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AS Geography - Abbi > Hazards (Seismic) > Flashcards

Hazards (Seismic) Flashcards

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1
Q

How are earthquakes formed?

A
  1. Plates move due to gravitational sliding, slab pull and convection currents
  2. Plates get stuck due to friction, pressure and tension building up
  3. When plates jerk past each other, they release this pressure and send shockwaves through the Earth’s crust
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2
Q

Distribution of seismic events

A
  • Found particularly along conservative and destructive plate boundaries
  • Ring of Fire accounts for 90% of Earth’s earthquakes
  • Alpine-Himalayan belt accounts for 5-6%
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3
Q

How does depth of focus affect magnitude of earthquakes?

A
  • Deeper focus earthquakes tend to be of a higher magnitude

- Deeper focus tend to do less damage as shock waves have to travel further which reduces the strength

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4
Q

Why do earthquakes have a higher magnitude at destructive plate boundaries?

A

Higher pressure builds between plates during subduction

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5
Q

Why are earthquakes lower magnitude at constructive plate boundaries?

A

Fracturing is more frequent so pressure doesn’t have as much time to build up

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6
Q

Three factors affecting magnitude of earthquakes

A
  • Type of plate boundary
  • Depth of focus
  • Rate of movement
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7
Q

Why might earthquakes occur away from a plate boundary?

A
  • Reactivation of old fault lines possibly from deferred stress release
  • Large dams or reservoirs cause pressure on underlying rock and reactivate old fault lines
  • Hydraulic fracturing
  • Subsidence of old mines
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8
Q

Body waves

A

Travel through the earth. Can be divided into primary and secondary waves.

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9
Q

Surface waves

A

Travel along the Earth’s surface. Cause most damage as they cause more ground movement. Travel slowly

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10
Q

Primary (P) waves

A

Alternately compress and expand. Particle movement is parallel to wave direction. Fastest wave and can travel through all substances.

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11
Q

Secondary (S) waves

A

Transverse. Movement of particles is perpendicular to wave movement. Cannot travel through air or water. Slow but causes more damage.

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12
Q

Rayleigh waves

A

Ground roll. Rocks move in elliptical motions as the wave passes and breaks up the surface.

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13
Q

Love waves

A

Horizontal shear waves. Move the ground from side to side at right angles to movement direction. Can damage infrastructure and buildings.

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14
Q

What does the Richter scale measure?

A

Measures the magnitude of an earthquake based on the amplitude of the secondary waves.

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15
Q

What does the Moment Magnitude scale measure?

A

Measures the magnitude of based on the total energy released. Distance a fault has moved x force taken to move it.

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16
Q

What does the Mercalli scale measure?

A

Intensity of an event and its impact. 12 point scale, subjective.

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17
Q

Epicentre

A

Point in the Earth’s surface directly above the focus, where the earthquake is first felt

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18
Q

Focus

A

Point in the Earth’s crust where the earthquake starts

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19
Q

Relationship between magnitude and frequency in seismic events

A

Negative relationship - as magnitude increases, frequency decreases

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20
Q

Primary impacts of seismic events

A
  • Ground rupture (displacement of the Earth’s surface along the fault line)
  • Ground shaking
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21
Q

Factors affecting the severity of ground shaking from seismic events

A
  • Dependent on magnitude
  • Depth of focus
  • Distance from the epicentre
  • Geological conditions (will affect likelihood of liquefaction)
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22
Q

How do earthquakes result in tsunamis?

A
  1. Earthquakes move the seabed up by several meters
  2. This displaces water above. The greater the movement, the greater wave produced.
  3. Large waves radiate outwards across the ocean away from the epicentre
  4. The water becomes shallower as it approaches the coast and the base is slowed by friction
  5. This forces the wave in a circular motion into an elliptical form. This heightens until it can’t be maintained and eventually breaks. This is called shoaling
  6. The tsunami hits the coast as a large wave
23
Q

How does the height of the wave affect intensity of tsunami impacts?

A

The greater the movement, the greater volume displaced and the greater the wave produced.

24
Q

How does the distance travelled by the wave affect the intensity of tsunami impacts?

A

Waves lose energy as they travel inland. The closer to the land it starts, the less energy lost and the more powerful the tsunami will be.

25
Q

How does the shape of the coastline affect the intensity of tsunami impacts?

A

Funnelled coastlines concentrate the energy on the bay. Irregular coastlines and offshore islands can accentuate the waveform.

26
Q

How does the relief of the coastline affect the intensity of tsunami impacts?

A

Cliffs present a natural barrier to the tsunami

27
Q

How does the presence of natural defences affect the intensity of tsunami impacts?

A

Coral and mangroves act as natural defences by dissipating wave energy through their large surface areas.

28
Q

How does population density affect the intensity of tsunami impacts?

A

High population density causes a greater intensity of impacts. Young and old are most vulnerable, may be gender disparity.

29
Q

What is liquefaction?

A
  • Weakening of water saturated sediment after an earthquake, causing it to act as a liquid.
  • May lead to eruptions of pressurised water and sand, called sandblow which could cause localised flooding
  • Causes lateral flow movement of the ground, damaging underground pipelines, causing building to collapse
30
Q

What conditions worsen impacts of liquefaction?

A
  • Geology: loose sand and silt
  • Water table close to surface
  • Lack of deep foundations to buildings
31
Q

How do seismic events result in landslides?

A
  • The earthquake destabilises hillsides and cause it to fall

- Can happen months or years later as it makes it more prone to fall during rainstorms etc.

32
Q

Why are landslides, that are caused by earthquakes, a hazard?

A
  • Cause debris flows

- Debris flows may result in natural dams and flash flooding when the dams break

33
Q

Secondary hazards of seismic events

A
  • Tsunamis
  • Landslides
  • Liquefaction
  • Fires
34
Q

Is prediction effective for seismic events?

A

No - regions at risk can be identified but it’s difficult to know when it will occur

35
Q

Methods of prediction for seismic events

A
  • Monitoring groundwater levels
  • Release of radon gas
  • Unusual animal behaviour
  • Using foreshocks
  • Tilt-meters to show ground formation
36
Q

Seismic Gap Theory

A

Over the long run, all parts of the fault must average the same level of movement. This can happen through a large number of very small earthquakes along the fault.

37
Q

Methods to attempt to prevent earthquakes.

A
  • Nuclear explosions at depth
  • Pumping water along the fault to stop plates from sticking
  • Use of boreholes to change soil properties and reflect energy waves.
38
Q

Methods of creating hazard resistant structures

A
  • Concrete weights on buildings to move in the opposite direction of the earthquake to counteract stress
  • Rubber shock absorbers in foundations
  • Cross bracing the structure to hold it together during shaking
39
Q

Evidence for the effectiveness of hazard resistant structures

A
  • Comparison of Loma Prieta and Armenia earthquakes. There were 63 deaths in California despite a higher magnitude and over 25,000 in Armenia as they had no structures protecting their buildings from collapsing.
  • Haiti: Badly constructed buildings, 230,000 killed. 7.0 Magnitude. Chile: 521 people killed. 8.8 magnitude. Building codes in place.
40
Q

Limiting factors of hazard resistant structures

A
  • Need to retrofit older buildings as well as new buildings

- Building regulations need to be enforced effectively for the structures to be successful.

41
Q

FEMA

A

Federal Emergency Management Agency

42
Q

Objectives of FEMA to reduce hazard risk

A
  • Promote understanding of the earthquakes and their risks
  • Work to identify earthquake risk
  • Improve earthquake resistant design and construction techniques
  • Encourage use of earthquake safety policies
43
Q

How does education minimise risk? (with example)

A

Minimises loss through use of drills, so people know how to respond in an emergency. Disaster Prevention Day in Japan (1st September)

44
Q

How does fire prevention minimise risk? (with example)

A

Smart meters cut off gas if a high enough magnitude occurs, reducing death by fire. Tokyo: has a network that transmits seismic information of when to switch off major pipelines

45
Q

How does emergency planning minimise risk? (with example)

A

organisation can tackle an minimise impacts. California: identifying which areas will need emergency services sent out first.

46
Q

How do early warning systems minimise risk?

A

Can give out warnings if foreshock or undersea earthquakes will lead to tsunamis, allowing people to take evasive action.

47
Q

How do sea walls minimise risk?

A

Protect the land from oncoming tsunamis. Can often be ineffective as large tsunamis often overwhelm them.

48
Q

How does insurance minimise risk?

A

People take out insurance to cover loss and recover faster.

49
Q

How does land use planning minimise risk?

A

Identifies most hazardous areas and regulates land use in these areas.

50
Q

Microzonation

A

Mappin out variability of hazards through urban areas to assess their geological and geophysical characteristics. This determines how hazardous they are likely to be

51
Q

Factors considered when land-mapping

A
  • Known fault lines
  • Geology: soft sediment amplifies magnitude
  • Topography: small features like hills and ridges can amplify damage
52
Q

Frequency of seismic events

A

Earthquakes are frequent and occur everyday at boundaries. Negative relationship between frequency and magnitude

53
Q

Regularity of seismic events

A

Earthquakes follow no pattern and are random so there is irregularity between events

54
Q

Predictability of seismic events

A

Earthquakes are almost impossible to predict. Magnitude is always impossible to predict.

AS Geography - Abbi (16 decks)

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