✅HAZARDS 3.1.5.4 - Seismic Hazards Flashcards

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1
Q

What are seismic hazards?

A

Earthquakes, tsunamis, landslides and liquefaction

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2
Q

What is seismicity?

A

The Earth’s shaking, can be human induced or natural

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3
Q

How are seismic waves formed?

A

Pressure builds up at a point where plates meet
Sudden release due to rock failure creates waves
The waves cause the ground to shake
Intensity of the shaking is dictated by the depth of focus and energy release

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4
Q

What are tsunamis caused by?

A

Earthquakes under the sea, moving a column of water above it, displacing it

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5
Q

What are the characteristics of tsunamis?

A

Waves less than 3ft tall, but are compress towards land, gaining height. A wave train which propagates form the earthquake.

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6
Q

What is liquefaction?

A

The mixing of sand or soil with groundwater during the shaking of a moderate or strong earthquake. Ground becomes very soft and similar to quicksand.

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7
Q

When is land susceptible to landslides?

A

It has experienced heavy rain
Fractured or unconsolidated rock
Fold mountains, unstable plate boundary
Steep relief

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8
Q

How can earthquakes be predicted?

A

Past seismic events
Remote sensing
Radon gas emissions

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9
Q

How can past seismic events be used to predict earthquakes?

A

Looking into past frequencies and magnitudes to infer future events can be used for prediction

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10
Q

How can remote sensing be used to predict earthquakes?

A

GPS can be used to monitor the smallest of movements by the plates, stationary plates suggest build up of pressure

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11
Q

How can radon gas emissions be used to predict earthquakes?

A

Radon escapes form cracks in the earth’s crust and a sudden increase may suggest that an earthquake may be imminent

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12
Q

Are earthquakes predictable?

A

No

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13
Q

What type of boundary to most earthquakes originate from?

A

Destructive - at subduction zones

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14
Q

Where is liquefaction a particularly dangerous hazard?

A

In areas where groundwater is near the surface and the soil is sandy

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15
Q

What is subsidence?

A

When the ground surface is lowered often during earthquakes

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16
Q

What are the characteristics of P waves?

A

Fastest waves
Earth moves backwards and forwards but surface stays flat
Moves through solids and liquids

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17
Q

What are the characteristics of S waves?

A

A ripple, up and down movement
Water wave, surface doesn’t stay flat
Shakes earth at right angles
Can’t move through liquid, like the core

18
Q

What are the characteristics of Rayleigh waves?

A

Up and down movement and side to side, moves like a snake
Low frequency, rolling motion
Radiate along the surface

19
Q

What are the characteristics of Love waves?

A

Slowest waves, surface
Causes sideways motion and most damage
One pulse, up and down and some side to side

20
Q

What is a seismograph used for?

A

To measure ground movement

21
Q

How does a seismograph work?

A

Pen attached to weight to track movements

22
Q

What is a seismometer used for?

A

Recording ground movement data digitally

23
Q

What are the issues with a seismometer?

A

So small that waves must be amplified

24
Q

What does the Mercalli scale range from and to?

A

1-12

25
Q

What is a 1 on the Mercalli scale?

A

Instrumental - not felt by many people unless in favourable conditions

26
Q

What is a 12 on the Mercalli scale?

A

Cataclysmic - total destruction

27
Q

What are the positives of the Mercalli scale?

A
Can be used for damage comparison
Relates to epicentre location
No equipment required
Idea of damage done, builds a picture
Perfect for response planning
Allows links for human need
28
Q

What are the negatives of the Mercalli scale?

A

Subjective, qualitative measure
Uncertainty and bias
Location of observer alters results
Compares effects, not the event

29
Q

What does the Richter scale measure?

A

Ground deformation and energy release

30
Q

What type of scale does the Richter follow?

A

Logarithmic

31
Q

What is the highest recorded value on the Richter scale?

A

8.9

32
Q

What is the Moment Magnitude Scale

A

Measures EQ
takes into account energy released, distance of movement of faults, rigidity of rock (softer rock deforms more), maps onto Richter scale

33
Q

Human activity that can cause EQs

A

Mining
Fracking
Nuclear Explosions

34
Q

Mining as a cause of EQs

A

Removal of material makes instability - 271 sites have been identified where it has caused EQs in last 150 years

35
Q

Fracking as a cause of EQs

A

29 sites from fracking
36 from post-fracking wastewater disposal (the high pressure can crack rocks and lubricate them)
Lots in Oklahoma

36
Q

What is elastic rebound theory

A

The concept that EQs are repeating patterns of pressure accumulation and release

37
Q

Evidence against Elastic rebound theory

A

EQs have acted uncharacteristically, not in a cycle:
EQs have hopped from fault to fault, 7.3 in California, 7.9 in Alaska, 2002
EQ expected in San Andreas based off 22 year cycle in 1993, next one was 2004

38
Q

Tsunami formation

A

Displacement of water, EQ
Oscillation of water
Wave height increases towards land due to friction with bathymetry (seabed)

39
Q

Tsunami features

A

approx. 90% are on the ring of fire
1 metre tall in ocean, up to 25 metres in shallower water
600-700km/hr

40
Q

Magnetic fields in Tsunamis

A

When lots of water moves, a magnetic field is created, can be detected to aid prediction and preparation

41
Q

Sea level and tsunamis

A

As climate change causes sea level rise, coasts shrink so tsunamis can travel further in land, therefore more damage