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Edmund G, Brown Jr., STATE OF CALIFORNIA-THE RESOURCES AGENCY

DEPARTMENT OF CONSERVATION

DIVISION OF MINES AND GEOLOGY DIVISION HEADQUARTERS RESOURCES BUILDING, ROOM 1341

1416 NINTH STREET SACRAMENTO, CA 95814

District Offices: LOS ANGELES

Junipero Serra Bldg., Rm. 1065 107 South Brood way 90012

W. Darwin Myers Contra Costa County Planning Department

SACRAMENTO Resources Bldg., Rm. l lS 1416 Ninth: Street 95814

County Administration Building, Horth Wing P.O. Box 951 Martinez, California 9lt553

Dear Mr. Myers:

SAN FRANCISCO

Ferry Building 94111

(lt15) 557-0li13

July 8, 1976

~~' Governor

A?-.242 Qi

We are placing on open file the following report, reviewed and submitted for the City of Concord In compliance with the Alqulst­Prlolo Special Studies Zones Act:

Report to James J, Campbell on geologic and seismic hazard Investigation for Concord self-service station, Concord; by Engeo, lne., June 11t, 1976.

EWH/mkr

cc: Gary Bonfiglio, City of Concord A-P fl le,__--

Sincerely yours,

EARL \I, HART Office of the State Geologist RG lt62 CEG 935

Contra Costa County /IP-24-2

r 'I Planning Commission Members

Andrew H. Young Alamo ~Chairman

William L. Milano Pittsburg - Vice.Chairman Donald E. Anderson Moraga

Anthony A. Dehaesus Director of Planning Albert R. Compaglia Martinez

Phone: 372-2024

July 6, 1976

Mr. Gary Bonfiglio City of Concord Planning Department 1950 Parkside Drive Concord, California 911519

Dear Gary,

Richard J. Jeha El Sobrante Jack Stoddard Richmond William V. Walton 111 Pleasant Hill

Pursuant to your request I reviewed the geologic report submitted by the applicant for the proposed self-service gasoline station, southeast corner cif Market Street an<l Concord Avenue. The study area is a rectangular-shaped parcel with approximately 100 feet of frontage on the south side of Concord Avenue and 130 feet on the east side of Market Street.

The geologic report was prepared by ENGEO and titled "Concord Self-Service Station, Concord, California, Geologic and Seismic Hazards Investigation" (report dated June 14, 1976). The proposed new construction is to consist of a small cashier's booth and underground tanks for the storage of gasoline.

Adequacy of Geology Report

Pursuant to the requirements of the Alquist-Priolo Act a geologic study was conducted on the site of a proposed self-service gasoline station (project area identified in Figure 7 of the ENGEO report). The study was performed under the supervision of a geologist registered in the State of California and the primary conclusion of the resulting report is that there is no evidence of an active fault trace passing through the site. The report goes on to indicate that available off-site surface information suggests that the main trace of the Concord fault passes approximately 180 feet northeast of the site (see Figure 4 of the ENGEO report). I have carefully reviewed the report as well as other pertinent mapping from nearby areas. The preponderance of evidence, particularly the on-site trenching done by the consultant support the conclusion that the site is free of evidence of active faulting.

There were anomalous features in the northeast portion of the trench. However, there was no evidence of shearing or definite indications of active faulting. Nevertheless, the consultant has recommended a setback and I concur with this recommendation. It is my opinion that the report satisfies the requirements of the Alquist-Priolo Act.

, Gary Bonfiglio Page 2 July 6, 1976

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Building Design Criteria

,

The geologic report recommends that due to the proximity of the site to the active Concord fault, the seismic design criteria should exceed code minimums (see page 15, recommendation 115 of the ENGEO report). I fully support this recommendation. The Building Inspection Department should review building plans in the light of this recommendation.

If you have any questions on the report or my comments, please contact me at 372-2031. I will submit a copy of the geologic report, along with this letter review to Mr. Earl W. Hart of the California Division of Mines and Geology.

WDM:jyl

cc: Robert Sharpe / Earl W. Hart v William Wigginton

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Sincerely yours,

Anthony A. Dehaesus, Director of Planning

'\l\j ~ '1:!...~ W. Darwin Myers.~\. Planning Geologist RG 3164, EG 946

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1 .. :i i:: 1·· ; · · '. ' .r: ,..., \ '-· •-" ' ~ J

' -- t_ ..

A?-242

CONCORD SELF-SERVICE STATION CONCORD, CALIFORNIA

Jmi ? q "11 •• u 2 ~8 py '~OLOGIC AND SEISMIC HAZARDS INVESTIGATION

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Af - 2 1-i 2.

RE FORT

TO

J ALES J. CA.l-1PBELL

ON

GEOLOGIC AHD SEIS~'.IC HAZASDS INVESTIGATION

FOR

CONCORD SELF-SERVICE STATION

COHCORD, CALIF03.NIA

I

tJdJ~~iJBY ::£___ WILLIAM B. WIGGIN"

C.E.G. #855

E:JGEO PROJEC'I' NO. N6-0740-B2

JUNE 14, 1976

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Pa<re StJ~FA.CE ?AULT S_lTP<T·u~E I!,J'i/ESTIGA'I'IOr·I

Purpose and Scope

Geologic Setting

Site Description and Conditions

Eethods of Investigation

co:JCLUSIONS

RECOl,WENDATIONS

SEL.ECTED REFEREl'JCES

A?PF.t,DIX

Fig·nre 1 Seisr'iC Hazard Zone

1

2

4

4

14

16

17

Figure 2 Surface Fault Features Concord Fault

Fi~re 3 Concord Fault - Sharp, 1973, U.S.G.S.

Fi gi.lre 4 SPcond9.ry Faul ts

Pigllre 5 Aerial Photograph 1952

?igure 6 Zickefoose' s Inferred "Fault" 1-Iap

Fii:rure 7 TrPnch Location

Fi~re B Eorthern Segrr.ent Concord Fault

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Fur~ose A.nd Scope

The p1;rDose of this study is to deterrr:ine the poten"':ial for surface

fa11l t rupture wit"'. in or in the negr vicinity of the Dronosed site.

This study is in accorde.nce with the Suggested Guidelines for Eval­

ua+: ina: the Hazard of Surface Fa1Jl t ::tupture, Note No. 49, published

by the California Di vis ion of !lines and Geology (October 19 75, Re­

ference 4). These '1;Uideline~ sua:irest thei procedures and report for­

~at needed to co~ply with thJ Alauist-Prialo Studies Zone Act of

1972. This la1·r directed the State '.}eolo.o:ist to delineate special

studies zones for California. Within these zones are napped poten-

tlally active R.vid active geoloo:ic f8,Ult traces. ':'he proposed site

is located ;·1i thin such a zone enco'.l:nassing the :capped trqces of the

Concord Fault (Fig;ure 1).

Our ini ti8.l Pha,se consisted of locating the '.'learest !::8DPed Concord

?a1J.l t t"."ace e.nd associated seconc.ary or cranchino; fault traces in

the vici'.'lity of the site. :re have Rlso revie11ed the literature data

~ertaining to the recency of activity along the Concord Fault. In

crr"Jdi ti0n to rP,1_,.ie1·1ing; t:his oerti.nent li ::er8.ture, ~'le conducted field

o'c~Prv .. P.tio!'ls ot' cl.1.rb o.ffset g_nd other fai.1lt creep featu_res near th_e

site.

·T·he sec0·'.ld p"1qse of Ollr i:ivesti':!'a+:ioY: consisti?r1 of t':LP interpretation

of 0.e"'."i"<l nYioto"raphs to detect if faul t-relat"ed toooo;raphy, vegeta-

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tional chg,,nrz:es, soil tonal co~~rP._sts nn<l lin~8.r features ( "linear.:!ents")

of Possible fault ori<Zi"l do exist- neqr the site.

Our third u>;ase included a 1°rnbs1J_rface explorqtion to Demit a detailed

8."ld direst observ"_+:ion of cont;in'lnusly exnosed geolo;:i;ic units xl'!ich

7lere then C9.".'efully loi:i;g:ed.

Ho geoohysical TI'ethods ·-;-ere used for this investigation because site

co"lditions were not favorable for their use due to existing facil-

ities and traffic.

B. Geologic Setting

To provide the background data and to follow guideline procedures,

Yle are giving the Geolo~ic Set tine; 1;rerbatiI!' fror:. the City of Co'.'."lcord

SeisTI'iC Safety Elei;:ent and other pertinent excerpts.

"Concord is loceted in an -".lluvial valley fo?T:ed by strear:'.s draining fro!I' the countains of the Diablo Ranire. Eastern Concord is dr1:1ined by the noTthwesterly flnwing 1-:t. Diablo Creek, while the western D1:1r+-. of the City is dr8.ined by Walnut Creek and its tributar.v, Pine Creek. fl. fourth stream, Galindo Creek, flows 'iestWEffd along the northeast side of Lin:e RVlo:e. These creeks have denosited several hundred feet of sediTr.ents on top of the bedrock to forl!' the valley floor, and firn hedrock at the surface is limited to the 'hillside areas of the i;aval Wea,.,ons Station on the flanks of Vt. Zion.

Bedrock Materials

The oldPst rocks in the Conco:rd p·-ea arA the Eesozic intru­sives (diabase) and Cretaceous sedi~entary rocks which are found on the flarks of l''t. Zion. In the recent geologic past, these rocks have bePn i_.,,_-.;rol"red i~ ~a.ssi1,re IandslidinftS, and it is likely th8.t the area continues to be unsta'.Jle at the present ~ime. Lanrlslides ~rea~er than one ~ile wide

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h.n.'re b2en :ca:p-oed. tlsirt~ 8.erial photo~raphs.

Yotln~er !'oc1<:s ~re friunrl on I..i!!!e RirJ..qe a.nd in thP, F:s.stern pH rt of Concord. ·These ::_'er-:- iar~r roc 1->'s 8.r~ ~:ren Of"in."1_!1."t-ly shales, cl9_ys+-o'!.es, p._ne!_ SP..Y1Ct:::-tor1es r;'i'"ith associ...R_ted volc9.ri.ics. Since +:heir forra 4 ion 35 to SO pi_] 1 io"1 Y 0 8.TS arco in ':ocene tire, they have been extensively fnlrled and f?ul~ed. As a r"sult, steeDl:V di"1)in"' beds 9re c 0 !!:I"on. Hillsirie aI'eas urcderl8.in b;r t~.~s~ roclcs, 1---:-~rt:ic11l.?.~lJr those of the T1:a.r~{e­ley :'or:nation, 8re hic;hly susceptil:ole +:o landsliding. Failure is of+:en due to the presence of interbedded ben­tonic ( shri nk-s1:ell) clay s ee.r.-.s.

quaternary Deposits

S<0veral types of Qua'::er;iary deposits: are found in Concord. including silty alluviat gravels, st;iff clays and bay mud. These d<0Dosits range in

1

thickness fr:om 150 feet in Clayton to over 300 feet in the northern part of Concord. Silty gra.Yel deposits occur over less than 10 percent of the City and are frequently fminc as lo>•, northwest-trending rid:,-es on the valley floor. Most of Concord is covered by stiff Rlluvi9l cle.ys which are locslly silty or sandy. Clean, saturated sand lavers, where p~·esent within 15 feet of the gro'.i"id mirface, ac-e a ootential liouef8.Ctio".l hazard, and are '--:nown to exist in tre •·re stern :oart of Concord. Sub­surface infortration for the eastern part of Concord is very sparse, and liquefiable sand lenses are urobably pre­sent, pa.rticula.rly alono:- Ht. Diab lo Creek.

Korth of Mallard Reservoir, the predocinant unit is bay cud. Bay "'nd is dark organic clay, is usually saturated, and iray contain peat and sand layers. ".'he presence of wet sandy layers sives tbe c2.y mud a high lic_uefaction po­ten+:ial, and ~he loose nat,ire of the sediments I".?.kes them highly corpressibl<0 a.n1 of low bearing:; stren;z:th.

The Concorc Fault

The Concord f'aul t is ir-por'cant to the City because the dal"aQ'.e caused during sn earthQUa1{e is generally concentra­ted within JO riles of the fault trace. It has been stu­died in detail by ~he U. s. Geolo;z:ical Survey and recent geologic investio:-ations suo:-gest that it is a steeply di n:'>i!lg reverse fault with the eastern side rovi!lg upward.

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N6-074C-~2

Other Faults

170 ct.her fa1J1ts 1-·1i+-.b ~\n0rNn eEtrthou2 1-\e r-otP!lti8.l exist 1-ii":;hin the City of Concord. The hills to the northeast and to the south are cut by a ff'l!!ber of faults, including the Walker Canyon, tcedanos, Clayton, and Kirker Fass Faults, but none of these have 'been shown to 'be active."

C. Site Description and Conditions

The site is located at the southeast corner of the intersection of

Concord Avenue and Harket Street (Figure 2). It is recta.Ylgular in

shape with 110 feet on Conco:rjd Ave".lue and,,130 feet on Narket Street. 1, I

At the time we conducted our ',trenching, the site was soil covered.

The site vras previously occupied by a >:ras station that was rerroved

about one year Pgo (1975).

l'Iethods of Investio:atlon

Literature and 1''.ap Review

R. V. Sharp re'realed evidence for right-lateral displace!'lent on the

Concord Fault (Reference 12; Figure 3). Various areas exhibiting

creep features such as offset curbs, en-ecr.elon P"-·rement fractures,

dis tressed tree lines, etc., served to delineate the r:8.in trace and

a.ssocia ted secon"ary or nr.Rnch ino: traces. BPrloorar, Long & Associates

in an investio:Rtio"l revealed. evide"lce of fault offset in exploratory

trenchln;z conducted on February 13, 14, 1974, within the LehI"er Olds-

!"Obile property at the corner of Fira \.'is':a Terrace and Concord Boul-

evard (Reference 1; Fi~ire 1). The location of this evidence co­

l incided with the Concord Fault TI8in trace revealed by Sharp's

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slio:'1.!"ent of offset anr1 col!:r:ressirmal waruing cur'is JO to 40 feet

north of !-1ira ' . ."ista 'Ierrf'lCe along Concord Boulevard.

In a soil investio-ation co"lduct.ed by ENGEO north of Concord Avenue

-9.nd west of Freeway "Lil), three tr"!nches located the rr.ain segn:ent of

the Co"lcord !<'ault (Reference 7; ?i>i;Ure 8). This trace is about 200

feet northeast of She.rp's ciueried fault trace (Reference 12; Figure

J}. The projection of t'iis trace n-eet:s wi+-h Sh8.rp's well-documented

main trace of the CoYJcord Fa.11lt in the city of Concord.

The prese'1t investio;ation is1, concerned with a site wrich is located I

1·-rest of the main segrr.ent trace of the Concord Fault (Reference 12).

Thus, of so!l'e concern to us, was the proximity and or projections

of the rapned seconrlary faults associated with the main Concord seg-

T:ient (Reference 12; Figure J). Based on evidence of a small com-

nression of railroad siding obtained by Sharp, 1973, a srr.all secondary

fault Point B TCBY be present 240 feet southwest of the site.

Air Photo Interpretation

Black a"ld '•-'l-:ite stereo aArial photo<rraphs of the site and its e•prirons

were studied to deterr.cine any evidence for an active fault on the pro-

perty ( Re::'erence 10, 11; Fi«ure 5). The aerial _choto"'."raphs were flown

on October 8, 1952, at a scale of 1 inch to approxi!l'.ately 1,667 feet.

The traces of recently ".c':ive fcults cal". generally be recognized on

9_e,...iP.l 0hoto7.ra:-hs bv contrasts in "'ITe;r;e"!:ation and soil types a.cross "' -- . _. v ,

the fault anrl by topo;i:raphic feat;ures such as scarps, troughs, notches

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in hillsi,,es, us,rallel rida:es, of·0 set dr2inP.Q'.e crwnnels, sg.g nonds and

closed deuressiol'ls. Our ''t1Jdy of the bl9.c1' and white aeriB.l 0hoto-

granhs did show evide"lce of lineations southeast and nor':hwest of the

site, trendinz N JOO W. The date, consisted of sag ponds, a low scarp

and a sharp tonal difference along a lineament. Our air photostudies

did not reveal any fault-related features crossing the site.

The photos did, however, show a s:irall northwest trending rise or ridge

that could be interpreted as• a fB.ult-related structure. The trend of

I this !".ore western linear feature crosses .Frel"ont Street just south

i of Co:'lcord Avenue Hnd then disanpe'l.rS to the north.

Northw.,st of the site, aerial photogranhs ( 1972, Figure 8) show a

distinct tonal contrast coincidina; with the ll'.ain tr9.ce of the Concord

Fault (Reference 11).

Geologic Reconnaissance - Surface Observations

The area in the vicinity of the site was examined for evidence of

recent faulting and fault creep along the suspected Concord segment

and 'l.ssociated secondary faults of the Concord Fault Trace (Reference

12). Beisinning at the intersection of Sunset and 11ira Vista Terrace

and exte'lcling to the intersection of Salvio and ~'ira Vista Terrace,

the fault trace is shm•n by offset curbs. This tyoe of displaceirent

occurs in a line JO to 40 feet northeast and p<trallel to ri'ira Vist:a

Terrace.

A diffet'C>l1Ce in elevation which is 8. orooable low fault scarp coincides

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1tiith the ~lli'..\n~e:'Lt nf c1J-rb offs-sts a.nd a line of distressed ~rees

south of Willow :p0ss 1.oad. Prior to its rel"oval, asphalt pavement

ir.. 8.n aban0on.~rl ~8.S st~tiori_ at the cornP.r of I'1!ira \Tista TerrA.ce and

'-"ill ow P0.ss RoRd exhibited en-echelon crR.ckin« also co inc id i>;g; with

tche project fault trg,ce (Fio:ure 2). In "<ddition, the S'U'!e crg,cking

is occurring in the pRrkin.o; lot of the King Auction Store at the

intersection of Salvio anr1 Broqclway Stre.,,ts and distress is occurring

in a convalescent hospital building: astride the f,;rnlt trace between

Harrison and Central Streets, (Darwin Myers, Contra Costa County Geol-i J

I

oi:tist, Personal Co'."!"'.unica.tion). A projec,tion of this fault would I

apnroximately trend north of the intersection of Fremont Street and

. . Concord Avenue (Figure 2).

A recheck of the inforrcation supplied by Darwin Myers, indeed, re,realed

en-echelon left stennin,;i; cracks in a zone 1 to 1~ feet wide. These

cracks "re about 1/10 to 1/4 of an inch in width and are .generally

6 to 12 inches lonR: orientec1 flbout 20 to 40 deo;rees to the trend of the

ranped main fault trace. ~he fractured zone is approximately 150 feet

long whose northern term.inns is qpnroxi!'ls.tely 30 to 50 feet froTI the

northwest pronerty line of the King Auction Store which is 14 years

old.

Our off-site survey also included an attecpt to 1) locate evidence of

the seconi:g,ry faults nearest the site and 2) obtain any additional

evidence for faultin., in the vicinity of the site.

Where the fault (Point A, Fi=re 4) crosses Harriso:'.1 Street, we were

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unable to confirm the off~et curbs of Sharp's 1973 findings. In

fact, th~re is np cnrb a+: this tiP'P O!'l t~e no"t"t:h~'iest sid_e of Harrison

be;z:innin« gt Sutter and c,Ytendin;;i; lCO to 120 feet ci.lona: Harrison. The

rest o!~ tl:e Ctlrbin~ sho-:-is no 0ffs2ts. T..-Ie v1ere 1.J.nable to f'i!1.d any

cree:o related f"!atures where the fault (Point A, Figure 4) crosses

13roacway Street. There are sone left lateral curb offsets on the

southeastern side of California Street about 175 feet north of Broad-

way. In our opinion, these offsets are not related to fault creep

but probably to differential. settle:cent arid minor translational move-1

ment. The fault (Point B, Figure 4) as mapped is short and presumably

its location was based on the compression of the railroad siding.

He were unable to confirn this creep feature and a search along a

southeastern projection of lts trace did not reveal any features sug-

gestlve of creen.

The reconnaissance close to the site was unable to confirn Sharp's

small-ri;;i;ht lateral curb offset on Concord Avenue east of Market

Street (Figure J). There are ~any new patches of curb and sidewalk

from when the ><;as stat.ion and driveways were re'l'_o•red. A possible

sli<rht bow of the curbline could be seen at the northeast corner of

the site and further to the east. A building on Fre=ont about 50 feet

south of Concord Avenue showed si;:,;ns of stress in the form of crack-

lng in the foundation le8.din;;c in to the interior (Figure 2). Accord-

ing to the owner, the building is at least fourty years of age.

Aoproxicately 150 feet north of the intersection of Fre'l'ont Street

and Concord Ave!lue, a spring extqts which, according; to the owner.

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flows continuo1rnly (171'1"1.lre 2). This sprino; 1·ro1.llrl be on the orojected

northward trace of the active Concord Fault and crnuld be abnut 250

feet !'lortheast fror.. t!:.e site.

In a geolog;ic lnvesti<i:a.tion on undeveloped nt'operty 115 feet south of

Concord Avenue on the east side of Fre,rnnt Street, Zlckefosse and

Associates renort;ed evidence of two poesible subparallel "secondary

faults, Z1 and Z2" of the Concord Fault running throuo:;h the Bisso and

3a=.ey parcels. In additioni Zickefoose J?roposes a secondary "fault",

' ' 23, about 350 feet south of the Bisso-Barney property and proposes a

tra.ce of the Concord Fault 140 feet northeast of the property.

"Fault Z3" is probably derived fron the conpressed railroad siding,

~·r'cich EHGEO couldn't verify, and is continued with a question mark

near Fremont Street. At California Street the location presence of

Z3 is based on possible fault-related foundation darrage. At best,

t'iis p-robable "fault" is hio;hly ouestionable.

For the suspected "fault Z2" the C'.ain basis for existence and location

co:r.es froc tre::i.ch data at the nor':h\-rest corner of the property inves-

tigated by Zickefo0se. Supoorting data is a rr:agnetic survey run down

?rercont Street. While +:he trench data apoeqrs to show a possible

fault c0ntact, the mao;netic data co1Jln be influer;ced by buried street

utilities. A bric~ wall that is 40 years old located on the west side

of Fre~ont Street in direct li~e of the suspected trace shows no dis-

tress or cracks. At Cal\fornia Street, Zic~efoose reports ri~ht

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13.teral offset where ENGEO saw only left 13.teral offsets. The "fault"

I stops short of Harrison with no TI'.ore positiv-e sig,-,s of faulting •

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.

While the trench lo<>: s'iows r;vidence for suspected Holocene faulting,

the extension west of Fre~ont Street is auesttonable.

"Fault Z1" is based on trench exoloration data and is not shown to

project into the site or intersect the site.

Very good evidence exists for a lengthy main trace of the Concord

Fault east of the site. Pos~ible short dtscontinuous secondary I I I '

faults, exhibiting localized isi<>:ns of str1ess, have been located with

'TRrying degrees of reliability. Where located these suspected short

segments are all west of the main trace and are mapped generally as

subparallel. There is no logical reason to propose that these secon-

dary features are perfectly linear and intersect the site.

Exploratory Trenching

The decision to trench was !1'.ade to check for any potentiqlly active

faults crossing the site. The trench was dug at a right angle to the

suspected trend of faults in the area. It started fro~ the northeast

corner striking N 600 E and went southwestward for 131 feet, then

turned N 200 E for the last 37 feet (Figure 7). The short bend r.oved

the trench from the potential location of the cashier's booth.

The trench was dug into a t.>iree to five foot layer of adobe clay.

Below ~his the soil is genprally a silty clay that grades into silty

sand and, in the western h8.lf of the trench, ,q;rades into sand. A

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caliche 10.yer runs i'1ter:-!ittently about a foot below the adobe.

The ado~e clay is a ~ark gray clay, slightly moist and stiff. Oc-

casional traces of fine grctvel, subangular in she.pe, about 2 to ?

inches in dianeter, can also be found in the clay. Below the clay

is a gr0.dational zone of about 4 to 6 inches which then becoroes a

broi-m silty clay which is slightly damp and medium stiff. The silty

clay grades into a li<5ht brown sandy silt, slightly darop and medium

de"lse, and in the W'lstern half I

colored sand, slightly damp fO I

of the tre;nch, gr8.des. into a tan

dry, loose/ to medium dense. I

~he strata below the clay contctins occasional fine gravel, subangular

tu subrounded, about 3 to 10 inches in dial':eter, with the longest

din ens ion. oft; en rouo:\o l~r oriented in a horizontal plane. The pebbles

are mostly nuartz and chert. Trace patc'1es of adobe clay foo1nd in

the silt:;' cl8.y also have the saIJ:e orientation with the long axis hor-

izon.tal.

The layers are horizontal with the exception of the first 20 feet of

the eA.stern end. A green rn.ottled clayey sand fo!T'.S a contact, under-

neath the brown sand sil-t:, with an orientRtio!l of N 30° W di99ing

400 to the west. The contact co,.,_es up 4 feet from the t;rench bottom,

and curves to a rmi.grly horizonta_l plaYie.

':'his fBat.ure is interpreted as an ano.rralous stratification. A.'1 ex-

planation for this discontinuity is t"1at the feature is related (un-

reflected in the adobe layer) or depositional. No change in d.aropness

EN GEO INCORPOR~TED

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N6-0740-B2

r.1r?~s seen across th is coYltRc t.. The -r-:1ai n ConcnrQ Fault is a trnoY.ln

o:roundwater table barrier. We in'"eroret the main br<J.nch of the Con-

cord Fault to he at least 200 to 250 fPet northeast of ter'TI'.inus of

this trench.

Some secondary or branch fault trace or traces rr.ay be located between

the nain trace "<.nd Stat ion 1 +53 of this trench, possibly the north-

ern extention of Zickefoose's 11c.apped f9.ults" Z1 and Zz (Figure 6).

Seismic Ha3ards 1 I I I

Ground shakin.o: is C8.USed by ~he transmiss1

ion of earthquake induced

bedrock vibrations up through the overlying soil deposit. Shaking in

turn is transmitted into the structure. The variables which dete=ine

the extent of shE>.king darr.a.ge are chatacteristics of the foundation ma-

terial, the design' and ':"aterlal of t'.-le structure, the T"agnitude and

epicenter of the earthquake and the intensity art& duration of th·e

shaking.

So!!le major earthciuakes which have occurred in the San Francisco Bay

Region are:

Date Fault

1868 Hayward

1906 San Andreas

1955 Concord

Fagnitude

Intensity X !1'.ercall i Scale

8.3 Richter See.le

5.4 Richter Scale

Estimated Maximum Ground Acceleration at Site

0.5 to o.6 g

0.25 to 0.35 g

0.3 to o.45 .s

The estimated 1"8.Xil'lum «ro11ni! Hccc:lerations shouli! not be construed as

EN GEO INCORPORATED

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N6-0740-B2

a design value.

The rr.aximum expec+:'able earthouake on the Co"lcord Fault is a Richter

l'cagni tude of 6. 6 which would result in naxi:nun ground acceleration

on the order of o.6 g. Relating this to a Building Code seismic

I pseudostatic design value of 0 .113 is not reasonable. Ma.-icirr:um ground

I I' I I; I I I I I I I I I

.

a.cceleration values in the field should not be considered to be build-

ing design values. Ho'.~ever, it is reasonable because of the prox-

imity to the active Concord ?ault to use a higher design value than I

given as a minircu"! by the Code. ' Typicalllf, in the Concord area,

earthouake design values of d.2 to 0.3 g are being used in the seismic

hazard zone as a precaution.

ENGEO INCORPORATED

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CONCLUSIONS

1. Our investio:gtion reconfir~s the presence of an acti,re fault trace

identified as the Concorrl Seg"lent of the Concord Fault (Reference 12)

in the vicinity of the site. This seglr'.ent is the easternmost knomi

trg,ce of the Conco!'d Fault Zone. Our studi"ls, primarily of curb off-

set and literature and rr.ap studies, would place this projected fault

trace about 200 to 250 feet from the site (Fi~re 8). A potentially

active branch or second.<iry f11-ult may exist at the extreme northeastern

corner of the property. No

in the trench •

I other I

I I

indicat,ions of faulting were seen

2. The Concoc-il Seo:rr.ent of the Concorrl Fault is a right lateral strike-

slip fault·. Its epicentral loce."ions wo11ld sugo:est a verticP.l to

steeply dipping fault. The rcaxil"um creilible ef'.rthauake for this fault

is Richter Magnitude 6.4 to ?.O which could result in an anticipated

offset of 2 to 4 feet ("best fit" regression line, Reference 3)

horizontally. Vertical displace"!ent woulil then be on the order of l

foot. The maximum probable or expectable earthquake would be on the

orde!' of 'lichter !•1a;z:nitude 5.8 to 6.6 .which is in a range where fault

rupture nay or l"ay not take place. If it did, the best fit regression

line would inl'lic<ite horizontal offset of 8 .inches to 2 feet and a

lesser figure vertic"llly (Reference 3). These offsets would be off-

site in the vicinity of the rrapoed trace.

site is in excess of 50 fee+: fro": the T'lain Concord Segnent trace

8.nd a discovered oossible seconct,,ry fe.ult at Station 1+53 is ".'.Ore than

EN GEO INCORPORATED

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N6-074C-B2

50 feet frorr a.ny pronosed s~ructure. The possibility for ground rup­

ture on the site"is unlikely. The a.no!!'alous soil profile of Stqtion

1+53 ray be fault relater1. It does not appear t<J ha,re offset the

adobe l~yer in the unper 5 feet.

J. LiauP.f8.ction occurs when low density saturated fine-grained

P-;ranular S<Jils are subjected to the shaking effect of an earthauake.

With the type of soil ::-resent at the site, liauefaction is unlikely.

The liauefaction notential of the soils in the upper 10 :feet is very , I

low. The Concord Seis:nic satety Element 1indicates that where clean

I

saturated sand layers are present, particularly in the upper 15 feet,

a liauefaction potential could exist. Our test trench at the site

revealed co'.!'pact soils in the upper 10 feet with ground water level

beneath this depth.

Lurch crackin~ and lateral snreadi115 potential at this site is minimal.

Since the site is on a flat area, landslide potential does not pertain.

4. The potential for loss of human life is minimal as a single atten­

dant will be used in a small bo<Jth. The canopy would be designed by

:c:odern codes.

ENGEO INCORPORATED

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RECO!'T'EXDATIONS

1. The Alnuist-Priolo Act renuires ""lininul!l setback of 50 feet for

any hE>.bitable structure unless a oualified Geoloo:ist specifies less

based on very detailed studies. We qre co'lfident that the proposed

structure is in excess of 50 feet from a hazardous fault.

2. The cashier's booth be located at the southwestern part of the site.

3. The . .:z:asoline tanks be lodated in the northwestern part of the site I :

at least 25 feet fro~ any • I po~ential fault 1

• Observe a setback of 25

feet froT'l Station 1+53 of the exploration trench (See Figure 7).

4. We do not believe additional o:eolo;z;lc studies are required for

this site.

5. The canopy and booth should be structurally designed using proba-

bly more than T'lini:r.:m!'l 1973 Code values.

ENGEO INCORPORATED

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N6-0740-:92

SELECTED REFEREI\"CES

(1) BerlOQ'Rr, Long & Associates, 1974, "Ern:rineering Geologic Inves­':io:ation, Leh!1'er Olds!"obile Property, Willow Pass Road and Con­cord Boule'·Rrd, Concord, California", unpublished report.

(2)

(3)

, 1974, "Engineering Geologic Inves-.,...,-~..,...,~~-=.,---..,-..,.-,.----.,,....,,-,-__,=

tigation, Vicinity·of the El Paseo Center, Concord, California", unpublished report.

Bonilla, Jv'.G., 1967, "Historic Surface Faul tin<>; in Continental United States a-nd Adjace-nt Parts of Mexico".

(4) California Division of Mines a-nd Geoloo:y, October 1975, "Guide­li-nes for EVfl.luating th~ Hazard of S1frface Fault Rupture", Note 49.

I

\ I , 1973, "Special Studies "'z,_o_n_e_s_,__,W..,.fl-. l=-n-u_,..t_ . ..,C,...r_e_e_,k--,7""'/;'.,..._ ""'r;"';.,..i n-u_,.t-e--=i:;-u_a..,d=-r-fl-. n-g-1=-e" •

(6) Earth Science Associates, 19 74, "Seisrric Safety EleI'lent, City of Concord, Draft Geoloa:ic Hazards Nap".

(7) ENGEO .• Inc., 1974, "Soil Investigation for Office-Warehouse Build­ing, Concord, California"~

(8) , i975, "Proposed Com."'.ercial-Industrial Facility, Geo-logic Hazard Study, Concord, California".

(9) , 1975. "Systron-Donner Corporation Geologic Hazard Study", unpublished report.

(10) Pacific Resources, Inc., 1952, Aerial Photographs AV-104-04-11 and AV-104-c4-12.

(11) Real Estate Atlas of Contra Costa County, California, 1973.

(12) Sharp, R.V., 1973. "Map Showing; Recent Tectonic Movement on the Concord Fault, Contra Costa and Solano Counties, California", U.S. GeoloQ;ical Survey, Miscellaneous Field Studies Map MF-505.

(13) Seed, H.B., and I.JV'. Idriss, Septel'lber, 1971, "Sin:olified Proce­dure for Evaluatina: Soil Liquefaction Potential", .Journal of the Soil }'echanics and ·Foundations Division, ASCE, Vol. 97, No. SM9, ProceecUngs Paper 8371, :op. 1249 - 1273.

(14) Zickefoose, N.H. & Associates, 1975, "Uni~proved Property Fre~ont Street ne'3.r Concord Avenue, Geologic Fault Study".

EN GEO INCORl'ORATED

® J

I I I I I APPENDIX

I Figure 1 Seismic Hazard Zone

I Figure 2

Figure 3

Surface Fault Features Concord Fault

Concord Fault - Sharp, 1973, U.S.G.S.

I Figure 4 Secondary Faults

Figure 5 Aerial Photograph 1952

I Figure 6 Zickefoose's Inferred "Fault" Map

•• Figure 7

Figure 8

Trench Location

Northern Segment Concord Fault

I I I I I I I I I ENGEO

INCORPORATED

®

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Snecial Studies Zone

CRlifornia Division of Fines an~ Geolo~y, July, 1974

Cali for:'lia Cor!trR Costa Co.

:"'ault Loca+:ed in ~rencr._ ~y Be!'lo~ail:", Lnn_~ & Asqoei·~.t;:.s, 3-15-74

ENGE·o self-service station site Concord, California

I N CO R p O R AT E D 1-------,,-_,_.,,..sE:;..::r'=s i:,-,n,:.,:.c-,-.cH::::cA;::cZA=--.,.B..-D_z_o_:rn ____ ---t ~S~C~A-'-L_E~_1~"-=~2_c_·o_c_,_· ........ ~~JOB ENGINEEl=IS ANO GEOLOGISTS

CONSULTANTS IN THE APPLIED EARTH SCIENCES DATE June, 197 NO. IJ6-074C-B2

FIGURE NO.

1

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 PHONE (415) 548-8800

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•• I I I I I I I I 1

CoY\cord Fault Trace

1) Offset Curb 2) Crack in Pavement

(PR Vi n;:i: rer' 01red) 3) ?Ernl t Scqrn 4) En-echelon'Left

Steppi~~ Fractures 5) Distress in Hoscital 6) Artesian Snrin~· 7) Crac'.-<:ino: in

Pou!lda t ion 8) Bmr i.n Curb Site Creep Features Veri fled by E'.~GEO

EN GEO

~--0 00 -------

'l:::J0 [)?6

Qolf-Q 0~~~c~ 9~a~1o~ Site Concord, California

SURFACE FAULT ?EATURES CONCORD FAULT

INCORPORATEDf------c-,,--___,~~-,.-~--c~~~ SCALE 1 "= 20 C'' JOB ENGINEERS AND GEOLOGISTS

CONSUL TAN TS JN THE APPLIED EARTH SCIENCES

i-:.-:~:.:;;....~~~~.,..,,...~--1

DATE uune, ~7 b NO. N6-074C-B2

FIGURE NO.

2

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE (415) 548-8800

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EN GEO Self-Service st~ti~~ Site Concord, California

I NCO R p 0 RATED f--c_o:_,;c_o~::in~~ ~F-Au=T~':'--u~"'"'_:A_RF_,~1-9_7J_._u_._s_. _G _· s_:_. ----1

ENGINEERS AND GEOLOGISTS CONSULTANTS IN THE APPLIED

EARTH SCIENCES N6-0740-B2

FIGURE NO.

J

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE {415) 548-8800

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r~· . . ~ .. 1ENGEO Self-Service Station Site

CoYJcord, Ca·lifornia

SECONDARY FAl'LTS

ENGINEERS AND GEOLOGISTS SCALE 1 "= 2r. 0' JOB coNsuL TANTS 1N THE APPLIED DATE JiJ>"le 1 G NO F6-C?40-B2

EARTH SCIENCES ' ~ t ./ ' •

FIGURE NO.

4

lINCORPORATED

~150 SH . .\TTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE t415) 548-8800

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EN GEO

Lineation - rise in topography inter?reted to be fs.ul t scarp or fault rel9ted

Self-Service Station Site Concord, California

AERIAL FHOTOG3APH 1952 INCORPORATED>--~~~~~~~~~~

SCALE 1" = 1 , 6 7" ~~,,--.,,,--~_,,...,"""~r-~-1J08 6 4 ENGINEERS AND GEOLOGISTS CONSULTANTS IN THE APPLIED

EARTH SCIENCES DATE Ju"'le, 1970 NO. N -07 C-32

FIGURE NO.

5

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE (415) 548-8800

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Self-Service Station Site Concord, California

I N CO R p OR ATE O ~"-'~ r_c_"~~'° ?_o_o_s ~~"' ·~s_n_;::<_E-..qR_. E_D_" "'_" -~_D_L_T'_' _r'._AP __ -1

SCALE 1 "= 200.

EN GEO ENGINEERS ANO GEOLOGISTS - JQ8 CONSU1E.1~'i-~Sslt'ili::tE'i,?PLIEO DATE Jll_'._'18' 1976 NO. r·~6-C740-B2

FIGURE NO.

6

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE (415) 548-8800

11 " I ,. i

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'• " f I I

r

SIT£

•

\'L ·--...l.---L- STA 1+53

25'

SETBACK NO BURIE GAS TANK NE OF TH t LINE

Self-Service Stq.tion Site Con.cord, C9-l1:'ornia EN GEO

I NC 0 R P 0 RATE D f-,,-___ _,,...,,..--n~.,..,.8~_::1r_c:_~ _Lo_c.--A'f._r_c;_; -~--

FIGURE NO.

7

ENGINEERS ANO GEOLOGISTS SCALE ' - JOB caNsULTANTs IN THE APPLIED NO r-.r6-0?4o-E,2

EARTH sc1ENcEs DATE June 19 6 · ~ 150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE (415) 548-8800

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Fault located iYL trench by ENGEO, 1974

Concord ?aul t - r:"1.ill ·J:r,,_ce

Aerial Photo 1972 'teal Esta.t" Atlas of Contra Costa Co.

EN GEO SRlf-Service Station Site Concord, California

:'JORT~3BN SE8!'/f3~JT CONCO:!OD FAULT

INCORPORATED~---___,.-:-:-:-~---r~~~~---i ENGINEERS AND GEOLOGISTS SCALE 1 "= 600' JOB N6-0740-32

CONSULTANTS IN THE APPLIED DA 6 NO EARTH sc1ENcEs TE June 19 7 ·

FIGURE NO.

8

2150 SHATTUCK AVENUE, SUITE 600, BERKELEY, CALIFORNIA 94704 · PHONE (415) 548-8800