Earthquakes Flashcards
What is a natural hazard
Naturally occurring process or event which has the potential to cause loss of lives, properties and injuries
What is a disaster
A hazard that has significant impact on people
Why is it important that we examine natural hazards in cities
Urban centres are densely populated and are heavily built up
About 40% of urban cities are in costal areas and are susceptible to natural hazards like tsunamis. Rising sea levels and climate change have increased flooding risk in costal cities like Bangkok, Manila and London
Layers of Earth
Crust
Mantle
Core
What is crust
Outermost layer of the earth made of solid rock. Ranges from 6 to 70 km thickness
What is mantle
Beneath the crust
Extending from 100 to 300km within the mantle is the asthenosphere which is semi-molten and can flow slowly
Made up of liquid magma and is about 2900km thick
What is core
At the centre of the Earth
Outer core is solid and inner core is liquid
How does mantle convection currents work
The mantle is heated up by energy released from the core
The mantle expands, becoming less dense and rises
Mantle moves just beneath the lithospheric plates, dragging it along
Cooling of the mantle causes it to contract, denser and sink
Plate divergence occurs as the mantle convection currents underneath move away from each other
Plate convergence occurs as the mantle convection currents underneath move towards each other
Types of plate movement
Transform (Passive/ Conservative plate margin)
Convergent (Destructive plate margin)
Divergent (Constructive plate margin)
Where do earthquakes occur
Along plate boundaries
All types of plate boundaries are susceptible to earthquake hazards
Cause of earthquakes
Stress is set up when mantle convection currents drag the plates but friction prevents movement of plates
Pressure accumulates overtime and deformation of rocks occurs
When friction can no longer hold the plates in place, rupture occurs and the stored energy is released, leading to an earthquake
The rocks rebound to the original undeformed shape after the release of pressure
How to measure earthquakes
Richter Scale (1 to 12)
Mercalli Scale
A seismograph measures the seismic waves released by an earthquake
A spring-mounted weight in the seismograph moves up and down as tremours are detected
the distance of vertical markings relate to the magnitude of the earthquake
Where does seismic waves come from
Radiate out from a point of suddenly energy release, called the focus. Point on the Earth’s surface directly above the focus is the epicentre. Most of the energy released by an earthquake travels along the surface of the earth, causing the ground to vibrate violently.
Roman numerals 1 to 12
i
ii
iii
iv
v
vi
vii
viii
x
ix
xi
xii
Earthquake hazards
Ground motion
Tsunami
Landslides
Fires
What is ground motion
Shaking of earth’s surface due to the arrival of earthquake waves
Severity of shaking will reduce as distance from the earthquake focus increases until it can only be recorded by extremely sensitive instruments
Many buildings collapse or tilt. People may be buried by debris and numerous deaths and injuries occur
What are tsunamis
A series of waves that can attack coastal areas are caused by earthquakes or underground volcanic eruptions. They can sweep away many people, livestock and houses
Although often referred as tidal waves, tides have nothing to do with these giant waves
How do tsunamis occur
Movement of ocean floor causes a displacement of ocean water
A series of waves travel outwards at a height of less than 1 metre on the open ocean
On reaching shallower water, greater friction causes the waves to slow down and forces the waves to increase in heights
At point of impact with coast, the waves may reach a height of 15 metres
What are landslides
Shaking of ground in earthquakes can result in weakening of slopes of hills and mountains. When these slopes are unstable, they can lead to landslides.
Landslides are rapid downward movement of rocks, soil and debris from a slope
In some places, dams collapse and flooding may occur. Mountainous and hilly places are some of the most common sites of earthquakes. The trembling ground can shake dirt, mud and rocks loose from steep terrain, creating landslides
What are fires
An earthquake can disrupt services like electricity, gas and water
Vibrating on the ground can snap (underground) pipes and break cables, causing outbreak of fires
Factors affecting extent of earthquake impact
Population Density
Level of preparedness
Tine of occurrence
Distance from epicentre
Magnitude
How does magnitude affect the extent of earthquake
Higher magnitude, more energy released by earthquake
More powerful earthquakes that will cause more destruction to properties and loss of lives
How does population density affect the extent of earthquake
Earthquakes that occur in sparsely populated areas are less likely to affect more people than denser populate cities
Earthquakes in cities are more likely to cause damage and casualties compared to an earthquake in the countryside
How does time of occurrence affect the extent of earthquake
Time of the day affects what people are doing and where they are, affecting their chances of survival
If an earthquake strikes at night when most people are sleeping, there is a higher chance people will be trapped in their own house
How does level of preparedness affect the extent of earthquake
The preparedness plans include evacuation plans, trained rescue workers and a range of action plans
The damage caused by an earthquake is more manageable if people are prepared for it
How does distance from epicentre affect the extent of earthquake
The damage to an area nearer to the epicentre is larger as there is more ground motion nearer to the epicentre of the earthquake
