921~15 - gmw.consrv.ca.govgmw.consrv.ca.gov/SHP/APSI_SiteInvestigation... · 6/20/1977 · Roseb...

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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. 106.5 l 07 South Broadwoy 90012

John r. Roseb raugh

SACRAMENTO

Resources Bldg., Rm. 118 1416 Ninth Street 9.5814

r,htef Building Inspector r,ounty of San Bernardino Bullrlln~ and S~f~ty Depart~ent 1111 l'Mt '1!11 :.~rcet

S<m Rorn.ardlno, CA 921~15

Dear '1r, Rosebraugh:

SAN FRANCISCO

Ferry Building 94111

J1Jly 14, 1977

\.le are olaclnq on open file the fol lo.i!ng report, reviewed and approved by the r,ountv of San Bernardino In COIT!fll iance with the Alqulst-Prlolo Special Studies Zones Act:

RONALD REAGAN, Governor

Enqlneerlnq qeology lnvestlqatlon, Tentative Tract Numbers 9001, Lots 1-18 and <l812, lots 1-20, Rialto, California; by Rasmussen and Associates: June 20, 1977.

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cc: A•P flJeV

Sincerely yours,

l'M.t W. HART Office of the State Geologist CEG 'l35

ENVIRONMENT AL IMPROVEMENT AGENCY ~~!(' County of San Bernardino

1····11!~~~~~~~~~~~~~~~·············\~;;;~,~·~n~NARDl~O i··············· 11 I\ ,1

BUILDING AND SAFETY DEPARTMENT """d;ng•ndsafotv Dopmmont

1111 East Mill Street • San Bernardino, CA 92415 • (714) 383-1212

July 8, 1977

Robert J. Mitton JohnC. "Jack" Rosebraugh

Director Chief Bldg. lnsp.

Calif. State Div. of Mines & Geology

I RECEIVFf' California Division of Mines and Geology Division Headquarters, Room 1341 I JUL 111977

I 1416 Ninth Street Sacramento, CA 95814 Sacramento, California

Attention: State Geologist

Enclosed is the fault risk evaluation of tract 9001, lots 1-18 and tract 9812, lots 1-20. This report was reviewed and approved by geologist John D. Felt.

Respectful~ _ I

~'-~· John c. Rosebraugh Chief Building Inspector

cc: File

JCR/eh

EARL GOODWIN County Administrative Officer

ROBERT 8. RIGNEY, Adrnin1strator Environmental Improvement Agency

:..- .-----·-

,.,, ..

Board ot Supenrisors DENNIS HANSBERGER Third 01stnct DAN! EL 0. M! KESEL l Seco0G Q,srrwt

Chairrnan V•ce Ch;:i•r1n2n JAMES L. MAYFIELD .first District ROBERT Q_ TOWNSEND Fourth D1stnct

NANCY E. SMITH Fifth District

Earth Science S Engineering

GEOLOGY

GEOPHYSICS

SEISMOLOGY

27595 Santa Fe Street

Hemet, California 92343

(714) 658-7509

County of San Bernardino Environmental Improvement Agency Building and Safety Department 1111 East Mill Street San Bernardino, CA 92415

Attention J. C. Rosebraugh

Gentlemen:

June 27, 1977

At your request we have reviewed a report by G. Rasmussen and Associates dated June 20, 1977 and entitled:

Engineering Geology Investigation, Tentative Tract No. 9001 (Lots 1-18) and 9812 (Lots 1-20), Rialto, California.

ENGINEERING

OCEANOGRAPHY

HYDROLOGY

There was an ambiguity in the report. On page 13 it recommends no human occupancy structures to be built northeast of a line through the end of the exploratory trench. On the 1" = 150' scale map a 50-foot set-back line (parallel to the above line) is indicated as the boundary of the restricted building zone. Mr. Rasmussen was contacted by phone and he stated the recommendation on page 13 was in error and should include a 50-foot set-back from the line through the end of the trench. The 1" = 150' scale map was correct.

With the above clarification, it is our opinion that the report satisfies the requirements of the amended Alquist-Priolo Act and tiie report is approved.

The county should note the restrictions in the recommendations of the report (pages 13, 14 and the 1'' = 150' scale map).

Respectfully, Earth Science & Engineering, Inc.

d~f~~~ Cert. Eng. Geologist No. 858

JF:jl Copy: G. Rasmussen

I I I I I I I I I I I I I I I I I I I

GARY S. RASMUSSEN & ASSOCIATES /ENGINEERING GEOLOGY

560 NO. ARROWHEAD AVENUE. SUITE 7 • SAN BERNARDINO, CA- 92401 • TELEPHONE (714l 868-2422

ENGINEERING GEOLOGY INVESTIGATION TENTATIVE TRACT NU"6ERS 9001, LOTS 1-18 AND.9812, ·LOTS 1-20

RIALTO. CALI FORNI A

. June 20, 1977

Project No. 1213

Prepared for

Pacific Foundation Engineers 2230 So. Riverside Avenue

Bloomington, California 92316


GARY S. RASMUSSEN & ASSOCIATES / l!.NGINl!.F.RING GEOLOGY

S60 NO_ ARROWHEAD AVENUE. SUITE 7 • SAN BERNARDINO. CA 92401 • TELEPHONE (714J 888-2422

June 20. 1977

Pacific Foundation Engineers 2230 So. Riverside Avenue Bloomington, California 92316

Attention: John R. Byerly

Project No. 1213

Subject: Engineering Geology Investigation of Tentative Tract Numbers 9001. Lots 1-18 and 9812, Lots 1-20, Northeast Side of Riverside Avenue, Northwest of Knollwood Avenue, Rialto, CA.

A subsurface geologic investigation of the two tracts has been conducted

at your request. The purpose of our investigation was to determine if

any active faults have ruptured the upper 10± feet of natural sediments

on the sites in order to satisfy Alquist-Priolo Special Studies Zone

Act requirements. The two tracts are contiguous, with the northwest

boundary of Tract Number 9812 being the southeast boundary of Tract

Number 9001. Both tracts are zoned R-1 and will be developed for single

family residential structures. Both tentative tract maps were prepared

by Lockwood Engineering and Surveying Company. Tract 9001 was plotted

on a 50-scale map dated July 29, 1976 and Tract 9812 was plotted on a

60-scale plan dated March 22, 1977. The approximate location of the two

tracts and their relation to the Alquist-Priolo Special Studies Zone

boundaries are shown on the index map on page 2. The location of our

trench is shown on the enclosed map.

SITE INVESTIGATION

The tracts were free of man-made structures at the time of our investigation,

other than buried concrete irrigation lines.and fences. Numerous mature

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ase Map USGS 1"=2000' scale Project No. 1213 .•'

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Gary S. Rasmussen & Associates, Inc.

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PFE - Tracts 9001 & 9812 - Rialto June 20, 1977

Project No. 1213

eucalyptus trees were growing along the north and south boundaries of

each tract, and a row of trees was also growing along the west bank of

the active channel of Lytle Creek. The lower portions of Tract 9812

contained large quantities of trash which had been dumped over a long

period of time. Tract 9001 had been cleared of vegetation and numerous

large trees and brush had been placed as fill, in the active channel

portion of the tract.

, Branches of the San Jacinto fault zone are suspected to lie within the

vicinity of the tract, in particular to the northeast within the active

channel of Lytle Creek. All of the suspected active faults are northwest

trending features in this area, therefore a single, long backhoe trench

was excavated across the southeasterly boundary of Tract 9812. The trench

was excavated in a manner that would intercept any northwest trending

faults parallel to the San Jacinto fault zone which would pass through

either of the tracts, other than within the active stream channel. The

trench walls were examined in detail for evidence of faulting and none

was found. The trench was excavated with a backhoe and the walls sloped

so that detailed examination of the trench walls could be facilitated.

The trench log is enclosed with this report. The northeasterly portion

of the two tracts was not investigated by subsurface trenching as currently

active channels of Lytle Creek are located immediately northeast of the

northeast end of our trench. Trenching within the active channel of

Lytle Creek to depths of 10 to 11 feet would probably only be observing

•''

-3-GARY S. RASMUSSEN & ASSOCIATES


PFE - Tracts 9001 & 9812 - Rialto Project No. 1213 June 20, 1977

sediments less than 50 years in maximum age. This portion of both tracts

will be restricted from residential building because of the potential for

flooding.

A geologic field reconnaissance of the site and surrounding area was con

ducted during June 1977. A single backhoe trench was excavated across

the southeast border of Tract 9812 to a depth of 11± feet, to determine

if any active faults pass through the tracts (no datable material was

found in the trench and the exact age of sediments at 11 feet is not

known, but is estimated to be of the order of a few thousand years). In

addition to the field work, our investigation included review of stereo

scopic aerial photographs flown in 1971 using color infrared film; review

of stereoscopic photographs flown in 1930, 1953, and 1968; review of

pertinent geologic literature and seismic information including personal

communication with other geologists having completed work along the

faults in the immediate area. A list of references is enclosed.

SITE GEOLOGY

The two tracts are located on alluvium adjacent to the wes~ side of Lytle

Creek. The northeasterly portion of both tracts extends into the active

portion of Lytle Creek. Two benches or terraces are located on both sites

above the currently active channel of Lytle Creek and probably represent

former ancient channels or flood plains of Lytle Creek. Numerous other

ancient, pre-existing drainages can be observed on the older photographs,

all associated with Lytle Creek, which emanates from the San Gabriel

-4-GA.RYS. RASMUSSEN & .ABSOCI.ATES


'( /'

PFE - Tracts 9001 & 9812 - Rialto Project No. 1213 June 20, 1977

Mountains northwest of the tracts. Th~ upper bench is older alluvium

with the lCMer bench considered to contain slightly younger alluvium.

The lower bench is part of the current flood plain of Lytle Creek channel.

The approximate location of the contacts and relationship of the geologic

units is shown on the enclosed geologic index map (Dutcher & Garrett, 1961).

The tracts are located at or very near the northeast boundary of the Perris

Block. The Perris Block is considered to be a relatively stable block of

land as opposed to the more unstable and subsiding San Bernardino Valley

Block (Fife, et. il·• 1976) located along its northeast margin in this

area. The northwest boundary of the Perris Block is generally considered

to be the Cucamonga fault and base of the San Gabriel Mountains whereas

the northeasterly boundary of the Perris Block is generally considered

to be the San Jacinto fault.

The active channel of Lytle Creek is currently located along the northeast

property lines of both tracts and farther northeast. It is confined to

those channels by earthen flood-control dikes to the northwest where the

river debouches from the San Gabriel Mountains. Our review of the older

photographs indicates two major stream channels were in existence in 1930.

The southwesterly most stream channel was located irrmediately northeast

of the northeast end of our trench and included the northeasterly portion

of both tracts. Our review of the older photographs indicates south and

southwest flowing streams were responsible for deposition of the sand and

gravels encountered under the tracts. Observation of the trench walls

-5-GARY S. RASMUSSEN & A.SSOCXATlCS



Project No. 1213

indicates several different episodes of deposition occurred resulting in

moderately well-defined bedding throughout the length of the trench.

Numerous episodes of channeling and the existence of former river banks

were observed.

SAN JACINTO FAULT ZONE

The location of the Claremont trace of the San Jacinto fault zone is

believed to lie within the active channel of Lytle Creek in this vicinity.

Previous geologic work done in the area has failed to locate a surface

expression of this active fault. The buried faults shown on the Special

Studies Zone map were originally identified as ground water barriers and

shown as such in Dutcher & Garrett's publication. The exact location of

the fault has never been determined in this area, whereas farther to the

southeast abundant surficial and subsurface evidence of the precise fault's

location is known. The absence of surface indications of the fault's

location in this vicinity is attributed to the. very active deposition and

channeling of Lytle Creek.

The trench for the foothill feeder, connecting the California Water

Project from Devil's Canyon to water users to the west, passes approxi

mately one mile north of the site. Detailed observation of the trench

excavated for this aqueduct, which extended to depths of approximately

50 feet below the natural ground surface, was observed by both Dick

Proctor of the Metropolitan Water District and Doug Morton of the U.S.

Geological Survey, independently of each other, and no evidence of

-6-GA.RYS. RASMUSSEN & ASSOCIATES



Project No. 1213

faulting was observed by either of them in this deep trench (personal

communication, 1973). Other trenches along the main aqueduct's crossing

of the San Jacinto fault, on its course through Colton, revealed abundant

evidence of near-surface rupture of relatively young alluvial sediments.

The absence of fault rupture in the Lytle Creek is attributed to the

very young age of sediments within Lytle Creek, the very coarse nature

of the sediments, and the large volumes of water during rainstorms which

scour and fill channels to great depths. Where Lytle Creek empties into

the Santa Ana River in southeast Colton, aluminum pop-top cans were found

at a depth of approximately 30 feet below the surface in the trench for

the aqueduct. This is evidence of scouring along the Lytle Creek drainage

to depths of at least 30 feet during the 1969 rains.

Of critical importance to siting human occupancy structures on or near

a fault is the fault's state of activity. A clearly inactive fault can

be built across with very little chance that the fault will move signifi

cantly during the life of the structure. An active fault, on the other

hand, has a relatively high chance of undergoing surface rupture during

the life of a human occupancy structure. In order to more precisely

define fault-rupture hazards, the State has defined three states of

activity for faults. The following definitions are excerpted from the

California Division of Mines and Geology Special Publication 42 (see

enclosure). "For purposes of delineating Special Studies Zones, any

fault considered to have been active during Quaternary time (last 2 to

3 million years)--on the basis of evidence of· surface displacement--is




Project No. 1213

considered by the State Geologist to be potentially active. An exception

is a Quaternary fault which is determined, from direct evidence, to have

become inactive before Holocene time (approximately the last 11,000 years).

Such faults are presumed to be essentially inactive and most have been

omitted from the maps. The State Mining and Geology Board, in their

Policies and Criteria (adopted 21 November 1973), defined only those

faults which have had surface displacement within Holocene time as 'active

and hence as cons ti tuti ng a potenti a 1 hazard'."

No surface expression of the San Jacinto fault has been found in the

immediate vicinity. The sediments observed in our trenches are estimated

to contain sediments in age estimated to range from late Pleistocene

(older alluvium on the upper bench) to Holocene for the lower bench (which

is estimated to be only a few thousand years old). Trenching of the

sediments is consistent with the current state of the art for investigating

recency of faulting for the type of use intended for the site. Trenching

of sediments within the active channel of Lytle Creek (immediately north

east of the northeast end of Trench 1) would not be meaningful from the

standpoint of potential rupture hazard as these sediments are estimated

to be less than 100 years old to depths up to several tens of feet.

No evidence of faulting was found in the trench, therefore, the potential

for fault rupture through the portion of the tract covered by subsurface

trenching is considered to be very low to non-existent. The absence of

active faulting beyond the northeast limits of our trench has not been ,.




Project No. 1213

proven and this portion of the two tracts should be considered a

restricted use zone from a fault rupture standpoint as well as from the

flooding hazard.

SEISMIC SETTING

The northwest trending; active San Jacinto fault is estimated to lie

within one mile northeast of the northeast edge of both tracts. The

San Jacinto fault is generally considered to be the most active fault

in southern California. The active San Andreas fault is approximately

412 miles to the northeast. Both of these northwest trending faults are

characterized by right-lateral movement.

The Cucamonga fault is an east-west trending fault located approximately

4 miles northwest of the tracts. This fault zone is characterized by

left-lateral and reverse movement.

Other active faults are located in the general region, such as the Elsinore

fault zone, but are considered less significant to the sites than the

nearby San Jacinto, Cucamonga, and San Andreas faults.

No indications of faulting through the tracts were observed in the field

or on the aerial photographs and surface faulting has not been recognized

close to the site. Buried faults are indicated adjacent to the northeast

portion of the tracts and farther northeast, based on ground water data

as previously discussed.

-9-GARY S. RASMUSSEN" & ASSOCIATES



Project No. 1213

A summary of major faults and their distances from the site is shown in

the following table:

Fault

San Jacinto

Cucamonga

San Andreas

Chino-Elsinore

Newport-Englewood

SEISMIC HISTORY

Distance from Site (miles)

Less than 1 NE

4 NW

4\, NE

24 SW

50 SW

Numerous earthquakes up to Richter magnitude 4.5 have occurred in the

vicinity and may have originated on the San Jacinto, Rialto-Colton or

Fontana faults. Location of epicenters is not precise enough to exactly

correlate earthquakes with certain faults when they are as close as these

three. A Richter magnitude 5.5 earthquake occurred within 5 miles of

the site in 1970, north of the south base of the San Gabriel Mountains.

This larger earthquake and most of the smaller ones appear to have

occurred along the San Jacinto fault zone.

Numerous earthquakes of Richter magnitude 6.0 to 7.0 have occurred along

the San Jacinto fault between the San Gabriel Mountains and Mexico during

historic time. The 1923 earthquake, estimated at Richter magnitude 6.0,

was centered south of Loma Linda along the San Jacinto fault zone and

resulted in severe structural damage to buildings in east San Bernardino.

-10-

0.ARY S. RASMUSSEN & .ASSOCIATES



Project No. 1213

A Richter magnitude 6.0± earthquake was centered very close to the site

in 1899. Earthquakes of Richter magnitude 6.0 to 7.0 have occurred

somewhere along the San Jacinto fault zone on the average of once every

5 to 6 years during historic times.

The San Andreas fault zone has been very quiet since the great Fort

Tejon earthquake of 1857, in southern California. This fault has the

tendency of almost no movement or activity for long periods of time

(100± years), then suddenly releasing large amounts of energy, such as

in the 1906 San Francisco and 1857 Fort Tejon earthquakes.

SEISMIC ANALYSIS

Significant earthquakes affecting the site are likely to occur on the

San Jacinto, San Andreas, and/or Cucamonga fault zones during the life

of the proposed structures. Recurrence intervals for maximum probable

earthquakes cannot yet be precisely determined from a statistical stand-

point, as recorded information on seismic activity does not encompass a

sufficient span of time. However, based on the information available

at this time, it is our opinion that the following maximum probable

earthquakes should be expected (at least 50 percent chance of occurrence

within the next 100 years); up to Richter magnitude 7.0 along the San

Jacinto fault; 7.5 ·along the San Andreas fault; and 6.5 along the Cucamonga

fault. Larger earthquakes could occur on any of these faults but their

probability of occurrence for that time period is low.

-11-GA.RYS. RASMUSSEN & ASSOCIATES



Project No. 1213

Earthquakes of these magnitudes and distances from the site should be

expected to produce maximum peak ground accelerations at the site of

approximately 0.7g in bedrock under the site (estimated to be 1,000 feet

deep). This acceleration should not be used as a design value as it is

a maximum peak acceleration, but should be regarded as an aid in the

structural evaluation of the buildings to be placed on the site.

Large earthquakes may occur on other faults not mentioned but are con

sidered less significant to the site because of their larger distance

or 1 ow er probability of occurrence.

Numerous earthquakes of Richter magnitudes 4.0 to 4.5 have occurred near

the site since detailed instrumentation began in 1932. Many of these

appear to concentrate along the west side of the San Jacinto fault and

may be related to the Rialto-Colton fault. These small earthquakes

normally do not result in surface rupture.

CONCLUSIONS

Active traces of the San Jacinto fault zone pass northeast of the north

east limit of our trench. No evidence of fault rupture was found in the

upper 11 feet of the trench excavated across the site; therefore active

faulting through the portions of the tracts southwest of the northeast

limits of our trench has not occurred since deposition of the upper 11

feet of materials. Fault rupture through the tracts southwest of this

limit during the life of the proposed structures is not likely and no



P FE - Tracts 9001 & 9812 - Rialto June 20, 1977

Project No. 1213

restriction on the location of structures on the site is necessary from

a fault rupture standpoint, southwest of this line.

Earthquake induced flooding of the tracts is not likely as there are no

bodies of water or reservoirs immediately upstream that would flood the

site if they failed.

The active channel of Lytle Creek passes through the northeasterly portions

of both tracts and is considered to be subject to recurring flooding during

periods of heavy precipitation. Surface water was flowing in a small

channel less than 100 feet northeast of the two tracts at the time of our

investigation.

Liquefaction and other shallow grollld water related hazards are not

expected as the ground water table is estimated to be more than 80 feet

below the surface.

Severe seismic shaking of the tract should be expected within the next

100 years from earthquakes along the San Jacinto, San Andreas, or

Cucamonga fault zones.

RECOMMENDATIONS

Human occupancy structures should not be placed northeast of a northwest

trending line through the northeast end of our trench as shown on the

enclosed map, due to the potential hazards from both flooding and fault

rupture.

-13-QA.RYS. RASMUSSEN & ASSOCIATES



Project No. 1213

The potential for flooding of the tracts should be closely evaluated

because of the proximity and elevation with respect to the adjacent

Lytle Creek Wash.

A maximum probable earthquake of Richter magnitude 7.0 along the San

Jacinto fault (less than one mile) and magnitude 7.5 along the San Andreas

fault (4~ miles) is expected in this area, therefore we recommend human

occupancy structures be designed accordingly.

GSR/mt

Encl: 150-scale map Alquist-Priolo data References Trench Logs

Respectfully submitted

GARY S. RASMUSSEN & ASSOCIATES, INC.

/? /f;y//fk4u~ Gary S. Rasmussen Engineering Geologist, EG 925

Dist: Pacific Foundation Engineers (6)

-14-

GARY S. RASMUSSEN & ASSOCIATES

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- - - - - - - - - - - - -GARY S. RASMUSSEN & ASSOCIATES Pacific Foundation En&ineers

Rial to, Cail.iforn1a 0 50 100 200 300feet Project No. 1213

6/20/ 77 SCALE 1"=150'

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BANK~ \ \\\\ FORMER RIVER \ \ \

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II 11 11 r I 11 I I f I 11 11 I f 1111111111111 r I I 1111111111111111 I 111 111 111111 I I

TRACT 9001

APPROXIMl\TE BOUNDARY--

TRACT 9812

IAL STUDIES ZONE AL UIST PRIOLO SP

RIVERSIDE AVENUE

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TRENCH l---1~

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EXPLANATION

W~B River-<:hannel depooit.s

Unc01t•olida~d coar•e gra.•el, ·•a.nd, and •ilt in major riwr channel•: l&iohl11 pe'"""4ble, l.a.roel'JI "naaturated, bait tran•mit• lar11e •eepa11e lo•••• from river• 11-nd •trcama to Uie ma.in watn' bod.11

LOCAL UNCONFORMITY

Duneaand llnam•olidated coar•e to fiu •and; oemra.ll11 a:nchored

b11 woeta.tion but in pa.rt activel11 drifting

LOCAL UNCONFORMITY

E1 Younger alluvium

Unc<mttolidatm coa.r•ie ora.vel, •aml, silt and cl.av,· con•i•t• of two ttniU in the San &rnardino Valley which are in pla.cea indiat:inguiahabU, bu.I loca.U11 the upper mfl'mber confine• tlwr ma.in water bod11: lower member where within the ~~ of water-level jZ1"tuationa ill hiahl'JI permeable and J(ielda wa.kr copimulv to wells

•• 0 •

oO~· •••

Older alluvium Unconaolidated coarae gravel, aand, ailt, and cla.11, in

places deeply ~athered with red and 11ellow soil zones; locaU11 contains thU:k cla11 beds which confine underlvina a.quifera; fractured b'I/ numerov.• faults and in place• •lightl11 foldedj 'flield• water copioual11 to wella that penetrate permeable member•

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NDEX MAP GEOLOGIC I_ Engineers dat1on Pacific Foun California

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Rialto, d

Legen ashed where Fault, ddotted where ------······ approx.

-- -- buried

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6 CALIFORNIA DIVISION OF MINES AND GEOLOGY SP 42

lhe area believed by lhe Slate Geologist to warrant. special geologic investigations to confirm the presence or absence or hazardous fault~.

Locations or special studies zone boundaries arc controlled by the position or the known traces or potentially acuvc faults (defined below). However, the faults shown on the special studies zones maps were not field checked during the compilation or these maps. Because available fault data range widely in quality, and the locations or some faults arc known imprecisely, the zone boundaries are positioned at a reasonable distance (about 660 reel, or 1/8 mile) on both sides or the trace of the nearest potentially active fault. However, zone

· boundaries generally arc more or less than 660 feel away from mapped faults because of I) curved or multiple fault traces. 2) the practical advantage in keeping the number or turning points .lo a reasonable minimum, and 3) the quality of the data dictates a narrower or wider zone.

Definitions

Fault and Fault Zone

A fault is defined as a fracture or zone of closely associated fractures along which rocks on one side have been displaced with respect to those on the other side. Most faults are the result of repeated displacement which may have taken place suddenly and/or by slow creep. A fault zone is a zone of related faults which commonly are braided and subparallel, but may be branching and divergent. A fault zone has sjgnificant width (with respect to the scale at which the fault is being considered, portrayed, or investigated), ranging from a few feel to several miles.

Fault Trace I . .

A fault trau is the line formed by the intersection of a fa tilt and the earth's surface. It is the representation of a fault as del'icted on a map, including maps of the special studies zones.

Potent/ally Active Fault

For the purposes of delineating special studies zones, any fault considered to have been active during Quaternarr time (last 2 to 3 million years, figure 2}-on the basis of evidence of surface displacement-is considered by the State Geologist to be potentially actiw:. An exception is a Quaternary fault which is determined, from direct evidence, to have become inactive before Holocene time (approximately the last 11,000 years). Such faults are presumed to be essentially inactive and most have been omitted from the maps. Although potentially active faults shown on the maps may have been active during any part of, or throughout, Quaternary time, evidence for their recency of displacement commonly is inco.mpletely preserved or equivocal. I In contrast, the State Mining and Geology Board, in their Policies and Criteria (adopted 21 November 1973), defined only those faults which have had surface displacement within Holocene time as "active and hence as constituting a potential hazard."

t In those cases where data are avaiJable from fie.Id and aerial pholographic investigations specifically directed al the problem of potcnlial surface faulting, the potentially active faults shown on the special studies zones maps may not be directly rclatablc to the entire Quaternary Period. For example, the .. recently active" fault traces depicted on some U.S. Geological Survey maps arc believed to be predominantly of late Quaternary age and may exclude most fauhs that have become inactive during or prior to early Quaternary time.

GEOLOGIC AGE YEARS BEFORE

En Period Epoch

1iisloric~

Holocene

QUATERNARY

" ~ Pfcisloceac

~ f'Uocenc

TERTIARY

pre-Pliocene

p-e..CENOZOJC time

l....--.i Heainniq ul JCOlcsic 1imt :

PRESENT (estimated)

200

11.000

2.000.000 - 3.000.000-

7.000.000 - 10.-000.000-

tiS.000.000

I <l.b00.000.oOO

}

F9ulls along which mowement hu occunlld during lhi• lntetval •• dl!lllned • 8CllN ~ Polides snd Cril•la DI U'• S.111• Mb'ling Ind Geology Board.

Faults defined • potenUaJly active tar lhe pur~ pose al delineating special sludi• xnes.

Figure 2. Geologic lime scale


11171> !'AULT HAZARD ZONES IN CALIFORNIA 7

Uses and Limitations of Special Studies Zones Maps

The special studies zones arc delineated to define those areas within which special studies arc required prior to building structures for human occupancy.

·Traces of potentially active faults are shown on the maps mainly to justify the locations ofzone boundaries. These fault tra~es are pinned as accurately as the sources of data permit; yet the plots arc not sufficiently accurate to be used as the bases for set-back requirements, and they should not be so used.

The State Geologist has identified potentially active faults in a broad sense, and the evidence for potential activity of some faults may be only weak or indirect.

The fault information shown on the maps is not sufficient to meet the requirement for special studies. Local governmental units must require the developer to evaluate specific sites within the special studies zones to determine if a potential hazard from any fault, whether

hcrctororc rccngnizod or not. exists with regard tn proposed structures and their occupants.

The surface fault ruptures associated with historic earthquake and creep events arc identified where known. However, no degree of relative potential for future surface displacement or degree of hazard is in1-plied for the faults shown. Surface ruptures resulting from the secondary effects of seismic shaking during historic time are omiued from the map and do 11ot · serve as a basis for zoning.

Model Ordinance and Code

In order to assist the cities and the counties to implement the Alquist-Priolo Act, a suggested Model Ordinance and Code was wrillcn by James E. Slosson, former State Geologist. This publication is available from the Division of Mines and Geology, P.O. Box 2980, Sacramento CA 95812. for 25 cents plus tax.

SUMMARY OF POLICIES AND CRITERIA PURSUANT TO THE ACT

Policies and Criteria, called for in the Alquist-Priolo Act, were intially adopted on November 21, 1973. by the State Mining and Geology Board following a review by all counties a_nd incorporated cities in California. The Policies and Criteria were subsequently expanded and slightly revised July J, 1974, and June 26, 1975. Future revisions may be anticipated in order to assist the cities and counties in implementing the Act.

Table 2. A summary of policies and criteria adopted by the State Mining and Geology Board. Complete text is in Appendix B.

POl.ll'l 1'S Sl'~.l"lFll" UHIHHA

I. Sp1....:ilit.~ 1ha1 1h1.• Acl is 11111 n.:1ruacli\·c. I. No s1ruc1un.•s tor hu1114111 occupan~y nrc pcnni1h.·t.l on 1hc lnH:c ut' un ac1ive 1·auh. 1lh1l,:ss proven •11her"is.i:. the 11rca \\·i1hin ~O

!. Svgges1s nu!thotls rcl;i1ini tu rcvie" nl" Prelitninary M:tps prior fec1 of an aetiv..: h1uh is. prc'!.u111cJ In h..: untlcrh1in hy an uc1ivc lu iSl'iuancc of ()tli.,;:i:1I Maps. l~1uhJ.

). P11licics anJ crilcria appl) uni~ . .. area wilhin th"· spccial , Kcquir"-s gr..•olugic '";purl Jir"·i:1"•J ill 1hc pr11hlc1n of p111c111ial S1uJi1i."1' :1:uni:s. surhn:c fau11ing. h1r all prujcc1s deli nee.I hy 1hc Ac:t tScc:1i11n

1h11.h). ~. t>clincs 1H·ti1·" tj111l1 (cqq;1ls p111cn1ial hazurJ) as a fuuh 1h;i1 h:u;

had surfac"" displ:11i.-...·1n1i.'111 during H11hK.-..."llC 1i1nc th1s1 11.DOO ). R"·quircs 1lnn g.c11l11g.ic r"·puns h..- pl;u,:1:d 1111 11pcn file fly the ycars). St;uc ( i1i.-.1lugis1.

~- M.1:quircs cities and cuuntics 111 rc\.iC\\ ;,1Jcquac)" ul g.r..-.1l11gi\:' rcrw1rt!I. suh111i11eJ "i1h requr..·s1s li~r de,·et11p1ucn1 Jli:nnits.

~. Pennil"" "·i1ie1rio :.ind cuunlies 111 e'!.lllhlish M:uulard!I. 1uorc rcs1ric~ th'-· 1·lt<111 1h1i.· pulicir..•)rr. illlll cri1erh1:

"· llelinc ... (ll) ··pru.iccl°·. (t'tJ Mruc1urc for hu1na11 occupancy. and (CJ 11ft,.°\\ real .: ... liuc '"'"'\t.."l11pn11.:n1.


REFERENCES

Allen, C.R., Saint Amand, P., Richter, C.F., and Nordquist J.M., 1965, Relationship between seismicity and geologic structure in the southern California region: Seismological Society of America Bulletin, V. 55, no. 4, p. 753-797.

California Department of Water Resources, 1964, Crustal strain and fault movement investigation-Faults and earthquakes in California: Department of Water Resources Bulletin 116~2.

California Department of Water Resources, 1970, Meeting water demands in the Bunker Hill-San Timoteo area: Department of Water Resources Bulletin 104-5.

Dutcher, L,C., and Garrett, A.A., 1963, Geologic a"nd Hydro logic features of the San Bernardino area, California, with special reference to under-flow· across the San Jacinto fault: U.S. Geologi1cal: :survey·Water.·,Supply Paper 1419.

Eckis, Rollin, 1934, Geology and ground water storage capacity of valley fill-South Coastal Basin Investigation: California Division of Water Resources Bulletin 45.

Fairchild Camera, 1930, Black and white stereoscopic aerial photos, Flight C910, no's. 77 & 78, l"= approx. 1700'.

Fife, D.L., et. al., 1976, Geologic hazards in southwestern San Bernardino County, California, California Division of Mines and Geology: Special Report 113.

Friedman, M.E., Whitcomb, J.H., Allen, C.R., and Hileman, J.A., 1976, Seismicity of the southern California region, l Jan. 1972 to 31 Dec. 1974, Seismological lab., California Institute of Technology.

Greensfelder, R.W., 1972, Crustal movement investigations in California: Their history, data, and significance: California Division of Mines and Geology Special Publication 37.

Greensfelder, R.W., 1974, Maximum credible rock acceleration from earthquakes in California: CDMG Map Sheet 23.

Hart, E.W., 1976, Fault hazard zones in California: California Division of Mines and Geology Special Publication 42, Revised Edition,

Hileman, J.A., Allen, C.R., Nordquist, J.M., 1974, Seismicity of the Southern California region, l January 1932 to 31 December 1972: Seismology Laboratory, California Institute of Technology, Pasadena, California

Jennings, C.W., 1975, Fault map of California with locations of volcanoes, thermal springs, and thermal wellS: - CDMG, 1 :750,000.'

GA.RYS. RASMUSSEN & ASSOCIATES


Hileman, J.A., Allen, C.R., Nordquist, J.M., 1974, Seismicity of the Southern California region, 1 January 1932 to 31 December 1972: Seismology Laboratory, California Institute of Technology, Pasadena, California.

Jennings, C.W., 1975, Fault map of California with locations of volcanoes, thermal springs, and thermal wells: CDMG, 1:750,000.

Lamar, D.L., Merifield, P.M. and Proctor, R.J., 1973, Earthquake recurrence intervals on major faults in southern California, in Moran, D.E. an others eds., 1973 Geology, seismicity and environmental impact: Association of Engineering Geologists Special Publication, p. 265-276,

Moreland, J.A., 1972, Artifical recharge in the Upper Santa Ana Valley southern California: U.S. Geological Survey Open-file Report.

Morton, D.M., 1975, Relations between major faults, eastern San Gabriel Mountains, southern California jn Abstracts with programs. Cordilleran section, 7lst Annual Meeting, Geological Society of America, p. 352-253.

National Aeronautics and Space Administ.ration. March 15, 1974, Color infra-red aerial photographs, Mission 74-041, 7591, 7591, Scale 1:125,000.

Proctor, R.J., 1973, Personal Communication

City of Rialto Planning Department, August, 1975, Seismic and pyblic safety element. Preliminary study.

Rogers, T.H., 1967, Geologic map of California, Olaf P. Jenkins edition San Bernardino Sheet: California Division of Mines and Geology, 1:250,000.

Schnable, P.B. and Seed, H.B., 1973,Accelerations in rock for earthquakes in the western United States: Seismological Soci~ty of America Bulletin v. 63, no. 2.

Sharp, R.V., 1972, ~ap showing recently active breaks along the San Jacinto fault zone between the San Bernardino area and Borrego Valley, California U.S. Geological Survey Miscellaneous Investigations Map 1-675.

Sharp, R.V., 1975, En echelon fault patterns of the San Jacinto fault zone in Crowell, J.C., ed., 1975, San Andreas Fault in southern California, a-guide to San Andreas fault from Mexico to Carrizo Plain: California Division of Mines and Geology Special Report 118, p. 147-152.

Thatcher, W.J., Hileman, J.A.,and Hanks, T.C., 1975, Seismic slip dislocations along the San Jacinto fault zone southern California and its implications: Geological Society of America Bulletin 86, p. 1140-1146.

Troxell, H.C. and others, 1942, Floods of March 1938, in southern California U.S. Geological Survey Water Supply Paper 844.

Unites States Department of Agriculture, January 23, 1953, Aerial photographs AXL-31K-136, 137.

0-.ARY S. RASMUSSEN & .ASSOCIATES


Wallace. R.E., 1970, Earthquake recurrence intervals on the San Andreas fault: Geological Society of America Bulletin, v. 81, p. 2875-2889.

Willingham, C.R., 1968, A gravity survey of the San Bernardino Valley southern California: unpublished M.A. thesis, University of California,

- Riverside, California.

GARY S. RASMUSSEN & ASSOCIATES

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