Site-Specific Seismic-Hazard Maps and Deaggregation 

in the Western United States Using the NGA Models 

for Ground-Motion Prediction

By Stephen Harmsen

Open-File Report 2011–1218

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Harmsen, Stephen, 2011, Site-specific seismic-hazard maps and deaggregation in the western United 

States using the NGA models for ground-motion prediction: U.S. Geological Survey Open-File Report 

2011–1218, 67 p.
iii

Contents

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

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

Site Response in GMPEs ........................................................................................................................................... 3

Rock versus Soil in the Hazard Integral...................................................................................................................... 4

Regional Scale Uniform Soil Hazard Maps................................................................................................................. 6

Methods for Estimating Vs30 over Broad Regions ..................................................................................................... 9

Regional Scale Site-Specific Hazard Maps ................................................................................................................ 9

Regional Scale Site-Specific Vs30 Maps for Central California .............................................................................. 9

Regional Scale Site-Specific Hazard Maps for Central California ......................................................................... 15

Regional Scale Site-Specific Vs30 Maps for Southern California.......................................................................... 27

Regional Scale Site-Specific Hazard Maps for Southern California ...................................................................... 31

Deaggregation of Seismic Hazard at Specific Sites.................................................................................................. 44

Case I. Soil Mode Same as Rock Mode................................................................................................................ 45

Case 2. Soil Mode Different from Rock Mode ....................................................................................................... 49

Caveats and Model Limits ........................................................................................................................................ 59

Discussion and Conclusions..................................................................................................................................... 63

Acknowledgments .................................................................................................................................................... 64

References ............................................................................................................................................................... 64

Figures

Figure 1. Schematic diagram of spectral acceleration distribution from two types of sources recorded on 

two types of sites.................................................................................................................................................. 5

Figure 2. Spectral acceleration ratio for sites in the western United States, using the new Next 

Generation of Attenuation ground-motion prediction equations ........................................................................... 7

Figure 3. Regional scale site-specific Vs30 maps for central California using three methods to

determine Vs30.................................................................................................................................................. 10

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz spectral acceleration ............. 16

Figure 5. Regional scale site-specific hazard maps for central California, 3-second spectral acceleration.......... 22

Figure 6. Regional scale site-specific Vs30 maps for southern California using three methods to 

determine Vs30.................................................................................................................................................. 28

Figure 7. Regional scale site-specific hazard maps for southern California, 5-hertz spectral acceleration .......... 32

Figure 8. Regional scale site-specific hazard maps for central California, 3-second spectral acceleration.......... 38

Figure 9. Map showing principal faults in the vicinity of the Los Angeles site over a base map showing

the site-specific 1-second spectral acceleration at 2 percent in 50 years probability of exceedance, 

using method 1 to estimate Vs30....................................................................................................................... 46

Figure 10. Map showing principal faults in the vicinity of a site in San Jose, California, over a contour

map showing site-specific 1-second spectral acceleration with 2 percent in 50 years probability of 

exceedance using method 1 to estimate Vs30................................................................................................... 48

Figure 11. Rock-site hazard near San Rafael: 5-hertz spectral acceleration with 2 percent in 50 years 

probability of exceedance for a site with Vs30 of 760 meters per second.......................................................... 51

Figure 12. Soil-site hazard near San Rafael: 5-hertz spectral acceleration with 2 percent in 50 years 

probability of exceedance for a site with Vs30 of 180 meters per second.......................................................... 52

Figure 13. Harbor Island 5-hertz hazard deaggregation assuming rock site condition....................................... 54
iv

Figure 14. Harbor Island 5-hertz hazard deaggregation assuming soil site condition........................................ 55

Figure 15. Harbor Island 1-second spectral acceleration assuming rock site condition..................................... 57

Figure 16. Harbor Island 1-second spectral acceleration assuming soil site condition ...................................... 58

Figure 17. Scattergram of Vs100 versus Vs30 at or near 50 southern California strong-motion sites ............... 62

Tables

Table 1. Important future earthquake sources in Los Angeles, California........................................................... 45

Table 2. Seismic hazard at San Jose, California. ............................................................................................... 49

Table 3. Comparison of Vs30 values measured by one of three methods (borehole, spectral analysis of 

surface waves, or refraction microtremor) and Vs30 values from three regional model values.......................... 60

List of Abbreviations Used in This Report

μ and σ median and logarithmic standard deviation, respectively, of ground-motion prediction equation

g gravitational acceleration at Earth surface, approximately equal to 9.8 m/s2

GMPE ground-motion prediction equation, also known as attenuation equation

Hz hertz (frequency in cycles per second of seismic waves or building oscillations)

IMW Intermountain West; a boundary that separates the central and western United States based on differences in 

attenuation of seismic waves in the Earth’s continental crust

IP Intermediate-period; generally in the range 0.5 to 2.0 seconds

km kilometer

LP long-period; generally in the range 2.5 seconds or greater

m meter

NEHRP National Earthquake Hazard Reduction Program

NGA Next Generation of Attenuation models (developed for tectonically active regions)

NSHMP National Seismic Hazards Mapping Project (Golden, Colorado)

PE probability of exceedence; probability that a specified ground acceleration or other intensity measure will be 

exceeded in a specified time, such as 50 years. Typically, PE values like 2% in 50 years or 10% in 50 years are 

considered. (Term coined for use in PSHA.)

PGA peak (horizontal) ground acceleration; generally, PGA is the geometric mean of the two horizontal components

PGV peak (horizontal) ground velocity

PSHA probabilistic seismic hazard analysis

SA 5% damped pseudo spectral acceleration

SASW spectral analysis of surface waves

SP short-period; generally in the range 0.01 to 0.4 seconds

Vs30 average shear-wave velocity in upper 30 meters of the soil column
1

Site-Specific Seismic-Hazard Maps and Deaggregation 

in the Western United States Using the NGA Models 

for Ground-Motion Prediction

By Stephen Harmsen

Abstract

The 2008 National Seismic Hazard Mapping Project (NSHMP) update for the 

conterminous United States employs several new ground-motion prediction equations which 

include modern empirical models of linear and nonlinear site response to local and regional

earthquakes. The recent availability of attenuation functions incorporating site conditions via

Vs30 values permits the calculation of site-specific hazard maps for a wide range of spectral 

accelerations. I compare alternative site specific hazard maps using Vs30 values estimated 

according to the methods of Wills and Clahan (2006), Wald and Allen (2007), and Yong and 

others (in press). These maps are presented for 5-hertz (Hz) and 3-second spectral accelerations 

having 2 percent probability of exceedance in 50 years for central California and the western part 

of southern California.

Because these attenuations incorporate nonlinear site response for the larger ground 

motions, the site-specific probabilistic ground motions for the western United States can show 

either increases or decreases with respect to the firm-rock site condition. Furthermore, the 

ground motions on soil can be different from those that are predicted by applying National 

Earthquake Hazard Reduction Program recommendations for adjusting rock values to account 

for the soil column. One finding of this investigation is that at high spectral frequencies, strong 

differences in the site’s Vs30 estimates often result in relatively small differences in probabilistic 

ground motion in western California or other tectonically active regions.

In addition, this report shows how incorporating geologic site condition information 

alters the values of the dominating magnitudes and distances in deaggregation—5-Hz values for 

a site near San Quentin, Calif., and 5-Hz and 1-Hz values for Harbor Island near Seattle, Wash. 

These deaggregations show that the modal event can shift from a larger closer source to a more 

distant, perhaps smaller source when nonlinear soil behavior is explicitly included in the hazard 

integral. The potential shift in the mode when considering the soil column’s effect ought to be 

carefully considered by engineers who select scenario events based in part on the distribution in 

magnitude, distance, and epsilon space.

Introduction

Seismic hazard analysis and seismic risk analysis have always been applied to sites with 

specific local site conditions, such as nearness to active faults, soil column properties, 

topography, and much more. The National Seismic Hazard Mapping Project (NSHMP) has 

traditionally focused its modeling effort on the estimation of probabilistic motion for a specific,

albeit generic, site condition, namely, rock having average shear-wave velocity of 760 meters per 

second (m/s) in the upper 30 m. Procedures for modifying probabilistic rock motion to account 
2

for local site conditions have been traditionally left to other agencies, such as the National 

Earthquake Hazard Reduction Program (NEHRP) and the Federal Emergency Planning Agency, 

and to members of the geotechnical consulting industry.

The NEHRP recommended provisions for seismic regulations include the standard guide 

for converting rock motions to soil motions, for example, table 4.1.2.4ab in Building Seismic 

Safety Council (BSSC) (1997). These tables list factors to apply to sites within NEHRP site 

classes when converting probabilistic rock motion (presumed to be associated with class B rock) 

to soil motion. There are two sets of factors: one for intermediate-period (IP) ground motion and 

one for short-period (SP) ground motion. These factors begin to address variability of response 

with Vs30 (through site classes) and spectral period (two-state period classification). The SP soil 

factors in that standard reference (Building Seismic Safety Council, 1997, table 4.1.2.4a) do not 

go below 0.9, that is, substantial damping of the input rock motion is not recognized. All IP soil 

factors (Building Seismic Safety Council, 1997, table 4.1.2.4b) are ≥ 1.3. The basic underlying 

idea of the BSSC tables is that Vs30 in broad classes (A, B, C, D, and E) controls site response 

and that the response generally goes up with NEHRP site class.

This report performs site-specific hazard analysis using the NSHMP 2008 source model 

and NGA (Next Generation of Attenuation) ground-motion equations for sites in parts of 

California and other western United States locations. Three new NGA equations are used to 

perform site-specific seismic hazard deaggregation. The first equation is the Boore and Atkinson 

(2008) NGA model, hereafter designated BA08. A modified version of the Choi and Stewart 

(2005) site-response model is used in BA08. The second equation is from Chiou and Youngs 

(2008), designated CY08. The CY08 equation incorporates a new site-response model that at 

each input period T captures many salient features of site amplification on soft and firm soil. The 

third equation is from Campbell and Bozorgnia (2008), designated CB08. The CB08 models site 

response using a functional form developed by Walling and others (2008). In contrast to the 

tables in BSSC (1997), μ and σ in the NGA ground-motion prediction equations (GMPEs)

exhibit continuous variation with Vs30, ground-motion input, and other factors. Next Generation 

of Attenuation GMPEs attempt to capture a broad range of observed soil response and in this 

way may be superior to earlier simplified models. In several NGA site-response models, IP soil 

amplification can be unity or less, and SP signal deamplification through the soil column can be 

substantial, in some instances reducing the probabilistic motion to half the input rock motion. To 

perform site-specific hazard analysis, several models of the local site condition, parameterized by 

Vs30, are considered and compared. Some caveats regarding the adequacy of this 

parameterization are discussed in a later section.

Many regional Probabilistic Seismic Hazard Analysis (PSHA) studies have incorporated 

soil response in various ways. Engineers have long requested more sophisticated soil-response 

models than those used in GMPEs predating the NGA. Earlier GMPEs, such as Boore and others

(1997), explicitly model site response as a monotonically increasing amplification with 

decreasing Vs30 but without the nonlinear damping feature. Other earlier GMPEs, such as 

Sadigh and others (1997) and McGuire (2004), include soil-damping features but categorize sites 

broadly (rock or soil) and are not sensitive to the degree of soil firmness or thickness, which are 

often important for assessing variation in soil amplification and damping. To add realism to site 

response modeling, many geotechnical analyses perform exhaustive scenario development, 

where the scenarios vary the soil column properties—for example, shear-wave modulus 

reduction and damping—over their ranges of uncertainty. Such studies tend to entail 

considerable expense and result in amplification factors that are applied to input rock motion.
3

While the approach taken in this study is not designed to replace comprehensive soil-column 

analysis, it exercises the most recently developed capabilities of the NGA equations. In 

particular, this analysis computes probabilistic ground motions “inside the hazard integral,” 

which in some ways is a superior method to subsequent modification of the rock probabilistic 

motion to account for local site conditions (Bazzurro and Cornell, 2004).

Site Response in GMPEs

Seismological investigators have long recognized that local geology underlying the site

and vicinity often strongly affects the character (amplitude, phase, duration) of ground motion 

recorded from near and distant earthquakes. The Loma Prieta earthquake of 1989 provided 

evidence that soft soils—for example, those at the man-made Treasure Island in San Francisco 

Bay—could behave in dramatically nonlinear ways, initially amplifying the seismic signal, then 

liquefying and deamplifying the signal (Chin and Aki, 1991; Hanks and Brady, 1991). A 

consensus view is that much of local geology’s influence may be captured by a few parameters 

such as Vs30, the average shear-wave velocity in the top 30 m, and depth to basement or 

bedrock, variously defined as the depth where 1,000–2,500 m/s sustained shear-wave velocity is 

found (Power and others, 2008). Decades of study of seismic response of many soil columns, in 

both in situ and laboratory settings, has resulted in the development of empirical soil-response 

models, such as Choi and Stewart (2005), whose purpose is to predict peak ground acceleration

(PGA) and spectral acceleration (SA) given the rock PGA at the base of the soil column, 

accounting for both damping dependent on ground motion and shear modulus reduction in the 

soil column.

The most recent NSHMP U.S. seismic-hazard update (Petersen and others, 2008) uses 

three NGA models to model ground vibration from sources in the western United States: BA08, 

CB08, and CY08. The soil-response parts of these three models (briefly outlined above), along 

with other features, are discussed by the respective authors. All of the NGA soil-response models 

contain nonlinear damping features. The BA08 model assumes that only the median ground

motion is affected by local geology, but the CB08 and CY08 models, however, indicate that

damping mechanisms in the soil column also reduce σT, the total aleatory uncertainty in ground 

motion. Heuristically, larger-than-median input motions at the base of the soil column dampen

more, and smaller-than-median ground motions dampen less as they traverse the soil column. A 

fourth NGA relation, that of Abrahamson and Silva (2008), also develops a new model of

nonlinear soil response, but their GMPE was not used in the NSHMP 2008 update (due to time 

constraints) nor is it used in this report.

Nonlinear soil behavior in the NGA models includes a limited degree of ground-vibration 

damping at intermediate spectral periods (IP, 0.5–2 s, and peak ground velocity [PGV]) and 

significant damping at short periods (SP, 0.01–0.3 s, and PGA). For motions in the IP band, soil 

amplification (that is, amplification of the seismic shear-wave input at the base of the soil 

column) is not as strong for high ground-motion input as for low ground-motion input, whereas 

for SP motions, soil de-amplification is significant (reduction of a factor of 2, for example). A 

summary of these effects is graphed in Campbell and Bozorgnia (2008, fig. 13).
4

Rock versus Soil in the Hazard Integral

The basic seismic-hazard equation associated with a specific GMPE may be written

ν[SA > sa0] = ∑ wiλi ∫ Pr[ sa3

sa0

n

i=1 s | Si, Rv]ds (1)

where n is the number of sources, indexed Si, and Rv is the receiver’s local site condition, 

parameterized as Vs30, depth to bedrock, and perhaps other variables.

Further, ν is the mean annual rate of ground-motion exceedances from the set of sources 

{Si} in the model. The integral factor is the probability of exceedance of a specific logged 

ground-motion value, here specified as sa0, and is fully computable for each GMPE. Spectral

acceleration, SA in equation 1, refers in this report to pseudo-spectral acceleration with 5 percent

damping, but more generally it may refer to PGA, PGV, or PGD (peak ground displacement),

along with other measures of motion. The integral’s upper bound is the maximum considered 

ground motion as a function of ground-motion uncertainty, here sa3 = μ + 3σ, and λi is an 

estimate of the rate of occurrence of Si, either the mean annual rate or an adjusted rate based on 

recent history of Si. The basic equation above is in practice made more elaborate by considering 

alternative models of sources that sample the range of informed belief about future hazardous 

earthquakes and their distribution (of magnitude, frequency, etc.), as well as multiple GMPEs

and their weights. In the above representation, a given fault or fault segment may host many 

ruptures, and each rupture is considered a specific source with known magnitude, sense of slip, 

fault geometry, and distance to site and with weight wi relative to other sources in the model. The 

need to separate weights and frequencies results from multiple scenarios of a given magnitude 

earthquake on a fault segment—for example, weights to apply to the set of magnitude (M) 6.5 

earthquakes on fault segment F. Alternatively, equation 1 may be written as a multiple integral 

with dM and dr components, where r is source-to-site distance. The left-side ν is a Poisson rate 

of exceedance and is converted to probability of one or more exceedances of sa0 in T years 

through the standard Poisson formula:

Pr[SA > sa0] = 1 − exp(−νT). (2)

The influence of the recurrence model on this probability is in the event-rate parameter λi, 

a linear factor. The influence of site geology is contained in the conditional probability, which is 

computed for the joint occurrence of Si and Rv. Nonlinearity occurs in the determination of the 

mean response on soil, which is a function of strength of shaking in bedrock at the base of the 

soil column. The NGA GMPEs all model the conditional probability as a normal density 

function, whose parameters depend on source geometry and distance, as well as on site 

properties. As mentioned before, σ is site- as well as source-dependent in most NGA GMPEs 

(exception, BA08).

There is no dynamic or time-varying element to site-response dependencies (for example, 

no potential for modeling degradation of shear strength of the soil column) in the PSHA model. 

For engineering applications that require such dynamic analysis, further geotechnical 

investigation is indicated.

Figure 1 schematically illustrates the joint influence of site and source on the probability 

of exceedance integral. On the left side (A), the bell curves correspond to short-period ground 

motion from a hypothetical close, large, reverse-slip source. The rock curve has ln(μ) and σ of 0 

and 0.6, and the soil curve has ln(μ) and σ of -0.693 and 0.5, respectively. That is, the soil mean 

is half the rock mean, resulting from significant nonlinear soil de-amplification. This graph
5

approximately corresponds to the upper tail of probabilistic 10-Hz SA from an M6.5 earthquake 

at 1-kilometer (km) distance, according to BA08. On the right side (B), the curves correspond to 

motion from a more distant, smaller source. The rock curve has ln(μ) and σ of -1.693 and 0.6, 

and the soil curve has ln(μ) and σ of -1.0 and 0.5, respectively. That is, the soil median is double 

the rock median. Figure 1B illustrates typical linear soil amplification and approximately 

corresponds to the upper tail of 10-Hz SA from an M5.5 earthquake at 10-km distance, according 

to BA08. The soil-site standard deviation σT is 20 percent lower than the rock-site σT in both 

cases, as in CY08.

Figure 1. Schematic diagram of spectral acceleration (SA) distribution from two types of sources recorded 

on two types of sites. A, Upper-tail distribution of short-period (SP) ground-motion from a relatively 

large, nearby source and firm-rock or soft-soil site condition. B, Upper-tail distribution of SP groundmotion from a relatively distant small source and same pair of site conditions. Text gives more concrete 

examples of earthquake size, distance, and spectral period where these distributions may 

approximately occur in nature (or ground-motion prediction equations that describe nature).

The site-response models embedded in the NGA equations are believed to be reasonably 

appropriate for application to crustal sources originating in the western United States. For plate 

subduction or interface sources and deep intraplate sources, earlier source-specific GMPEs are 

used in the NSHMP 2008 PSHA update (Petersen and others, 2008). Interface GMPEs invariably 

use less sophisticated equations for modeling soil nonlinear response than those used in the NGA 

equations. For this article, and at the 2008 deaggregation Web site, we apply the Geomatrix 

interface and intraplate equations (Youngs and others, 1997) at reference Vs30, but we modify 

site response to correspond to the model of BA08 for nonreference Vs30. Other interface 

GMPEs were left as originally written. Several interface GMPEs have site-class sensitivity (for 

example, NEHRP D, CD, C, BC, and B site classes) but not continuous variation with respect to

Vs30. Where different types of sources, such as crustal versus interface, make comparable 

contributions to the hazard, readers should keep in mind that these different levels of 

sophistication in GMPE site-response estimation may influence the relative contributions of 
6

different classes of sources (crustal, interface, intraplate). In a more ideal modeling environment, 

all GMPEs would display comparable nonlinear site response behavior.

Regional Scale Uniform Soil Hazard Maps

The third revision to the 2008 U.S. National Seismic Hazard Map 

(http://earthquake.usgs.gov/hazards/products/conterminous/2008/update_201001/), maintained 

and hosted online by the U.S. Geological Survey (USGS), provides to the public hazard curves 

and hazard values at specific probabilities of exceedance. These PSHA data all utilize source and 

ground-motion models of the most recent NSHMP update (Petersen and others, 2008). For sites 

in the western United States, they are computed for a variety of uniform soil conditions, with 

Vs30 of 180, 259, 360, 537, 760, and 1,150 m/s, using the NGA-West GMPEs and other modern 

GMPEs for noncrustal earthquakes. These Vs30 values were chosen to sample NEHRP site class 

central values and site class boundaries. Besides variation in Vs30, deep-basin information helps 

to define the site response in several of the NGA GMPEs, but this information was not available 

for most sites. The Chiou-Youngs equation requires the depth at which Vs becomes greater than

1 km/s. This depth is denoted Z1 and is, at best, approximately known at most locations. This 

report adopts a recommended default value, which is a function of Vs30, given in equation 1 of 

CY08 (Chiou and Youngs, 2008). The Campbell-Bozorgnia GMPE requires the depth at which 

Vs becomes greater than 2.5 km/s, called Z2.5. For these calculations, we use neutral values

(Z2.5 of 1–2 km) in the absence of comprehensive knowledge of this depth. These neutral values 

are those which yield 0 additional basin effect (CB08; Campbell and Bozorgnia, 2008, equation 

12).

Using the new NGA GMPEs, we compute ratios of probabilistic motion at a given site, 

where the numerator and denominator values result from different Vs30, show the effect of soil 

nonlinear damping, and provide information on differences between NGA predictions and, for 

example, 1997 NEHRP Recommended Provisions for Seismic-Resistant Design. Figure 2A

below is a map showing the ratio of probabilisitic 5-Hz SA with 2 percent in 50-year probability 

of exceedence (PE), where the numerator model assumes a uniform 259 m/s Vs30 and the 

denominator model assumes a uniform 760 m/s Vs30. Figure 2B below shows the corresponding 

ratio of 3.0-s SA. Hazard calculations used in these maps stop at the Intermountain West (IMW), 

because Central and Eastern U.S. (CEUS) earthquake sources are important east of this boundary

and no widely accepted GMPEs are currently available for modeling soil ground motion from 

these sources. Figure 2A shows that sites near the most active faults of California, where 

predicted probabilistic ground motion is largest, have the greatest amount of high-frequency 

ground-motion damping. At such sites, the soil probabilistic motion is about 70 percent to 75

percent of the rock value. On the other hand, the soil column typically amplifies the rock motion 

by 1.5 times or more at sites with lower expected rock ground motion, that is, away from the 

most active faults. Figure 2B shows that even when considering long-period motion, less soil site

amplification is expected to occur near the most seismically active locations than elsewhere. 

Numerous maps available at the 2008 U.S. National Seismic Hazard Map Web site 

(http://earthquake.usgs.gov/hazards/products/conterminous/2008/update_201001/) consider

other spectral periods and soil-to-soil comparisons.
7

Figure 2. Spectral acceleration (SA) ratio for sites in the western United States, using recently developed 

ground-motion prediction equations. The numerator model has uniform Vs30 of 259 m/s and 

denominator has uniform Vs30 of 760 m/s. Ground motions are computed for the 2 percent in 50 year 

probability of exceedance, from the seismic-hazard model of Petersen and others (2008). (IMW,

Intermountain West; east of the IMW boundary, hazard at soil sites is not computed)

A, 5-Hertz or 0.2-second SA ratio.
8

Figure 2. Spectral acceleration (SA) ratio for sites in the western United States, using recently developed 

ground-motion prediction equations. The numerator model has uniform Vs30 of 259 m/s and 

denominator has uniform Vs30 of 760 m/s. Ground motions are computed for the 2 percent in 50 year 

probability of exceedance, from the seismic-hazard model of Petersen and others (2008). (IMW, 

Intermountain West; east of the IMW boundary, hazard at soil sites is not computed)—Continued

B, 3.0-second, or 0.33-Hertz SA ratio. Note: although the color tiles are the same as those of figure 2A, 

their meanings are different.
9

Methods for Estimating Vs30 over Broad Regions

In the United States, California leads the way in the development of statewide Vs30 

maps, using geologically mapped units to estimate Vs30 (Wills and Clahan, 2006) and soil 

hazard probabilistic seismic hazard assessments (Kalkan and others, 2010). Investigators at the 

California Geological Survey continue to refine this model, and the latest available revision of 

the Wills and Clahan model as of April 2010 that is used in this report is a hybrid of the original 

geologic-unit and topographic-slope methods. This hybrid is referred to as “method 1” in this 

report. Wald and Allen (2007) developed a different method (“method 2”) of estimating Vs30 

based on topography and topographic slope, and their method is routinely applied globally, for 

example, in the production of near real-time shake maps following major earthquakes. Wald and 

Allen (2007) discuss differences and merits of method 2 versus the original method 1.

Investigators at the USGS Pasadena office developed a terrain-based shear-wave velocity 

estimation method (“method 3”) that is used to compute Vs30 in California (Yong and others, in 

press). Method 3 uses the average measured or inferred Vs30 in each of 16 terrain classes to 

predict Vs30 anywhere that the terrain class can be determined. In this report, no strong 

preference is attached to any of these regional Vs30 models, and seismic-hazard maps based on 

each of them are presented separately, along with ratio maps.

Regional Scale Site-Specific Hazard Maps

Using Vs30 maps derived from methods 1, 2, and 3, we compute the site-specific hazard 

in two regions: (1) central California, and (2) southern California. Similar site-specific hazard 

mapping is possible in other western U.S. States using method 2, where soil GMPEs are

available. Locally, sometimes significant variations from the site-class and topographic slope 

model Vs30s are known from borehole and related studies, but these Vs30 renderings are

believed to capture the primary features of sedimentary rock and soil Vs30 distribution in 

California, and their differences capture some of the uncertainty, given sparse data. One caveat is 

that Vs30 in these models has an upper limit of 760 m/s, whereas shallow crystalline rock in

parts of the Sierra Nevada Mountains and elsewhere is known to have faster shear-wave velocity 

(see section on “Caveats and Model Limits”).

Regional Scale Site-Specific Vs30 Maps for Central California

Figure 3A–E, below, exhibits Vs30 in central California at a 0.01 ̊(roughly 1-km2) 

sampling. Figure 3A corresponds to the method 1 (primarily geologic-unit) approach, (R. Chen, 

California Geological Survey, written commun., 2010). Figure 3B corresponds to method 2, the 

topographic-slope approach (Wald and Allen, 2007). Figure 3C corresponds to method 3, the 

terrain-based approach (Yong and others, in press). In figure 3A, note the low Vs30 values at 

several locations, for example, Holocene bay mud in the South Bay area and north of San Pablo 

Bay, and the large patch of low-velocity soils in the floodplain of the Sacramento River, in the 

Great Valley south of Sacramento and northwest of Stockton. Several of these low-Vs regions 

are also seen in figure 3B, whereas figure 3C has a higher minimum Vs30 and less correlation 

with figures 3A and 3B. Several significant differences are apparent in figures 3A and 3B, such as

in the Great Valley, where there is little agreement on where the slowest sediments may be 

found, and in the western foothills of the Sierra Nevada Mountains, where method 2 predicts 

considerably slower media.
10

In general, method 2 finds far less extensive area of rock at or near the NEHRP B/C

(Vs30>680 m/s) boundary than method 1. While there may be more soil in the foothills of the 

Sierra Nevada Mountains than is suggested by figure 3A, it is likely to be thin at most locations, 

incapable of amplifying IP to long-period (LP) seismic vibrations. In mountains and hills with 

relatively thin Quaternary soil cover, method 1 for estimating Vs30 may be more appropriate for 

LP amplification, while the USGS method 2 may be more appropriate for SP amplification, 

although it may be that the soil column is too thin to amplify even the SP signal at some such 

locations. The upper limit of Vs30 in method 3 is about 550 m/s, equivalent to a NEHRP middleC site class. Terrain-class Vs30 estimates are defined as the average measured Vs30 in that class 

(Yong and others, in press). The 550 m/s upper limit is likely to be the result of sampling bias;

geotechnical boreholes into harder rock are relatively infrequent. Method 3 is likely to 

underpredict Vs30 in many mountain environments, and therefore overpredict LP site response 

at those sites.

Figure 3. Regional scale site-specific Vs30 maps for central California. (km, kilometers; m/s, meters per 

second; FVC, Filoli Visitor Center, Woodside; MTNS, Mountains)

A, Vs30 map using method 1.
11

Figure 3. Regional scale site-specific Vs30 maps for central California. (km, kilometers; m/s, meters per 

second; FVC, Filoli Visitor Center, Woodside; MTNS, Mountains)—Continued

B, Vs30 map from topographic slope, method 2.
12

Figure 3. Regional scale site-specific Vs30 maps for central California. (km, kilometers; m/s, meters per 

second; FVC, Filoli Visitor Center, Woodside; MTNS, Mountains)—Continued

C, Vs30 map using method 3.
13

Ratio maps of the estimated Vs30 for the region of figure 3 are shown in figures 3D and 

3E. Figure 3D is the ratio Vs30_Wald / Vs30_Wills, and figure 3E is the ratio Vs30_Wald / 

Vs30_Yong. In these figures, ratios that are off scale on the high side plot as dark brown, and 

ratios that are off scale on the low side plot as violet.

Figure 3. Regional scale site-specific Vs30 maps for central California. (km, kilometers; FVC, Filoli Visitor 

Center, Woodside; MTNS, Mountains)—Continued

D, Ratio of Vs30, method 2 divided by method 1, at each location. Violet zones are where the method 2 

estimate is less than 75 percent of the method 1 estimate. Dark brown zones are where the method 2

estimate is more than 140 percent of the method 1 estimate.
14

Figure 3. Regional scale site-specific Vs30 maps for central California. (km, kilometers; FVC, Filoli Visitor 

Center, Woodside; MTNS, Mountains)—Continued

E, Ratio of Vs30, method 2 model over method 3 model. Violet zones are where the method 2 estimate 

is less than 75 percent of the method 3 estimate. Dark brown zones are where the method 2 estimate 

is more than 140 percent of the method 3 estimate.
15

Regional Scale Site-Specific Hazard Maps for Central California

The Vs30 values corresponding to the above maps are used in the hazard calculation 

according to equation 1 to produce the 2 percent in 50-year probability of exceedance (PE) maps 

for 5-Hz spectral acceleration (short-period example) and for 3-s spectral acceleration (longperiod example). Figure 4A shows the 5-Hz map corresponding to the Vs30 of figure 3A, and

figure 4B corresponds to figure 3B. Comparing figures 3A and 4A, we see at best a weak 

correlation of probabilistic ground motion with Vs30 at most sites. For example, on the two sides 

of the San Andreas fault (SAF), the 5-Hz motion is similar, even though Vs30 can be quite 

different. Amplification effects of the very low Vs30 sediments in the Great Valley and near the 

shoreline of South San Francisco Bay are conspicuous for their absence (that is, predicted 

deamplification) in figures 4A and 4B. In figure 3C, the very-low-velocity Holocene Bay muds 

are not modeled with lower Vs30 than surrounding sediments, and consequently, in figure 4C

those locations do not exhibit deamplified short-period strong ground motion. In high-hazard

regions, the effect of relatively soft sediments on the 2 percent in 50-year SP motion is quite 

limited, even though the effect of these sediments on related hazards, such as liquefaction, is 

quite pronounced. One current concern is the safety of the levee system on the Sacramento River 

and tributaries and its ability to withstand seismic events.
16

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz (Hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; FVC, Filoli Visitor Center, 

Woodside; MTNS, Mountains; SAF, San Andreas fault)

A, Site-specific 5-Hz SA for central California, using method 1 Vs30 model.
17

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz (Hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains)—Continued

B, Site-specific 5-Hz SA for central California, using method 2 Vs30 model. 
18

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz (Hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains)—Continued

C, Site-specific 5-hz SA for central California, using method 3 Vs30 model.
19

Figure 4D below is the 5-Hz SA ratio, where the numerator results from using the Wald 

and Allen (2007) model of site-specific Vs30 and the denominator results from using the latest 

available revision of the Wills and Clahan (2006) model of Vs30 in the PSHA calculations. 

Figure 4E has the same numerator, but the denominator is now the result of using the Yong and 

others (in press) Vs30 model in the PSHA calculations. Comparing figure 4D with figure 3D

shows that at high frequencies and in high-SA regions, strong differences in the site’s Vs30 

estimates result in relatively small differences in probabilistic ground motion, but result in large 

differences in low-SA regions. These high- and low-SA regions depend strongly on the SA 

return time, here 2,500 years.

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz (Hz) spectral 

acceleration (SA). (MTNS, Mountains)—Continued

D, Ratio of Vs30, method 2 divided by method 1, at each location. 
20

Figure 4. Regional scale site-specific hazard maps for central California, 5-hertz (Hz) spectral 

acceleration (SA). (km, kilometers; MTNS, Mountains)—Continued

E, Ratio of Vs30, method 2 divided by method 3, at each location. Violet zones are where the method 2 

estimate is less than 75 percent of the method 3 estimate.
21

Figures 5A–C show the 3-s maps corresponding to the Vs30 values of figures 3A–C,

respectively. Three-second SA is a period that may roughly correspond to 30-story-tall building 

resonance. Comparing figures 3A and 5A, or figures 3B and 5B, we see much stronger correlation 

of LP probabilistic ground motion with Vs30 at most sites, compared to SP ground motion and 

Vs30. For example, LP motion is now asymmetric on the two sides of the SAF where soil 

conditions differ markedly. In San Francisco, where tall, long-period buildings are not 

uncommon, there is a strong correlation of Vs30 with probabilistic LP motion. Alluvial valleys 

in the western foothills of the Sierra Nevada Mountains are prominently visible in figure 5A, as 

are the soil patches containing very low Vs30, less than 180 m/s (NEHRP soil class E) in the

Sacramento River floodplain and elsewhere. The floodplain region of the Sacramento River may 

have a few intermediate-to-long-period structures, such as bridges. Most of these soft-soil sites, 

however, are unlikely to have engineering infrastructure with long-period resonance because 

they are mostly farmland and riparian forest. Near San Francisco Bay, multi-use pressures such 

as ecosystem restoration may discourage such construction at many locations. The thickness of 

the very-low-Vs Holocene Bay mud may not be much greater than 30 m in most parts of the San 

Francisco Bay shoreline. For example, the bottom of Bay Mud at well-studied Treasure Island 

occurs at about 29 m depth (Darragh and Idriss, 1997). Intermediate-period SA, such as that at T

(time) = 1.0 s, exhibits stronger correlation with Vs30 than short-period SA but weaker 

correlation with Vs30 than long-period SA, such as the 3-s SA illustrated here.
22

Figure 5. Regional scale site-specific hazard maps for central California, 3-second spectral acceleration 

(SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains; darkest browns, in excess 

of 0.6 g)

A, Site-specific 3-second SA map for central California, using model 1 Vs30 model. 
23

Figure 5. Regional scale site-specific hazard maps for central California, 3-second spectral acceleration 

(SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains; darkest browns, in excess 

of 0.6 g)—Continued

B, Site-specific 3-second SA map for central California, using model 2 Vs30 model.
24

Figure 5. Regional scale site-specific hazard maps for central California, 3-second spectral acceleration 

(SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains; darkest browns, in excess 

of 0.6 g)—Continued

C, Site-specific 3-second SA map for central California, using model 3 Vs30 model.
25

Figure 5D shows the ratio of 2 percent in 50-year PE ground motion at 3-s spectral 

period, where the numerator model uses the site specific Vs30 model of figure 3B and the 

denominator uses the Vs30 model of figure 3A. The mean value of the ratio is 1.58, indicating 

that the substantially lower Vs30 at several locations in the figure 3B model is mapping into 

higher expected LP ground vibrations. Comparing figure 5D and 4D shows that the low-velocity 

sediments amplify the LP signal and deamplify the SP signal at many locations in the 

Sacramento River floodplain, SF Bay shoreline, and elsewhere, according to the NGA siteresponse models used in these calculations. Figure 5E uses the Yong and others (in press) model 

in the denominator instead of the Wills and others (2006) model. The mean value of the 

Wald/Yong ratio is 1.18.

Figure 5. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; MTNS, Mountains)—Continued

D, Ratio of 3-s probabilistic motion in central California, where numerator is calculated using model 2

Vs30 (fig. 3B) and denominator is calculated using model 1 Vs30. The average value of the ratio is 

1.58.
26

Figure 5. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; MTNS, Mountains)—Continued

E, Ratio of 3-s probabilistic motion in central California, where numerator is calculated using method 2

Vs30 (fig. 3B) and denominator is calculated using method 3 Vs30 (fig. 3C). The average value of the 

ratio is 1.18.
27

Regional Scale Site-Specific Vs30 Maps for Southern California

Patterns of correlation of ground motion with Vs30 that are illustrated above are equally 

evident in southern California hazard maps. Figure 6A shows Vs30 for a region in southern 

California from method 1, figure 6B shows Vs30 from method 2, and figure 6C shows Vs30

from method 3. Some of the lowest-velocity sediments in this mapped region are found in the 

eastern Coachella Valley northwest of the Salton Sea and southwest of the SAF. These low 

seismic velocities are associated with the low-lying Salton Trough, which extends to the Mexico 

border. From a risk perspective, some of the most important low-Vs30 areas of figure 6 include 

the Los Angeles and San Bernardino basins, each with Vs30 of about 220 to 300 m/s. Method 2 

predicts greater Vs30 than method 1 in the San Bernardino basin and predicts more rock at or 

near the B/C boundary in the southern part of the map region.
28

Figure 6. Regional scale site-specific Vs30 maps for southern California. (km, kilometers; m/s, meters per 

second; #LA, Los Angeles site [discussed below]; CCD, Carbon Canyon Dam; COE, Colton 

Interchange E.; FT, Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring 

St.; LBW2, Long Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood strikeslip fault system; OSI, Osito Audit–Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, 

Palos Verdes strike-slip fault; TAR, Tarzana; TOV, Thousand Oaks [discussed below])

A, Vs30 map for a region in southern California, using method 1.
29

Figure 6. Regional scale site-specific Vs30 maps for southern California. (km, kilometers; m/s, meters per 

second; #LA, Los Angeles site [discussed below]; CCD, Carbon Canyon Dam; COE, Colton

Interchange E.; FT, Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring 

St.; LBW2, Long Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood strikeslip fault system; OSI, Osito Audit–Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, 

Palos Verdes strike-slip fault; TAR, Tarzana; TOV, Thousand Oaks [discussed below])—Continued

B, Vs30 map for a part of southern California, from the topographic slope method 2.
30

Figure 6. Regional scale site-specific Vs30 maps for southern California. (km, kilometers; m/s, meters per 

second; #LA, Los Angeles site [discussed below]; CCD, Carbon Canyon Dam; COE, Colton 

Interchange E.; FT, Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring 

St.; LBW2, Long Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood strikeslip fault system; OSI, Osito Audit–Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, 

Palos Verdes strike-slip fault; TAR, Tarzana; TOV, Thousand Oaks [discussed below])—Continued

C, Vs30 for southern California when using method 3.
31

Regional Scale Site-Specific Hazard Maps for Southern California

Figures 7A–C, below, exhibit the site-specific 5-Hz SA with 2 percent in 50-year PE for 

the same region as figure 6. As in central California, the 5-Hz probabilistic motion is reasonably 

symmetric around the San Andreas fault, even though media on the two sides of the fault may be 

firm rock versus relatively soft soil. Significant deamplification of the 5-Hz motion is evident in 

the eastern Coachella Valley on the southwest side of the SAF, where sediment Vs30 is less than 

220 m/s. Probabilistic motion at 5 Hz is relatively symmetric on the two sides of the much less

active Elsinore fault, even though at several locations the shallow portions of the respective 

blocks are rock and soil. The probabilistic 5-Hz motion is not amplified in the Los Angeles basin 

compared to neighboring regions with higher Vs30 values, in spite of the relatively low shallow 

shear-wave velocity in the LA basin. Figure 7D shows the ratio of 5-Hz SA using the data of 

figure 7B divided by the data of figure 7A to define the ratio. There are few locations where 

figures 7A and 7B differ significantly, even though there are many places where the 

corresponding Vs30 values differ significantly. It is not surprising that where one method 

predicts rock and the other predicts soil, differences can be significant. Rock at the B/C boundary 

has little nonlinear SP attenuation, whereas most soils have substantial SP attenuation of strong 

ground motion. Damping mechanisms in soil counter the amplification associated with lower 

rigidity and lower Vs30, yielding a reasonably stable SP motion estimate (0.9 < ratio < 1.1) in 

figure 7D. Figure 7E also compares 5-Hz SA, where the numerator again uses the method 2

Vs30 and denominator uses the method 3 Vs30.
32

Figure 7. Regional scale site-specific hazard maps for southern California, 5-hertz (Hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; OSI, Osito Audit–Castaic Lake Dam; PHBT, 

Puente Hills blind thrust fault system; TOV, Thousand Oaks)

A, Site-specific 5-hz SA in southern California corresponding to figure 6A (method 1 Vs30 model).
33

Figure 7. Regional scale site-specific hazard maps for central California, 5-Hertz (hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

B, Site-specific 5-hz SA in southern California corresponding to figure 6B (method 2 Vs30 model).
34

Figure 7. Regional scale site-specific hazard maps for central California, 5-Hertz (hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

C, Site-specific 5-hz SA in southern California corresponding to figure 6C (method 3 Vs30 model).
35

Figure 7. Regional scale site-specific hazard maps for central California, 5-Hertz (hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

D, Ratio of probabilistic site specific 5-hz SA, where numerator uses Vs30 from method 2 and 

denominator uses Vs30 from method 1.
36

Figure 7. Regional scale site-specific hazard maps for central California, 5-Hertz (hz) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

E, Ratio of probabilistic site specific 5-hz SA, where numerator uses Vs30 from method 2 and 

denominator uses Vs30 from method 3.
37

Figures 8A–C exhibit the site-specific 3-s SA for 2 percent in 50-year PE for a part of

southern California. These figures, like the corresponding figures for central California, often 

show an asymmetric ground motion distribution around the major faults that is strongly 

correlated with Vs30. For example, the 3-s motion in the eastern Coachella Valley southwest of 

the SAF is significantly greater than at corresponding distances northeast of the SAF, at least in 

figures 8A and 8B. The LP probabilistic motions at sites between the SAF and San Jacinto fault

are some of the highest in the state, but the corresponding short-period motions are only high, not 

extremely high. The Coachella Valley is a region of special concern because the Coachella 

segment of the SAF is believed to be near the end of its seismic cycle, likely to rupture in the 

next few decades (Fialco, 2006; Weldon and others, 2008). Between the Elsinore and San Jacinto 

faults (the Perris block), various soil patches with an approximate 5-km radius are quite evident 

in the 3-s maps but are not apparent in the 5-Hz maps. In the vicinity of the San Andreas, San 

Jacinto, and Elsinore faults, Vs30 differences evident in figures 6A and 6B map into LP groundmotion differences in figures 8A and 8B. Near Long Beach, Calif., figures 8A and 8B exhibit

several locations with higher motion than figure 8C because of the low-Vs sediments in figures

6A and 6B, but not in 6C.
38

Figure 8. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)

A, Site-specific 3.0-s SA in southern California, using method 1 Vs30.
39

Figure 8. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

B, Site-specific 3.0-s SA in southern California, using method 2 Vs30.
40

Figure 8. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

C, Site-specific 3.0-s SA in southern California, using method 3 Vs30.
41

To further explore sensitivity of LP motion to differences in Vs30 estimates, figures 8D

and 8E exhibit 3-s SA ratios for this southern California region. Figure 8D, SA from method 2 

over SA from method 1, shows 5–40 percent larger LP SA in the greater Los Angeles urban 

region when using the Wald and Allen (2007) estimate of Vs30 versus using the hybrid Wills 

and Clahan (2006) estimate. The greatest differences in the greater LA region occur between the 

Newport-Inglewood and Palos Verdes faults. Strong LP SA differences, sometimes in excess of 

100 percent (dark brown spots in figure 8D) are found in parts of the Perris block and elsewhere. 

Figure 8E, SA from method 2 over SA from method 3, shows less variability in much of the 

greater Los Angeles region between Wald and Allen (2007) and Yong and others (in press). 

Again, the Wald and Allen method predicts lower Vs30 and therefore greater 3-s SA than Yong 

and others between the Newport-Inglewood and Palos Verdes faults.
42

Figure 8. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

D, Ratio of site-specific 3.0-s SA, where numerator uses method 2 to estimate Vs30 and denominator 

uses method 1.
43

Figure 8. Regional scale site-specific hazard maps for central California, 3-second (s) spectral 

acceleration (SA). (km, kilometers; g, acceleration of gravity, 9.8 m/s2; CCD, Carbon Canyon Dam; FT, 

Fort Tejon site; IKF, Idyllwild Keenwild Fire Station; LAFR, Los Angeles–N. Faring St.; LBW2, Long 

Beach; LLS, Fountain Valley [next to Santa Ana River]; N-I, Newport-Inglewood fault; OSI, Osito Audit–

Castaic Lake Dam; PHBT, Puente Hills blind thrust fault system; PV, Palos Verde fault; SAF, San 

Andreas fault; TOV, Thousand Oaks)—Continued

E, Ratio of site-specific 3.0-s SA, where numerator uses method 2 to estimate Vs30 and denominator 

uses method 3.
44

Deaggregation of Seismic Hazard at Specific Sites

The strong effect that local site conditions have on probabilistic motion, generally for 

lower velocity sediments to amplify strong long-period motion and to deamplify strong shortperiod motion, suggests that the influence of different earthquake sources is highly dependent on 

the site condition and that this influence is also highly dependent on the spectral period or 

periods of interest. Thus, seismic hazard deaggregations should explicitly consider local shallowsoil effects (and deep basin effects as well) wherever possible. The USGS Geologic Hazards 

Science Center maintains an easy-to-use online seismic hazard deaggregation tool in which local 

Vs30 is one of the input parameters: https://geohazards.usgs.gov/deaggint/2008/. The source and 

GMPE models of the NSHMP model described in Petersen and others (2008) are built into this

tool.

An early working hypothesis of engineers who required site-specific seismic hazard 

deaggregations is that the deaggregation for a soil site should be essentially the same as that for a 

rock site at the same location but with appropriately scaled ground motion, for example, using 

the NEHRP soil factors above. Implicit in this hypothesis is the assumption that nonlinear soil 

response should affect contributions from all sources in the model, if at all, to approximately the 

same degree. However, nonlinear soil damping is a far more important factor when predicting 

strong motion from the closer, larger source than weaker motion from the more distant or smaller 

source, as schematically shown in figure 1. In tectonically active regions with many well 

characterized faults, the nearest fault to the site is often the dominant source in seismic hazard 

deaggregations on rock. While explicit consideration of the soil column’s effect does not tend to 

overturn this generalization, soil’s nonlinear response often significantly modifies the relative 

contribution of this modal source. The ratio of the modal source rate to the total rate of 

exceedances is called the mode participation factor (MPF). In some instances, the modal (M,R) 

shifts from this nearest large source to a more distant source. This report examines some of these 

instances of reduced MPF and cases involving a significant shift in the modal (M,R).

As illustrated in figure 1, the probability of exceeding exp(0.0) = 1.0 g (acceleration of 

gravity, 9.8 m/s2

) is considerably greater for the rock site (left; 0.5 versus 0.083), but it is 

considerably greater for the soil site (right; 0.023 versus 0.0024). The modal event is the source 

that produces the most ground-motion exceedances, which from equation 1 is a product of 

frequency of the event and probability of exceedance, given the occurrence of that event. Under 

the assumption that the frequency of the smaller, more distant source is greater than that of the 

larger source, the SA (and return time) where the modal event switches to the larger event is 

delayed when considering site-specific soil conditions because of SP vibration damping in the 

soil column. For the hypothetical data of figure 1, this switch may occur at about 1-g SA at a soil 

site if the smaller, more distant source has about 21 times the annual frequency of the larger, 

closer source. At a rock site, with all other things equal, this switch will occur at about 0.44 g. If 

the mean return time of 0.44 g is 100 years at the rock site, the mean return time of 1 g at the soil 

site (given just these two sources) is 182 years, representing a significant delay in the modalevent switchover to the larger, closer source. In the NSHMP 2008 PSHA model, nonlinear

effects are not quite as dramatic as those illustrated in figure 1, which uses round numbers to

keep the discussion simple.

This section describes two general cases encountered in the deaggregation of the NSHMP 

2008 SH model: (1) sites where the modal event for soil and rock site conditions are the same, 

but the relative contribution of the soil-site mode is less than the rock-site mode, and (2) sites 
45

where the modal event actually shifts to a more distant source when site condition is taken into 

account inside the integral (equation 1) instead of assuming the default rock site condition.

Case I. Soil Mode Same as Rock Mode 

Current seismic-resistant design practice bases the design spectrum on a spectral

amplitude at two periods, T=0.2 s (SP) and T=1.0 s (IP). In this article, we select these two 

periods to illustrate soil versus rock deaggregations. The online tool mentioned above allows 

users to specify T, from 0.1 to 5.0 s, and PGA.

The first example of soil versus rock probabilistic seismic hazard deaggregation 

corresponds to a site in the Los Angeles Basin. The California Geological Survey compiled 

several versions of Vs30 for the entire State; soil-site Vs30 estimates are taken from the latest of 

these models (Branum and others, 2008). The site is at 34.05 ̊N, 118.25 ̊W, near the Disney 

Concert Hall. Vs30 is estimated to be 285 m/s (NEHRP class D soil). The Vs30 estimate from 

figure 6B is 360 m/s (NEHRP class C/D). Borehole measurements at a nearby California 

Geological Survey strong-motion site, LA Temple and Hope at 34.059 ̊N, 118.246 ̊W, yield

Vs30 of 376 m/s. Table 1 shows the spectral accelerations having 2 percent in 50-year PE and 

the principal fault sources contributing to SP and IP hazard at this site, given rock or soil site 

conditions. Also tabulated are the percentage of SA exceedances contributed by the principle 

sources conditioned on rock or soil site and the corresponding epsilon (εo) values. Figure 9

shows the hazardous faults in the vicinity of the LA Temple and Hope site. The longest brown 

curve in figure 9 is the Puente Hills Connected fault.

Table 1. Important future earthquake sources in Los Angeles, California.

[m/s, meters per second; SA, spectral acceleration; s, second; g, acceleration of gravity; Char., characteristic rupture 

model; avg.(εo), average epsilon; GMPE, ground-motion prediction equation]

Site Vs30 (m/s) 760 285

SA at 0.2 s (g) 2.306 1.868

Main source: Elysian Park Char. 27.1% 20.8%

Main source 2: Puente Hills 1&2 Char.* 17.3% 12.6%

Avg(εo): all sources, all GMPEs 1.03 1.36

SA at 1.0 s (g) 0.673 1.188

Main source: Elysian Park Char. 27.1% 29.7%

Main source 2: Puente Hills 1&2 Char.* 17.7% 16.7%

Avg(εo): all sources, all GMPEs 1.10 1.16

*Puente Hills 1&2 is a pooled contribution from the Los Angeles segment and the all-segment Puente Hills 

characteristic rupture models.
46

Figure 9. Map showing principal faults in the vicinity of the Los Angeles (LA) site. Traces of surface 

rupturing faults are depicted in blue; blind thrusts are brown. Base map shows the site-specific 1-

second spectral acceleration at 2 percent in 50 year probability of exceedance, using method 1 to 

estimate Vs30. (km, kilometer; g, acceleration of gravity, 9.8 m/s2; PH, Puente Hills blind thrust fault 

system)

For SP hazard, table 1 shows that the biggest contributor (perhaps the modal source,

depending on how the deaggregation is performed) and the second biggest contributor both have 

significantly reduced influence on the soil hazard compared to the rock hazard. The higher 

avg(εo) for soil versus rock motion with specified probability of exceedance is a manifestation of 

the reduction of importance of nearby sources on soil: more distant (higher εo) sources take up 

the slack. For the IP hazard, the biggest contributor’s percentage actually increases, whereas the 

second biggest contributor’s percentage decreases slightly. The reason for this unusual behavior 

is that the Elysian Park characteristic event has a magnitude (M) of 6.58 whereas the Puente 

Hills sources measure M6.87 and M7.05, respectively (averaged over logic tree branches). These 

larger-magnitude sources at about the same distance as the Elysian Park source produce reduced 

amplification in the soil column compared to more distant and (or) smaller sources for IP 
47

response as well as SP response. (The Whittier Narrows M5.9 mainshock of October 1987 

ruptured part of the Elysian Park fault.) What takes up the slack as the main contributors decline 

in importance because of nonlinear damping of spectral response? At this site, more distant 

sources, such as the Raymond fault, the Puente Hills blind thrust–Santa Fe Springs segment, the 

Hollywood fault, and others become more important contributors when the soil response is 

included in the probability integral (equation 1). All of these neighboring faults have comparable 

activity rates, or recurrence intervals, as the ones listed. Smith and Harmsen (2010) present a 

new graphical tool that shows the changing relative influence of different sources with increasing 

ground motion, which at soil sites is caused in large part by nonlinear site response, at least for 

shorter-period hazard.

The second example of rock versus soil deaggregation corresponds to a site in San Jose, 

Calif., in the Santa Clara Valley. Like Los Angeles, San Jose has several active Quaternary faults 

in the immediate vicinity, though not as many as L.A. In contrast to L.A., where much of the 

hazard is from blind thrusts and other reverse faults, most of the hazard in San Jose is associated 

with right-lateral strike-slip faults. Less than 2 percent of the hazard comes from the Monte 

Vista/Shannon reverse fault system. The site is at 37.35 ̊N, 121.9 ̊W. The Vs30 estimate is 285 

m/s, as in L.A. The three main faults contributing to hazard at San Jose are (1) the HaywardRodgers Creek, (2) the northern San Andreas, and (3) the Calaveras. Figure 10 shows the site 

and the neighboring hazardous faults. For the purposes of discussion, the figure combines several 

rupture-model alternatives for each of these: Hayward N+Hayward S and Hayward S for the 

Hayward, all Calaveras ruptures containing the central segment for the Calaveras, and the most 

important a priori and moment-balanced models, respectively, for the northern San Andreas 

fault. These fault models are discussed further in Field and others (2009). Table 2 shows the 

contributions assuming the site is underlain by class BC rock or class D soil.
48

Figure 10. Map showing principal faults in the vicinity of a site in San Jose, California. The contour map 

shows site-specific 1-second spectral acceleration (SA) with 2 percent in 50 years probability of 

exceedance using method 1 to estimate Vs30. (N, north; S, south; km, kilometer; g, acceleration of 

gravity, 9.8 m/s2)
49

Table 2. Seismic hazard at San Jose, California.

[m/s, meters per second; SA, spectral acceleration; s, second; g, acceleration of gravity; SAF, San Andreas fault; 

avg.(εo), average epsilon; GMPE, ground-motion prediction equation]

Site Vs30 (m/s) 760 285

SA at 0.2 s (g) 1.794 1.792

Source 1: Hayward 24.8% 24.4%

Source 2: SAF 9.1% 9.0%

Source 3: Calaveras 24.5% 27.2%

Avg(εo): all sources, all GMPEs 1.90 1.98

SA at 1.0 s (g) 0.550 1.023

Source 1: Hayward 24.9% 24.7%

Source 2: SAF 24.6% 24.7%

Source 3: Calaveras 16.1% 16.2%

Avg(εo): all sources, all GMPEs 1.81 1.83

Table 2 indicates remarkably little change in the primary three faults contributing to 

hazard at San Jose when explicitly including the local site condition versus using the firm-rock 

(default) site condition. The contrast with Los Angeles is due to fact that other sources with 

comparable recurrence intervals are relatively abundant in the vicinity of L.A., but such potential 

substitutes are uncommon around San Jose. The sameness of the 0.2-s SA for soil and rock at 

San Jose indicates that the 5-Hz GMPEs predict approximately unity soil amplification for most 

of the important sources. San Jose’s deaggregations also exhibit very little change in the average 

εo between rock and class D soil site condition. 

Case 2. Soil Mode Different from Rock Mode

The degree of soil nonlinear damping is expected to increase as sites approach the rupture 

for a given Vs30 and other soil column properties presumed to be the same (and uniform source 

properties, which are not likely in nature). In practice soil properties vary, generally having the 

lowest Vs30 near estuaries, bays, lakes, rivers and reservoirs and also greatest propensity to 

liquefy in these low lying regions. The greater expected damping of the seismic signal on softer 

soils reduces the seismic hazard, but it is frequently associated with high risk from liquefaction, 

lateral spreading, and other ground damaging phenomena. Culturally speaking, one only needs to 

remember the extensive damage in the waterfront region of Oakland and San Francisco from the 

Loma Prieta earthquake—or more recently, damage in Port-au-Prince, Haiti, from the January 

2010 mainshock and in Christchurch, South Island, New Zealand, from the September 2010 

mainshock—to realize that damping of the high-frequency signal by soft soils such as 

unengineered fill is more likely to be an indication of troubles to come rather than a benefit to the 

urban built environment. Similar experience at soft-soil sites from the Northridge earthquake is 

documented in USGS Open-File Report 96-0263 (Updike, 1996), available on the Web at 

http://pubs.usgs.gov/of/1996/ofr-96-0263/. That said, this section examines sites where the modal 

source significantly shifts from one fault or source type to another when explicitly considering 

the soil column’s influence on vibratory ground motion in the hazard integral (equation 1).
50

The first example in this section is near San Rafael or Greenbrae, Calif., a site whose 

seismic hazard is primarily controlled by the San Andreas, Rodgers Creek, and north Hayward 

fault ruptures. The site is at 37.9375 ̊N, 122.525 ̊W. The Vs30 for method 1 is 160 m/s at this 

location. The site’s 5-Hz seismic hazard deaggregation, assuming rock for local geology, is 

shown in figure 11.

In figure 11, the dominant hazard is associated with northern San Andreas fault multisegment ruptures, west of the site, M8.0±∆M, where ∆M is uncertainty in characteristic-event 

magnitude. Secondary hazard is associated with Rodgers Creek and North Hayward fault 

segments, east of the site, M7.0±∆M. Because of the nearness of the north SAF to this site, 

relatively large nonlinear soil damping is expected in the soft soil column. Because of the greater 

distance and lower magnitude of the Rodgers Creek and North Hayward ruptures, less nonlinear 

signal damping is expected in the soft soil column from these sources. This result is confirmed in 

figure 12, which exhibits the SH deaggregation at the same site but assumes a 180-m/s Vs30, or 

soft soil site condition (Vs30 lower limit for the above-mentioned deaggregation online tool is 

180 m/s, which is at the NEHRP class D/E boundary [soft soil]).
51

Figure 11. Rock-site hazard near San Rafael (and San Quentin prison): 5-hertz (Hz) spectral acceleration (SA) with 2 percent in 50 years probability 

of exceedance (PE) for a site with Vs30 of 760 meters per second (m/s).
52

Figure 12. Soil-site hazard near San Rafael: 5-hertz (Hz) spectral acceleration (SA) with 2 percent in 50 years probability of exceedance (PE) for a 

site with Vs30 of 180 meters per second (m/s).
53

A comparison of figures 11 and 12 shows a clear shift in the tallest magnitude/distance 

bin from a northern San Andreas fault source of about M8.0 (rock-site modal-source) to a 

Rodgers Creek fault source of about M7.0 (soil-site modal-source). The very distant Cascadia 

subduction megathrust source is also visible in figure 12 but is comparatively unimportant at this 

site. When determining sources to use as inputs to software for seismic analysis of buildings and 

other structures, the initial reaction from examining figure 12 might be to select the M7.0 source. 

If soil liquefaction is a potential issue at this site, the San Andreas scenarios may be more likely 

to produce extensive liquefaction and lateral spreading than the Hayward/Rodgers Creek 

scenarios because of a longer duration of strong shaking. Thus, other criteria besides height of 

deaggregated-source columns may often provide important supplementary information in 

comprehensive hazard assessments at many soil sites. For intermediate- to long-period hazard, 

the soil deaggregation (M,R,percent) distribution is quite similar to the rock deaggregation, with 

the SAF clearly dominating the hazard in all such instances.

While several California cities are located between active Quaternary faults, few cities 

exhibit the balance-of-hazard shifts as dramatic as that illustrated in the above example near San 

Rafael. A more well-known city which exhibits a SP modal-source shift is Seattle, Wash. 

Downtown Seattle and South Seattle have many soft-sediment sites near the Puget Sound 

waterfront, at which infrastructure and commercial building damage was extensive in the 

Nisqually earthquake of 2002 and the Seattle earthquake of 1965. Many of these sites are near 

the en echelon Seattle faults, which are south dipping reverse faults. The relatively high annual 

frequency of the Puget Sound intraplate mainshocks suggests that they may be modal-event 

sources, but their uncertain locations (they are background sources, not associated with any 

known faults) and their relatively great depth (about 50 km) suggest that they may not be

important when stacked up against the Seattle fault. The next figures (figs. 13–14) show the 5-Hz 

hazard for a site on Harbor Island, a major port facility for Seattle and the Pacific Northwest. The 

180-m/s Vs30 value (fig. 14) is for illustration only. In practice, the actual site Vs30 may be 

larger or smaller and will be affected by soil remediation efforts.
54

Figure 13. Harbor Island 5-hertz hazard deaggregation assuming rock site condition.
55

Figure 14. Harbor Island 5-hertz hazard deaggregation assuming 180 meters per second Vs30 (relatively soft) soil site condition.
56

Figures 13 and 14 indicate a shift in modal-event distance from less than 5 km (Seattle 

fault) to more than 50 km (deep intraplate background seismicity) when explicitly considering 

the effect of the soil column on the 5-Hz vibratory intensity; also note the increasing contribution 

from Cascadia subduction in figure 14 (M9 at R>80 km). The above example shows that the 

probabilistic 5-Hz SA on soil, 1.62 g, is greater than the corresponding value on rock, 1.40 g. 

The reduction of median motion from the very nearby Seattle fault compared to increases in the 

medians for the more distant deep and interplate sources and the reduction in aleatory σ (on 

average) associated with Seattle fault motion as it might be recorded on soil versus rock are the 

driving causes of this large change in the distribution of SP hazard at Harbor Island. Groundmotion prediction equations associated with intraplate earthquakes do not have reduced soil σ, 

and this feature is a factor that may bias the probabilistic hazard towards intraplate and interface 

earthquakes.

Figures 15 and 16 show the corresponding rock and soil deaggregations at Harbor Island

for the 1-s SA. The effect of less amplification of the IP signal from nearby relatively large 

sources in the soil column compared to that at rock outcrops means that for this site the Seattle 

fault will have a reduced hazard contribution (mode participation factor), while the soil condition 

at Harbor Island is explicitly included in the hazard integral (equation 1). Figure 16 shows two 

modal events, one for the tallest bin in R and M and the other for the tallest bin in R, M, and ε. 

The (R,M,ε*

) mode corresponds to a Cascadia subduction event, whereas the (R,M) mode 

corresponds to a Seattle fault source. For the 1-s spectral period, the intraplate source is of 

reduced importance whether the Seattle site is rock or soil. While Seattle has been repeatedly 

subjected to strong ground motion from intraplate events, it has not experienced significant 

shaking from either Cascadia or Seattle fault sources; thus, the PSHA model and its 

deaggregation are based on heavily extrapolated information.
57

Figure 15. Harbor Island 1-second spectral acceleration assuming rock site condition. Seattle fault sources dominate the hazard. Cascadia sources 

are second in importance, deep intraplate are third.
58

Figure 16. Harbor Island 1-second spectral acceleration assuming soil site condition. National Earthquake Hazards Reduction Program (NEHRP) 

DE soil is soil with Vs30 of 180 m/s, at the NEHRP boundary between D- and E- soils. Seattle fault sources dominate the hazard in distance and 

magnitude. Cascadia interface sources remain secondary but are closer in their total hazard contribution, and are dominant when considering 

distance, magnitude, and epsilon bins.
59

Caveats and Model Limits

Regional mapping of Vs30 using various techniques can yield substantially different 

results, none of which are guaranteed to be close to the true site-specific Vs30. This section of 

the report compares estimates from the above three methods with measurements of Vs30 at 

several sites. Measured Vs30 data are from Gibbs and others (2000), Kayen and others (2005),

and Louie (2005). The first uses borehole travel time measurements, the second uses spectral 

analysis of surface waves (SASW) methods (see 

http://walrus.wr.usgs.gov/geotech/methods.html), and the third uses refraction microtremor 

(“ReMi”) methods. Table 3 presents results for more than a dozen sites shown in figures 3B and 

6B. These examples show that taking the median or average or any statistic from a set of biased 

estimates does not necessarily improve the estimate. The expectation is that Cretaceous granites 

(Kgr) and Franciscan mélange (KJf) will be relatively fast compared to most other geologic units 

in western California; however, reality may not always conform to expectation and, when it 

does, it is still unknown how much faster they are. The use of the real variable Vs30 in sitespecific PSHA rather than broad class variables such as NEHRP site class adds the responsibility 

to estimate Vs30 well. (However, how does one measure how well Vs30 is estimated? Within 10

percent? 20 percent?)

Table 3 shows some near blatant misses in Vs30 estimation. At almost co-located sites in 

Long Beach, both borehole and ReMi measurements were made, and the model Vs values are 

within the range of these measurements. In the case of a high Vs30 at Thousand Oaks (TOV), a 

thin layer of Quaternary alluvium, which seems to drive the model estimates, is underlain by fast 

Mesozoic rock. In the case of the a Vs30 at Fort Tejon, one might speculate that repeated major 

earthquakes such as the 1857 mainshock may have fractured the surficial rock significantly, 

lowering Vs30 from that which would be expected in a tectonically quieter environment. The 

same argument might be applied to the Filoli Visitor Center, very close to a part of the San 

Andreas fault that ruptured during the 1906 mainshock. A second speculation is that the Carbon 

Canyon Dam Vs30 estimates may be improved by sampling the topography and geology in the 

vicinity at a finer scale than that available when preparing this report (May 2010). Sampling 

Vs30 and other Earth-surface variables at 250 m instead of 1 km or 0.01 ̊ has been suggested as a 

possible strategy to better capture small-scale site-response variability in urban environments. 

Except for the cost, this suggestion makes equal sense in rural environments. An added concern 

is that different field techniques for estimating Vs30 are known to yield sometimes substantially 

different results, for example, ReMi analysis compared to ROSRINE (Resolution of site response 

issues from the Northridge Earthquake) borehole analysis (Louie, 2007), although ReMi Vs30 

generally is within 20 percent of the more expensive borehole value. In table 3, Vs30 estimates 

are sometimes interpolated from the nearest two samples.
60

Table 3. Comparison of Vs30 values “measured” by one of three methods (borehole, spectral analysis of 

surface waves, or refraction microtremor) and Vs30 values from three regional model values (Wills and 

Clahan, 2006; Wald and Allen, 2007; Yong and others, in press), in meters per second. [NEHRP, National Earthquake Hazard Reduction Program; OSI, Osito Audit–Castaic Lake Dam; LA, Los Angeles; 

E., east; LBW2, Long Beach Water Reclamation Plant 2; WTP, Long Beach Water Treatment Plant; LLS, Ellis; KJf, 

Franciscan mélange Cretaceous; Kgr, Cretaceous granite; Qal, Quaternary alluvium; Qoa, Older Quaternary 

alluvium; Qya, Younger Quaternary alluvium]

Site Lat. Long. Vs30 

measured

Vs30 

Wills

Vs30 

Wald

Vs30 

Yong

Geologic 

unit

NEHRP 

site 

class

Filoli Visitor Center, 

Woodside 37.468 -122.308 240 470 470 490 KJf DFort Tejon 34.87 -118.90 314 760 727 532 Kgr D+

OSI 34.613 -118.725 424 437 436 524 CIdyllwild 33.708 -116.717 902 653 630 528 Kgr BThousand Oaks (TOV) 34.157 -118.821 884 397 470 455 Qal, thin BCarbon Canyon Dam 33.912 -117.838 235 457 457 424 Qal, thin DLA Faring Rd (LAFR) 34.089 -118.435 255 354 520 464 Qoa DObregon Park (OBG) 34.037 -118.178 348 368 361 372 D+

Tarzana (TAR) 34.160 -118.533 256 399 416 437 Qal, thin DColton Interchange E., (COE) 34.064 -117.289 250 310 307 370 Qya DLong Beach LBW2 33.799 -118.088 279 241 246 246 Qal D

Long Beach WTP 33.798 -118.088 225 241 246 246 Qal D

Fountain Valley LLS 33.687 -117.943 220 262 261 275 Qal DFullerton (FUL) 33.872 -117.923 306 323 324 327 D+

Mojave Oak Creek Canyon 35.042 -118.377 320 530 607 n/a ? D+
61

While Vs30 (or perhaps a finer grained category variable such as NEHRP site class with 

steps within class, as in the above table) is an important element in the characterization of the 

local site condition, knowledge of several other variables is required to perform a reasonably 

comprehensive site-specific hazard analysis. A second potentially important but here neglected 

consideration is layering in the sediment stack under the site, which produces shear-wave 

resonance at narrow bandwidth periods. For example, borehole analysis at the Tarzana (TAR) 

site in table 3 indicates a sharp S-velocity increase at 13 m depth. The average Vs in the top 13 m 

is about 180 m/s (Gibbs and others, 2000), implying a wavelength of 54 m for 0.3-s body waves. 

Quarter-wavelength theory predicts that 0.3 s may be a resonant period for S waves at the 

Tarzana site, and spectral analysis of strong-motion seismograms from the Northridge and 

Whittier Narrows mainshocks demonstrates that periods in the neighborhood of 0.3 s are strongly 

amplified at TAR. (Other factors have also been considered. Analyzing TAR’s seismic behavior 

was a cottage industry following the Northridge mainshock.) When one or more soil-column 

resonant periods match structure resonant periods, the building must withstand an above-average 

shaking intensity. 

A third important consideration is the effect of deep sedimentary basins on the seismic 

wave field. Basin fill, geometry, and relative location affects interaction with source properties 

such as focal depth, radiation pattern, and source directivity. Such effects are quantified in 

scenario studies, such as in southern California (Day and others, 2008) and in central California 

(Harmsen and others, 2008). Site-response models, such as those embedded in the NGA 

equations, frequently focus on the vertically propagating shear wave (one-dimensional analysis) 

and do not necessarily capture important basin-related amplification features that are observed in 

three-dimensional simulations and in many strong-motion records.

A fourth consideration is topographic amplification, which can increase risk to structures 

on ridges and hilltops in spite of relatively high Vs30 at many such sites. Topographic 

amplification of the seismic signal has been studied in several theoretical reports and in several 

empirical data reports, such as McCrink and others (2010). Procedures for inserting a defensible 

model of topographic amplification into NSHMP seismic-hazard codes probably could be 

developed by convening a workshop of experts. 

Intermediate- to long-period site response may better correlate with Vs100 or Vs200 (the 

average shear-wave velocity in the top 100 to 200 m of the sedimentary basin, respectively) than 

with Vs30. As is frequently observed (for example, Louie and Scott, 2009), at intermediate to 

long periods, Vs30 works as well as it does as a predictor variable because of its correlation with 

Vs100 or Vs200. The correlation of Vs30 and Vs30 is not discussed, however, for the strongmotion data used in the NGA regressions (Chiou and others, 2008). Figure 17 illustrates the 

correlation of Vs100 with Vs30 at 50 sites in southern California, based on refraction 

microtremor data from Louie (2005). Places where the correlation may break down include 

alluvial valleys around intermittent streams that drain the Sierra Nevada Mountains, such as the 

ones visible in figures 3 and 5. Slow sediment thickness is expected to be quite limited in these 

high-erosion-rate environments. Here, the long-period signal is probably not much amplified by 

the shallow sediments around the streams. In figure 17, data labeled “A” are suggestive of sites 

where some short-period site amplification may not imply significant LP site amplification

(Vs100>>Vs30). Whether there are at-risk long-period structures in such valleys needs to be 

determined before potential Vs30-model bias at those locations may be important from a risk 

perspective. 
62

The importance of deep-basin features to seismic hazard, such as depth of hard rock with 

Vs=2,500 m/s or so and the nature of the sediment column above that depth, is generally 

underappreciated by the current generation of NGA and other GMPEs used in PSHA. One way 

in which apparent model sensitivity is in actuality absent is the depth to relatively hard rock with 

Vs of 1 km/s, or Z1, according to the Chiou and Youngs (2008) relation. Assuming that the site 

Vs30 is 200 m/s (soft soil) and using the default depth of Z1=336.2 m that CY08 suggests

instead of a much shallower Z1 of 50 m, as might be more realistic in the western foothills of the 

Sierra Nevada Mountains, results in differences in estimated median 3-s ground motion from a 

M7 earthquake of less 0.1 percent, at all source-to-site distances. This miniscule difference in the 

CY08 model, versus what may be a first-order site-effect difference in reality, indicates that 

NGA models LP site-response features may benefit from further revision. The figures of this 

report would not be visibly altered if more realistic Z1 values had been available and used in the 

two study regions

Figure 17. Scattergram of Vs100 versus Vs30 at or near 50 southern California strong-motion sites (from 

Louie, 2005, table 1). A standard linear regression is plotted in red (ρ=0.859). Data labeled A probably 

correspond to thin layer of soil underlain by rock with relatively high shear-wave velocity. (m/s, meters 

per second; Vs30, average shear-wave velocity in top 30 m; Vs100, average shear-wave velocity in top 

100 m)
63

Site-response models contained in the NGA equations were developed mostly from data 

and simulations based on soil response in western California; they are average or generic models. 

Because of the many simplifications of the NGA GMPE site-response models, the “site-specific” 

label needs to be taken with a grain of salt. The minimum Vs30 where the NGA equations are 

believed to be valid is about 180 m/s, or soil at the NEHRP class D/E boundary. For the 

calculations presented in this report, sites with Vs30 < 180 m/s were revised to 180 m/s. Soil 

liquefaction and lateral spreading effects are not predicted by NGA site-response models.

True site-specific hazard analysis has other potentially important differences from 

regional hazard analysis such as is performed by NSHMP. Anderson and Brune (1999) point out 

that the uncertainty in ground motion at a single site is estimated from the scatter in data for a 

variety of sites having a range of Vs30 and other physical properties. Many of these data are 

likely to be unrepresentative of the motion that may be expected at a specific site, even when 

sites are restricted to a limited range of Vs30. For example, Tarzana’s high recorded ground 

motion from several southern California mainshocks may be unrepresentative at other soil sites 

with different shallow Vs gradients. If and when better site- and path-specific data are available, 

the total aleatory sigma, σT, associated with specific sites should decrease substantially, perhaps 

20 to 25 percent, as suggested by John Anderson (Univ. of Nevada at Reno, 2010, written 

commun.). A reduction of 25 percent in σT can easily map into more than 15 percent reduction in 

probabilistic motion at the 2 percent in 50-year PE.

Discussion and Conclusions

Site-specific hazard mapping and deaggregation, using recently developed NGA 

equations for predicting ground motion, provide new perspective and potential realism to the 

assessment of seismic hazard compared to the ubiquitously available corresponding products for 

the bedrock site condition, that is, rock with Vs30=760 m/s (for example, Petersen and others, 

2008). At short periods, and for relatively large probabilistic (or deterministic) ground motion, 

the soil column significantly deamplifies the expected ground motion, according to NGA models 

employed in the 2008 NSHMP hazard analysis. That probabilistic motion in many instances 

remains the same or goes down as NEHRP site class goes up (B→C→D→DE) is apparent in the 

T=0.2-s results presented here and in other periods (see 

http://earthquake.usgs.gov/hazards/products/conterminous/2008/update_201001/). This result is 

at odds with portions of the BSSC site response tables (for example, Building Seismic Safety 

Council, 1997) and may be the most important finding of this investigation. (However, this 

conclusion is not new; see Trifunac and Todorovska, 1998). For LP response, the standard view 

that probabilistic motion increases inversely with Vs30 is of course upheld, but only under the 

assumption that shallow sediment properties such as low rigidity continue to much greater depth. 

At some locations, abrupt and sustained increases in rock rigidity with depth result in low LP site 

amplification, regardless of Vs30. Regional mapping of Vs100 or Vs200 is suggested to identify 

locations where LP response is not well predicted using Vs30.

The aleatory scatter associated with repeated occurrences is believed to be significantly 

less than the scatter at rock sites for a given PSHA source model (Abrahamson and Silva, 2007). 

At long periods, the expected amplification of spectral acceleration in the soil column is not 

much affected by strength of input motion, although the scatter may continue to be less than that 

at corresponding rock sites. In California and in some urban sedimentary basins elsewhere, 

several competing models of regional Vs30 are available; most are sampled at about 1 km2

. They 

have significant differences. Where multiple regional estimates for Vs30 are available, it would 
64

be helpful to develop consensus Vs30 models. Increasing the spatial sampling in urban areas to 

about 250-m squares also makes sense because 0.01° squares are not adequate to capture 

important variations. Hazard mapping at the 250-m scale is currently being done in Japan’s 

metropolitan areas (Wakamatsu and others, 2006). Even finer-scale geology is mapped in some 

urban areas, such as Oakland/Alameda Island, and some applications may require even finer 

sampling than 250 m to sample known soil hazards (Holzer and others, 2005).

As a general rule, the modal event, at least as determined in the deaggregation of 2

percent in 50-year ground motions, tends to be reasonably stable as a function of Vs30, although 

its contribution diminishes at sites over softer soils compared to firmer soils and bedrock. A few 

exceptional sites—where the modal event shifts from a nearby, relatively large earthquake to a 

more distant, perhaps smaller earthquake and when the local site condition is explicitly included 

in the hazard integral—are known, although many remain to be discovered. Site-specific case 

studies, such as those discussed in this report, may be important to consider as engineers design 

buildings and bridges to withstand scenario earthquakes.

The USGS and some State geological surveys are developing and improving State-wide 

Vs30 maps, or at least urban sedimentary basin Vs30 maps, and similar site-specific hazard 

products to allow computations of site-specific hazard maps and curves, such as is now routinely 

done in California. These site-specific probabilistic motions could ultimately replace the NEHRP 

soil amplification factors currently used in building design and retrofit codes. In California, 

continued interest in site amplification and its relation to Vs30 have resulted in additional 

studies, among them Wills and Gutierrez (2009), that find that small changes in sedimentary 

basin slope may be a helpful predictor of Quaternary alluvium Vs.

Acknowledgments

Rui Chen, of the California Geological Survey, provided the data file corresponding to 

the Wills version of Vs30 for the state of California. Alan Yong, of USGS Pasadena, provided 

his version of Vs30 for the two study regions. Maps were produced from the Generic Mapping 

Tools 3.4.3, Wessel and Smith (1991). Several NGA team members provided critical help 

implementing their models, with special thanks to Brian Chiou and Ken Campbell. Insightful 

reviews by Mark Petersen, Daniel Garcia, David Perkins, and David Wald (all with the USGS)

improved the manuscript.

References

Abrahamson, Norman, and Silva, Walter, 2007, Abrahamson and Silva NGA ground motion 

relations for the geometric mean horizontal component of peak and spectral ground motion 

parameters: Pacific Earthquake Engineering Research Center, draft report.

Abrahamson, Norman, and Silva, Walter, 2008, Summary of the Abrahamson and Silva NGA 

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