Earthquakes, Processes, Hazards + Their Impacts Flashcards
Define earthquakes
Violent shaking of the ground, caused by movements of the Earth’s crust usually along pre-existing faults
Where do earthquakes occur?
- Mainly: along plate boundaries
- Also: locations where human activity causes low magnitude earthquakes (e.g result of fracking or nuclear explosions)
What percentage of earthquakes occur along the Pacific Ring of Fire?
80%
What type of plate boundaries can earthquakes occur at?
All
Outline how earthquakes are formed at converging (destructive) plate boundaries
Most common earthquake location
Varying foci depth
- Caused by friction along the Benioff Zone as slab pull occurs which triggers build up then release of stress
Outline how earthquakes are formed at diverging (constructive) plate boundaries
Shallow focus
- Caused by build up and release of stress at transform faults
What are transform faults?
Faults that open up near the main fault due to plate movements (occur at diverging plate boundaries)
Outline how earthquakes are formed at converging collision plate boundaries
Shallow focus
- Cause by build up and release of stress as plates move together as orogenies occur
What are orogenies?
Mountain building
Outline how earthquakes are formed at conservative plate boundaries
Can cause strong earthquakes
- Caused by: lateral movements cause friction, causing tension to build up then be released when plates slip past each other
Outline how earthquakes occur
- As crust moves, pressure builds up in rocks in underground fault zones
- Pressure becomes so great that rock shifts violently and deforms (internal deformation)
- When the rock deforms, seismic waves are released from the focus
- Afterwards, the rock rebounds to its original shape, but in a new geographical position (elastic rebound)
Define fault zones
The location deep underground where pressure builds up in rocks due to crust movement
Define internal deformation
When pressure in rocks becomes so great that the rock deforms and releases shockwaves
Define elastic rebound
When deformed rock rebounds to its original shape in a new geographical position
Define focus
The point of rock deformation, which seismic waves are released from
What is a ‘shallow focus’ depth?
70km or less
What is an ‘intermediate focus’ depth?
70km -> 300km
What is a ‘deep focus’ depth?
Over 700km
Define epicentre
The projection of the focus onto the Earth’s surface
What causes a series of small magnitude earthquakes?
When pressure is released in stages
What causes one major earthquake?
When pressure is released all at once
Define seismic waves
Waves released from earthquakes
What are the four types of seismic waves?
- P waves (Primary)
- S waves (Secondary)
- L waves (Love)
- R waves (Rayleigh)
What are the two ‘body waves’?
- P waves (Primary)
- S waves (Secondary)
What are the two ‘surface waves’?
- L waves (Love)
- R waves (Rayleigh)
What are ‘body waves’?
- P + S
- Travel through Earth’s interior
- High frequency
- Less damaging
- Arrive before surface waves
What are ‘surface waves’?
- L + R
- Travel through Earth’s crust/surface
- Low frequency
- More damaging
- Arrive after body waves
List the properties of P waves (Primary)?
- Body wave
- Fastest/first to arrive
- Less damaging than S waves
- Can move through: solids + liquids
- Pattern: compressions + expansions of rock particles (longitudinal)
List the properties of S waves (Secondary)
- Body waves
- Arrive after P waves but before surface waves
- More damaging than P waves
- Can move through: solids
- Pattern: rock particles move at right angles to wave travel (transverse)
List the properties of L waves (Love)
- Surface wave
- Faster than R waves, slower than body waves
- Often most damaging
- Pattern: rock particles moved horizontally from side to side
List the properties of R waves (Rayleigh)
- Surface wave
- Slowest wave, arriving last
- Can be largest, resulting in most shaking felt
- Pattern: rock particles moved elliptically, so ground moves horizontally + vertically
List the 4 seismic waves from fastest to slowest
- P waves
- S waves
- L waves
- R waves
What are the two key uses of seismic waves?
- To locate Moho Discontinuity
- To locate earthquake’s epicentre
Outline how seismic waves have been used to locate the Moho Discontinuity
- Seismographs record body waves
At the discontinuity… - P waves: refract, slower velocity in mantle (partially liquid)
- S waves: don’t pass through the mantle
- This point of P waves refracting + S waves stopping is recorded
What is the Moho Discontinuity?
Boundary between crust + mantle
How can seismic waves be used to locate an earthquake’s epicentre?
- Record time taken for waves to reach seismograph station
- Already know speed of each type of wave
- Do speed x time to work out distance (between epicentre + station)
- Can work out where epicentre is by triangulating the distance circles of three seismograph stations
Define a primary impact of earthquakes
A direct result of the ground shaking
List some examples of primary environmental impacts of earthquakes
- Destruction of physical environment: e.g. treefall
- Destruction of built environment: e.g. collapsed buildings
List some examples of primary social impacts of earthquakes
- Deaths
- Injuries
Define secondary impacts of earthquakes
An impact caused indirectly as a result of the ground shaking
What hazards can be triggered by earthquakes - causing secondary impacts?
- Liquefaction
- Landslides, Avalanches, Rock falls (MASS MOVEMENTS)
- Tsunamis
List some examples of secondary environmental impacts of earthquakes
- Flooding
- Damage of buildings + infrastructure after the earthquake, as a result of secondary hazards
- Pollution (e.g. atmospheric pollution from fires triggered)
List some examples of secondary social impacts of earthquakes
- Homelessness (due to loss of buildings)
- Unemployment (due to loss of business buildings + infrastructure)
- Spread of diseases from contaminated water + conditions in temporary camps (e.g. cholera)
- Lawlessness
- Lack of education + health facilities (due to loss of trained staff)
List some examples of secondary economic impacts of earthquakes
- Need for costly repairs
- Costs of aid (often comes from abroad)
- Loss of businesses + lower economic activity lowers GDP
Define liquefaction
Water-saturated material loses strength + behaves as a liquid when subjected to ground movement
(A secondary hazard triggered by earthquakes)
How does liquefaction occur?
- Ground shakes
- Increase in pressure on ground
- Water forced from pore spaces when under pressure
- Cohesiveness of ground’s internal structure decreases
- Ground material is able to move as a liquid
What can liquefaction cause?
Structural instability in buildings, as their foundations sink + they collapse
What happens to liquefacted land post-earthquake?
Water sinks back into ground + surface firms
Give an example of the secondary hazard liquefaction
Christchurch earthquake (2011)
- Liquefaction caused buildings to collapse
- About 20,000 residential properties had severe damage due to this
Define landslides/avalanches/rock falls
Mass movements that occur after land shakes in earthquakes
A secondary hazard triggered by earthquakes
How do earthquakes trigger mass movements?
- Ground shakes
- Stresses are created
- Natural slopes weakened by the stresses + collapse in mass movement
What do mass movements cause?
A significant hinderance in rescue efforts, as accessibility is usually reduced
What influences the chances that the secondary hazard of mass movements is triggered?
- Rainfall
- Relief of land
- Land use
- Geology
Give an example of a secondary hazard mass movement
Alps (often)
- Avalanches often triggered by earthquake activity
Define tsunamis
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