Seismic Hazard Analysis (Task 6)

Task Overview

Objective

The objective of this task is to develop seismic hazard curves associated with the plant site.

Purpose

A seismic hazard curve presents the annual frequency (or rate) of exceedance for different values of a selected ground motion parameter (acceleration, velocity, and so on). Hazard curves, which are used as input data in an SPRA, are the final output of a PSHA (probabilistic seismic hazard analysis) representing the mean annual rate of exceedance for the selected motion parameter and the associated uncertainty of this rate at a particular site.

Guidance

An example seismic hazard depicting the annual frequency of exceedance for different values of peak ground acceleration is shown in Figure 6-1. The probability of exceedance of ground motion x means the total probability of the occurrence of all ground motion levels equal to and greater than x. This probability reflects the randomness of future earthquakes; that is, the location, magnitude, and time of the next earthquake to affect the plant is not known. There are generally a number of equally plausible models and interpretations supported by different seismology and geology experts. Each such model produces a different seismic hazard curve. Thus, there is uncertainty about any given result. But the compilation of all such curves would fully represent the state of knowledge about the seismic hazard at the site.

This figure includes the mean hazard curve (average), the 50^th percentile curve (which, by definition, indicates that half of the seismic hazard results are below it and half are above), and two typical fractile hazard curves (95% and 5%). The 95th percentile curve indicates that only 5% of the results are above this curve. The 95^th percentile curve can be characterized as the confidence (95%) in the hazard estimate (that is, the probability of exceedance of a given ground motion level, x); and 5% of the time, the hazard could be higher.

Figure 6-1 Example Seismic Hazard

As shown in Figure 6-2 shows the three key elements that make up the seismic hazard calculation.

• Seismic source characterization describing earthquake source rates, magnitudes, and locations
• Ground motion attenuation characterizing the site ground motion based on the earthquake sources (magnitude, style-of-faulting), the wave propagation (distance), and a general site response (site class or Vs₃₀)
• Site response characterization providing site response functions that account for the site-specific response of the soils below the plant

Figure 6-2 Seismic Hazard Elements (from EPRI 1025287)

Seismic sources are typically identified as fault sources or area sources. Fault sources are at identified locations where there are rock fractures or a zone of rock fractures that are known to produce earthquakes. In a seismic source model, they are identified with locations, and a probabilistic distribution of expected magnitudes and recurrence rates. Area sources are regions where earthquake can occur with without defined faults. These area sources are also characterized with a location, or geometric area and a probabilistic distribution of expected magnitudes and recurrence rates.

Seismic ground motion models characterize how earthquake vibrations decay (attenuate) with distance as they travel through rock from the hypocenter to the nuclear plant site. The model also characterizes the resulting ground motion amplitudes (e.g., accelerations), given a specific earthquake magnitude. The ground motion models are also characterized as probabilistic distributions.

A site response model characterizes how the vibrations propagate through the local soils, amplifying or attenuating the motions. These models are developed using site investigations that study the types of soils at the site using bore holes to extract vertical columns of soil as well as various types of vibration propagation measurements. In some studies, the seismic sources are combined with the ground motion model to produce a rock hazard and then the rock hazard is amplified by the site response functions. In more recent studies, the site response is combined with the seismic sources and ground motion model in a composite calculation.

Seismic hazard analyses frequently involve a combination of a number of hazard experts and a variety of potential interpretations. These interactions are frequently performed using a process developed by the Senior Seismic Hazard Analysis Committee (SSHAC) in NUREG/CR-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. The goal of the SSHAC process is to determine the center, body, and range (CBR) of the technically defensible interpretations (TDI), as depicted in Figure 6-3 from NUREG-2213, Updated Implementation Guidelines for SSHAC Hazard Studies.

Figure 6-3 Representation of the center, body, and range of technically defensible interpretations (from NUREG-2213)

Supplemental Guidance

Related Element of ASME/PRA Standard

Part 5, Seismic Hazard Analysis (SHA)

EPRI Guidance

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, (1013105)

Central and Eastern United States Seismic Source Characterization for Nuclear Facilities, (EPRI 1021097 / NUREG-2115 / DOE/NE-0140)

Seismic Evaluation Guidance: Screening, Prioritization and Implementation Details (SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic, (1025287)

Central and Eastern United States Seismic Source Characterization for Nuclear Facilities: Review for Reservoir-Induced Seismicity (RIS) in the Southeast and Earthquakes in South Carolina Near the 1886 Charleston Earthquake, (3002005288)

Central and Eastern United States Seismic Source Characterization for Nuclear Facilities: Maximum Magnitude Distribution Evaluation, (3002005684)

EPRI (2004, 2006) Ground-Motion Model (GMM) Review Project, (3002000717)

Seismic Hazard Research: Kappa and High-Frequency Ground Motion Effects at Hard-Rock Sites, (3002020750)

Other Guidance

NUREG/CR-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts

NUREG/CR-6728, Technical Basis for Revision of Regulatory Guidance on Ground Motions: Hazard- and Risk-consistent Ground Motion Spectra Guidelines

Regulatory Guide 1.208, A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion

NUREG-2213, Updated Implementation Guidelines for SSHAC Hazard Studies

Pacific Earthquake Engineering Research Center (PEER) 2018/08, Central and Eastern North America Ground-Motion Characterization: NGA-East Final Report