Probabilistic Seismic Hazard Analysis Project
12-05-2012
Origins and Objectives
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BC has all four types of tectonic regions1.Active Crustal: WNA2.Stable Continental: CENA3.Subduction Zones: Cascadia4.Volcanic: Volcanic Arc/Cascades
>80 dams18 Extreme Consequence6 > 10,000 PAR
Background
• BC Hydro has been involved in probabilistic seismic hazard analysis (PSHA) since the 1980s.
• Evolution in Approach:
• 1984 – first PSHA for Lower Mainland & Vancouver Island, using GSC seismic source model
• 1991-92 – Provincial Seismic Hazard Review; developed BCH seismic source model
• 1997 – Introduced “multiple model” approach, with BCH and GSC “H” & “R” source models as alternates, & alternate ground motion models
• Post-1997 – Ongoing introduction of new ground motion models
• Results have been used for seismic performance evaluations and designs for dams and the electric system as they have become available
Concerns About BCH PSHAs in 2000s
• Seismic source and ground motion models becoming outdated
• Cascadia subduction zone not incorporated into probabilistic model
• Large computed ground motions
• Large uncertainties
• Sensitivity of computed ground motions to some model inputs (especially subduction zone ground motion models)
• Stability and consistency of hazard estimates, same site and site-to-site
• Dr. C. A. Cornell retained in 2007.
• Alternative approaches to update BCH’s PSHA were considered.
4
Project Objectives and Startup
• Develop an up-to-date and technically sound PSHA model for the BC Hydro service area.
• Achieve technical stability of the PSHA model and its results for 10-15 years.
• Model to be used by BC Hydro for seismic hazard assessment and characterization at all sites.
• Initial work undertaken mainly with internal resources
• Review by Dr. Cornell indicated that more rigour required to meet Project Objectives
• Dr. Cornell recommended a SSHAC III approach
Probabilistic Seismic Hazard Analysis Project
+$10M and 4 years later… .
Results and Policies for Use
PSHA Model – ‘Shallow’ Seismicity
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8
PSHA Model – ‘Deep’ Seismicity
Subduction slab represented by 4 source zones which become increasingly deeper to the northeast.
Source zone boundary uncertainty is included in the model
The Nootka Fault is represented as a separate source
PSHA Model – Evaluated & Selected GMPEs
Active Crustal Stable Continental
Subduction Zone
Abrahamson & Silva (2008)
Silva et al (2003) Youngs et al (1997)
Chiou & Youngs (2008)
Campbell (2003) Atkinson & Boore (2003)
Campbell & Bozorgnia (2008)
Atkinson & Boore (2006, 2011)
Garcia et al (2005)
Boore & Atkinson (2008)
Atkinson (2008, 2011)
Zhao et al (2006)
Idriss (2008) Kanno et al (2006)
Macias & Atkinson (2009)
Lin and Lee (2010)
Volcanic Arcs: Evaluated McVerry et al (2011) but did not use it.
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PSHA Model: BC Hydro Subduction Model
ln(Sa) 1 4 * C1 (2 14 * Fevent 3 * (M 7.8)) * ln R C4 * exp[(M 6) * 9 ] 6 * R 10 * Fevent fMag (M ) fdepth (Zh ) fFABA (R)
fsite (PGA1000,VS30)
fMag (M) 4 * (M (C1 C1)) 13 * (10 M)2 for M C1 C1
5 * (M (C1 C1)) 13 * (10 M)2 for M C1 C1
fFABA (R) [7 8 *Ln(
max[R,85]
40)]*FFABA for Fevent 1
[15 16 *Ln(max[R,100]
40)]*FFABA for Fevent 0
fsite (PGA1000 ,Vs30m)
12 * Ln(Vs
*
Vlin) b* Ln(PGA1000 c)
b* Ln(PGA1000 c * (Vs
*
Vlin)n )
for VS30 Vlin
12 * Ln(Vs
*
Vlin) b* n * Ln(
Vs*
Vlin) for VS30 Vlin
fdepth (Zh ) 11 * (Zh 60) *Fevent
Vs*
1000.0 for VS30 1000
VS30 for VS30 1000
Uncertainty in PGA Hazard
0.00001
0.0001
0.001
0.01
0 20 40 60 80
Ann
ual E
xcee
danc
e Fr
eque
ncy
PGA (%g)
0.95
0.85
MEAN
0.50
0.15
0.05
Now what??
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Now that we understand the uncertainty…
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Peak Ground Acceleration (g)
Pro
ba
bil
ity
(%
)
84th percentileMeanMedian
• what to use?
• Different philosophies at different sites
Typical Results (actual data : Site C)
Rel
ativ
e co
nfid
ence
Lev
el (
%)
13
PSHA Policy
•For High, very High and extreme Consequence dams, ground motion hazards at a mean 10-4 AEF to be a starting point
•Best representation of the uncertainties
•Use full spectrum of results for risk analyses
•Consider the potential for more extreme events, and the risks associated with those events
•Policy decision to embrace risk informed decision making (finally…)
•Consider combinations of events. For example, if a dam is damaged due to an earthquake, would it be possible to subsequently pass the mean annual flood (or larger flood) safely?
•Incorporate system thinking and reliability principles in design
Effects of PSHA project and policy
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Effects of PSHA project and policy
15
LaJoie
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Effect of PSHA Policy and Results
Different effects at different sites
Site C : increase in ground motion (median to mean)
HLK: decrease in ground motion (84th to mean)
RUS : decrease in seismic hazard
JHT : decrease in PGA, but increase in hazard
JOR : increase in ground motion
Probabilistic Seismic Hazard Analysis Project
Giving away the hard-earned IP….
12-05-2012
Sharing the Work
Requests for Model/Model ComponentsAgency Request PurposeStanding Committee on Earthquake Design (SCED)
PSHA Model National Building Code of Canada
USGS Subduction GM Model
USGS National Seismic Hazard maps
Pacific Northwest National Laboratory, WA
Cascadia Subduction Model
Hanford
URS Corporation Subduction GMPE Consulting
PEERBerkeley, CA
Subduction GMPE and Data
Research & Development
Shannon& WilsonSeattle, WA
Subduction GMPE Consulting
GeoMotions, LLCLacey, WA
Subduction GMPE Incorporation into Commercial Software
Geosyntec ConsultantsHouston, TX
Source ModelSubduction GMPE
Consulting
Golder AssociatesBurnaby, BC
PSHA Model Consulting
Klohn Crippen BergerVancouver, BC
PSHA Model Consulting
Miscellaneous Individuals PSHA Model -------------
Sharing the Model
Project documents ALMOST signed off :
- GIS based Seismic Source Model
logic trees that characterize the various source areas
- Ground Motion model (attenuation formulae)
logic trees
IP will be shared by Publication of scientific papers
Access to hardcopy reports
Actual software code held by AMEC
With access to IP, others can develop alternative codes
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20
end
What Has Changed Since 1997?
• Seismic Data Base
– Another 15 years of data : increased data base of western Canada recordings
– no large, “surprise” earthquakes
– Several important worldwide earthquakes, some with strong motion records
• Improved understanding of BC seismotectonics, e.g.
– greatly advanced knowledge of 3D structure & geological evolution of Canada’s continental landmass and its margins.
– Improved understanding of the configuration & behaviour of the Cascadia subduction zone, its seismic history and seismic potential
• Ground Motion Models
– Ongoing development & publication of new attenuation relationships, e.g.
STEP 1Seismogenic Zone Model
STEP 2Recurrence Model
STEP 3Ground Motion
Attenuation
STEP 4Ground Motion
Hazard
Magnitude M1
Site
Fault
Area Sources
Acc
eler
atio
n
Distance
Uncertainty inAttenuation
M2
Alternate seismogeniczone models
Alternate recurrencecurves for each
seismogenic zone
Alternate Mx valuesfor each recurrence
curveAlternate attenuation
relationships
BCH
GSC-HGSC-R
Upper
Best EstimateLowerExtreme
Lower
Extreme Upper
Best Estimate
Upper
Lower
Sadigh et al.
Campbell
BJF
Idriss
Cum
ulat
ive
Rat
e (E
Q >
M/y
r)Magnitude (M)
Mx Fre
quen
cy o
f E
xcee
danc
e
Acceleration
ConfidenceLimits
Mean
Elements of PSHA – Cornell Methodology
Identify and model sources of aleatory (random) and epistemic (model and parameter) uncertainty
Available information often supports multiple, credible (scientifically sound) interpretations
SSHAC : The goal is to develop inputs that represent the composite distribution of the informed scientific community
STEP 1Seismogenic Zone Model
STEP 2Recurrence Model
STEP 3Ground Motion
Attenuation
STEP 4Ground Motion
Hazard
Magnitude M1
Site
Fault
Area Sources
Acc
eler
atio
n
Distance
Uncertainty inAttenuation
M2
Cum
ulat
ive
Rat
e (E
Q >
M/y
r)Magnitude (M)
Mx Fre
quen
cy o
f E
xcee
danc
e
Acceleration
ConfidenceLimits
Mean
Elements of PSHA – Cornell Methodology
Seismic Source Characterization Model Ground Motion Model
Ivan Wong led the SSC group!
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PSHA Policy Review
Peer Reviewed as part of Project
April 28, 2011 at Stanford U Marty McCann
Ivan Wong
Kevin Coppersmith
All agreed on use of mean : entire scientific process of hazard analyses revolves around the understanding of uncertainty
Use of mean already considered best practice in the US
Confirmed that policy is defendable
Use of Risk Informed approach most welcome :
“Dam community is two decades behind the Nuclear industry”
USBR and USACE have now moved to risk-informed decision making
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PSHA Policy : Use of the Mean
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Peak Ground Acceleration (g)
Pro
ba
bil
ity
(%
)
84th percentileMeanMedian
• Broadly representative of the distribution
• Means add to means mathematically for combined risk analyses
• Accounts for uncertainty
Preliminary Resultsfor Site C
Uncertainty not accounted for if Median is adopted
Rel
ativ
e co
nfid
ence
Lev
el (
%)
26
PSHA Policy : Use of the Mean
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Peak Ground Acceleration (g)
Pro
ba
bil
ity
(%
)
84th percentileMeanMedian
• Broadly representative of the distribution
• Means add to means mathematically for combined risk analyses
• Accounts for uncertainty
Preliminary Resultsfor Site C
Do we really need this little uncertainty?
Rel
ativ
e co
nfid
ence
Lev
el (
%)
27
PSHA Policy : Use of the Mean
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Peak Ground Acceleration (g)
Pro
ba
bil
ity
(%
)
84th percentileMeanMedian
• Broadly representative of the distribution
• Means add to means mathematically for combined risk analyses
Use of mean supported by ICOLD draft bulletin
Rel
ativ
e co
nfid
ence
Lev
el (
%)