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Tectonics and Lithosphere Dynamics > (1.2) Global Seismology and Potential Field Techniques > Flashcards

(1.2) Global Seismology and Potential Field Techniques Flashcards

1
Q

How are earthquake first motion pulls represented on a diagram?

A

White

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2
Q

How are earthquake first motion pushes represented on a diagram?

A

Black

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3
Q

What is a lower focal hemisphere plot?

A

“Beach-ball” diagram - plotting first motion pull and pushes of an earthquake

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4
Q

What is the correct name for a diagram showing first motion pulls and pushes of an earthquake?

A

Lower Focal Hemisphere Plot

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5
Q

What does the red line show on the following diagram?

A

Dextral fault trending NE

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6
Q

What does the red line show on the following diagram?

A

Normal, steep fault downthrowing to SW

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7
Q

What does the red line show on the following diagram?

A

Thrust. steep fault with top to NW

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8
Q

What does the red line show on the following diagram?

A

Normal plus Strike slip on shallow fault tranding NNE

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9
Q

What does the blue line show on the following diagram?

A

Sinistral fault trending SE

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10
Q

What does the blue line show on the following diagram?

A

Normal, shallow fault downthrowing to NE

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11
Q

What does the blue line show on the following diagram?

A

Thrust, shallow fault with top to SE

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12
Q

What does the blue line show on the following diagram?

A

Normal plus Strike slip on shallow fault tranding NNW

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13
Q

What is an Anisotropic Medium?

A

A medium which has a preferred direction of seismic wave travel

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14
Q

What is an Isotropic Medium?

A

A medium which acts the same in all directions

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15
Q

Give two prominent examples of an Anisotropic Mediums in modern Earth?

A

Olivine crystals which have undergone strain, Fluid filled voids with a preferential direction

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16
Q

What can anisotropic Olivine give an indication of?

A

Strain in upper mantle

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17
Q

How can the theory of anisotropy be used for reservoir identification in basins?

A

The presence of preferentially orientated fractures containing hydrocarbons

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18
Q

Give a modern day example where the presence of magma is aiding rifting by reducing friction

A

East African Rift Valley

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19
Q

How does the theory of anisotropy relate to the East African Rift Valley

A

There is a preferred orientation of fractures running parallel to the rift which contain magma in abundance

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20
Q

Why are nearly all of the biggest earthquakes occurring around the edges of the Pacific Ocean?

A

Rocks are stronger in compression than tension, so store more energy before brittle failure occurs. Pacific margins are areas of compression and so release higher amounts of energy than a spreading centre

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21
Q

What was the biggest ever earthquake?

A

Chile, 22/05/1960, Mw 9.5

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22
Q

What is the CTBT and why are seismometers important to it?

A

Comprehensive Test Ban Treaty for Nuclear Weapons Development. These types of explosions actually have a unique signature in the seismological record and thus can be recognised over a normal seismic release

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23
Q

What did the study by Earle and Shearer (1994) set out to achieve?

A

Creation of an automatic phase picker for P and S wave arrival times

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24
Q

What was the main advantage of the algorithm created by Earle and Shearer (1994)?

A

Lots of data can be processed quickly and efficiently to determine source of earthquake

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25
Q

Who were the researchers whom created the arrival time algorithm?

A

Earle and Shearer (1994)

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26
Q

What was the main disadvantage of the arrival time algorithm created by Earle and Shearer (1994)?

A

Humans are better able to destinguish the arrival time manually

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27
Q

What is the name of the researchers whom studied the Northern Ethiopian Rift?

A

Kendall et al (2005)

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28
Q

What did the model created by Kendall et al. (2005) determine?

A

Very likely that melt induced anistropy aided in rift

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29
Q

Who were the researchers whom observed six plumes stretching into the lowermost mantle?

A

Montelli et al. (2010)

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30
Q

Which were the six plumes which Montelli et al. (2010) determined streched into the lowermost mantle?

A

Ascension, Azores, Canary, Easter, Samoa, Tahiti

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31
Q

What did Montelli et al. (2010) suggest for those plumes which were restricted to the upper mantle?

A

The possibility of the existance of two depth regiemes in plume development

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32
Q

How did Montelli et al. (2010) suggest that mantle plumes effected the entire heat regieme of the earth?

A

Plumes contribute towards a substantial fraction of internal heat escape

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33
Q

How large did Montelli et al. (2010) suggest mantle plumes are based on their tomographic findings?

A

Several hundreds of km’s

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34
Q

Who were the researchers whom used tomography to determine the process of the Indian plate collision with Asia?

A

Replumaz et al. (2004)

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35
Q

What did Replumaz et al. (2004) suggest happened to the Indian Plate’s mantle?

A

Over-ridden by the Indian plate, not significantly thrusted beneath Tibet

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36
Q

How much deformation was absorbed by the Asian plate during the Indian collision according to Replumaz et al. (2004)?

A

~1500km

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37
Q

What does the Indian plates slab look like at 1100 km and 700 km?

A

Linear at 1100 km, becomes more deformed with shallowing - so 700 km highly deformed

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38
Q

What do the components of the following equation mean?

A

F = Force, G is the Universal Gravitational Constant (~6.7 x10^-11), r is the seperation between m1 and m2, the masses which are interacting gravitationally

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39
Q

What happens to gravity with increasing seperation? What is another name for this law?

A

Rapidly decreases with increasing seperation - inverse square law

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40
Q

Substitute Newton’s second Law into the following equation and simplify.

F= G m1 m2 / r^2

A

F = ma where m is the smaller second mass, m2, giving:

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41
Q

How much does “g” vary in gravitational surveys?

A

up to 100 micrometer s-2

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42
Q

What are the two controlling factors influencing changes in “g” in gravitational surveys?

A

Densities of rocks, pore fluids

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43
Q

What does 1 Gal =?

A

0.01 m s-2

44
Q

Who were the researchers whom observed amagmatic ridge segments?

A

Dick et al. (2003)

45
Q

What is the cause of amagmatic ridge segments according to Dick et al. (2003)?

A

Ultraslow spreading ridges

46
Q

What defines a ultraslow spreading ridge according to Dick et al. (2003)?

A

12 mm yr-1 to 20 mm yr-1

47
Q

What are the two examples of ultraslow spreading ridges given by Dick et al. (2003)?

A

Southwest Indian and Arctic ridges

48
Q

What are the two features of ultraslow spreading ridges given by Dick et al. (2003)?

A

Intermittent volcanism and lack of transform faults

49
Q

What is special about the formation of amagmatic segments and their potential relationship to transform faults according to Dick et al. (2003)?

A

Assume any orientation and hence have no requirement for transform faults

50
Q

What is the Free Air Anomaly correlated to? (3)

A

Topography, components of mass burial, non-spheroidal shape of Earth

51
Q

What is the Bouguer Anomaly?

A

What remains after gravitational effects of topography taken into account

52
Q

Why does the Bouguer Anomaly have an inverse correlation with topography?

A

Isostatic compensation at deeper levels

53
Q

How can we correct the Bouguer Anomaly so that the signals due to mass variations in the uppermost mantle are isolated?

A

Using models of crustal thickness

54
Q

What would be a good measurement scale for a gravimeter?

A

1 micro m Gal

55
Q

What do gravimeters measure?

A

Change in gravity of a mass at various locations

56
Q

Why does altitude need to be corrected for when measuring gravity?

A

Takes the measurement further away from the center of mass (Earth)

57
Q

What is a common datum used in the Free Air Anomaly?

A

Sea level due to good approximation of geoid

58
Q

How do we correct the geoid using the free air correction?

A

Free air correction is 0.3086 mGal/m * the height in m above sea levels giving the answer in mGal

59
Q

Why does the Free Air Anomaly retain a strong positive correlation with topography?

A

Density contrast between rocks and air is large, so need to correct using Bouguer Anomaly

60
Q

What are the two steps to calculating the Bouguer Anomaly?

A

Slab correction then Terrain correction

61
Q

What is the slab correction in calculating the Bouguer Anomaly?

A

A hypothetical slab of rock is added ontop of the Free Air anomaly which equates to the height of the site of measurement (2_G_h ms-2)

62
Q

What is the terrain correction in calculating the Bouguer Anomaly?

A

Removes the effect of terrain variablilty - i.e. compensates for the slab correction stage

63
Q

What is the standard product of most gravity surveys?

A

Bouguer Anomaly

64
Q

Why is the Bouguer Anomaly useful?

A

With elevation and terrain effects removed, it highlights rock density variations within the crust and mantle

65
Q

Why might the Bouguer Anomaly be less useful over larger scales, such as those including both Oceanic and Land surveys?

A

Oceanic particularly dense, thus anticorrection occurs

66
Q

What might an image of the Bouguer Anomaly show over a wide expanse of oceanic mantle?

A

Density differences due to expanded/less dense/hotter mantle

67
Q

How can we attain a image of the Bouguer Anomaly of the oceanic mantle? Why is this harder to achieve with Continental imagary?

A

Oceanic lithosphere generally uniform density - can remove it’s effect. Doesn’t work with continental due to mountain roots

68
Q

What are the four corrections needed when using the sea as an estimation of the geoid?

A

Salinity, Currents, Waves, Ice

69
Q

How do we attain gravity information for the geoid?

A

Radar altimeters fitted to satelites

70
Q

What resolution is given for the geoid using satellite imagary?

A

15km resoultion

71
Q

Why are satellite missions with altimeters not a full picture of the geoid?

A

They rely on the geoid to compute the height of the orbit

72
Q

What are the two satellite missions set to overcome the geoid altimeter problem?

A

GRACE and GOCE

73
Q

How does GRACE work?

A

Two satellites, on the same orbit, small distance appart - first satellite crosses a mass and speeds away from second - overall shape computed

74
Q

Why is a global view of the geoid useful? Give one example

A

Water content in the amazon - changes in water can depress geoid by 12mm in a wet season

75
Q

How does GOCE work?

A

Flies closer to the surface so smaller gravitational changes are easier to sense

76
Q

Aside from water changes in the Amazon, give five other uses of the GRACE mission

A

Changes in mass in the polar ice caps, Exchanges of water vapour between oceans and atmosphere, Earthquake movements, Lava in volcanoes, Minerals prospecting

77
Q

Why are gravity anomaly surveys not useful on their own?

A

A gravity signature could be created from a combination of different densities and depths - often these are preliminary to seismic surveys

78
Q

Who were the authors who reconstrcuted paleo-plate movement in the South Pacific from gravity anomaly data?

A

Eagles et al (2004)

79
Q

What did Eagles et al (2004) manage to observe?

A

Gravity anomalies in the south pacific which lead to the reconstruction of plate movements

80
Q

What are the units of magnetic field strength?

A

Tesla

81
Q

How does magnetic field strength vary with distance seperating the bodies?

A

Inverse square

82
Q

Because anomalies in rocks are so small, literature tends to use ____ as a unit

A

nanoTesla

83
Q

Magnetic field strength is a ___ quanitiy, as it has both direction and magnitude

A

Vector

84
Q

Why do we assume that the core has an electrical origin to drive it’s magnetism?

A

Permanent magnitisation cannot exist as the core is too hot

85
Q

What is the name for the overall drive of Earth’s magnetic field?

A

Geodynamo

86
Q

What must have initiated Earth’s geodynamo?

A

Strong external magnetic field - possibly young sun?

87
Q

What are the names of the satellites which have recorded changes in the magnetic core field between 1980 and 2000?

A

Oersted and Magsat

88
Q

What kind of timescale does magnetism operate on, and what is the cause of the shortest timescale?

A

Very short geological timescales, with solar wind causing hourly and daily fluctuations

89
Q

Low amplitude magnetic anomalies tend to have a ___ source, where as ___ anomalies are more typical of ____ and ___ rocks found in ___.

A

Deeper, strong anomales, igneous and metaorphic basin rocks

90
Q

What does this map of magenic anomales generally indicate about Scotland and England/Wales?

A

Scotland has higher amplitude anomalies, thus likely to be dominated by shallow igneous/metamorphic. England dominated by deeper basement rocks

91
Q

Basalt has a high ___ ___ giving the ability to trase magnetic stripes on the sea floor

A

Remnant magnetism

92
Q

What is used to measure magnetic susceptibility over large areas, and on what platform?

A

Helicopters dragging Magnotometers

93
Q

What is the problem with helicopters having magnetometers on board, and how is it overcome?

A

Helicopters are slightly magnetically susceptable, thus magnotometer is usually dragged on a tow line (or left in place)

94
Q

Who are the authors who published on using gravity anomalies to determine the depth and location of the Chicxulub Crater?

A

Hildebrand et al. (1998)

95
Q

What did Hildebrand et al. (1998) find in reguards to gravity anomalies in the crater fill?

A

Strong negative anomaly in the fill

96
Q

Name some of the uses that Okuma et al. (2001) has described for aeromagnetic surveying in Japan (5 total)

A

Faulting/fault zones, debris avalance deposits, volcanic activity, subsurface ancient volcanoes, subsurface faulting

97
Q

Why is care needed when observing gravity and magnetic anomalies on maps?

A

May be a result of data smearing with distance from anomaly OR may be result of less intense/wider mapping

98
Q

What happens to a magnetically susceptible body when it interacts with Earth’s inclined field?

A

The resulting field from the body will also be inclined

99
Q

The field strength variation of a magnetically susceptable body will ___ and ___ components which are centered where?

A

Positive and negative - not centered over buried body

100
Q

When will dipole effects not occur?

A

When the inducing field is vertical, or if the induced body strikes parallel to azimuth of inducing field lines

101
Q

What is “reduction to the pole”?

A

A mathematical proess where magnetic anomalies appear as if they have been recorded in a region with a vertical inducing field - dipole component is removed

102
Q

Why is “reduction to the pole” useful?

A

Anomalies become centered over their source bodies allowing for easier interpretation

103
Q

When may the dipole component remain when using “reduction to the pole”?

A

When there is already remanent magnetisation

104
Q

Give an example where reduction to the pole can be used

A

Over a volcanoe - can see higher magnetisation and thus fresher lava ridge

105
Q

What is upward continuation?

A

A mathematical technique in which potential field anomalies decrease in amplitude and increase in width with seperation between detecotr and source body increases

Tectonics and Lithosphere Dynamics (2 decks)

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