NGA-West2 GMPE Average Site Coefficients for Use in 

Earthquake-Resistant Design

By Roger D. Borcherdt

Open-File Report 2015–1116

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Borcherdt, R.D., 2015, NGA-West2 GMPE average site coefficients for use in earthquake-resistant 

design: U.S. Geological Survey Open-File Report 2015–1116, 27 p., http://dx.doi.org/10.3133/ofr20151116.

ISSN 2331-1258 (online)
iii

Acknowledgments

This study was motivated by the NGA-West2 project coordinated by the Pacific Earthquake 

Engineering Research Center and by discussions with members of the Provision Update Committee for 

ASCE/SEI 7-16. Special acknowledgments are due to all of the NGA-West2 project participants and the 

developers for the GMPEs. Special notes of thanks are due Dr. Charles Kircher and Dr. Sanaz Rezaeian 

for their generous contributions and many keen insights. Dr. Kircher kindly provided his Excel

calculations and preprints of his results. Dr. Rezaeian kindly provided independent MATLAB estimates 

of the spectral amplifications versus period for each GMPE. Emel Seyhan’s excellent Excel compilation 

of the NGA-W2 GMPEs on the Pacific Earthquake Engineering Research Center web site is gratefully 

acknowledged. Suggestions by PUC members and review comments provided by Dr. Kircher, Dr. Sanaz 

Rezaeian, Dr. Mehmet Celebi, and Mr. Keith Knudsen are sincerely appreciated.
iv

Contents

Acknowledgments.......................................................................................................................................................iii

Contents .....................................................................................................................................................................iv

NGA-West2 GMPE Average Site Coefficients for Use in Earthquake-Resistant Design ............................................ 1

Abstract ...................................................................................................................................................................... 1

Introduction................................................................................................................................................................. 1

NGA-West2 Site Coefficients...................................................................................................................................... 2

NGA-W2 Amplification as a Function of Period .......................................................................................................... 4

Comparison of NGA-W2 and NGA-W1 Amplification Factors with NEHRP Coefficients ............................................ 4

NGA-W2 Scaling Factors to Reduce Conservatism in Design Spectra and to Scale Site Amplification 

Coefficients to Other Reference Site Conditions....................................................................................................... 12

Implications of NGA-W2 for ASCE 7-10 (NEHRP) Site Coefficients......................................................................... 18

Comparison of NGA-W2 Coefficients with Those Proposed for ASCE 7-16............................................................. 19

Conclusions .............................................................................................................................................................. 24

References Cited...................................................................................................................................................... 25

Figures

1. Graphs showing NGA-W2 and NGA-W1 spectral amplifications with respect to site class B for site class E at 

PGA levels of 0.1 g and 0.4 g together with corresponding average short-period and mid-period spectral 

amplifications................................................................................................................................................. 5

2. Graphs showing NGA-W2 and NGA-W1 spectral amplifications with respect to site class B for site class D at 

PGA levels of 0.1 g and 0.4 g together with corresponding average short-period and mid-period spectral 

amplifications................................................................................................................................................. 6

3. Graphs showing NGA-W2 and NGA-W1 spectral amplifications with respect to site class B for site class C at 

PGA levels of 0.1 g and 0.4 g together with corresponding average short-period and mid-period spectral 

amplifications................................................................................................................................................. 7

4. Graphs showing short-period and mid-period average amplification values for site class E for each NGA-W2 

GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @ 0.2 s , 1.0 s, 

and NEHRP site coefficients as a function of PGA........................................................................................ 8

5. Graphs showing short-period and mid-period average amplification values for site class D for each NGA-W2 

GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @ 0.2 s , 1.0 s, 

and NEHRP site coefficients as a function of PGA........................................................................................ 9

6. Graphs showing short-period and mid-period average amplification values for site class C for each NGA-W2 

GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @ 0.2, 1.0 s, 

and NEHRP site coefficients as a function of PGA...................................................................................... 10

7. Graphs showing short-period coefficients and mid-period coefficients at PGA=0.1 g as a function of vS30 for 

each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients......... 13

8. Graphs showing short-period coefficients and mid-period coefficients at PGA=0.2 g as a function of vS30 for 

each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients......... 14

9. Graphs showing short-period coefficients and mid-period coefficients at PGA=0.4 g as a function of vS30 for 

each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients......... 15
v

10. Graphs showing short-period and mid-period average amplification values as a function of PGA for site class 

E for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 SD), 

NEHRP site coefficients, SS14/1050 factors, and SS14/760 factors ........................................................... 20

11. Graphs showing short-period and mid-period average amplification values as a function of PGA for site class 

D for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 SD), 

NEHRP site coefficients, SS14/1050 coefficients, and SS14/760 coefficients............................................. 21

12. Graphs showing short-period and mid-period average amplification values as a function of PGA for site class 

C for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 SD), 

NEHRP site coefficients, SS14/1050 coefficients, and SS14/760 coefficients............................................. 22

13. Graphs showing short-period and mid-period average amplification values as a function of PGA for site class 

B for NGA-W2 Avg, SS14 /760, SS14/1050 SP, and MP coefficients ......................................................... 23

Tables

1. Percent difference average NGA-W2 coefficients exceed average NGA-W1 coefficients ........................... 11

2. Average NGA-W2 SP and MP Avg spectral amplifications and standard deviations for vS30=760 m/s and 900 

m/s as inferred with respect to 1,050 m/s .................................................................................................... 16

3. Mean NGA-W2 SP and MP scale factors as inferred from table 2............................................................... 16

4. NGA-W2 SP and MP Avg site coefficients referenced to vS30=1,050 m/s.................................................... 16

5. NGA-W2 SP and MP Avg site coefficients referenced to vS30=900 m/s....................................................... 17

6a. NGA-W2 SP Avg site coefficients referenced to vS30=760 m/s in parentheses and corresponding Fa site 

coefficients adopted in table 11.4-1 ASCE/SEI 7-10.................................................................................... 17

6b. NGA-W2 MP Avg site coefficients referenced to vS30=760 m/s in parentheses and corresponding Fv site 

coefficients adopted in table 11.4-2, ASCE/SEI 7-10................................................................................... 17

7. Percent difference between NGA-W2 Avg/760 short- and mid-period coefficients and those proposed for 

ASCE 7-16 (SS14/760)................................................................................................................................ 19
1

NGA-West2 GMPE Average Site Coefficients for Use in 

Earthquake-Resistant Design

By Roger D. Borcherdt

Abstract

Site coefficients corresponding to those in tables 11.4–1 and 11.4–2 of Minimum Design Loads 

for Buildings and Other Structures published by the American Society of Civil Engineers (Standard 

ASCE/SEI 7-10) are derived from four of the Next Generation Attenuation West2 (NGA-W2) 

Ground-Motion Prediction Equations (GMPEs). The resulting coefficients are compared with those 

derived by other researchers and those derived from the NGA-West1 database. The derivation of the 

NGA-W2 average site coefficients provides a simple procedure to update site coefficients with each 

update in the Maximum Considered Earthquake Response MCER maps. The simple procedure yields 

average site coefficients consistent with those derived for site-specific design purposes. The NGA-W2 

GMPEs provide simple scale factors to reduce conservatism in current simplified design procedures.1

Introduction

The Next Generation Attenuation West2 (NGA-W2) project, coordinated by the Pacific 

Earthquake Engineering Research Center, has resulted in major advancements in the estimation of

earthquake ground motions for purposes of earthquake-resistant design (Bozorgnia and others, 2014). 

These advancements include development of an extensive international strong-motion database (Ancheta

and others, 2014) and a corresponding set of Ground-Motion Prediction Equations (GMPEs) developed 

by teams of researchers (Gregor and others, 2014). The NGA-W2 GMPEs have been used to revise the

Maximum Considered Earthquake Response (MCER) maps for a uniform ground condition (Petersen and 

others, 2014) for a future version of the US building codes to be published by the American Society of 

Civil Engineering (ASCE Standard ASCE/SEI 7-16).

This paper provides rigorous estimates of site coefficients implied directly by the updated 

NGA-W2 database and four of the NGA-W2 GMPEs used to derive the recent MCER maps (Petersen and 

others, 2014). The paper provides a simple procedure for deriving the site coefficients from the average 

spectral amplifications, implied as a function of period by each Ground-Motion Prediction Equation 

(GMPE). The procedure is intended to provide coefficients that can be easily derived by consultants for 

independent site-specific estimates of site response. Simple and robust scale factors, derived from the

NGA-W2 GMPEs, allow a reduction in conservatism associated with current simplified design 

procedures.

The NGA-W2 Average (Avg) coefficients derived herein are compared with those derived from 

the NGA-West1 (NGA-W1) database using similar procedures (Borcherdt, 2014a,b). They are also 

 1

Keywords: site coefficients; building code; soil response; next generation attenuation, site class
2

compared to similar NGA-W2 Avg coefficients derived using MATLAB codes by Rezaeian (written

commun., 2015; Rezaeian and others, 2014) and those derived at 0.2 s and 1.0 s by Kircher and others

(written commun., 2014). The coefficients are compared with those recently proposed for ASCE 7-16.

The NGA-W2 Avg scale factors are derived and recommended for use in procedures to estimate 

simplified design spectra. The scale factors may be interpreted as factors to reduce conservatism 

associated with specifying base input motion levels at a vS30 value more typical of rock sites in site class B 

or alternatively they may be interpreted as scale factors for normalization of the site coefficients to a lower 

reference vS30 value. The procedure to develop the site coefficients and the scale factors is intended as a 

simple procedure that can be used with each update in the MCER maps and corresponding update of the 

site coefficients.

NGA-West2 Site Coefficients

The Next Generation Attenuation (NGA) Ground-Motion Prediction Equations (GMPEs), based 

on the NGA-W2 database, were used to update the MCER maps for ASCE/SEI 7-16. Four of these 

equations, ASK14 (Abrahamson, Silva, and Kamai, 2014), BSSA14, (Boore and others, 2014), CB14

(Campbell and Bozorgnia, 2014), and CY14 (Chiou and Youngs, 2014) are used here to derive NGA-W2

site coefficients. The I14 GMPE (Idriss, 2014) was not used to infer the site coefficients, because the I14 

vS30 model does not account for site classes with soft-soil conditions.

Spectral amplifications are derived as a function of period for site classes B, C, D, and E for peak

ground acceleration (PGA) levels of 0.1 g, 0.2 g, 0.3 g, and 0.4 g for each GMPE for M6, 7, and 8 events. 

The amplifications for each period were calculated using default parameters for sediment depth (Z1 and 

Z2.5) and other model parameters for the California region as provided by Seyhan (2014). For consistency 

with the concept of simplified site classes as specified in terms of vS30, each site class is represented by the 

vS30 value at the corresponding midpoint value of the vS30 interval that defines the site class. The spectral 

amplifications were inferred with respect to the reference site class B for corresponding PGA levels of 0.1

g, 0.2 g, 0.3 g, and 0.4 g also determined at the midpoint of site class B. Short-period (SP) and mid-period 

(MP) coefficients were derived from the spectral amplifications for each GMPE by computing the 

average spectral amplification at equally-spaced period values in the short-period band (0.1–0.5 s) and the 

mid-period band (0.5–2.0 s). These average amplification coefficients were inferred directly from each 

GMPE for site class B (vS30=1,050 m/s) PGA levels of 0.1 g, 0.2 g, 0.3 g, and 0.4 g for site classes E, D, C, 

and B as represented by the midpoint vS30 values of 150 m/s, 290 m/s, 540 m/s, and 1,050 m/s, respectively

(Borcherdt, 1992; 1994). The coefficients at 0.5 g were extrapolated using a power law fit to the NGA-W2 

SP and MP coefficients at 01 g, 0.2 g, 0.3 g, and 0.4 g. Scaling factors are derived to scale the inferred 

NGA-W2 coefficients to other reference vS30 values, including 760 m/s.

An important issue concerns the correct procedure to infer the spectral amplifications for the site 

classes. A detailed justification of the procedure used here is provided in Borcherdt (2014a,b). The 

procedure is similar to that used to derive the initial NEHRP (National Earthquake Hazard Reduction 

Program) site coefficients (Borcherdt, 1992; 1994; Dobry and others, 2000) as adopted in the 1994 and in 

several subsequent code provisions. In brief, the short-period coefficients (Fa) and the mid-period 

coefficients (Fv) were intended to be average spectral factors that could be multiplied by corresponding 

short- and mid-period band spectral estimates on rock to obtain a two-band simplified design spectrum for 

sites on other classes of site conditions. Site classes (A–E) were defined unambiguously, in terms of vS30

intervals implied by detailed correlations between geotechnical physical properties, measured shear 

velocities, and empirical spectral amplifications observed at more than 200 sites in the San Francisco and 

Los Angeles regions (Borcherdt, 1970; Borcherdt and Gibbs, 1976; Fumal, 1978; Borcherdt, 1994; 

Rogers and others, 1984; Borcherdt, 2012). To account for nonlinearity in the response of the site classes,
3

the corresponding site class amplification factors were inferred from numerical models for PGA levels of 

0.2–0.4 g with respect to a reference rock site condition (site class B) assumed to have a linear response at 

all PGA levels (Seed, 1992; Dobry and others, 1992). Empirical correlations between average spectral 

amplifications and vS30 imply that the average amplification for each site class is best represented by the 

amplification at a vS30 value near or at the midpoint of the vS30 interval for each site class, including the 

reference rock site class B with unity amplification at vS30~1,050 m/s (Borcherdt, 1992; 1994).

Changes in code terminology and in GMPE databases have led to suggestions and opportunities to 

modify some aspects of the procedures. Some of these suggestions include: (1) compute spectral 

amplifications with respect to the boundary between site classes C and B (vS30=760 m/s) for 

ground-motion levels specified at the midpoint (vS30~1,050 m/s) of site class B; (2) infer site coefficients 

from a single NGA GMPE, for example, SS14; (3) compute spectral amplifications with respect to the 

boundary between site classes C and B (vS30=760 m/s) for ground-motion levels specified at the boundary 

(vS30~760 m/s); (4) use median vS30 values to represent the spectral amplifications for each site class

instead of a value near the midpoint; (5) infer spectral amplification factors only at periods of 0.2 s and 1.0 

s corresponding to the periods for the spectral estimates used for the MCER maps instead of using 

averages for the short- and mid-period bands; (6) infer the short-period spectral amplification factors at 

spectral accelerations for 0.2 s at levels of Ss=0.25, 0.50, 0.75, 1.0, 1.25, and 1.50; infer the mid-period 

spectral amplifications at spectral accelerations for the mid-period band of S1=0.1, 0.2, 0.3, 0.4, 0.5 g.

Implications of these suggestions are explored in more detail for the NGA-W2 dataset in 

subsequent sections and briefly summarized here.

Suggestions (1) and (2) lead to significant reductions in the site coefficients and the need to adjust 

the site coefficients to larger values based on lack of data arguments (Stewart and Seyhan, 2013). The

resulting adjusted coefficients are no longer referenced to vS30=760 m/s and are not consistent with the 

coefficients and associated epistemic uncertainty implied by averages of four of the NGA-W2 GMPEs. 

Suggestion (3) leads to coefficients which are significantly less than those inferred with respect to 

vS30~1,050 m/s, but when scaled with W2, implied scale factors are comparable to those derived with 

respect to vS30~1,050 m/s.

Suggestion (4) to use the median vS30 value to represent the site classes is shown to yield results 

that differ from those implied by the midpoint value by only a few percent for site class E, D, and C, but 

use of the median value for site class B will be shown to imply a change of 10 to 13 percent in the site class 

B coefficient, which, if used as the reference ground condition, implies a similar change in each of the 

other site-class coefficients. Considering that the site class vS30 intervals are defined by independent 

evidence and that median values can be biased by procedures used to infer vS30, representation of the site 

classes by the midpoint vS30 value is preferred and used here.

Suggestions (5) and (6) evolved from changes in terminology used in the codes to specify the 

reference ground-motion levels in terms of spectral accelerations instead of PGA. This change was 

implemented by changing the terminology for the reference ground motion from levels of PGA=0.1,..., 

0.5 g to corresponding levels of spectral acceleration (SA), SA=Ss=0.25,..., 1.25 for the short-period 

factor, Fa, and to levels of SA=S1=0.1, ..., 0.5 for the mid-period factor, Fv, without changes in the values 

for the Fa and Fv factors. These changes in terminology were implemented to make the code terminology 

for specification of ground-motion levels for site classes consistent with that for the MCER maps. It will 

be shown in the next section that the amplification factors, inferred at 0.2 s, are a few percent less than 

those inferred as the average over the short-period band. Similarly, those inferred at 1.0 s tend to be

slightly greater that those inferred as the average over the mid-period band. Preliminary evaluations based 

on the NGA-W2 GMPEs indicate that the coefficients inferred with respect to a single specified PGA 

level for both coefficients may differ significantly from those inferred for the corresponding specified SA 
4

levels for the short- and mid-period bands. Considering that a relationship between the PGA and SA 

levels has not been established yet from the four NGA-W2 GMPEs, the procedure used here is to infer the 

coefficients for specified PGA levels, then label these using the correlation with SA specified in the code. 

This is an issue, however, that can yield different amplifications and merits further evaluation.

NGA-W2 Amplification as a Function of Period

The NGA-W2 amplification coefficients, inferred as a function of period as implied directly by 

each of the NGA-W2 GMPEs, are shown as solid curves in figures 1, 2, and 3 for site classes E, D, and C 

for input ground-motion PGA levels of 0.1 g and 0.4 g. The plots shown in figures 1, 2, and 3 were 

computed using the Excel formulation for the GMPEs developed by Seyhan (2014) as modified herein to 

infer amplification factors. They also were computed independently using a MATLAB formulation of the 

GMPEs by Rezaeian (written commun., 2015; Rezaeian and others, 2014). The amplification factors 

derived from the two formulations for each site class and ground-motion level were in exact agreement 

confirming each of the numerical algorithms.

Figures 1, 2, and 3 show the dependence of NGA-W2 amplification on period as inferred for each 

GMPE and for each site class. They illustrate that a significant amount of epistemic or model uncertainty 

exists for the NGA-W2 models. They show that the amplification coefficients for each site class first 

decrease with period for periods less than about 0.1 s, then increase rapidly with period. They also show a 

tendency for the rate of increase to decrease for some of the GMPEs for periods longer than about 1.0 s.

Figures 1, 2, and 3 also show the average of the average amplification factors inferred for the 

short-period band (NGA-W2 SP Avg) and mid-period band (NGA-W2 MP Avg) from each of the four 

GMPEs. The figures illustrate that an average amplification factor inferred at a single period of 0.2 s from 

each set of four GMPEs would be less than the corresponding SP-Avg coefficient. They also illustrate that 

the average amplification factors inferred at the single period of 1.0 s from the four GMPEs would tend to 

be slightly greater than the corresponding MP-Avg coefficient.

Comparison of NGA-W2 and NGA-W1 Amplification Factors with NEHRP 

Coefficients

Spectral amplifications as inferred from both the NGA-W2 GMPEs (solid curves) and the 

NGA-W1 GMPEs (dashed curves) are plotted as a function of period in figures 1, 2, and 3. The spectral 

amplifications plotted for the NGA-W1 database were inferred from the W1 GMPEs compared by 

Abrahamson and others (2008), by Stewart and Seyhan (2013), and reported on by Borcherdt (2014b). 

The plots of spectral amplification as a function of period show that a considerable amount of epistemic or 

model uncertainty exists for both the NGA-W1 and NGA-W2 sets of GMPEs. The spectral amplifications 

inferred from each of the W2 GMPEs are in general distinctly larger for each period than those inferred 

from the W1 database. Similarly, the SP Avg and MP Avg inferred from the W1 GMPEs are distinctly 

smaller than those for the W2 GMPEs. Additional evidence of this difference between amplifications 

inferred from the W1 and W2 GMPEs is provided in figures 4, 5, and 6.

Average short-period (NGA-W2 SP Avg) and mid-period (NGA-W2 MP Avg) amplification 

factors are plotted as a function of base PGA level for each GMPE in figures 4, 5, and 6. The figures also 

show the mean (W2 SP Avg, W2 MP Avg, solid blue) and the mean + 2 standard deviation (SD) values

(dashed blue) inferred from the four NGA-W2 GMPEs. These figures also show the corresponding mean 

values for the NGA-W1 data set (purple curves) (see tables II-1 and II-2, in Borcherdt, 2014b) are shown 

together with the corresponding Fa and Fv NEHRP coefficients (black curves). The figures also show the

NGA-W2 Avg coefficients as extrapolated to 0.5 g using the illustrated power law regression curve.
5

Figure 1. Graphs showing NGA-W2 (solid curves) and NGA-W1 (dashed curves) spectral amplifications with 

respect to (wrt) site class B for site class E at PGA levels of 0.1 g (A) and 0.4 g (B) together with corresponding 

average short-period (SP Avg, 0.1–0.5 s) and mid-period (MP Avg, 0.5–2.0 s) spectral amplifications inferred from 

mean of corresponding averages for each of the four NGA-W2 and NGA-W1 GMPEs. Curves indicate epistemic 

uncertainty associated with each set of GMPEs and show that spectral amplifications for NGA-W2 database are 

significantly larger than those derived from NGA-W1 database.

0

1

2

3

4

5

6

7

8

9

0.00 0.50 1.00 1.50 2.00 2.50

NGA Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

W1 MP Avg

NGA-W2 & W1 Amplification versus Period

(Site Class E, 0.1 g)

0

1

2

3

4

5

6

7

8

9

0.00 0.50 1.00 1.50 2.00 2.50

NGA Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

W1 MP Avg

NGA-W2 & W1 Amplification versus Period

(Site Class E, 0.4 g)

A

B
6

Figure 2. Graphs showing NGA-W2 (solid curves) and NGA-W1 (dashed curves) spectral amplifications with 

respect to (wrt) site class B for site class D at PGA levels of 0.1 g (A) and 0.4 g (B) together with corresponding 

average short-period (SP Avg, 0.1–0.5 s) and mid-period (MP Avg, 0.5–2.0 s) spectral amplifications inferred from 

mean of corresponding averages for each of four NGA-W2 and NGA-W1 GMPEs. Curves indicate epistemic 

uncertainty associated with each set of GMPEs and show that spectral amplifications for NGA-W2 database are 

significantly larger than those derived from NGA-W1 database.

0

1

2

3

4

5

0.00 0.50 1.00 1.50 2.00 2.50

NGA Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

NGA Amplification versus Period

(Site Class D, 0.1g)

NGA W2 & W1Amplification versus Period

(Site Class D, 0.1 g)

0

1

2

3

4

5

0.00 0.50 1.00 1.50 2.00 2.50

NGA Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

W1 MP Avg

NGA-W2 & W1 Amplification versus Period

(Site Class D, 0.4 g)

B

A
7

Figure 3. Graphs showing NGA-W2 (solid curves) and NGA-W1 (dashed curves) spectral amplifications with 

respect to (wrt) site class B for site class C at PGA levels of 0.1 g (A) and 0.4 g (B) together with corresponding 

average short-period (SP Avg, 0.1–0.5 s) and mid-period (MP Avg, 0.5–2.0 s) spectral amplifications inferred from 

mean of corresponding averages for each of four NGA-W2 and NGA-W1 GMPEs. Curves indicate epistemic 

uncertainty associated with each set of GMPEs and show that spectral amplifications for NGA-W2 database are 

significantly larger than those derived from NGA-W1 database.

0

1

2

3

0.00 0.50 1.00 1.50 2.00 2.50

Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

W1 MP Avg

NGA-W2 & W1 Amplification versus Period

(Site Class C, 0.1 g)

0

1

2

3

0.00 0.50 1.00 1.50 2.00 2.50

Amplification wrt Site Class B

Period (s)

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 MP Avg

AS08

BA08

CB08

CY08

W1 SP Avg

W1 MP Avg

NGA-W2 & W1 Amplification versus Period

(Site Class C, 0.4 g)

B

A
8

Figure 4. Graphs showing short-period (A) and mid-period average amplification (B) values for site class E for 

each NGA-W2 GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @ 0.2 s, 

1.0 s, and NEHRP site coefficients as a function of PGA.

y = 0.6598x-0.605

0

1

2

3

4

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP Fa

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NGA W1 SP Avg

W2 SA @ 0.2 s

W2 SP 0.5g

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class E) 

y = 3.2042x-0.244

0

1

2

3

4

5

6

7

8

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP Fv

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

NGA W1 MP Avg

W2 SA @ 1.0 s

W2 MP 0.5g

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class E) 

B

A
9

Figure 5. Graphs showing short-period (A) and mid-period average amplification (B) values for site class D for 

each NGA-W2 GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @ 0.2 s, 

1.0 s, and NEHRP site coefficients as a function of PGA.

y = 1.321x-0.225

0

1

2

3

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP Fa

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NGA W1 SP Avg

W2 SA @ 0.2 s

W2 SP Avg 0.5g

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class D) 

y = 2.8165x-0.077

0

1

2

3

4

5

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP Fv

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

NGA W1 MP Avg

W2 SA @ 1.0 s

W2 MP Avg 0.5g

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class D) 

A

B
10

Figure 6. Graphs showing short-period (A) and mid-period average amplification (B) values for site class C for 

each NGA-W2 GMPE, NGA-W2 Avg, and 95 percent NGA-W2 limits (W2 Avg +2 SD), NGA-W1 Avg, W2 SA @0.2 s, 

1.0 s, and NEHRP site coefficients as a function of PGA.

y = 1.4579x-0.041

0

1

2

3

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP Fa

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

NGA W1 SP Avg

W2 SA @ 0.2 s

W2 SP Avg 0.5g

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class C) 

y = 1.8977x-0.007

0

1

2

3

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP Fv

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

NGA W1 MP Avg

W2 SA @ 1.0 s

W2d MP Avg 0.5g

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class C) 

A

B
11

Superimposed on figures 4, 5, and 6, are amplification factors (olive green curves) inferred by 

Kircher and others (written commun., 2014,) at periods of 0.2 s (W2 SA@0.2 s/1,050) and 1.0 s (W2 

SA@1.0 s) from the geometric mean of the response spectra for the various site classes referenced to 

vS30=762 m/s for magnitude and distance parameters yielding (Ss, S1) pairs of (0.25, 0.1), ... (1.25, 0.5) at 

a site condition of vS30=762 m/s. For comparison with the coefficients derived here, the Kircher and others

coefficients are scaled to yield unity amplification at the midpoint (1,050 m/s) of site class B (solid olive 

curves). The dependence of the W2 SA@0.2 s/1,050 factors (Kircher and others, written commun., 2014) 

on amplitude of ground motion is in good agreement with the W2 SP Avg factors (solid blue curves). The

factors at 1.0 s (W2 SA@1.0 s) are consistently about 5 to 10 percent larger than the corresponding W2

MP Avg coefficient. The consistency in the two sets of factors helps confirm the calculations used in each 

procedure.

Figures 4, 5, and 6 show that, for each site class, the NGA-W1 factors (solid purple curves) are 

significantly less than the NGA-W2 amplification factors inferred herein and those inferred for the 

NGA-W2 data set by Kircher and others. The amplification factors, as plotted in figures 1, 2, and 3 by 

Rezaeian (written commun., 2015; Rezaeian and others, 2014) using MATLAB formulations for both the 

NGA-W1 and NGA-W2 GMPEs used to prepare the 2008 and 2014 MCER maps (not shown), also 

indicate that the W2 factors are significantly larger than the W1 factors. This difference also is apparent in 

plots of median Pseudo Spectral Acceleration (PSA) estimates as a function of period provided by the 

NGA developers based on their 2008 and 2014 GMPEs (see, for example, fig. 10, Campbell and 

Bozorgnia, 2014; figs. 6 and 7, Abrahamson and others, 2014).

The percent difference between the coefficients inferred from the NGA-W2 database and those 

from the NGA-W1 database using the same procedures (Borcherdt, 2014b) are summarized in table 1. 

The percent differences vary from 14 to 28 percent for the short-period band and from 19 to 42 percent for 

the mid-period band. These large increases in the amplification factors have significant implications and 

merit further evaluation depending on intended application of the NGA-W2 coefficients.

Table 1. Percent difference average NGA-W2 coefficients exceed average NGA-W1 coefficients.

vS30 (m/s) / PGA (g) 0.1 0.2 0.3 0.4

SP Coefficient Percent Differences

1,050 0 0 0 0

540 14 15 14 15

290 24 24 23 23

150 28 27 26 25

MP Coefficient Percent Differences

1,050 0 0 0 0

540 19 19 19 19

290 30 30 29 29

150 39 41 42 42

The significant difference between the amplification factors implied by the NGA-W2 GMPEs and 

those implied by the NGA-W1 GMPEs is expected to primarily result from changes in the NGA-W2 

GMPE models associated with linear and nonlinear shallow site response (vS30) and changes to account 

for sediment depth. Any increases in the W2 amplification coefficients associated with the larger amount 

of data in the NGA-W2 database could possibly be expected for site class E, but not for other site classes 
12

well populated with W1 data. Additional evaluation is needed to better understand the significant 

increases in site coefficients implied by the recent soil response models used in the NGA-W2 GMPEs and 

their implications for future changes in building codes.

Figures 7, 8, and 9 provide corresponding plots of the NGA-W2 coefficients as a function of 

average shear velocity (v̅

s or vS30) for PGA levels of 0.1 g, 0.3 g, and 0.4 g at the midpoint of site class B.

With the exception of the short-period coefficient (W2 SP Avg) for site class E, the figures illustrate that 

W2 coefficients for site classes are significantly greater than the corresponding NEHRP coefficients.

NGA-W2 Scaling Factors to Reduce Conservatism in Design Spectra and to Scale 

Site Amplification Coefficients to Other Reference Site Conditions

Following adoption of the NEHRP site coefficients in the 1994 edition of the code provisions 

(BSSC, 1994), revised sets of seismic hazard and MCER type design maps were adopted in the 1997 and 

subsequent editions of the code provisions. The v̅

s(vS30) value chosen to specify the site condition for the 

subsequent MCER maps was v̅

s=760 m/s. This value was chosen in 1997 (Frankel and others, 1997) 

because it was considered then as a conservative but typical value to represent firm rock sites (Frankel, 

written commun., 2012). Subsequent compilation of the NGA-W2 database shows that the median value 

for rock sites in site class B is near 913 m/s (Stewart and Seyhan, 2013). Hence, a vS30 value near 900 m/s 

is more representative of a typical firm rock condition than is 760 m/s.

Considering that the concept of simplified site classes as defined in terms of vS30 implies that the 

site classes be represented by the midpoint of each site class and referenced to the midpoint of the 

reference site class B with unity amplification (Borcherdt, 1992; 1994), choice of vS30=760 m/s for the 

MCER maps implies that estimates of simplified design spectra as defined in ASCE/SEI 7-10 are 

conservative. This conservatism can be reduced by specifying the vS30 value for the maps at a vS30 value 

more typical of firm to hard rock sites that comprise site class B, for example vS30=900 m/s. A simple 

procedure in lieu of changing the maps is to apply a simple scaling factor to estimates of simplified design 

spectra. The NGA-W2 GMPEs permit the rigorous derivation of such scale factors to allow the MCER

motions to represent other ground conditions, such as those corresponding to vS30=900 m/s or vS30=760

m/s.

Considering that the simplified design spectra as defined in ASCE/SEI 7-10 are the product of the 

MCER motions and the site coefficients in tables 11.4.1 and 11.42 of ASCE/SEI 7-10, a scale factor 

chosen to scale the MCER input motions may also be regarded as a factor to scale the site coefficients to a 

different reference ground condition. Corresponding scale factors and site-class amplification coefficients 

for reference ground conditions of 1,050 m/s, 900 m/s, and 760 m/s are provided in tables 2–6.

Table 2 provides estimates of the average NGA-W2 short-period (SP) and mid-period (MP) 

spectral amplifications inferred at 760 m/s and 900 m/s with respect to the reference site condition of 

vS30=1,050 m/s for PGA=0.1–0.4g. Table 2 shows the W2 SP Avg and W2 MP Avg values are nearly 

constant as a function of reference PGA level with the estimates for 900 m/s showing the least variation. 

The corresponding standard deviations (SD) inferred from the four NGA-W2 Avg estimates indicate the 

epistemic or model uncertainty in the 900 m/s estimates is small and about 50 to 60 percent less than that 

for 760 m/s. The table shows that the scale factors show little or no dependence on PGA level. Hence for 

purposes of maintaining a simplified procedure, the mean of the scale factors derived at each PGA level, 

as shown in the right hand column of table 3, provides a robust estimate of the desired scale factors. A 

summary of the mean scaling factors for the short- and mid-period bands, as derived from the reciprocal 

of the corresponding values in table 2, are provided in table 3.
13

Figure 7. Graphs showing short-period coefficients (A) and mid-period coefficients (B) at PGA=0.1 g as a function 

of vS30 for each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients.

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Short-Period Amplification 

vS30 (m/s)

Short-Period Avg. Amplification versus Vs30

(PGA=0.1 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Mid-Period Amplification 

vS30 (m/s)

Mid-Period Avg. Amplification versus Vs30

(PGA=0.1 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

A

B
14

Figure 8. Graphs showing short-period coefficients (A) and mid-period coefficients (B) at PGA=0.2 g as a function 

of vS30 for each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients.

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Short-Period Amplification 

vS30 (m/s)

Short-Period Avg Amplification versus vS30

(PGA=0.2 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Mid-Period Amplification 

vS30 (m/s)

Mid-Period Avg Amplification versus vS30

(PGA=0.2 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

A

B
15

Figure 9. Graphs showing short-period coefficients (A) and mid-period coefficients (B) at PGA=0.4 g as a function 

of vS30 for each NGA-W2 GMPE, NGA-W2 Avg, NGA-W2 Avg +2 SD, and NEHRP Fa and Fv site coefficients.

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Short-Period Amplification 

vS30 (m/s)

Short-Period Avg Amplification versus vS30

(PGA=0.4 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Mid-Period Amplification 

vS30 (m/s)

Mid-Period Avg Amplification versus vS30

(PGA=0.4 g) 

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

NEHRP

A

B
16

Table 2. Average NGA-W2 SP and MP Avg spectral amplifications and standard deviations for vS30=760 m/s and 

900 m/s as inferred with respect to 1,050 m/s.

PGA 0.1 g 0.2 g 0.3 g 0.4 g Mean W2 Avg

vS30 (m/s) W2 SP 

Avg SD W2 SP Avg SD W2 SP Avg SD W2 SP Avg SD SP Avg

760 1.27 0.05 1.26 0.05 1.25 0.05 1.25 0.06 1.26

900 1.12 0.02 1.11 0.02 1.12 0.02 1.12 0.02 1.12

vS30 (m/s) W2 MP 

Avg SD W2 MP Avg SD W2 MP Avg SD W2 MP Avg SD MP Avg

760 1.37 0.04 1.37 0.04 1.37 0.04 1.37 0.04 1.37

900 1.15 0.03 1.15 0.03 1.15 0.03 1.12 0.03 1.15

Table 3. Mean NGA-W2 SP and MP scale factors as inferred from table 2.

Ratio Mean W2 Avg W2 SP Avg W2 MP Avg

W2 Avg 900/W2 Avg 1,050 0.90 0.87

W2 Avg 760/W2 Avg 1,050 0.80 0.73

W2 Avg 760/W2 Avg 900 0.89 0.84

Table 4. NGA-W2 SP and MP Avg site coefficients referenced to vS30=1,050 m/s.

Site Class/PGA 0.1 g 0.2 g 0.3 g 0.4 g 0.5 g

SP Avg site coefficients

A 0.80 0.80 0.80 0.80 0.80

B 1.00 1.00 1.00 1.00 1.00

C 1.60 1.56 1.53 1.51 1.50

D 2.20 1.92 1.74 1.60 1.54

E 2.61 1.80 1.38 1.12 1.00

MP Avg site coefficients

A 0.80 0.80 0.80 0.80 0.80

B 1.00 1.00 1.00 1.00 1.00

C 1.93 1.92 1.91 1.91 1.91

D 3.35 3.20 3.10 3.01 2.97

E 5.57 4.82 4.32 3.96 3.79
17

Table 5. NGA-W2 SP and MP Avg site coefficients referenced to vS30=900 m/s.

Site Class/PGA 0.1 g 0.2 g 0.3 g 0.4 g 0.5 g

SP Avg Site coefficients

A 0.71 0.72 0.72 0.72 0.72

B 0.89 0.90 0.90 0.90 0.90

C 1.43 1.41 1.37 1.35 1.34

D 1.96 1.74 1.56 1.44 1.38

E 2.33 1.62 1.24 1.00 0.90

MP Avg site coefficients

A 0.69 0.69 0.69 0.69 0.69

B 0.87 0.87 0.87 0.87 0.87

C 1.67 1.66 1.66 1.66 1.65

D 2.90 2.78 2.68 2.61 2.58

E 4.83 4.18 3.75 3.43 3.29

Table 6a. NGA-W2 SP Avg site coefficients referenced to vS30=760 m/s in parentheses and corresponding Fa site 

coefficients adopted in table 11.4-1 ASCE/SEI 7-10.

Site Class Ss<0.25 Ss=0.50 Ss=0.75 Ss=1.00 Ss>1.25

A (0.63) 0.8 (0.64) 0.8 (0.64) 0.8 (0.64) 0.8 (0.64) 0.8

B (0.79) 1 (0.79) 1 (0.80) 1 (0.80) 1 (0.80) 1

C (1.26) 1.2 (1.24) 1.2 (1.22) 1.1 (1.21) 1.0 (1.20) 1.0

D (1.74) 1.6 (1.53) 1.4 (1.39) 1.2 (1.28) 1.1 (1.23) 1.0 

E (2.06) 2.5 (1.43) 1.7 (1.10) 1.2 (0.89) 0.9 (0.80) 0.9

F see section 11.4.7

Use straight-line interpolation for intermediate values of Ss.

Use straight-line interpolation for intermediate values of vS30 using site class mid-point value for vS30 from chapter 20.

Table 6b. NGA-W2 MP Avg site coefficients referenced to vS30=760 m/s in parentheses and corresponding Fv site 

coefficients adopted in table 11.4-2, ASCE/SEI 7-10.

Site Class S1<0.1 S1=0.20 S1=0.30 S1=0.40 S1>0.50

A (0.58) 0.8 (0.58) 0.8 (0.59) 0.8 (0.59) 0.8 (0.59) 0.8

B (0.73) 1 (0.73) 1 (0.73) 1 (0.73) 1 (0.73) 1

C (1.41) 1.7 (1.40) 1.6 (1.40) 1.5 (1.40) 1.4 (1.40) 1.3

D (2.45) 2.4 (2.34) 2.0 (2.26) 1.8 (2.20) 1.6 (2.17) 1.5

E (4.07) 3.5 (3.53) 3.2 (3.16) 2.8 (2.90) 2.4 (2.78) 2.4

F see section 11.4.7

Use straight-line interpolation for intermediate values of S1.

Use straight-line interpolation for intermediate values of vS30 using site class mid-point value for vS30 from chapter 20.
18

Table 3 shows that average scaling factors applied to reduce the MCER short- and mid-period 

motions to that at the midpoint of site class B (1,050 m/s) or equivalently average scale factors applied to 

reduce the NGA-W2 Avg site coefficients to that at 760 m/s would imply that the resultant design spectra 

would be reduced by 20 percent in the short-period band and 27 percent in the mid-period band. Table 3 

shows the corresponding average scale factors to reduce the MCER short- and mid-period motions to a 

more typical rock condition of 900 m/s would imply reductions of 11 and 16 percent, respectively.

Tables 4, 5, and 6 provide NGA-W2 Avg site coefficients for reference ground conditions 

specified by vS30=1,050 m/s, 900 m/s, and 760 m/s as inferred using the scale factors specified in table 2 

for various PGA levels. The tables show that referencing the SP W2 site coefficients to 900 m/s and 760 

m/s reduces the corresponding SP coefficients referenced to 1,050 m/s by average percentages of 10 and 

20 percent, respectively. Corresponding average reductions in the MP coefficients are 13 and 27 percent, 

respectively.

Applying scale factors to reference either the MCER motions to 900 m/s or the W2 Avg site 

coefficients in table 4 to 900 m/s reduces some of the conservatism in the design spectral estimates, but 

not all. Applying scale factors to reference the MCER motions and the W2 Avg site coefficients to the 

same reference ground condition eliminates all conservatism in design spectral estimates. Using the scale 

factors to reference both the MCER motions and the W2 Avg coefficients to either 760 m/s or 900 m/s will 

yield the same design spectral estimate.

Implications of NGA-W2 for ASCE 7-10 (NEHRP) Site Coefficients 

With the exception of the NEHRP Fa site coefficients at 0.1 g and 0.2 g for site class E (vS30=150 

m/s), figures 4–9 and table 4 show that the NGA-W2 Avg amplification coefficients referenced to 

vS30=1,050 m/s are significantly greater than the NEHRP coefficients and the corresponding coefficients 

inferred from the NGA-W1 database. Similar conclusions derived by Kircher (2015) based on results 

derived independently suggest further evaluation of the W2 results derived using default sediment depth 

parameters for each of the NGA-W2 GMPEs may be needed. If further evaluation establishes that the

NGA-W2 results derived using the default parameters are justified, tables 4–6 provide alternatives for 

consideration in future versions of the codes depending on the consensus of the engineering community. 

Table 6 provides W2 SP Avg and W2 MP Avg coefficients in parentheses as scaled to a reference site 

condition of 760 m/s in the format of tables 11.4.-1 and 11.4-2 of ASCE 7-10 together with the 

corresponding NEHRP coefficients as adopted in ASCE 7-10.

The procedures used here to derive the NGA-W2 Avg site coefficients as normalized to 760 m/s, 

900 m/s, and 1,050 m/s are intended to be consistent with the concepts of unambiguous site classes and a

simplified design spectrum as adopted in ASCE/SEI 7-10 (2010). The procedures capture the full 

epistemic or model uncertainty associated with the NGA-W2 GMPEs. 

Tables 4–6 imply that if site coefficients are desired as a continuous function of site conditions as 

specified by (v̅

s) as opposed to discrete values as specified for the five site classes, then the site 

coefficients should be inferred by straight-line interpolation using the site coefficients as specified at the 

midpoint of each site class interval. Linear interpolation provides a simple, but reasonably good 

approximation for the coefficients at intermediate values of v̅

s. It also has the advantage that no additional 

inconsistencies are introduced between adopted site coefficients and those implied directly by simple 

averages of the values predicted by the four NGA-W2 GMPEs. This simple procedure ensures that site 

coefficients predicted continuously will agree exactly with those tabulated at the midpoint v̅

s value for 

each site class. It ensures that the proposed NGA-W2 site coefficients are consistent with the MCER maps. 
19

Comparison of NGA-W2 Coefficients with Those Proposed for ASCE 7-16

Site amplification factors derived from a single GMPE SS14 (Stewart and Seyhan, 2013; Seyhan 

and Stewart, 2014) have been recommended for adoption in ASCE 7-16. Figures 10–13 provide plots of 

coefficients proposed for ASCE 7-16, which are labeled “SS14/760” (solid pink curves). These 

coefficients, scaled to a reference vS30 value of 1,050 m/s, are labeled “SS14/1050” (dashed pink curve). 

The percent differences between the SS14/ 760 and NGA-W2 Avg coefficients scaled to 760 m/s (tables 

6a and 6b) are tabulated in table 7.

Table 7. Percent difference between NGA-W2 Avg/760 short- and mid-period coefficients and those proposed 

for ASCE 7-16 (SS14/760).

vS30 (m/s) / PGA (g) 0.1 0.2 0.3 0.4 0.5

SP coefficient percent differences

1,050 −13.9 −13.4 −12.9 −12.6 −12.6

540 −2.8 −4.8 1.7 0.7 -0.1

290 7.8 8.3 13.4 14.2 19.0

150 −16.5 −19.0 −17.8 −23.0 −12.2

MP coefficient percent differences

1,050 −13.9 −13.4 −12.9 −12.6 −12.6

540 −2.8 −4.8 1.7 0.7 −0.1

290 7.8 8.3 13.4 14.2 19.0

150 −16.5 −19.0 −17.8 −23.0 −12.2

For site class E, figure 10 shows that the SS14/760 short-period factors referenced to 760 m/s are 

nearly the same as the NEHRP Fa and NGA-W2 Avg SP/1050 coefficients, both of which are normalized 

to 1,050m/s. The SS14/760 short-period factors scaled to 1,050 m/s (SS14/1050) for site class E are larger 

than the corresponding NGA-W2 factors derived with respect to 1,050m/s for each of the individual 

NGA-W2 GMPEs. The SS14/760 factors are less than the corresponding NGA-W2 Avg/1050 factors for 

the mid-period band for site class E (figure 10) and they show a stronger nonlinear trend, than indicated by 

NGA-W2 GMPEs. Tables 7 indicates that the SS14/760 coefficients proposed for ASCE 7-16 site class E 

differ from the corresponding NGA-W2 Avg/760 coefficients by 17 to 23 percent for the short period 

band and by 3 to 21 percent for the mid-period band depending on base PGA level. 

For site class D, figure 11 also shows that the SS14/760 coefficients differ from those implied by 

the NGA-W2 GMPEs. The scaled SS14/760 coefficients, namely SS14/1050 are less than the 

corresponding individual NGA-W2 Avg coefficients. Table 7 shows that the SS14/760 coefficients 

proposed for ASCE 7-16 site class D differ from the corresponding NGA-W2 Avg coefficients by 8 to 19

percent for the short period band and by 2 to 17 percent for the mid-period band depending on base PGA 

level.

For site class C, figure 12 shows that the SS14/760 coefficients show some significant differences 

from those inferred from the NGA-W2 mid-period coefficients. Table 7 shows that the SS14/760 

coefficients proposed for ASCE 7-16 site class C differ from the corresponding NGA-W2 Avg 

coefficients by 0 to 5 percent for short period band and by 6 to 8 percent for the mid-period band

depending on base PGA level.
20

Figure 10. Graphs showing short-period (A) and mid-period average amplification (B) values as a function of PGA

for site class E for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 

SD), NEHRP site coefficients, SS14/1050 factors, and SS14/760 factors proposed by Stewart and Seyhan, (2014) for 

ASCE 7-16. Curves show that SS14 results differ significantly from NGA-W2 Avg coefficients for site class E.

0

1

2

3

4

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

SS14 SP/760

SS14 SP/1050

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class E) 

0

1

2

3

4

5

6

7

8

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

SS14 MP/760

SS14 MP/1050

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class E) 

B

A
21

Figure 11. Graphs showing short-period (A) and mid-period average amplification (B) values as a function of PGA 

for site class D for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 

SD), NEHRP site coefficients, SS14/1050 coefficients, and SS14/760 coefficients proposed by Stewart and Seyhan, 

(2014) for ASCE 7-16. Percent differences between SS14/760 curves and NGA-W2 Avg/760 coefficients are 

tabulated in table 7.

0

1

2

3

Short

0.1 0.2 0.3 0.4 0.5 0.6

-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

W2 SP Avg+2*SD

SS14 SP/1050

SS14 SP/760

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class D) 

0

1

2

3

4

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

SS14 MP/1050

SS14 MP/760

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class D) 

B

A
22

Figure 12. Graphs showing short-period (A) and mid-period average amplification (B) values as a function of PGA 

for site class C for each NGA-W2 GMPE, NGA-W2 Avg coefficients, 95 percent NGA-W2 limits (NGA-W2 Avg +2 

SD), NEHRP site coefficients, SS14/1050 coefficients, and SS14/760 coefficients proposed by Stewart and Seyhan 

(2014) for ASCE 7-16. Percent differences between SS14/760 curves and NGA-W2 Avg/760 coefficients are 

tabulated in table 7.

0

1

2

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 SP Avg

W2 SP Avg-2*SD

SS14 SP/1050

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class C) 

0

1

2

3

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP

ASK14

BSSA14

CB14

CY14

W2 MP Avg

W2 MP Avg-2*SD

W2 MP Avg+2*SD

SS14 MP/1050

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class C) 

A

B
23

Figure 13. Graphs showing short-period (A) and mid-period average amplification (B) values as a function of PGA 

for site class B for NGA-W2 Avg, SS14 /760, SS14/1050 SP, and MP coefficients proposed by Stewart and Seyhan, 

(2014) for ASCE 7-16. Percent differences between the SS14/760 curves and the NGA-W2 Avg/760 coefficients are 

tabulated in table 7.

0.5

1.0

1.5

0.1 0.2 0.3 0.4 0.5 0.6

Short-Period Amplification (Avg 0.1

–0.5 s)

PGA (g)

NEHRP

W2 SP Avg

SS14 SP/1050

SS14 SP/760

NGA-W2 Average Short-Period Amplification versus PGA

(Site Class B) 

0.5

1.0

1.5

0.1 0.2 0.3 0.4 0.5 0.6

Mid-Period Amplification (Avg 0.5

–2.0 s)

PGA (g)

NEHRP

W2 MP Avg

SS14 MP/1050

SS14 MP/760

NGA-W2 Average Mid-Period Amplification versus PGA

(Site Class B) 

B

A
24

For site class B, figure 13 shows that the SS14/1050 coefficients differ significantly from the 

corresponding NGA-W2 Avg coefficients. Table 7 shows that the SS14/760 coefficients proposed for 

ASCE 7-16 site class B differ from the corresponding NGA-W2 Avg coefficients scaled to 760 m/s

(tables 6a and 6b) by 9 percent for the short-period band and by 13 to 14 percent for the mid-period band 

depending on base PGA level.

Conclusions

A detailed evaluation of the implications of NGA-W2 Avg site coefficients for possible 

modification of the corresponding NEHRP site coefficients as adopted in US building codes (ASCE/SEI 

7-10, 2010) is provided herein. The NGA-W2 Avg coefficients are inferred using procedures that are 

intended to be consistent with the simplified design spectra and the simplified unambiguous site classes 

having a reference site class with unity amplification as adopted in ASCE/SEI 7-10 (2010). The proposed 

coefficients derived from the average of the short- and mid-period amplifications derived from each 

NGA-W2 GMPE capture the full epistemic or model uncertainty associated with the NGA-W2 GMPEs. 

The procedure used to derive the coefficients is suggested as a simple procedure that can be applied each 

time the MCER maps are updated, so as to maintain consistency between the maps and the GMPEs used to 

derive the site coefficients. The NGA-W2 Avg coefficients are consistent with those previously reviewed 

and adopted in numerous versions of the codes.

The site coefficients inferred from the GMPEs for the NGA-W2 database using default parameters 

for sediment depth (Z1 and Z2) are significantly larger than those inferred using a similar procedure 

applied to the GMPEs developed from the NGA-W1 database. These results are consistent with 

independent results derived by Kircher (2015), Rezaeian (written commun., 2015), and those shown in 

figures of Campbell and Bozorgnia (2014) and Abrahamson and others (2014). The reasons for this 

increase merit further investigation.

The NGA-W2 GMPEs are used to develop scaling factors that can be applied to scale the MCER

motions or the NGA-W2 site coefficients to other reference ground conditions. The scale factors, applied 

to scale either the MCER motions or the W2 site coefficients to a reference condition of 900 m/s, reduce 

some, but not all of the conservatism in current design spectral estimates. The scale factors applied to 

convert both the MCER motions and the W2 coefficients to the same reference ground condition eliminate

current conservatism in design spectral estimates. Tables 4, 5, and 6 provide NGA-W2 site coefficients 

referenced to 1,050 m/s, 900 m/s, and 760 m/s.

Site coefficients proposed for ASCE 7-16 are compared in detail with those inferred from each of 

the NGA-W2 GMPEs and the NGA-W2 Avg coefficients. Percent differences between the coefficients 

proposed for ASCE 7-16 and the corresponding NGA-W2 Avg coefficients scaled to 760 m/s (tables 6a 

and 6b) are shown to be significant as summarized in table 7.
25

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