tsunamis are caused by shallow focus earthquakes that by shallow focus earthquakes that occur under the sea flooroccur under the sea floor
Some are caused by particularly Some are caused by particularly violent volcanic eruptions or violent volcanic eruptions or landslides under the sea.landslides under the sea.
The dramatic tremor at the sea floor produces big waves.
The height of the waves rises dramatically as they reach the shore, posing a great threat to coastal settlements and population.
Where and why do tsunamis occur?
Where are tsunamis usually found?
Along coastal areas/islands in the oceans
Coincide with major earthquakes/volcaniczones
Where and why do tsunamis occur?
Earthquakes/volcanic eruptions cause vibrations on the ocean floor and trigger huge waves.
Concentrated in the .
The Pacific Ocean is surrounded by unstable plate boundaries, mainly and .
Pacific Ocean
WHY?WHY?
destructive conservative
Earthquakes/volcanic eruptions here are more violent.
Why do tsunamis have such a distribution pattern?
Where and why do tsunamis occur?Why does a tsunami generated from an Alaskan earthquake affect the Hawaiian Islands in the Pacific Ocean? Click the diagram below to view the animation.
Where and why do tsunamis occur?
Why are tsunamis almost absent in the Atlantic and Indian Oceans?
These two oceans do not lie in the earthquake zone.
Earthquakes/Volcanic eruptions are less violent there.
They are not as large as the Pacific Ocean.
There is no compressional force there.Y
N
N
Y
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