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Physical Geography > Seismicity- Content > Flashcards

Seismicity- Content Flashcards

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1
Q

What are the three different earthquake depths?

A
  1. Shallow focus
  2. Intermediate focus
  3. Deep focus
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2
Q

How deep is a shallow focus earthquake likely to be

A

0-70km

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3
Q

How deep is an intermediate focus earthquake likely to be

A

70-300km

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4
Q

How deep is a deep focus earthquake likely to be

A

300-700km

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5
Q

Which type of focus has the greatest damage?

A

Shallow focus (0-70km)

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6
Q

What are seismometers?

A

Seismometers are sensitive instruments that measure seismic waves. They give us information about earthquakes and the internal structure of the Earth.

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7
Q

How does a seismometer work?

A
  1. The heavy mass doesn’t move
  2. The whole seismometer moves as the earth it is attached to shakes
  3. The recording device measures how far the rest of the seismometer has moved
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8
Q

What are the two main types of seismic wave?

A
  1. Primary waves

2. Secondary waves

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9
Q

What are the characteristics of primary waves

A
  • Shake ground up then down in longitudinal movement
  • can also causes back and forth movement
  • relatively weak
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10
Q

What are the characteristics of secondary waves

A
  • travel slower through crust
  • crust moves side to side at right angles to outward motion of the main wave
  • also called transverse waves
  • cause most damage
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11
Q

What are characteristics of the Richter scale

A
  • logarithmic scale (each unit is a 10 fold increase)
  • destructive earthquakes tend to be above 6
  • measures the amount of energy released
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12
Q

What are the characteristics of the Mercalli scale

A
  • 12 point scale runs from Level 1 (detected by few people) to Level 12 (total destruction)
  • measures the intensity of an earthquake and its impact
  • seismic detectors only date back to 1848; the Mercalli scale works on observations on the ground of the actual impacts
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13
Q

Why do people often fear aftershocks as much as an initial earthquake?

A

People often fear aftershocks as much as an initial earthquake as they may last for months afterwards and can be quite destructive

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14
Q

What is an example of destructive aftershocks

A

The 9.3 magnitude Indonesian earthquake of 2004 was followed by a series of aftershocks, one of which was a 6.1 in magnitude

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15
Q

What are the 7 main factors that influence the impact of earthquakes?

A
  1. Magnitude and depth of the earthquake
  2. Geological conditions
  3. Distance from the epicentre
  4. Population density, preparation and education
  5. Design and strength of buildings
  6. Time of day/week
  7. Impact of indirect hazards
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16
Q

How can magnitude and depth affect the impact

A
  • the deeper the focus, the less damage is caused as waves lose energy as they travel through the crust and by the time they reach the surface, the damage can be minimised
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17
Q

How do geological conditions affect the impact

A
  • if the area is solid rock there is usually less damage than sands and clays
  • on clays liquefaction can occur where water penetrates between the clay particles creating a quicksand-like substance into which buildings sink
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18
Q

How can population density, preparation and education affect the impact

A
  • the more densely populated an area, the more potential for loss of life and damage to property
  • the more educated, the better prepared a population can be for an earthquake
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19
Q

How can design and strength of buildings affect the impact

A
  • buildings can be built to withstand shaking of the earth and to limit loss of life and damage
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20
Q

How can time of day/week affect the impact

A
  • weekends mean families are usually together which reduces people trying to move around
  • midday during the week, night times and rush hours can result in more damage
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21
Q

What are examples of indirect hazards caused by an earthquake

A
  1. Fires
  2. Landslides
  3. Tsunamis
22
Q

What are the primary effects of earthquakes

A
  1. Tsunamis
  2. Soil liquefaction
  3. Landslides and avalanches
  4. Effects on people and their build environment
23
Q

What is soil liquefaction?

A
  • when violently shaken, soils with a high water content lose their mechanical strength and behave like a fluid
  • can cause movement of groundwater
  • it can come to the surface through cracks e.g. soil volcanoes
  • buildings can sink as a result
24
Q

What is an example of when soil liquefaction occurs

A

San Francisco 1989;

- the reclaimed Bay Area cost 100,000 million US dollars due to liquefaction

25
Q

What are tsunamis?

A

Tsunamis are giant sea waves generated by shallow focus underwater earthquakes

26
Q

What is an example of a tsunami

A

Indonesia December 2004

27
Q

What are landslides and avalanches and what can they lead to

A
  • slope failure as a result of ground shaking
  • further deaths and injuries
  • flooding from blocked rivers creating ‘quake lakes’
28
Q

What are the primary effects on people and their built environment

A
  • buildings, roads and bridges collapse

- disruption to gas, electricity and water supplies

29
Q

What are the secondary effects on people and their built environment

A
  • scarce or contaminated water supplies
  • fires from ruptured gas mains
  • loss of grade from damage to industries
30
Q

What are the long term effects of earthquakes?

A
  • permanent disruption to natural drainage patterns
  • problems with restoring trust in civil authorities/neighbours
  • repair and reconstruction of buildings/infrastructure can take years
  • higher unemployment; not all businesses recover
  • longer term illness and reduced life expectancy
  • trauma and grief may take months and years for recovery
  • loss of farmland and food production
31
Q

What is an example of somewhere that had problems restoring trust in civil authorities after an earthquake

A

Civil disorder in Haiti

32
Q

How does a tsunami form

A
  • the tsunami is generated by low focus earthquake underwater
  • this leads to displacement of the water
  • the wave can travel at speeds greater than 700km per hour
  • the wave rises in height when it reaches shallower water
  • it causes an initial reduction in sea level
33
Q

What are the locations of tsunamis

A
  • around 90% are generated within the Pacific basin
  • associated with tectonic activity around the edges of the Pacific basin, mostly convergent plates where subduction is occurring
  • 25% occur off the Japan-Taiwan island arc
34
Q

What are factors affecting the impact of a tsunami

A
  • coastal land use and population density
  • coastal physical geography
  • height of waves and distance they have travelled
  • length of the earthquake that caused it
  • extent to which warnings were given
35
Q

What are the likely effects of a tsunami

A
  • boats and wooden coastal structures washed inland
  • drowning and injuries
  • buildings, roads, bridges, harbour structures and soil washed away
36
Q

What are warning systems of a tsunami like

A
  • the first visual sign of a tsunami (apparent draining away of the sea) will usually be too late
  • many hours warning of approaching waves following important, major or serious seismic events
37
Q

What is an example of a tsunami warning system

A

The Pacific Tsunami Warning System in Hawaii

38
Q

What are the 5 main methods of earthquake prediction

A
  1. Seismic records
  2. Radon gas emissions
  3. Groundwater
  4. Remote sensing
  5. Low frequency electromagnetic activity
39
Q

How can seismic records be used to predict earthquakes

A

Studying patterns of earthquakes and using these to predict the next event

40
Q

How can radon gas emissions be used to predict earthquakes

A

Radon is released from rocks such as granite at a faster rate when they are fractured by deformation and movement

41
Q

How can groundwater be used to predict earthquakes

A

Deformation of the ground can cause water levels to rise or fall

42
Q

How can remove sensing be used to predict earthquakes

A

There is some evidence that electromagnetic disturbances in the atmosphere directly above places about to have an earthquake can be detected

43
Q

How can low frequency electromagnetic activity be used to predict an earthquake

A

Strong correlations have been found between certain types of electromagnetic activity and the most seismically active zones on Earth. A sudden change in electron density and temperature was recorded a week before a 7.1 magnitude earthquake in Japan in September 2004

44
Q

What are the three main types of protection against earthquakes

A
  1. Raising public awareness
  2. Making buildings and cities more earthquake resistant
  3. Improving earthquake prediction
45
Q

How can public awareness be raised

A
  • insurance dedicated to earthquake damage is available
  • in places such as Tokyo, citizens are advised to keep supplies such as water, food, blankets, first aid and emergency tools
  • police information sheets are provided
46
Q

How can buildings/cities be made more earthquake resistant

A
  • better evacuation routes and sites to recover evacuees
  • building of fire resistant buildings
  • all new buildings must meet strict earthquake resistant standards e.g. single storeys more resistant to ground shaking, soft storeys designed to collapse at the base
  • land use planning e.g. Schools and hospitals away from high risk areas
47
Q

What are ways in which buildings can be earthquake proofed

A
  1. Base isolation
  2. Vibration control
  3. Building materials
48
Q

What is base isolation

A

Isolation units resting on the foundations of the building to provide separation from the building and shaking ground

49
Q

What is vibration control

A

A set of technical means for reducing seismic loads and Improving seismic performance of buildings e.g. tuned mass dampers (masses that away at the top of a building in opposition to the building sway)

50
Q

How can building materials help

A

Un-reinforced brick or concrete is dangerous because due to the weight, the walls tip over or crumble and the roof will fall like a house of cards.

51
Q

How is an earthquake caused?

A
  1. Earthquakes originate along faults
  2. Parts of the crust are being forced in opposite directions. The rocks get stuck but forces on them continue, building up stresses
  3. Eventually the strain overcomes friction and the rocks move which releases large amounts of energy.
  4. Energy is transferred to the surrounding rocks as seismic waves and it decreases as the waves spread outwards.

Physical Geography (12 decks)

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