5.4: Seismic Hazards Flashcards

1
Q

What is an earthquake?

A

As the crust of the Earth is constantly moving, there tends to be a slow build up of stress within the rocks. When this pressure is released, parts of the surface experience, for a short period, an intense shaking experience.

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2
Q

What is the focus of an earthquake?

A

The point at which the pressure release occurs within the crust.

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3
Q

What is the epicentre?

A

The point immediately above the focus on the Earth’s surface.

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4
Q

How deep is a shallow focus earthquake?

A

0-70km

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5
Q

How deep is an intermediate focus earthquake?

A

70-300km deep

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6
Q

How deep is a deep focus earthquake?

A

300-700km deep

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7
Q

What kind of earthquake causes the greatest damage (shallow, intermediate or deep focus earthquake)?

A

Shallow focus (0-70km deep).

Accounts for 75% of all the earthquake energy released.

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8
Q

Where do the vast majority of earthquakes occur?

A

Along destructive plate margins.

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9
Q

Name the most famous fault line (constructive plate margin) that can cause earthquakes.

A

The San Andreas Fault in California.

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10
Q

What are the different ways in which the magnitude of earthquakes can be measured?

A
  • Richter scale
  • Moment magnitude scale (MMS)
  • Mercalli scale
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11
Q

Outline the Richter scale.

A

Measures the magnitude of earthquakes. Uses logarithmic scale on a scale of 1-10.

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12
Q

Outline the the MMS (moment magnitude scale)

A

Has replaced the Richter scale to identify the energy release from earthquakes - thus measuring magnitude.

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13
Q

Outline the Mercalli scale.

A

Mercalli scale measures the intensity of an event and its impact. On a 12 point scale (from ‘I. not felt’ to ‘XIII. extreme’).

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14
Q

Give examples of events that occur just before an earthquake strikes.

A
  • bulging of the ground
  • microquakes before the main tremor
  • raised groundwater levels
  • curious animal behaviour
  • increased argon gas content in the soil
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15
Q

How can GIS be useful to earthquakes?

A

GIS are used to prepare hazard maps that show the areas at greatest risk, and thus can planning of urban growth and development.

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16
Q

Outline some primary effects of earthquakes.

A
  • ground shaking: causing buildings to collapse, windows to shatter, sewers to fracture, power lines to collapse.
  • ground rupture: the visible breaking and displacement of the Earth’s surface.
  • immediate deaths and injuries from crushing, falling glass and transport accidents.
17
Q

Outline some secondary effects of earthquakes.

A
  • soil liquefaction: when violently shaken, soils with a high water content lose their mechanical strength and start to behave like a fluid.
  • tsunamis: giant sea waves generated by shallow focus underwater earthquakes involving movements of the sea bed.
  • fires: due to broken gas pipes and collapsed electricity transmission systems.
  • landslides: slope failure due to ground shaking.
  • effects on people: collapsing buildings, destruction of transport links, flooding, disease spreading, food heritages
18
Q

What is a tsunami?

A

Giant sea waves generated by shallow focus underwater earthquakes, violent volcanic eruptions, underwater debris slides and landslides into the sea.

19
Q

Do tsunamis have a short or long wavelength in open ocean?

A

Long wavelength

20
Q

Do tsunamis have a low or high wave height in open ocean?

A

Low wave height (under 1metre)

21
Q

How quickly to tsunamis travel in the open ocean?

A

700km per hour

22
Q

What happens to the wave height of tsunamis as they reach shallow water bordering land?

A

Rapidly increase in height

23
Q

What is the first warning sign of a tsunami?

A

The wave trough in front of the tsunami, which results in a reduction in sea level (known as a drawdown)

24
Q

Once on land, how high can a tsunami reach?

A

In excess of 25m.

25
Q

When a tsunami reaches land, its effects will depend upon…

A
  • the height of the waves and distance they’ve travelled
  • the extent to which warnings could be given
  • coastal land use and population density
26
Q

How can earthquakes be predicted?

A

Regions at risk can be identified through plate tectonics.

Attempts to predict the event a few hours before are based upon monitoring groundwater levels, release of radon gas and unusual behaviour.

27
Q

What is a seismic gap? And how can this help to predict an earthquake?

A

An area (on fault line) which has not had any real seismic activity for the past 20 years

28
Q

How can it be possible to prevent an earthquake?

A

Schemes look at keeping plates sliding past each other (rather than sticking and releasing, which is the main cause of earthquakes).

Could be done by lubricating this movement using water and oil, or nuclear explosions at depth.

29
Q

How can protection help to manage earthquakes?

A

For example, in the USA, the Federal Emergency Management Agency’s programme has ten following objectives:

  • to promote understanding or earthquakes and their effects
  • to work better to identify earthquake risk
  • to improve earthquake resistant design and construction technique
  • to encourage the use of earthquake safe policies and planning practices.
30
Q

In what was can protection manage the seismic hazard of earthquakes?

A

They prepare for the event by modifying the human and built environments in order to decrease vulnerability. Also includes attempts to modify the loss by insurance and aid.

31
Q

Outline the 8 ways to protect the environment for an earthquake (seismic hazard).

A
  • hazard resistant structures
  • education
  • fire prevention
  • emergency services
  • land use planning
  • insurance
  • aid
  • tsunami protection
32
Q

Outline how hazard resistant structures are used as protection for managing seismic hazards.

A

Buildings designed to be aseismic, so earthquake resistant.

  • putting large rubber shock absorbers in foundations which allow some movement of the building.
  • putting a large concrete weight on too often building which will move, with the use of computer programme, in the opposite direction to the force of the earthquake to counteract stress.
  • adding cross bracing to the structure to hold it together better when it shakes.

Older buildings can be retrofitted with such devices to make them more earthquake proof.

33
Q

Outline how education is used as protection

A

Instructions are issued by authorities in how to prepare for events by securing homes, appliances and heavy furniture, and assembling earthquake kits.

Children in schools, as well as offices and factories have earthquake drills.

After the Loma Prieta event, the American Red Cross issued a list of supplies that people should keep at hand including water, foodstuffs (high energy, canned), clothing first aid kit, radio, torch batteries, matches, fire extinguisher etc.

34
Q

Outline how fire prevention can be used as protection for managing seismic activities.

A

‘Smart meters’ have been developed that can cut off the gas if an earthquake of sufficient magnitude occurs.

In Tokyo, the gas company has a network that transmits seismic info to a computer which then informs employees where to switch off major pipelines, so reducing the number of fires.