Probabilistic Seismic Hazard Analysis Project

Probabilistic Seismic Hazard Analysis Project

12-05-2012

Origins and Objectives

2

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• Seismic source and ground motion models becoming outdated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

7

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.

10

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??

12

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 (

%)

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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 (

%)