Geology AS > Module 2 - Earthquakes > Flashcards

Module 2 - Earthquakes Flashcards

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1
Q

What is an Earthquake?

A

A vibration in the rocks of the crust and upper Mantle caused by a sudden dislocation of the rocks along a fault

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2
Q

What are the vibrations that cause Earthquakes called?

A

Earthquake waves or Seismic waves

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3
Q

What is a seismic wave?

A

A wave that travels through the Earth, generally as the result of a tectonic earthquake. Occasionally by big explosions like a nuclear bomb.
Particles of rock vibrate, transmitting energy from one particle to the next, away from the source of the earthquake

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4
Q

What are body waves?

A
  • They travel through the interior of the Earth
  • follow paths bent by the varying density and rigidity of the layers within the Earth
  • similar to the refraction of light waves
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5
Q

What are the two types of body waves?

A

P and S waves

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6
Q

Where do P waves get their name from?

A

Primary - travel fastest and arrive first (4-7km/s)
Push - longitudinal or compressional waves so the vibration of the rock particles are back and forth (like sound waves), they can travel through any material
Pressure - the particles alternately move together (compression) and apart (rarefaction) in the direction the wave is traveling

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7
Q

What are the properties of rock that affect the speed of P waves?

A
  • Density - the denser the material, the harder it is for a wave to pass through it (the more it slows down)
  • Incompressibility - a material with high incompressibility will rebound quicker and P waves will travel faster
  • Rigidity - the same as shear strength or how much a material resists a bending force. A liquid has zero rigidity.
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8
Q

What is the formula to work out the velocity of a P wave?

A

Vp = square root of (incompressibility+4/3xrigidity)/(density)

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9
Q

What is the formula for the velocity of an S wave?

A

Vs = square root of rigidity/density

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10
Q

Where do S waves get their name from?

A

Secondary - travel slower than P waves (about 60% of the P waves velocity - 2-5km/s)
Shear - the movement of particles is sideways, in a shearing motion at right angles to the direction the wave is traveling (transverse wave)
Several times larger in amplitude than P waves

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11
Q

What are surface waves?

A

Travel just below the Earth’s surface. Move more slowly than body waves due to their low frequency, long duration and large amplitude.
Most destructive type of seismic wave

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12
Q

What are the two types of surface waves?

A

Rayleigh waves - Vertical movement

Love waves - horizontal movement

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13
Q

Where do L waves get their name from?

A

Long - longer wavelength
Last - arrive to the scene last
Love - named after the scientist who identified them

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14
Q

What is the focus of an Earthquake?

A

The focus is the point within the Earth at which the earthquake originates. It occurs along fault planes as one section of crust moves alongside another

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15
Q

What are the different classifications of the Earthquakes and their depth?

A
  • Shallow focus - 0-70km
  • Intermediate - 70-300km
  • Deep focus - 300-700km
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16
Q

What is the epicenter of an earthquake?

A

The epicenter of an earthquake is the point on the Earths surface directly above the focus (the area where the greatest amount of damage is likely to be done)

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17
Q

How can you find the epicenter of an earthquake?

A

A map showing areas of equal seismic intensities is compiled from either observations or readings from seismometers.

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18
Q

What is the special name of the lines that join up points of equal intensity?

A

Isoseismal lines

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19
Q

What is the definition of intensity?

A

Intensity is a measure of the surface damage caused by an Earthquake

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20
Q

What does the Mercalli scale measure?

A

Measures the intensity of an earthquake and is based on the effects that are felt in the area

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21
Q

What is magnitude?

A

Magnitude is a measure of the amount of strain energy released by an earthquake

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22
Q

What does the effects of surface vibrations in an area rely on?

A
  • the strength of the earthquake
  • the distance from the focus and epicenter
  • the nature of the rocks and sediments that the buildings are constructed on
  • the standard and design of the buildings
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23
Q

How many points are there on the Mercalli scale?

A

12 point scale

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24
Q

What are the points of the Mercalli scale?

A
  1. Instrumental - not normally felt - <3.5
  2. Feeble - only felt by a few people - 3.5-4.0
  3. Slight - vibrations like a large Lorry passing 3.5-4.0
  4. Moderate - felt indoors, cars rock - 4.0-5.0
  5. Rather strong - sleepers wakened, windows broken - 4.0-5.0
  6. Strong - small bells ring, trees sway - 5.0
  7. Very strong - hard to stand up - 5.5
  8. Destructive- partial collapse of buildings- 6.0
  9. Ruinous - ground cracks, buildings shift - 6.5
  10. Disastrous - landslides, buildings damaged - 7.0
  11. Very disastrous - bridges destroyed - 7.5
  12. Catastrophic - total damage - >8.1
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25
Which rocks allow vibrations to pass through easily?
Competent rocks such as granite or limestone | Don't cause too much damage
26
Which rock is the damage greater?
Unconsolidated rocks
27
What does the Richter scale measure?
Magnitude (from recording the amplitude) | Logarithmic scale
28
What do scientists use today to measure earthquakes?
Seismic moment - based on the idea of the moment of leverage developed on the two sides of the fault
29
What is the definition of Stress?
Stress is the force per unit area acting on or within a body
30
What is the definition of strain?
Strain is the change in shape of a body in response to stress
31
What is the elastic rebound theory?
* 2 parts of a body of rock are under stress due to opposing forces (tectonic plates moving in opposite directions) * the rock now under strain has energy applied to it and is stored as potential or strain energy in the rock * the deformation continues until the stress overcomes the strength of the rock and fractures * the 2 parts of the rock moves and their is displacement along the fault (earthquake occurs)
32
What does the seismometer do?
A seismometer detects and records ground motion. One part is fixed to the ground and the other part is able to freely move
33
What is the relative movement between the two parts recorded as?
It's recorded as a seismogram (Modern seismometers can record smaller than 1nm A paper or electronic record made by a seismograph
34
What is a seismograph?
A device that receives and records seismic vibrations
35
What does a seismometer do?
Receives seismic vibrations and converts them into a signal which can be transmitted and recorded
36
What is the effect of ground movement?
* cause little permanent damage if the intensity is low * stronger large amplitude earthquakes are more damaging * intensity scale 12 - 'the ground moves as a series of waves, with cracks opening at the crest and closing at the troughs'
37
Damage to structures by earthquakes?
* bricks and stonework separate along the mortar causing walls to collapse * floors separate from supporting walls causing them to 'pancake' on top of each other * bridges built in sections separate from their supporting piers * sections of gas, water and drainage pipes separate from each other and leak
38
What is liquefaction?
The vibration in bedrock is transmitted into superficial deposits at the surface . In wet sand and other unconsolidated deposits, the water separates from the solid particles and rises to the surface.
39
Where is liquefaction most likely to occur?
In loose to moderately saturated granular soils with poor drainage e.g. Silty sands/sands and gravels capped or containing seams of impermeable sediments such as clay
40
What are landslips?
On steep slopes made unstable by high rainfall, the vibration may trigger landslides and mudflows, partly assisted by liquefaction.
41
What are aftershocks?
The main movement along the fault releases the most amount of energy, but subsequent movements, minutes/hours/days later cause aftershocks
42
What are Tsunamis?
A tsunami is a water wave which is set up at the same time as the earthquake but is not a seismic wave. Caused by the bodily displacement of a large volume of water by the movement of a large section of crust.
43
What amplitude does a tsunami wave have in open water?
About a metre, a wavelength of several hundred kilometers and a speed of up to 700km/hr
44
When do Tsunamis also result?
From the displacement of water by large landslides e.g. When a large volcano collapses into the sea
45
How can you reduce the effects of Tsunamis?
Creating a barrier of trees/other vegitation/building the houses back from the shoreline/ place mid ocean detectors for maximum warning
46
What kills people during earthquakes?
Earthquakes themselves rarely kill it is the collapsing of dwellings, fires and disease that can cause hundreds of thousands of casualties
47
What can still be disrupted years after an earthquake?
* Housing and industrial buildings can be damaged * Communications dislocated * services disrupted
48
What are the social an economical effects of earthquakes before it occurs?
* Frequency and magnitude of earthquakes in the area | * quality of forward planning and training
49
Social and economic effects during the earthquake?
* population density and the number of people affected * magnitude of the earthquake and the proximity to the epicenter * amount of damage caused
50
What affects the rescue speed after earthquakes?
* quality of the emergency services (fire/search and rescue teams/policing in the area) * availability of medical services and supplies * availability of emergency food and water, shelter and sanitation * viability of communications
51
What affect on the area does the aftermath of an earthquake have?
* health care, hygiene and sanitation * restoration of water and electricity * rebuilding costs, materials, labour * employment opportunities
52
What do historical record for a particular fault show?
The period of elapsed time between earthquakes
53
What can be done if the pattern is regular?
The date of the next earthquake can be predicted
54
What is Seismic Gap theory?
It is the theory of regular earthquakes in the area and predicting the next one. Seismic theory is applied to sections of a fault that have been quiet for some time (the gap between active periods) the fault is ready for a high magnitude earthquake.
55
What does the seismic gap theory assume?
That frictional forces and other properties are constant. Which they aren't.
56
What causes minute cracks to develop that increase the volume and allows inward percolation of water and gases which can be detected?
Rocks under stress
57
What can be placed in boreholes to measure the deformation and therefore increase in stress?
Strain gauges
58
What material percolates through microcracks suggesting an earthquake may be in the future?
Radon, a radioactive decay product of uranium
59
Why do the number of foreshocks increase before a main event?
P wave velocities decrease and increase before the quake. Water increases the electrical conductivity of the ground lowering its resistivity.
60
What are the social consequences of earthquake prediction?
You don't know the exact date, place or magnitude of the earthquake. Also the reaction from the people you are telling to evacuate because there MAY be an earthquake
61
How can you reduce the impact of earthquakes?
* Planning * Building design * Ground or bass isolation systems * Resisting shear forces * Absorbing sway * Services
62
What can planning do to reduce the impact of earthquakes?
Local authorities can ban building from being built on the fault itself and areas that suffer from liquefaction and areas that suffer from landslides. They can also plan emergency procedures
63
What can building design do to reduce the impact of earthquakes?
Wooden structures are flexible and accept a certain amount of strain. You can use steel-reinforced concrete as opposed to brick
64
What are Ground or base isolation systems?
Buildings that test on large rollers/rubber pads/springs/sliders coated with non stick Teflon
65
How can you resist shear forces?
Each part of the structure can be connected to to other parts to prevent collapse. Diagonal bracing by cables/rigid girders
66
What does Absorbing sway involve?
Tall buildings are designed to sway, absorbing the energy through flexible supports
67
What can flexible piping be used to avoid?
Broken gas mains, power lines and fractured water mains
68
What does seismic loading mean?
Application of an earthquake-generated agitation
69
How can you calculate the epic centre of an earthquake?
At least 3 seismometers plotted apart from each other and you can calculate their distance from the earthquake using an s-p time graph. Where they overlap is the epicentre of the earthquake
70
What are the Earthquake wave relationships?
* earthquake waves slow down as rock density increases * earthquake waves speed up as the rocks become more rigid or incompressible * generally with increased depth from the Earths surface, earthquake waves increase in velocity because the increase in rigidity outweighs the density increase of the rocks
Geology AS
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