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