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Geohazards > Earthquakes > Flashcards

Earthquakes Flashcards

1
Q

What is the difference between deterministic and probabilistic hazard?

A 
Deterministic = for an expected scenario
Probabilistic = probability that an event of certain size occurs in a given spatio-temporal window and incurs specific effects
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2
Q

What are the requirements of unstable sliding/ nucleating earthquakes?

A
  1. velocity-weakening frictional properties (as sliding starts the fault weakens)
  2. low material stiffness (slow unloading compared to weakening rate)
  3. High enough strength to store elastic energy (needs to accumulate)
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3
Q

What constraints do we use to constrain source zones?

A

zones of homogenous seismicity characteristics
faults/ fault segments
etc

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4
Q

What is strain?

A

deformation expressed as the relative change in length or shape.

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5
Q

What ways can we describe earthquake amplitude?

A

PGA (peak ground acceleration)
PGV (peak ground velocity)
duration of shaking

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6
Q

Name 4 secondary hazards of earthquakes

A

tsunamis, land slides, fires and liquefaction

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7
Q

What is the difference between forecasting and prediction?

A

forecasting - fairly imprecise statement of time, place and size etc

prediction- relatively precise statement defining a narrow spatial, temporal and magnitude window in which an event of a certain type is expected, allowing for evaluation

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8
Q

What are ‘static’ stress changes?

A

long-term changes in loading due to stress release on nearby faults or segments

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9
Q

What are ‘dynamic’ stress changes?

A

temporary changes in loading due to passing earthquake waves larger than static stress changes

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10
Q

What does Omori’s law describe?

A

clustering observed as aftershock sequences

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11
Q

What is the best first-order approximation of recurrence behaviour over large time an spatial scales?

A

Poissonian- random with no correlation with previous or next event

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12
Q

What does ‘conditional’ probability mean?

A

time-dependent

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13
Q

What are arguments for earthquakes being a self-organised critical system?

A

Observed powerlaw magnitude-frequency distribution
Observed powerlaw fault size-frequency distribution
Earthquake stress drops are only around 1% of tectonic stresses

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14
Q

What are arguments against earthquakes being a self-organised critical system?

A

largest earthquakes don’t always follow power law

stress changes have deterministic effect on seismicity

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15
Q

What criteria should precursors satisfy to be useful?

A

occur and can be observed, unambiguous indication of impending activity, know timescale, plausible physical relationship between precursory activity and event magnitude

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16
Q

Common earthquake precursors…

A

foreshocks (precursory strain), opening and closing of cracks, seismic quiescence, radon gas release, changes in water table height, animal behaviour, changes in seismic travel times etc

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17
Q

what does the seismic moment describe?

A

the strength of the force couple that would be required to move the fault

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18
Q

How does seismic coupling effect magnitude of earthquakes?

A

If its low earthquakes magnitudes are low in the area or don’t occur at all

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19
Q

What is uniformitarianism?

A

view that geology is shaped by slow long-term processes, in small steps that we can observe everyday

20
Q

What is catastrophism?

A

view that geology is dominantly shaped by rare, large events such as impacts, megafloods and catastrophic volcanic eruptions

21
Q

Why is PGA not always a good measure for how much damage will occur?

A

peak accelerations may only occur during a short period meaning structures may have too little time to respond to them (?) (page13)

22
Q

Why is PGV often better than PGA at indicating how much damage will occur?

A

velocity is related to an integrated force input so it is therefore a lower frequency (longer duration) characterisation of ground motion

23
Q

Summarise the 4 steps needed to build a hazard map:

A
  1. define source zones
  2. define magnitude-frequency distribution for zone
  3. build a shaking map using attenuation relationships full waveform calculations
  4. Assign a probability to each potential event
24
Q

Issues with Byerlee’s Law in predicting an earthquake?

A

We can’t measure absolute stress deep in the earths crust.

when the required stress is exceeded it doesn’t necessarily imply an earthquake as sliding could be slow and stable

25
Q

What is the ‘state’ effect?

A

a fault weakening effect due to the fault ‘wearing down’ as slip progresses.
occurs after the characteristic slip length

26
Q

When the rate effect dominates the state effect the dynamic friction exceeds the static friction leading to a net effect of velocity-strengthening or weakening?

A

velocity-strengthening

27
Q

When the state effect dominates the rate effect the dynamic friction is lower than the static friction leading to a net effect of velocity-strengthening or weakening?

A

velocity-weakening

28
Q

What does the state variable represent?

A

the extent of bonding between 2 fault surfaces

29
Q

What properties encourage velocity-weakening behaviour?

A

higher overburden pressure/normal stress, low porosities, quartz-rich compositions, smooth surfaces and low amounts of fault gouge

30
Q

What properties encourage velocity-strengthening behaviour?

A

low overburden pressure, high temperatures and the presence of soft, ductile minerals

31
Q

What properties encourage velocity-strengthening behaviour?

A

low overburden pressure, high temperatures and the presence of soft, ductile minerals

32
Q

Name a method that is being tested for intermediate-term hazard assessment

A

Monitoring of locking and loading

33
Q

What does periodic mean?

A

regular time intervals between subsequent events

34
Q

What does clustered mean?

A

events occur in groups (spatially and/or temporally??)

the probability of an event is highest right after the previous one

35
Q

What is Conditional Probability?

A

the probability that an event will occur in gap-time interval [T_0,g:T_g], given the event hasn’t happened in gap time T_0,g since the last event

36
Q

What rough distance in metres does 1 degree of lat or lon equal?

A

100km

37
Q

How does the cumulative no. of events per year (n) (with >=M_w) relate to recurrence time?

A

recurrence time(t) = 1/n

38
Q

For long term probabilistic hazard analyses one needs to know what?

A

event recurrence statistics

39
Q

What is an EQ?

A

A sudden unstable deformation that happens quickly enough to emit seismic waves with periods from less than a second to hours

40
Q

What determines if sliding is stick-slip or stable frictional sliding?

A 
2nd order fricitonal properties
How coefficient of friction changes with:
-Pressure
-Temperature
-Fluids
-Material
41
Q

What is required for unstable sliding?

A

Velocity weakening is dominant

42
Q

What is the state variable?

A

Represents the the extent of bonding between between the two fault surfaces

43
Q

Does frictional ageing increase or decrease static friction?

A

increase . (probs due to chemical bonding and etc…) page 19

44
Q

Discuss 2 aspects of of an earthquake rupture mechanism that make it efficient at generating tsunami.

A

They can rupture at low frequencies (300-1000s)-frequency matches that of many tsunamis so they can be generated with low slip velocities and not high magnitude earthquakes

45
Q

How do you induce failure using byerlees law?

A

Increase shear stress

Increase pore fluid pressure

46
Q 
Seismogenic widths of?
Subduction
transform
ridges
intraplate
A

50-200 km

10-15 km

1-5 km

10-50 km

Geohazards (4 decks)

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