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EOSC 114 > Earthquakes 5 > Flashcards

Earthquakes 5 Flashcards

1
Q

Identify location and type of tectonic boundaries in Cascade region

A

Cascadia Subduction Zone ~ 1000km long convergent boundary

  • Juan de Fuca Plate + ridge (young oceanic plate) to the left
  • North American continental plate overriding on the right
  • they’re locked, so stress builds up = elastic deformation…
  • fault strength is overcome = brittle slip = kaboom!
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2
Q

List and describe modern and historical evidence for large megathrust EQ beneath cascadia

A

Makah Legend (1868)
Vancouver Island (1946)
Pachena Bay (1991)
Nisqually EQ (2001)

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3
Q

Prediction VS Forecast

A

Prediction
- EQ of a given magnitude will occur in this region within a specific period of time

Forecast
- less specific, probability statement

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4
Q

Examples of some precursors that have been observed

A
  • change in seismic activity
  • rapid ground deformation
  • water table changes
  • radon gas emissions
  • electromagnetic disturbances
  • animal behaviour
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5
Q

Do large earthquakes have precursor signals?

A

nope most don’t

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6
Q

Why is it difficult to accurately predict EQs?

A

predicting based on precursory signals generally unsuccessful cuz:

  • most large EQ don’t have precursor signals
  • signals can be mixed
  • some signals monitored aren’t particularly reliable
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7
Q

Stress rate is _____ and regional rock properties _____

A
  • relatively constant (plate tectonics)
  • don’t vary much (fault strength)
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8
Q

How does forecasting work?

A
  • use evidence of past quakes
  • apply statistic to determine recurrence interval for given magnitude
  • yields probability/risk of an EQ of a magnitude in some time
  • allow us to perform risk assessment, endure preparedness, and follow good construction paractice
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9
Q

Megathrust EQ Pachena Bay

A

(Vancouver Island)
1991
- EQ in night + flooding

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10
Q

Megathrust EQ Makah Legend

A

(Olympic Peninsula)
1868
- clear description of tsunami
- canoes destroyed by falling out of trees
- escaping by boat separated community

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11
Q

How were the oral histories consistent?

A
  • winter
  • same moon phase
  • same time of day
  • quite recent
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12
Q

How do earthquakes create a sedimentary record?

A

tsunami deposits leave records in sediment

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13
Q

return period of cascadia M9+ EQs

A

~ 500yrs per event

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14
Q

return period of cascadia M8+ EQs

A

~ 250yrs per event

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15
Q

the amount of cascadia M9+ and M8+EQs in the past 10,000 yrs

A

19 M9+
41 M8+

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16
Q

Geodynamic modelling models ____ (3 things)

A
  • plate boundary deformation (in space & time)
  • how stresses change after an EQ
  • how stress changes may affect ruptures on adjacent faults
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17
Q

Land level changes caused by

A

elastic deformation (due to compression)
-> gradual uplift & subsidence

18
Q

How can we measure deformation of earth’s crust?

A
  • satellites
  • ground based systems
    (phase differences of radio waves)
19
Q

Earthquake monitoring

A

CNSN networks monitors
- change of seismicity levels
- compilation of EQs show plate structure

20
Q

seismic gaps

A

areas along strike slip faults showing “locked zones” in otherwise active faults

21
Q

what do seismic gaps indicate?

A

potential for future EQs

22
Q

What country has a very dense (older) regional network for EQ monitoring?

A

Japan
-> rapid early warning system = immediate prediction!

23
Q

EQ Early Warning Systems: when do alarm go out?

A

when p waves are detected by seismometers near epicenter

24
Q

EQ Early Warning System: what is the warning time?

A

difference between P and S wave arrive times

-> seconds to minutes
ex/ BC offshore = 90 sec

25
Q

Why would we stop trains when a EQ warning is issued?

A

so it doesn’t jump off its tracks mid travel

26
Q

What are the three types of subduction zone EQs?

A
  1. Megathrust EQ
    - up to Mw 9.4
    - thrust faulting
  2. Crustal EQ
    - up to Mw 7.5
  3. Deep Earthquakes
    - up to Mw 7.0
    - to 100km depth
27
Q

When and where was Canada’s most damaging EQ?

A

Vancouver Island, 1946
Magnitude 7.3

  • intensity VIII
  • crustal EQ within NA plate
28
Q

If a megathrust EQ happens off BC coast, what will Japan feel?

A

probably just a tsunami
-> surface waves disappear across the globe

29
Q

Nisqually Earthquake

A

Feb 2001
Mw 6.8

  • 57km deep beneath Olympia
  • crustal EQ
  • $13 billion damage, 1 fatality
30
Q

What type of EQ is worst for Vancouver?

A) M9.0 megathrust
B) M7.0 deep crustal (60km) in subducting plate
C) M7.5 shallow crustal (<5km) close by

A

C

&laquo_space;refer to slides for seismograph&raquo_space;

A)
- 1-5 min shaking
- moderate intensity (Mercali VII)
- wide range of predicted damage, significant problems

B)
- 15-30 seconds shaking
- moderate intensity (Mercali VII)
- minor problems

C)
- 15-60 seconds shaking
- VIOLENT intensity (Mercali IX)
- old buildings destroyed, tall buildings ok

31
Q

Compare the risks of a megathrust vs a nearby shallow EQ

A

Smaller + shallow and close EQ
- may locally be just as damaging!
- shorter return period

Megathrust
- affect huge region
- tsunami damage have potential for greater economic impact

32
Q

Risk is ____ x _____

A

hazard (chance) x vulnerability (fatality)

33
Q

Chance of big EQ happening in your lifetime

A

20-25% ish

34
Q

Is your chance of dying in megathrust EQ high?

A

nope, around 2 in 10k
- much less than death by car

35
Q

What should you do before an EQ?

A
  • preparation like EQ kits
  • locate safe spots and anchor potentially dangerous objects
36
Q

What should you do during an EQ?

A
  • stay calm
  • get out of kitchen
  • stay under strong tables or desks
  • stand under archways or in corners
  • avoid doorways
  • don’t move until shaking stops
37
Q

What should you do after an EQ?

A
  • help
  • get out
  • do not use phone unless necessary
  • be prepared for 3+ days without help
38
Q

How good are we at predicting: immediate warning (<1 min)

A

Good! But expensive

39
Q

How good are we at predicting: short term (hours to weeks)

A

Nope!

40
Q

How good are we at predicting: intermediate (10 yrs ish)

A

Nope but improving
-> seismic gaps show potential for strike slip
-> not yet for subduction

41
Q

How good are we at predicting: long (10-30 yrs)

A

Pretty good for active faults
- not so good for big faults cuz less stats

42
Q

How good are we at predicting: very long (hundreds of yrs)

A

Excellent!

EOSC 114 (30 decks)

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