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U.S. NUCLEAR REGULATORY COMMISSION March 2007

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 1.208

(Draft was issued as DG-1146, dated October 2006)

A PERFORMANCE-BASED APPROACH TO DEFINE

THE SITE-SPECIFIC EARTHQUAKE GROUND MOTION

A. INTRODUCTION

This regulatory guide provides an alternative for use in satisfying the requirements set forth in

Section 100.23,“Geologic and Seismic Siting Criteria,” of Title 10, Part 100, of the Code of Federal

Regulations (10 CFR Part 100), “Reactor Site Criteria” (Ref. 1). The NRC staff incorporated

developments in ground motion estimation models; updated models for earthquake sources; methods for

determining site response; and new methods for defining a site-specific, performance-based earthquake

ground motion that satisfy the requirements of 10 CFR 100.23 and lead to the establishment of the safe

shutdown earthquake ground motion (SSE). The purpose of this regulatory guide is to provide guidance

on the development of the site-specific ground motion response spectrum (GMRS). The GMRS

represents the first part of the development of the SSE for a site as a characterization of the regional and

local seismic hazard. The final SSE must satisfy both 10 CFR 100.23 and Appendix S, “Earthquake

Engineering Criteria for Nuclear Power Plants,” to 10 CFR Part 50, “Domestic Licensing of Production

and Utilization Facilities” (Ref. 2).

In 10 CFR 100.23, paragraph (c), “Geological, Seismological, and Engineering Characteristics,”

and paragraph (d), “Geologic and Seismic Siting Factors,” require that the geological, seismological, and

engineering characteristics of a site and its environs be investigated in sufficient scope and detail to

permit an adequate evaluation of the proposed site, provide sufficient information to support evaluations

performed to arrive at estimates of the SSE, and permit adequate engineering solutions to actual or

potential geologic and seismic effects at the proposed site. Data on the vibratory ground motion; tectonic

and non-tectonic surface deformation; earthquake recurrence rates, fault geometry and slip rates, site

foundation material; seismically induced floods and water waves; and other siting factors and design

conditions should be obtained by reviewing pertinent literature and carrying out field investigations.
RG 1.208, Page 2

10 CFR 100.23, paragraph (d)(1), “Determination of the Safe Shutdown Earthquake Ground

Motion,” requires that uncertainty inherent in estimates of the SSE be addressed through an appropriate

analysis, such as a probabilistic seismic hazard analysis (PSHA).

In Appendix A, “General Design Criteria for Nuclear Power Plants,” to 10 CFR Part 50, General

Design Criterion (GDC) 2, “Design Bases for Protection Against Natural Phenomena,” requires, in part,

that nuclear power plant structures, systems, and components (SSCs) important to safety must be

designed to withstand the effects of natural phenomena (such as earthquakes) without loss of capability

to perform their safety functions. Such SSCs must also be designed to accommodate the effects of, and

be compatible with, the environmental conditions associated with normal operation and postulated

accidents.

Appendix S to 10 CFR Part 50 specifies, in part, requirements for implementing GDC 2 with

respect to earthquakes. Paragraph IV(a)(1)(i), of Appendix S to 10 CFR Part 50, requires that the SSE

must be characterized by free-field ground motion response spectra at the free ground surface. In view of

the limited data available on vibratory ground motions of strong earthquakes, it will usually be

appropriate that the design response spectra be smoothed spectra compatible with site characteristics. In

addition, the horizontal motions in the free field at the foundation level of the structures consistent with

the SSE must be an appropriate response spectrum with a peak ground acceleration of at least 0.1g.

The 10 CFR Part 50 licensing practice required applicants to acquire a construction permit (CP),

and then apply for an operating license (OL) during construction. Alternatively, the requirements and

procedures applicable to the NRC’s issuance of early site permits (ESPs) and combined licenses (COLs)

for nuclear power facilities are set forth in Subparts A and C of 10 CFR Part 52, “Licenses, Certifications,

and Approvals for Nuclear Power Plants” (Ref. 3), respectively.

This regulatory guide has been developed to provide general guidance on methods acceptable to

the NRC staff for (1) conducting geological, geophysical, seismological, and geotechnical investigations;

(2) identifying and characterizing seismic sources; (3) conducting a probabilistic seismic hazard

assessment (PSHA); (4) determining seismic wave transmission (soil amplification) characteristics of soil

and rock sites; and (5) determining a site-specific, performance-based GMRS, satisfying the requirements

of paragraphs (c), (d)(1), and (d)(2) of 10 CFR 100.23, and leading to the establishment of an SSE to

satisfy the design requirements of Appendix S to 10 CFR Part 50. The steps necessary to develop the

final SSE are described in Chapter 3, “Design of Structures, Components, Equipment and Systems,” of

NUREG-0800, “Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power

Plants” (Ref. 4). Regulatory Position 5.4 of this guide provides a more detailed description of the

development of the final SSE.

This regulatory guide contains several appendices that address the objectives stated above. 

Appendix A contains a list of definitions. Appendix B contains a list of abbreviations and acronyms. 

Appendix C discusses a procedure to characterize site geology, seismology, and geophysics. Appendix D

describes the procedure to determine the controlling earthquakes. Appendix E describes the procedure

for determining seismic wave transmission (soil amplification) characteristics of soil and rock sites.

Appendix F provides criteria for developing and evaluating modified ground motions used for soil

response analyses.

This regulatory guide may address information collections that are covered by the requirements

of 10 CFR Part 50, 10 CFR Part 52, and 10 CFR Part 100, which the Office of Management and Budget

(OMB) approved under OMB control numbers 3150-0011, 3150-0151, and 3150-0093, respectively. The

NRC may neither conduct nor sponsor, and a person is not required to respond to, an information

collection request or requirement unless the requesting document displays a currently valid OMB control

number.
The CEUS and WUS Time History databases are provided in NUREG/CR-6728 (Ref. 6).

1

RG 1.208, Page 3

B. DISCUSSION

Background

Regulatory Guide 1.165 (Ref. 5) provides general guidance on procedures acceptable to the NRC

staff to satisfy the requirements of 10 CFR 100.23. This regulatory guide provides alternative guidance

that incorporates developments in ground motion estimation models; updated models for earthquake

sources; methods for determining site response; and new methods for defining a site-specific,

performance-based GMRS. Therefore, the NRC staff is providing this regulatory guide as an alternative

to Regulatory Guide 1.165 (Ref. 5) to satisfy, in part, the requirements of 10 CFR 100.23 and Appendix

S to 10 CFR Part 50.

The general process to determine a site-specific, performance-based GMRS includes the

following:

(1) site- and region-specific geological, seismological, geophysical, and geotechnical investigations

(2) a probabilistic seismic hazard analysis (PSHA)

(3) a site response analysis to incorporate the effects of local geology and topography

(4) the selection of appropriate performance goals and methodology

Geological, seismological, and geophysical investigations are performed to develop an up-todate, site-specific, earth science database that supports site characterization and a PSHA. The results of

these investigations will also be used to assess whether new data and their interpretation are consistent

with the information used in probabilistic seismic hazard studies previously accepted by the NRC.

The PSHA is conducted with up-to-date interpretations of earthquake sources, earthquake

recurrence, and strong ground motion estimation. The site seismic hazard characteristics are quantified

by the seismic hazard curves from a PSHA and the uniform hazard response spectra (UHRS) that cover a

broad range of natural frequencies. The hazard curves are developed in part by identifying and

characterizing each seismic source in terms of maximum magnitude, magnitude recurrence relationship,

and source geometry. The rock-based ground motion at a site resulting from the combined effect of all

sources can then be determined through the use of attenuation relationships.

Seismic wave transmission (site amplification) procedures are necessary to obtain appropriate

UHRS at the free-field ground surface if the shear wave velocity of the surficial material is less than the

generic rock conditions appropriate for the rock-based attenuation relationships used in the PSHA. A

database of earthquake time histories on rock for both the central and eastern United States (CEUS) and

the western United States (WUS) has been developed to determine the dynamic site response for the soil

or rock site conditions.1
The reference probability is computed as the median probability obtained from the distribution of median probabilities 2

of exceeding the SSE at 29 sites in the CEUS. The sites selected were intended to represent relatively recent (to the

study) designs that used the Regulatory Guide 1.60 (Ref. 8), design response spectrum, or similar spectrum, as their

design bases. The use of the reference probability ensures an adequate level of conservatism in determining the SSE

consistent with recent licensing decisions.

The PF P D, R , and H criteria corresponding to Seismic Design Category (SDC) 5 in Reference 7 (which are equivalent to 3

nuclear power plant requirements) are used.

RG 1.208, Page 4

A performance-based approach is described in Chapters 1 and 2 of ASCE/SEI Standard 43-05

(Ref. 7), instead of the reference probability approach described in Appendix B to Regulatory Guide

1.165 F(Ref. 5). The performance-based approach employs Target Performance Goal (P ), Probability 2

Ratio P D (R ), and Hazard Exceedance Probability (H ) criteria to ensure that nuclear power plants can 3

withstand the effects of earthquakes with a desired performance, the desired performance being

expressed as the target value of 1 E-05 for the mean annual probability of exceedance (frequency) of the

onset of significant inelastic deformation (FOSID). This approach targets a performance criteria for

SSCs which is defined relative to the onset of inelastic deformation; instead of relative to the occurrence

of failure of the SSC. The mean annual probability of exceedance associated with this performance

target was chosen based on this performance criterion.

One of the objectives in developing the performance-based GMRS is to achieve approximately

consistent performance for SSCs, across a range of seismic environments, annual probabilities, and

F

structural frequencies. The intent is to develop a site-specific GMRS that achieves the P that ensures

that the performance of the SSCs related to safety and radiological protection is acceptable.

Basis for Regulatory Positions

Site- and Region-Specific Investigations to Characterize Site Geology, Seismology, and Geophysics

The primary objective of geological, seismological, and geophysical investigations is to develop

an up-to-date, site-specific earth science database that supports site characterization and a PSHA. The

results of these investigations will also be used to assess whether new data and their interpretation are

consistent with the most recent information used in probabilistic seismic hazard studies accepted by the

NRC. If the new data, such as new seismic sources and new ground motion attenuation relationships, are

consistent with the existing earth science database, updating or modification of information used in the

hazard analysis is not required. It will be necessary to update seismic sources and ground motion

attenuation relationships for sites where there is significant new information resulting from the site

investigations.

Features Discovered During Construction

It should be demonstrated that deformation features discovered during construction (particularly

faults, potential soft zones, or other features of engineering significance) do not have the potential to

compromise the safety of the plant. Applying the combined licensing procedure to a site could result in

issuing a license prior to the start of construction. During the construction of nuclear power plants

licensed in the past, previously unknown faults were often discovered in site excavations. Before

issuance of the OL, it was necessary to demonstrate that faults exposed in the excavation posed no hazard

to the facility. Under the combined license procedure, these kinds of features should be mapped and

assessed as to their rupture and ground motion generating potential while the excavations’ walls and

bases are exposed, and the NRC staff should be notified when excavations are open for inspection.
RG 1.208, Page 5

Probabilistic Seismic Hazard Analysis Methods

A PSHA has been identified in 10 CFR 100.23 as a means to address the uncertainties in the

determination of the SSE. Furthermore, the rule recognizes that the nature of uncertainty and the

appropriate approach to account for uncertainties depend on the tectonic setting of the site and on

properly characterizing input parameters to the PSHA, such as the seismic source characteristics, the

recurrence of earthquakes within a seismic source, the maximum magnitude of earthquakes within a

seismic source, and engineering estimation of earthquake ground motion through attenuation

relationships.

Every site is unique; therefore, requirements for analyses and investigations vary. It is not

possible to provide procedures for addressing all situations. In cases that are not specifically addressed

in this regulatory guide, prudent and sound engineering judgment should be exercised.

Probabilistic procedures were developed specifically for nuclear power plant seismic hazard

assessments in the CEUS, also referred to as the Stable Continent Region (SCR). Experience has been

gained by applying this methodology at nuclear facility sites, both reactor and non-reactor sites,

throughout the United States. The experience with these applications also served as the basis for the

guidelines for conducting a PSHA in Reference 9. The guidelines detailed in Reference 9 should be

incorporated into the PSHA process to the extent possible. These procedures provide a structured

approach for decision making with respect to site-specific investigations. A PSHA provides a framework

to address the uncertainties associated with the identification and characterization of seismic sources by

incorporating multiple interpretations of seismological parameters. A PSHA also provides a means for

evaluation of the likelihood of SSE occurrence during the design lifetime of a given facility, given the

recurrence interval and recurrence pattern of earthquakes in pertinent seismic sources. Within the

framework of a probabilistic analysis, uncertainties in the characterization of seismic sources and ground

motions are identified and incorporated in the procedure at each step of the process for estimating the

GMRS. The role of geological, seismological, and geophysical investigations is to develop geoscience

information about the site for use in the detailed design analysis of the facility, as well as to ensure that

the seismic hazard analysis is based on up-to-date information.

Experience in performing seismic hazard evaluations in active plate-margin regions in the WUS

(for example, the San Gregorio-Hosgri fault zone and the Cascadia Subduction Zone) has also identified

uncertainties associated with the characterization of seismic sources (Refs. 10–12). Sources of

uncertainty include fault geometry, rupture segmentation, rupture extent, seismic-activity rate, ground

motion, and earthquake occurrence modeling. As is the case for sites in the CEUS, alternative

hypotheses and parameters must be considered to account for these uncertainties.

Uncertainties associated with the identification and characterization of seismic sources in

tectonic environments in both the CEUS and the WUS should be evaluated. While the most recent

characterization of any seismic source accepted by the NRC staff can be used as a starting point for

analysis of a new facility, any new information related to a seismic source that impacts the hazard

calculations must be evaluated and incorporated into the PSHA as appropriate based on the technical

information available.
RG 1.208, Page 6

Central and Eastern United States

The CEUS is considered to be that part of the United States east of the Rocky Mountain front, or

east of Longitude 105E West (Refs. 13, 14). A PSHA in the CEUS must account for credible alternative

seismic source models through the use of a decision tree with appropriate weighting factors that are

based on the most up-to-date information and relative confidence in alternative characterizations for each

seismic source. Seismic sources identified and characterized by the Lawrence Livermore National

Laboratory (LLNL) (Refs. 13–15) and the Electric Power Research Institute (EPRI) (Refs. 16, 17) were

used for studies in the CEUS in the past. In addition to the LLNL and EPRI resources, the United States

Geological Survey also maintains a large database of seismic sources for both the CEUS and the WUS. 

The characterization of specific seismic sources found in these databases may still represent the latest

information available at the time that a PSHA is to be undertaken. However, if more up-to-date

information is available, it should be incorporated.

In the CEUS, characterization of seismic sources is more problematic than in active plate-margin

regions because there is generally no clear association between seismicity and known tectonic structures

or near-surface geology. In general, the observed geologic structures are a result of response to tectonic

forces that may have little or no correlation with current tectonic forces. Therefore, it is important to

account for this uncertainty by the use of multiple alternative source models.

The identification of seismic sources and reasonable alternatives in the CEUS considers

hypotheses currently advocated for the occurrence of earthquakes in the CEUS (e.g., the reactivation of

favorably oriented zones of weakness or the local amplification and release of stresses concentrated

around a geologic structure). In tectonically active areas of the CEUS, such as the New Madrid Seismic

Zone, where geological, seismological, and geophysical evidence suggest the nature of the sources that

generate the earthquakes, it may be more appropriate to characterize those seismic sources by using

procedures similar to those normally applied in the WUS.

Western United States

The WUS is considered to be that part of the United States that lies west of the Rocky Mountain

front, or west of approximately 105E West Longitude. In some regions of the WUS, significant research

into seismic sources is ongoing and the results of this research should be evaluated and addressed.

The active plate-margin regions include, for example, coastal California, Oregon, Washington,

and Alaska. For the active plate-margin regions, where earthquakes can often be correlated with known

faults that have experienced repeated movements at or near the ground surface during the Quaternary,

tectonic structures should be assessed for their earthquake and surface deformation potential. In these

regions, at least three types of sources may exist:

(1) faults that are known to be at or near the surface

(2) buried (blind) sources that may often be manifested as folds at the earth’s surface

(3) subduction zone sources, such as those in the Pacific Northwest and Alaska

The nature of surface faults can be evaluated by conventional surface and near-surface

investigation techniques to assess orientation, geometry, sense of displacements, length of rupture,

Quaternary history, etc. Possibilities of multiple ruptures should be considered, as appropriate.

Buried (blind) faults are often associated with surficial deformation such as folding, uplift, or

subsidence. The surface expression of blind faulting can be detected by mapping the uplifted or downdropped geomorphological features or stratigraphy, survey leveling, and geodetic methods. The nature of

the structure at depth can often be evaluated by deep core borings and geophysical techniques.
RG 1.208, Page 7

Continental U.S. subduction zones are located in the Pacific Northwest and Alaska. Seismic

sources associated with subduction zones are sources within the overriding plate, on the interface

between the subducting and overriding lithospheric plates, and in the interior of the downgoing oceanic

slab. The characterization of subduction zone seismic sources should consider the three-dimensional

geometry of the subducting plate, rupture segmentation of subduction zones, geometry of historical

ruptures, constraints on the up-dip and down-dip extent of rupture, and comparisons with other

subducting plates worldwide.

The Basin and Range region of the WUS, and to a lesser extent the Pacific Northwest and the

central United States, exhibit temporal clustering of earthquakes. Temporal clustering is best

exemplified by the rupture histories within the Wasatch fault zone in Utah, where several large, late

Holocene coseismic faulting events occurred at relatively close intervals (for hundreds to thousands of

years) that were preceded by long periods of quiescence that lasted thousands to tens of thousands of

years. Temporal clustering should be considered in these regions, or wherever paleoseismic evidence

indicates that it has occurred.

Ground motion attenuation relationships for the WUS are also continually being developed and

revised. Significant effort has been undertaken by the Pacific Earthquake Engineering Research Center’s

Lifelines program to develop Next Generation Attenuation (NGA) relationships for shallow crustal

earthquakes in active tectonic regions based on the large database of earthquake recordings now available

for this tectonic setting. The suitability of the NGA relationships or other WUS relationships for a

particular site will be evaluated on a case-by-case basis.

Lower-Bound Magnitude Cutoff

Current seismic hazard analysis methods generally utilize a lower-bound body wave magnitude

cut-off value for earthquakes to be included in the PSHA. This lower-bound magnitude cut-off level is a

conservatively defined value based on research studies whose objective was to estimate the damage

potential of smaller earthquakes. Reference 18 establishes an appropriate distribution of low-magnitude

earthquakes in the seismic hazard analysis through the use of the cumulative absolute velocity (CAV)

model, in place of a lower-bound magnitude cutoff.

Several ground motion measures such as peak ground acceleration, Arias intensity, root mean

square acceleration, and CAV were evaluated to determine the single ground motion measure that is best

correlated with the threshold of potential damage. The CAV was determined to be the best parameter

correlating damage with the Modified Mercalli Intensity Scale, and a CAV value of 0.16 g-sec was found

to be a conservative characterization of the threshold between damaging and non-damaging earthquake

motions for buildings of good design and construction. An empirical model for estimating CAV in terms

of magnitude, peak ground acceleration (PGA), and duration is needed because the PSHA calculation

does not use time histories directly.

Spectral Frequency Range Considered in the Probabilistic Seismic Hazard Analyses

The frequency range considered in the PSHA should extend from a low frequency that can be

reliably obtained from the current strong-motion data set from the WUS, principally California, to a highfrequency limit that permits the ratio of spectral acceleration to PGA to reach nearly 1 for hard rock site

conditions. For the CEUS this high-frequency limit is approximately 100 Hz. For soft rock conditions,

the ratio of spectral acceleration to PGA will reach 1 at about 40–50 Hz. To obtain a smooth plot of

spectral response across the frequency range of interest, the hazard assessment should be provided for a

minimum of 30 frequencies, approximately equally spaced on a logarithmic frequency axis between 100

and 0.1 Hz.
RG 1.208, Page 8

Deaggregation of Mean Hazard

Deaggregation provides information useful for review of the PSHA and insight into the seismic

sources that most impact the hazard at a particular site. It can also be used to determine the controlling

earthquakes (i.e., magnitudes and distances) which can be used to perform dynamic site response

analyses.

Analysis of multiple ground motion levels are used to obtain a more complete understanding of

the earthquake characteristics (i.e., mean magnitudes and distances) that contribute to the high-frequency

(5 and 10 Hz) and low-frequency (1 and 2.5 Hz) hazard, than could be obtained from a single ground

motion level (e.g., 1 E-05/yr).

Choice of Epsilon in Probabilistic Seismic Hazard Analyses

Epsilon, the number of standard deviations included in defining the distribution of ground

motions for each magnitude and distance scenario, can have a significant impact on the results of the

PSHA. Care should be taken in choosing a value for epsilon large enough such that natural aleatory

variability in ground motions is adequately addressed. A study conducted by EPRI and the U.S.

Department of Energy (DOE) (Ref. 19) found no technical basis for truncating the ground motion

distribution at a specified number of standard deviations (epsilons) below that implied by the strength of

the geologic materials. Even though very large ground motions may have a low probability of

occurrence, when the hazard is calculated for low annual frequencies of exceedance, low probability

events need to be considered.

Site Response Analysis

For rock or soil sites where the free-field ground surface shear wave velocity is less than the

generic rock conditions defined by the attenuation relationships used in the PSHA, seismic wave

transmission (site amplification) procedures should be used to obtain appropriate UHRS at the free-field

ground surface. Earthquake time histories and time-domain procedures are one method that may be used

to determine the dynamic site response for the soil or rock site conditions. When using this method,

enough earthquake time history records should be used to get consistent mean results. Reference 6

contains a database of recorded time histories on rock for both the CEUS and WUS that can be used as

seed records for input time histories for analyses. The records provided in Reference 6 should be used

consistently with the recommendations for use in Reference 6 (e.g., the records were developed to be

used as seed motions for spectral matching programs, rather than to be used directly). The database is

divided into magnitude and distance bins, with each bin having a minimum of 15, three-component (two

horizontal and one vertical) sets.
RG 1.208, Page 9

Performance-Based Method

A performance-based approach is described in Chapters 1 and 2 of ASCE/SEI Standard 43-05

(Standard) (Ref. 7). The Standard was developed to provide performance-based criteria for the seismic

design of safety-related SSCs for use in nuclear facilities, covering a broad range of facilities that

process, store, or handle radioactive materials. The goal of the Standard is to present seismic design

criteria using a graded approach commensurate with the relative importance to safety and safeguards,

which will ensure that nuclear facilities can withstand the effects of earthquakes with desired

performance. The performance-based approach used for this regulatory guide is based on the premise

that the seismic demand and structural capacity evaluation criteria laid out by the NRC’s Standard

Review Plan (SRP) (NUREG-0800) ensure sufficient conservatism to reasonably achieve both of the

following:

(1) less than about a 1% probability of unacceptable performance for the site-specific response

spectrum ground motion

(2) less than about a 10% probability of unacceptable performance for a ground motion equal to

150% of the site-specific response spectrum ground motion

The Standard, F for the most demanding seismic design category (SDC 5), recommends using a P

= 1 E P D -05 for the FOSID, R = 10, and H = 1 E-04 for the minimum structural damage state, which is

described as essentially elastic behavior. Specifically, essentially elastic behavior means that localized

inelasticity might occur at stress concentration points, but the overall seismic response will be essentially

elastic.

The performance-based approach combines a conservative characterization of ground motion

hazard with equipment/structure performance (fragility characteristics) to establish risk-consistent

GMRS, rather than only hazard-consistent ground shaking, as occurs using the hazard reference

probability approach in Appendix B to Regulatory Guide 1.165 (Ref. 5). The performance target (the

mean annual probability of SSCs reaching the limit state of inelastic response) results from the

modification of the UHRS at the free-field ground surface by a design factor to obtain the performancebased site-specific GMRS. The design factor achieves a relatively consistent annual probability of plant

component failure across the range of plant locations and structural frequencies. It does this by

accounting for the slope of the seismic hazard curve, which changes with structural frequency and site

location. The design factor ensures that the site-specific response spectrum is equal to or greater than the

mean 1 E-04 UHRS.

The flow chart in Figure 1 depicts the procedures described in this regulatory guide.
RG 1.208, Page 10

Geological, Geophysical, Seismological, and

Geotechnical Investigations

Regulatory Position 1 Appendix C

Seismic Sources Significant to the

Site Seismic Hazard

Regulatory Position 2 Appendix C

Probabilistic Seismic Hazard Analysis

(PSHA) Procedures

Regulatory Position 3

Deaggregation of Mean Hazard

Regulatory Position 3.5 Appendix D

Seismic Wave Transmission

(Soil Amplification) Characteristics of the Site

Regulatory Position 4 Appendices E & F

Performance-Based Site-Specific Earthquake

Ground Motion Procedure

Regulatory Position 5

Figure 1. Flow Diagram
RG 1.208, Page 11

C. REGULATORY POSITION

1. Geological, Geophysical, Seismological, and Geotechnical Investigations

1.1 Comprehensive Site Area and Region Investigations

Comprehensive geological, seismological, geophysical, and geotechnical engineering

investigations of the site area and region should be performed. For existing nuclear power plant sites

where additional units are planned, the geosciences technical information originally used to validate

those sites may need to be updated. Depending on (1) how much new or additional information has

become available since the initial investigations and analyses were performed, and (2) the complexity of

the site and regional geology and seismology, additional data may be required. This technical

information should be used, along with all other available information, to plan and determine the scope

of additional investigations. The investigations described in this regulatory guide are performed

primarily to gather information needed to confirm the suitability of the site and to gather data pertinent to

the safe design and construction of the nuclear power plant. Appropriate geological, seismological, and

geophysical investigations are described in Appendix C to this regulatory guide. Geotechnical

investigations are described in Regulatory Guide 1.132 (Ref. 20). Another important purpose for the sitespecific investigations is to determine whether there are any new data or interpretations that are not

adequately incorporated into the existing PSHA databases. Appendix C also describes a method for

assessing the impact of new information, obtained during the site-specific investigations on the databases

used for the PSHA.

These investigations should be performed at four levels, with the degree of detail based on (1)

distance from the site, (2) the nature of the Quaternary tectonic regime, (3) the geological complexity of

the site and region, (4) the existence of potential seismic sources, and (5) the potential for surface

deformations, etc. A more detailed discussion of the areas and levels of investigations and the bases for

them are presented in Appendix C to this regulatory guide. General guidelines for the levels of

investigation are characterized as follows.

1.1.1 Within a Radius of 320 Kilometers (200 mi) of the Site (Site Region)

Conduct regional geological and seismological investigations to identify seismic sources. These

investigations should include literature reviews, the study of maps and remote sensing data and, if

necessary, onsite ground-truth reconnaissance.

1.1.2 Within a Radius of 40 Kilometers (25 mi) of the Site (Site Vicinity)

Geological, seismological, and geophysical investigations should be carried out in greater detail

than the regional investigations, to identify and characterize the seismic and surface deformation

potential of any capable tectonic sources and the seismic potential of seismogenic sources, or to

demonstrate that such structures are not present. Sites with capable tectonic sources within a radius of 40

kilometers (km) (25 mi) may require more extensive geological and seismological investigations and

analyses [similar in detail to investigations and analysis usually preferred within an 8 km (5 mi) radius].

1.1.3 Within a Radius of 8 Kilometers (5 mi) of the Site (Site Area)

Detailed geological, seismological, geophysical, and geotechnical engineering investigations

should be conducted to evaluate the potential for tectonic deformation at or near the ground surface and

to assess the transmission characteristics of soils and rocks in the site vicinity.
RG 1.208, Page 12

1.1.4 Within a Radius of Approximately 1 Kilometer (0.6 mi) of the Site (Site Location)

Very detailed geological, geophysical, and geotechnical engineering investigations should be

conducted to assess specific soil and rock characteristics, as described in Reference 20.

1.2 Expanding the Areas of Investigation

The areas of investigation may need to be expanded beyond those specified above in regions that

include capable tectonic sources, relatively high seismicity, or complex geology, or in regions that have

experienced a large, geologically recent earthquake identified in historical records or by paleoseismic

data.

1.3 Features Discovered During Construction

It should be demonstrated that deformation features discovered during construction (particularly

faults, potential soft zones, or other features of engineering significance) do not have the potential to

compromise the safety of the plant. The newly identified features should be mapped and assessed as to

their rupture and ground motion generating potential while the excavations’ walls and bases are exposed. 

A commitment should be made, in documents (safety analysis reports) supporting the license application,

to geologically map all excavations of significant size and to notify the NRC staff when excavations are

open for inspection. This should apply to excavations for construction of all Seismic Category I

structures, as a minimum.

1.4 Justifying Assumptions and Conclusions

Data sufficient to clearly justify all assumptions and conclusions should be presented. Because

engineering solutions cannot always be satisfactorily demonstrated for the effects of permanent ground

displacement, it is prudent to avoid a site that has a potential for surface or near-surface deformation.

Such sites normally will require extensive additional investigations.

1.5 Characterize Lithologic, Stratigraphic, Hydrologic, and Structural Geologic Conditions

For the site and for the area surrounding the site, the lithologic, stratigraphic, hydrologic, and

structural geologic conditions should be characterized. The investigations should include the

measurement of the static and dynamic engineering properties of the materials underlying the site as well

as an evaluation of the physical evidence concerning the behavior during prior earthquakes of the

surficial materials and the substrata underlying the site. The properties needed to assess the behavior of

the underlying material during earthquakes should be measured. These include the potential for

liquefaction and the characteristics of the underlying material in transmitting earthquake ground motions

to the foundations of the plant (such as seismic wave velocities, density, water content, porosity, elastic

moduli, and strength).
RG 1.208, Page 13

2. Seismic Sources Significant to the Site Seismic Hazard

2.1 Evaluation of New Seismic Sources

For sites in the CEUS, existing databases may be used to identify seismic sources to perform

PSHA. Previously unidentified seismic sources that were not included in these databases should be

appropriately characterized and sensitivity analyses performed to assess their significance to the seismic

hazard estimate. The results of investigation discussed in Regulatory Position 1 should be used, in

accordance with Appendix C to this regulatory guide, to determine whether the seismic sources and their

characterization should be updated. The guidance in Regulatory Positions 2.2 and 2.3 (below) and the

methods in Appendix C to this regulatory guide may be used if additional seismic sources are to be

developed as a result of investigations.

2.2 Use of Alternative Seismic Sources

When existing methods and databases are not used or are not applicable, the guidance in

Regulatory Position 2.3 should be used for identification and characterization of seismic sources. The

uncertainties in the characterization of seismic sources should be addressed. “Seismic sources” is a

general term that is equivalent to capable tectonic sources.

Identification and characterization of seismic sources should be based on regional and site

geological and geophysical data, historical and instrumental seismicity data, the regional stress field, and

geological evidence of prehistoric earthquakes. Investigations to identify seismic sources are described

in Appendix C to this regulatory guide. The bases for the identification of seismic sources should be

described. A general list of characteristics to be evaluated for seismic sources is presented in Appendix

C.

2.3 Characterizing Seismic Potential

As part of the seismic source characterization, the seismic potential for each source should be

evaluated. Typically, characterization of the seismic potential consists of four equally important

elements:

(1) selection of a model for the spatial distribution of earthquakes in a source

(2) selection of a model for the temporal distribution of earthquakes in a source

(3) selection of a model for the relative frequency of earthquakes of various magnitudes, including

an estimate for the largest earthquake that could occur in the source under the current tectonic

regime

(4) a complete description of the uncertainty

For example, truncated exponential and characteristic earthquake models are often used for the

distribution of magnitudes. A stationary Poisson process is used to model the spatial and temporal

occurrences of earthquakes in a source. For a general discussion of evaluating the earthquake potential

and characterizing the uncertainty, refer to NUREG/CR-6372 (Ref. 9).
RG 1.208, Page 14

2.3.1 Characterizing Seismic Potential When Alternative Methods and Databases Are Used

For sites in the CEUS, the seismic sources and data accepted by the NRC in past licensing

decisions may be used as a starting point, along with the data gathered from the investigations carried out

as described in Regulatory Position 1.

Generally, the seismic sources for the CEUS are broad areal source zones because there is

uncertainty about the underlying causes of earthquakes. This uncertainty is caused by a lack of active

surface faulting, a low rate of seismic activity, or a short historical record. The assessment of earthquake

recurrence for CEUS area sources commonly relies heavily on catalogs of historic earthquakes. Because

these catalogs are incomplete and cover a relatively short period of time, the earthquake recurrence rate

may not be estimated reliably. Considerable care must be taken to correct for incompleteness and to

model the uncertainty in the rate of earthquake recurrence. To completely characterize the seismic

potential for a source, it is also necessary to estimate the largest earthquake magnitude that a seismic

source is capable of generating under the current tectonic regime. This estimated magnitude defines the

upper bound of the earthquake recurrence relationship.

Primary methods for assessing maximum earthquakes for area sources usually include a

consideration of the historical seismicity record, the pattern and rate of seismic activity, the Quaternary

geologic record (1.8 million years and younger), characteristics of the source, the current stress regime

(and how it aligns with known tectonic structures), paleoseismic data, and analogs to sources in other

regions considered tectonically similar to the CEUS. Because of the shortness of the historical catalog

and low rate of seismic activity, considerable judgment is needed. It is important to characterize the

large uncertainties in the assessment of the earthquake potential (Refs. 9 and 16).

2.3.2 Characterizing Seismic Potential for Western United States Sites

For sites located within the WUS, earthquakes can often be associated with known tectonic

structures with a high degree of certainty. For faults, the earthquake potential is related to the

characteristics of the estimated future rupture, such as the total rupture area, the length, or the amount of

fault displacement. Empirical relations can be used to estimate the earthquake potential from fault

behavior data and also to estimate the amount of displacement that might be expected for a given

magnitude. It is prudent to use several different rupture length or area-versus-magnitude relations to

obtain an estimate of the earthquake magnitude. When such correlations are used, the earthquake

potential is often evaluated as the mean of the distribution. The difficult issue is the evaluation of the

appropriate rupture dimension to be used. This is a judgmental process based on geological data for the

fault in question and the behavior of other regional fault systems of the same type.

In addition to maximum magnitude, other elements of the recurrence model are generally

obtained using catalogs of seismicity, fault slip rate, and other data. Known sources of uncertainty

should be appropriately modeled.

2.3.3 Characterizing Seismic Potential for Sites Near Subduction Zones

For sites near subduction zones, such as in the Pacific Northwest and Alaska, the maximum

magnitude must be assessed for subduction zone seismic sources. Worldwide observations indicate that

the largest known earthquakes are associated with the plate boundary interface, although intraslab

earthquakes may also have large magnitudes. The assessment of plate interface earthquakes can be based

on estimates of the expected dimensions of rupture or analogies to other subduction zones worldwide.
RG 1.208, Page 15

3. Probabilistic Seismic Hazard Analysis Procedures

A PSHA should be performed to allow for the use of multiple source models to estimate the

likelihood of earthquake ground motions occurring at a site and to systematically take into account

uncertainties that exist in various parameters (such as seismic sources, maximum earthquakes, and

ground motion attenuation). Alternative hypotheses are considered in a quantitative fashion in a PSHA.

They can be used to evaluate the sensitivity of the hazard to the uncertainties in the significant

parameters and to identify the relative contribution of each seismic source to the hazard.

The following steps describe a procedure acceptable to the NRC staff for performing a PSHA.

3.1 Perform Regional and Site Investigations

Perform regional and site geological, seismological, and geophysical investigations in

accordance with Regulatory Position 1 and Appendix C to this regulatory guide.

3.2 Evaluation of New Information

For CEUS sites, perform an evaluation of seismic sources, in accordance with Appendix C to this

regulatory guide, to determine whether they are consistent with the site-specific data gathered in

Regulatory Position 3.1 or if they require updating. The PSHA should be updated if the new information

indicates that the current version of the seismic source model underestimates the hazard or if new

attenuation relationships are available. In most cases, limited-scope sensitivity studies are sufficient to

demonstrate that the existing database in the PSHA envelops the findings from site-specific

investigations. Any significant update should follow the guidance of NUREG/CR-6372 (Ref. 9).

3.3 Conduct a Probabilistic Seismic Hazard Analysis

Perform a site-specific PSHA with up-to-date interpretations of earthquake sources, earthquake

recurrence, and strong ground motion estimation using original or updated sources as determined in

Regulatory Positions 3.1 and 3.2. CAV filtering can be used in place of a lower-bound magnitude cutoff

of 5.0 (Ref. 18). For the CEUS, the understanding of single- and double-corner ground-motion models is

evolving, and the PSHA should fully document the appropriateness of using either or both of these

models. Characterize epistemic uncertainties in a complete and defensible fashion. Aleatory variability

of the ground motion as expressed in the choice of standard deviation of natural log of ground motion

used in the PSHA should be considered carefully and described. The ground motion estimates should be

made in the free field to develop the seismic hazard information base discussed in Appendix D to this

regulatory guide.

3.4 Hazard Assessment

Report fractile hazard curves at the following fractile levels (p) for each ground motion

parameter: 0.05, 0.16, 0.50, 0.84, and 0.95, as well as the mean. Report the fractile hazard curves in

tabular as well as graphical format. Also, determine the mean UHRS for annual exceedance frequencies

of 1 E-04, 1 E-05, and 1 E-06 at a minimum of 30 structural frequencies approximately equally spaced on

a logarithmic frequency axis between 100 and 0.1 Hz.
RG 1.208, Page 16

3.5 Deaggregate the Mean Probabilistic Hazard Characterization

Deaggregate the mean probabilistic hazard characterization at ground motion levels

corresponding to the annual frequencies of 1 E-04, 1 E-05, and 1 E-06 in accordance with Appendix D to

this regulatory guide. Deaggregation provides information useful for review of the PSHA and insight

into the seismic sources that most impact the hazard at a particular site. It can also be used to determine

the controlling earthquakes (i.e., magnitudes and distances) and document the hazard information base as

discussed in Appendix D.

4. Seismic Wave Transmission Characteristics of the Site

The hazard curves from the PSHA are defined for general surficial rock conditions that are

consistent with the definition of rock for the attenuation relationships used in the PSHA. For example,

existing attenuation relationships for the CEUS typically define generic rock conditions as materials with

a shear wave velocity (Vs) of 2.8 km/sec (9,200 ft/sec). Because the surficial and subsurface shear wave

velocities at nuclear power plant sites are generally less than those for the generic rock conditions, the

following procedures should be used to define the UHRS at the free-field ground surface. Additional

discussion pertaining to this regulatory position is provided in Appendix E to this regulatory guide.

For both soil and rock sites, the subsurface model should extend to sufficient depth to reach the

generic rock conditions as defined in the attenuation relationships used in the PSHA.

4.1 Site and Laboratory Investigations and Testing

Perform site and laboratory investigations and testing to obtain data defining the static and

dynamic engineering properties of the site-specific soil and rock materials. Consideration should be

given to spatial distribution both within individual geologic strata and across the site. Methods

acceptable to the NRC staff are described in Regulatory Guide 1.132 (Ref. 20), Regulatory Guide 1.138

(Ref. 21), and Subsection C.2.2.2 of Appendix C to this regulatory guide. Sufficient site property data

should be developed to adequately estimate the mean and variability of the properties in the site response

calculations. When soil models are deduced from laboratory studies, the sampling and testing programs

should be critically peer reviewed to ensure that the generated soil dynamic nonlinear properties are

appropriate to properly characterize site response.

4.2 Dynamic Site Response

Using the site-specific dynamic properties of the soil and information on hazard levels from the

rock-based site-specific UHRS, the dynamic site response should be determined through analyses based

on either time history or random vibration theory (RVT). If a time history-based procedure is used, it is

necessary to determine an appropriate set of ground motions or earthquake time histories for each of the

target response spectra to be analyzed. A sufficient number of time histories should be used to obtain

consistent average behavior from the dynamic site response analyses. The target response spectra may be

based on Controlling Earthquakes, as discussed in Appendix E.

RVT methods characterize the input rock motion using a Fourier amplitude spectrum instead of

time domain earthquake input motions. This spectrum is propagated through the soil to the surface using

frequency domain transfer functions and computing peak ground accelerations or spectral accelerations

using extreme value statistics. In the past, traditional site response analyses using time histories has been

most prevalent. However, RVT methods are acceptable as long as the strain dependent soil properties

are adequately accounted for in the analysis.
RG 1.208, Page 17

Reference 6 contains a database of recorded time histories on rock for both the CEUS and WUS

to be used as seed motions for site response analyses. The database is divided into magnitude and

distance bins, with each bin having a minimum of 15, three-component (two horizontal and one vertical)

sets. If these records are used as seed motions, they should be developed consistent with the

recommendations in Reference 6. Appendix F to this regulatory guide provides criteria acceptable to the

NRC staff for developing and evaluating sets of modified ground motions used for soil response and

structural analyses. The time histories are scaled or spectrally matched to match the response spectrum

and have characteristics appropriate for the corresponding target response spectrum.

Often vertically propagating shear waves are the dominant contributor to free-field ground

motions at a site. In these cases, a one-dimensional equivalent-linear analysis or nonlinear analysis that

assumes vertical propagation of shear waves may be appropriate. However, site characteristics (such as a

dipping bedrock surface, topographic effects, or other impedance boundaries), regional characteristics

(such as certain topographic effects), and source characteristics (such as nearby dipping seismic sources)

may require that analyses are also able to account for inclined waves.

A Monte Carlo or equivalent procedure should be used to accommodate the variability in soil

depth, shear wave velocities, layer thicknesses, and strain-dependent dynamic nonlinear material

properties at the site. A sufficient number of convolution analyses should be performed using scaled

earthquake time histories to adequately capture the effect of the site subsurface variability and the

uncertainty in the soil properties. Generally, at least 60 convolution analyses should be performed to

define the mean and the standard deviation of the site response. The results of the site response analyses

are used to develop high- and low-frequency site amplification functions as detailed in Appendix E to

this regulatory guide.

4.3 Site Amplification Function

Based on the site response analyses described in Regulatory Position 4.2, site amplification

functions are calculated. To determine the UHRS at the free-field ground surface for a specific annual

probability of exceedance, multiply the rock-based UHRS by either the high-frequency or low-frequency

site amplification functions as appropriate for each frequency. The appropriate site amplification

function is determined for each frequency based on which of the controlling or input earthquakes has a

spectral value that is closest to the rock UHRS at that frequency. At high frequencies, this will be the

high-frequency spectral amplification function, and at low-frequencies, the low-frequency site

amplification function. By applying the appropriate (high- or low-frequency) amplification function for

each frequency to the mean rock-based UHRS, the site-specific soil-based UHRS is calculated for the

free ground surface. The surface 1 E-04 and 1 E-05 UHRS are site-specific earthquake ground motions

that form the basis of the performance-based site GMRS.

5. Performance-Based Site-Specific Earthquake Ground Motion Procedure

5.1 Horizontal Spectrum

The performance-based, site-specific earthquake ground motion is developed using a method

analogous to the development of the ASCE/SEI Standard 43-05 (Ref. 7) design response spectrum (DRS)

F

that achieves the annual FOSID target performance goal (P ) of 1 E-05, and hazard exceedance

probability D (H ) of 1 E-04, described in Chapters 1 and 2 of Reference 7, and also discussed in Section B

of this guide. The horizontal site-specific, performance-based GMRS are obtained by scaling the sitespecific mean UHRS by a design factor (DF), or
RG 1.208, Page 18

GMRS = UHRS x DF Equation (1)

where UHRS is the mean 1 E-04 UHRS derived in accordance with Regulatory Position 4.3, and

DF (from Reference 7) is

DF R = max { 1.0, 0.6(A ) } Equation (2) 0.8

whe Rre A is the ground motion slope ratio of spectral accelerations, frequency by frequency,

from a seismic hazard curve corresponding to a 10-fold reduction in hazard exceedance frequency;

therefore,

AR = mean 1 E-05 UHRS ÷ mean 1 E-04 UHRS Equation (3)

Figure 2 provides a comparison of the performance-based, site-specific horizontal GMRS and the

mean 1 E-04 and 1 E-05 UHRS.

The above is based on the assumption that the hazard curves are approximated by a power law

equa Rtion (i.e., linear on a log-log plot) in the range of 1 E-04 to 1 E-05. If A is greater than 4.2, then this

assumption is not valid. In these cases, it is acceptable to use a value equal to 45 percent of the mean 1

E-05 UHRS. Using this method, results in a GMRS that achieves the annual FOSID target of 1 E-05.

5.2 Vertical Spectrum

Vertical response spectra are developed by combining the appropriate horizontal response

spectra and the most up-to-date V/H response spectral ratios appropriate for either CEUS or WUS sites.

Appropriate V/H ratios for CEUS or WUS rock and soil sites are best determined from the most up-todate attenuation relations. However, as there are currently no CEUS attenuation relations that predict

vertical ground motions, appropriate V/H ratios for CEUS rock sites provided in Reference 6 may be

used. For CEUS soil sites, References 6 and 22 describe a procedure to determine a WUS-to-CEUS

transfer function that may be used to modify the WUS V/H soil ratios. Other methods used to determine

V/H ratios may also be appropriate.

5.3 Location of the Site Ground Motion Response Spectrum

The horizontal and vertical GMRS are determined in the free field on the ground surface. For

sites with soil layers near the surface that will be completely excavated to expose competent material, the

GMRS are specified on the outcrop or hypothetical outcrop that will exist after excavation. Motions at

this hypothetical outcrop should be developed as a free surface motion, not as an in-column motion.

Although the definition of competent material is not mandated by regulation, a number of reactor designs

have specified a shear wave velocity of 1,000 fps as the definition of competent material. When the

GMRS are determined as free-field outcrop motions on the uppermost in-situ competent material, only

the effects of the materials below this elevation are included in the site response analysis.
RG 1.208, Page 19

5.4 Determination of Safe Shutdown Earthquake

The SSE is the vibratory ground motion for which certain structures, systems, and components

are designed to remain functional, pursuant to Appendix S to 10 CFR Part 50. The SSE for the site is

characterized by both horizontal and vertical free-field ground motion response spectra at the free ground

surface. The purpose of this regulatory guide is to provide guidance on the development of the

site-specific GMRS. The GMRS satisfy the requirements of 10 CFR 100.23 with respect to the

development of the SSE. In the performance-based approach described in this guide, the GMRS are

based on the site-specific UHRS at the free-field ground surface modified by a design factor to obtain the

performance-based site-specific response spectrum. The design factor achieves a relatively consistent

annual probability of plant component failure across the range of plant locations and structural

frequencies.

The steps necessary to develop the final SSE are described in Chapter 3, “Design of Structures,

Components, Equipment, and Systems,” of NUREG-0800, “Standard Review Plan.” This process is

defined by two distinct options. The first option is for COL applicants using a standard design nuclear

power plant. The standard designs use broad-band smooth response spectra, referred to as certified

seismic design response spectra (CSDRS). The CSDRS are site-independent seismic design response

spectra scaled to a peak ground motion value. Under the first option or combined license procedure, the

GMRS are used to determine the adequacy of the CSDRS for the site. If adequate, the CSDRS become

the SSE for the site. Alternatively, under the second option an applicant would develop a site-specific

nuclear power plant design based on a site SSE. For some sites the GMRS may not contain adequate

energy in the frequency range of engineering interest and, therefore, may not be appropriate as the

site-specific plant SSE. For these situations the GMRS could either be broadened or enveloped by a

smoothed spectrum.

During the design phase, an additional check of the ground motion is required at the foundation

level. According to Appendix S to 10 CFR Part 50, the foundation level ground motion must be

represented by an appropriate response spectrum with a peak ground acceleration of at least 0.1g. The

steps necessary to determine the final SSE, such as the acceptability of the CSDRS for a given site and

the minimum seismic design-basis requirements, are described in Chapter 3 of NUREG-0800.
RG 1.208, Page 20

Figure 2. Comparison of the Mean 1 E-04 and 1 E-05 Uniform Hazard Response 

Spectra and the Performance-Based Ground Motion Response Spectrum (GMRS) 

Mean UHRS 1E-5

Mean UHRS 1E-4

Performance-Based GMRS
RG 1.208, Page 21

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants regarding the NRC staff’s

plans for using this regulatory guide. No backfitting is intended or approved in connection with its

issuance.

This regulatory guide identifies methods that the NRC staff considers acceptable for (1)

conducting geological, geophysical, seismological, and geotechnical investigations, (2) identifying and

characterizing seismic sources, (3) conducting probabilistic seismic hazard analyses, (4) determining

seismic wave transmission characteristics of soil and rock sites, and (5) determining a performance-based

site-specific GMRS, for satisfying the requirements of 10 CFR 100.23. A brief overview of techniques

to develop the SSE in order to satisfy the requirements of Appendix S to 10 CFR Part 50 is also included

in this guide and in NUREG-0800, “Standard Review Plan,” Section 3.7.1.

REGULATORY ANALYSIS / BACKFIT ANALYSIS

The regulatory analysis and backfit analysis for this regulatory guide are available in Draft

Regulatory Guide DG-1146, “A Performance-Based Approach to Define the Site-Specific Earthquake

Ground Motion” (Ref. 23). The NRC issued DG-1146 in October 2006 to solicit public comment on the

draft version of Regulatory Guide 1.208.
All NRC regulations listed herein are available electronically through the Public Electronic Reading Room on the 4

NRC’s public Web site, at http://www.nrc.gov/reading-rm/doc-collections/cfr/. Copies are also available for inspection

or copying for a fee from the NRC’s Public Document Room at 11555 Rockville Pike, Rockville, MD; the PDR’s

mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737 or (800) 397-4209; fax (301)

415-3548; email PDR@nrc.gov.

All NUREG-series reports listed herein were published by the U.S. Nuclear Regulatory Commission. Copies are

5

available for inspection or copying for a fee from the NRC’s Public Document Room at 11555 Rockville Pike,

Rockville, MD; the PDR’s mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737 or

(800) 397-4209; fax (301) 415-3548; email PDR@nrc.gov. Copies are also available at current rates from the U.S.

Government Printing Office, P.O. Box 37082, Washington, DC 20402-9328, telephone (202) 512-1800; or from the

National Technical Information Service (NTIS) at 5285 Port Royal Road, Springfield, Virginia 22161, online at

http://www.ntis.gov, by telephone at (800) 553-NTIS (6847) or (703) 605-6000, or by fax to (703) 605-6900.

NUREG-0800 is also available electronically through the Electronic Reading Room on the NRC’s public Web site, at

http://www.nrc.gov/ reading-rm/doc-collections/nuregs/.

All regulatory guides listed herein were published by the U.S. Nuclear Regulatory Commission. Where an accession 6

number is identified, the specified regulatory guide is available electronically through the NRC’s Agencywide

Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html. All other

regulatory guides are available electronically through the Public Electronic Reading Room on the NRC’s public Web

site, at http://www.nrc.gov/reading-rm/doc-collections/reg-guides/. Single copies of regulatory guides may also be

obtained free of charge by writing the Reproduction and Distribution Services Section, ADM, USNRC, Washington,

DC 20555-0001, or by fax to (301) 415-2289, or by email to DISTRIBUTION@nrc.gov. Active guides may also be

purchased from the National Technical Information Service (NTIS) on a standing order basis. Details on this service

may be obtained by contacting NTIS at 5285 Port Royal Road, Springfield, Virginia 22161, online at

http://www.ntis.gov, by telephone at (800) 553-NTIS (6847) or (703) 605-6000, or by fax to (703) 605-6900. Copies

are also available for inspection or copying for a fee from the NRC’s Public Document Room (PDR), which is located

at 11555 Rockville Pike, Rockville, Maryland; the PDR’s mailing address is USNRC PDR, Washington, DC 20555-

0001. The PDR can also be reached by telephone at (301) 415-4737 or (800) 397-4209, by fax at (301) 415-3548, and

by email to PDR@nrc.gov.

RG 1.208, Page 22

REFERENCES

1. U.S. Code of Federal Regulations, Title 10, Energy, Part 100, “Reactor Site Criteria.”4

2. U.S. Code of Federal Regulations, Title 10, Energy, Part 50, “Domestic Licensing of Production

and Utilization Facilities.”

5

3. U.S. Code of Federal Regulations, Title 10, Energy, Part 52, “Licenses, Certifications, and

Approvals for Nuclear Power Plants.”5

4. NUREG-0800, “Standard Review Plan for the Review of Safety Analysis Reports for Nuclear

Power Plants,” U.S. Nuclear Regulatory Commission, Washington, DC.5

5. Regulatory Guide 1.165, “Identification and Characterization of Seismic Sources and

Determination of Safe Shutdown Earthquake Ground Motion,” U.S. Nuclear Regulatory

Commission, Washington, DC.6
Copies are available for inspection or copying for a fee from the NRC’s Public Document Room at 11555 Rockville

7

Pike, Rockville, MD; the PDR’s mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737

or (800) 397-4209; fax (301) 415-3548; email PDR@nrc.gov. In addition, copies are available at current rates from

the U.S. Government Printing Office, P.O. Box 37082, Washington, DC 20402-9328, telephone (202) 512-1800; or

from the National Technical Information Service (NTIS) at 5285 Port Royal Road, Springfield, Virginia 22161, online

at http://www.ntis.gov, by telephone at (800) 553-NTIS (6847) or (703) 605-6000, or by fax to (703) 605-6900.

Copies may be purchased from the American Society for Civil Engineers (ASCE), 1801 Alexander Bell Drive, Reston, 8

VA 20190 [phone: 800-548-ASCE (2723)]. Purchase information is available through the ASCE Web site at 

http://www.asce.org/bookstore/book.cfm?isbn=0784407622

Copies are available for inspection or copying for a fee from the NRC’s Public Document Room at 11555 Rockville

9

Pike, Rockville, MD; the PDR’s mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737

or (800) 397-4209; fax (301) 415-3548; email PDR@nrc.gov.

RG 1.208, Page 23

6. McGuire, R.K., W.J. Silva, and C.J. Costantino, “Technical Basis for Revision of Regulatory

Guidance on Design Ground Motions: Hazard- and Risk-Consistent Ground Motion Spectra

Guidelines,” NUREG/CR-6728, U.S. Nuclear Regulatory Commission, Washington, DC,

October 2001.7

7. ASCE/SEI 43-05, “Seismic Design Criteria for Structures, Systems, and Components in Nuclear

Facilities,” American Society of Civil Engineers/Structural Engineering Institute, 2005.8

8. Regulatory Guide 1.60, “Design Response Spectra for Seismic Design of Nuclear Power Plants,”

U.S. Nuclear Regulatory Commission.7

9. Budnitz, R.J., et al., “Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on

Uncertainty and Use of Experts,” NUREG/CR-6372, Volumes 1 and 2, U.S. Nuclear Regulatory

Commission, Washington, DC, April 1997.8

10. Pacific Gas and Electric Company, “Final Report of the Diablo Canyon Long-Term Seismic

Program: Diablo Canyon Power Plant,” Docket Nos. 50-275 and 50-323, 1988.9

11. Rood, H., et al., “Safety Evaluation Report Related to the Operation of Diablo Canyon Nuclear

Power Plant, Units 1 and 2,” NUREG-0675, Supplement No. 34, U.S. Nuclear Regulatory

Commission, Washington, DC, June 1991.

8

12. Sorensen, G., Washington Public Power Supply System, Letter to Document Control Branch,

U.S. Nuclear Regulatory Commission, Washington, DC, Subject: “Nuclear Project No. 3,

Resolution of Key Licensing Issues, Response,” February 29, 1988.10

13. Bernreuter, D.L., et al., “Seismic Hazard Characterization of 69 Nuclear Plant Sites East of the

Rocky Mountains,” NUREG/CR-5250, Volumes 1–8, U.S. Nuclear Regulatory Commission,

Washington, DC, January 1989.8

14. Sobel, P., “Revised Livermore Seismic Hazard Estimates for Sixty-Nine Nuclear Power Plant

Sites East of the Rocky Mountains,” NUREG-1488, U.S. Nuclear Regulatory Commission,

Washington, DC, April 1994.8

15. Savy, J.B., et al., “Eastern Seismic Hazard Characterization Update,” UCRL-ID-115111,

Lawrence Livermore National Laboratory, June 1993 (Accession #9310190318 in the NRC’s

Public Document Room).10
Draft Regulatory Guide DG-1146 is available electronically under Accession #ML063030291 in the NRC’s 10

Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html.

Copies are also available for inspection or copying for a fee from the NRC’s Public Document Room (PDR), which is

located at 11555 Rockville Pike, Rockville Maryland; the PDR’s mailing address is USNRC PDR, Washington, DC

20555-0001. The PDR can also be reached by telephone at (301) 415-4737 or (800) 397-4209, by fax at (301) 415-

3548, and by email to PDR@nrc.gov.

RG 1.208, Page 24

16. Electric Power Research Institute (EPRI), “Probabilistic Seismic Hazard Evaluations at Nuclear

Power Plant Sites in the Central and Eastern United States,” NP-4726, All Volumes, 1989–1991.

17. Electric Power Research Institute (EPRI), “The Earthquakes of Stable Continental Regions,”

Volume 1: Assessment of Large Earthquake Potential, EPRI TR-102261-V1, 1994.

18. Electric Power Research Institute (EPRI) and U.S. Department of Energy (DOE), “Program on

Technology Innovation: Use of Minimum CAV in Determining Effects of Small Magnitude

Earthquakes on Seismic Hazard Analyses,” Report 1012965, December 2005.

19. Electric Power Research Institute (EPRI) and U.S. Department of Energy (DOE), “Program on

Technology Innovation: Truncation of the Lognormal Distribution and Value of the Standard

Deviation for Ground Motion Models in the Central and Eastern United States,” Report 1013105,

February 2006.

20. Regulatory Guide 1.132, “Site Investigations for Foundations of Nuclear Power Plants,” U.S.

Nuclear Regulatory Commission, Washington, DC.7

21. Regulatory Guide 1.138, “Laboratory Investigations of Soils and Rocks for Engineering Analysis

and Design of Nuclear Power Plants,” U.S. Nuclear Regulatory Commission, Washington, DC.7

22. McGuire, R.K., W.J. Silva, and C.J. Costantino, “Technical Basis for Revision of Regulatory

Guidance on Design Ground Motions: Development of Hazard- and Risk-Consistent Seismic

Spectra for Two Sites,” NUREG/CR-6769, U.S. Nuclear Regulatory Commission, Washington,

DC, April 2002.8

23. Draft Regulatory Guide DG-1146, “A Performance-Based Approach to Define the Site-Specific

Earthquake Ground Motion,” U.S. Nuclear Regulatory Commission, Washington, DC.10
Appendix A to RG 1.208, Page A-1

APPENDIX A

DEFINITIONS

Accepted PSHA Model. An accepted PSHA model is a method of conducting a Probabilistic Seismic

Hazard Analysis (including the seismic sources and ground motion equations) that has been developed

using Senior Seismic Hazard Analysis Committee (SSHAC) guidelines and that has been reviewed and

accepted by the NRC in the past either for generic application (e.g., the 1989 studies by LLNL and EPRI,

with the inherent seismic source description for the CEUS) or as part of an ESP or COL application. 

Accepted PSHA models are starting points for developing probabilistic seismic hazard calculations for

new ESP or COL applications, yet must be updated with new information on seismicity, geology,

geophysics, and ground motion equations, as appropriate for a site that is being reviewed. The term

accepted PSHA model should not be assumed to imply that the model can be used without updates or

reviews as discussed RG 1.208.

Capable Tectonic Source. A capable tectonic source is a tectonic structure that can generate both

vibratory ground motion and tectonic surface deformation such as faulting or folding at or near the earth's

surface in the present seismotectonic regime. It is described by at least one of the following:

characteristics:

(1) presence of surface or near-surface deformation of landforms or geologic deposits of a recurring

nature within the last approximately 500,000 years or at least once in the last approximately

50,000 years

(2) a reasonable association with one or more moderate to large earthquakes or sustained earthquake

activity that are usually accompanied by significant surface deformation

(3) a structural association with a capable tectonic source that has characteristics of either item a or b

(above), such that movement on one could be reasonably expected to be accompanied by

movement on the other

In some cases, the geological evidence of past activity at or near the ground surface along a potential

capable tectonic source may be obscured at a particular site. This might occur, for example, at a site

having a deep overburden. For these cases, evidence may exist elsewhere along the structure from which

an evaluation of its characteristics in the vicinity of the site can be reasonably based. Such evidence is to

be used in determining whether the structure is a capable tectonic source within this definition.

Notwithstanding the foregoing paragraphs, the association of a structure with geological structures that

are at least pre-Quaternary, such as many of those found in the central and eastern regions of the United

States, in the absence of conflicting evidence, will demonstrate that the structure is not a capable tectonic

source within this definition.

Certified Seismic Design Response Spectra (CSDRS). Site-independent seismic design response

spectra that have been approved under Subpart B of 10 CFR Part 52 as the seismic design response

spectra for an approved certified standard design nuclear power plant. The input or control location for

the CSDRS is specified in the certified standard design.
Appendix A to RG 1.208, Page A-2

Combined License. A combined construction permit and operating license with conditions for a nuclear

power facility issued pursuant to Subpart C of 10 CFR Part 52.

Controlling Earthquakes. Earthquakes used to determine spectral shapes or to estimate ground motions

at the site for some methods of dynamic site response. There may be several controlling earthquakes for

a site. As a result of the probabilistic seismic hazard analysis (PSHA), controlling earthquakes are

characterized as mean magnitudes and distances derived from a deaggregation analysis of the mean

estimate of the PSHA.

Cumulative Absolute Velocity (CAV). For each component of the free-field ground motion, the CAV

should be calculated as follows: (1) the absolute acceleration (g units) time-history is divided into

1-second intervals, (2) each 1-second interval that has at least 1 exceedance of 0.025g is integrated over

time, and (3) all the integrated values are summed together to arrive at the CAV. The CAV is exceeded

if the calculation is greater than 0.16 g-second. The application of the CAV in siting requires the

development of a CAV model (Ref. 18) because the PSHA calculation does not use time histories

directly.

Deaggregation. The process for determining the fractional contribution of each magnitude-distance pair

to the total seismic hazard. To accomplish this, a set of magnitude and distance bins are selected and the

annual probability of exceeding selected ground acceleration parameters from each magnitude-distance

pair is computed and divided by the total probability for earthquakes.

Design Factor. The ratio between the site-specific GMRS and the UHRS. The design factor is aimed at

achieving the target annual probability of failure associated with the target performance goals.

Early Site Permit. A Commission approval, issued pursuant to Subpart A of 10 CFR Part 52, for a site

or sites for one or more nuclear power facilities.

Earthquake Recurrence. The frequency of occurrence of earthquakes as a function of magnitude.

Recurrence relationships or curves are developed for each seismic source, and they reflect the frequency

of occurrence (usually expressed on an annual basis) of magnitudes up to the maximum, including

measures of uncertainty.

Frequency of Onset of Significant Inelastic Deformation (FOSID). The annual probability of the

onset of significant inelastic deformation (OSID). OSID is just beyond the occurrence of insignificant

(or localized) inelastic deformation, and in this way corresponds to “essentially elastic behavior.” As

such, OSID of a structure, system, or component (SSC) can be expected to occur well before seismically

induced core damage, resulting in much larger frequencies of OSID than seismic core damage frequency

(SCDF) values. In fact, OSID occurs before SSC “failure,” where the term failure refers to impaired

functionality.

Ground Motion Response Spectra (GMRS). A site-specific ground motion response spectra

characterized by horizontal and vertical response spectra determined as free-field motions on the ground

surface or as free-field outcrop motions on the uppermost in-situ competent material using

performance-based procedures. When the GMRS are determined as free-field outcrop motions on the

uppermost in-situ competent material, only the effects of the materials below this elevation are included

in the site response analysis.
Appendix A to RG 1.208, Page A-3

Ground Motion Slope Ratio. Ratio of the spectral accelerations, frequency by frequency, from a

seismic hazard curve corresponding to a 10-fold reduction in hazard exceedance frequency. (See

Equation 3 in Regulatory Position 5.1.)

In-column Motion. Motion that is within a soil column, as opposed to the motion at the surface or

treated as if it is at the surface.

Intensity. The intensity of an earthquake is a qualitative description of the effects of the earthquake at a

particular location, as evidenced by observed effects on humans, on human-built structures, and on the

earth’s surface at a particular location. Commonly used scales to specify intensity are the Rossi-Forel,

Mercalli, and Modified Mercalli. The Modified Mercalli Intensity (MMI) scale describes intensities with

values ranging from I to XII in the order of severity. MMI of I indicates an earthquake that was not felt

except by a very few, whereas MMI of XII indicates total damage of all works of construction, either

partially or completely.

Magnitude. An earthquake’s magnitude is a measure of the strength of the earthquake as determined

from seismographic observations and is an objective, quantitative measure of the size of an earthquake. 

The magnitude can be expressed in various ways based on seismographic records (e.g., Richter Local

Magnitude, Surface Wave Magnitude, Body Wave Magnitude, and Moment Magnitude). Currently, the

most commonly w used magnitude measurement is the Moment Magnitude, M , which is based on the

seismic moment computed as the rupture force along the fault multiplied by the average amount of slip,

and thus is a direct measure of the energy released during an earthquake.

Maximum Magnitude. The maximum magnitude is the upper bound to earthquake recurrence curves.

Mean Site Amplification Function. The mean amplification function is obtained for each controlling

earthquake, by dividing the response spectrum from the computed surface motion by the response

spectrum from the input hard rock motion, and computing the arithmetic mean of the individual response

spectral ratios.

Nontectonic Deformation. Nontectonic deformation is distortion of surface or near-surface soils or

rocks that is not directly attributable to tectonic activity. Such deformation includes features associated

with subsidence, karst terrain, glaciation or deglaciation, and growth faulting.

Response Spectrum. A plot of the maximum responses (acceleration, velocity, or displacement) of

idealized single-degree-of-freedom oscillators as a function of the natural frequencies of the oscillators

for a given damping value. The response spectrum is calculated for a specified vibratory motion input at

the oscillators’ supports.

Ring Area. Annular region bounded by radii associated with the distance rings used in hazard

deaggregation (Table D.1, “Recommended Magnitude and Distance Bins,” in Appendix D to this

regulatory guide).

Safe Shutdown Earthquake Ground Motion (SSE). The vibratory ground motion for which certain

structures, systems, and components are designed, pursuant to Appendix S to 10 CFR Part 50, to remain

functional. The SSE for the site is characterized by both horizontal and vertical free-field ground motion

response spectra at the free ground surface.
Appendix A to RG 1.208, Page A-4

Seismic Wave Transmission (Site Amplification). The amplification (increase or decrease) of

earthquake ground motion by rock and soil near the earth’s surface in the vicinity of the site of interest. 

Topographic effects, the effect of the water table, and basin edge wave-propagation effects are sometimes

included under site response.

Seismogenic Source. A portion of the earth that is assumed to have a uniform earthquake potential

(same expected maximum earthquake and recurrence frequency), distinct from that of surrounding

sources. A seismogenic source will generate vibratory ground motion but is assumed to not cause surface

displacement. Seismogenic sources cover a wide range of seismotectonic conditions, from a well-defined

tectonic structure to simply a large region of diffuse seismicity.

Spectral Acceleration. Peak acceleration response of an oscillator as a function of period or frequency

and damping ratio when subjected to an acceleration time history. It is equal to the peak relative

displacement of a linear oscillator of frequency, f, attached to the ground, times the quantity (2Bf) . It is

2

expressed in units of gravity (g) or cm/second .

2

Stable Continental Region (SCR). An SCR is composed of continental crust, including continental

shelves, slopes, and attenuated continental crust, and excludes active plate boundaries and zones of

currently active tectonics directly influenced by plate margin processes. It exhibits no significant

deformation associated with the major Mesozoic-to-Cenozoic (last 240 million years) orogenic belts. It

excludes major zones of Neogene (last 25 million years) rifting, volcanism, or suturing.

Stationary Poisson Process. A probabilistic model of the occurrence of an event over time (or space)

that has the following characteristics: (1) the occurrence of the event in small intervals is constant over

time (or space), (2) the occurrence of two (or more) events in a small interval is negligible, and (3) the

occurrence of the event in non-overlapping intervals is independent.

Targe Ft Performance Goal (P ). Target annual probability of exceeding the 1 E-05 frequency of onset

of significant inelastic deformation (FOSID) limit state.

Tectonic Structure. A large-scale dislocation or distortion, usually within the earth’s crust. Its extent

may be on the order of tens of meters (yards) to hundreds of kilometers (miles).

Uniform Hazard Response Spectrum (UHRS). A plot of a ground response parameter (for example,

spectral acceleration or spectral velocity) that has an equal likelihood of exceedance at different

frequencies.

Within Motion. An earthquake record modified for use in a site response model. Within motions are

developed through deconvolution of a surface recording to account for the properties of the overburden

material at the level at which the record is to be applied. The within motion can also be called the

“bedrock motion” if it occurs at a high-impedance boundary where rock is first encountered.
Appendix B to RG 1.208, Page B-1

APPENDIX B

ABBREVIATIONS

A peak ground acceleration

ADAMS Agencywide Documents Access and Management System (NRC)

AR ground motion slope ratio

ASCE American Society of Civil Engineers

ASTM American Society of Testing and Materials

CAV cumulative absolute velocity

CFR Code of Federal Regulations

CP construction permit

CEUS central and eastern United States

COL combined license

CSDRS certified seismic design response spectra

D distance

D peak ground displacement

DF design factor

DOE U.S. Department of Energy

DRS design response spectrum

EPRI Electric Power Research Institute

ESP early site permit

FIRS foundation input response spectra

FOSID frequency of onset of significant inelastic deformation

GMRS ground motion response spectra

HD mean annual hazard exceedance frequency/probability

IAEA International Atomic Energy Agency

LLNL Lawrence Livermore National Laboratory

M magnitude

MMI Modified Mercalli Intensity

NGA Next Generation Attenuation

NRC U.S. Nuclear Regulatory Commission

OL operating license

OMB Office of Management and Budget

OSID onset of significant inelastic deformation
Appendix B to RG 1.208, Page B-2

PFtarget performance goal

PDR Public Document Room (NRC)

PGA peak ground acceleration

PGV peak ground velocity

PSHA probabilistic seismic hazard analysis

RP probability ratio

RVT random vibration theory

SCDF seismic core damage frequency

SCR stable continental region

SDC seismic design category

SRP Standard Review Plan (NUREG-0800)

SSCs structures, systems, and components

SSE safe shutdown earthquake ground motion

SSHAC Senior Seismic Hazard Analysis Committee

Standard ASCE/SEI Standard 43-05 (Ref. 7)

UHRS uniform hazard response spectrum

USNRC U.S. Nuclear Regulatory Commission

V peak ground velocity

V/H ratio of vertical to horizontal spectral accelerations

Vsshear wave velocity

WUS western United States
Appendix C to RG 1.208, Page C-1

APPENDIX C

INVESTIGATIONS TO CHARACTERIZE

SITE GEOLOGY, SEISMOLOGY AND GEOPHYSICS

C.1 Introduction

As characterized for use in probabilistic seismic hazard analyses (PSHA), seismogenic sources

are zones within which future earthquakes are likely to occur. Geological, seismological, and

geophysical investigations provide the information needed to identify and characterize source zone

parameters, such as size and geometry, and to estimate earthquake recurrence rates and maximum

magnitudes. The amount of data available about earthquakes and their causative sources varies

substantially between the western United States (WUS) (west of the Rocky Mountain front) and the

central and eastern United States (CEUS), or stable continental region (east of the Rocky Mountain

front). Furthermore, there are variations in the amount and quality of data within these regions.

Seismogenic sources in active tectonic regions such as the WUS are generally readily identified

because of their relatively high activity rate. In the CEUS, identifying seismogenic sources is more

difficult because their activity rate is relatively low and because most seismic sources are covered by

thick deposits. However, several significant tectonic structures exist and some of these have been

interpreted as seismogenic sources (e.g., the New Madrid fault zone, Nemaha Ridge, and Meers fault).

In the CEUS, it is most likely that the determination of the properties of the seismogenic source,

whether it is a tectonic structure or a broad areal source zone, will be inferred rather than demonstrated

by strong correlations with seismicity or geologic data. Moreover, it is not generally known what

relationships exist between observed tectonic structures in a seismic source within the CEUS and the

current earthquake activity that may be associated with that source. The historical seismicity record, the

results of regional and site studies, and expert judgment play key roles in characterizing a source zone.

If, on the other hand, strong correlations and data exist suggesting a relationship between seismicity and

seismic sources, approaches used for more active tectonic regions can be applied. Reference C.1 may be

used to assess potential for large earthquakes.

The primary objective of geological, seismological, and geophysical investigations is to develop

an up-to-date, site-specific earth science database that supports site characterization and a site-specific

PSHA. The results of these investigations will also be used to assess whether new data and their

interpretation are consistent with the information used in recent probabilistic seismic hazard studies

accepted by NRC staff. If new data, such as new seismic sources and new ground motion attenuation

relationships, are consistent with the existing earth science database, updating or modification of the

information used in the site-specific hazard analysis is not required. It will be necessary to update

seismic sources and ground motion attenuation relationships for sites where there is significant new

information provided by the site investigation.
Appendix C to RG 1.208, Page C-2

The following are to be evaluated for seismic source characterization:

• Evaluation of seismic source location and geometry (location and extent, both surface and

subsurface). This evaluation will normally require interpretations of available geological,

geophysical, and seismological data in the source region by multiple experts or a team of experts.

The evaluation should include interpretations of the seismic potential of each source and

relationships among seismic sources in the region to express uncertainty in the evaluations. 

Seismic source evaluations generally develop four types of sources: (1) fault sources, (2) area

sources representing concentrated historic seismicity not associated with known tectonic

structure, (3) area sources representing geographic regions with similar tectonic histories, type of

crust, and structural features, and (4) background sources. Background sources are generally

used to express uncertainty in the overall seismic source configuration interpreted for the site

region. Acceptable approaches for evaluating and characterizing uncertainties for input to a

seismic hazard calculation are contained in NUREG/CR-6372 (Ref. C.2).

• Evaluations of earthquake recurrence for each seismic source, including recurrence rate and

recurrence model. These evaluations normally draw most heavily on historical and instrumental

seismicity associated with each source and paleoearthquake information. Preferred methods and

approaches for evaluating and characterizing uncertainty in earthquake recurrence generally will

depend on the type of source. Acceptable methods are described in NUREG/CR-6372 (Ref.

C.2).

• Evaluations of the maximum earthquake magnitude for each seismic source. These evaluations

will draw on a broad range of source-specific tectonic characteristics, including tectonic history

and available seismicity data. Uncertainty in this evaluation should normally be expressed as a

maximum magnitude distribution. Preferred methods and information for evaluating and

characterizing maximum earthquakes for seismic sources vary with the type of source. 

Acceptable methods are contained in NUREG/CR-6372 (Ref. C.2).

• Other evaluations, depending on the geologic setting of a site, such as local faults that have a

history of Quaternary (last 1.8 million years) displacements, sense of slip on faults, fault length

and width, area of faults, age of displacements, estimated displacement per event, estimated

earthquake magnitude per offset event, orientations of regional tectonic stresses with respect to

faults, and the possibility of seismogenic folds. Capable tectonic sources are not always exposed

at the ground surface (such as blind thrust faults) in the WUS as demonstrated by the buried

reverse causative faults of the 1983 Coalinga, 1988 Whittier Narrows, l989 Loma Prieta, and

1994 Northridge earthquakes. These examples emphasize the need to conduct thorough

investigations not only at the ground surface but also in the subsurface to identify structures at

seismogenic depths. Regional topographic features should also be closely studied. Whenever

faults or other structures are encountered at a site (including sites in the CEUS) in either outcrop

or excavations, it is necessary to perform adequately detailed specific investigations to determine

whether or not they are seismogenic or may cause surface deformation at the site. Acceptable

methods for performing these investigations are contained in NUREG/CR-5503 (Ref. C.3).
Appendix C to RG 1.208, Page C-3

C.2. Investigations To Evaluate Seismic Sources

C.2.1 General

Investigations of the site and region around the site are necessary to identify capable tectonic

sources and to determine their potential for generating earthquakes and causing surface deformation. If it

is determined that surface deformation need not be taken into account at the site, sufficient data to clearly

justify this determination should be presented in the application for an early site permit or license.

Generally, any tectonic deformation at the earth’s surface within the Site Area [8 km (5 mi)of the site]

will require detailed examination to determine its significance. Potentially active tectonic deformation

within the seismogenic zone beneath a site will have to be assessed using geological, geophysical and

seismological methods to determine its significance. Engineering solutions are generally available to

mitigate the potential vibratory effects of earthquakes through design. However, engineering solutions

cannot always be demonstrated to be adequate for mitigation of the effects of permanent ground

displacement phenomena such as surface faulting or folding, subsidence, or ground collapse. For this

reason, it is prudent to select an alternative site when the potential exists for permanent ground

displacement at the proposed site (Ref. C.4).

The level of detail for investigations should be governed by knowledge of the current and late

Quaternary tectonic regime and the geological complexity of the site and region. The investigations for

determining seismic sources should be carried out in all the following levels, with areas described by

radii of 320 km (200 mi) (Site Region), 40 km (25 mi) (Site Vicinity), and 8 km (5 mi) (Site Area) and 1

km (0.6 mi) (Site Location) from the site. The investigations should provide increasingly detailed

information as they proceed from the regional level down to the site (e.g., from Site Region to Site

Location). Whenever faults or other structures are encountered at a site in either outcrop or excavations,

it is necessary to perform many of the investigations described below to determine whether or not they

are capable tectonic sources.

C.2.1.1 Site Region Investigations [within radii of 320 km (200 mi) – 40 km (25 mi) of the site]

The Site Region investigations should be planned to identify seismic sources and describe the

Quaternary tectonic regime. The data should have a resolution consistent with a map developed at a

scale of 1:500,000 or larger. The investigations are not expected to be extensive or in detail, but should

include a comprehensive literature review supplemented by focused geological reconnaissances based on

the results of the literature study (including topographic, geologic, aeromagnetic, and gravity maps, and

airphotos). Some detailed investigations at specific locations within the region may be necessary if

potential seismic sources that may be significant for determining the safe shutdown earthquake ground

motion (SSE) are identified.

The large size of the area for the regional investigations is necessary because of the possibility

that all significant seismic sources, or alternative configurations, may not have been captured or

enveloped by existing PSHA source databases. Thus, the large area will increase the chances of (1)

identifying evidence for unknown seismic sources that might extend close enough for earthquake ground

motions generated by that source to affect the site, and (2) confirming the PSHA’s database. 

Furthermore, because of the relative rarity of seismic activity in the CEUS, the area should be large

enough to include as many historical and instrumentally recorded earthquakes for analysis as reasonably

possible. The specified area of study is expected to be large enough to incorporate any previously

identified sources that could be analogous to sources that may underlie or be relatively close to the site.

In past licensing activities for sites in the CEUS, it has often been necessary, because of the absence of

datable horizons overlying bedrock, to extend investigations out many tens or hundreds of kilometers
Appendix C to RG 1.208, Page C-4

from the site along a structure or to an outlying analogous structure to locate overlying datable strata or

unconformities so that geochronological methods could be applied. This procedure has also been used to

estimate the age of an undatable seismic source in the Site Vicinity by relating its time of last activity to

that of a similar, previously evaluated structure, or a known tectonic episode, the evidence of which may

be many tens or hundreds of kilometers away.

It is necessary to evaluate the geological, seismological, and geophysical data obtained from the

site-specific investigations to demonstrate that the data are sufficient and consistent with the PSHA

methodology. Specifically, the evaluation may need to include (but is not limited to) seismic source

characterization, ground motion attenuation relationships and other PSHA-related contents (such as lowmagnitude threshold and magnitude conversion relationships). If new information identified by the sitespecific investigations were to result in a significant increase in the hazard estimate for a site, and if this

new information were validated by a strong technical basis, the PSHA might have to be modified to

incorporate the new technical information. Using sensitivity studies, it may also be possible to justify a

lower hazard estimate with an exceptionally strong technical basis. However, it is expected that large

uncertainties in estimating seismic hazard in the CEUS will continue to exist in the future, and

substantial delays in the licensing process will result from trying to justify a lower value with respect to a

specific site.

C.2.1.2 Site Vicinity Investigations [within radii of 40 km (25 mi) – 8 km (5 mi) of the site]

Reconnaissance-level investigations, which may need to be supplemented at specific locations by

more detailed explorations such as geologic mapping, geophysical surveying, borings, and trenching,

should be conducted for the Site Vicinity; the data should have a resolution consistent with a map

developed at a scale of 1:50,000 or larger.

C.2.1.3 Site Area Investigations [within radii of 8 km (5 mi) – 1 km (0.6 mi) of the site]

Detailed investigations should be carried out within the Site Area, and the resulting data should

have a resolution consistent with a map developed at a scale of 1:5,000 or larger. The level of

investigations should be in sufficient detail to delineate the geology and the potential for tectonic

deformation at or near the ground surface. The investigations should use the methods described in the

following subsections that are appropriate for the tectonic regime to characterize seismic sources.

The areas of investigations may be asymmetrical and may cover larger areas than those described

above in regions of late Quaternary activity, regions with high rates of historical seismic activity (felt or

instrumentally recorded data), or sites that are located near a capable tectonic source such as a fault zone.
Appendix C to RG 1.208, Page C-5

C.2.2 Contents of Site Vicinity and Site Area Investigations

The following methods are suggested but they are not all-inclusive and investigations should not

be limited to them. Some procedures will not be applicable to every site, and situations will occur that

require investigations that are not included in the following discussion. It is anticipated that new

technologies will be available in the future that will be applicable to these investigations.

C.2.2.1 Surface Investigations

Surface exploration to assess the geology and geologic structure of the Site Area is dependent on

the Site Location and may be carried out with the use of any appropriate combination of the geological,

geophysical, seismological, and geotechnical engineering techniques summarized in the following

paragraphs. However, it is not necessary for all of these methods to be carried out at a given site.

C.2.2.1.1 Geological interpretations should be performed of aerial photographs and other

remote-sensing imagery, as appropriate for the particular site conditions, to assist in identifying rock

outcrops, faults and other tectonic features, fracture traces, geologic contacts, lineaments, soil conditions,

and evidence of landslides or soil liquefaction.

C.2.2.1.2 Mapping topographic, geomorphic, and hydrologic features should be performed

at scales and with contour intervals suitable for analysis and descriptions of stratigraphy (particularly

Quaternary), surface tectonic structures such as fault zones, and Quaternary geomorphic features. For

coastal sites, or sites located near lakes or rivers, this includes topography, geomorphology (particularly

mapping marine and fluvial terraces), bathymetry, submarine landslides, geophysics (such as seismic

reflection), and hydrographic surveys to the extent needed to describe the site area features.

C.2.2.1.3 Vertical crustal movements should be evaluated using (1) geodetic land

surveying and (2) geological analyses (such as analysis of regional dissection and degradation patterns),

shorelines, fluvial adjustments (such as changes in stream longitudinal profiles or terraces), and other

long-term changes (such as elevation changes across lava flows).

C.2.2.1.4 Analysis should be performed to determine the tectonic significance of offset,

displaced, or anomalous landforms such as displaced stream channels or changes in stream profiles or the

upstream migration of knickpoints; abrupt changes in fluvial deposits or terraces; changes in paleochannels across a fault; or uplifted, down-dropped, or laterally displaced marine terraces.

C.2.2.1.5 Analysis should be performed to determine the tectonic significance of

Quaternary sedimentary deposits within or near tectonic zones, such as fault zones, including (1) faultrelated or fault-controlled deposits such as sag ponds, graben fill deposits, and colluvial wedges formed

by the erosion of a fault paleo-scarp and (2) non-fault-related, but offset, deposits such as alluvial fans,

debris cones, fluvial terrace, and lake shoreline deposits.

C.2.2.1.6 Identification and analysis should be performed of deformation features caused

by vibratory ground motions, including seismically induced liquefaction features (sand boils, explosion

craters, lateral spreads, settlement, soil flows), mud volcanoes, landslides, rockfalls, deformed lake

deposits or soil horizons, shear zones, cracks, or fissures.
Appendix C to RG 1.208, Page C-6

C.2.2.1.7 Analysis should be performed of fault displacements, including the interpretion

of the morphology of topographic fault scarps associated with or produced by surface rupture. Fault

scarp morphology is useful in estimating the age of last displacement [in conjunction with the appropriate

geochronological methods described in NUREG/CR-5562 (Ref. C.5)], approximate magnitude of the

associated earthquake, recurrence intervals, slip rate, and the nature of the causative fault at depth.

C.2.2.2 Subsurface Investigation Contents

Subsurface investigations at the site to identify and describe potential seismic sources and to

obtain required geotechnical information are described in Regulatory Guide 1.132 (Ref. C.6). The

investigations include, but may not be confined to, the following.

C.2.2.2.1 Geophysical investigations useful in the past include magnetic and gravity

surveys, seismic reflection and seismic refraction surveys, bore-hole geophysics, electrical surveys, and

ground-penetrating radar surveys.

C.2.2.2.2 Core borings to map subsurface geology and obtain samples for testing such as

determining the properties of the subsurface soils and rocks and geochronological analysis.

C.2.2.2.3 Excavating and logging of trenches across geological features to obtain samples

for the geochronological analysis of those features.

C.2.2.2.4 At some sites, deep unconsolidated material/soil, bodies of water, or other

material may obscure geologic evidence of past activity along a tectonic structure. In such cases, the

analysis of evidence elsewhere along the structure can be used to evaluate its characteristics in the

vicinity of the site.

In the CEUS it may not be possible to reasonably demonstrate the age of youngest activity on a

tectonic structure with adequate deterministic certainty. In such cases, the uncertainty should be

quantified; the NRC staff will accept evaluations using the methods described in NUREG/CR-5503 (Ref.

C.3). A demonstrated tectonic association of such structures with geologic structural features or tectonic

processes that are geologically old (at least pre-Quaternary) should be acceptable as an age indicator in

the absence of conflicting evidence.

C.2.3 Site Location Investigations [within 1 km (0.6 mi)]

Data from Site Location investigations should have a resolution consistent with a map developed

at a scale of 1:500 or larger. Important aspects of the site investigations are the excavation and logging

of exploratory trenches and the mapping of the excavations for the plant structures, particularly those that

are characterized as Seismic Category I. In addition to geological, geophysical, and seismological

investigations, detailed geotechnical engineering investigations as described in Regulatory Guide 1.132

(Ref. C.6), should be conducted at the site.
Appendix C to RG 1.208, Page C-7

C.2.4 Surface-Fault Rupture and Associated Deformation at the Site

Sites that have a potential for fault rupture at or near the ground surface and associated

deformation should be avoided. Where it is determined that surface deformation need not be taken into

account, sufficient data or detailed studies to reasonably support the determination should be presented.

The presence or absence of Quaternary faulting at the site needs to be evaluated to determine whether

there is a potential hazard that is caused by surface faulting. The potential for surface fault rupture

should be characterized by evaluating (1) the location and geometry of faults relative to the site, (2) the

nature and amount of displacement (sense of slip, cumulative slip, slip per event, and nature and extent of

related folding and/or secondary faulting), and (3) the likelihood of displacement during some future

period of concern (recurrence interval, slip rate, and elapsed time since the most recent displacement). 

Acceptable methods and approaches for conducting these evaluations are described in NUREG/CR-5503

(Ref. C.3); acceptable geochronology dating methods are described in NUREG/CR-5562 (Ref. C.5).

For assessing the potential for fault displacement, the details of the spatial pattern of the fault

zone (e.g., the complexity of fault traces, branches, and en echelon patterns) may be important as they

may define the particular locations where fault displacement may be expected in the future. The amount

of slip that might be expected to occur can be evaluated directly based on paleoseismic investigations or

it can be estimated indirectly based on the magnitude of the earthquake that the fault can generate.

Both non-tectonic and tectonic deformation can pose a substantial hazard to a nuclear power

plant, but there are likely to be differences in the approaches used to resolve the issues raised by the two

types of phenomena. Therefore, non-tectonic deformation should be distinguished from tectonic

deformation at a site. In past nuclear power plant licensing activities, surface displacements caused by

phenomena other than tectonic phenomena have been confused with tectonically induced faulting. Nontectonic structures are those found in karst terrain and those resulting from growth faulting. The nature

of faults related to collapse features can usually be defined through geotechnical investigations and can

either be avoided or, if feasible, adequate engineering fixes can be provided.

Glacially induced faults generally do not represent a deep-seated seismic or fault displacement

hazard because the conditions that created them are no longer present. However, residual stresses from

Pleistocene glaciation may still be present in glaciated regions, although they are of less concern than

active tectonically induced stresses. These features should be investigated with respect to their

relationship to current in situ stresses.

Large, naturally occurring growth faults as those found in the coastal plain of Texas and

Louisiana can pose a surface displacement hazard, even though offset most likely occurs at a much less

rapid rate than that of tectonic faults. They are not regarded as having the capacity to generate damaging

vibratory ground motion, can often be identified and avoided in siting, and their displacements can be

monitored. Some growth faults and antithetic faults related to growth faults and fault zones should be

investigated in regions where growth faults are known to be present. Local human-induced growth

faulting can be monitored and controlled or avoided.

If questionable features cannot be demonstrated to be of non-tectonic origin, they should be

treated as tectonic deformation.
Appendix C to RG 1.208, Page C-8

C.2.5 Site Geotechnical Investigations and Evaluations

C.2.5.1 Geotechnical Investigations

The geotechnical investigations should include, but not necessarily be limited to, (1) defining site

soil and near-surface geologic strata properties as may be required for hazard evaluations, engineering

analyses, and seismic design; (2) evaluating the effects of local soil and site geologic strata on ground

motion at the ground surface; (3) evaluating dynamic properties of the near-surface soils and geologic

strata; (4) conducting soil-structure interaction analyses; and (5) assessing the potential for soil failure or

deformation induced by ground shaking (liquefaction, differential compaction, and land sliding).

Subsurface conditions should be investigated by means of borings, soundings, well logs,

exploratory excavations, sampling, geophysical methods (e.g., cross-hole, down-hole, and geophysical

logging) that adequately assess soil and ground-water conditions and other methods described in

Regulatory Guide 1.132 (Ref. C.6). Appropriate investigations should be made to determine the

contribution of the subsurface soils and rocks to the loads imposed on the structures.

A laboratory testing program should be carried out to identify and classify the subsurface soils

and rocks and to determine their physical and engineering properties. Laboratory investigations and

testing practices acceptable to the NRC staff are described in Regulatory Guide 1.138 (Ref. C.7).

Laboratory tests for both static and dynamic properties (e.g., shear modulus, damping, liquefaction

resistance, etc.) are generally required. The dynamic property tests should include cyclic triaxial tests,

cyclic simple shear tests, cyclic torsional shear tests, and resonant column tests, as appropriate. Both

static and dynamic tests should be conducted as recommended in American Society for Testing and

Materials (ASTM) standards or test procedures acceptable to the staff. The ASTM specification

numbers for static and dynamic laboratory tests can be found in the annual books of ASTM Standards,

Volume 04.08. Sufficient laboratory test data should be obtained to allow for reasonable assessments of

mean values of soil properties and their potential variability.

C.2.5.2 Ground Failure Evaluations

Liquefaction is a soil behavior phenomenon in which cohesionless soils (sand, silt, or gravel)

under saturated conditions lose a substantial part or all of their strength because of high pore water

pressures generated in the soils by strong ground motions induced by earthquakes. Potential effects of

liquefaction include reduction in foundation bearing capacity, settlements, land sliding and lateral

movements, flotation of lightweight structures (such as tanks) embedded in the liquefied soil, and

increased lateral pressures on walls retaining liquefied soil. Guidance in Regulatory Guide 1.198,

“Procedures and Criteria for Assessing Seismic Soil Liquefaction at Nuclear Power Plant Sites,” (Ref.

C.8), should be used for evaluating the site for liquefaction potential.

Investigations of liquefaction potential typically involve both geological and geotechnical

engineering assessments. The parameters controlling liquefaction phenomena are (1) the lithology of the

soil at the site, (2) the ground water conditions, (3) the behavior of the soil under dynamic loadings, and

(4) the potential severity of the vibratory ground motion.

In addition to liquefaction, a number of other forms of ground failure or ground settlement

should be investigated. These forms of ground failure and ground settlement include localized soft

zones, karst features, seismic settlement of non-saturated soils, all forms of mass wasting, and any other

form of ground deformation that has the capacity to impact foundation performance or design.
Appendix C to RG 1.208, Page C-9

C.3 Evaluation of New Information Obtained from the Site-specific Investigations

The first step in reviewing the new information obtained from the site-specific investigations

with previous interpretations is determining whether the following existing parameters are consistent

with the new information: (1) the range of seismogenic sources as interpreted by the seismicity experts

or teams involved in the study, (2) the range of seismicity rates for the region around the site as

interpreted by the seismicity experts or teams involved in the studies, (3) the range of maximum

magnitudes determined by the seismicity experts or teams, and (4) attenuation relations. The new

information is considered not significant and no further evaluation is needed if it is consistent with the

assumptions used in the PSHA, no additional alternative seismic sources or seismic parameters are

needed, or it supports maintaining the site mean seismic hazard.

APPENDIX C REFERENCES

C.1 Electric Power Research Institute, “The Earthquakes of Stable Continental Regions,” Volume 1:

Assessment of Large Earthquake Potential, EPRI TR-102261-V1, 1994.

C.2 Budnitz, R.J., et al., “Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on

Uncertainty and Use of Experts,” NUREG/CR-6372, Volumes 1 and 2, U.S. Nuclear Regulatory

Commission, Washington, DC, April 1997.

C.3 Hanson, K.L., et al., “Techniques for Identifying Faults and Determining Their Origins,”

NUREG/CR-5503, U.S. Nuclear Regulatory Commission, Washington, DC, 1999.

C.4 International Atomic Energy Agency (IAEA), “Earthquakes and Associated Topics in Relation to

Nuclear Power Plant Siting,” Safety Series No. 50-SG-S1, Revision 1, 1991.

C.5 Sowers, J.M., et al., “Dating and Earthquakes: Review of Quaternary Geochronology and Its

Application to Paleoseismology,” NUREG/CR-5562, U.S. Nuclear Regulatory Commission,

Washington, DC, 1998.

C.6 Regulatory Guide 1.132, “Site Investigations for Foundations of Nuclear Power Plants,” U.S.

Nuclear Regulatory Commission, Washington, DC.

C.7 Regulatory Guide 1.138, “Laboratory Investigations of Soils and Rocks for Engineering Analysis

and Design of Nuclear Power Plants,” U.S. Nuclear Regulatory Commission, Washington, DC.

C.8 Regulatory Guide 1.198, “Procedures and Criteria for Assessing Seismic Soil Liquefaction at

Nuclear Power Plant Sites,” U.S. Nuclear Regulatory Commission, Washington, DC.
Appendix D to RG 1.208, Page D-1

APPENDIX D

DEVELOPMENT OF SEISMIC HAZARD INFORMATION BASE AND

DETERMINATION OF CONTROLLING EARTHQUAKES

D.1 Introduction

This appendix discusses site-specific development of seismic hazard levels and elaborates on

methods to meet the goals discussed in Regulatory Position 3 of this regulatory guide. This appendix

describes a methods for undertaking probabilistic seismic hazard analysis (PSHA), performing

deaggregation of the results of the PSHA, and determining the controlling earthquakes that are developed

for comparison with identified local and regional seismic sources. The controlling earthquakes can also

be used to develop appropriate ground motions for the site response analysis. The deaggregation of the

mean probabilistic seismic hazard analysis (PSHA) results determines the contribution of individual

magnitude and distance ranges to the overall seismic hazard. Information from the deaggregation is

useful to both review the PSHA and understand the seismic sources that contribute most to hazard at the

site.

The controlling earthquakes are developed from the deaggregation of the PSHA results at ground

motion levels corresponding to the annual frequencies of 1 E-04, 1 E-05, and 1 E-06 and are based on the

magnitude and distance values that contribute most the to hazard at the average of 1 and 2.5 Hz and the

average of 5 and 10 Hz. While the ASCE/SEI Standard 43-05 (Ref. D.2) approach only requires

calculation of ground motion levels with mean annual probabilities of 1 E-04 and 1 E-05, NRC

confirmatory activities also specify calculation of ground motion levels with a mean annual probability of

1 E-06. Using the controlling earthquakes, Regulatory Position 4 and Appendix E to this regulatory

guide describe one acceptable procedure for determining the appropriate ground motion for the site

response analysis to determine the performance-based safe shutdown earthquake ground motion (SSE).

D.2 Probabilistic Seismic Hazard Assessment

Site-specific PSHA should be performed using the methodology described in Regulatory

Positions 3.3 and 3.4 of this regulatory guide. Steps taken relative to the PSHA methodology are

summarized as follows:

• Perform the PSHA for actual or assumed rock conditions with up-to-date interpretations of

earthquake sources, earthquake recurrence, and strong ground motion estimation using original

or updated sources as determined in Regulatory Positions 3.1 and 3.2.

• CAV filtering can be used in place of a lower-bound magnitude cutoff (Ref. D.1).

• Conduct the hazard assessment at a minimum of 30 frequencies, approximately equally spaced

on a logarithmic frequency axis between 100 and 0.1Hz.

• Report fractile hazard curves at the following fractile levels (p) for each ground motion

parameter: 0.05, 0.16, 0.50, 0.84, and 0.95, as well as the mean. Report the fractile hazard

curves in tabular as well as graphical format.

• Determine the mean uniform hazard response spectra (UHRS) for annual exceedance frequencies

of 1 E-04, 1 E-05, and 1 E-06.
The recommended magnitude and distance bins and procedure used to establish controlling earthquakes were

1

developed for application in the CEUS, where the nearby earthquakes generally control the response in the 5 to 10 Hz

frequency range, and larger but distant events can control the lower frequency range. For other situations, alternative

binning schemes as well as a study of contributions from various bins are necessary to identify controlling earthquakes,

consistent with the distribution of the seismicity.

Appendix D to RG 1.208, Page D-2

D.3 Deaggregation of PSHA Results

Deaggregation is undertaken as a way to better understand the earthquakes that contribute most to hazard

at the site of interest. It is also used to assess the reasonableness of the PSHA results and is further used

to develop controlling earthquakes as described in the section below. Deaggregation can be performed

using the following steps.

Deaggregation Step 1

(a) Using the annual probability levels of 1 E-04, 1 E-05, and 1 E-06, determine the ground motion

levels for the spectral accelerations at 1, 2.5, 5, and 10 Hz from the total mean hazard obtained in

Step 1.

(b) Calculate the linear average of the ground motion level for the 1 and 2.5 Hz and the 5 and 10 Hz

spectral acceleration pairs.

Deaggregation Step 2

Perform a deaggregation of the PSHA for each of the spectral accelerations and annual

probability levels as enumerated in Step 1, and provide a table (such as illustrated in Table D.1 ) for each 1

of the 12 cases. The deaggregation is performed to obtain the relative contribution to the hazard for each

of the magnitude-distance bins for each case.

Table D.1

Recommended Magnitude and Distance Bins

Moment Magnitude Range of Bins

Distance

Range of Bin

(km)

5 – 5.5 5.5 – 6 6 – 6.5 6.5 – 7 >7

0 – 15

15 – 25

25 – 50

50 – 100

100 – 200

200 – 300

>300
Appendix D to RG 1.208, Page D-3

Deaggregation Step 3

If the deaggregated results for each of the 12 cases is not in terms of the fractional contribution to

the total hazard then perform the steps as described below. Otherwise, average the low-frequency (1 and

2.5 Hz) and high-frequency (5 and 10 Hz) deaggregation results.

From the deaggregated results of Step 2, the mean annual probability of exceeding the ground

motion levels of Step 2(a) (spectral accelerations at 1, 2.5, 5, and 10 Hz) are determined for each

magni mdf tude-distance bin. These values are denoted by H .

Using mdf H values, the fractional contribution of each magnitude and distance bin to the total

hazard for 1 the average of 1 and 2.5 Hz, P(m,d) , is computed according to the following:

P 1 mdf mdf (m,d) = [( 3 H ) / 2 ] ÷ [ 3 3 ( 3 H ) / 2 ] (Equation 1)

 

f = 1,2 m d f = 1,2

where f = 1 and f = 2 represent the ground motion measure at 1 and 2.5 Hz, respectively.

The fractional contribution of each magnitude and distance bin to the total hazard for the average

of 2 5 and 10 Hz, P(m,d) , is computed according to the following:

P 2 mdf mdf (m,d) = [( 3 H ) / 2 ] ÷ [ 3 3 ( 3 H ) / 2 ] (Equation 2)

 

 f = 1,2 m d f = 1,2

where f = 1 and f = 2 represent the ground motion measure at 5 and 10 Hz, respectively.
Appendix D to RG 1.208, Page D-4

D.4 Procedure To Determine Controlling Earthquakes

The following approach is acceptable to the NRC staff for determining controlling earthquakes

which are used by the staff to assess the reasonableness of the PSHA results and can be used for the site

response analyses. This procedure is based on a deaggregation of the mean probabilistic seismic hazard

in terms of earthquake magnitudes and distances. When the controlling earthquakes have been obtained,

the site specific earthquake ground motion can be developed according to the simplified procedures

described in Appendix E. The SSE response spectrum is then determined according to the procedures

described in Regulatory Position 5.

Step 1 for Determining Controlling Earthquakes

Review the magnitude-distance distribution for the average of 1 and 2.5 Hz to determine whether

the contribution to the hazard for distances of 100 kilometer (km) (63 mi) or greater is substantial (on the

order of 5 percent or greater).

If the contribution to the hazard for distances of 100 km (63 mi) or greater exceeds 5 percent,

additional calculations are needed to determine the controlling earthquakes using the magnitude-distance

distribution 1 for distances greater than 100 km (63 mi). This distribution, P > 100 (m,d) , is defined by

the following:

P 1 1 1 > 100 (m,d) = [ P(m,d) ] ÷ [ 3 3 P(m,d) ] (Equation 3)

 

m d > 100

The purpose of this calculation is to identify a distant, larger event that may control

low-frequency content of a response spectrum.

The distance of 100 km (63 mi) is chosen for CEUS sites. However, for all sites the results of

full magnitude-distance distribution should be carefully examined to ensure that proper controlling

earthquakes are clearly identified.

Step 2 for Determining Controlling Earthquakes

Calculate the mean magnitude and distance of the controlling earthquake associated with the

ground motions determined in Step 1 of the deaggregation (above) for the average of 5 and 10 Hz. The

following relation is used to calculate the mean magnitude using results of the entire magnitude-distance

bins matrix:

Mc 2 (5 – 10 Hz) = 3 m 3 P(m,d) (Equation 4)

 

m d

where m is the central magnitude value for each magnitude bin.

The mean distance of the controlling earthquake is determined using results of the entire

magnitude-distance bins matrix:

Ln {Dc 2 (5 – 10 Hz)} = 3 Ln(d) 3 P(m,d) (Equation 5)

 

d m

where d is the centroid distance value for each distance bin.
Appendix D to RG 1.208, Page D-5

Step 3 for Determining Controlling Earthquakes

If the contribution to the hazard calculated in Step 5 for distances of 100 km (63 mi) or greater

exceeds 5 percent for the average of 1 and 2.5 Hz, calculate the mean magnitude and distance of the

controlling earthquakes associated with the ground motions determined in Step 2 for the average of 1 and

2.5 Hz. The following relation is used to calculate the mean magnitude using calculations based on

magnitude-distance bins greater than distances of 100 km (63 mi) as discussed in Step 1:

Mc 1 (1 – 2.5 Hz) = 3 m 3 P > 100 (m,d) (Equation 6)

 

m d > 100

where m is the central magnitude value for each magnitude bin.

The mean distance of the controlling earthquake is based on magnitude-distance bins greater than

distances of 100 km (63 mi) as discussed in Step 5 and determined according to the following:

Ln {Dc 2 (1 – 2.5 Hz)} = 3 Ln(d) 3 P > 100 (m,d) (Equation 7)

 

d > 100 m

where d is the centroid distance value for each distance bin.

When more than one earthquake magnitude-distance pair contributes significantly to the spectral

accelerations at a given frequency, it may be necessary to use more than one controlling earthquake for

determining the spectral response at the frequency.

Step 4 for Determining Controlling Earthquakes

Document the high- and low-frequency controlling earthquakes for each target annual probability

of exceedance in the tabular form illustrated in Table D.2.
Copies may be purchased from the American Society for Civil Engineers (ASCE), 1801 Alexander Bell Drive, Reston, 2

VA 20190 [phone: 800-548-ASCE (2723)]. Purchase information is available through the ASCE Web site at 

http://www.asce.org/bookstore/book.cfm?isbn=0784407622

Appendix D to RG 1.208, Page D-6

Table D.2

High- and Low-Frequency Controlling Earthquakes

Hazard Magni btude (m ) Distance

Mean 1 E-04

High-Frequency (5 and 10 Hz)

Mean 1 E-04

Low-Frequency (1 and 2.5 Hz)

Mean 1 E-05

High-Frequency (5 and 10 Hz)

Mean 1 E-05

Low-Frequency (1 and 2.5 Hz)

Mean 1 E-06

High-Frequency (5 and 10 Hz)

Mean 1 E-06

Low-Frequency (1 and 2.5 Hz)

APPENDIX D BIBLIOGRAPHY

D.1 Electric Power Research Institute (EPRI), and U.S. Department of Energy (DOE), “Program on

Technology Innovation: Use of Minimum CAV in Determining Effects of Small Magnitude

Earthquakes on Seismic Hazard Analyses,” Report 1012965, December 2005.

D.2 ASCE/SEI 43-05, “Seismic Design Criteria for Structures, Systems, and Components in Nuclear

Facilities,” American Society of Civil Engineers/Structural Engineering Institute, 2005.2

D.3 Sobel.P., “Revised Livermore Seismic Hazard Estimates for Sixty-Nine Nuclear Power Plant

Sites East of the Rocky Mountains,” NUREG-1488, U.S. Nuclear Regulatory Commission,

Washington, DC, April 1994.

D.4 Savy, J.B., et al., “Eastern Seismic Hazard Characterization Update,” UCRL-ID-115111,

Lawrence Livermore National Laboratory, June 1993.

D.5 Bernreuter, D.L., et al., “Seismic Hazard Characterization of 69 Nuclear Plant Sites East of the

Rocky Mountains,” NUREG/CR-5250, Volumes 1–8, U.S. Nuclear Regulatory Commission,

Washington, DC, January 1989.

D.6 Electric Power Research Institute (EPRI), “Probabilistic Seismic Hazard Evaluations at Nuclear

Power Plant Sites in the Central and Eastern United States,” NP-4726, All Volumes, 1989–1991.
Appendix D to RG 1.208, Page D-7

D.7 Electric Power Research Institute (EPRI), “The Earthquakes of Stable Continental Regions,”

Volume 1: Assessment of Large Earthquake Potential, EPRI TR-102261-V1, 1994.

D.8 Electric Power Research Institute (EPRI), et al., “CEUS Ground Motion Project Final Report,”

Report 1009684, 2004.
Appendix E to RG 1.208, Page E-1

APPENDIX E

SEISMIC WAVE TRANSMISSION ANALYSIS

E.1 Introduction

The horizontal and vertical safe shutdown earthquake ground motion (SSE) should be

determ sined in the free field on the ground surface. For sites that have materials with V less than the

assumed generic rock shear wave velocity, as defined in the probabilistic seismic hazard analysis

(PSHA), a site response analysis should be performed. This appendix discusses the site-specific

information needed for these analyses and elaborates on one of the acceptable site response methods

discussed in NUREG/CR-6728 that can be used to meet the intent of Regulatory Position 4 of this

regulatory guide. The method described is a simplified approach and, as such, some sites may require

more detailed analyses as described in NUREG/CR-6728.

The input ground motion used in the analyses are specified based on the PSHA, which assumes

that an outcrop or a hypothetical outcrop exists at the free-field surface. If the rock layer (as defined by

shear wave velocity) first occurs at depth, the input ground motions are deconvolved to develop “within”

motions and imposed at the generic rock horizon.

The purpose of the method described in this appendix is to develop the uniform hazard response

spectrum (UHRS) at the free-field ground surface. The hazard curves from the PSHA are defined for

generic rock conditions as defined by the attenuation relationships used in the PSHA. In the central and

eastern united sstates (CEUS), rock is usually defined as that material with a shear wave velocity (V ) of

about 2.8 km/sec (9,200 ft/sec). To properly address amplification or deamplification effects of the soils

between the generic rock horizon, the ground surface, and other interfaces in between, the following

procedure should be used:

(1) Develop the site-specific soil profile.

(2) Develop appropriate modified earthquake time histories to be used in site response analyses.

(3) Perform a suite of site response analyses to determine mean site amplification functions for a

range of frequencies.

(4) Develop the UHRS at the free ground surface based on the generic rock-based PSHA and the

mean site amplification functions.

E.2 Site-Specific Soil Profile

Site and laboratory investigations and testing are performed to obtain data defining the static and

dynamic engineering properties of soil and rock materials, and their spatial distribution. The procedures

identified in Regulatory Guide 1.132 (Ref. E.1), Regulatory Guide 1.138 (Ref. E.2), and subsection

C.2.2.2 of Appendix C to this regulatory guide are acceptable to the NRC staff.
Appendix E to RG 1.208, Page E-2

The seismic wave transmission characteristics (spectral amplification or deamplification) of the

materials overlying bedrock at the site should be described over a range of frequencies that include the

significant structural frequencies. The wave transmission characteristics should be developed for levels

of seismic motion ranging from those causing very small strains (i.e., no soil degradation) to those

consistent with strain levels anticipated in the SSE. The following material properties should be

determined for each stratum under the site: (1) thickness, (2) compressional velocity, (3) shear wave

velocity, (4) bulk densities, (5) soil index properties and classification, (6) shear modulus and damping

variations with strain level, and (7) the water table elevation and its variation throughout the site.

Internal friction angle, cohesive strength and over-consolidation ratio for clay are also needed for nonlinear analyses. Site specific soil profiles should be deep enough to fully capture the amplification to the

lowest structural frequency of interest and should extend to the depth at which the input ground motion is

to be imposed.

The strain-dependent shear modulus and damping curves are developed based on site-specific

testing results and supplemented as appropriate by published data for similar soils. One source of

published data is the EPRI 93 curves (Ref. E.3). The strain-dependent shear modulus and damping

curves incorporated into the analysis should be justified against observed data for each soil at the site.

When the site-specific laboratory data are developed, the result should be compared to earthquake

recordings on similar soils. A maximum critical damping value of 15 percent is allowed in site response

analyses. The effects of confining pressures (that reflect the depths of the soil) on these strain-dependent

soil dynamic characteristics are assessed and considered in site response analyses. The variability in

these properties is accounted for in the site response analyses.

Often vertically propagating shear waves are the dominant contributor to free-field ground

motions at a site. In these cases, a one-dimensional equivalent-linear analysis or nonlinear analysis that

assumes vertical propagation of shear waves may be appropriate. However, site characteristics (such as a

dipping bedrock surface or other impedance boundaries), regional characteristics (such as topographic

effects), and source characteristics (such as nearby dipping seismic sources) may require that analyses be

able to also account for inclined waves. Multi-dimensional soil models may be needed if complex

geologic and geotechnical conditions exist.

E.3 Site Response Analysis

Due to the non-linear nature of site response, the choice of input ground motions has a significant

impact on the amplification of motion observed in the soil column. One way to approach this problem is

to use controlling earthquakes determined from deaggregation of the PSHA results, as described in

Appendix D. The controlling earthquakes are developed to focus the analyses on the types of scenarios

most likely to impact the site. However, for some sites additional earthquake scenarios may be needed to

adequately reflect the full UHRS, particularly in the intermediate frequency range.

The response spectra for each of the controlling earthquakes should be developed using either

appropriate attenuation relationships or the appropriate response spectrum from NUREG/CR-6728. The

CEUS spectral shapes described in NUREG/CR-6728 are for both a single corner and double corner

source model. These two models should be evenly weighted and accounted for in the development of the

input time histories. The resulting response spectra are then scaled to match the site rock spectral

accelerations at 7.5 Hz (high-frequency), 1.75 Hz (low-frequency), and an intermediate frequency range

as appropriate.
Appendix E to RG 1.208, Page E-3

The fundamental frequency of the site can be approximated prior to analysis using simplified

methods. Once the spectra for the input earthquakes are determined and scaled, the spectra are plotted

and compared against the fundamental frequency of the soil to ensure that the spectra from at least one

key earthquake scenario has sufficient energy content at the natural frequency of the site as determined

by the full rock-based UHRS. Because the non-linear behavior of the soil will decrease the fundamental

frequency of the site, this comparison should be verified upon completion of the analyses. The soil

response is typically highest at the natural period of the soil. Therefore, applying a suite of cases that are

deficient in this range may not adequately predict the true site specific UHRS. A visual comparison of

the spectra from the controlling earthquakes, the UHRS spectra, and the natural frequency of the soil

column should be provided.

After determining the scaled response spectra from each of the input earthquakes, corresponding

acceleration time histories are identified and their spectra compared to the target earthquakes spectra. 

The acceleration time histories are modified to match the target spectra. The scaled time histories, which

were recorded as outcrop motions, are imposed in the developed subsurface model (and are typically first

converted to equivalent “within” motions at the bedrock level). The imposed motions propagate through

the analytical model of the site soils to determine the free-field surface ground motions (Regulatory

Position 4.2). Only acceleration time histories modified from recorded earthquakes are used for this

purpose. A sufficient number of time histories should be used to get reasonably consistent average

results.

As a minimum, the results of the site response analysis should show the input motion (rock

response spectra), output motion (surface response spectra), and spectral amplification function (site

ground motion transfer function) at the surface. It is common practice to also provide a plot showing the

peak accelerations in the soil as a function of depth. In addition to developing the spectral amplifications

functions at the surface, it is also useful to simultaneously develop the spectral amplification functions

for the foundation level for use in development of the SSE. However, because the location of the

foundation is not always known at the time at which the analyses are undertaken, the functions may be

determined at several depths including the deepest depth from the Plant Parameter Envelope. In addition,

determining and providing the spectral amplification functions at a variety of depths may support review

of the results used in determining the soil UHRS and the SSE.

Due to the non-linear nature of the analyses and the heterogeneity of the soil, sufficiently

capturing the variability of the soil response at the site requires that at least 60 randomized shear velocity

profiles are paired with 60 sets of randomized shear modulus and damping curves (i.e., one shear velocity

profile with one set of modulus reduction and damping curves). The use of 60 profiles is generally

adequate to determine a reliable estimate of the standard deviation of the site response. To determine the

UHRS at the free-field ground surface, the site amplification functions (spectral ratios) for each input

earthquake are computed. The mean site amplification function is obtained for each input earthquake

scenario by dividing the response spectrum from the computed surface motion by the response spectrum

from the input hard-rock surface motion, and computing the arithmetic mean of these 60 individual

response spectral ratios.
Appendix E to RG 1.208, Page E-4

Once the site amplification functions are developed, they are applied to the free-field rock-based

UHRS. To determine the site-specific soil UHRS at the free-field ground surface, for each of the annual

exceedance frequencies (1 E-04, 1 E-05, and 1 E-06), multiply the rock-based UHRS by either the highor low-frequency site amplification function value at each frequency by the site amplification function

that is most appropriate for that frequency. The appropriate site amplification function is determined for

each frequency based on which of the input earthquakes has a spectral value that is closest to the rock

UHRS at that frequency. At high frequencies, this will be the high-frequency spectral amplification

function, and at low frequencies, the low-frequency site amplification function. By applying the

appropriate (high- or low-frequency) amplification function for each frequency to the mean rock-based

UHRS, the site-specific soil-based UHRS is calculated for the free ground surface. Figure E.1 shows a

comparison of the results of each of the soil amplification functions applied to the free-field rock UHRS

and the final free-field soil UHRS.

As a minimum, the results of the site response analysis should show the input motion (rock

response spectra), output motion (surface response spectra), and spectral amplification functions (site

ground motion transfer functions) at the surface. It is common to also provide a plot showing the peak

accelerations in the soil as a function of depth. In addition, determining and providing the spectral

amplification functions at a variety of depths supports review of the results used in determining the soil

UHRS and the SSE.

Figure E.1. Free-Surface Soil Uniform Hazard Response Spectra

Compared to Spectra Resulting from Scaling of Mean Rock UHRS

by Low- and High-Frequency Amplification Functions
Appendix E to RG 1.208, Page E-5

E.4 Free-Field Ground Surface Uniform Hazard Response Spectra

In accordance with Regulatory Position 4.3 of this regulatory guide, the UHRS at the free-field

ground surface is determined to form the basis of the SSE. In the method detailed above, the UHRS for

the site is developed by scaling the appropriate UHRS at the rock surface by the appropriate mean site

amplification function developed for either the low- and high-frequency ranges. These smooth UHRS

are used in Regulatory Position 5 of this regulatory guide to develop the performance-based horizontal

and vertical response spectra.

APPENDIX E REFERENCES

E.1 Regulatory Guide 1.132, “Site Investigations for Foundations of Nuclear Power Plants,” U.S.

Nuclear Regulatory Commission, Washington, DC.

E.2 Regulatory Guide 1.138, “Laboratory Investigations of Soils and Rocks for Engineering Analysis

and Design of Nuclear Power Plants,” U.S. Nuclear Regulatory Commission, Washington, DC.

E.3 Electric Power Research Institute (EPRI), “Guidelines for Determining Design-Basis Ground

Motions,” EPRI TR-102293, Palo Alto, California, 1993.
Appendix F to RG-1.208, Page F-1

APPENDIX F

CRITERIA FOR DEVELOPING TIME HISTORIES

This appendix provides criteria for developing and evaluating modified ground motions used for

soil response. Actual recorded earthquake ground motion or modified recorded ground motions should

be used for the site response analysis. Synthetic motions that are not based on seed recorded time

histories should not be used.

The general objective is to generate a modified recorded accelerogram that achieves

approximately a mean-based fit to the target spectrum. That is, the average ratio of the spectral

acceleration calculated from the accelerogram to the target, where the ratio is calculated frequency by

frequency, is only slightly greater than one. The aim is to achieve an accelerogram that does not have

significant gaps in the Fourier amplitude spectrum, but which is not biased high with respect to the target. 

Time histories biased high with respect to a spectral target may overdrive (overestimate damping and

stiffness reduction) a site soil column or structure when nonlinear effects are important. Ground motions

or sets of ground motions that are generated to “match” or “envelop” given design response spectral

shapes should comply with the following six steps:

(1) The time history should have a sufficiently small time increment and sufficiently long durations.

Time histories should have a Nyquist frequency of at least 50 Hz (e.g., a time increment of at

most 0.010 seconds) and a total duration of 20 seconds. If frequencies higher than 50 Hz are of

interest, the time increment of the record must be suitably reduced to provide a Nyquist

y t t

frequency (N = 1/(2 ) ), where ) = time increment) above the maximum frequency of interest. 

The total duration of the record can be increased by zero packing to satisfy these frequency

criteria.

(2) Spectral accelerations at 5 percent damping are computed at a minimum of 100 points per

frequency decade, uniformly spaced over the log frequency scale from 0.1 Hz to 50 Hz or the

Nyquist frequency. If the target response spectrum is defined in the frequency range from 0.2 Hz

to 25 Hz, the comparison of the modified motion response spectrum with the target spectrum is

made at each frequency computed in this frequency range.

(3) The computed 5 percent damped response spectrum of the average of all accelerograms should

not fall more than 10 percent below the target spectrum at any one frequency. To prevent spectra

in large frequency windows from falling below the target spectrum, the spectra within a

frequency window of no larger than ± 10 percent centered on the frequency should be allowed to

fall below the target spectrum. This corresponds to spectra at no more than nine adjacent

frequency points defined in (2) above from falling below the target spectrum.

(4) The mean of the 5 percent damped response spectra should not exceed the target spectrum at any

frequency by more than 30 percent (a factor of 1.3) in the frequency range between 0.2 Hz and

25 Hz. If the spectrum for the accelerogram exceeds the target spectrum by more than 30 percent

at any frequency in this frequency range, the power spectral density of the accelerogram needs to

be computed and shown to not have significant gaps in energy at any frequency over the

frequency range. An additional check over the 0.2 to 25 Hz frequency range is that the mean

ratio (ratio of the average record value to the target value) be greater than 1.0.
The directional correlation coefficient is a measure of the degree of linear relationship between two earthquake 1

accelerograms. For accelerograms X and Y, the directional correlation coefficient is given by the following equation:

 n _ _

ñ x y i i x y = { (1 / n) ( 3 [ (X - x ) (Y - y) ] ) } ÷ { ó ó }

 

i = 1 

 _ _

where n is x y the number of discrete acceleration-time data points, x and y are the mean values, and ó and ó are the

standard deviations of X and Y, respectively.

Appendix F to RG 1.208, Page F-2

(5) Because of the high variability in time domain characteristics of recorded earthquakes of similar

magnitudes and at similar distances, strict time domain criteria are not recommended. However,

modified motions defined as described above should typically have durations (defined by the 5

percent to 75 percent Arias intensity), and ratios V/A and AD/V (A, V, and D are peak ground 2

acceleration, ground velocity, and ground displacement, respectively), which are generally

consistent with characteristic values for the magnitude and distance of the appropriate controlling

events defining the uniform hazard response spectra. However, in some tectonic settings these

criteria may need to be altered to accommodate phenomena, such as, directivity, basin effects,

and hanging wall effects.

(6) To be considered statistically independent, the directional correlation coefficients between pairs

of time histories should not exceed a value of 0.16 . Simply shifting the starting time of a given 1

accelerogram does not constitute the establishment of a different accelerogram.