Further Development of Site Response in
NGA Models
PEER Lifelines Program
NGA-West2 Project
Topic #8
Working Group Meeting
Kickoff Meeting
April 20, 2010
Agenda
• 10 am: Welcome/logistics (Stewart/Bozorgnia)
• 10:10 am: NGA-West2 overview and project objectives
(Bozorgnia)
• 10:30 am: Background on NEHRP site factors (Borcherdt)
• 11:00 am: Results from previous NGA Site Factor Working
Group (Power)
• 11:30 am: Comparison of NGA-West site terms to NEHRP
(Stewart/Seyhan)
• Noon: Lunch
• 1 pm: Open discussion of project objectives, scope,
deliverables, & schedule
Project Management
• Yousef Bozorgnia, PEER, NGA-West2 Project Manager
• Jonathan P. Stewart, UC Los Angeles (Working Group
Chair)
• Emel Seyhan, UC Los Angeles (GSR)
Working Group Committee (WGC)
• Don Anderson, CH2MHill, Bellevue, WA (Geotechnical engineer, TS3
Member)
• Roger Borcherdt, USGS, Menlo Park, CA (Engineering seismology,
developer of NEHRP site factors)
• C. B. Crouse, URS Corporation, Seattle, WA (Geotechnical engineer,
TS3 Chair and PUC Member)
• R.W. Graves, URS, Pasadena, CA (Seismologist, ground motion
simulation and basin effects)
• I.M. Idriss, UC Davis, Santa Fe, NM (Geotechnical engineer, GMPE
developer)
• Maury Power, AMEC Geomatrix, Oakland, CA (Geotechnical engineer,
for TS3 Chair)
• Walter Silva, PEA, El Cerrito, CA (Seismologist, NGA database
manager, GMPE developer)
• Thomas Shantz, Caltrans, Sacrament, CA (Geotechnical engineer)
Working Group Committee (WGC)
• Do we need others?
Logistics
•
•
•
•
•
Database provided by NGA-West2 (PEA)
Technical work by UCLA researchers
Oversight of work direction/results by WGC
Oversight of WGC by …?
Deliverables:
– Check of NGA-W models (trends with Vs30,
nonlinearity, sigma)
– Evaluate basin depth effects
– Develop technical basis for, and consensus behind,
revisions to NEHRP site factors
Agenda
• 10 am: Welcome/logistics (Stewart/Bozorgnia)
• 10:10 am: NGA-West2 overview and project objectives
(Bozorgnia)
• 10:30 am: Background on NEHRP site factors (Borcherdt)
• 11:00 am: Results from previous NGA Site Factor Working
Group (Power)
• 11:30 am: Comparison of NGA-West site terms to NEHRP
(Stewart/Seyhan)
• Noon: Lunch
• 1 pm: Open discussion of project objectives, scope,
deliverables, & schedule
NGA-West2 Overview and
Project Objectives
Yousef Bozorgnia
Background on NEHRP Site
Factors
Roger Borcherdt
Results from Previous NGA Site
Factor Working Group
Maury Power
Agenda
• 10 am: Welcome/logistics (Stewart/Bozorgnia)
• 10:10 am: NGA-West2 overview and project objectives
(Bozorgnia)
• 10:30 am: Background on NEHRP site factors (Borcherdt)
• 11:00 am: Results from previous NGA Site Factor Working
Group (Power)
• 11:30 am: Comparison of NGA-West site terms to NEHRP
(Stewart/Seyhan)
• Noon: Lunch
• 1 pm: Open discussion of project objectives, scope,
deliverables, & schedule
Outline
• Evaluation of Site Factors
– Available approaches
– Approaches adopted in NEHRP and NGA
GMPEs
– Input parameters
• Comparison of NGA & NEHRP site terms
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
Output
Vs
G/GMax
D
g
Rock Att.
Input
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
Day et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
– Randomized soil
properties and input
motions (1D only)
Walling et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
– Randomized soil
properties and input
motions (1D only)
Walling et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
– Randomized soil
properties and input
motions (1D only)
– Site factors from
simulation results
Walling et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
– Randomized soil
properties and input
motions (1D only)
– Site factors from
simulation results
Day et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Computational model,
1D or 3D
– Randomized soil
properties and input
motions (1D only)
– Site factors from
simulation results
Day et al. 2008
Evaluation of Site Factors
• Simulation driven:
– Not used as “stand alone” factors for active regions
– Used to constrain certain aspects of “hybrid” models
Evaluation of Site Factors
• Simulation driven
• Empirical (reference
site approach)
– Single event
– IMsoil/IMref: evaluate
dependence on site
condition and PGAref
Borcherdt 2002
Evaluation of Site Factors
• Simulation driven
• Empirical (reference site
approach)
• Empirical (non-reference
site approach)
– Multiple events
– Analysis of residuals
– Evaluate dependence on
site condition, PGArock,
etc.
Choi & Stewart 2005
Evaluation of Site Factors
Simulation
Empirical:
Ref. Site
Empirical:
Non-Ref. Site
Evaluation of Site Factors
NEHRP Factors
Nonlinearity
Simulation
Weak Motion
Amplification
Empirical:
Ref. Site
Empirical:
Non-Ref. Site
Evaluation of Site Factors
NGA: AS, CB
Amplification
level; Vs30dependence
Nonlinearity
Simulation
Empirical:
Ref. Site
Empirical:
Non-Ref. Site
Evaluation of Site Factors
NGA: BA, CY
Nonlinearity;
Amplification
level; Vs30dependence
Simulation
Empirical:
Ref. Site
Empirical:
Non-Ref. Site
Evaluation of Site Factors
• Input parameters:
–
–
–
–
–
NEHRP: Vs30, Ss, S1
AS: VS30, Median PGA1100
BA: VS30, Median PGA760
CB: VS30, Median PGA1100
CY: VS30, Median + i (Sa)1130
NGA-NEHRP Comparisons
• In natural log units, site term = Fx(Vs30, Ax)
– Fx=amplification relative to Vs30=x site
condition
– Ax=ground motion amplitude for reference site
condition of Vs30=x
• Use Vs30=150, 270, 560, 760, and 1100 m/s
• Evaluate F at T=0.3 and 1.0 sec.
NGA-NEHRP Comparisons
• Ax:
– A=median PGA for AS, BA, CB
– A=Sa at period of interest for CY (median + i);
Take Sa(0.3)=2.5×PGA and Sa(1.0)=1.0×PGA
• Adopt reference condition of 760 m/s
– F760(Vs30, Ax)=Fx(Vs30, Ax)-Fx(760, Ax)
S a Vs30 Sa Vs30 Sa x


 exp Fx Vs 30 , Ax   Fx 760, Ax 
Sa 760 Sa x Sa 760
NEHRP E
D
C
B
B
Suggestions on NEHRP-NGA
comparisons
• Average Sa across all NGA periods in the
ranges for Fa and Fv. Use average of logs +
limits. Exact middle of the range too (e.g.
0.3 sec for Fa, 1.2 sec for Fv)
• Average across velocities in site class: use
wt average based on histograms of Vs30.
• Ask Brian how he plotted F vs PGA
• Check coding using full models.
• Check NGA errata for AS
Residuals Analysis
• WS: run NGA for 760 with NEHRP factors
and look at residuals.
– All sites and estimates Vs only
• Use GMRotI50
• New data:
– Use June 2010 version of NGA flat file
– Alan Yong, USGS: he has funding to perform
site investigations. His site list has been
approved, previous efforts to provide input on
sites to investigate have been resisted
Residuals Analysis
• New data:
– Residuals analysis, including Baja earthquake
data, focusing on low Vs30 range.
Other recommendations
• USGS maps for all Vs30s, with interpolation (next
generation of mapping)
• When we have the new factors, compare hybrid
predictions with them & old factors with full psha
for various sites in Nor Cal and So Cal.
• Check sites without Vs using Virtual Geotechnical
Data Center (TS will send url and password).
• Google earth file with CA stations, see where E
stations are (look at other categories too).
NGA-NEHRP Comparisons
Differences:
• NEHRP Fv high
– Esp for C to E
• NEHRP nonlinearity
stronger for C to D
• NEHRP Fa and Fv high
for rock (Class B)
Working Group Objectives &
Scope
Objectives
• Input to NGA-West2 developers
• Develop technical basis for, and consensus
behind, revisions to NEHRP site factors
– Problems with medians
– Different sigmas
Task Order Scope
• Database