Earthquakes 5 Flashcards
Identify location and type of tectonic boundaries in Cascade region
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!
List and describe modern and historical evidence for large megathrust EQ beneath cascadia
Makah Legend (1868)
Vancouver Island (1946)
Pachena Bay (1991)
Nisqually EQ (2001)
Prediction VS Forecast
Prediction
- EQ of a given magnitude will occur in this region within a specific period of time
Forecast
- less specific, probability statement
Examples of some precursors that have been observed
- change in seismic activity
- rapid ground deformation
- water table changes
- radon gas emissions
- electromagnetic disturbances
- animal behaviour
Do large earthquakes have precursor signals?
nope most don’t
Why is it difficult to accurately predict EQs?
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
Stress rate is _____ and regional rock properties _____
- relatively constant (plate tectonics)
- don’t vary much (fault strength)
How does forecasting work?
- 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
Megathrust EQ Pachena Bay
(Vancouver Island)
1991
- EQ in night + flooding
Megathrust EQ Makah Legend
(Olympic Peninsula)
1868
- clear description of tsunami
- canoes destroyed by falling out of trees
- escaping by boat separated community
How were the oral histories consistent?
- winter
- same moon phase
- same time of day
- quite recent
How do earthquakes create a sedimentary record?
tsunami deposits leave records in sediment
return period of cascadia M9+ EQs
~ 500yrs per event
return period of cascadia M8+ EQs
~ 250yrs per event
the amount of cascadia M9+ and M8+EQs in the past 10,000 yrs
19 M9+
41 M8+
Geodynamic modelling models ____ (3 things)
- plate boundary deformation (in space & time)
- how stresses change after an EQ
- how stress changes may affect ruptures on adjacent faults
Land level changes caused by
elastic deformation (due to compression)
-> gradual uplift & subsidence
How can we measure deformation of earth’s crust?
- satellites
- ground based systems
(phase differences of radio waves)
Earthquake monitoring
CNSN networks monitors
- change of seismicity levels
- compilation of EQs show plate structure
seismic gaps
areas along strike slip faults showing “locked zones” in otherwise active faults
what do seismic gaps indicate?
potential for future EQs
What country has a very dense (older) regional network for EQ monitoring?
Japan
-> rapid early warning system = immediate prediction!
EQ Early Warning Systems: when do alarm go out?
when p waves are detected by seismometers near epicenter
EQ Early Warning System: what is the warning time?
difference between P and S wave arrive times
-> seconds to minutes
ex/ BC offshore = 90 sec
Why would we stop trains when a EQ warning is issued?
so it doesn’t jump off its tracks mid travel
What are the three types of subduction zone EQs?
- Megathrust EQ
- up to Mw 9.4
- thrust faulting - Crustal EQ
- up to Mw 7.5 - Deep Earthquakes
- up to Mw 7.0
- to 100km depth
When and where was Canada’s most damaging EQ?
Vancouver Island, 1946
Magnitude 7.3
- intensity VIII
- crustal EQ within NA plate
If a megathrust EQ happens off BC coast, what will Japan feel?
probably just a tsunami
-> surface waves disappear across the globe
Nisqually Earthquake
Feb 2001
Mw 6.8
- 57km deep beneath Olympia
- crustal EQ
- $13 billion damage, 1 fatality
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
C
«_space;refer to slides for seismograph»_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
Compare the risks of a megathrust vs a nearby shallow EQ
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
Risk is ____ x _____
hazard (chance) x vulnerability (fatality)
Chance of big EQ happening in your lifetime
20-25% ish
Is your chance of dying in megathrust EQ high?
nope, around 2 in 10k
- much less than death by car
What should you do before an EQ?
- preparation like EQ kits
- locate safe spots and anchor potentially dangerous objects
What should you do during an EQ?
- 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
What should you do after an EQ?
- help
- get out
- do not use phone unless necessary
- be prepared for 3+ days without help
How good are we at predicting: immediate warning (<1 min)
Good! But expensive
How good are we at predicting: short term (hours to weeks)
Nope!
How good are we at predicting: intermediate (10 yrs ish)
Nope but improving
-> seismic gaps show potential for strike slip
-> not yet for subduction
How good are we at predicting: long (10-30 yrs)
Pretty good for active faults
- not so good for big faults cuz less stats
How good are we at predicting: very long (hundreds of yrs)
Excellent!
