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Probabilistic Ground Motions
for Scoggins Dam, Oregon
Chris Wood
Seismotectonics & Geophysics Group
Technical Service Center
July 2012
Scoggins Dam
• 45 km east of locked
part of CSZ interface
(red)
– M 9.0 megathrust
earthquakes
• 60 km above slab (blue,
green)
– M 7.5 deep earthquakes
Reclamation Dam Safety
Guidelines
• Use risk-based approach for decision making
that evaluates the following risks:
– annual probability of failure
– annualized loss of life
• Consider a wide range of loading conditions to
determine risk
Probabilistic seismic loadings are needed for input
into engineering analyses of how dams respond
to seismic loadings
– Specifically, we need scenario ground motion time
histories for a wide range of return periods
How probabilistic ground motions were
developed for Scoggins Dam
• Perform a Probabilistic Seismic Hazard Analysis (PSHA)
to get hazard curves, uniform hazard spectra (UHS), and
disaggregation of hazard
• Determine response spectra for scenario earthquakes
using the Conditional Mean Spectrum (CMS) method
• Select initial time histories that are representative of the
main contributors to the hazard
– from historical records (e.g., M 9.0 Tohoku, Japan)
– from Empirical Green’s Function (EGF) models
• Spectrally match the selected initial time histories to the
scenario earthquake response spectra
Seismic sources
• Subduction
zone
interface
• Intra-slab
(deep)
earthquakes
• Crustal:
mapped
faults and
shallow
background
seismicity
Source: USGS
PSHA
• Probabilistic seismic hazard analyses
(PSHA) conducted by R. LaForge, Fugro
Wm. Lettis & Assoc. (FWLA) for
Reclamation, 2006, 2009, 2011
• Considered commonly recognized
earthquake sources (CSZ interface,
intraslab, and crustal)
Earthquake Sources used for PSHA
Puget intraslab
CZS Interface
Portland intraslab
Crustal faults
Cascadia (Interface), Intraslab
& Crustal seismic sources
PHA Hazard
Scoggins Dam PHA Hazard
1.E-01 10 • 10,000-yr PHA =
Total 1.4 g
Interface-only
Intraslab Sources • Mean hazard
1.E-02 Crustal Sources 100 contributors (for
return periods of
Return Period, years
Annual Frequency
interest):
1.E-03 1,000 1) Cascadia SZ
Interface
2) Portland
1.E-04 10,000
Intraslab
3) Crustal sources
(Gales Creek
fault)
1.E-05 100,000
0.00 0.50 1.00 1.50 2.00
PHA, g
Contributions by source – PHA
Scoggins Dam PHA Hazard Contribution by Source
1
Interface
Intraslab
0.8 Crustal
Relative Contribution
0.6 50,000-yr
5,000-yr
10,000-yr
0.4
1,000-yr
0.2
0
0.00 0.50 1.00 1.50 2.00 2.50
PHA, g
0.75-sec. SA Hazard – All Sources
Scoggins Dam 0.75-Sec SA Hazard
1.E-01
Total
10
• 10,000-yr
Interface-only
Intraslab Sources
SA = 1.7 g
• Mean
1.E-02 Crustal Sources 100
Return Period, years
Annual Frequency
hazard
1.E-03 1,000
contributors
(for return
1.E-04 10,000
periods of
interest):
1.E-05 100,000
0.00 0.50 1.00 1.50 2.00 2.50 1) Cascadia
0.75-Sec SA, g
SZ
Interface
2) Crustal
Contributions by source – 0.75-sec. SA
Scoggins Dam 0.75-Sec SA Hazard Contribution by Source
1
Interface
Intraslab
0.8 Crustal
Relative Contribution
50,000-yr
0.6 10,000-yr
5,000-yr
1,000-yr
0.4
0.2
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00
0.75-sec SA, g
Computed
hazard
strongly
depends
on
subjective
GMPE
weights
Ground Motion Prediction
Equations (GMPEs) and Weighting
• Cascadia Subduction Zone Interface
– Atkinson & Boore, 2003 (worldwide): 0.1 (predicts lowest ground motions for R <
150 km)
– Atkinson & Macias, 2009: 0.1
– Youngs et al, 1997: 0.3
– Zhao et al, 2006: 0.5 (predicts highest ground motions for R < 150 km)
• Cascadia Subduction Zone Intraslab
– Atkinson & Boore, 2003 (worldwide): 0.17
– Youngs et al, 1997: 0.0
– Zhao et al, 2006: 0.83
• Shallow Crustal Sources
– Abrahamson & Silva, 2008 (NGA): 0.25
– Boore & Atkinson, 2008 (NGA): 0.25
– Campbell & Bozorgnia, 2008 (NGA): 0.25
– Chiou & Youngs, 2008 (NGA): 0.25
Note: Same GMPEs and weights used both for PSHA and for development of
scenario response spectra
Median CSZ interface deterministic spectra
Deterministic spectra, median
• ZEA06 gives
1.0
AB03 - ε=0 much larger
AM09 - ε=0
0.8
YEA97 - ε=0 estimated
Spectral Acceleration, g
0.6
ZEA06 - ε=0
Mean
accelerations
0.4
0.2
0.0
0.01 0.1 1 10
Period, sec.
Comparison
of M 9.0
Tohoku,
Japan EQ
observatio
ns with
GMPE
predictions
Scenario Response Spectra
Development
• Scenario spectra were developed using the Conditional
Mean Spectrum (CMS) method developed by Baker &
Cornell, 2006
• Requires specification of one or more critical spectral response periods
• If the critical periods of a structure are poorly known, may require target
spectra from numerous trial periods to fully envelope total hazard
defined by UHS
• Provides a more realistic response spectrum for a single earthquake
than the UHS
• For Scoggins, simplified to long- and short-period scenario
spectra
– Results in larger ground motions, but fewer time histories to analyze
• Scenario spectra constraints
– Scenario spectra for a specified return period envelope the UHS
– No scenario spectrum is allowed to exceed the UHS
Uniform Hazard Spectra (UHS)
Scoggins Dam UHS
6.0
Total - 50k UHS
Total - 10k UHS
5.0 Total - 5k UHS
Total - 1k UHS
Total - 500-yr UHS
4.0
Spectral Acceleration, g
3.0
2.0
1.0
0.0
0.01 0.1 1 10
Period, sec.
Development of CMS Spectra
CMS Method - AM09, 10k Scenario, Tc=0.75 sec
3.0 4.0
Rrupt = 47 km 10k UHS
M = 9.0, ε = 2.13
Spectral Acceleration, g
SA(ε=2.13)
SA(ρ?ε)
2.0 3.0
ρ(Tc=0.75,T)
1.0 2.0
ε = 2.13
ε = 1.14
ε = 0.76
0.0 1.0
Correlation
0.75-sec.
Critical
-1.0 Period 0.0
0.01 ρ =0.1
0.54 ρ=1 1 10
ρ = 0.36
Period, sec.
CMS – CSZ Interface, 10k, Tc=0.75 sec.
CMS Method - 10k Interface scenario - Tc = 0.75 sec.
ZEA06
3.0 YEA97
AM09
AB03
Interface CMS
Spectral Acceleration, g
10k UHS
2.0
1.0
0.0
0.0 0.1 1.0 10.0
Period, sec.
CMS – Crustal sources, 10k, Tc=0.75 sec.
CMS Method - 10k crustal scenario - Tc=0.75 sec.
AS08
BA08
3.0
CB08
CY08
Crustal CMS
Spectral Acceleration, g
10k UHS
2.0
1.0
0.0
0.0 0.1 1.0 10.0
Period, sec.
Time History Development
• Obtain initial time histories for scenario
earthquakes from historical or synthetic records
with magnitudes, distances and characteristics
representative of the Cascadia interface,
crustal, and intraslab sources
• Use a wavelet-based spectral matching method
to modify the initial time histories so that the
response spectrum of the final time history
matches that of the scenario spectrum
• Match spectra for 500, 1,000, 5,000, 10,000,
and 50,000 year return periods
CSZ interface source characteristics
(M, RRupt,ε) for PHA, 10k, YEA97
• Largest
contribution
from M 9.0,
and RRupt
45-50 km
• Similar
results for
other return
periods,
response
spectral
periods, and
GMPEs
Cascadia interface initial time
histories
• Historical records from 3/11/2011 M 9.0
Tohoku, Japan earthquake
• Synthetic M 8 to 9 Cascadia interface
earthquake records based on Empirical
Green’s function (EGF) method.
HKD070
EGF
HKD070 EGF
• R = 104 km
• M 6.4
aftershock of
2003 M 8.0
Tokachi-oki,
Japan
earthquake
• Soil site
• Simulation
using EGF
with R = 100
km
• Rupture length
is 600 km
• Hypocenter in
center
• HKD070 (soil)
• Duration ~ 130
sec
Spectrally-matched
5k time history for
CSZ interface
earthquake source
• Initial time history: M
9.0 EGF simulation
using HKD070 EGF
Spectral Match • Target spectrum: 5k
interface, CMS
• PHA = 0.56 g
• Duration = 180 sec
• IA = 30.2 m/s
Comparison of target and matched response
spectra
• Target and
matched
response
spectra, 5k
CSZ
interface
from M 9.0
EGF
simulation
using
HKD070
EGF
Non-interface Time Histories
• Initial time histories representing crustal
earthquake source (Gales Creek)
• Initial time histories representing the
Portland intraslab
• Spectrally matched to CMS non-interface
scenario spectra
• Initial time histories selected from
historical strong-motion records based on
PSHA disaggregation results
Initial
Spectrally-matched
5k time history for
crustal earthquake
source
• Initial time history: M
6.5, 1992, Big Bear,
Spectral Match
CA, Big Bear Lake,
R=9 km
• Target spectrum: 5k
non-interface, CMS
• PHA = 0.96 g
• Duration = 10 sec
• IA = 11.2 m/s
Scenario Earthquake Time History
Summary
• Seven sets of 3-component time histories for
each scenario spectra.
– Time histories for CSZ interface, CSZ intraslab, and
crustal fault scenarios
– Five return periods: 500, 1,000, 5,000, 10,000 and
50,000 years
– Baseline-corrected acceleration, velocity and
displacement
• Free-surface acceleration time histories
• Velocity time histories for use as compliant base
input to FLAC model
