ML19294C940
| ML19294C940 | |
| Person / Time | |
|---|---|
| Site: | Humboldt Bay |
| Issue date: | 09/10/1980 |
| From: | Alterman I Office of Nuclear Reactor Regulation |
| To: | Rolonda Jackson Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19294C939 | List: |
| References | |
| NUDOCS 8009190639 | |
| Download: ML19294C940 (20) | |
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UNITED STATES i
NUCLEAR REGULATORY COMMISSION
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WASHINGTON. 3.0.20555
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SEp 1 0 ISSO MEN RANDLM FOR:
Rcbert E. Jackscn, Chief Geosciences Branch, CE car THRU:
Richard 3. McMullen, Acting Leader Geology Sectien, 353, CE FRCM:
Ina 3. Alterman, Geologist Geology Section, GSB, CE
SUBJECT:
FIELD AND TRENCH CSSERVATICNS IN THE VICIE TY CF HUMBCL37 BAY NUCLEAR PCWER :LANT f3 (JULY I4-17,1930)
PLANT LCCATICN: Eureka, CA CCCXET NO..
50-133 RESFCNSISLE 3 RANCH: Caerating React:rs Brancn f2 (CL)
CPERAT!NG CONCIT!CN: Shu Ocwn LICENSING ACTICN: PG&E respense :: ASL3 Creer of License Mcdification AttachedisareportofmyYisittotneHumecidtSayArea. It describes the field trip with :G&E and their geclogical censultants, Wcccward/Clyde, during which ae examinec the regicnal, lccal and site geclegy of the Humboldt Say Nuclear ?cwer ?lant 13, including three trenenes en the plant site. The trip teck place fecm July 14-I7,1980.
I::cluced in the report is new infcrmation en the age of the deposits, evidence that :cints to the capability of most of the nearby faults including the Little Salmen Fault, and a description of a newly-discovered, aroba ly capaole, f ault by Wecdward-Clyde ;eclagis*.s that is 7CO ft. frem the plant.
It shculd ce pointed cut that nc written reacrt has yet been submitted dccumenting their findings and interpretations. Until a far nal review of sucn a rescrt is undertaken, no change in the NRC Geosciences 00sitien regarding the License Mcdification and plant shutdcwn is c:ntemplated.
U.~:
Ina 3. A1:erman, 3eclegist Geology Cecticn Gecsciences Branch, CE
Attachment:
As stated cc: w/attacnment S. Goldberg J. Cavis, CCMG
?. Ami-ota, CCMG 8009190l/57
Field and Trench Observations in the '/icinity of Humboldt Say Nuclear Power Plant 13 Backorcund: Secause of unresolved geologic and seismic issues related to the tectonic instability of the area in wnich the Humboldt Say Nuclear Power Plant is located,an Crder for Modificaticn w
_icense was issued in May 1976, and the clant shut down until satisf actory ccmpliance with the Order has been determined.
In March 1977 PG&E submitted an updated geologic report by their censultants, Earth Science Asscciates (ESA),
in partial fulfillment of the Order requirements, followed by a procosal for a license stendment that would allow restart of the clant. The review of the report by the NRC Geosciences Staff (August 10,1977) concluded that the evidence provided was insufficient to prove that either of the two f aults under consideration were non-cacaole and reccamended that the plant remain in shutdcwn ccnditicn until more ccnclusive information is provided.
In September 1978, PG&E engaged Wocdward/Clyde Censultants (W/C) to undertake a 2-year program of geologic and seismic in' estigations to resolve the issues. Preliminary results of their study were presented to the NRC staff at a meeting in Bethesda on May 7, 1980, which was attended by representatives of NRC, PG&E and Woodward /Clyde. While they presented no written material, maps, or trench logs, there was general agreement that a geologist frcm NRC would have to examine the trenches on the plant site before a formal review of their petition to restart is undertaken. Sucn first-hand informatien is necessary in assessing the safety of the plant site.
. Issues: The geologic and seismic issues in the Order of Modification License (paragraph E.2) that had to be addressed included:
a.
The Bay Entrance Fault: the location, age, attitude, amount of displacement and possibility of surf ace f aulting.
b.
Little Salmen Fault: same informaticn as above.
Origin e' displacements in the ravine at Humboldt Hill and the c.
quarry at Fields Landing.
d.
Seismic potential of the Freshwater and Table Bluff f aults und their relation to the Little Salmen Fault.
e.
Seismic monitoring network in the vicinity of the plant, f.
Past and future earthquake monitoring to determine if any local faults are sources of earthquakes around the plant.
The Bay Entrance and Little Salmen faults were the primary focus of the investigation. The concerns relating to these two f aults arose from the fact that both appeared to ccme within one mile of the plant and their capability was not determined. The first reports fecm ESA concluded that the Little Salmon fault was truncated by an overlying undisturbed sedimentary unit of Plio-Pleistocene age (acproximately 1-2 million years) and therefore not capable. The NRC staff, however, found that there was nc evidence to prove the age of the caoping sedimen.s, ner tnat the fault was indeed capped.
Also, the location and orientation of the f ault were not proven to :he Staff's satisfaction.
. The same reports were not able to show conclusively that the Bay Entrance Fault was not capable, nor were they able to show convincingly the location of the trace with respect to the plant.
One of the major tasks therefore in resolving the issues is dating the sedimentary strata in the area to determine when the fault movements occurred and the age Jf the oldest undisturbed units.
A second major task is determining the likelihood of a new f ault occurring under the plant in the event of an earthquake focused on an active fault in the area of the plant site, should any of the local f aults mentioned in :na Order prove to be sources of seismic activity.
Particicants: The site visit arranged with PG&E also included a fielc trip to see the regicnal geology. W/C geologists thought this important so that the NRC geologists could see the site geology of the trench in the context of the regional and local geologic situation.
Frank 3rady, Humcoldt 3ay Nuclear Power Plant Project Manager, represented PG&E, Lloyd Cluff of W/C, led a team of field geologists on this project, Tem Stevens, Kevin Coppersmith and Burt Swan.
I was the only representative from NRC. A geologist fecm the U.S.G.S., NRC consultant on Humboldt Say, did not appear as originally anticipated, but visited the site three weeks later.
Itinerary: In keeping with their view that a regional perspective was essential a an appreciation of the significance of the site geology, the first day and part of the sec:nd day were spent examining geologic features up to 20 miles frcm the sita.
-4 On Monday, July 14, 1980 I met with F. Brady, L. Cluff and the three field geologists at the Eureka Airport in McKinleyville. We spent that af ternoon and evening examining structures and surf ace characteristics in Area 1 of the attached map (Attachment 1). This area is about 20 miles north of the clant site. Structures and associated features seen here include the McKinleyville Fault, tne Trinidad Fault, the Big Lagcon Fault (from a distance) and several terrace levels.
The morning of Tuesday, July 15, 1980, was s:ent in Area 2A, acout 15 miles southeast of the plant, examining a former trencn site (now backfilled) in the Gcose Lake Fault Icne. This was followed by a visit to the still-open " Brazil" trench on Humcoldt Hill in Area 25, about 2 miles south of the plant. We examined structures associated with the Little Salmon Fault. The rest of the afternocn was spent in the three trenches at the plant site.
The last field visit was the morning of Wednesday, July 16, when we examined the quarry in Humboldt Hill in Area 23, about one mile south of the plant site.
CBSET/ATIONS It should be noted that, because of time limitations and complicaticns resulting frca terraces of differing ages and ::cograchic levels, a systematic field review of the stratigraphy was not possible on this brief field trip. Therefore, all strata ages and identifications mentioned in this recort were these recorted by W/C and have not yet been cocumentad.
. Faults:
McKinleyville:
This fault, about 15 miles north of the plant site, and approximately paralleling the Little Salmon Fault at NaCW, is believed to be the :cne of the 1954 Eureka earthquake. The cnly identifiable surf ace manifestaticn of this f ault was a '1W-trending, south-f acing escarpment several feet high, close to the airport.
A trench that originally crossed the escar; ment at right angles is ncw backfilled.
Accceding to the trench logs and discussicns at this locale, the trench shcws a thrust f ault shear zcne with at least two sets of events. The lcwer unit exposed in the trench, the Crannell Sands, thought to be 300- to 600,000 yrs, has shear fractures both parallel with and conjugate to the fault. Amount of offset on these is not known because of lack of marker becs in this sand unit. These fractures are truncated by an overlying gravel unit interpreted to be about 82,C00 yrs.
Super-imposed on this earlier set of fractures is a thrust fault bringing the old.r unit over the younger grahels with a vertical disolacement of 10 meters.
Boreholedata,accordingtotheW/Cgeologists,showthathertical disclacement is cumulative, increasing with death to at least 25 meters of offset.
According to their drilling informatien, the shear :cne within whicn the movement occurred is 10 meters aide. They claim it is a narrow zone, hignly deformed on the upper plate at the major shear, with fracturing and otner manifestations of defonation dying out away from it :2 the upper thrust plate.
. Trinidad Fault: At the north end of the en-land segment of the Trinidad Fault we observed snear fractures on a sea-cliff face. The surface manifestation of this fault, on the Anderson Ranch, was a prcminent, straight-line, soutn-f acing escar; ment seheral feet high. Cnly the afore-menticned gravel unit that cherlies the Crannell sands is exposed.
It is sandy with gravel beds that are offset about i 1/2 meters of apparent net dip slio. Basement Franciscan Formation rocks are exacsec en the upper plate here,theOnlyplacethiswasObserhedCnthistrip. Theherticaldisplacement here therefore is interpreted to be 20 m.
An older set of nearly tertical, filled fractures were Offset a few cm by younger conjugate shears dipping parallei with and at a high angle to the fault surface. Here cver a distance of a!:out 30 feet the parallel shear fractures died out and disaopeared more racidly than the conjugate set.
At the furthest exposure frca the projected fault, Only fractures that were conjugate to the main fault orientation aere visible.
31a Lagcon F3 ult:
At Patrick's Point State Park abcut 35 mi N of the plant site we observed the coastal features fr:m a vantage point. Big Lagcen at the far distance separates a rocky unterraced coast of Franciscan rocks en the north side of the lagoon fr m a sicping terrace :n the south side.
The terrace is the youngest in the area, uith hori: ental terrace descsits 32,C00 years old unccnformably overlying 7C0,000 year-cid strata di::cing north-westward. The straight line of the 3ig Lagoon and the juxtacosition of the Mesozoic Franciscan Rocks and 31eistocene terrace decasits suggests a W-trending thrust fault here, essentially parallel with those seen further south, that post-dates the terrace depcsits.
y
, Goose Lake Fault Zone:
About 15 miles southeast of the plant site near the town of Hydesville (in Area 2A of the enclosed map) a north-facing escarpment and associated linears parallel the Little Salmon Fault.
A trench through the escarpment, new backfilled, showed that, despite the suggestien of uplift on the south side, the f aults and offsets seen in the trench showed uplift north over scuth. They did not have an explanation f0r this contradiction but suggested scme tentative geomorphic and mechanical possibilities.
The trench logs again indicate multiple faulting episodes. Here a late Pleistocene 111uhium,theYagerCreek,liesunconformablyona5CO-to700,0C0 year-old unit, the Carlotta, and truncates a variety of deformational structures in two major z nes of faulting about 10 m apart. Alcng the major thrusts seen in the trench, the Carlotta has been thrust over the Yager Creek alluvium. These in turn have been thrust over pond or lake deposits with C
dates of S-9,CCO years.
p Here again the geologists emphasized the multiple fault movements occurring repeatedly in the same narrow shear zone.
Trench 'lisits:
3razil Trench: The first trench seen on this trip was the Brazil" trench,
located about 2 miles south of tne plant on Humboldt Hill, in Area 23 of tne attached map. This locality is significant because a quarry and ravine at the north end of this hill, which is just about one mile south of tne Humboldt Say Nuclear Plant, figured significantly into the uncertainties that the NRC Geosciences staff considered in their position of August 1977 not to reccamend reopening. Severe deformation of what appeared to be very young material in these two places in Humcoldt Hill suggested recent f aulting, possibly associated with the Bay ntrance and/or Little Salmon Faults,
,,i.
3 both of which were said to be non-capable by PG&E censultants, ESA, in an earlier report.
The wesi;-f acing slope of this hill parallels the Little Salmen Fault, the trace of which had been oreviously orojected by W/C to emerge in the bay just SW of the hill.
The trench, the smaller of two parallel trenches, and the only cne still open, is cut perpendicular to tne hill slope en a broad bench abcut 50 feet above the Redwood Highway. At the trench bott0m, the oldest material is a rather uniform, massive gray clay withcut noticeable markers beds, 7,tified as the Rio Cell Formation. Overlying the clay on an irregular erosion surface is a gravel unit interpreted by W/C as abrasion platforn deposits representing the base of a marine terrace.
The age of these younger terrace deposits is unknown. As the Rio Cell is interpreted from magnetic stratigraony to be greater than 700,0C0 years old, this trench does not help to resolve the age of faulting. There is however clear evidence of thrusting of Rio Cell over the terrace deposits.
Abrasion-platform gravel layers have been offset up to 3 m along thrust faults dipping ncetheastward abcut 10 and striking MO-50W, approximately the orientation of the Little Salmon Fault. At least two gravel units have been deformed by drag-folding alcng two of the icnger faults at about the 8 m station of the trench.
The visible displacements do not, hcwever, preclude greater offset at depth, and therefore may be regarded as minimum cffset. As the younger material has not been dated, the age of the f aulting is unknown.
.g.
However, the deformation is regarded as part of the shear zone on the upper plate of the Little Salmon Fault. The f ault surf ace is interpreted as emerging about one-third of the way up the sicpe of Humboldt Hill below the trench, which would be at an elehation of about 30' above sea lehel. This is a change in interpretation, as W/C previously mapped the fault trace in the 3ay close to the shore.
Humboldt Hill Guarry:
The oxidized brcwn sands and intercalated gravel bedsexposedinthisquarryarebeliehedtobeatleast2000ftdownsecticn of an important marker bed,the Unit F clay. Palecmagnetic studies shcw normal polarity which brackets its age between 200,000 yrs (approx, age of Unit F) and 700,000 yrs (Matuyama reversed epoch, the minimum age of the Rio Cell below). A great many offsets and drag of beds brcugnt this quarry to the attention of NRC geologists in their review of the geclogic and seismic issues at the time of the issuance of the license modificaticn.
W/C contends that aerial photography, gecmcrphic study and sterecnet plots of shears all point to this part of the hill as part of a landslice, close to the toe. The geologists pointed out that all stereo plots of measured conjugate shears in fault zones in the regicn shcw two clearly defined concentrations of poles to planes, while a plot of poles to planes in the quarry shows a randem distribution. This nas ccnvinced tnem tnat even if some of the fractures were tectonically deheloped, landsliding has rotated and otherwise recriented them and, due to radial extension, caused displacements which are randcm in orientation.
. The present c ndition of the quarry with its deeply incised Badlands topogra::hy was not conducihe to any shaluation in a brief hisit. Only a few small offsets were obserhed, most showing ner al displacement, a few showing rehersed displacement, while ncne a;;peared to continue through the strata.
In places some fractures offset earlier ones.
Site Trenches: All three site trenches were scill open, although scme collapse had begun at the eastern end of the main trench, and both the main trench (trench 11 on Attachment 2) and the 'ncse" trench (trench 13 of attachment 2) had varying amounts of water en the trench bottom.
The main trench trends slightly north of west from close to the containment building towards the "ncse" of the terrace on which the plant is built. This trend is approximately normal to the Bay Entrance Fault. At 145 m aest of the plant the trenen takes a right angle turn shif ting the west-trending line of the trench abcut 35 meters to the south, then continues aestward for ancther 82 meters.
Trench 12 is a steep-ficored trench which cuts through the stratigraphic column at the edge of the terrace and trends cbliquely tc the Bay Entrance f ault. The ficcr descends about IS m in a horizontal distance of 35 m.
rench f3 is at a " nose" of the terrace and also descends tnrcugh the T
stratigraphic section beginning about one meter below the lowest exposed stratum of the " steep" trench. The lowest level of this trench was inundated by seepage of water so that the disturced strata below water lehel were not cbservable during this site visit.
. Main Trench (#1)
Tne " main" trench had a total E-W 1ength of 22S m (740 ft) beginning 35 m (about 110 ft) west of the containment building.
It was entirely within the uppermost terrace deposit, a thick, apparently uniform, gray clay-silt containing wood fragments of harying sizes, pyritized plant stems, and some incorporated grassy material. This unit, interpreted to be in the "Hcokten" formation, had a tentative age determination based en C dates en the wccd, wn'ch went to the limit of carben dating of abcut 35,C00 yrs. This therefore establishes the minimum age for the clay. Careful obserhation showed the a:parently uniform gray-silt clay to have thin silt layers which were brought cut in places byatechniqueW/Cgeologistsdehelopedofpcuringbucketsofwater carefully as sheets or films dcwn the trench sides. The result was " instant" differential erosion which brought cut the fi ne alternating laminae of this unit. No shears or offsets of any type were evident through most of the trench.
Steeo Trench 42 This trench rehealed the stratigraphy beneath the plant, showing in the 13 m vertical descent about 2CO,CCO yrs of deposition. At least three unconformities were recognized, each overlain oy dcwnward coarseningsequencefromfineclayatthetcptocoarsesandsandgrahels resting on the unconformity.
. For most of the length of the trench there was no discernable deformation of the strata. Exceptions to this were at the very beginning of the trench,atitshighestelehation(designated 32-35mhorizontaldistance en the trench log).
Small ncrmal f aults and graben-like f aults with a few centimeters of offsets did not appear to go anywhere. The extensional nature of these is in contradiction to the interpretation of a ccepressional tectonic setting characterized by thrust faulting. W/C geologists believe these are non-tectonic and simply reflect dcwn-sliding extension on the free-face sicpe of the terrace.
These faults were not seen in any of the units below. Another exception was the set of shear fractures with reverse displacements of 1-2 cm in the silt unit, seen within the last 10 m at the lower end of the trench.
Theunccnformityabovethisshearedunitwascherlainbyundisturbed coarsesandandgrahelstrata. While I thcught I saw faint traces of planes which seemed to continue upward into the coarse and coorly censolidated sand / gravel unit en the unconformity frcm the lower shear fractures, there was no offset af the unconformity anywhere in the trench.
The significance and age of these shears are uncertain.
Significance: While the shears strike essentially parallel with the regional pattern of f aulting, they dip to the SW rather than to the NE and die out toward the plant, according to W/C. They do not apcear to relate to any known faults in the area, although they might be conjugate snears en the Bay Entrace upper plate. W/C geologists reject this interpretatien because the trench location is at least 1000 ft frca the f ault. This, they say, is too f ar away for any deformation en the ucper plate, because in their
. experience with the regional f aulting pattern, deformaticn is confined to narrow shear zones and both primary and conjugate shears related to the faults die out and disappear within a few tens of meters.
Another possibility is that they may be conjugate shears on the icwer plateofafaultnewlydiscoheredbyW/Cgeolcgiststhatwillbecescribed later in this report.
W/C has also censidered the possibility that these fractures relate to the flexing of brittle strata during the dcwnwarp of the broad syncline that characterizes Buhne point structure.
Age:
Tne ages of the strata in which the fractures are found, the unconformity,andtheoverlyinggraYelshaYenotbeencreciselydetermined yet.
The fractured silt is part of a downward coarsening sequence resting on an unconformity directly above the Unit F clay, a distinctiYe unit that is recognized easily as the "ga:mia spike unita of their berehole data studies.
In a road cut one mile south of the plant a similar sequence aboYe the Unit F clay has calcareous fossils which have been dated by amino acid racemizaticn ac 200,0C0 yrs.
W/C geologists are presently attempting to ascertain the correlation of the trench strata with the readcut. IftheydotheywillhaYe a bracketed date between 35,CC0 and 200,000 yrs for the age of these fractures.
.. Nose Trench 43 Stratigraphically, the "ncse" trench begins, at its upper level, about one meter below the lowest stratum of the steep trench but in the same unit.
It descends 12 m through a horizontal distance of 26 m beginning in the fractured unit of the steep trencn and ending in the Unit F clay.
The fractures which are present in the steep trench are not recognized in the same units of the nose trench. Tnis is acparently tecause the nose trench walls are oriented almost parallel with this set of fractures.
The trench contains the Unit F Clay as the icwest exposed unit and its overlying unconformity, both dipping gently to the 5cutheast.
The bottom of the trench at its lowest elevation was filled with a few feet of water, so I did not see the exposure. However, the W/C geologists inforned me, and the trench logs indicate, that the lowest exposed part of the Unit F Clay has three shears with nornal displacements of up to 6 mm.
The dip of the beds in this sheared part of Unit F is steeper than the dips further up the trench where no fractures are discernable.
As the slip is towards the free face of the terrace escarpment and the steecer dips of the lower beds are in the proper sense of rotation, W/C geologists believe these fractures resulted from extensicn associated with slump or sliding of the free-face escarpment.
.. Buhne Point Fault:
Afterthetrenchhisits,W/Cgeologistsinforedmethatthroughsubsurface exploration they have discoYered another fault close to the plant. The search began in response to reehaluation of a data dropout in a seismic reflection profile. Crilling a series of boreholes across the data drepout line and then parallel to it led to the identification of a icw-angle thrust faul; apparently similar in geometry (strike, dip, and direction of displacement) to tne Little Salmon Fault. TheycouldnotgivedefinitiYe data on orientaticn, they said, because they had confirmed the presence of the fault only a few days before my Yisit and had not had time to evaluate the information.
ThedetailspresentedherearetentatiYe,accordingtoW/Cgeologists, and subject to modification with further study. The fault strikes approximately U
N45W, dips 30 N, and the trace passes 7CO ft west of the plant. The f ault plane is about 700 ft belcw the plant and comes to within 500 ft of the plant at its closest approach.
Borehole data indicates that the lowest unit downhole, the top of the Rio Dell (1 million yrs), has vertical displacement of 230 ft., an unconformi ty above the Rio Dell dated at 700,000 yrs shows 160 ft of vertical displacement; a 500,000 yr unit (Unit K) is offset 80 ft; and Unit F (200,000 yrs) shows 20 ft of vertical separaticr..
The increase in vertical displacement with depth indicates continued fault movement for at least million years.
As there has been recurring
. movement within the past 500,000 years, this fault must be considered a capable fault as defined in Appendix A Part 100 to 10 CFR.
Other details reported by W/C are a fault surface with variable dip out averaging about 30*; and a narrow shear zone about 10 ft. wide.
SL'MMARY Regional Geolccy: Regionally, the area is characterized by a series of en echelon NW-striking, NE-dipping thrust faults along the coast of Northern Cali fornia. The faults shcw repeated movements through tine. W/C geologists emphasized that the chief characteristic of these faults is the narrow :one of deformation along which repeated movements appear to be restricted, with deformation confined to the upper plate. These charac-teristics were noted by W/C in the Little Salmon and newly discovered Buhne Point Faults.
Ea ';.;g : At the site the sediments range in age between Site and Local. 3..#y, 35,000 and 200,000 j:
while locally they are up to 1 million yrs. The s
trenches show no major deformation in any units, an'd only minor displacements in sediments 200,000 yrs or older. The displacements do not clearly relate to any of the kncwn faults, and some may be related to slumping on the free faces of the terrace.
The Little Salmon Fault is now recognized by W/C as a capable fault with the fault trace at the lower third of the southwest slope of Humboldt Hill.
They are considering the possibility that the Bay Entrance Fault, tne trace of which lies between the seaward projection of the Little Salmon Fault and the plant, is a splay of the Little Salmon.
-17 The newly mapced Buhne Point Fault, also capable, emerges at the surface 700 ft west of the plant between the plant and the trace of the Bay Entrance Fault.
W/C geologists pointed out that they do not have any evidence for er against a similar f ault plane surfacing directly under the plant, and they could not state with any certainty whether or not such a fault exists.
NRC Gecsciences Branch Position:
It should be emphasized that all geologic interpretations and opinions expressed in this reacrt are those presented by PG&E's consulting geologists to me during the field trip.
I made note of those features I observed directly but made no judgeaents concerning the validity of their interpretations. Until PG&E and their consultants present a documented written report and a fornal ?0 View by the Geosciences Branch of NRC has been undertaken, no change in our position is contemplated nor can any judgement be made concerning their findings.
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