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Geo: Tectonic Hazards > 2 - What are the main hazards of earthquakes? > Flashcards

2 - What are the main hazards of earthquakes? Flashcards

1
Q

How often do earthquakes happen world wide?

(Its a lot)

A

1 every 30 seconds

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2
Q

What percentage of seismic activity takes place at plate boundaries?

A

99.9%

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3
Q

What percentage of seismic activity is located at the ring of fire?

A

95%

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4
Q

What does SKAATES stand for?

A

Source
Key
Appropriate to the question?
Author
Timescale
Earthquakes and volcanoes?
Spatial scale

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5
Q

What is the definition of an earthquake?

A

A sudden and violent shaking of the ground causing great destruction, as a result of seismic activity due to tectonic plates or volcanic activity

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6
Q

What is the focus of the earthquake?

A

The focus is the origin of the earthquake where there has been a slip in the fault line and the energy dissipates from

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7
Q

What is the epicentre of an earthquake?

A

The point on the earths surface directly above the focus.

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8
Q

When and where was the ‘Loma prieta earthquake’

A

California, San Andreas Fault, 1989

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9
Q

What type of plate boundary is the San Andreas Fault

A

Conservative

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10
Q

What are the properties of shallow focus earthquakes?

A
  • Surface down to 70km
  • Often occur in brittle rocks
  • Generally release low levels of energy but high energy shallow earthquakes can cause severe impacts
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11
Q

What are the properties of deep focus earthquakes?

A
  • 70km to 700km deep (normally at destructuive plate boundaries)
  • Increasing depth leads to high temperature and pressure
  • Less frequent but very powerful
  • Full understanding is evolving: water and minerals may be contributing factors
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12
Q

What are the impacts of shallow vs deep quakes?

A
  • Shallow quakes tend to be more damaging
  • Deep quakes are more widely felt
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13
Q

What order do seismic waves arrive in?

A
  • P-waves (primary)
  • S-waves (Sheer)
  • Surface Waves
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14
Q

What are the properties of different seismic waves?

A
  • Primary waves are faster, longitudinal, and do not cause much damage due to having a smaller amplitude
  • Sheer waves shake the ground side to side (transversal) and are slower. They cause more damage
  • Surface waves shake in the direction of propogation and perpendicular so cause the most damage. They are the slowest.
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15
Q

How does geology impact ground shaking?

A
  • Solid rock shakes less
  • softer ground slows the waves and increases the amplitude making them more damaging
  • Saturated and softer permeable rock can liquify under shaking
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16
Q

What is a fault?

A

A fault is a fracture in the Earth’s crust along which two blocks of the crust have slipped with respect to each other.

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17
Q

What is a fault?

A

A fault is a fracture in the Earth’s crust along which two blocks of the crust have slipped with respect to each other.

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18
Q

What are the three types of faults?

A

Normal, reverse, strike-slip

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19
Q

Explain a normal fault.

A
  • Divergent
  • Hanging wall slides down below the footwall
  • Usually at constructive plate boundaries
  • Leads to the formation of a fault scarp
  • Causes the extension of the Earth’s crust
  • Can be observed in the western US and along oceanic ridges
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20
Q

Explain a reverse fault.

A
  • Convergent
  • Hanging wall is forced above the footwall
  • Usually at destructive or collision plate boundaries
  • Leads to the formation of fault scarps
  • Causes the compression of the Earth’s crust
  • Thickens and shortens rocks
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21
Q

Explain strike-slip faults.

A
  • Not convergent or divergent
  • Plates run parrallel
  • Usually at conservative plate boundaries or collision (long ones found in the Himilayas)
  • E.g San Andreas Fault
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22
Q

Where is the East African Rift Valley found?

A

On the African Plate. It is splitting into two smaller plates.

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23
Q

What are the names of the two plates that the African Plate is splitting into?

A

Nubian Plate moving north west and the somalian plate moving south east

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24
Q

What landforms has the East Afrcian Rift valley lef to the formation of?

A
  • Escarpments (horsts) and rift valleys (graben)
  • It is split into the Eastern Rift and the Western Rift
  • The Western Rift borders the Congo and has a series of deep water lakes such as Lake Edward and Tanganyika
  • The Eastern Rift doesn’t have deepwater lakes, more shallow lakes and volcanic activity as the crust has thinned due to extension.
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25
Q

How long is the East African Rift Valley and when did it form?

A

4000km and formed 25-30million years ago.

26
Q

How are fold mountains formed?

A

When two plates carrying continental crust are colliding at a collision plate boundary, the crust folds up and forms mountains.

27
Q

Why does continental crust not subduct?

A

Because it is not dense enough

28
Q

What are examples of fold mountain ranges?

A

Himalayas, Urals, the Alps

29
Q

How did the Himalayas form as a result of plate tectonics?

A

The Indian and Eurasian plates moved together to cause the closing of the ocean between them. Once the ocean had fully closed, the continental crust was not dense enough to subduct and therefore this has caused the formation of fold mountains.

30
Q

What is an example of a conservative fault.

A

The San Andreas Fault

31
Q

What is the most famous peice of visible evidence for the San Andreas Fault?

A

The offset in Wallace Creek

32
Q

How are earthquakes measured based on damage to buildings?

A

Using the Modified Mercalli Scale

33
Q

How does a seismometer work?

A

It measures the shaking of the earth up and down by tracking the relative movement of a hanging weight on a spring against a rotating drum.

34
Q

Why don’t we currently use the Richter Scale?

A

It underestimated the size of larger earthquakes.

35
Q

What is the standard way to measure the amount of energy released in an earthquake?

A

Moment Magnitude Scale

36
Q

What scale are the magnitudes of earthquakes measured on?

A

Richter magnitude scale

37
Q

What is the difference between magnitude and intensity?

A
  • Magnitude is a measure of the amplitude of seismic waves
  • Magnitude is the same everywhere in an earthquake
  • Intensity is how severe the ground shaking is, and depends on the depth of the focus, geology, and distance from the epicentre.
38
Q

How does geology impact seismic waves?

A
  • Seismic waves are amplified by a factor of five when they pass from bed rock to saturated bay mud.
  • Seismic waves were amplified by a factor of two when they pass from bed rock to sand and gravels (alluvium)
39
Q

What are the hazards generated by an earthquake?

A
  • Ground shaking (primary)
  • Avalanche
  • Landslide
  • Flooding
  • Tsunami
  • Liquefaction
40
Q

Why is groundshaking a primary hazard?

A

Because it is directly caused by an earthquake, whereas the other hazards are caused by groundshaking.

40
Q

Why is groundshaking a primary hazard?

A

Because it is directly caused by an earthquake, whereas the other hazards are caused by groundshaking.

41
Q

What is a natural hazard?

A

It is a natural event with the potential to harm people and their property

42
Q

What is a natural disaster?

A

The realisation of a natural hazard.

UN definition: a serious disruption of the functioning of a community or a society involving widespread human, material, economic or environmental losses and impacts, which exceeds the ability of the affected community or society to cope using its own resources.

43
Q

Why is a disaster hard to define?

A

Because it is hard to know what to base it on. Money, death toll and number of people affected are usually considered.

44
Q

What model is used to show the intersection of people and natural hazards?

A

The Degg Model

45
Q

Why is groundshaking considered an earthquake hazard?

A

If shaking is intense enough, it can damage infrastructure. Crucially, if buildings collapse then it puts humans at risk. “Earthquakes don’t kill people, buildings do.” - Professor Iain Stewart

46
Q

What makes people particularly vulnerable to ground shaking?

A
  • Vulnerable buildings
  • Time of day. Night is most dangerous in EDCs and LIDCs but day is more dangerous in ACs
47
Q

How many people did the Nepal 2015 avalanche kill and what magnitude was it?

A

7.8 magnitude caused an avalanche that killed around 200 people

48
Q

What are some impacts of avalanches and landslides?

A

· Disruption to transport links.
· Settlements destroyed.
· Farmland covered in mud and rocks.
· Tourist infrastructure damaged.
· Death of people in landslide or avalanche.
· Block rivers.
· Remove vegetation from hillsides.
· Removes communication infrastructure.
· Damages water & electricity infrastructure.

49
Q

What is vulnerability?

A

Some places are more vulnerable than others to tectonic hazards. This means they are more likely to suffer significant damage as a result of a tectonic hazard. This can be both human and physical reasons.

50
Q

Why was nepal vulnerable to landslides and avalanches

A
  • Lack of defences - buildings and people are less protected from the threat of an avalanche or landslide
  • Deforestation - there are less trees to stop rocks and snow falling down slopes, and hold the soil together
  • Steep slopes - it is easier for snow/rocks to slip as they have more potential energy
  • Monsoonal rainfall - makes soil heavier as rain is absorbed. Also lubricates the soil/snow.
  • Cold climate - more snow is able to build up, meaning it is heavier and could fall more easily.
  • Geology - rocks are easily dislodged
  • Seismic activity - earthquakes can dislodge snow/rocks and cause them to fall more easily
  • Level of development - low level of development means there are less stringent safety precautions, and buildings are a lower quality.
51
Q

What are quake lakes

A

Landslides can block the course of rivers in mountainous areas. Because of the constraints of the relief (there are mountains either side of it), a ‘quake lake’ forms behind an unstable barrier made of the landslide debris. When this collapses a huge rush of water downstream that floods areas. This happened in Sichuan 2008.

52
Q

How can earthquakes cause flooding

A
  • Formation of quake lakes, which then burst and cause flooding.
  • Natural flood risks increase, movement of ground can collapse river banks, trap water on the surface and reduce infiltration rates. This happens in Christchurch where shaking caused subsidence of 0.5 to 1m.
53
Q

How are Tsunamis formed

A

Underwater landslides: A landslide draws water downwards and this causes water to be displaced upwards, causing waves to propagate outwards. E.g. Palu, Indonesia, 2018 where 1000 people died. Conservative plate boundary so unexpected, and warning systems failed.

Seabed Uplift: One plate moves under the other, and occasionally the top plate slips up if too much pressure builds up. This causes enormous energy to be released and the column of water above to be forced up. E.g. Japan 2011 where 20,000 people died, and there was $360bn in damages. Tsunami walls saved lives but couldn’t stop the tsunami.

54
Q

Why are Tsunami waves not dangerous out at sea?

A

They have very long wavelength and small aplitude, but when they reach the coast they are forced up and can reach 10s of meters high.

55
Q

What is a good example of when natural factors have overcome human factors?

A

Japan 2011 Tsunami

56
Q

What is liquefaction and how is it caused by earthquakes?

A

Secondary hazard to ground shaking. Short term phenomenon where saturated ground made up of unconsolidated sediment behaves like a liquid due to shaking. During an earthquake, soil looses its strength as shaking destabilizes the sediment and increased pore pressure causes the water table to rise to the surface.

57
Q

What are the two case studies for where liquefaction occured

A

Christchurch and Haiti

58
Q

What was the cause and effect of liquefaction in Christchurch 2011

A

Causes:
- Shallow focus of only 5km below ground and high magnitude of 6.3 caused sever shaking
- Building codes dont mitigate against liquefaction
- Housing and buildings located near river on low lying floodplains which are saturated and made up of silty sands

Effects:
- People relocated and houses were bulldozed -almost 7000 in total - River Avon Red Zone
- Underground pipes float to surface and 80% of local water and sewage was damaged, with hamrful chemicals going into the river
- Damage to infrastructure, cars and property due to sinking.

59
Q

What was the cause and effect of liquefaction in Haiti 2010

A

Causes:
- Port was located on reclaimed land from sea so was low lying and saturated.
- High magnitude earthquake of 7.0
- Port was not built on bedrock, but mainly on unconsolidated sediment soil.

Impacts:
- Wharff frontage and jetty collapsed into sea
- Various damages to cargo and ships
- Limited the aid that could reach the country as the only other trade route in was the single runway airport
- Only the port was damaged, but it caused economic suffering and reduced aid

Geo: Tectonic Hazards (7 decks)

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