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Natural Hazards – Earthquakes, Volcanoes and Tsunamis
Lower six – Ms Shalto
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What are natural hazards?
A hazard is an event that threatens or causes destruction to human ________ and ________. lives property Hazards caused by nature are ______________. natural hazards
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Types of natural hazards Which types of hazards do they belong to?
Climatic Geological Biological Tectonic Caused by Examples Climatic extremes Changes of slope Living things Plate movements Blizzards Avalanches Pests Earthquakes Frost Landslides Poisoning Volcanic eruptions Droughts Mudflows Bacteria and diseases Floods Tsunamis Tornadoes Typhoons Can you give some examples of common natural hazards found in Hong Kong? Which types of hazards do they belong to? Climatic and geological. Typhoons, floods, landslides, etc.
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What are natural hazards?
Are all hazards natural? Can you give one more example of man-made hazards? NO! Sometimes, hazards are intensified or caused by human activities. Example Deforestation in China and Bangladesh Flooding
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San Francisco 日本 台灣 1. Earthquake
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Earthquake, volcanic eruption and tsunami are the major tectonic hazards resulted from endogenetic forces Most of the these natural hazards are found along plate boundaries Describe the distribution of tectonic hazards
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What are natural hazards?
Sometimes, one hazard may trigger another. Can you give an example? Which type of hazard do these three hazards belong to? Tectonic hazard. Earthquakes Volcanic eruptions Tsunamis
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1. Structure of the Earth The earth can be divided into different layers. They include the core, the mantle Outer core Inner core and the crust. Hence, the earth is like an egg. The yolk is the core. The egg white is the mantle and the shell is the crust.
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There are continental plates which make up continents and oceanic plates which make up ocean floors.
Mantle Ocean
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IndoAustralian plate Arabian plate Cocos plate Nazca plate
Eurasian plate N. American plate Arabian plate Philippine plate Pacific plate Cocos plate African plate Nazca plate IndoAustralian plate S. American plate Antarctic plate What are plate boundary Plate boundaries are where two plates meet. How do plate move? They are dragged by convection currents of magma in the mantle.
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Where and why do earthquakes occur?
Where do earthquakes frequently occur?
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Where and why do earthquakes occur?
Earthquakes are found along plate boundaries Earthquakes WHY? Plate movement Friction Energy stored is released Pressure builds up in the rocks Rock breaks Strength of the rocks ＞ Great pressure
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An earthquake is the sudden shaking of the ground due to the movement of tectonic plates.
The convection currents in the mantle causes the plates to move away or towards each other, or slide past each other. Frictions occur along plate boundaries and build up pressure in the rocks.
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When the pressure exceeds the strength of the rocks, the rocks breaks, releasing the stored energy through seismic waves travelling This sudden release of stress is called an earthquake.
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The energy released is propagated in the form of seismic waves
The place where fracture suddenly occurs and releases energy waves is called the focus The point vertically above the focus on the earth’s surface is known as the epicentre. The energy released is propagated in the form of seismic waves Seismic waves Fault Epicentre Fault Focus
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Focus vs. magnitude Relationship between…
Shallow earthquake focus at 0-70km beneath ground Medium Deep >300 Relationship between… The deeper the focus, the greater the amount of energy stored, the higher the magnitude. So earthquakes with a deep focus result in greater magnitude. (Richter Scale 1-9)
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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
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Damage of earthquakes If two earthquakes are of the same magnitude, the one with a shallow focus is more more destructive as the same amount of seismic waves are spread over a larger area
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A powerful quake measuring 8.0 on the Richter scale jolted northern Japan, the meteorological agency said.(AFP)
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Magnitude vs Intensity
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.
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What is the type of earthquake for A and B respectively
What is the type of earthquake for A and B respectively? Which one is of a higher magnitude? B 25 km A 150km
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If both A and B are of Richter Scale 5, which one is more destructive?
25 km A 150km
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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
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Along destructive plate boundaries
Earthquakes of all foci occur along the subduction zone where one oceanic crust collides with one continental crust
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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
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Along destructive plate boundaries
Shallow earthquakes occur along the destructive plate boundaries where two continental crusts collide with each other
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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
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Along constructive plate boundaries
Shallow earthquakes occur along the edge of the mid-oceanic ridge where two oceanic crusts move away from each other
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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
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Along conservative plate boundaries
Shallow earthquakes occur along the fault where two crusts slide past each other
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Where and why do earthquakes occur?
Are earthquakes at different types of plate boundaries of similar strength and frequency? Do earthquakes happen at all types of plate boundaries? Yes, they do. Earthquakes occur at all types of plate boundaries. NO! As pressure builds up more rapidly and easily under and forces, earthquakes are stronger and more frequent at and plate boundaries. compressional lateral destructive conservative
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Major belts for earthquakes and volcanoes
Alpine-Himalayan Belt Circum-Pacific Belt
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2. Volcanic 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.
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2. Volcanic 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.
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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
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+ Produce/extend cracks in the plates Cracks reach the magma chambers
Why are volcanic eruptions closely related to earthquakes? Volcanic eruptions Produce/extend cracks in the plates Cracks reach the magma chambers Pressure builds up in the magma High temperature Weight of the plate above + Earthquakes When there is enough pressure in the magma and the presence of lines of weakness (e.g. faults), volcanic eruptions can occur even earthquakes are absent.
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Destructive plate boundary
Do you know at which type of plate boundary will eruptions be generally more active and violent? Destructive plate boundary WHY? Along destructive plate 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
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How Volcanoes are formed at plate margins
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Tsunami is a Japanese word which means ‘harbour wave’. 3. Tsunami
The majority of tsunamis are caused by shallow focus earthquakes that occur under the sea floor Some are caused by particularly violent volcanic eruptions or landslides under the sea. 3. Tsunami
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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.
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Where and why do tsunamis occur?
Where are tsunamis usually found? Along coastal areas/islands in the oceans Coincide with major earthquakes/volcanic zones
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Why do tsunamis have such a distribution pattern?
Where and why do tsunamis occur? Why do tsunamis have such a distribution pattern? Earthquakes/volcanic eruptions cause vibrations on the ocean floor and trigger huge waves. Concentrated in the Pacific Ocean WHY? The Pacific Ocean is surrounded by unstable plate boundaries, mainly and destructive conservative Earthquakes/volcanic eruptions here are more violent.
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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.
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Why are tsunamis almost absent in the Atlantic and Indian Oceans?
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. N Earthquakes/Volcanic eruptions are less violent there. Y They are not as large as the Pacific Ocean. N There is no compressional force there. Y
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