Earthquakes Flashcards
What is the difference between deterministic and probabilistic hazard?
Deterministic = for an expected scenario Probabilistic = probability that an event of certain size occurs in a given spatio-temporal window and incurs specific effects
What are the requirements of unstable sliding/ nucleating earthquakes?
- velocity-weakening frictional properties (as sliding starts the fault weakens)
- low material stiffness (slow unloading compared to weakening rate)
- High enough strength to store elastic energy (needs to accumulate)
What constraints do we use to constrain source zones?
zones of homogenous seismicity characteristics
faults/ fault segments
etc
What is strain?
deformation expressed as the relative change in length or shape.
What ways can we describe earthquake amplitude?
PGA (peak ground acceleration)
PGV (peak ground velocity)
duration of shaking
Name 4 secondary hazards of earthquakes
tsunamis, land slides, fires and liquefaction
What is the difference between forecasting and prediction?
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
What are ‘static’ stress changes?
long-term changes in loading due to stress release on nearby faults or segments
What are ‘dynamic’ stress changes?
temporary changes in loading due to passing earthquake waves larger than static stress changes
What does Omori’s law describe?
clustering observed as aftershock sequences
What is the best first-order approximation of recurrence behaviour over large time an spatial scales?
Poissonian- random with no correlation with previous or next event
What does ‘conditional’ probability mean?
time-dependent
What are arguments for earthquakes being a self-organised critical system?
Observed powerlaw magnitude-frequency distribution
Observed powerlaw fault size-frequency distribution
Earthquake stress drops are only around 1% of tectonic stresses
What are arguments against earthquakes being a self-organised critical system?
largest earthquakes don’t always follow power law
stress changes have deterministic effect on seismicity
What criteria should precursors satisfy to be useful?
occur and can be observed, unambiguous indication of impending activity, know timescale, plausible physical relationship between precursory activity and event magnitude
Common earthquake precursors…
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
what does the seismic moment describe?
the strength of the force couple that would be required to move the fault
How does seismic coupling effect magnitude of earthquakes?
If its low earthquakes magnitudes are low in the area or don’t occur at all
What is uniformitarianism?
view that geology is shaped by slow long-term processes, in small steps that we can observe everyday
What is catastrophism?
view that geology is dominantly shaped by rare, large events such as impacts, megafloods and catastrophic volcanic eruptions
Why is PGA not always a good measure for how much damage will occur?
peak accelerations may only occur during a short period meaning structures may have too little time to respond to them (?) (page13)
Why is PGV often better than PGA at indicating how much damage will occur?
velocity is related to an integrated force input so it is therefore a lower frequency (longer duration) characterisation of ground motion
Summarise the 4 steps needed to build a hazard map:
- define source zones
- define magnitude-frequency distribution for zone
- build a shaking map using attenuation relationships full waveform calculations
- Assign a probability to each potential event
Issues with Byerlee’s Law in predicting an earthquake?
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
What is the ‘state’ effect?
a fault weakening effect due to the fault ‘wearing down’ as slip progresses.
occurs after the characteristic slip length
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?
velocity-strengthening
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?
velocity-weakening
What does the state variable represent?
the extent of bonding between 2 fault surfaces
What properties encourage velocity-weakening behaviour?
higher overburden pressure/normal stress, low porosities, quartz-rich compositions, smooth surfaces and low amounts of fault gouge
What properties encourage velocity-strengthening behaviour?
low overburden pressure, high temperatures and the presence of soft, ductile minerals
What properties encourage velocity-strengthening behaviour?
low overburden pressure, high temperatures and the presence of soft, ductile minerals
Name a method that is being tested for intermediate-term hazard assessment
Monitoring of locking and loading
What does periodic mean?
regular time intervals between subsequent events
What does clustered mean?
events occur in groups (spatially and/or temporally??)
the probability of an event is highest right after the previous one
What is Conditional Probability?
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
What rough distance in metres does 1 degree of lat or lon equal?
100km
How does the cumulative no. of events per year (n) (with >=M_w) relate to recurrence time?
recurrence time(t) = 1/n
For long term probabilistic hazard analyses one needs to know what?
event recurrence statistics
What is an EQ?
A sudden unstable deformation that happens quickly enough to emit seismic waves with periods from less than a second to hours
What determines if sliding is stick-slip or stable frictional sliding?
2nd order fricitonal properties How coefficient of friction changes with: -Pressure -Temperature -Fluids -Material
What is required for unstable sliding?
Velocity weakening is dominant
What is the state variable?
Represents the the extent of bonding between between the two fault surfaces
Does frictional ageing increase or decrease static friction?
increase . (probs due to chemical bonding and etc…) page 19
Discuss 2 aspects of of an earthquake rupture mechanism that make it efficient at generating tsunami.
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
How do you induce failure using byerlees law?
Increase shear stress
Increase pore fluid pressure
Seismogenic widths of? Subduction transform ridges intraplate
50-200 km
10-15 km
1-5 km
10-50 km
