Deaggregation of U.S. Seismic Hazard 

Sources: The 2002 Update 

by S.C. Harmsen, A. D. Frankel, and M. D. Petersen 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial 

standards or with the North American Stratigraphic Code. 

Any use of trade, firm, or product names is for descriptive purposes and does not imply endorsement by the 

U. S. Government.

Open-File Report 2003-03-440 

U.S. Department of the Interior 

U.S. Geological Survey 
Deaggregation of U.S. Seismic Hazard Sources: The 2002 Update 

by S. C. Harmsen, A. D. Frankel, and M. D. Petersen, USGS 

INTRODUCTION 4

ESSENTIAL BACKGROUND MATERIAL 4 

DEFINITIONS OF TERMS 5 

BINNING 6

QUANTIFICATION OF GROUND-MOTION UNCERTAINTY FROM FUTURE 

EARTHQUAKES 7

RESULTS 9

WUS SEISMIC HAZARD DEAGGREGATION MAPS: THE MODE 9 

WUS SEISMIC HAZARD DEAGGREGATION MAPS: THE MEAN 11 

California-Nevada Region Maps of Modal Source Parameters 13 

Oregon-Washington Region Maps of Modal Source Parameters 15 

A Closer Look at Seismic Hazard in Portland, Oregon 17 

Maps of Modal Source Parameters in the Intermountain Seismic Belt 18 

CENTRAL AND EASTERN U.S. MODAL SOURCE PARAMETER MAPS 19 

CENTRAL AND EASTERN U.S. MEAN SOURCE PARAMETER MAPS 21 

New Madrid Seismic Zone Modal Source Parameters 22 

South Carolina Modal Source Parameters 23 

SUMMARY 25

CONCLUSIONS AND DISCUSSION 25 

ACKNOWLEDGEMENTS: 27

2
REFERENCES CITED 27 

FIGURES 29

3
Introduction 

This document provides a nation-wide overview of seismic sources that 

contribute to the 2001-2002 update of the USGS Probabilistic Seismic Hazard 

Assessment (PSHA) (Frankel et al., 2002) in the form of deaggregated mean and 

modal sources for the 2% in 50 year probability of exceedance (PE) and the 10% 

in 50 year PE for sites in the conterminous USA. The primary purpose of this 

document is to provide information to urban planners and seismic-resistant 

design engineers about seismic sources that might sensibly be considered when 

planning and designing. 

Mean and modal seismic sources vary with spectral period. Source 

deaggregations associated with peak horizontal ground acceleration (PGA), and 

with 2.0-s, 1.0-s. 0.2-s and 0.1-s spectral acceleration (SA) are performed, and 

maps are presented and contrasted for these periods of motion. Maps of mean 

and modal source parameters for 0.3-s and 0.5-s SA are also available from the 

first author (harmsen@usgs.gov). 

Essential Background Material 

Documentation for the 2002 Update of the National Seismic Hazard Maps (OFR 

02-420) (A. Frankel and others, 2002) discusses the new models of seismic 

hazard, including newly recognized sources, as well as new treatments of 

previously recognized sources. The reader should be familiar with that document, 

available on the web at http://pubs.usgs.gov/of/2002/ofr-02-420, in order to better 

understand the components of the PSHA model that affect mean and modal 

source parameters. The OFR also discusses the way modifications came to be 

made from the previous USGS PSHA (Frankel et al, 1996), mostly through a 

series of regional workshops and follow-up meetings, where experts provided 

current models of seismic hazards for their regions and current models of strongmotion attenuation. Other changes to the PSHA model made after publication of 

OFR 02-420, such as the extension of the maximum distance (R) for calculating 

hazard for California A-faults from 200 km to 250 km, are also included in the 

calculations of this report. This report's calculations include all modifications 

through October 2003 (version 6 of the PSHA model). 

On national and broad regional map scales, deaggregation analysis can help to 

highlight changes from the previous PSHA maps, but site-specific hazard 

deaggregation is necessary to gain a better understanding of newly recognized 

sources and revised treatments of previously recognized sources. The USGS 

WWW URL, http://eqint.cr.usgs.gov/eq/html/deaggint2002.html allows the visitor to 

deaggregate the seismic hazard at any location in the conterminous USA using 

the 2002 PSHA model, for a wide variety of spectral periods and return times. 

4
That web site exhibits contributions from all sources at a specific site, along with 

the mean and modal source information provided in this report. 

Definitions of Terms 

This document uses deaggregation analysis definitions given in Harmsen et al., 

1999; Harmsen and Frankel, 2001; and Harmsen, 2001. Three primary 

deaggregated source parameters are considered, (1), source-to-site distance, 

(2), magnitude, and (3), epsilon, or ground-motion (g.m.) uncertainty. These are 

denoted R, M, and ε0 , respectively. R initially is one of Rseis, Rrup, or Rjb 

(Abrahamson and Shedlock, 1997), or in some cases, R is distance to the center 

of a cell within which a seismicity rate is defined. Rseis is the slant distance from 

the site to the nearest point on the part of the fault believed capable of generating 

strong motion. Rrup is the slant distance from the site to the nearest location on 

the modeled fault or portion of the fault that ruptures. Rjb is the nearest distance 

to the vertical projection of the fault (or portion of the fault that ruptures) to the 

free surface. A fourth distance metric, based on projecting the fault downdip into 

the Earth's mantle, is used to determine the hanging-wall term for the 

Abrahamson and Silva (1997) attenuation model. After binning takes place, R is 

a hybrid distance. M is bold to indicate moment magnitude. The mean R,M, ε0 at 

a site averages over all sources considered in the PSHA model, and is 

independent of any binning definitions. The modal-event source is determined 

from the magnitude and distance bin that yields the largest contribution to the 

specified hazard. The mode is the mean R,M, ε0 of sources in that R,M bin. The 

mode is dependent on binning definitions. The largest bin contribution in this 

report ranges from less than 5% of the total to over 80% of the total, depending 

on site location, spectral period, and PE. Whether knowing the mode means 

knowing the model is strongly spatially dependent for "typical" PEs (2% in 50 

years or 10% in 50 years, for example) and specified spectral period. For the 

most part such statements are overgeneralizations likely to lead to 

misunderstanding when applied uncritically to seismic mitigation decision 

making. 

The primary summation in PSHA is of mean rate of g.m. exceedance, which is 

mean rate of occurrence times conditional probability of g.m. exceedance given 

the occurrence of a seismic source, and given a set of attenuation models that 

predict the distribution of future ground motion from seismic sources. A 

"contribution" for a given PE, then, is the mean rate of exceedance for sources in 

a specific R,M bin divided by the rate implied by the given PE. A hazard 

contribution can be expressed as a fraction, as a percent, or in a more qualitative 

manner. Thus, the brief statement "the mode represents 50% of the hazard" is 

intended to mean, "potential sources in the modal R,M bin will produce, on 

average, half of the ground motions that exceed some specified g.m. level (i.e., 

"exceedances") in a randomly selected 50-year period." There is no claim about 

the relative likelihood of the mode versus other considered (or unconsidered) 

5
seismic hazard sources for any specific 50-year period, such as the next 50 

years. 

In this document, SA is pseudo-spectral acceleration with 5% damping (see 

Hudson, 1979). PGA is peak horizontal ground acceleration. SA, like PGA, is a 

peak-horizontal-motion indicator. Units for acceleration are percent g, where g is 

the standard acceleration due to gravity, 9.8 m/s2

. For this report, lower-case sa 

is the logarithm of SA. The logarithm is a pervasive operator in PSHA because 

sa, when thought of as a random variable, is approximately normally distributed, 

i.e., SA is lognormally distributed. 

Most USGS seismic hazard maps, including those of this report, pertain to 

motion at a site with firm-rock conditions. Firm rock is rock with average shearwave velocity of 760 m/s in the upper 30 meters, putting it at the NEHRP B-C 

boundary. The word "motion" in this report generally refers to SA, although it may 

refer to PGA in some instances. In this document these terms are implicitly 

preceded by the adjective "probabilistic," if the authors are discussing future 

rather than recorded motion. 

Binning 

For the analysis underlying the maps presented below, and at the interactive 

deaggregation web site, the magnitude bin width is 0.2 Mw units, reduced from 

0.5 Mw units of previous studies cited above. The magnitude-bin centers are at 

M 4.6,4.8, 5, 5.2,..., 9. 

For this report (but not for the interactive deaggregation web site) the distance 

annular width, ∆R, is 10 km from 0 km to 100 km, 50 km from 100 to 300 km, 

200 km from 300 to 500 km, and 500 km from 500 to 1000 km. 1000 km is the 

maximum considered distance for seismic hazard in the CEUS. The 1000-km 

limit is double that of the 1996 maps for random seismicity, but is the same for 

New Madrid Seismic Zone (NMSZ) sources. 200 km is the maximum considered 

source-to-site distance in the WUS west of the Rocky Mountains for most 

sources other than Cascadia sudduction, which has a 1000-km limit, and 

California A-faults, such as the San Andreas and Imperial faults. The maximum 

considered distance for hazard from California A-faults is 250 km. 

In some cases, such as Cascadia subduction and NMSZ mainshocks, locations 

of distant sources may be reasonably well known. In other cases, such as 

Central and Eastern U.S. random seismicity, potentially significant earthquakes 

may occur at a wide range of distances and azimuths. The relative importance of 

distant sources to regional and local sources is assessed by varying ∆R, the 

distance bin width, with R. For some regions that have very little earthquake 

threat, such as Minnesota and Wisconsin, we will see that the modal-source 

distance, given the above bin definition, can exceed 500 km, with or without 

6
contributions from NMSZ. At the interactive deaggregation web site, the distancebin annular width remains constant. ∆R is 10 km for seismically more active 

areas, and is 25 km elsewhere. Thus, the mode reported here and that reported 

at the web site may differ, due exclusively to different ∆R definitions. The mean 

M, R, and ε0, however, are the same here as at the web site for a given site, SA, 

and PE due to their definitions, given in Harmsen et al (1999). The mean is 

independent of bin details. 

Quantification of Ground-Motion Uncertainty from Future 

Earthquakes 

A variety of methods have been proposed for deaggregating ground-motion 

uncertainty given R and M for the exceedance of a specified ground motion. Two 

measures of ground-motion uncertainty are discussed in this report. These are 

(1), modal ε0, or ε 

ˆ 

0 , and (2), the average ε0 for all sources that contribute to 

hazard, denoted mean ε0 or ε 0 . 

As in McGuire (1995), we define 

ε0Α (S) = (sa0 - μA) / σA , (1)

that is, the standardized value of sa0 =log(SA0), or the number of standard 

deviations, from sa0 to the logarithmic mean ground motion, μA, for a specific 

source S, and attenuation model, A. SA0 is the ground motion associated with a 

specific PE, for example, the 2% in 50 year PE. SA0 is what is mapped in reports 

such as Frankel et al. (1996) and Frankel et al. (2002). In general, any remarks 

about mean or modal sources implicitly contain the phrase, "for a given 

probability of exceedance (PE)," because these sources change, often 

dramatically, with PE. 

When examining maps of mean or modal ε0 it is helpful to be able to translate the 

mapped ε0 values into simple, easy to communicate numbers. For concreteness, 

but in an approximate sense, if the modal-source ε0 = 1 then the probabilistic 

motion is about 1.8 times the median motion expected from that source, whereas 

if ε0= -1 then the median motion of that source is about 1.8 times the probabilistic 

motion. If ε0=0, then the probabilistic motion equals the median motion of that 

source. Initially, ε0 is a quantity that is determined from each attenuation model 

for a given source. However, a reported or binned or mapped ε0=0 not only 

averages over sources in a (R,M) bin, but also weighs ε0 that is computed for 

each attenuation model, A. Mathematically, epistemic weighting is the same thing 

as averaging. We average effects of the attenuation models for sources in a 

(R,M) bin. 

For the modal-event R, M, ε0, denoted ( , it is reasonable to suppose that 

sa0 approximately equals μ

ˆ R , M ˆ ,ε 

ˆ 

0 )

Α+ε 

ˆ 

0σΑ, or, 

7
SA0 ≈ exp(μ A + ε 

ˆ 

0σ A) (2)

where, as stated before, μΑ and σΑ are mean and standard deviation of the 

normal distribution of sa given and ˆ R Mˆ . The right-hand side of (2) should be 

computed using an attenuation model A that is used in the PSHA. The accuracy 

of the approximation in Eqn. (2) should improve as ∆R and ∆M (bin widths) 

decrease. However, for the mean R,M, ε0 , (which is the mode for largest 

possible bins), the relation of SA0 to the conditional distribution parameters is less 

likely to fit an equation like the following 

SA0 ≈ exp(μ ′

A + ε 0σ ′A) . (3) 

where μ'

Α and σ'Α are the mean and standard deviation of the ground-motion 

prediction equation, respectively, given R and M . For a randomly chosen site in 

the U.S.A., and a randomly chosen attenuation model used in the PSHA, the 

most likely relationship to actually hold is 

exp(μ ′ A + ε 0σ ′ A ) < exp(μ A + ε 

ˆ 

0σ A ) < SA0, (4) 

although for some sites and for some specifically chosen attenuation models, it is 

possible that 

exp(μ ′ A + ε 0σ ′ A ) > exp(μ A + ε 

ˆ 

0σ A ) > SA0. (5) 

The left side of inequality (4) is at many sites and for several attenuation models 

less than half the right side, a substantial difference. Inequality (5) is much less 

likely to be true at a randomly chosen site for a typical PE. One region where 

inequality (5) is sometimes true is western Oregon for the 1s SA, the 2% in 50 

year PE, and the Sadigh et al. (1997) attenuation model. In this instance, the 

difference between the left and right sides of Eqn. (5) is a few percent. 

The distribution of probabilistic seismic sources for many if not most sites in the 

U.S. is multi-modal. That is, the distribution does not tend to concentrate strongly 

around any single magnitude and distance. At sites where the difference 

between exp(μ+ε 0σ) and SA0 is small, we may usually infer that the distribution of 

probabilisitic sources is approximately uni-modal. For the western Oregon region 

just mentioned, the mode is a M8.3 or M9 subduction earthquake, and other 

sources tend to contribute little to the seismic hazard. 

How significant is the modal event? A probabilistic answer might be given as the 

ratio of the modal-source contribution to the total hazard. This ratio whether for 

the mode or any other source is sometimes called the "source contribution 

factor." The modal source contribution factor, or MCF, is mapped in the 

California-Nevada section below. 

Mean ε0 rises monotonically with probabilistic ground motion, which of course 

increases in a mean sense as the probability of exceedance decreases. For a 

given modal-source bin, modal ε0 rises with probabilistic ground motion until a 

new contributing source achieves dominance, at which moment modal ε0 falls

abruptly. The distinctly different ways in which the mean and the mode vary with 

probabilistic ground motion is examined in the section on Portland Oregon below. 

8
The USGS interactive deaggregation web sites provides considerable detail 

about the distribution of ε along with magnitude and distance for probabilistic 

sources at any specified site in the USA. The "1996 site" includes Alaska and 

Hawaii. USGS PSHA for Alaska and Hawaii will be updated in the future. Also, 

Puerto Rico site PSHA deaggregation is now possible using the new USGS 

model (Mueller et al., 2003) and is also available at the 2002 interactive 

deaggregation web site. 

Results 

Deaggregated seismic hazard mean and modal R M and ε0 are presented for the 

WUS and for sub-regions of the WUS, and for the CEUS and sub-regions of the 

CEUS (100o

 West defines the boundary between west and east for these maps). 

Although the maps are presented separately for these three parameters, the 

modal-source (R,M, ε0) is a single entity, i.e., the modal R M and ε0 maps should 

be considered together for a correct understanding of the modal source. For 

example, for a site in eastern Washington, M9 may be the modal source 

magnitude and several hundred km may be the modal source distance. Such a 

combination implies that a Cascadia megathrust event is the dominant source, 

i.e., the source that contributes the largest fraction of ground-motion 

exceedances in a random 50-year interval to that site, at that spectral period, for 

that PE. 

For those who are interested in the names of and details about the sources that 

contribute to the seismic hazard in the USGS model, the interactive 

deaggregation web site cited above should be helpful. At that site, one can learn 

the relative contributions of all the major contributors to hazard at the site. Also, 

all individual fault sources (if any) that contribute more than 5% of the 

exceedances of ground motion are listed along with those faults' mean 

magnitude, distance and ε0 at the site location specified. Also, at that web site, 

one can perform a geographic deaggregation to look at the locations of faults and 

other sources that contribute to the hazard at the site, and look at the relative 

contributions plotted on a map centered at the site. 

WUS Seismic Hazard Deaggregation Maps: The Mode 

Maps of the modal-event magnitude, or Mˆ , for the WUS are shown in Figure 1. 

The left side of Figure 1 exhibits Mˆ for 1-s SA and the right side exhibits Mˆ for 

0.2-s SA. All modes are with respect to the 2% in 50 year PE. By plotting two 

periods' data together we can see how in general, large-M sources tend to 

remain modal to greater distances for longer-period waves. For example, M8.3 

and M9 Cascadia sources dominate the hazard in most of Oregon and 

Washington for 1-s SA, but dominate only in western Washington, principally, the 

9
Olympic Peninsula and Columbia River Gorge, for 0.2-s SA. The Seattle fault 

dominates the 1s hazard for many sites in Seattle. The M9 Cascadia source also 

dominates northwestern California's seismic hazard, extending far into the Great 

Valley for the 1-s SA. Figure 1 shows that at sufficient distance from the Pacific 

coast states, M6.5 to M6.9 sources tend to dominate the hazard at 1 s, and M5 to 

6 tend to dominate the hazard at 0.2 s, with several locations having highermagnitude modes, and some lower. In California, modal-event magnitude is often 

associated with a nearby fault, the most prominent of which is the San Andreas. 

San Andreas fault modal magnitudes are as great as 8 in northern California and 

7.9 in southern California, although alternate-scenario magnitudes can exceed 

M8.3. California and Pacific Northwest modal events will be shown in greater 

detail in figures that follow. 

Modal-source distance, or , for 1-s and 0.2-s spectral periods is shown in ˆ R 

Figure 2. Units for R are km. is often less than 5 to 10 km at sites near WUS 

Quaternary faults. is computed using a nearest-distance-to-fault metric. can 

be large, as great as 1000 km, if the megathrust source dominates the hazard at 

a particular site, as is the case at a few locations in northern Montana. In this 

figure, as in several figures that follow, a saturation color (in this case dark blue) 

represents parameter values greater than the maximum shown in the legend 

(here 500 km). 

ˆ R 

ˆ R ˆ R 

Modal-source ε0, or ε 

ˆ 

0 , for those spectral periods is shown in Figure 3. In much of 

Nevada, Arizona, New Mexico, Colorado, Idaho, Utah, and Oregon, the dominant 

source is a relatively long-recurrence-time fault. The probabilistic ground motion 

can be low for sites on or very near these faults. The ε0 map uses warm and hot 

colors (reds and browns) to indicate probabilistic ground motion well below the 

median of the modal source for those sites. The ε0 topic is discussed more fully in 

Harmsen (2001) and in the conclusions to this report. 

Modal magnitude maps are again shown for the 2% in 50 year PE for the WUS in 

Figure 4. The left-side map shows data for the 2-s spectral period and the right 

side for 0.1-s. Two s and 0.1 s are the longest and shortest SA periods, 

respectively, calculated by the USGS national seismic hazard mapping project 

(excluding PGA). A comparison of Figure 4 and Figure 1 indicates that a slightly 

greater region encompassing California's Great Valley has a modal San Andreas 

source for the 2-s compared to the 1-s SA. In the east part of the WUS, an M7.7 

New Madrid source everywhere dominates the 2-s hazard out to the maximum 

source-to-site distance of 1000 km, but in only a few places for the 1-s hazard. 

For the 0.1-s SA mode compared to the 0.2-s SA mode, the modal event 

magnitude tends to be lower for the shorter-period motion, and frequently Mˆ ≈5 

east of the Rocky Mountains. Except for these differences, the maps of Figure 4

are a recapitulation of those of Figure 1, and therefore, WUS PSHA for these 

periods will not be considered further here. 

10
The PGA modal-event M is shown for the WUS in Figure 5, the left side for the 

2% in 50 year PE, and the right side for the 10% in 50 year PE. The PGA modalevent R for the WUS is shown in Figure 6. By showing two return-period modalevent maps together, we can see the effect of increasing or decreasing the 

probabilistic motion (which rises with mean return time) on the mode. Long 

recurrence-time faults, common in Nevada and parts of Arizona, New Mexico, 

Utah, and Colorado, are frequently modal sources at the 2% in 50 year PE, but 

cease to be as significant for the 10% in 50 year PE. Note also that Cascadia 

subduction M9 sources tend to extend their modal influence further east for the 

10% in 50 year PE, because Cascadia subduction has an estimated recurrence 

time that is relatively short compared to that of many of the local sources of 

eastern Washington, Idaho, and elsewhere, which often dominate the 2% in 50 

year PE modal-event maps, at least at short spectral periods. In general, 

decreasing the PE tends to increase the influence of local sources compared to 

more regional sources (Harmsen et al, 1999) in terms of contributions to the 

probabilistic motion. As long as the mean source recurrence time is relatively 

short compared to the return period under consideration, that source will tend to 

be important in the PSHA, often modal if close to the site, .large in magnitude, or 

both. 

Figures 5 and 6 indicate that in many parts of the WUS, there is little change in 

the modal-source description associated with PGA for the 475-year and 2475-

year return times. However, there are many sites at which closer smaller sources 

may contribute comparably to a given probabilistic motion as more distant larger 

sources for a given PE. At such sites, going from this PE to another in a 

deaggregation analysis will often result in a shift in the mode. Decisions about

appropriate scenario earthquakes at these sites contend with multi-modal 

seismic hazard distributions for a given frequency of motion. One purpose of the 

interactive deaggregation web site is to assist in the determination if any given 

U.S. site has such a distribution of probabilistic sources. 

WUS Seismic Hazard Deaggregation Maps: The Mean 

The mean M or M (read, Mbar) for sites in the WUS is shown in Figure 7, for 1-s

SA on the left and for 0.2-s SA on the right. These maps are for the 2% PE in 50 

years. M is larger at 1 Hz than at 5 Hz because larger sources tend to produce 

seismograms that are enriched in the longer period vibrations. If the mean 

magnitude, distance pair is close to the mode, we can usually infer that the 

distribution of hazard sources is reasonably symmetric about the mean. At typical 

PE levels, such as 2% and 10% in 50 years, most sites receive contributions 

from a broad distribution of source magnitudes and distances. The approximate 

equality of the mean and mode does not imply that only one source strongly 

dominates the seismic hazard at a site, although this statement becomes 

increasingly true at very low PE levels. 

11
Like the modal M, the mean M for 1-s SA is often significantly greater than that 

for 0.2-s SA for a given PE. This is true for much of the WUS, as indicated in 

Figure 7. This can be partly understood by comparing median motion for 1s with 

median motion for PGA and a given PE. In this case μ(M7+, 1s) > μ (M7+,PGA) 

but μ (M6.5-,PGA)> μ (M6.5-,1s), where μ is the median predicted motion for a 

given attenuation model and a given source recorded at a given site. These 

inequalities are meant to indicate that the ratio of median motion for longer period 

SA to shorter period SA progressively rises with magnitude, where other 

conditions are fixed. Thus, at a given level of probabilistic ground motion, larger 

sources are more important contributors at longer periods. 

A comparison of Figure 7 with Figure 1 suggests that Cascadia subduction 

sources, although modal, represent only a small fraction of the seismic hazard in 

much of Washington and Oregon. In other words, Cascadia subduction 

earthquakes often have a low MCF in much of the Pacific Northwest. The fact 

that the mean M is 7 for 1-s SA in much of eastern Washington, while the modal 

M is 9 indicates that relatively low-magnitude local sources (M5 to 6) are quite 

important contributors to the 1-s SA hazard. This is not surprising, because those 

lower-magnitude sources can be modal for the 0.2-s SA in eastern Washington. 

Similar kinds of inferences can be made at many other locations of the WUS by 

comparing Figure 7 with Figure 1. 

Approximate locations of fault lines often may be inferred better from M maps for 

the high-frequency (5 Hz, or 0.2 s) SA of Figure 7 than for the intermediatefrequency (1 Hz, or 1 s) SA. Mainly, lower magnitude gridded sources are more 

significant (i.e., produce more ground-motion exceedances) at short periods than 

at longer periods for a given site and PE, producing a larger contrast in M maps

such as Figure 7 as site location moves away from the fault, whose magnitude 

distribution is typically larger than those of gridded sources. Also, a lesser spatial 

magnitude variation in the longer-period map is explained in part by the fact that 

most attenuation models have larger aleatory σ for longer period SA, tending to 

smear or mix source influences more as spectral period increases. Fault traces 

will be shown on figures that follow. 

Mean R or R for sites in the WUS is shown in Figure 8, for 1-s SA on the left and 

for 0.2-s SA on the right. These maps again show the greater influence of more 

distant sources for longer period probabilistic SA than for shorter period SA. 

Inland from the west coast of the U.S., investigators such as Youngs et al (1997) 

develop their models from ground motion records from subduction sources 

elsewhere, such as Latin America, and the national seismic hazard mapping 

project uses these models to predict Cascadia strong motion. For WUS sites at 

relatively great distances from the Pacific coast, the mean source description is 

far less sensitive than the mode to potential errors in predicted intermediate to 

long-period motion from these Cascadia sources. Taken collectively, local 

gridded sources tend to be far more significant to the intermediate to long-period 

hazard than Cascadia subduction sources east of about 120o W, even though 

12
individual gridded-source bins may be relatively small. Newly included faults such 

as the Saddle Mountain fault in central Washington, while often not modal at the 

2% in 50 year PE, make significant contributions to the mean hazard at many 

sites in the Pacific Northwest. 

Mean ε0 or ε 0 for sites in the WUS is shown in Figure 9, for 1-s SA. On the right 

side of Figure 9 we show, for the first time in this report, a comparison of the 2002 

map data with the 1996 data, by subtracting ε 0 for 1996 data from ε 0 for 2002 

data. A resulting difference greater than 0 implies that (1), new sources are being 

considered, (2), mean recurrence time estimates have decreased, (3), mean 

magnitude has increased, or (4), newly included attenuation models predict that 

the source is more likely to produce a larger ground motion. For example, if a site 

is on the hanging wall of thrust fault, median motion is higher for some 

attenuation models for a given (R,M), a feature that is present in the 2002 hazard 

calculations but not in the 1996 calculations. If ε 0 ('02)- ε 0 ('96) is less than zero, 

one or more of the opposite conditions occurs. For example, at a few spots in 

Nevada, the blue zones along fault traces indicate that mean recurrence rate 

estimates of sources on those faults may have decreased (recurrence times 

increased) compared to 1996 estimates. Reddish colors associated with Nevada 

faults may indicate an increase in estimated recurrence rate. Green-colored 

locations in Figure 9 imply little change or canceling changes since 1996. One 

case of canceling changes is on the hanging wall of the Sierra Madre (SM) thrust 

fault, where a potential increase in hazard associated with new attenuation 

models is more than offset by a decrease in mean recurrence rate of 

earthquakes on the fault resulting from an increase in characteristic M. When 

taken with other changes in the PSHA model, there is a net decrease in 1s 

probabilistic motion (typically about 10% reduction) at such SM hanging wall sites 

in the 2002 update but essentially no change in ε 0 . 

California-Nevada Region Maps of Modal Source Parameters 

Figure 10 shows the modal-source magnitude for the 1-s SA with 2% PE in 50 

years for the states of California and Nevada. Modal-source magnitudes in this

region range from about M6 mostly near Lake Mead to M9 in northern California. 

The white color in Figure 10 indicates where an M9 subduction event dominates 

the hazard, and shades of gray indicate where a large-magnitude San Andreas

fault (SAF) source dominates the hazard. Seismic-wave attenuation models 

extend the modal influence of the megathrust source more than two degrees 

southeast of the Mendocino escarpment, the southern edge of the subducting 

oceanic plate. Similarly, an M8 source on the SAF contributes enough to seismic 

hazard at many sites in the Great Valley, almost 200 km east of fault trace, to be 

modal. The 2% in 50 year probabilisitic ground motions are relatively low in most 

of the Great Valley compared to other parts of western California. This implies 

that more local sources have low probability of occurrence compared to SAF 

main shocks. There is a paucity of recorded strong motion data at larger 

13
magnitudes and distances that might aid in validating large-M and large-R 

attenuation model predictions empirically. 

In central California on a section of the SAF southeast of Monterey Bay, Figure 10

indicates that a M6.5 ± source on the fault dominates the multi-segment rupture, 

higher magnitude, SAF scenarios. Relatively low-magnitude yet modal SAF 

events rupture either the creeping section or the Parkfield segment frequently. 

Compared to the multi-segment SAF sources, these single-segment ruptures 

have high recurrence rates or recurrence times on the order of a few dozen 

years, locally allowing the low-M to dominate the high-M SAF source. 

Figure 11 shows the modal-source distance, or , for the 1-s SA with 2% PE in 

50 years for the states of California and Nevada. The modal-source distance is 

typically less than 5 km at sites near active faults, shown as white lines in Figures 

ˆ R

10, 11 and 12. Even when a site is directly over a fault, the modal-source 

distance can be more than 5 km. This > 5 km phenomenon frequently occurs 

for less-active faults of Nevada and also occurs for a few faults in California. 

Sources inside the 0-to-10 km annulus other than those on the fault may make a 

comparable contribution to the hazard as those on the fault, in which case the 

average distance in that bin can be greater than 5 km. Very distant sources can 

also be modal in California. Greens and blues in the California maps indicate

that either Cascadia or SAF sources are the dominant hazard at distances of 100 

to 200+ km. Cascadia M9 sources also dominate the seismic hazard in parts of 

northern Nevada. 

ˆ R

ˆ R 

Figure 12 shows the modal-source ε0 for the 1-s SA with 2% PE in 50 years for 

the states of California and Nevada. The generally blue color of California's 2% in 

50 year modal event ε0 map (Figure 12) means that the 2%/50year 1-s SA is 

about 1.8 times the median from the modal source, often called a "deterministic" 

motion. If the modal source occurs in Nevada, the yellow to orange to red colors 

of the map (Figure 12) indicate that the probabilistic 1-s SA is lower than the 

median ("deterministic") motion. In western California, the best example of a 

Quaternary fault that has low probabilistic SA associated with it is the longrecurrence-time Rinconada fault within the Salinian block southeast of Monterey 

Bay. The 2% in 50 year 1-s SA is considerably lower than the median for a 

characteristic earthquake on that fault. The rate of random seismicity determined 

from seismic network monitoring is low in the vicinity of the Rinconada fault (Hill 

et al, 1991), resulting in uniquely low, for western California, 1-s ε0 for sites near 

that fault. 

Figures 13, 14, and 15 show the modal-source Mˆ , , and ˆ R ε 

ˆ 

0 , respectively, for

the 0.2-s SA with 2% PE in 50 years for California and Nevada. The modal 

M>7.7 SAF source is far more restricted areally for the 5-hz SA than for 1-hz SA 

(c.f., Figure 10 and Figure 13 gray regions), except in the southern CaliforniaArizona border region, where it dominates seismic hazard sometimes at 

14
distances approaching 200 km. The dominance of Cascadia M9 extends further 

into California's Great Valley for 5-hz SA than for 1-hz SA. This inversion of the 

usual distance effect occurs because subduction-source attenuation models 

attenuate higher frequency vibration less rapidly with distance than crustalsource attenuation models. The ε0 map, Figure 15, indicates that in general, the 

2% in 50 year probabilistic 5- hz motion is about 1.4 to 1.8 times the modal-event 

median for sites west of the SAF, and is somewhat less for many sites east of the 

SAF. The 2% in 50 year 5 Hz motion for sites near the Great Valley blind thrust 

(GVBT) fault system is generally about median for rupture on those faults, except 

for sites on/near the northernmost GVBT, GV1. The probabilistic motion near 

GV1 is significantly less than median, due to its long (9600 year) mean 

recurrence time. Indeed GV1 has about the longest recurrence time of any 

California fault in the data base, but it is comparable to that of many Nevada

faults. The vicinity of such faults is illuminated with red and brown colors in Figure

15. The Reno-Carson City corridor has higher probabilistic ground motion than 

most other parts of Nevada, mostly due to the relatively short recurrence time on 

the Carson Range (Mchar, 970 year) and other fault systems near that important 

urban area. 

For California and Nevada, the modal source contribution factor or MCF is 

plotted in Figure 16. The 1-s SA MCF is shown in the lower left map of Figure 16, 

the 0.2-s SA MCF is shown in the upper left map, and PGA MCF is shown in the 

upper right map, all for the 2% PE in 50 years. Also, the 1-s SA MCF for the 10% 

PE in 50 years is shown in the lower right map of Figure 16. The MCF achieves 

local maxima at sites near several faults of western California, sometimes 

exceeding 0.7 (70% or more of the hazard). For any spectral period, the MCF 

decreases with PE. In Figure 16, this is more evident in Nevada than in California. 

The MCF for 1-s SA and 10% in 50 year PE is generally less than 0.05 (< 5% of 

the hazard) for most sites in Nevada. The answer to the question, "how 

significant is the modal event?" at least with respect to the probabilistic definition, 

is that it is highly dependent on site location and on PE or exposure time. One 

can of course increase bin sizes to increase the MCF of the modal event. It is 

better, however, to recognize that a small MCF often indicates a multi-modal 

seismic-source distribution, and further investigation of non-modal sources might 

be worthwhile for many applications. 

Oregon-Washington Region Maps of Modal Source Parameters 

Various geophysical investigations in Washington and Oregon have yielded 

information on urban-area faults and models of Cascadia subduction that alter 

the USGS PSHA model significantly from that of 1996. Many of these reports are 

referenced in Frankel et al.,2002. Modal-event parameters assist in evaluating 

the relative importance of these sources to Pacific Northwest seismic hazard. 

Figure 17 shows the modal M for the 2% in 50 year PE 1-s SA for Washington, 

Oregon, and parts of Idaho. Fault traces are gray or white in this and subsequent 

15
maps. For 1-s SA, M9 megathrust is the dominant source through most of the 

Pacific NW, except for sites near the few characterized faults, principally, the 

Seattle and S. Whidbey Island faults in the Puget Sound area, and various faults 

in southern and eastern Oregon. Note that many of the Quaternary faults of 

western Oregon and eastern Washington do not provide modal events to the 1-s 

SA hazard at the 2% in 50 year PE, because of their very low recurrence rates 

compared to those of Cascadia sources. 

Figure 18 shows the 1-s modal distance ( ) for the Pacific Northwest region for 

the 2% in 50 year PE. The irregular boundary between the eastern region where 

local seismicity dominates the hazard and the western region where Cascadia 

subduction dominates the hazard has the appearance of being a prominent 

feature of the PSHA model. In fact, this boundary is nothing more than the 

location where there is an exact balance between binned Cascadia hazard and 

binned local hazard, i.e., where the hazard distribution is bimodal (or perhaps 

multimodal). 

ˆ R 

Figure 19 shows the 1-s modal-event ε0 for the region. In western Washington

and Oregon the 2% in 50 year motion generally exceeds median motion for the 

modal source. In eastern Washington and Oregon, the probabilistic motion can 

be more than double the median motion from a Cascadia megathrust source. 

Characteristic earthquakes on some of the Quaternary faults of southeastern 

Washington and Oregon, or on the Seattle fault, for sites sufficiently near those 

faults, yield 2% in 50 year motions that can be below the median motion from 

those sources. 

Figure 20 shows for 5-hz SA for the 2% in 50 year PE for Washington, Oregon, 

and parts of Idaho. A comparison of Figures 

ˆ R 

18 and 20 indicates that Cascadia 

subduction spatially dominates the seismic hazard to a lesser extent for 5-hz SA 

(or for PGA) than for 1-s SA. In eastern Oregon and Washington local random 

sources dominate the hazard, as well as sources on some, but not all, of the 

Quaternary faults in the 2002 model (shown as white lines). Note in Figure 20 the 

mixture of fault sources that do or do not dominate Cascadia subduction sources 

in western Oregon. This behavior is due to a wide range of estimated recurrence 

times for activity on these faults. 

Local random sources dominate the 5-hz SA hazard at Bellingham, Washington, 

a change from the 1996 maps, where Cascadia subduction earthquakes were 

modeled closer to Bellingham, and dominated the hazard. The nearness to 

Bellingham of the boundary between local and M9 Cascadia sources implies that 

5-hz SA (and PGA) has a multi-modal seismic hazard distribution at Bellingham, 

at least when data are binned as above. In fact the distribution of seismic 

sources is trimodal in Bellingham, with a third prominent peak associated with 

deep intraplate seismicity along with local and subduction sources, for higherfrequency SA. Deep intraplate sources like the destructive earthquakes of April 

16
1949, April 1965, and the Nisqually earthquake of February 2001 play a 

prominent role in PSHA at most sites in the vicinity of Puget Sound. 

A Closer Look at Seismic Hazard in Portland, Oregon 

Figure 21 examines the deaggregated seismic hazard source parameters for a 

site in Portland Oregon. Figure 21 is composed of four graphs. The bottom left 

graph of Figure 21 is a hazard curve for 1-s SA, where SA ranges from less than 

0.05 g to about 0.8 g. The vertical axis is mean rate of SA exceedances per year, 

according to the 2002 USGS PSHA model. The solid-line graph is the hazard 

curve, i.e., the curve resulting from all considered sources. The dash-line graph 

is the hazard contribution from the modal or most-likely source. In general, and in 

Portland, these curves are far apart at low SA levels, but converge for high SA 

levels. The upper left graph of Figure 21 is the graph of mean (solid-line) and 

modal (dash-line) distance as a function of 1-s SA at Portland. Note that mean 

distance is a smooth curve that decreases monotonically, while modal distance is 

a step function. At this site, with coordinates 45.5 o

 N, 122.65 o W., the modal 

source distance is initially about 140 km for low SA, and changes significantly 

twice. The upper right graph of Figure 21 is mean and modal M for the site, 1-s 

SA. For low SA, the modal M is 8.3, then jumps to 9.0, then decreases to about 

6.7. The first two modal M values correspond to Cascadia subduction, either a 

partial rupture, or a nearly entire oceanic plate megathrust, from the Mendocino 

escarpment to Vancouver Island. The final modal M corresponds to rupture on 

the Portland Hills fault system. The squares on the graphs are located at the 2% 

in 50 year ground motion, i.e., where the mean rate of exceedance is 0.000404. 

The modal M at this PE corresponds to a characteristic source on the Portland 

Hills fault. 

The lower right graph of Figure 21 is mean and modal ε0 for 1-s SA at Portland. 

Note that mean ε0 increases monotonically with SA, but modal ε0 has a sawtooth 

pattern, dropping sharply at each SA-value where the modal source (R,M) 

changes significantly, then ramping up (≈ linearly with log SA). For the 2% in 50 

year 1-s SA, the ε0 corresponding to the Portland Hills modal source is about -

0.5. Note that ε0 for Cascadia sources in Portland is much higher, ≈ 1 for the M9 

source, at that PE. The sawtooth behavior of ε 

ˆ 

0 with sa is an important feature to 

consider when considering maps of ε 

ˆ 

0 and how they might be applied in specific 

contexts. For example, a low ε 

ˆ 

0 does not imply that non-modal sources also 

have low ε0. 

 

With respect to the relationship between SA0 and ε 

ˆ 

0 for this example, the 1-s SA0

for the 2% in 50 year PE at this site in Portland is 0.375 g. ε 

ˆ 

0 = −0.567. Modal 

(R,M) is (3.06 km, 6.76). The Portland Hills fault is a reverse fault that dips 

southwest, and the site is east of the fault. Using the Campbell and Bozorgnia 

(C&B, 2003) attenuation model, 0.375 g is less than the median motion for a 

17
reverse-slip earthquake with the stated (R,M). Specifically, ε0 from C&B is −0.40 

given that ground motion and (R,M) pair. Note that ε0C&B is close to ε 

ˆ 

0 . However, 

mean (R,M,ε0) for that PE and ground motion is (49.7 km, 7.5, 0.76). ε0= 2.56 for 

that R,M pair using the Campbell and Bozorgnia relation. Portland is one of many 

cities where ε0 computed for the mean (R,M) and any utilized attenuation model 

is substantially larger than ε 0 . In other words, you will not even approximately 

recover the probabilistic ground motion by the quantity exp(μ+ε 0σ) where μ is 

determined using the mean (R,M) (Equation 3 above) but you will approximately 

recover the probabilistic ground motion by the quantity exp(μ+ε 

ˆ 

0σ) where μ is 

determined using the modal (R,M) and a dominant attenuation model (Equation 2 

above). One point of this argument is that efforts to generate seismograms from 

scenario earthquakes that have the mean magnitude and distance and which are 

scaled by the mean ε0 may yield results whose amplitudes are significantly lower 

than the probabilistic SA. 

Maps of Modal Source Parameters in the Intermountain Seismic Belt 

The rapidly growing Salt Lake City-Ogden urban corridor is near the Wasatch 

front fault system, which has long been known to be a substantial earthquake 

threat. Return times for characteristic earthquakes on these faults is 1500 to 

2000 years. Some of the largest conterminous U.S. earthquakes in the latter half 

of the twentieth century occurred in the ISB region, although not on the Wasatch 

front. These include Hebgen Lake, an M7.3 earthquake that occurred in 1959,

and Borah Peak, an M6.9 earthquake that occurred in 1983. 

Figure 22 shows the 1-s SA for the 2% in 50 year PE for the ISB region. The left 

side shows the update-map SA level, in percent g. The 1-s SA can be in excess 

of 60% g for sites near the Wasatch frontal fault system and near faults in 

western Wyoming. The right side shows the change in SA, i.e., the 1-s SA of the 

2002 map minus that of the 1996 map, in the same units. This map shows that 

there has generally been less than 10% g increase or decrease in 1-s SA from 

1996 to 2002. One of the largest changes occurs for sites near the Joes Valley 

fault system, designated JV in Figure 22. For sites near JV, the increase in 1-s SA 

can exceed 25% g. The east-dipping JV fault has a relatively high slip rate, 0.75 

mm/year, which has increased significantly from that used in the 1996 PSHA. 

The broad regional increase on the order of 5 to 10% g in the 1-s SA for 2002 

compared to 1996, evident in Figure 22, results primarily from newly included 

attenuation models. The 0.2-s SA in the ISB region (not shown) generally 

decreases compared to that of 1996, except near a few faults on which rate 

estimates have increased. The regional decrease in 0.2-s SA is due, again, to 

new attenuation models, most notably, Spudich et al (1999). Sufficiently large 

increases in random seismicity rate estimates (a-grid values) can increase the 

0.2-s SA from that of 1996, more than offsetting the lowering effect of new 

18
attenuation models. Pocatello, Idaho, is one city where local a-grid increases 

result in about a 5 to 10% g increase in 0.2-s SA. Changes in a-grid values have 

little influence on the 1-s motion, however, at least at the 2% in 50 year PE. 

Figure 23 shows the 1-s modal-source magnitude, or Mˆ , on the left-side map and 

modal-source distance, or , on the right side, for the 2002 update PSHA and 

the 2% in 50 year PE. 

ˆ R 

Mˆ is generally about M7.1 in the Salt Lake City-Ogden 

urban corridor. At sites near the Provo section of the Wasatch fault system, the 

modal-source M can exceed 7.3. For sites in this urban area, is often less than 

5 km. The PSHA crustal attenuation model changes from a WUS high 

attenuation to a CEUS low attenuation in the region shown in Figures 

ˆ R 

22 and 23. 

The approximate longitude where this change occurs at these latitudes is about 

109o to 110o W. East of this transition, relatively distant random seismicity can 

dominate the hazard, resulting in >150 km, indicated with light to dark blue 

colors in the right-side map of 

ˆ R 

Figure 23. West of this transition longitude, 

relatively close random seismicity, in the 20 to 30 km range, indicated with an 

orange color, can dominate the hazard, at least where no mapped Quaternary 

faults are located in the vicinity of the site. 

ˆ R

Central and Eastern U.S. Modal Source Parameter Maps

Figure 24 shows 1-s and 0.2-s SA modal-source magnitude, or Mˆ , for the 2% in 

50 year PE for the CEUS. As was the case in 1996, the NMSZ mainshock often 

dominates the 1-s CEUS seismic hazard, to the maximum considered source-toreceiver distance of 1000 km. However, modal M for NMSZ mainshocks is now 

7.7 rather than 8.0, and there is a distribution of other possible magnitudes in the 

hazard model. The NMSZ source is located on one of the three S-shaped 

strands that cross the Mississippi River at about 36 to 36.5o

 N, shown as white 

lines. 

The other major seismic hazard source in the CEUS is a mainshock in the 

Coastal Plain of South Carolina, similar to the 1886 earthquake. The modalsource M is 7.3, as it was in 1996, although in 2002, there is a distribution on 

possible magnitudes from 6.8 to 7.5. M7 sources on the Meers fault of southwest 

Oklahoma are modal through much of Oklahoma and Texas, especially for the 

0.2-s SA. The Meers fault, shown as a faint white line, is the only Quaternary 

fault with known location in the region mapped in Figure 24 that is recognized as a 

fault source in the 2002 update (same as in 1996). A few other possible CEUS 

tectonic Quaternary faults are discussed in Crone and Wheeler (2000). 

According to the hazard depicted in Figure 24, several pockets of local seismicity

with mean M < 5 dominate the 0.2-s hazard from eastern Tennessee north to 

Ohio and northeast to Maine, although none of these low-M sources dominates 

the 1-s hazard anywhere in the CEUS. Another prominent source zone on both 

19
1-s and 0.2-s Mˆ maps is the Charlevoix-St. Lawrence River region north of 

Maine. At Charlevoix, the concentric circles of different M modal sources indicate 

that when using a dM=0.2 bin size, sources having a variety of magnitudes from 

about 6.8 to 7.3 may contribute almost equally to the site's hazard. 

Figure 25 shows 1-s and 0.2-s SA modal-source distance, or , for the 2% in 50 

year PE for the CEUS. 

ˆ R

Figure 25 again shows that NMSZ mainshocks can 

dominate the 1-s hazard out to 1000 km in Texas, Minnesota, Wisconsin, and 

several other states. The darker gray regions beyond the 1000-km limit of NMSZ 

hazard indicate that more than 500-km-distant random seismicity dominates the 

hazard at many locations in North Dakota, Minnesota, Pennsylvania, Florida, and 

several other states. Bear in mind that this result is a function of the large 

annulus, R from 500 km to 1000 km, used to evaluate the relative importance of 

distant sources to the seismic hazard when preparing these maps. The next 

largest R-bin, from 300 to 500 km radial distance, also provides a modal source 

to many locations in the CEUS. Figure 25 shows that the modal distance to the 

Charleston source can be quite small, at some locations less than 10 km, for 

sites in the "narrow" zone discussed in greater detail below and in Frankel et al

(2002). 

Many of the red spots in the 0.2-s CEUS map of ˆ R Figure 25 are the result of 

determining an a-value from one or two earthquakes in the CEUS earthquake 

data-base. One such location is the Keweenaw peninsula of northern Michigan, 

whose 0.2-s SA level has been slightly elevated by an earthquake with epicenter 

under Lake Superior. 

The left and right sides of Figure 26 show the modal-source ε0 for the CEUS for 1-

s and 0.2-s SA with 2% in 50 year PE. The 1-s map shows that for much of the 

CEUS the 2% in 50 year probabilistic motion is about 1.8 times the median 

motion from the modal or most likely source. However, at some CEUS sites the 

2%/50 year motion is somewhat less than the median ground motion from the 

modal source. Such site locations are indicated with yellow, orange, red, and 

brown colors in Figure 26. The 0.2-s map also shows large regions for which the 

2% in 50 year motion is greater than the modal-source median motion. Sites in 

the western Long Island/eastern New Jersey region, for example, have 5-hz 

probabilistic motion about 1.2 to 1.4 times the median motion from the modal 

source. Exceptional regions are found near possible locations of the Charleston, 

S.C., mainshock, and the Meers, Oklahoma, fault. At these locations the 

probabilistic motion can be significantly lower than the median of the 

characteristic earthquake. 

Figure 27 is a map of Mˆ for 2-s SA (left side) and 0.1 s SA (right side) for the 

CEUS and the 2% in 50 year PE. Comparing Figure 27 with Figure 24, we see that 

the 2-s Mˆ is usually the same as or very similar to the 1-s Mˆ . A Meers fault 

earthquake dominates the 1-s hazard over a broader region than the 2-s hazard. 

This is due to the fact that the 2-s motion from the NMSZ mainshock is less 

20
rapidly attenuated with distance than the 1-s motion. The local source 

contributes relatively more ground-motion exceedances compared to the regional 

source for shorter-period motion, broadening the modal influence of the nearer 

source in southwestern Oklahoma and parts of Texas. The 10-hz map shows 

even more clearly the dominance by local sources when compared to the 5-hz 

map. Whereas Meers-fault M7 characteristic earthquakes supplant NMSZ M7.7 

characteristic earthquakes in much of Oklahoma for 1-s SA compared to 2-s SA, 

local sources in the M4.5 to 5 range supplant Meers M7 sources in the same 

region for 10-hz SA compared to 5-hz SA. For example, the blue spot 200 km 

northeast of the Meers fault on the right-side map of Figure 27 is the result of a 

locally elevated seismicity rate, partly driven by a mbLg 5.1 earthquake on April 9, 

1952. Similar hierarchies of seismic hazard from local, more regional, more 

distant sources can be found in many other states of the CEUS. In other words, 

there are many CEUS sites for which three or more distinct nearly modal bins in 

the magnitude, distance distribution will exist for higher frequency probabilistic 

motion. 

Figure 28 shows modal-source ε0 (ε 

ˆ 

0 ) for the CEUS PGA at 2% in 50 year PE, left 

side, and 10% PE in 50 years, right side. ε 

ˆ 

0 can be higher for the 10% in 50 year 

motion than for the 2% in 50 year motion, but this cannot occur for ε 

0 . ε 

ˆ 

0 drops 

when the modal-source M or R changes over some considered range of 

probabilities. Recall, in the Portland, Oregon seismic hazard section above, two 

significant changes to modal (R,M) occur, one at a ground motion between the 

10% in 50 year and the 2% in 50 year motions. These PGA ε0 map comparisons 

are presented to remind us that modal event R,M, and ε0 should be considered 

as a single vector entity rather than separately, as scalar statistics. 

Central and Eastern U.S. Mean Source Parameter Maps 

Figure 29 shows the mean-source magnitude, M , units Mw, for the CEUS and for 

1-s and 0.2-s SA, and the 2% PE in 50 years. M for 1-s SA, shown on the left 

map, is usually less than 6.9 but greater than 6.5 except in the lowest-hazard 

regions. M rises in the South Carolina coastal plain to about 7.3, and rises in 

the NMSZ to about 7.7. M for 0.2-s SA, shown on the right map, is lower for the 

most part, less than 6 in many parts of the map. In the central part of the NMSZ, 

M rises to about 7.7. In the northeast US, the 0.2-s SA M is in the 6 to 6.5 

range for the most part. 

Figure 30 shows the mean source distance, or R , in km, for the CEUS and for 1-s

and 0.2-s spectral periods. The mean distance from CEUS sites to CEUS 

sources is generally greater than R in the WUS, and the 1-s mean distance (left 

side) is generally significantly greater than the 0.2-s SA mean distance. The 

former fact is primarily due to the lower rate of attenuation of seismic waves in 

the CEUS crust compared to WUS crust. The changeover longitude is modeled 

21
as a sharp boundary - one which varies with latitude - but a more gradual 

transition is probably more likely. Epicenter concentrations that have produced 

higher than average a-values show up fairly well in the 0.2-s SA M map, but they

are generally not visible at intermediate to long period SA. The most prominent 

exception to this observation is the Charlevoix- St Lawrence River seismicity, 

which is important to local and some regional seismic hazard at all spectral 

periods. 

Figure 31 shows CEUS-region maps of the mean-source ε0, the left map for the 

1-s and the right map for the 0.2-s spectral periods, for the 2% in 50 year PE. 

These maps show that over broad regions the 2% in 50 year motion is greater 

than median motion from the mathematically defined but otherwise nebulous 

"mean" source, even for sites near the NMSZ, shown as three white S-shaped 

curves. The only geographically extensive region of Figure 31 where 2% in 50 

year motion does not approach the median motion from the mean source is in 

the vicinity of the Meers fault of southwest Oklahoma, shown as a white line in 

Figure 31. 

New Madrid Seismic Zone Modal Source Parameters 

The NMSZ has the highest probabilistic ground motions in the CEUS. The 1811-

1812 mainshocks have estimated M ranging from 7 to 8 and greater. In the 1996 

PSHA maps, the NMSZ characteristic sources were modeled with M=8. In the

2002 maps, the characteristic source is modeled with a distribution of M from 7.3 

to 8. The modal magnitude tends to be M7.7 for a wide range of spectral periods 

and return times for most sites in the vicinity of the NMSZ. In 1996, the estimated 

mean recurrence time of this source was 1000 years. In 2002 the estimated 

mean recurrence time is 500 years (Frankel et al., 2002), based on recent 

research findings of paleoseismologists. The simultaneous lowering of M and 

raising of recurrence rate produce offsetting effects on probabilistic motion for the 

2% in 50 year PE. The higher rate does however increase the 10% in 50 year PE 

SA levels considerably from those of 1996. 

Figure 32 exhibits some details of the new PSHA model for the NMSZ region and 

the 5-hz SA, for the 2% in 50 year PE. The upper left map of Figure 32 is the 5-hz 

SA in percent g. The white lines represent the range of locations used to model 

the NMSZ fault(s). The central fault trace follows the epicenter trend of monitored 

microseismicity (Frankel et al., 2002). The probabilistic motion within 10 to 20 km 

of the central trace exceeds 300% g and the motion within a few km of the 

eastern and western traces exceeds 250%g. At a distance of 150 km, the 5-hz 

probabilistic SA has dropped to about 40% g, an 8-fold decrease compared to 

the maximum on the central trace. For rock sites in Memphis, Tennessee, the 

probabilistic motion exceeds 110% g. 

22
The upper right map of Figure 32 compares the probabilistic 5-hz SA for 2002 with 

that of the 1996 maps by plotting their ratio, SA(2002)/SA(1996) (1 represents no 

change). In general there is a slight increase in g.m. for 2002, rarely exceeding 

30% g in the region shown. At most locations in the mapped region, the increase 

is less than 10% g. The gray fault traces in this figure are at the locations of the 

three NMSZ faults in the 1996 PSHA, while the white fault traces correspond to 

the 2002 PSHA. The change in location of the eastern trace as well as the 

greater weight assigned to the middle trace in the 2002 model explain the 

increase in 5-Hz SA shown in this figure, which in two spots exceeds 20%. 

The modal-event magnitude everywhere in the mapped region of Figure 32 is 

M7.7. The lower left map of Figure 32 is the modal-event ε0 for the 5-hz SA and 

the 2% in 50 year PE. The three NMSZ possible fault locations are again shown 

as white lines. At most locations ε0 is greater than 0, meaning that the 2% in 50 

year motion exceeds median motion from potential NMSZ M7.7 mainshocks. 

There are yellow bands just outside the eastern and western traces, indicating

locations where modal-source ε0 is slightly less than zero. 

The lower right map of Figure 32 is modal-source distance ( ) for the 5-hz SA 

and the 2% in 50 year PE. This map indicates that can be as low as 10 km for 

sites near the central NMSZ fault trace (the central white line). The NMSZ faults 

are modeled such that the top or shallowest edge is at 10 km depth. This fact 

coupled with the use of a Rrup-type distance metric yields a 10 km modal-source 

distance for sites such as Cairo Illinois over or almost over the central fault trace. 

Sites in Memphis are about 30 to 40 km from the nearest location on the eastern 

trace shown. M7.7 sources on the eastern trace are modal for sites in Memphis, 

even though the eastern and western fault sources are given only 25% weight 

each in the PSHA model of Frankel et al (2002). 

ˆ R 

ˆ R

South Carolina Modal Source Parameters 

Parts of South Carolina have relatively high probabilistic ground motion, second 

only to that at sites near the NMSZ in the CEUS. Large probabilistic motion 

results from the relatively short mean recurrence time of the 1886 -like 

characteristic earthquake, 550 years in the 2002 PSHA model. Owing to the low 

rate of seismic attenuation in the CEUS, the influence on probabilistic ground 

motion of the Charleston source is great throughout South Carolina and other 

southeast U.S. states. With respect to impact on PSHA maps, the main new 

feature of the 2002 update in the vicinity of the South Carolina coastal plain is the 

inclusion - with 50% weight - of a narrow zone of Charleston-like seismic 

sources, areally the same as the south Zone of River Anomalies, or ZRA of 

Marple and Talwani (2000). The updated hazard model mean recurrence time of 

the Charleston-like source is shorter than the 650-year estimate used in the 1996 

model calculations, also resulting in an increase in probabilistic SA and PGA. 

23
Figure 33 is a four-map examination of the new seismic hazard for 0.2-s SA, and 

2% in 50 year PE in South Carolina and vicinity. The upper left map is the 5-Hz 

probabilistic SA, which can exceed 200% g in the ZRA (the "narrow zone"). The 

upper right map is the ratio SA(2002)/SA(1996). The most notable feature is the 

increase in 5-Hz SA in the narrow zone, a change of up to 40%, or in units, 60% 

g to 80% g. This map also shows that in spite of the shorter Charleston 

mainshock recurrence time, several factors more than counterbalance the rate 

effect to produce a net decrease in 5 Hz probabilistic motion by a small amount 

(generally 0 to 10% g) in most of the state away from the ZRA. These factors 

include (1), a more comprehensive set of ground-motion attenuation models, (2), 

the greater average distance to the Charleston source for sites sufficiently far 

from the ZRA, and (3), truncation of sa at μ+3σ. 

In the 2002 update, there is a distribution of magnitude (i.e., logic tree) for 

Charleston sources, ranging from M6.8 to M7.5, discussed in Frankel et al

(2002), whereas in the 1996 PSHA, Charleston-source M was fixed at 7.3. It is 

not transparent whether the 2002 M distribution increases or decreases 

probabilistic motion relative to the fixed-M model. Analysis indicates that for the 

2% in 50 year PE, the distribution of M with weights as defined in the 2002 

update decreases probabilistic SA and PGA by up to a few percent compared to 

using the fixed-M Charleston source model, for sites throughout South Carolina 

and vicinity. The decrease is greatest in the Coastal Plain. 

The lower left map of Figure 33 is 5-hz SA modal-source magnitude, or Mˆ , for the 

2002 PSHA model. Mˆ is M7.3 to M7.36 in most of the state. Mˆ > 7.3 results 

from the addition to the modal bin of random seismicity sources with M as great 

as 7.46 in the extended margin (Frankel et al., 2002). M7.5 in the extended 

margin is the same Mmax as that assumed for the 1996 hazard calculations; 

however, in the stable craton Mmax was raised in the 2002 calculations, from 6.5 

to 7.0. Only the northwest corner of South Carolina has a substantially lower Mˆ 

than M7.3, due to the influence of the eastern Tennessee seismic zone (ETSZ), 

which is the most active region, seismically, of all of the Appalachian Mountains. 

Clemson, S.C., is located in a region where the probabilistic source distribution is 

bimodal, with (R,M) peaks associated with an 1886-like M7.3 earthquake with 

epicenter in the Atlantic Coastal Plain and with an ETSZ M5± earthquake. 

The lower right map of Figure 33 is the 5-hz SA modal-source distance, or , in 

km, for South Carolina and the 2% in 50 year PE. , i.e., the distance to the 

most likely source, is between 5 and 10 km in a cigar-shaped region 

corresponding to, but lengthier than, the ZRA. Finite-length sources with strike 

parallel to the main axis of the ZRA with notional fault centers in the ZRA can 

extend outside the ZRA, whose boundaries are shown in Figure 3 of Frankel et 

al. (2002). These finite notional sources are the reason for the long zone where 

 is less than 10 km. for sites in Charleston, S.C. is about 15 to 20 km. 

ˆ R 

ˆ R

ˆ R ˆ R 

24
Summary

This report exhibits maps of modal and mean source parameters for the 2% in 50 

year PE according to the new USGS seismic hazard model documented in 

Frankel et al. (2002). These maps cover the conterminous U.S. More detailed 

maps of a few sub-regions with higher than average probabilistic ground motion 

are also presented. In a few cases, the 10% in 50 year modal-event maps have 

been shown alongside the 2% in 50 year maps for comparison. 

At one city, Portland, Oregon, graphs of mean and modal R, M, and ε0 are 

presented with the probabilistic seismic hazard curve for 1-s SA to demonstrate 

how modal source parameters vary discontinuously with increasing ground 

motion, while mean source parameters vary smoothly with ground motion. In 

Portland, the modal source may be a Cascadia subduction earthquake or rupture 

on a local fault, depending on exposure time, or level of ground motion. At many 

locations, a similar variation in the dominant source with exposure time may be 

expected. Typically, a more frequently occurring source at a more regional 

distance dominates the hazard (i.e., is modal) for low SA levels, and a closer, but 

less frequent (longer mean recurrence time) source dominates the hazard at 

higher SA levels. 

Conclusions and Discussion 

Sensitivity studies about features of regionally dominant sources, such as that of 

Cramer (2001), suggest that such analysis can provide useful information to 

those who make decisions on research priorities and hazard mitigation 

strategies. That is, increments in knowledge and understanding of some features 

of such dominant sources, such as fault location, recurrence rate, and so on, 

may have greater impact on estimates of probabilistic ground motions than other 

increments, and may therefore merit more research effort. Deaggregation 

analysis can be used to determine these sources, the degree of dominance, for 

example MCF maps like Figure 16, and the basis for their dominance, including 

the ground-motion prediction equations that may be pivotal in making such a 

determination. 

The 2002 USGS PSHA update often yields the same or similar modal sources as 

the 1996 USGS PSHA. This may be verified by comparing the maps of this 

report with those of Harmsen et al (1999) and Harmsen and Frankel (2001). 

Many hazard details have changed, some of which are visible on nation-scale 

maps, but most of which are best appreciated by performing site-specific seismic 

hazard deaggregation. At the 2002 interactive deaggregation web site, 

http://eqint.cr.usgs.gov/eq/html/deaggint2002.html, the user may perform these 

25
deaggregations and learn about new and modified sources. Several new or 

previously unused attenuation models have also been included in the 2002 

USGS PSHA. Although their effects are in the results, we have not attempted in 

this report to deaggregate hazard from different attenuation models. 

PSHA maps may change significantly when additional research and 

understanding indicate the need to modify strong-motion attenuation models 

and/or seismic sources, e.g., their location, their size, and their likelihood of 

occurring in a random or specific future period. The most obvious way to 

document the impact of these changes is to present maps of differences or ratios 

of probabilistic motion, as has been done in s. 22, 32, and 33 of this report. The 

change in mean ε0 , such as that presented in Figure 9, is a less obvious indicator 

of changes in PSHA map input data. However, maps such as Figure 9 may 

complement other comparison maps, such as ratios of R , differences of M , or 

other contrasts that one might consider when trying to evaluate potential impact 

on seismic safety decisions of significant changes in our understanding of 

seismic hazard in the USA. Ideally, deaggregation products will be considered

when incorporating an evolving understanding of the seismic hazard into the 

design and retrofit decision process. Considering maps such as Figure 9 that 

compare "old" and "new" probabilistic source parameters provides a more 

comprehensive basis of background information for making informed decisions 

than just considering differences in probabilistic motion that result from updates 

of the USGS or any other PSHA.

One purpose of deaggregation analysis is to find plausible (R,M) pairs from 

which to choose accelerograms, A(t), for input to seismic design programs for 

structural response. If one chooses A(t) corresponding to the mean (R,M), at 

many sites and for the 2% in 50 year (or other) PE, the accelerograms will tend 

to yield spectral accelerations that are significantly lower than the probabilistic 

spectral acceleration, as in Eqn. (5) above. Therefore, it is frequently necessary 

to scale seismograms to the probabilistic motion. 

While the mean is an important parameter for characterizing a statistical 

distribution, its usefulness has been challenged in the seismic-resistant design 

application for another reason. For sites that have multimodal hazard 

distributions, the mean magnitude, distance pair can represent a source that has 

little or no probability of occurrence. For example, at Portland Oregon, the mean 

(M, R) for 1-s SA and the 2% in 50 year PE is (7.5, 50 km), an "amalgamation" of 

local sources at lower magnitudes and much more distant subduction sources 

having M8.3 or M9. This multi-modal feature could conceivably result in an 

undesirable choice of earthquake A(t) to resist if design decisions are tied very 

closely to resisting certain (M,R) pairs, such as the mean (M,R), but not others 

for a given SA and PE. The maps of mean magnitude, distance, and ε0 presented 

here are intended to convey information about the distribution of probabilistic 

seismic sources rather than to provide prescriptions or suggestions for seismic 

sources to use in building design or retrofit projects. 

26
The information of deaggregation analysis can and perhaps should be 

considered in a complex seismic-resistant design decision-making environment. 

Acknowledgements: 

This document was reviewed by E.V. Leyendecker, U.S.G.S. Nancy Dickman 

helped prepare the web-viewable version of the document. Most figures were 

prepared from GMT by Wessel and Smith. 

References Cited 

Abrahamson, N. A. and K. M. Shedlock, 1997. Some comparisons between 

recent ground-motion relations, Seism. Res. Lett., 68, pp 9-23. 

Abrahamson, N. A. and W. J. Silva, 1997. Empirical response spectral 

attenuation relations for shallow crustal earthquakes, Seism. Res. Lett., 68, pp 

94-127. 

Campbell, K.W., and Y. Bozorgnia, 2003. Updated near-source ground motion 

(attenuation) relations for the horizontal and vertical components of peak 

ground acceleration and acceleration response spectra, . Bull. Seism. Soc. 

Am. , 93, 314-331. 

Cramer, C. H., 2001. The New Madrid seismic zone: Capturing variability in 

seismic hazard analysis. Seism. Res. Lett., 72, 664-672. 

Crone, T. and R. Wheeler, 2000. Data for Quaternary faults, liquefaction 

features, and possible tectonic features east of the Rocky Mountains. USGS 

OFR 2000-0260, at URL http://pubs.usgs.gov/of/2002/ofr-00-0260/. 

Frankel, A., C. Mueller, T. Barnhard, D. Perkins, E. Leyendecker, N. Dickman, S. 

Hanson, and M. Hopper (1996), National seismic-hazard maps: 

documentation, June 1996, U.S. Geol. Surv. Open-File Rept., 96-532, 110 pp. 

Frankel, Arthur D., Mark D. Petersen, Charles S. Mueller, Kathleen M. Haller, 

Russell L. Wheeler, E.V. Leyendecker, Robert L. Wesson, Stephen C. 

27
Harmsen, Chris H. Cramer, David M. Perkins, and Kenneth S. Rukstales 

(2002). Documentation for the 2002 Update of the National Seismic Hazard 

Maps, U.S. Geological Survey Open-File Report 02-420. 

Harmsen, S., 2001. Mean and modal epsilon in the deaggregation of probabilistic 

ground motion, Bull. Seism. Soc. Am. 91, 1537-1552. 

Harmsen, S., D. Perkins, and A. Frankel, 1999. Deaggregation of Probabilistic 

Ground Motions in the Central and Eastern United States, Bull. Seism. Soc. 

Am. 89, 1-13. 

Harmsen, S. and A. Frankel, 2001. Geographic deaggregation of seismic hazard 

in the United States. Bull. Seism. Soc. Am. 91, 13-26. 

Hill, D.P., J.P. Eaton, W.L. Ellsworth, R.S. Cockerham, F.W. Lester, and E.J. 

Corbett, 1991. The seismotectonic fabric of central California, ch. 8 of 

Neotectonics of North America: Decade Map Volume.Geol. Soc Am., Boulder, 

CO. pp. 107-132. 

Hudson, D. E., 1979. Reading and Interpreting Strong Motion Accelerograms, 

Earthquake Engineering Research Institute, Berkeley, CA. 112 p. 

Marple, R. T. and P. Talwani, 2000. Evidence for a buried fault system in the 

Coastal Plain of the Carolinas and Virginia - Implication for neotectonics in the 

southeastern United States, Geol. Soc. Am. Bull.,112, p. 200-220. 

McGuire, Robin K. (1995). Probabilistic seismic hazard analysis and design 

earthquakes: closing the loop. Bull. Seism. Soc. Am., 85, 1275-1284. 

Mueller, C. S., A. D. Frankel, M. D. Petersen, and E. V. Leyendecker, 2003. 

Probabilistic Seismic hazard maps for Puerto Rico and the U.S. Virgin Islands,

Seism. Res. Lett. 74, p. 207. 

Sadigh, K., C.-Y. Chang, J. A. Egan, F. Makdisi, and R.R. Youngs, 1997. 

Attenuation relationships for shallow crustal earthquakes based on California 

strong-motion data, Seism. Res. Lett., 68, 180-189. 

28
Spudich, P., W.B. Joyner, A.G. Lindh, D.M. Boore, B.M. Margaris, and J.B. 

Fletcher, 1999. SEA99: A revised ground motion prediction relation for use in 

extensional tectonic regimes, Bull. Seism. Soc. Am., 89, 1156-1170.

Youngs, R.R., S.-J. Chiou, W.J. Silva and J.R. Humphrey, 1997. Strong groundmotion relationships for subduction zone earthquakes, Seism. Res. Lett., 68, 

pp. 58-73. 

Figures 

Figure 1. Maps of modal-event magnitude (or Mhat) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 2. Maps of modal-event distance (or Rhat) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 3. Maps of modal-event ε0 (or epsilon-sub-zero) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 4. Maps of modal-event magnitude in the western U.S. for the PSHA 

model of Frankel et al. (2002), for the 2% in 50 year probability of exceedance. 

Left side, for 2.0-s spectral acceleration. Right side, for 0.1-s, or 10-Hz, spectral 

acceleration. 

Figure 5. Maps of modal-event magnitude in the western U.S. for the PSHA 

model of Frankel et al. (2002), for peak horizontal ground acceleration, or PGA.

Left side, for the 2% in 50 year probability of exceedance. Right side, for the10% 

in 50 year probability of exceedance. 

Figure 6. Maps of modal-event distance in the western U.S. for the PSHA model 

of Frankel et al. (2002), for peak horizontal ground acceleration, or PGA. Left 

side, for the 2% in 50 year probability of exceedance. Right side, for the10% in 

50 year probability of exceedance.

Figure 7. Maps of mean-event magnitude (or Mbar) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

29
exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 8. Maps of mean-event distance (or Rbar) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 9. Maps of mean-event ε0 (or epsilon-sub-zero) in the western U.S. for the 

PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for the change 

in mean ε0 from that associated with the previous PSHA model of Frankel et al. 

(1996). 

Figure 10. Map of modal-event magnitude (or Mhat) in the California-Nevada

region for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance and for 1.0-s spectral acceleration. Light gray is used 

to denote areas where the dominant seismic-hazard source is Cascadia M9 

megathrust. Medium and dark gray are used to denote regions where the 

dominant seismic-hazard source is a large San Andreas fault earthquake. 

Figure 11. Map of modal-event distance (or Rhat) in the California-Nevada region 

for the PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance and for 1.0-s spectral acceleration. Gray is used to denote regions 

where a nearby fault is the dominant source of seismic hazard. 

Figure 12. Map of modal-event ε0 (or epsilon-sub-zero hat) in the CaliforniaNevada region for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance and for 1.0-s spectral acceleration. 

Figure 13. Map of modal-event magnitude (or Mhat) in the California-Nevada

region for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance and for 0.2-s or 5-Hz spectral acceleration. 

Figure 14. Map of modal-event distance (or Rhat) in the California-Nevada region 

for the PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance and for 0.2-s or 5-Hz spectral acceleration.

Figure 15. Map of modal-event ε0 (or epsilon-sub-zero hat) in the CaliforniaNevada region for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance and for 0.2-s or 5-Hz spectral acceleration. 

Figure 16. Maps of mode-contribution-factor for California-Nevada region. Bottom 

left, for 1-s SA and for 2% in 50 year probability of exceedance. Bottom right, for 

1-s SA and for 10% in 50 year probability of exceedance. Top left, for 0.2-s SA 

and for 2% in 50 year probability of exceedance. Top right, for PGA and for 2% 

30
in 50 year probability of exceedance. Yellow to orange colors indicate that one 

source or a narrow range of sources (in magnitude and distance) contribute the 

majority of the seismic hazard. A light blue color indicates that many sources, 

over a broad range of magnitudes and/or distances, contribute to the seismic 

hazard at the site. 

Figure 17. Map of modal-event magnitude (or Mhat) in the Pacific Northwest, 

principally Washington and Oregon, for the PSHA model of Frankel et al. (2002), 

for the 2% in 50 year probability of exceedance and for 1.0-s spectral 

acceleration. A white color is used to denote areas where the dominant seismichazard source is Cascadia M9 megathrust. Quaternary fault traces are gray. 

Figure 18. Map of modal-event distance (or Rhat) in the Pacific Northwest, 

principally Washington and Oregon, for the PSHA model of Frankel et al. (2002), 

for the 2% in 50 year probability of exceedance and for 1.0-s spectral 

acceleration. Quaternary fault traces are white. 

Figure 19. Map of modal-event ε0 (or epsilon-sub-zero hat) in the Pacific 

Northwest, for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance and for 1.0-s spectral acceleration. Quaternary fault 

traces are white. 

Figure 20. Map of modal-event distance (or Rhat) in the Pacific Northwest, 

principally Washington and Oregon, for the PSHA model of Frankel et al. (2002), 

for the 2% in 50 year probability of exceedance and for 0.2-s spectral 

acceleration. Quaternary fault traces are white. 

Figure 21. Graphs depicting probabilistic seismic-hazard for a site in Portland, 

Oregon using the PSHA model of Frankel et al. (2002). All graphs are for 1-s 

spectral acceleration. Lower left, mean seismic hazard from all sources (solid 

curve) and for the modal source (dashed curve). Upper left, mean-event distance 

(km) (solid curve) and modal-event distance (dashed curve). Upper right, meanevent magnitude (M) (solid graph) and modal-event magnitude (dashed curve). 

Lower right, mean-event epsilon-sub-zero (ε0) (solid curve) and modal-event 

epsilon-sub-zero (dashed curve). 

Figure 22. Maps of probabilistic 1-s spectral acceleration (or SA, units: g) for the 

Intermountain Seismic Belt region and for the 2% in 50 year probability of 

exceedance. Left side, for the PSHA model of Frankel et al. (2002). Right side, 

change in 1-s SA from the model of Frankel et al. (1996), mapped as the 

difference SA1 (2002) – SA1(1996). 

Figure 23. Maps of deaggregated seismic hazard parameters for the 

Intermountain Seismic Belt region for 1-s SA, and for the 2% in 50 year 

probability of exceedance. Left side, modal-event magnitude. Right side, modalevent distance (km). Quaternary fault traces are white. 

31
Figure 24. Maps of modal-event magnitude (or Mhat) in the central and eastern 

U.S. for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance. Left side, for 1.0-s spectral acceleration. Right side, 

for 0.2-s, or 5-Hz, spectral acceleration. Possible New Madrid fault locations are 

shown as three white traces. 

Figure 25. Maps of modal-event distance (R) in the central and eastern U.S. for 

the PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 26. Maps of modal-event ε0 (or epsilon-sub-zero hat) in the central and 

eastern U.S. for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance. Left side, for 1.0-s spectral acceleration. Right side, 

for 0.2-s, or 5-Hz, spectral acceleration. 

Figure 27. Maps of modal-event magnitude (or Mhat) in the central and eastern 

U.S. for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance. Left side, for 2.0-s spectral acceleration. Right side, 

for 0.1-s, or 10-Hz, spectral acceleration. Possible New Madrid fault locations are 

shown as three white traces. 

Figure 28. Maps of modal-event ε0 (or epsilon-sub-zero hat) in the central and 

eastern U.S. for the PSHA model of Frankel et al. (2002) for peak horizontal 

ground acceleration, or PGA. Left side, for the 2% in 50 year probability of 

exceedance. Right side, for the 10% in 50 year probability of exceedance. 

Figure 29. Maps of mean-event magnitude (or Mbar) in the central and eastern 

U.S. for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance. Left side, for 1.0-s spectral acceleration. Right side, 

for 0.2-s, or 5-Hz, spectral acceleration. 

Figure 30. Maps of mean-event distance (or Rbar) in the central and eastern U.S. 

for the PSHA model of Frankel et al. (2002), for the 2% in 50 year probability of 

exceedance. Left side, for 1.0-s spectral acceleration. Right side, for 0.2-s, or 5-

Hz, spectral acceleration. 

Figure 31. Maps of mean-event ε0 (or epsilon-sub-zero bar) in the central and 

eastern U.S. for the PSHA model of Frankel et al. (2002), for the 2% in 50 year 

probability of exceedance. Left side, for 1.0-s spectral acceleration. Right side, 

for 0.2-s, or 5-Hz, spectral acceleration. 

Figure 32. Deaggregated seismic hazard for 0.2-s spectral acceleration with 2% 

in 50 year probability of exceedance in the New Madrid Seismic Zone and 

vicinity. Top left, spectral acceleration (units, g) for PSHA model of Frankel et al.

32
(2002). Top right, difference in spectral acceleration from the PSHA model of 

Frankel et al. (1996). Bottom left, modal-event ε0 (or epsilon-sub-zero hat). 

Bottom right, modal-event distance (or Rhat, units: km). Possible locations of 

NMSZ fault system are shown as three white traces. 

Figure 33. Deaggregated seismic hazard for 0.2-s spectral acceleration (or SA) 

with 2% in 50 year probability of exceedance in South Carolina and vicinity. Top 

left, spectral acceleration (units, g) for PSHA model of Frankel et al. (2002). Top 

right, difference in spectral acceleration from the PSHA model of Frankel et al.

(1996) expressed as the ratio SA(2002)/SA(1996). Bottom left, modal-event 

magnitude. Bottom right, modal-event distance (units: km). Rivers are shown as 

blue lines. 

33