2.296 Idriss (2008)

• Ground-motion model is:

where PSA is in g, α1 = 3.7066 and α2 = -0.1252 for M 6.75, α1 = 5.6315 and α2 = -0.4104 for 6.75 < M 8.5, β1 = 2.9832, β2 = -0.2339, γ = 0.00047, ϕ = 0.12 and σ = 1.28 + 0.05ln(T) - 0.08M. σ for M < 5 equals σ at M5 and σ for M > 7.5 equals σ at M7.5. σ for T < 0.05 equals σ for T = 0.05s. Correction factor for V S30 > 900ms Δα1(T) = ln[(1 + 11T + 0.27T2)(1 + 16T + 0.08T2)] for 0.05 T 10s [Δα1(T) for T < 0.05s equals Δα1(0.05)].

• Use two site classes (may derive model for 180 V S30 < 450ms in future):
1.
V S30 > 900ms. 45 records. Since not enough records from stations with V S30 > 900ms derive correction factor, Δα1(T), to α1 based on residuals for these 45 records. Find no trends in residuals w.r.t. M, R or V S30.
2.
450 V S30 900ms. 942 records (333 from stations with measured V S30).

Notes that only 29% of stations have measured V S30; the rest have inferred V S30s. Examine distributions of measured and inferred V S30s and concluded no apparent bias by using inferred values of V S30.

• Uses two mechanism categories:
Strike-slip
Rake within 30 of horizontal. Includes records from normal events (rake within 30 of vertical downwards) because insufficient data to retain as separate category. F = 0.
Reverse
Rake within 30 of vertical upwards. Includes records from reverse oblique and normal oblique events (remaining rake angles) because insufficient data to retain as separate categories. F = 1.
• Uses the PEER Next Generation Attenuation (NGA) database (Flat-File version 7.2).
• Excludes (to retain only free-field records): i) records from basements of any building; ii) records from dam crests, toes or abutments; and iii) records from first floor of buildings with 3 storeys.
• Excludes records from ‘deep’ events, records from distances > 200km and records from co-located stations.
• Only retains records with 450 V S30 900ms for regression. Notes that initial analysis indicated that ground motions not dependent on value of V S30 in this range so do not include a dependency on V S30.
• Uses 187 records from California (42 events), 700 records from Taiwan (Chi-Chi, 152 records, and 5 aftershocks, 548 records) and 55 records from 24 events in other regions (USA outside California, Canada, Georgia, Greece, Iran, Italy, Mexico and Turkey).
• Only 17 records from R 5km and 33 from R 10km (for M 7 only 3 records from California for these distance ranges) (all site classes). Therefore, difficult to constrain predictions at short distances, particularly for large magnitudes.
• States that, from a geotechnical engineering perspective, use of V S30 bins is more appropriate than use of V S30 as an independent parameter.
• Does not investigate the influence of other parameters within the NGA Flat-File on ground motions.
• Uses PSA at 0.01s for PGA (checked difference and generally less than 2%).
• Divides data into magnitude bins 0.5 units wide and conducts one-stage regression analysis for each. Compares observed and predicted PGAs at distances of 3, 10, 30 and 100km against magnitude. Find that results for each magnitude bin generally well represent observations. Find oversaturation for large magnitudes due to presence of many records (152 out of 159 records for M > 7.5) from Chi-Chi. Does not believe that this is justified so derive α1 and α2 for M > 6.75 by regression using the expected magnitude dependency based on previous studies and 1D simulations.
• Examines residuals w.r.t. M, R and V S30 and concludes that for 5.2 M 7.2 model provides excellent representation of data. Examine residuals for 5 Chi-Chi aftershocks and find that for R > 15km there is no bias but for shorter distances some negative bias.
• Compares predictions to observations for Hector Mine (M7.1), Loma Prieta (M6.9), Northridge (M6.7) and San Fernando (M6.6) events w.r.t. R. Finds good match.
• Comments on the insufficiency of V S30 as a parameter to characterise site response due to soil layering and nonlinear effects.