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1B Earth A > 9-14 > Flashcards

9-14 Flashcards

1
Q

Three stages of earthquake cycle

A
  1. Interseismic strain accumulation
  2. Coseismic Slip
  3. Postseismic deformation
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2
Q

Interseismic strain accumulation

A

Cool upper crust warped elastically - strain builds up.

  • depends on rate of motion across fault zone and the slip from the resulting earthquake
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3
Q

Coseismic slip

A

The earthquake

  • elastic strain 10-4 to 10-5
  • cannot be supported elastically
  • break of rocks - seconds to minutes depending on size
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4
Q

Postseismic deformation

A

stress changes due to slip are relaxed - causing deformation

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5
Q

Study earthquake cycles - 3 ways

A
  1. Location of event + map ruptures
  2. Satellite data - measure ground motion
  3. Seismology - so the waves produced
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6
Q

How to find location of earthquake with seismology?

A

P wave - faster than s

  • so differential time can show how far it is
  • TRIANGULATE FROM 3
  • though they can use the direction of first wave motion as on Mars and only use one, but not very accurate
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7
Q

How to find earthquake depth

A

depth phases - waves travelling upward from source - p or s - bounce off surface near source and follow same path as direct wave to seismometer
- so difference from depth phases to direct waves indicate the depth

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8
Q

How to find magnitude of earthquake

A

mainly use moment magnitude
seismic moment
=Mw = 2/3(M0) - 10.73 where M0=μAu

μ= shear modulus
A = fault area
u = average fault slip
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9
Q

What is a faults geometry

A

describe fault plane with strike and dip

- direction of motion = rake = relative strike direction in the plane of the fault

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10
Q

Rake of strike-slip faults

A

0 or 180 degrees

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11
Q

what rake represents thrust faulting

A

90

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12
Q

What rake is normal faulting

A

270 or -90

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13
Q

Auxiliary plane

A

plane that runs at 90 degrees to fault plane

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14
Q

Νodal planes

A

fault and auxiliary planes

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15
Q

P and T axes

A

orientation with most compression and extension

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16
Q

Hotspot reference frame

A

The theory that cause use the movement of plates over hotspots in order to determine absolute movement of plates
- atm its just relative plate motion

  • just dont know if plumes are fixed in place
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17
Q

Triple junction

A

point at which three plates meet

18
Q

Stability lines

A

lines that tell us where the triple junction can sit on a vector diagram

19
Q

2 reasons why configurations of plate boundaries change over time

A
  1. one plate entirely subducted under another - like what will happen to Juan de Fuca and Nazca plates
  2. change in the distribution og forces acting on the plate
20
Q

4 plate driving forces

A
  1. Slab pull
  2. Ridge push
  3. Mountain range buoyancy forces
  4. Basal drag
21
Q

What is slab pull

A

the force resulting from the cold and dense subducting slabs sinking into the mantle - pull the plate along

22
Q

What is ridge push

A

mid-oceans ridges are elevated above older and cooler sea floor
- so exert an outward force that pushes the bounding plates away from the ridge

23
Q

What are mountain range buoyancy forces

A

the tendency of mountains to spread under their own weight

- the gravitational potential energy differences

24
Q

What is basal drag

A

the shear force on the plate base due to relative motion between plate and underlying mantle

25
Q

example of abandoned plate boundaries

A

central Indian ocean - wharton ridge abandoned when convergence rate between india and asia rapidly decreased - changing forces on the entire plate

26
Q

ways of accommodating continental convergence

A
  1. Mountain building (continental thickening)

2. Lateral expulsion

27
Q

why can deep earthquakes (depths of over 600km) even exist - theories x 3

A
  1. Presence of high-pressure fludis that have been brought down with the slab, or released by metamorphic dehydration reactions, reduced effective normal stress - allow faulting to occur
  2. Mineral volumetric changes due to metamorphic reactions generate large enough stresses to cause faulting
  3. a thermal run away effect - any fault motion pressure so high that friction causes melting of plane - so slip continues
    - but that needs slip to start
28
Q

why do passive margins often have layers of salt

A

when little extension has occured, like in east african rift, above sea level with some lakes in the rift valleys which can lead to evaporates

29
Q

Lifecycle of extension - doesnt always run to completion

A
  1. Continental stretching - East African Rift
  2. Surface drops below sea level due to isostacy - The Aegean
  3. Seafloor spreading - Red seas and Gulf of Aden
30
Q

What are characteristic differences between slow and fast spreading ridges?

A

slow = mid atlantic ridge - well-developed central rift valley and many normal faulting earthquakes

fast = east pacific rise - lake a central valley and have few moderate or large mag earthquakes

31
Q

how thick is oceanic crust

A

roughly 7 km

32
Q

how thick is continental crust

A

30km

33
Q

why is the crust below iceland thick

A

becuase it sits over a mantle plume, meaning there is increased mantle temperatures and increased volume of melt - this cools and causes thicker crust

34
Q

how are natural non-tectonic earthquakes produced

A

motion of fluids - either melt or hydrous fluids

  1. can be at high temperatures and reduce effective stress on pre-existing faults
    - or produce new extensional fractures
  2. fluids can move fast - strain rates can be high
35
Q

Human induced earthquakes

A
  1. reservoir filling
  2. Fracking
  3. Mining
  4. Disposal of wastewater in subsurface
36
Q

How can reservoir filling cause earthquakes

A
  1. example = Thrust-faulting mag 5 in Thompson Australis 1996
    - suggest increased pore-fluid pressure - because of thrust faulting which would not occur without as pressure on surface
37
Q

What is a hazard

A

something that can cause harm

38
Q

What is risk

A

The likelihood of harm from a hazard

39
Q

How do we work out hazard by earthquakes

A
  1. recent earthquakes - time between

2. tectonic geomorphology - signs of past earthquakes in the landscaps

40
Q

How to reduce earthquake risk

A
  1. policy and making buildings better

2. warning systems

1B Earth A (5 decks)

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