Plate Tectonics: Seismicity

Define 'earthquake'
A sudden vibration or trembling in the Earth resulting from a build up and release of energy after friction has taken place.
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How many tremors occur annually that are strong enough to be felt by humans?
over 150,000
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How many earthquakes on average occur each year with a magnitude of 7.0 or over?
18
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Where do the most severe earthquakes occur?
At Destructive and Conservative plate boundaries
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Why do the most severe earthquakes take place at destructive and conservative boundaries?
Here, large masses of rock that are moving past each other can become locked due to friction. Friction is overcome when the accumulating stress has enough force to cause a sudden slippage of the rock masses, making the ground shake violently.
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What is the focus of an earthquake?
The point at which energy is released within the crust
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What is the epicentre of an earthquake?
Th point immediately above the focus on the Earth's surface. It receives the most energy and is the most hazardous location.
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How deep is a shallow focus earthquake?
0-70km
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What is magnitude determined by? (3 points)
The quantity of stress built up because of friction, the distance the rock, the ability of the rock to transmit the energy contained in the seismic waves.
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What are aftershocks?
The continued release of tensional stress and by the Earth settling back into place after the original displacement.
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Why are aftershocks dangerous?
Despite usually being considerably smaller than the main seismic event, they can cause further damage to already weakened structures.
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What are the 3 types of seismic wave?
P waves (primary), S waves (secondary), and L waves (Long)
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Which waves are body and which waves are surface?
Body waves are P and S, Surface waves are L
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Describe P waves
Travel fastest at 5-7km per second. They are longitudinal and compressional, vibrating in the direction in which they travel. Can travel through liquids and solids.
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Describe S waves
Travel at half the speed of P waves, arriving immediately after. Shear shock by vibrating at right angles to the direction of travel, transverse waves. Moves through solid materials. They do more damage because they move horizontally & vertically.
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Describe L waves
Travel slowest, at or near the Earth's surface, they produce a rolling or swaying motion causing the Earth's surface to behave like waves. Despite their slow speed, these cause considerable ground movement and so can be particularly destructive.
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How many people died at Killari, India
10,000 people
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When was the Killari earthquake
1993
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Why was the Killari earthquake so unusual
It occurred far away from the Himalayan collision belt. It is believed that a reservoir and dam recently completed 20km to the west caused an old fault line to be lubricated
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What does the Richter scale measure?
Both the strength of an earthquake and the energy released by measuring shockwaves
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What is used to measure the Richter scale
A seismometer. A weighted pen stays still whilst the rest of the apparatus moves with the Earth during the quake. This results in zig-zagging lines on the paper, the amplitude of which belies the scale.
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Describe the Richter scale
The scale is infinite in theory, although the highest ever recorded was a 9.5 in Chile, 1960. It is logarithimic, so each unit represents a 10 fold increase in strength and a 30 fold increase in energy.
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What is a minor/major event on the Richter scale?
A 2.0 is the smallest that can be felt by humans, a 7.0 would be a major seismic event.
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What does the Mercalli scale measure?
The intensity of the event via its impact and effects, running from 1-12, with 1 being extremely minimal and 12 being total destruction.
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What is used to measure the Mercalli scale?
It does not use graphs or specialist equipment, so is less precise, and is instead in relation to the visible impacts on humans and infrastructure.
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What is a minor/major event on the Mercalli scale?
A 5 means some small unstabel objects being displaced and some windows and pipes cracking slightly. A 10 means intense damage, with serious damage to earthquake-resistant structures and most unreinforced structures being destroyed.
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Why do some places suffer more from an earthquake than others? 6 reasons
Time (higher loss of life on week days during working hours or at night when people are sleeping), magnitude, distance of epicentre, geology, preparation of infrastructure, population size.
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Can earthquakes be predicted?
It is difficult to predict earthquakes exactly; we can identify the regions most at risk due to plate tectonic theory and previous events, but we cannot predict when and precisely where there will be an earthquake.
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What do they do in Tokyo to reduce seismic hazards?
They have installed smart meters which cut off gas supplies to the city to prevent fires.
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What are 4 seismic prediction techniques
Radon gas counters (the amount dissolved in groundwater may increase), Laser Reflector (detect small movements along a fault), GPS (Ground deformation as rocks are strained under increasing pressure), seismometers (can measure foreshocks)
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List 3 ways of making architecture aseismic.
Single storey buildings are more resistant to ground shaking. Large rubber shock absorbers built into foundations. Add cross-bracing to the structure to hold it together when it shakes.
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An example of an LEDC aseismic building
Istanbul's Sabiha Gokcen airport (World's largest earthquake-resistant building) Built to withstand an 8.0 quake and remain fully operational.
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An example of an MEDC aseismic building
Transamerica Pyramid, San Francisco (49-storey office block)
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case study of an LEDC earthquake
Bam, IRan, 2003, 5 AM
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Case study of an MEDC earthquake
Central Italy 2016
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The word 'tsunami' comes from where?
The Japanese for 'Harbour wave'
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What percentage of tsunamis happen within the Pacific Ring of Fire?
80%
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Define tsunami
A giant sea wave casued by the displacement of large volumes of water, usually resulting as a secondary hazard of a submarine earthquake.
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Why is a tsunami more powerful if tsunamigenesis is closer to land?
Because the wave loses energy as it travels, especially as it travels inland.
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what is a tsunami wave like in the open ocean?
Long wavelength of 200km with a small wave height of 1m. High speeds of 500-950kmph.
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What is a tsunami wave like close to land?
Wavelength decreses to less than 20m, wave height (amplitude) increases to over 30m tall. wave speed decreases to 80kmph.
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Describe ocean drawback
This is where there appears to be an unusally low low-tide, whereby coastal water is sucked seaward, exposing the seabed. Important warning sign of a tsunami, gives up to a 5 minute warning.
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What is a wave train?
A tsunami is usually composed of a series of eaves called a wave train, so its destructive force may be compounded as successive waves reach the shore.
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What is an example of a tsunami warning system?
The Pacific Tsunami Warning System is a coalition of 26 nations, headquartered in Hawaii, that maintains a web of seismic equipment and water level gauges to identify tsunamis at sea.
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Where is the most developed regional tsunami warning system?
In Japan, where they aim to issue a warning within 20 minutes of a tsunamigenic earthquake within 600km of the Japanese coastline. TOkyo network of 150 seismic detectors, a main computer which calculates the size of the tsunami & issues warnings
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Why can it be difficult to issue tsunami warnings?
The initial earthquake may have destroyed power lines and communications in the region. The area covered may be too large. Events may be too quick as there may only be 30 minutes or less between formation and landfall.
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What is the downside of tidal gauges ?
They give little to no warning of tsunamis as they are so close to shore, and so can only really record the details of the event
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What does the Pacific Tsunami Warning System comprise of?
30 seismic and 70 tidal stations
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how much does a false warning cost Hawaii
$30million
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what are the risks of tsunami warnings?
Not all submarine earthquakes result in tsunamis, false warnings cost a lot of money. The more false warnings there are, the more complacent the public will become.
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What are some methods to protect against tsunamis?
Planting trees and mangroves on the coast, defensive engineering works eg onshore walls and offshore breakers, specific building design such as open ground floors to allow waves to pass through.
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Tsunami case study
2004 boxing day Indian ocean tsunami
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Other cards in this set

Card 2

Front

How many tremors occur annually that are strong enough to be felt by humans?

Back

over 150,000

Card 3

Front

How many earthquakes on average occur each year with a magnitude of 7.0 or over?

Back

Preview of the front of card 3

Card 4

Front

Where do the most severe earthquakes occur?

Back

Preview of the front of card 4

Card 5

Front

Why do the most severe earthquakes take place at destructive and conservative boundaries?

Back

Preview of the front of card 5
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