[Federal Register Volume 90, Number 126 (Thursday, July 3, 2025)]
[Notices]
[Pages 29587-29594]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2025-12432]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-275 and 50-323; NRC-2024-0148]
Pacific Gas & Electric Company; Diablo Canyon Nuclear Power
Plant, Units 1 and 2; Petition
AGENCY: Nuclear Regulatory Commission.
ACTION: Director's decision under 10 CFR 2.206; issuance.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) has issued a
director's decision with regard to a petition dated March 4, 2024, as
supplemented on June 7, 2024, July 17, 2024, and October 31, 2024,
filed by San Luis Obispo Mothers for Peace, Friends of the Earth, and
Environmental Working Group (the petitioner), requesting that the NRC
take action with regard to Pacific Gas & Electric Co. (the licensee).
The petitioner's requests and the director's decision are included in
the SUPPLEMENTARY INFORMATION section of this document.
DATES: The director's decision was issued on June 26, 2025.
ADDRESSES: Please refer to Docket ID NRC-2024-0148 when contacting the
NRC about the availability of information regarding this document. You
may obtain publicly available information related to this document
using any of the following methods:
Federal Rulemaking Website: Go to https://www.regulations.gov and search for Docket ID NRC-2024-0148. Address
questions about Docket IDs in Regulations.gov to Bridget Curran;
telephone: 301-415-1003; email: [email protected]. For technical
questions, contact the individual listed in the For Further Information
Contact section of this document.
NRC's Agencywide Documents Access and Management System
(ADAMS): You may obtain publicly available documents online in the
ADAMS Public Documents collection at https://www.nrc.gov/reading-rm/adams.html. To begin the search, select ``Begin Web-based ADAMS
Search.'' For problems with ADAMS, please contact the NRC's Public
Document Room (PDR) reference staff at 1-800-397-4209, at 301-415-4737,
or by email to [email protected]. For the convenience of the reader,
instructions about obtaining materials referenced in this document are
provided in the ``Availability of Documents'' section.
NRC's PDR: The PDR, where you may examine and order copies
of publicly available documents, is open by appointment. To make an
appointment to visit the PDR, please send an email to
[email protected] or call 1-800-397-4209 or 301-415-4737, between 8
a.m. and 4 p.m. eastern time (ET), Monday through Friday, except
Federal holidays.
FOR FURTHER INFORMATION CONTACT: Dennis J. Galvin, Office of Nuclear
Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington,
[[Page 29588]]
DC 20555-0001; telephone: 301-415-6256; email: [email protected].
SUPPLEMENTARY INFORMATION: The text of the director's decision is
attached.
The documents identified in the following table are available to
interested persons through ADAMS.
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Document description Adams accession No.
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Petition submitted by San Luis Obispo Mothers for Peace, ML24067A066.
Friends of the Earth, and Environmental Working Group,
dated March 4, 2024.
NRC SECY Order referring the petition to the 10 CFR 2.206 ML24072A529.
process, dated March 12, 2024.
NRC Management Directive 8.11, ``Review Process for 10 CFR ML18296A043.
2.206 Petitions,'' dated March 1, 2019.
NRC email to Petitioner's, dated March 28, 2024............ ML24088A238.
Email from the NRC to the Petitioners regarding Diablo ML24136A162.
Canyon Seismic Core Damage 2.206 petition--Initial
Assessment, dated May 15, 2024.
Petition Supplement, dated June 7, 2024.................... ML24162A079.
Public Meeting Petitioner Presentation to the PRB, dated ML24198A105.
July 17, 2024.
Public Meeting Official Transcript of Proceedings, dated ML24218A164.
July 17, 2024.
NRC Acknowledgement Letter, dated August 27, 2024.......... ML24205A066.
PG&E Voluntary Submittal of Information, dated October 24, ML24298A234.
2024.
Petition Supplement, dated October 30, 2024................ ML24305A187.
Supplement Acknowledgement Letter, dated December 5, 2024.. ML24317A038.
Response to request for additional information regarding ML15071A046 (Package).
seismic models developed by PG&E, dated March 11, 2015.
Response to request for additional information, regarding ML12056A046 (Package).
NRC's 50.54(f) request, dated March 12, 2012.
NRC staff assessment and conclusion, dated December 21, ML16341C057.
2016.
NUREG/KM-0017, ``Seismic hazard Evaluations for U.S. ML21344A126.
Nuclear Power Plants; Near-Term Task Force Recommendation
2.1 Results,'' published December 2021.
NUREG-2213, ``Updated Implementation Guidelines for SSHAC ML18282A082.
Hazard Studies,'' published October 2018.
NRC staff review of Seismic Probabilistic Risk Assessment, ML18254A040.
dated December 22, 2019.
Proposed Director's Decision, dated April 10, 2025......... ML24264A091 (Package).
Proposed Director's Decision, dated April 10, 2025......... ML24302A148.
Attachment to the Proposed Director's Decision: Figures 1 ML25093A060.
through 9.
Proposed Director's Decision letter to Petitioners, dated ML24302A153.
April 10, 2025.
Proposed Director's Decision letter to Licensee, dated ML24302A154.
April 10, 2025.
Comments on Proposed Director's Decision, dated May 15, ML25136A355.
2025.
Director's Decision DD-25-01, dated June 26, 2025.......... ML25160A132 (Package).
Director's Decision DD-25-01, dated June 26, 2025.......... ML25160A125.
Attachment to the Director's Decision: Figures 1 through 9. ML25161A264.
Director's Decision letter to Petitioners, dated June 26, ML25160A124.
2025.
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Dated: June 30, 2025.
For the Nuclear Regulatory Commission.
Dennis Galvin,
Project Manager, Plant Licensing Branch IV, Division of Operating
Reactor Licensing, Office of Nuclear Reactor Regulation.
Attachment--Director's Decision Under 10 CFR 2.206
DD-25-01
United States of America
Nuclear Regulatory Commission
Office of Nuclear Reactor Regulation
Michael X. Franovich, Deputy Director
In the Matter of Pacific Gas and Electric Co. Diablo Canyon,
Nuclear Power Plant, Units 1 and 2
Docket Nos. 50-275 and 50-323
License Nos. DPR-80 and DPR-82
Director's Decision Under 10 CFR 2.206
I. Introduction
On March 4, 2024, the San Luis Obispo Mothers for Peace, Friends
of the Earth, and Environmental Working Group petitioned the U.S.
Nuclear Regulatory Commission (NRC) requesting that the NRC exercise
its supervisory authority to order the immediate closure of Diablo
Canyon Nuclear Power Plant, Units 1 and 2 (Diablo Canyon), due to
``the unacceptable risk of a seismically induced severe accident''
(Agencywide Documents Access and Management System Accession No.
ML24067A066). On March 12, 2024 (ML24072A529), the Office of the
Secretary of the Commission referred the petitioners' request to the
enforcement petition process under Title 10 of the Code of Federal
Regulations (10 CFR) Section 2.206, ``Requests for action under this
subpart.''
The basis for the petitioners' request, as included in the
original petition and in the petitioners' supplements, is summarized
below:
Concern 1: Thrust faulting is neglected by Pacific Gas &
Electric Company's (PG&E) 2015 Seismic Source Characterization (SSC)
model \1\ because the model assumes that a majority of large
earthquakes affecting Diablo Canyon are strike-slip and disregards
the significant contribution of thrust faulting earthquake sources
under the Diablo Canyon site and the adjacent Irish Hills. In
addition, the hazard characterization performed by PG&E did not use
a hanging-wall term for the modeling of potential ground motions
from the Los Osos and San Luis Bay thrust faults.
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\1\ In 2015, PG&E developed the Diablo Canyon seismic source
characterization (SSC) model and the ground motion characterization
(GMC) model and documented them in reports, which are referred to
herein as the PG&E 2015 SSC Report and the PG&E 2015 GMC Report.
These reports are available on the PG&E website https://www.pge.com/en/about/pge-systems/nuclear-power/seismic-safety-at-diablo-canyon.html#tabs-2967acbbcb-item-1b0b13e766-tab.
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Concern 2: The January 2024 magnitude 7.5 earthquake centered in
the Noto Peninsula (Japan), with an average slip of 2 meters on the
fault, is analogous to future potential thrust mechanism earthquakes
beneath Diablo Canyon. Based on the slip rate of an ``inferred''
offshore thrust fault proposed by the petitioners, which is located
beneath the Irish Hills adjacent to Diablo Canyon and the slip of
the Noto earthquake, large ground motions from thrust fault
earthquakes will occur, on average, every 715 years near the Diablo
Canyon site.
Concern 3: PG&E's SSC model does not account for an ``inferred''
offshore thrust fault that has the potential for producing a
magnitude 7.5 earthquake. Based on regional stratigraphy, gravity
modeling and global positioning system (GPS) modeling, the total
thrust faulting slip rate beneath the Irish Hills is between 2.0 to
2.8 millimeter per year (mm/yr), which is not accounted for in
PG&E's SSC model.
Concern 4: Seismic core damage frequency, estimated by PG&E in
2018 to be 3x10-5, should be 1.4x10-3 per year
(about once every 715 years) based on this higher recurrence rate
for thrust earthquakes.
In accordance with the handbook for NRC Management Directive
(MD) 8.11, ``Review
[[Page 29589]]
Process for 10 CFR 2.206 Petitions,'' dated March 1, 2019
(ML18296A043), Section III, ``Petition Review Board (PRB),'' NRC
staff promptly deliberated on the request for immediate action and
began the screening process. On March 28, 2024 (ML24088A238), the
petitioners were informed that the NRC staff concluded that no
immediate action is necessary, that the concerns expressed in the
petition were screened into the 2.206 process, and that a PRB would
be assembled to evaluate the concerns.
In an email dated May 15, 2024 (ML24136A162), the petition
manager informed the petitioners that the PRB's initial assessment
was that the petition did not meet the criteria in MD 8.11 for
accepting petitions under 10 CFR 2.206 because ``the issues raised
have previously been the subject of a facility-specific or generic
NRC staff review'' and the petition does not provide significant new
information that the staff did not consider in a prior review.
On June 7, 2024, the petitioners submitted a supplement to the
petition (ML24162A079).
The NRC held a public meeting with the petitioners on July 17,
2024. The petitioners' presentation (ML24198A105) and the meeting
transcript (ML24218A164) are considered supplements to the petition.
This supplemental information provided by the petitioners is
addressed below as part of the NRC staff's response to Concern 3.
On August 27, 2024 (ML24205A066), the NRC issued an
acknowledgement letter informing the petitioners that the concerns
raised in the petition, as supplemented, now meet the criteria in MD
8.11 for accepting petitions under 10 CFR 2.206 and that the
concerns would undergo further review by the PRB. The letter also
informed petitioners that the PRB determined that there is no
imminent safety concern that warrants immediate shutdown of Diablo
Canyon.
On October 24, 2024, PG&E provided a voluntary submittal
(ML24298A234) to the NRC related to the PRB review of the petition.
On October 31, 2024, the petitioners submitted a supplement to the
petition (ML24305A187) in response, in part, to the October 24,
2024, PG&E voluntary submittal.
On December 5, 2024 (ML24317A038), the NRC issued a supplemental
acknowledgement letter informing the petitioners that two concerns
from the October 31, 2024, supplement would be included in the
ongoing PRB review. These supplemental concerns provided by the
petitioners are addressed below as part of the NRC staff's response
to Concern 3. The letter also informed petitioners that the PRB
determined that there is no imminent safety concern that warrants
immediate shutdown of Diablo Canyon.
Publicly available documents created or received at the NRC are
accessible electronically through ADAMS in the NRC's Library at
http://www.nrc.gov/reading-rm/adams.html. Persons who do not have
access to ADAMS, who encounter problems in accessing the documents
located in ADAMS, or who want to inspect publicly available
documents at the NRC's Public Document Room at 11555 Rockville Pike,
Rockville, MD 20852 should contact the NRC's PDR reference staff by
telephone at 1-800-397-4209 or 301-415-4737, or by email to
[email protected].
II. Discussion
Concern 1: Thrust faulting is neglected by PG&E's 2015 SSC model
because the model assumes that a majority of large earthquakes
affecting Diablo Canyon are strike-slip and disregards the
significant contribution of thrust faulting earthquake sources under
the Diablo Canyon site and the adjacent Irish Hills. In addition,
the hazard characterization performed by PG&E did not use a hanging-
wall term for the modeling of potential ground motions from the Los
Osos and San Luis Bay thrust faults.
Based on its previous review in 2016 of the seismic models
developed by PG&E, which are summarized in PG&E's seismic hazard and
screening report (ML15071A046),in response to the NRC's 50.54(f)
request (ML12056A046), the PRB disagrees with the petitioners'
claims that thrust faulting was not adequately accounted for by PG&E
in its seismic source model and that a hanging-wall term was not
implemented in the seismic ground motion model. The PRB supports the
NRC's conclusion in 2016 (ML16341C057) that PG&E adequately
accounted for reverse or thrust faulting in the alternative fault
geometry models developed for the SSC model and that a hanging-wall
term was implemented for the Ground Motion Characterization (GMC)
model, which increased the ground motion as expected. The bases for
the PRB conclusions are provided below.
Diablo Canyon is located on the southwest slope of the Irish
Hills in the northern part of the San Luis Range in central coastal
California. The current tectonic setting for the region around
Diablo Canyon is a transform plate boundary that accommodates
horizontal relative motions consisting of strike-slip faults with
transpressional deformation, resulting in localized areas of uplift
and folding alongside the major fault zone. Strike-slip faults
display predominantly horizontal movement, usually along a nearly
vertical fault surface, and transpression refers to a type of
strike-slip deformation where shortening (compression) occurs
perpendicular to the fault plane because of the presence of bends
along the fault line. The San Luis Range in central coastal
California is a topographic and structural elevation high (maximum
elevation of 1,784 meters) that formed within this region of reverse
and oblique slip faults due to this transpressional deformation. A
reverse fault is a fault where the upper side of the fault, called
the hanging wall, moves up and over the lower or foot wall side of
the fault, and an oblique slip fault is a fault in which the two
sides of the fault simultaneously move both vertically and
horizontally. For its SSC model, developed in response to the NRC's
50.54(f) request, PG&E modeled the uplift of the Irish Hills,
located in the San Luis Range adjacent to Diablo Canyon, assuming
rigid block uplift resulting from reverse faulting on the moderate
to steeply dipping (45 to 80 degrees) Los Osos and San Luis Bay
faults rather than from folding deformation on a low-dip angle (25
degrees) ``inferred'' offshore thrust fault, as postulated by the
petitioners. Thrust faulting is a type of reverse faulting with a
dip angle of 45 degrees or less. To develop the SSC model, PG&E used
recently acquired offshore and onshore two- and three-dimensional
seismic reflection data, multibeam bathymetric data, geodetic data,
and an updated seismicity catalog to better define the location,
geometry, and slip rate of the faults in the area around Diablo
Canyon. Modeling the uplift of the Irish Hills as a rigid block is
based on this extensive geologic field work and geophysical surveys,
which PG&E has been supporting for many years, going back to the
1980s.
PG&E's reevaluation of the seismic hazard in response to the
NRC's 50.54(f) request determined that four faults contribute to the
majority of the seismic hazard at Diablo Canyon. These four faults
are the Hosgri, Los Osos, San Luis Bay, and Shoreline faults. The
Hosgri and Shoreline faults are near-vertical strike-slip faults,
and the Los Osos and San Luis Bay faults are reverse faults that
border the northeastern and southern margins of the Irish Hills,
respectively. Figure 1,\2\ from the PG&E 2015 SSC Report and
incorporated into NUREG/KM-017, ``Seismic Hazard Evaluations for
U.S. Nuclear Power Plants: Near-Term Task Force Recommendation 2.1
Results'' (ML21344A126), shows the location of Diablo Canyon
relative to the Irish Hills and the four faults that contribute the
most to the hazard.
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\2\ See the ``Attachment to the Director's Decision: Figures 1
through 9'' in ML25161A264 for Figures 1 through 9.
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As specified in the 50.54(f) request, PG&E implemented the SSHAC
approach in NUREG-2213, ``Updated Implementation Guidelines for
SSHAC Hazard Studies'' (ML18282A082), to develop the SSC and GMC
models used to determine the seismic hazard for the Diablo Canyon
site. The SSHAC approach is focused on two critical activities:
evaluation and integration. The evaluation activity is defined as an
assessment of the complete set of data, models, and methods that are
relevant to the hazard analysis as proposed by the larger technical
community, consisting of geologists and seismologists with expertise
in coastal California tectonics. The SSHAC guidelines provide a
method for facilitating interactions with the SSHAC team and members
of the larger technical community to exchange viewpoints and to
challenge proponents of differing hypotheses. The integration
activity is the development of SSC and GMC models that capture all
technically defensible interpretations, as informed by the
evaluation activity. There are four SSHAC study levels, with the
higher levels involving a greater number of participants and a
longer duration to more fully assess available data, models, and
methods. A key element of the SSHAC approach is participatory peer
review from an outside panel (Participatory Peer Review Panel or
PPRP) to ensure that the full range of data, models, and methods are
considered in the evaluation phase and that the center (median),
body (16th to 84th percentile), and range (5th to 95th percentile)
of technically defensible interpretations are integrated into the
seismic source and ground motion
[[Page 29590]]
models to capture the uncertainty in seismic hazard as required by
10 CFR 100.23, ``Geologic and seismic siting criteria.''
In response to the 50.54(f) request, PG&E performed a Level 3
SSHAC study for its reevaluation of the seismic hazard for the
Diablo Canyon site. Development and documentation of the source and
ground motion models were performed from June 2011 to March 2015 and
included three formal workshops conducted in San Luis Obispo,
California, which were open to the public. Dr. Bird, expert witness
for the petitioners, participated in the second public workshop in
2012 and presented his views ``on both strike-slip and compressional
deformation rates affecting the region'' (PG&E 2015 SSC Report).
Elements of Dr. Bird's views (uplift of Irish Hills due to slip on
low-angle reverse or thrust faults) were incorporated into the SSC
model developed by the SSHAC team; however, other alternative models
were also developed by the SSHAC team in order to capture the
uncertainty in the local faulting mechanisms and underlying
tectonics. As observers of the three formal public workshops in
2012, the NRC staff viewed the wide range of hypotheses proposed by
the experts for the current regional transpressional tectonic
setting around Diablo Canyon.
To accommodate the multiple hypotheses proposed by the experts,
PG&E developed three alternative fault geometry models to capture
the range of potential mechanisms driving uplift of the Irish Hills.
These models include the Outward-Vergent, Southwest-Vergent, and
Northeast-Vergent models. The Southwest-Vergent model considers the
San Luis Bay fault as a reverse or thrust fault with a dip angle of
45 degrees and incorporates aspects of theories proposed by the
petitioners. In addition to the three fault geometry models, the SSC
model also accounts for earthquakes potentially occurring on
previously unidentified faults by developing a ``background''
seismic source zone surrounding Diablo Canyon. This background zone
considers the possibility of low-angle (35 degrees) thrust or
reverse faults with fault lengths of 50 kilometers (km) and
magnitudes as high as moment-magnitude (Mw) 7.1. In its review of
the SSC model, the NRC staff concluded that PG&E adequately
implemented the SSHAC process and developed multiple alternative
models for the uplift of the California Coast Ranges that are based
on the modeling of geological and geophysical field data
(ML16341C057). This conclusion was also supported by the SSHAC PPRP
in its project closure letter, which states that ``the data, models,
and methods within the larger technical community have been properly
evaluated, and the center, body, and range of the technically
defensible interpretations have been appropriately represented in
the SSC model'' (appendix B of the PG&E 2015 SSC Report).
In addition to stating that PG&E neglected the potential for
thrust faulting, the petition claims that PG&E did not use a
hanging-wall term for the modeling of potential ground motions from
the Los Osos and San Luis Bay reverse faults. The ``hanging-wall''
effect is the increase in ground motion at a site located on top of
the hanging wall side of the fault due to the site being located
directly above the fault and closer to the rupture area. Based on
its review of the SSHAC Level 3 GMC model (appendix C of the PG&E
2015 GMC Report), the NRC staff determined that a hanging-wall term
was implemented and that this term increased the ground motion as
expected (ML16341C057).
In summary, based on its previous review in 2016 of the seismic
models developed by PG&E in response to the NRC's 50.54(f) request,
the PRB disagrees with the petitioners' claims that thrust faulting
was neglected by PG&E in its seismic source model and that a
hanging-wall term was not implemented in the seismic ground motion
model.
Concern 2: The January 2024 magnitude 7.5 earthquake centered in
the Noto Peninsula (Japan), with an average slip of 2 meters on the
fault, is analogous to future potential thrust mechanism earthquakes
beneath Diablo Canyon. Based on the slip rate of the ``inferred''
offshore thrust fault proposed by the petitioners, which is located
beneath the Irish Hills adjacent to Diablo Canyon, and the slip of
the Noto earthquake, large ground motions from thrust fault
earthquakes will occur, on average, every 715 years near the Diablo
Canyon site.
Due to differences in the primary tectonic driving forces, the
types of earthquake focal mechanisms, rate of seismic activity, and
the lack of direct observations from geophysical surveys of a major
``inferred'' thrust fault off the coast of central California in the
vicinity of Diablo Canyon, the PRB concludes that the January 2024
magnitude 7.5 Noto Peninsula earthquake is highly unlikely to be
analogous to a future potential thrust mechanism earthquake beneath
Diablo Canyon. The bases for the PRB conclusion are provided below.
The Noto Peninsula in Japan is located on the eastern margin of
the Sea of Japan on the west coast of Honshu (largest island of
Japan) and was formed as a result of back-arc rifting arising from
subduction of the Pacific Plate beneath the Eurasian plate along the
Japan Trench. Back-arc rifting is a process that occurs when one
tectonic plate subducts beneath another, causing the overlying plate
to stretch and thin, forming a back-arc basin. Subsequent to back-
arc rifting during the Pliocene Epoch (3 million years ago), the
tectonic regime along the west coast of Honshu shifted to
compression, which reactivated older rift faults as reverse or
thrust faults, causing uplift of former basins on the peninsula. The
west coast of Honshu is now a convergent boundary between the
Amurian (eastern edge of the Eurasian plate) and Okhotsk
microplates, with convergence rates ranging from 14 to 16.5 mm/
yr.\3\ Figure 2 shows the location of the Noto Peninsula relative to
the boundary between the two converging microplates. Several large
earthquakes and tsunamis have occurred along this convergent
boundary between the two microplates including the most recent Mw
7.5 earthquake on January 1, 2024. The 2024 Noto earthquake occurred
on a shallow reverse or thrust fault with the rupture extending over
100 km in length from the southwestern portion of the Noto Peninsula
to Sado Island along a southeast-dipping fault.\4\ Figure 3, from
the U.S. Geological Survey's (USGS) website for the 2024 Noto
earthquake, shows the distribution of slip based on the finite fault
model developed by USGS for the 2024 Noto earthquake. According to
the USGS model, slip occurred mostly beneath the peninsula with the
zones of largest slip occurring to the southwest of the earthquake
hypocenter and with earthquake rupture propagating from the
peninsula to the seafloor.\5\
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\3\ Ito, C., T. Hiroaki, and O. Mako, ``Estimation of
convergence boundary location and velocity between tectonic plates
in northern Hokkaido inferred by GNSS velocity data,'' Earth,
Planets and Space, 71.1: 1-8, 2019.
\4\ U.S. Geological Survey, ``M 6.5-10 km NE of San Simeon,
California.'' Accessed December 1, 2024. https://earthquake.usgs.gov/earthquakes/eventpage/us6000m0xl/executive.
\5\ Id.
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In contrast to the Noto Peninsula tectonic regime of
compression, the tectonic setting for the region surrounding Diablo
Canyon is a transform plate boundary that produces horizontal
relative motions along strike slip faults with transpressional
deformation. The tectonic setting for the central coastal California
region is roughly triangular with the San Andreas fault on the east,
the San Gregorio-San Simeon-Hosgri fault on the west, and the
Western Transverse Ranges on the south.\6\ Figure 4, from
Langenheim,\7\ shows this triangular region that bounds the
California Coast Range with the numerous north-northwest striking
faults that cut through Cenozoic Era (approximately 66 million years
ago until today) sedimentary rocks that overlie older Mesozoic Era
(approximately 252 to 66 million years ago) basement rocks such as
the Franciscan Complex metamorphosed rock.
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\6\ Langenheim, V.E., R.C. Jachens, R.W. Graymer, J.P. Colgan,
C.M. Wentworth, and R.G. Stanley, ``Fault geometry and cumulative
offsets in the central Coast Ranges, California: Evidence for
northward increasing slip along the San Gregorio-San Simeon-Hosgri
fault,'' Lithosphere, 5(1), 29-48, 2013.
\7\ Id.
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Although faulting is primarily strike-slip, steady uplift over
at least the past 125,000 years has occurred along a 400-km long
portion of the central California coast. Along the San Gregorio-
Hosgri fault system, late Quaternary age (2.6 million years ago) to
modern reverse fault slip rates are on the order of 10 to 30 percent
of the strike-slip fault slip rates (O'Connell and Turner, 2023).
Near Diablo Canyon, the San Luis Range forms the core of the San
Luis-Pismo Block, a structural block that trends northwest to
southeast. It is bounded by strike slip fault zones on the west
(Hosgri fault) and east (Oceanic-West Husana fault), and by a series
of reverse faults to the northeast (Los Osos fault) and southwest
(Southwestern Boundary Zone including the San Luis Bay fault).
Geologic field studies \8\ show that the San Luis Range is
uplifting at rates between 0.1
[[Page 29591]]
mm/yr to 0.2 mm/yr. According to PG&E's SSC model, slip rates near
Diablo Canyon are estimated to be:
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\8\ Hanson, K.L., J.R. Wesling, W.R. Lettis, K.I. Kelson, and L.
Mezger, ``Correlation, Ages, and Uplift Rates of Quaternary Marine
Terraces, South-Central California,'' I.I. Alterman, R.B. McMullen,
L.S. Cluff, and D.B. Slemmons, eds., Seismotectonics of the Central
California Coast Range, Geological Society of America Special Paper
292. Pp. 45-72, 1994.
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Hosgri strike-slip fault: 1 to 2 mm/yr
Los Osos reverse fault: 0.2 to 0.4 mm/yr
San Luis Bay reverse fault: 0.1 to 0.3 mm/yr
For comparison, horizontal slip rates on the San Andreas fault
in central California, located approximately 85 km northeast of
Diablo Canyon, are estimated to be 25 to 36 mm/yr.
The tectonic differences between the Noto Peninsula and central
coastal California are further demonstrated by the types of
earthquakes in the two regions as evidenced by the focal mechanisms
of the earthquakes. In the Noto Peninsula, the earthquake focal
mechanisms are predominantly reverse, whereas in the vicinity of
Diablo Canyon, the focal mechanisms are a mixture of strike-slip,
reverse and oblique mechanisms. Figure 5 shows that the focal
mechanisms for earthquakes near Diablo Canyon exhibit this mixture
of different types of fault slip and orientations (PG&E 2015 SSC
Report).
The other major difference between the Noto Peninsula in Japan
and the Irish Hills in the western part of the San Luis Range in
central coastal California are the historical earthquake recurrence
rates. In addition to the 2024 Mw 7.5 Noto earthquake, several other
large earthquakes have recently occurred beneath the Noto Peninsula,
including an earthquake swarm for the last 3 years with the largest
earthquake being a Mw 6.3 earthquake occurring on May 5, 2023. This
earthquake swarm was preceded by the Mw 6.9 2007 Noto Hanto
earthquake, which occurred at a depth of 10 km near the west coast
of the Noto Peninsula. Similar to the Noto Peninsula, along central
coastal California and in the Transverse Ranges there have been
numerous earthquakes in the Mw 5 to Mw 7 range, including the 2003
Mw 6.5 San Simeon earthquake and the 1927 Mw 7.0 Lompoc earthquake.
However, near Diablo Canyon, in the vicinity of San Luis Bay and the
Irish Hills, McLaren and Savage \9\ document only two M5 events in
1913 and 1916. Figure 6, from the PG&E 2015 SSC Report, shows the
locations of historical earthquakes in central coastal California.
In summary, the historical rate for large earthquakes in the
vicinity of Diablo Canyon is much smaller than the rate for the Noto
Peninsula.
---------------------------------------------------------------------------
\9\ McLaren, M.K. and W.U. Savage. ``Seismicity of South-central
Coastal California: October 1987 through January 1997,'' Bulletin of
the Seismological Society of America, Vol. 91, Issue 6., pp. 1,629-
1,658, 2001.
---------------------------------------------------------------------------
An additional issue with the existence of the petitioners'
``inferred'' offshore thrust fault capable of producing an Mw 7.5
earthquake similar to the 2024 Mw 7.5 Noto Peninsula earthquake is
the lack of evidence from the recently acquired offshore and onshore
two- and three-dimensional seismic reflection data and multibeam
bathymetric data. Based on recent fault length versus magnitude
relationships for reverse or thrust faults, such as Thingbaijam et
al.,\10\ the length of the ``inferred'' offshore thrust fault would
need to be on the order of 70 to 100 km. In addition, the
petitioners assert, as described below in Concern 3, that the slip
rate of this ``inferred'' offshore thrust fault is between 2.0 to
2.8 mm/yr. That a thrust fault of this length and this relatively
high activity rate would go undetected considering the numerous
geophysical surveys and detailed studies of the regional seismicity
(e.g., Hardebeck \11\) is highly unlikely. However, to account for
the possibility of earthquakes occurring on previously unidentified
faults, PG&E developed a background seismic source zone for its SSC
model that includes 18 virtual offshore and onshore faults with
lengths of 50 km, magnitudes as high as Mw 7.1, and activity rates
based on the regional seismicity catalog. The style of faulting for
these virtual faults includes the possibility for both strike-slip
and reverse or thrust faulting on low-angle (35 degrees) dipping
faults. Figure 7, from the NRC staff's confirmatory analysis of
PG&E's hazard models, shows the Hosgri, Shoreline, Los Osos, San
Luis Bay, and the 18 virtual faults used to systematically account
for the possibility of earthquakes on previously unidentified faults
near Diablo Canyon (ML16341C057).
---------------------------------------------------------------------------
\10\ Thingbaijam, K.K. S., P.M. Mai, and K. Goda, ``New
empirical earthquake source[hyphen]scaling laws,'' Bulletin of the
Seismological Society of America, 107(5), 2225-2246, 2017.
\11\ Hardebeck, Jeanne L., ``Seismotectonics and fault structure
of the California Central Coast,'''' Bulletin of the Seismological
Society of America, 100.3: 1031-1050, 2010.
---------------------------------------------------------------------------
In summary, due to differences in the primary tectonic driving
forces, the types of earthquake focal mechanisms, rate of seismic
activity, and the lack of direct observations from geophysical
surveys of a major thrust fault off the coast of central California
in the vicinity of Diablo Canyon, the PRB concludes that the January
2024 magnitude 7.5 Noto Peninsula earthquake is highly unlikely to
be analogous to a future potential thrust mechanism earthquake
beneath Diablo Canyon.
Concern 3: PG&E's SSC model does not account for an ``inferred''
offshore thrust fault that has the potential for producing a
magnitude 7.5 earthquake. Based on regional stratigraphy, gravity
modeling and global positioning system (GPS) modeling, the total
thrust faulting slip rate beneath the Irish Hills is between 2.0 to
2.8 mm/yr, which is not accounted for in PG&E's SSC model.
The PRB concludes that the stratigraphic profile, gravity
anomalies, and GPS modeling used by the petitioners do not provide
adequate evidence to support the existence of a major ``inferred''
offshore thrust fault that extends beneath the Irish Hills with a
fault length of 70 to 100 km and a slip rate between 2.0 to 2.8 mm/
yr. The bases for the PRB conclusion are provided below.
The petitioners assert that folding beneath the Irish Hills
within the San Luis-Pismo block has been ongoing for the past 5 to 6
million years due to low-angle thrust faulting and that this has
resulted in the uplift of the Irish Hills. Based on this hypothesis,
the petitioners estimate that there has been 1.6 to 2.2 km of
vertical throw of the Obispo Formation over the past 5 million years
and that this vertical offset can be used to arrive at a ``minimum
total thrust'' fault slip rate of 1.5 to 2.1 mm/yr beneath the Irish
Hills. Figure 8, from the petitioners' July 17, 2024, presentation,
shows the petitioners' interpretation of the location and geometry
of the faults beneath the Irish Hills in red overlain on Figure 13-
17 from the PG&E 2015 SSC Report. On Figure 13-17 from the PG&E 2015
SSC Report, the petitioners have redrawn each of the more steeply
dipping faults to be at 25 degrees and added the ``inferred''
offshore thrust fault, which extends from just offshore towards the
eastern edge of the San Luis-Pismo block. The petitioners also added
a vertical line in the upper left to show throw of the Obispo
Formation (designated Tmo), which is depicted as the light blue
layer.
The Obispo Formation is a marine deposit made up of lavas and
tuffs that is about 20 million years old. The Obispo Formation is
present beneath several younger rock formations in the offshore and
onshore Santa Maria Basins and the onshore Pismo Basin. From the
Miocene to the early Pliocene (20 to 5 million years ago), normal
faulting on the margins of these basins resulted in subsidence of
the basins. The location, thickness, and offset of rock formations
across these basins is highly uncertain, especially for the older
formations such as the Obispo Formation. Therefore, the use of the
vertical offset of the Obispo Formation across multiple basins to
determine the slip rate on a previously unidentified ``inferred''
thrust fault beneath the Irish Hills is questionable. The
petitioners relied on the use of vertical offset of a single rock
layer (Obispo Formation) to support the hypothesis for low-angle
thrust faulting beneath the Irish Hills at a slip rate nearly twice
that of the strike-slip Hosgri fault, which is the most active fault
near Diablo Canyon. In contrast, PG&E relied on several geologic
studies performed in the region that use geomorphic evidence (i.e.,
study of landforms and landform evolution) to demonstrate that
folding within the San Luis-Pismo block has ceased or continued at a
very slow rate during the past half million years and that the
current vertical deformation of the Irish Hills is associated with
brittle failure and block uplift (Killeen,\12\ and Lettis et
al.\13\). Killeen states that, ``marine terraces, and stream
profiles show low, zero, to almost zero rates of Quaternary activity
around the Pismo syncline,'' and ``Data from paleo stream terrace
gradients suggest that synclinal folding of the Pismo syncline has
ceased, and that block uplift is the dominant style of
deformation.'' The Pismo syncline forms the core of the San Luis-
Pismo block. Lettis et al., state that elevations of dated marine
terraces show rigid uplift at a near constant rate of 0.1 to 0.2 mm/
yr during the late Quaternary in the northwestern part of
[[Page 29592]]
the block. This geologic evidence of block uplift of the Irish Hills
is not consistent with the petitioners' hypothesis of ongoing low-
angle faulting over the past 5 to 6 million years on an ``inferred''
offshore thrust fault. However, as stated previously, the SSC model
developed by the SSHAC team includes multiple alternative fault
geometries to capture the range of potential mechanisms driving
uplift of the Irish Hills. One of these alternative fault geometry
models (Southwest-Vergent) considers the possibility of thrust
faulting as the primary driving force for the uplift of the Irish
Hills.
---------------------------------------------------------------------------
\12\ Killeen, K. M., ``Timing of folding and uplift of the Pismo
syncline, San Luis Obispo County, California,'' University of
Nevada, Reno, 1989.
\13\ Lettis, W.R., K.L. Hanson, J.R. Unruh, M. McLaren, W.U.
Savage, and M.A.Keller, ``Quaternary tectonic setting of south-
central coastal California,'' US Geological Survey, 2004.
---------------------------------------------------------------------------
To further support the slip rate estimate for the ``inferred''
offshore thrust faulting beneath Diablo Canyon, the petitioners
propose the use of the Airy isostatic gravity model in which the
Earth's crust floats on the denser mantle with variations in crustal
thickness compensating for surface topography. Under this model,
mountains have thicker crustal roots extending deeper into the
mantle to balance the mass of the elevated terrain. This balancing
mechanism is called isostasy, with a buoyant iceberg floating in
water used as an analogy for the Earth's crust floating on the
denser mantle below. Based on a negative isostatic gravity anomaly
across the Irish Hills, the petitioners assert that ``the topography
of the Irish Hills is not just isostatically compensated, it is
over-compensated by crustal thickening.'' The petitioners then use
an ``Airy ratio of 6:1'' to calculate a slip rate of 2.8 mm/yr for
the ``inferred'' thrust fault under the Irish Hills. An Airy ratio
of 6:1 implies that for every 1 meter of vertical uplift of the
Irish Hills, the crustal root beneath grows downward by 6 meters.
The gravity profile that the petitioners use to support their claim
of an extensive crustal root beneath the Irish Hills is from an
American Geophysical Union meeting abstract \14\ that shows a
gravity low over the Irish Hills along coastal California near
Diablo Canyon. In a peer-reviewed paper published in the journal
Lithosphere, Langenheim et al.\15\ provides their interpretation for
the gravity low previously shown in the gravity map at the American
Geophysical Union (AGU) meeting. Langenheim et al. combines gravity
data together with aeromagnetic data to conclude that the gravity
low across the Irish Hills originates from rock density contrasts
within the upper 10 to 15 km of the crust rather than a deep
extensive crustal root extending into the mantle. Specifically, the
authors conclude that the gravity low across the Irish Hills is due
to the density contrast between the low density (D=2270 kilograms
per cubic meter (kg/m\3\)) younger sedimentary rock that overlies
the higher density (D=2710 kg/m\3\) older basement rock. This
conclusion is also supported by the aeromagnetic data gathered over
the Irish Hills that shows ``fairly'' magnetic rocks underlie the
upper younger sedimentary rocks.\16\ Figure 9, from Langenheim et
al.,\17\ shows the gravity and magnetic models across the Irish
Hills along with a geologic cross-section that provides the density
and magnetic susceptibility values for each of the rock units. The
low likelihood of a massive crustal root beneath the Irish Hills is
further supported by the conclusions of Lowry and P[eacute]rez-
Gussiny[eacute],\18\ which use a coherence analysis of gravity and
topography to estimate an effective elastic thickness of 10 to 15 km
along central coastal California. Under the simple Airy isostatic
model, the crust has no flexural rigidity, and its effective elastic
thickness is assumed to be zero.
---------------------------------------------------------------------------
\14\ Langenheim, V.E., R.C. Jachens, R.W. Graymer, and C.M
Wentworth, ``Implications for Fault and Basin Geometry in the
Central California Coast Ranges from Preliminary Gravity and
Magnetic Data,'' In AGU Fall Meeting Abstracts (Vol. 2008, pp.
GP43B-0811), 2008.
\15\ Langenheim, V.E., R.C. Jachens, R.W. Graymer, J.P. Colgan,
C.M. Wentworth, and R.G. Stanley, ``Fault geometry and cumulative
offsets in the central Coast Ranges, California: Evidence for
northward increasing slip along the San Gregorio-San Simeon-Hosgri
fault,'' Lithosphere, 5(1), 29-48, 2013.
\16\ Id.
\17\ Id.
\18\ Lowry, A. R., and M. P[eacute]rez-Gussiny[eacute], ``The
role of crustal quartz in controlling Cordilleran deformation,''
Nature, 471(7338), 353-357, 2011.
---------------------------------------------------------------------------
Finally, the petitioners use modeling of GPS data in the region
to develop a third independent estimate for the total thrust fault
slip rate beneath the Irish Hills. This estimate is not based on
actual GPS measurements near Diablo Canyon as only the direction of
shortening or compression (N15[deg]E) is known in the region near
the site. Despite this limitation, the petitioners used deformation
modeling to determine a shortening rate of 2.0 mm/yr across the
Irish Hills. In their presentation to the NRC staff, the petitioners
acknowledged that the deformation models rely on ``low resolution''
finite element grids in the Irish Hills region (ML24198A105).
Despite the low resolution of the model grid, the petitioners
allocate all the 2.0 mm/yr of shortening across the Irish Hills to
the ``inferred'' offshore thrust fault to determine a total thrust
fault slip rate of 2.2 mm/yr beneath the Irish Hills. The amount of
shortening (2.0 mm/yr), as well as the allocation of all the
shortening across the Irish Hills to a single ``inferred'' thrust
fault, is questionable as there are other known active faults in the
region that could accommodate the shortening.
The NRC staff's review of the PG&E 2015 SSC Report documented
PG&E's consideration of GPS geodetic velocities useful for site-
specific hazard estimation (ML16341C057). PG&E considered geodetic
data and associated analyses to inform patterns and rates of
deformation. Geodetic data and models were presented in 2012 at
SSHAC Workshop 2 and in 2014 at SSHAC Workshop 3. The NRC staff's
review concluded that the SSHAC team used available geodetic data to
provide regional constraints on the slip budget available for the
study region (ML16341C057). However, the SSHAC team did not use
geodetic data and numerical deformation models to directly assign
slip rates to specific faults or rupture sources. Instead, the SSHAC
team used the numerous geologic field studies and data gathered from
geophysical surveys to estimate fault slip rates. The decision to
rely primarily on geologic and geophysical data for seismic source
characterization and to use GPS data as a secondary source of
information to constrain the slip budget in the study region is
justified because of the limited number of onshore GPS stations and
the lack of offshore GPS stations in the region surrounding Diablo
Canyon. Johnson et al.,\19\ provides an overview of the deformation
modeling approaches and concludes that deformation models have not
reached sufficient maturity and require further research to identify
uncertainties associated with these models.
---------------------------------------------------------------------------
\19\ Johnson, K.M., W.C. Hammond, and R.J. Weldon, ``Review of
geodetic and geologic deformation models for 2023 US National
Seismic Hazard Model,'' Bulletin of the Seismological Society of
America, 114(3), pp.1407-1436, 2024.
---------------------------------------------------------------------------
In summary, the PRB concludes that the stratigraphic profile,
gravity anomalies, and GPS modeling used by the petitioners do not
provide adequate evidence to support the existence of a major
``inferred'' offshore thrust fault that extends beneath the Irish
Hills with a fault length of 70 to 100 km and a slip rate between
2.0 to 2.8 mm/yr.
Concern 4: Seismic core damage frequency, estimated by PG&E in
2018 to be 3x10-5, should be 1.4 x 10-3 per
year (about once every 715 years) based on this higher recurrence
rate for thrust earthquakes.
Based on its assessment of the petitioners' concerns, described
above in Concerns 1 to 3, the PRB concludes (1) that the seismic
models developed by PG&E do not neglect the potential for thrust or
reverse faulting beneath Diablo Canyon, (2) the tectonic setting
along central coastal California differs substantially from that for
the Noto Peninsula, and (3) the existence of a 70- to 100-km long
``inferred'' offshore thrust fault adjacent to Diablo Canyon with a
slip rate between 2.0 to 2.8 mm/yr is highly unlikely. Therefore,
the PRB concludes that a recurrence interval of 715 years for large
ground motions from a Noto Peninsula type earthquake beneath Diablo
Canyon and subsequent seismic core damage frequency (SCDF) of 1.4 x
10-3 per year are not credible values. The bases for the
PRB conclusion are provided below.
Using an average slip of 2 m from the 2024 Mw 7.5 Noto Peninsula
earthquake and slip rates ranging from 2.0 to 2.8 mm/yr for the
``inferred'' offshore thrust fault, the petitioners estimate a
recurrence interval of between 715 to 1000 years for an analogous
event beneath Diablo Canyon. Based on the assumption that peak
ground accelerations would be extremely large from this earthquake
at Diablo Canyon, the petitioners assume that seismic core damage
would occur and, therefore, the SCDF should be 1.4 x 10-3
per year (1/715 year). This SCDF value is about 47 times higher than
the SCDF value (3 x 10-5 per year) determined by PG&E
(ML18120A201) from its seismic probabilistic risk assessment (SPRA),
performed in response to the NRC's 50.54(f) request. The SPRA
performed by PG&E used the hazard curves from its implementation of
the SSHAC Level 3 SSC and GMC models in a probabilistic seismic
hazard analysis (PSHA) to assess the frequency of seismic core
damage at Diablo Canyon. The NRC staff reviewed the SPRA performed
by PG&E and concluded that it adequately characterized
[[Page 29593]]
the risk of seismic damage for Diablo Canyon (ML18254A040). As
previously stated in the NRC staff's response to Concern 1, based on
its review of the SSC and GMC models, the NRC staff concluded that
PG&E adequately captured the uncertainty in the data, models, and
methods through use of the structured SSHAC approach (ML16341C057).
Based on its assessment of the petitioners' concerns, described
above in Concerns 1 to 3, the NRC staff concludes that (1) the
seismic models developed by PG&E do not neglect the potential for
thrust or reverse faulting beneath Diablo Canyon, (2) the tectonic
setting along central coastal California differs substantially from
that for the Noto Peninsula, and (3) the existence of a 70- to 100-
km long ``inferred'' offshore thrust fault with a slip rate greater
than 2 mm/yr is highly unlikely.
In summary, the PRB concludes that a recurrence interval of 715
years for a Noto Peninsula type earthquake beneath Diablo Canyon and
subsequent SCDF of 1.4x10\-3\ per year are not credible values.
III. Conclusion
As a result of the PRB review of the petitioners' concerns, the
NRC has denied the petitioners' request. The request to shut down
Diablo Canyon is denied because the PRB concludes that (1) the
seismic models developed by PG&E do not neglect the potential for
thrust or reverse faulting beneath Diablo Canyon, (2) the tectonic
setting along central coastal California differs substantially from
that for the Noto Peninsula, (3) the existence of a 70- to 100 km
long ``inferred'' offshore thrust fault adjacent to Diablo Canyon
with a slip rate greater than 2 mm/yr is highly unlikely, and (4)
the return period of 715 years for seismic core damage is not
justified. Therefore, there is an insufficient basis on which to
take enforcement action against PG&E, and the petitioners' request
is denied.
In accordance with 10 CFR 2.206(c), a copy of this director's
decision will be filed with the Secretary of the Commission for
Commission review. As provided for by this regulation, the decision
will constitute the final action of the Commission 25 days after the
date of the decision unless the Commission, on its own motion,
institutes a review of the decision within that time.
Dated at Rockville, Maryland, this 26th day of June 2025.
For the Nuclear Regulatory Commission.
/RA/
Michael X. Franovich, Deputy Director, Office of Nuclear Reactor
Regulation.
Attachment
Petitioners' Comments on Proposed Director's Decision and NRC Response
Attachment
Petitioners' Comments on Proposed Director's Decision and Nuclear
Regulatory Commission Response
By letter dated May 15, 2025 (Agencywide Documents Access and
Management System Accession No. ML25136A355), the petitioners
provided comments to the U.S. Nuclear Regulatory Commission (NRC) on
the proposed director's decision regarding seismic core damage
frequency for Diablo Canyon Nuclear Power Plant, Units 1 and 2
(Diablo Canyon), dated April 10, 2025 (ML24302A153).
The petitioners' comments do not alter the staff's conclusions
in the proposed director's decision and therefore do not require
modification of the final director's decision. This attachment
provides the petitioners' comments on the proposed director's
decision and the NRC's responses to the comments.
The NRC staff determined that most of the comments involve
restatements of the petitioners' concerns or otherwise involved
information considered in the development of the proposed director's
decision. However, the NRC staff identified five of the petitioners'
assertions associated with Concerns 1 and 3 in the proposed
director's decision that are new or clarify the previous concerns.
The NRC staff considered and addressed these assertions, as
described below. Note that the discussion of Concern 1 in the
director's decision briefly covers the regional tectonic setting
encompassing Diablo Canyon, which provides useful context for the
following responses.
Assertion 1 on Proposed Director's Decision Concern 3
``My model of the total thrust seismicity of the Irish Hills
does not depend on the existence of the Inferred Coastline thrust;
the same total could be met by combined activity on the Los Osos
thrust, the San Luis Bay thrust, and other unmapped and unmodelled
thrust faults in the basement Franciscan Complex,'' and ``I have
proposed that the San Luis Bay thrust fault continues northwest to
connect to the Hosgri fault as a blind thrust fault (`Inferred
Coastline thrust'), and that these two segments would naturally
rupture together in a large event.''
Response to Assertion 1
The Senior Seismic Hazard Analysis Committee (SSHAC) Seismic
Source Characterization (SSC) model characterizes (section 7.0 of
Pacific Gas & Electric Company's (PG&E) 2015 SSC Report) \20\ a
number of different complex rupture events under the three fault
geometry models (FGMs). These complex rupture events capture the
potential for larger magnitude and longer ruptures that have
historical analogs in actual earthquakes. Under the Southwest-
Vergent (SW) FGM, rupture source SW-04 models an earthquake that
involves both the San Luis Bay (SLB) fault and the Hosgri fault
(table 9-5 of the PG&E 2015 SSC Report). Rupture source SW-04 is
assigned a combined set of magnitudes (table 10-11 of the PG&E 2015
SSC Report) for a primary rupture on the Hosgri fault (M6.9, M7.5,
M8.0) and a secondary rupture on the SLB fault (M6.4); the rupture
source is also allocated a portion of the total slip rate (section
9.3 of the PG&E 2015 SSC Report). Most of this complex rupture
source entails a near-vertical strike-slip rupture on the Hosgri
fault with subsequent reverse faulting on the much shorter SLB
fault. In summary, the FGMs developed as part of the SSC model
include a rupture event that includes both the Hosgri and SLB
faults, which the petitioners assert is the ``Inferred Coastline
thrust'' even though the SSC model captures the majority of this
rupture event as a strike-slip earthquake on a near-vertical fault.
---------------------------------------------------------------------------
\20\ In 2015, PG&E developed the Diablo Canyon seismic source
characterization (SSC) model and documented it in a report, which is
referred to herein as the PG&E 2015 SSC Report. This report is
available on the PG&E website https://www.pge.com/en/about/pge-systems/nuclear-power/seismic-safety-at-diablo-canyon.html#tabs-2967acbbcb-item-1b0b13e766-tab.
---------------------------------------------------------------------------
Assertion 2 (on Proposed Director's Decision Concern 3.)
Neither the PG&E's 2015 Level 3 SSHAC SSC Report nor the 2024
Updated Level 1 SSHAC SSC Report mentions the rate of crustal
shortening (compression); the reports only give the direction, and
PG&E is attempting to ``divert attention'' from this ``damning
evidence.''
Response to Assertion 2
Assertion (2) claims that PG&E has attempted to obfuscate the
rate of shortening across the Irish Hills. However, the SW FGM
captures the potential for crustal shortening across the Irish Hills
in which uplift is accommodated by more gently dipping reverse (or
thrust) faults. In addition, Page ES-3 within the Executive Summary
of the PG&E 2015 SSC Report states the following:
Geodetic data and inversions of earthquake focal mechanisms show
that the contemporary tectonic setting of the San Luis Range and
surroundings is one of transpressional dextral shear with localized
areas of crustal shortening and thickening such as within the Irish
Hills near Diablo Canyon. Geodetic data constrain regional crustal
velocities in the vicinity of Diablo Canyon to 1-3 mm/yr
[millimeters per year] of dextral shear subparallel to the San
Andreas fault zone, with comparable rates of crustal shortening
permissible orthogonal to the plate boundary west of the San Andreas
fault zone [emphasis added].
Section 5.2 of the PG&E 2024 Updated SSC Report (ML24298A234)
describes the literature review performed by the SSHAC team to
evaluate recent data and models not included in the 2015 PG&E SSC
model. Specifically, the SSHAC team reviewed the five Western United
States Earthquake Rupture Forecast 2023 deformation models, which
developed fault slip rates based, in part, on global positioning
system (GPS) geodetic data. Table 5-11 in the updated PG&E 2024
Updated SSC Report compares the fault slip rates developed by the
2015 PG&E SSC model with the 2023 GPS based fault slip rates from
the deformation models (including the Shen-Bird model) showing
general agreement between the older and newer estimates for the
fault slip rates for the primary faults near Diablo Canyon. In
summary, PG&E used GPS data to inform the development of the 2015
PG&E SSC model and to assess the continued viability of the PG&E
2024 Updated SSC model, and the analysis considered the potential
for crustal shortening.
[[Page 29594]]
Assertion 3 (on Proposed Director's Decision Concern 1)
``In fact, there is no geologic or geophysical evidence for
strike-slip tectonics within the Irish Hills,'' and ``The SSW-NNE
direction of compressive stress (shown by the World Stress Map
dataset) and of compressive strain-rate (shown by relative GPS
velocities) is incompatible with strike-slip and indicates pure
thrusting.''
Response to Assertion 3
Assertion (3) states that there is no geological or geophysical
evidence for strike-slip faulting beneath the Irish Hills and that
only thrust faulting is possible. The petitioners use this assertion
to support the use of the Noto Peninsula as an analog to the local
tectonic setting for Diablo Canyon. However, this assertion is not
supported by the earthquake focal mechanisms shown in Figures 5-24
and 13-12 of the PG&E 2015 SSC Report, which clearly show a mix of
strike-slip and thrust faulting events. In summary, there are
numerous strike-slip mechanisms onshore within the Irish Hills
consistent with the general result that focal mechanisms in south-
central coastal California are a mix of reverse and strike-slip
consistent with dextral transpressional deformation and that the
tectonic setting for the Noto Peninsula differs from that of coastal
California near Diablo Canyon.
Assertion 4 (on Proposed Director's Decision Concern 3)
The Local Area Source (LAS) zone modeled by PG&E to capture the
potential for earthquakes occurring on faults that were not already
characterized in its Seismic Source Characterization (SSC) model has
three major flaws:
(a) the recurrence rate for earthquakes within LAS zone is based
on seismicity catalog rather than moment rates from tectonic
deformation models,
(b) faulting in the LAS zone should not include the possibility
for strike-slip events, and
(c) the maximum magnitude of the LAS zone should range from M
[Magnitude] 8.07 to M8.67.
Response to Assertion 4
Assertion (4a) states that the recurrence rates for the LAS zone
should be based on tectonic deformation models rather than
seismicity. The use of local and regional seismicity catalogs to
determine earthquake recurrence rates for seismic areal source zones
is consistent with the guidance in American Nuclear Society (ANS)/
American National Standards Institute 2.29, ``Probabilistic Seismic
Hazard Analysis,'' and is followed for characterizing the seismic
hazards for both critical facilities and commercial facilities.
Assertion (4b) states that strike-slip faulting is not possible
within the Irish Hills, but this assertion is contradicted by
earthquake focal mechanisms showing a mix of strike-slip and reverse
faulting onshore within the Irish Hills (see the response to
Assertion 3). Assertion (4c) states that the maximum magnitude for
the LAS zone should be greater than M8. As described in the response
to Assertion (1), the FGMs that constitute PG&E's SSC model include
multiple large complex ruptures on adjoining faults that capture the
potential for M8 plus events. Rather than duplicating this set of
complex large-magnitude ruptures, the purpose of the LAS zone is to
characterize the potential for moderate to large (M6.6 to M7.1)
earthquakes on previously unmapped faults. Similarly, the maximum
magnitudes for the background zone developed by the U.S. Geological
Survey (USGS) National Seismic Hazard Model, which encompasses
Diablo Canyon range from M6.5 to M7.0. In addition, the use of
regional seismicity catalogs to develop the recurrence rate for
earthquakes within areal source zones is consistent with the
standard stationarity assumption made for the probabilistic seismic
hazard analysis that seismic hazard at a location remains constant
over relatively shorter time periods (e.g., 50 years).
Assertion 5 (on Proposed Director's Decision Concern 3)
``Given that Langenheim was also first-author on the high-
resolution isostatic gravity anomaly map that we now rely on it is
reasonable to assume that he [sic] incorporated his [sic] knowledge
of low surface rock densities into that isostatic correction.''
Specifically, this assertion states that the gravity modeling by
Langenheim et al.\21\ incorporates the low surface rock densities
into the isostatic correction to develop the isostatic residual
gravity map; the petitioner asserts that therefore, the gravity low
across the Pismo syncline, which underlies the Irish Hills, is due
to crustal thickening with an Airy ratio of 6:1.
---------------------------------------------------------------------------
\21\ Langenheim, V.E., R.C. Jachens, R.W. Graymer, J.P. Colgan,
C.M. Wentworth, and R.G. Stanley, ``Fault geometry and cumulative
offsets in the central Coast Ranges, California: Evidence for
northward increasing slip along the San Gregorio-San Simeon-Hosgri
fault,'' Lithosphere, 5(1), 29-48, 2013.
---------------------------------------------------------------------------
Response to Assertion 5
Assertion (5) postulates that the density contrast between the
lower density (D) (D = 2,270 kilograms per cubic meter (kg/m\3\)
younger sedimentary rock that overlies the higher density (D = 2,710
kg/m\3\) older basement rock has been accounted for in the isostatic
correction performed by Langenheim et al. (2013). The petitioner
therefore attributes the gravity low across the Pismo syncline to
crustal thickening with an Airy ratio of 6:1. However, the
petitioners' conclusion is not supported by the analysis in the
referenced paper. Langenheim et al. (2013) state that the gravity
measurements used to create an isostatic residual gravity map were
based on an isostatic correction with a sea-level crustal thickness
of 25 kilometers (km), a crustal density of 2670 kg/m\3\, and a
mantle-crust density contrast of 400 kg/m\3\ to remove the long-
wavelength effect of deep crustal and/or upper mantle masses that
isostatically support regional topography. Regarding the gravity low
across the Pismo syncline along profile F-F', Langenheim et al.
(2013) state that ``Lastly, model F-F' highlights structures that
bound the Irish Hills. The model crosses the Pismo syncline,
characterized by a gravity low originating in Miocene and younger
sedimentary rocks [emphasis added].'' This statement by Langenheim
et al. (2013) clarifies that shallow, low density material is the
cause of the gravity low across the Irish Hills, and that the
gravity low is not the result of a crustal root. In addition, the
rapid change in gravity anomaly over a relatively short distance (10
to 15 km) from negative to positive values supports this
interpretation. In addition, the SSC model developed by PG&E was
informed by both gravity and magnetic field data to model the
crustal structure in the Diablo Canyon area (see section 2.3.2 of
the PG&E 2015 SSC Report).
Conclusion
In conclusion, based on the PRB review of the petitioners'
comments on the proposed director's decision, the PRB determined
that no further actions were needed, and the NRC made no changes to
the final director's decision as a result of the petitioners'
assertions. The SSC model developed by PG&E in response to the NRC's
request under Title 10 of the Code of Federal Regulations (10 CFR)
50.54(f) captures multiple interpretations of the local geologic
setting through the use of alternative FGMs. The SSC model is based
on multiple geological and geophysical field measurements and was
developed, as specified in the 10 CFR 50.54(f) request, using the
SSHAC approach, as described in NUREG-2213, ``Updated Implementation
Guidelines for SSHAC Hazard Studies,'' issued October 2018
(ML18282A082). The SSHAC approach focuses on two critical
activities: evaluation and integration. The evaluation activity is
defined as an assessment of the complete set of data, models, and
methods that are relevant to the hazard analysis, and the
integration activity is the development of an SSC model that
captures all technically defensible interpretations, as informed by
the evaluation activity. A key element of the SSHAC approach is
participatory peer review from an outside panel to ensure that the
full range of data, models, and methods is considered in the
evaluation phase and that all technically defensible interpretations
are integrated into the SSC model to capture the uncertainty in
seismic hazard as required by 10 CFR 100.23, ``Geologic and seismic
siting criteria.'' Independent panels of experts in coastal
California geology and seismology reviewed and approved both PG&E's
2015 SSC model and its 2024 update (ML24298A234).
In summary, the PRB concludes that the petitioners' concerns,
including the assertions in its comments on the proposed director's
decision, are either (1) already incorporated as one of the
alternative scenarios in the SSC model, (2) technically inconsistent
with available information, or (3) inconsistent with standard
approaches for the seismic hazard characterization for a nuclear
power plant. Therefore, the PRB determined that no further actions
were needed, and the NRC made no changes to the final director's
decision as a result of the petitioners' comments.
[FR Doc. 2025-12432 Filed 7-2-25; 8:45 am]
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