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1B ESA > Tectonics and Geodynamics (L9-14) > Flashcards

Tectonics and Geodynamics (L9-14) Flashcards

1
Q

What makes up the earthquake cycle?

A

Interseismic strain accumulation
Coseismic slip
Postseismic deformation

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2
Q

Outline the interseismic strain accumulation part of the earthquake cycle

A

Cool upper crust warped elastically and strain builds up

Can take decades to tens of thousands of years

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3
Q

Outline the coseismic slip part of the earthquake cycle

A

Elastic strains built up enough to not be supported elastically
Rocks break along a fault
Elastic strains relieved by fault slip

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4
Q

Outline the postseismic deformation part of the earthquake cycle

A

Stress changes from slip is relaxed

Minor deformation

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5
Q

What are the three ways to study earthquake cycles?

A

Field observations
Seismology
Satellite data

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6
Q

How can the location of an earthquake be estimated from seismology?

A

Time delay between P- and S-waves relates to distance travelled in the Earth
Using three or more seismometers gives a location

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7
Q

How can the depth of an earthquake be estimated from seismology?

A

Use depth phases: waves that travel up from the source and bounce off the surface and travel directly to the seismometer
Time delay between direct wave and depth phase gives the earthquake depth

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8
Q

How can the magnitude of an earthquake be estimated from seismology?

A

Higher-magnitude earthquakes produce higher-amplitude waves

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9
Q

What are the three measurements of fault geometry?

A 
Strike and dip of the fault plane
Direction of motion on the fault plane = rake
Rake = 0/180 = strike-slip
Rake = 90 = thrust
Rake = 270 = normal
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10
Q

How are focal mechanisms represented on a stereonet?

A

White quadrant = ground moving away = dilatational
Coloured quadrant = ground moving towards = compressional
Fault + auxilliary plane = nodal planes = no P-waves received

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11
Q

What does the focal mechanism of a normal-faulting earthquake look like?

A

White in the centre

Colour on either side

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12
Q

What does the focal mechanism of a thrust-faulting earthquake look like?

A

Colour in the centre

White on each side

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13
Q

What does the focal mechanism of a strike-slip-faulting earthquake look like?

A

Four visible quadrants

2 coloured, 2 white

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14
Q

What tool is used to estimate plate motions?

A

Velocity vector diagrams

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15
Q

For plate motion, what would a velocity vector of B_V_A represent?

A

The motion of plate A relative to plate B

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16
Q

How can the geometry of plate motion be expressed on a sphere?

A

A rotation about an axis that passes through the centre of the Earth (an Euler pole)

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17
Q

How can a mid-ocean ridge be used to establish the location and rate of an Euler pole?

A

Transform faults are perpendicular to the local direction of the Euler pole
Lines perpendicular to transform faults meet at the Euler pole

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18
Q

How can the location and rate of an Euler pole be measured using satellite data?

A

If a large enough area of both plates is above sea-level, satellites can measure relative motion

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19
Q

What are stability lines used for in a velocity vector diagram?

A

Motion of a triple junction relative to the bounding plates

Whether the triple junction is stable

20
Q

What are the rules for the stability line of an extensional plate boundary?

A

Triple junction must move at the same rate relative to both plates, as motion is symmetric
Stability line is perpendicular to AB

21
Q

What are the rules for the stability line of a strike-slip plate boundary?

A

Triple junction must move along the boundary between the two plates
Stability lines is along AB

22
Q

What are the rules for the stability line of a compressional plate boundary?

A

If plate A is consumed: triple junction can only move along a line parallel to strike of subduction passing through B

23
Q

What classifies as an unstable triple junction?

What must happen?

A

If there is no coherent motion relative to all three plates

Evolves to stable geometries if the plates keep moving

24
Q

Why don’t stable triple junctions exist forever?

A

One of the plates is being consumed (subduction)

Changes to the force balance driving the plates

25
Q

What are the most important forces acting on plates?

A

Ridge push
Slab pull
Mountain range buoyancy
Basal drag

26
Q

What is slab pull?

A

Force from a cold, dense, subducting slab pulling the plate down into the mantle

27
Q

What is ridge push?

A

Force from bounding plates being pushed from the ridge

28
Q

What is mountain range buoyancy?

A

Force from mountain range pushing against bounding plates to spread out due to its weight

29
Q

What is basal drag?

A

Shear force on base of plates from relative motion between the plate and underlying mantle

30
Q

What is the consequence of an earthquake on individual normal faults?

A

Subsidence of the hangingwall

Uplift of the footwall

31
Q

What controls the widths of basins and mountains produced on normal faults?

A

Thickness of the brittle upper crust

32
Q

What are the two ways present-day faulting can be distributed?
What is the implication of this?

A

Closely following old deformation belts
Spatially distributed
If pre-existing strength contrasts are too low, wide regions stretch
If strength contrasts are large then isolated rift valleys form along pre-existing weaknesses

33
Q

What is the progression of extension to mid-ocean ridge formation?

A

Continental stretching
Crustal thickness reduces
Surface sinks below sea level due to isostasy
Extension continues enough for mid-ocean ridge formation

34
Q

Why is the extension in ocean basins focused at the ridge axis?

A

Large strength contrast between a ridge axis and older oceanic lithosphere

35
Q

What is the morphology of a mid-ocean ridge affected by?

How so?

A

Rate of magma production thus rate of extension
Slow-spreading: well-developed central rift valley and lots of normal-faulting earthquakes
Fast spreading: no central valley, often only earthquakes at the transform faults

36
Q

What does the thickness of oceanic crust produced at a mid-ocean ridge depend on?

A

Temperature of underlying mantle that is upwelling and undergoing decompression melting
Hotter mantle = deeper solidus intersection = more melt

37
Q

What is the implied temperature of underlying mantle from normal oceanic crust?

A

Normal oceanic crust = 7km

Therefore mantle has a potential temperature of 1300

38
Q

What else does mantle potential temperature have an effect on?

A

The nature of the passive margins left behind on the edges of the continents after seafloor spreading begins

39
Q

How does mantle potential temperature effect the nature of ocean-continent passive margins?

A

Normal-temp mantle = mantle too cool to melt = no large scale magmatism pre spreading= continental margin of thinned continental crust with rift sediments
Hotter mantle = significant magmatism pre spreading = intrusive + extrusive igneous rocks in sediments

40
Q

What are natural non-tectonic earthquakes produced by?

A

Motion of fluids (melt or hydrous fluids) through the rocks

41
Q

What are the two main effects of fluids moving through rocks in relation to natural non-tectonic earthquakes?

A

Fluids can be at high pressures = reduces effective normal stress to very low value or if above lithostatic pressure then produce new extensional fractures
Fluids can move very fast = very high strain rates

42
Q

What is the result of the effects of fluids moving through rocks in relation to natural non-tectonic earthquakes?

A

Earthquakes associated with magma intrusion and hydrothermal circulation can occur in tectonically unexpected places

43
Q

What are the ways humans induce earthquakes?

A

Fluids: reservoirs of water, moving fluids in the subsurface

Mine-related

44
Q

What are the two effects of fluids moving in the subsurface?

A

Pore fluid pressure change

Volume/mass change results in stress changes in surrounding material

45
Q

What are the two main settings in which earthquakes are intentionally produced by fluid injection?

A

Fracking for hydrocarbon extraction

Opening fractures for geothermal energy production

46
Q

How does mining induce earthquakes?

A

Intentional blasts in active mines

Collapse of old mines

1B ESA (10 decks)

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