abrahamson & silva nga model › static › lfs › nshm › ...abrahamson & silva nga model...
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Abrahamson & Silva NGAModel
Sep 25, 2006
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Recommendation
• Report not submitted on time• Not sufficient documentation to use model
in national maps• Weights for A&S 97 should go to 3 other
NGA models
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Regression Approach
• Random Effects– Accounts for correlation between residuals of
data from a single earthquake• e.g. an earthquake may produce systematically high
or low ground motions• Well recorded earthquakes do not dominate the
event scaling (e.g. mag, style-of-faulting, depth totop)
• Well recorded earthquakes do dominate thedistance, VS30, HW scaling
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Data Set Approach• Include ground motions from shallow crustal
earthquakes in active regions around the world– R < 100 km outside of WUS– R< 200 km for WUS– 130 earthquakes– 2810 recordings
• Include aftershocks– 1258 out of 2810 are from Chi-Chi aftershocks
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Size of Data Set
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Changes to Previous Versions• Simplified model
– Removed aspect ratio– Removed source-site angle (for HW)
• Changes to functional form– Added Depth to top effects at all magnitudes– Changed to a cubic term in magnitude– Added SCEC 3-D basin effects– Added Q term for small magnitudes– Changed treatment of 4 chi-chi aftershocks– Added break in VS30 scaling
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Chi-Chi Aftershocks
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Magnitude Saturation (PGA)
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Higher Order Mag Scaling
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Higher Order Mag Scaling(T=2)
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Higher Order Mag ScalingOnly fit to M>6.5 (T=5)
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Depth to Top (PGA)
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Style-of-Faulting (PGA)
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Style-of-Faulting (PGA)no mag dependence
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HW factor from A&S (1997)
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HW Factor - Dip
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HW Factor - Distance
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HW Factor - Depth to Top
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Break in VS30 scaling for large T
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Non-Linear Site (PGA)
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Non-Linear Site (T=0.2)
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Non-Linear Site
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3-D Basin Effects• Considered several parameters:
– Basin depth– Distance from edge– Basin edge locations
• Results parameterized in terms of median amplification as a functionof the depth to a VS isosurface– Z1.0 = depth to VS=1.0 km/s– Z1.5 = depth to VS=1.5 km/s– Z2.5 = depth to VS=2.5 km/s
• Variability scale factors were not parameterized• The Z1.0, Z1.5, and Z2.5 parameters were added to NGA data base
where available– Not available for 80% of the data in NGA data set
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3-D Basin Effects
• Used 3-D Basin results from SCECsimulations
• Issues of Implementation– Z1.0
• Similar to typical definitions of “engineering rock”• Can be reasonably estimated for projects
– Z2.5• Better metric to use, but difficult to obtain for
engineering projects
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Scaling with Basin DepthT= 3 sec
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Issues with Using 3-D ModelingResults
• Model is in terms of Z1.0• Correlation of Z1.0 and Vs30
– Some of the scaling with Z1.0 is alreadyincluded in the empirical model Vs30 scaling
• Need to remove the Vs30 scaling effects• Need to normalize to the median Z1.0 for a
given Vs30
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Correlation of Vs30 and Z1.0(SCEC Model only)
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Scaling with Basin Depth
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Correlation of Z1.0 and Vs30(SCEC Model only)
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Scaling with Z1.0, T=3
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Scaling with Z1.0 Vs270T=3
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Strong Smoothing of FicticousDepth
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Depth to Top Factor
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Style-of-Faulting Factor
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HW Scaling(Surface, 45 dip, M>6.5)
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HW Factor: 45 dip, Top=5
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HW Factor: 45 dip, Top=0
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Report Status
• Need to add:– Smooth cross-correlation of PGA and SA
residuals– Plots of residuals– Comparison with A&S 97