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1/35

STUDY

OF

SEISMIC ACTIVITY BY SELECTIVE TRENCHING ALONG THE

ELSINORE

FAULT

ZONE, SOUTHERN CALIFORNIA

D . L . Lamar and

S . C .

Swanson

Lamar-Merifield, Geologists

1318 Second Street,

Suite

2 7

Santa Monica,

California 90401

USGS

CONTRACT

N O . 14-08-000 1-19 144

Supported by

the

EARTHQUAKE HAZARDS REDUCTION PROGRAM

OPEN-FILE

N O .

81-882

U.S. Geological

Survey

OPEN

FILE REPORT

This

report was

prepared under

contract to the

U.S.

Geological

Survey

and has not been reviewed for conformity

with

USGS editorial standards

and stratigraphic nomenclature. Opinion s and conclusions expressed

her ein d o not necessarily represent

those

of the USGS. Any use

of

trade

names i s

for

descriptive purposes

only and does not imply

endorsement by

the USGS.


2/35

ABSTRACT

Published literature

and

engineering

geology reports

bearing

on

potential seismicity

of

the Elsinore fault

zone

were reviewed. A field

reconnaissance was

accomplished

t o

locate promising sites where trenches

might reveal datable Holocene fault displacement history,

and

possibly

times

between major seismic

events.

Nine

sites which

have

the

potential

of

revealing

ruptured

Holocene sediments

across

strands o f the Elsinore

fault zone have been identified.

Permission

t o trench

three

sites

has

been obtained and

a

site on

the

south branch o f

the Wildomar

fault,

a

strand o f the Elsinore

fault zone southeast

o f

Lake

Elsinore,

has

been

trenched.

Radiocarbon

dating

of

sediments disrupted

by the

south

branch of

the

Wildomar fault

indicates activity

within the

past

4120

+

260

years.

The

relationship

between

the

dated

sediment and

two distinct

sets

o f

secondary

faults

which were active

before

and after the deposition

of a

gravel

layer

indicates that one or possibly

two

seismic events

occurred

since deposition

o f

the dated sediments. T he lack of correlation

of

sediments on

opposite

sides

o f the main

fault, the orientation

o f

a

sub-parallel

drag fold and

slickensides in

a

silty

clay layer

suggest an

important

but unknown component

of

right-slip.

1 1


3/35

CONTENTS

Page

ABSTRACT ii

INTRODUCTION

1

DISPLACEMENT

HISTORY

3

PRESENT

INVESTIGATION 9

Proposed

Trench Locations ................

9

Glen

Ivy Fault,

Site 1 ................. 9

Faults on

Margins

o f Lake Elsinore,

Sites

2 - 5 ..... 1 3

Wildomar Fault,

Site

6

.................

1 3

Wolf Valley

Fault,

Site 7 ...............

1 3

Wolf

Valley Fault, Site 8

...............

1 3

South

Branch

o f

Elsinore

Fault,

Site 9

.........

1 5

Results

o f Trenching

Across

Wildomar Fault, Site 6 .... 1 5

CONCLUSIONS ........................

2 2

FUTURE PLANS

22

ACKNOWLEDGMENTS 2 2

REFERENCES ............ ............. 23

APPENDIX

A : Annotated list

o f

engineering geology reports

on file with

Riverside County relating

t o the

Elsinore fault zone

..............

2 8

APPENDIX B : Letter from Teledyne Isotopes, dated 5

Janu

ary 1 9 8 1 , with

radiocarbon

ages on sample

o f sediment

from

trench a t Site 6

.......

3 1

111


4/35

ILLUSTRATIONS

Page

Figure 1 Map showing principal

faults

in southern Cali

fornia

and

areas covered by

detailed

maps

along

Elsinore fault zone 2

Figure

2

Map showing

Elsinore

fault

zone

between Banner

Canyon

and

Aqua

Caliente

Springs ..........

5

Figure 3 Map showing Elsinore fault

zone

between Corona

and Lake

Elsinore

and

excavation

sites

....... 1 0

Figure 4 Map

showing

Elsinore fault zone between

Lake

Elsinore

and

Temecula area and

excavation

sites . .

11

Figure 5 Map showing

trenches

across closed depression

bounded

by strands

o f

Wildomar fault, Site 6

.... 14

Figure 6

Generalized

trench log and detailed plan

view of

fault

zone,

south branch

of

Wildomar

fault

..... 1 6

Figure Explanation

of

lithologic symbols on Figs.

6 ,

8 and

9 .........*............. 1 7

Figure 8

Detailed

trench log between 6.5 and 12.0 meters . .

18

Figure 9 Detailed trench log between

12.0 and 16.5 meters . 1 9

TABLE

Table 1 Data on possible

excavation sites

on

recently

active strands

of

Elsinore fault

.......... 1 2


5/35

INTROD U CTION

California's historic record i s t o o short t o estimate earthquake recur

rence because the first

recorded

earthquake

occurred in

1 7 6 9 , and instrumental

seismology first began

in 1933

(Alien

^t

a l ,

1 9 6 5 ) .

In

an

attempt

t o

extend

t h e seismic record, t h e magnitude and fault displacements during historic

earthquakes

and t h e

long-term

slip

rates based on geologic data have been used

t o

estimate earthquake recurrence intervals on

t h e

San Andreas

and

other

major

faults in

southern California

(Wallace, 1 9 7 0 ;

Lamar,

Merifield

and Proc

t o r , 1 9 7 3 ) . This approach may provide a

rough

determination o f t h e long-term

average recurrence intervals. However, such estimates are subject t o error

because

o f uncertainty

in

dating t h e

displaced

rocks (Greensfelder, 1 9 7 4 )

and large scatter i n data

relating earthquake

magnitude and fault

displacement

(Bonilla and Buchanan,

1970).

Significant local (Ambraseys,

1 9 7 0 )

and world

wide

(Davies and Brune, 1 9 7 1 ) time

variations in

the

level

o f

seismicity also

reduce

t h e

reliability o f this method.

Information

on

t h e

displacement

o f late Quaternary sediments may

provide

t h e

best

means

o f

evaluating the

seismic

hazard on individual faults ( A l i e n ,

1 9 7 5 ) . Dating o f Holocene

materials in trench exposures

across

fault ruptures

has

revealed evidence

o f

prehistoric earthquakes

on the

S a t i Jacinto

fault

( C l a r k , e t a l , 1972), t h e San

Fernando

segment o f the

Sierra

Madre fault

system

(Bonilla, 1973)

and

along

t h e

south-central reach

o f

the San

Andreas

fault

( S i e h and Jahns, 1 9 7 6 ; S i e h , 1 9 7 7 , 1978). Additional trench

exposures

in

late

Quaternary

materials across other active

and potentially active faults

i n

southern California are required

t o

predict the approximate times and

magnitudes

of

future earthquakes

t o

be generated by individual

faults.

The

northwest

trending Elsinore fault

zone extends a

distance

o f 200 km

( 1 2 5

miles) from directly north o f

the Mexican

border t o

the

northern end

o f t h e

Santa

Ana

Mountains

( F i g .

1 ) .

At the northern end o f t h e Santa Ana

Mountains,

the

Elsinore

fault appears

t o

split

into the

Chino

fault,

which

continues north-northwest

near

the

eastern

margin o f the

Puente Hills, and

the Whittier fault, which

continues

west-northwest along

t h e

southwest side

o f t h e Puente Hills ( G r a y ,

1 9 6 1 ;

Weber, 1 9 7 7 ) . The Whittier fault

extends

over a distance o f

3 2

kilometers ( 2 0 miles) from

t h e

Santa Ana River t o t h e

northwest

end

o f t h e Puente

Hills.

At t h e northwest end o f t h e Puente Hills,

the Workman Hill and Whittier

Heights

faults

appear

t o split

from

t h e Whittier


6/35

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7/35

fault (Daviess and Woodford,

1 9 4 9 ) .

The continuation o f these faults

t o

the

northwest

i s

obscured by

the recent

alluvium

o f the San

Gabriel Valley.

Farther northwest

in the

Elysian Park-Repetto Hills area, several northwest-

trending faults form

an 8-kilometer

( 5 - m i l e )

wide

zone of

complex structure;

i t h a s

been suggested

(Lamar, 1 9 6 1 , 1 9 7 0 ) that this zone

represents

a

widened

northwest continuation o f the Whittier fault z o n e . To

the

northwest,

t h e

Eagle Rock

and Whitney-Verdugo faults may

represent

principal strands o f

the

Whittier-Elsinore fault zone

( L a m a r , 1 9 7 0 ;

Proctor e t a . 1 ,

1 9 7 2 ) .

Although

the

Elsinore fault zone i s a major strand of the San Andreas

fault system in southern California (Crowell, 1962), it has not been the site

o f a major earthquake in

historic

t i m e .

Based on length

and

evidence

o f late

Pleistocene

or

Holocene displacement, Greensfelder

( 1 9 7 4 ) has

estimated

that

t h e Elsinore fault zone i s reasonably capable o f generating

an

earthquake

with a magnitude a s large a s

7 . 5 .

The Whittier-Elsinore fault zone must be

considered

t o

have

the potential for

generating

a major earthquake within

or

i n

close proximity

t o the

southern California

metropolitan a r e a .

However,

information on prehistoric earthquakes

i s

inadequate

t o estimate t h e probable

recurrence interval

and magnitude o f

future

events.

This

report summarizes

previous published

data on

displacement on

t h e

Elsinore fault zone and

d e

scribes t h e

results

t o

date o f

an attempt

t o better define the

seismic

potential

by

review

o f

engineering geology reports

on

seismic hazards prepared

t o

satisfy the

Alquist-Priolo Special Studies

Zones

Act

o f

1972

( H a r t , 1 9 8 0 )

and

study

o f

displaced Holocene sediments in trench exposures across fault

ruptures

within

the Elsinore fault z o n e .

D ISPLACEMENT HIS T ORY

Apparent offset o f facies and

thicknesses

within Paleocene sediments

along

the Whittier-Elsinore fault i n t h e

area northwest

o f

Lake Elsinore

( F i g .

1 ) suggests

3 0

t o

40

kilometers

( 2 0

t o

2 5

miles)

of

right-slip ( L a m a r ,

1 9 6 1 ; Yerkes and Campbell, 1 9 7 1 ; Sage 1973).

However,

Woodford e t a l

( 1 9 7 2 )

believe that large post-Cretaceous strike-slip on

the

Whittier-Elsinore fault

zone

i s

precluded by

the

distribution

o f

distinctive

Upper

Cretaceous

and

Paleocene strata on opposite sides

o f this

fault z o n e . Based on t h e

distribution

o f

older rocks o f

the

southern California batholith

and

associ

ated

metamorphic rocks, Weber ( 1 9 7 7 )

has estimated that the total

right-

slip

along the Elsinore

fault

in

the

same area

i s

about 9-11 km

(6-6%

miles).


8/35

Woodford ( 1 9 6 0 )

and

Woodford

e t

al

( 1 9 7 1 )

believe

that

post-Miocene

right-slip

on

the Elsinore-Whittier-Chino fault zone has been 5

km

( 3 miles)

or l e s s , and Yerkes and Campbell ( 1 9 7 1 ) suggest

t h a t

most o f t h e right-slip

occurred i n the middle Miocene. However,

stratigraphic relations

o f upper

Miocene and Pliocene rocks along the Whittier fault in the Puente Hills

in

dicate

4600 meters (15,000

f e e t ) o f

right-slip

(Yerkes, 1972). This i s

similar t o

the

5 kilometer ( 3 - m i l e ) offset

suggested by Lamar

( 1 9 6 1 )

for

lower

Pliocene

marine fans described by Conrey ( 1 9 5 9 , 1 9 6 7 ) , and the 5-km ( 3 - m i l e )

offset

o f a

facies boundary between sandstone

and

conglomerate in

a n

unnamed

Pleistocene

unit between

Lake

Elsinore and Murrieta described by Kennedy

( 1 9 7 7 ) . An

ash

horizon

within this

unit

has been correlated

with the 0.7

m . y .

old Bishop

Ash

(Merriam and Bischoff, 1 9 7 5 ;

and

personal communication

from A .

Sarna-Wojcicki, in

Kennedy, 1977,

page

5 ) .

Much smaller displacements have been

reported

along

t h e

southeast seg

ment

o f

the Elsinore

fault shown

on

F i g .

2 . Weber

( 1 9 6 3 ) has

noted

that

basement

rocks

in

the

Julian area

( F i g .

2 ) display only about 600 meters

( 2 0 0 0

f e e t )

o f right separation. To

the

southeast, Gastil and Bushee ( 1 9 6 1 )

and Hart ( 1 9 6 4 , 1 9 7 4 )

show right separations

o f 7 6 0 t o

2400 meters

(2500 t o

8000

f e e t )

on the

margins o f a

Bonsall Tonalite body exposed

in

Rodriquez

Canyon ( F i g .

2 ) .

Moyle ( 1 9 6 8 ) ,

Hart ( 1 9 7 4 )

and

Lowman ( 1 9 8 0 ) question

whether

t h e

Elsinore fault can be a continuous feature between Mason and

Vallecito

Valleys.

A

continuous

fault

would

have

t o

c u t

bedrock

on

a

ridge

between

the

valleys

at

Campbell Grade ( C G , F i g . 2 ) . Lowman ( 1 9 8 0 ) shows

that

foliation

in

metamorphic rocks exposed along

this

ridge

i s

uninter

rupted

across the

inferred Elsinore fault

t r a c e , a s

shown on previous maps

( M e r r i a m , 1955;

Strand,

1 9 6 2 ;

Weber, 1963). Farther

southeast, D r .

Robert

V . Sharp ( 1 9 6 8 ; personal

communication,

1 9 7 2 )

reported that displaced cata-

clastic zones

within

plutonic rocks along t h e margins of Vallecito Valley

( F i g . 2 )

limit

t h e

amount

o f

right-slip

on the Elsinore fault t o about

5

kilometers

( 3 miles)

or

l e s s . The

cataclastic

zones are

probably

at

least

a s

old a s Middle

Cretaceous

( S h a r p , 1 9 6 7 ) . Although t h e exact re

lationships

are obscured by alluvium,

the

north

end o f

the Thing Valley

fault south of Agua Caliente Hot

Springs

appears t o be displaced

700-1300

meters (2300-4300 f e e t ) in a right-lateral sense by the south branch

o f

t h e

Elsinore

fault, a s mapped by Merriam

( 1 9 5 5 )

and

Buttram

( 1 9 6 2 ) .

The

Thing

Valley

fault

was identified by study o f satellite imagery (Merifield

and Lamar, 1 9 7 6 ;

Lamar and Merifield,

1 9 7 6 ) .


9/35

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10/35

The

discrepancy

between t h e pre-Pliocene

displacement northwest

o f

Lake

Elsinore and the

displacement

i n t h e Julian-Vallecito Valley area ( F i g . 2 )

could

easily be accounted

for in t h e 1 0 0

kilometers ( 6 0

miles)

which separate

the

areas. Allison

(1974abc) has

suggested

that

a portion o f the right-slip could be

distributed on the Chariot

Canyon

fault

which

branches from

the

Elsinore

fault

( F i g . 2 ) .

Several northwest

t o

east-west trending

faults also

appear

t o splay

off

from t h e Elsinore fault southeast o f Lake Elsinore

( F i g .

1 )

( M a n n , 1 9 5 5 ; Rogers, 1965). Mann ( 1 9 5 5 )

described

stream offsets and

hori

zontal striae on slickensided surfaces along these faults in the Temecula

a r e a . The faults

studied

by M a n n , and

other

faults (Rogers, 1 9 6 5 ) ,

continue

for a

number

o f

kilometers

t o t h e southeast;

additional

right-slip on the

northwest

portion o f the

Elsinore

fault zone could

be distributed on these

faults. Detailed published maps o f t h e area southeast

o f t h a t

described by

Mann are not available.

However, Rogers

( 1 9 6 5 )

shows

abrupt changes

in

basement rock type across t h e

Agua

Caliente, Lancaster and

Aguanga

faults.

Lowman ( 1 9 8 0 )

has suggested

that the

discrepancy in the

amount o f

right-

slip on

t h e

northern and

southern

portions of

t h e

Elsinore fault may

be

accounted

for

by

the

down-faulting

o f the

Perris Basin,

Warner Valley, and

perhaps

Mason

Valley, a s pull-aparts (Crowell and Sylvester, 1 9 7 9 , p . 1 4 5 ) .

More o r

less

continuous d isplacement along

t h e

Whittier-Elsinore fault

during late

Miocene,

Pliocene and Quaternary

i s

indicated by t h e following:

1 . Diabasic intrusive

rocks

o f early late

Miocene

age are

associated

with

the

Whittier fault and were probably intruded along the

fault (Durham and Yerkes, 1964).

2 . Locally

along

the south side o f the Whittier fault i n the

Esperanza

area folded

late

Miocene

rocks are overlain

unconformably by early Pliocene strata

( L a m a r , 1 9 6 1 ) .

The

folding dies

out with

distance from

t h e

fault

and i s inter

preted t o be

the

result

o f

drag secondarily related t o post

late Miocene,

pre-early Pliocene

slip

on the

Whittier

fault.

Similar relations were observed near

a

possible branch

o f

the

Whittier fault

i n t h e

Repetto Hills area ( L a m a r ,

1 9 7 0 ) .

3 . Several unconformities are present in upper Miocene strata

in the

Sansinena Oil

Field

adjacent

t o t h e Whittier fault

in

the Puente Hills; these unconformities and the distribution

o f

coarser sediments

in

upper

Miocene

rocks strongly suggest

fault movement

o r

folding during late Miocene time

(Woodward,

1958).


11/35

4 . Foraminiferal studies i n t h e

Sansinena Oil

Field

indicate

considerable Miocene landslide debris i n Pliocene sediments

i n t h e

down-thrown

south

block

o f t h e Whitter fault (Wood

ward, 1958); this

could

b e

explained by fault displacement

during

t h e Pliocene.

5 . Kundert ( 1 9 5 2 ) also suggests that unusual sedimentary clasts

i n

the Repetto Formation

(lower

Pliocene) south o f

t h e

Whit-

tier

fault

may

have

been

deposited by

turbidity currents

originating

on

a

steep slope

formed

by

movement along t h e

Whittier f a u l t .

6 .

An

angular unconformity between

the

upper Pliocene Pico For

mation and Pleistocene

La

Habra Formation (Kundert, 1 9 5 2 )

may also be

t h e result o f

post upper

Pliocene

movement

and

drag folding

along

t h e

Whittier fault.

7 .

Basalts with an average age o f 9.6

0 . 7

m.y.

( K e n n e d y , 1 9 7 7 )

have been relatively down-dropped

1000

meters

( 3 3 0 0 f e e t ) i n

the Murrieta

graben

along t h e

Elsinore

fault zone ( M a n n , 1955).

8 . Post Pleistocene

displacement

i s

indicated

by

t h e

fault con

tact between

t h e

La Habra Formation and older rocks and by

the steep dips in the

La

Habra Formation along t h e south edge

o f

the

Puente Hills

( Y e r k e s ,

1972).

Mann

( 1 9 5 5 )

and

Kennedy

( 1 9 7 7 ) also report folded and

faulted

Pleistocene strata

along

the Elsinore fault z o n e ,

southeast

o f Lake Elsinore, and

Jahns ( 1 9 5 4 )

shows northwest

trending

faults

north

o f

Lake

Henshaw which cut

valley

f i l l .

9 . Right lateral offset o f stream courses o f up

t o

2600 meters

( 8 8 0 0

f e e t ) have been

reported by

Durham and Yerkes ( 1 9 6 4 )

on

the

Whittier

fault

along t h e

south

edge o f the Puente Hills.

Along much

o f

its length, the

Elsinore

fault

underlies

long

straight canyons s o

that

right-slip would not

be

reflected

i n displaced minor

drainage courses. However,

right lateral off

s e t o f streams

along

strands o f t h e Elsinore fault zone between

the

Santa Ana River

and

Lake Elsinore

have been reported

by


12/35

L a r a a r

( 1 9 5 9 )

and Weber

( 1 9 7 7 ) , in t h e

Wildomar-Temecula

area by Kennedy ( 1 9 7 7 ) ,

and in the

Julian area by Lamar and

Merifield

( 1 9 7 6 ) , and

along

t h e southeastern part by Clark

( 1 9 7 5 ) .

1 0 . Scarps and offset o f

colluvium

and alluvium have been

reported

along

the

Whittier

fault

by

Durham

and

Yerkes

( 1 9 6 4 ) ,

Nicoll

( 1 9 7 0 ) and Hannan

e t _ a l

( 1 9 7 9 ) , t h e Chino fault by Lewis ( 1 9 4 1 ) ,

Lamar

( 1 9 5 9 ) ,

Gray

( 1 9 6 1 )

and Weber

( 1 9 7 7 a ) ,

and

t h e

Elsinore

fault by Weber

( 1 9 7 7 a ) ,

Kennedy

( 1 9 7 7 ) ,

Jahns

( 1 9 5 4 ) , and

Buttram

( 1 9 6 2 ) . Carbon 1 4 age dating of

organic

material

within displaced

alluvium indicates

movement on

t h e

Whittier

fault

within

t h e past

2185

+

1 0 5 years

(Hannan

t

a l , 1 9 7 9 ) .

1 1 . Sags

or closed

depressions have been reported along strands

of

the

Elsinore

fault zone by

Jahns

( 1 9 5 4 ) ,

Engle

( 1 9 5 9 ) ,

Kennedy ( 1 9 7 7 ) ,

and

Sharp ( 1 9 7 8 ,

p .

1 9 4 ) .

Based

on study

o f Los

Angeles

County

Engineer

level l i n e s ,

Lamar and

Lamar

( 1 9 7 3 ) have noted

t h a t t h e

synclinal area south

o f t h e

Whittier fault

between the Puente

and

Montebello Hills and Santa Fe Springs-Coyote Hills

trend

i s

subsiding

a t . 3 t o 1 . 2 cm/year ( . 0 1 t o . 0 4 ft/year). The relative

motion across

the

Whittier fault could be caused by withdrawal

o f

ground

water o r compaction

of

sediments rather than tectonic movement.

The

tabulation

o f

earthquakes

o f

Richter magnitude

4.0

and greater

between 1934 and 1961 and larger earthquakes since 1906 by the California

Department

o f

Water Resources ( 1 9 6 4 ) indicates

t h a t

a destructive earthquake

has not occurred along the Whittier-Elsinore fault in historic t i m e . However,

Townley and Alien

( 1 9 3 9 )

describe

an

earthquake

which apparently

centered in

t h e

Lake

Elsinore

region

May

1 5 ,

1 9 1 0 , as

follows:

At

Corona,

Riverside County,

a chimney was

shaken

down and plastering

f e l l . In

Cold

Water

Canyon

in

Temescal Mountains,

the

shock

was

exceedingly

heavy.

At Temescal i t was the

most

severe for at least

twenty years, toppling chimneys

and overturning

chairs.

At

Wildomar,

rocks

rolled

down

hillsides,

bricks

fell

from chimneys, and books were thrown

from

shelves. Elsinore reported the

hardest

earth

quake i n years, but no damage Corona newspaper.


13/35

Richter

( 1 9 5 8 , p . 5 3 3 ) has

suggested

that this earthquake originated on the

Elsinore

fault and

estimated

the magnitude

a s

6 or

greater.

A

net

of

portable seismometers

was

operated

in the

Puente Hills along

t h e

Whittier fault between July 1971 and April

1972 ( L a m a r , 1 9 7 2 ;

Lamar and

Stewart,

1973).

Epicenters

o f 3 1 microearthquakes with a maximum magnitude

o f 3 . 0 were

determined. Sufficient

data were

available

t o

determine

hypo-

central

depths

for 1 7 events. Assuming a range o f 6 0

t o

7 0 degrees north

d i p on the Whittier fault,

8

o f t h e 1 7

hypocenters

lie

on

the

subsurface

projection o f the Whittier f a u l t . Langenkamp and Combs ( 1 9 7 4 ) also monitored

microearthquakes along

the

Elsinore fault zone between Corona and just north

o f

t h e Mexican

border.

PRESENT INVESTIGATION

Proposed Trench Locations

The Alquist-Priolo Special

Studies Zones Act

o f

1972 requires engineer

i n g

geology seismic hazards

reports

for development

o n property

situated

on

strands o f the

Elsinore

fault zone ( H a r t , 1 9 8 0 ) . Such reports

on

file with

Riverside County were reviewed

for

trench and other data on

recent

displace

ment. The pertinent data from these

reports

are summarized in Appendix A ;

t h e locations o f t h e trenches relative t o

segments

o f

t h e

Elsinore fault

are

shown

on

Figures

3 and 4 . Data

on

possible

excavation

sites for this

study are

summarized

on

Table

1

and

located

on

Figures

2 - 4 .

The

data

were

derived

from

a literature search and

the

review

o f

engineering geology

r e p o r t s ,

Additional information was obtained during a

field

reconnaissance in July 1 9 8 0 .

Glen

Ivy North Fault, Site 1 :

Based on previous studies

(Table

1 ) and our field observations, trench

i n g a t Site 1 ( F i g . 3 ) has t h e potential o f

revealing

disrupted Holocene

sediments

within

an

elongate

s a g pond along the

Glen

Ivy North f a u l t .

How

e v e r , during our field reconnaissance in

July

1 9 8 0 , the sag

pond

contained

standing

water

which

had overflowed

from irrigation o f adjacent orchards.

Permission t o

trench

has been obtained from

t h e

property

owner. This

site

may

be

trenched i f

sufficient funds

are

available

after t w o more

promising

sites

are excavated and i f groundwater

conditions

permit.

Previous investigators

( G P - 4 5 , Appendix

A ;

Sharp, 1 9 7 8 ,

p .

1 9 4 )

have

suggested

that

cracks in

t h e

asphalt o f Lawson Road at the eastern

margin

o f the main

sag pond area are due

t o fault creep. The cracks are

located


14/35

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15/35

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16/35

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.


17/35

a t , and westward from the

cut-fill contact

over a width o f about 1 0 meters.

About

half

o f t h e cracks

are

oriented

in a northwest

trending

en echelon

pattern and t h e

remainder

are

semi-circular or trend northeast. The cracks

may

be

t h e

result o f differential settlement o f

road

fill rather than aseismic

c r e e p .

Faults on Margins

o f

Lake

Elsinore, Sites

2 - 5 :

As summarized

on

Table

1 ,

previous

work

by Weber

( 1 9 7 7 ) suggests that

Sites 2 - 5

( F i g .

3 ) on the

usual

margins

o f

Lake Elsinore would be favorable

locations

t o find

disrupted Holocene

lake s e d i m e n t s .

During

July 1 9 8 0 , these

sites

were

under

water due t o the abnormally high lake level caused by

heavy

rainfall. These sites

will

not be suitable until

t h e

lake

and

water table

fall t o

previous

levels.

Wildomar

Fault, Site

6 :

A

previous engineering

geology

investigation (Table 1 )

revealed

dis

rupted

beds o f the

Pauba

Formation (late Pleistocene,

Kennedy,

1 9 7 7 )

on

branches

o f t h e Wildomar fault located

on

t h e

northeast

and

southwest sides

o f

a closed

depression

( F i g s .

4 and 5 ) . In July

1 9 8 0 , standing

water was

limited

t o the lowest

part

o f the

depression,

and vegetation consisted o f

dry grass on

t h e

slopes and was absent on the bottom

o f

the depression.

Permission

t o

trench across

t h e

depression was obtained from

t h e

property

owner

and was

accomplished during September

1 9 8 0 .

The

results

o f

the

trench

i n g

are described i n the next section.

Wolf Valley Fault, Site 7 ;

During our field reconnaissance,

a

small oval swamp

o f

low relief

containing standing

water and thick vegetation was observed along a strand

o f the

Wolf Valley fault

at

Site

( F i g . 4 ) . The fault

trace

i s

expressed

a s

an alignment

o f

scarps and shallow

depressions. A low scarp i s

located^

directly

northeast o f the

swampy

area between t h e site and

Pala

R o a d .

This

site i s

considered

less

favorable

than

other

sites which

will

be

investi

gated

first.

Wolf

Valley Fault, Site 8 ;

Kennedy's

( 1 9 7 7 )

map and our reconnaissance indicate

t h a t faults

bound

ing a closed depression are reflected in scarps

and

topographic saddles

adjacent

t o t h e

depression

and

along strike. Permission

t o

trench Site 8

13


18/35

F i g .

5 - M a p

s h o w i n g

t r e n c h e s a c r o s s c l o s e d

d e p r e s s i o n b o u n d e d b y

s t r a n d s o f W i l d o m a r f a u l t ,

S i t e

6 . L o c a t i o n o f s i t e

s h o w n o n F i g .

4 .

14


19/35

( F i g . 4 )

has

been

obtained

and work will begin i n t h e

near

future.

South Branch

o f

Elsinore

Fault, Site 9 :

Site

9 ( F i g . 2 )

has

not

been

visited during t h e

present

investigation.

Trenching at this location has a low priority because of

t h e

great distance

from urban areas.

Results

o f

Trenching Across

Wildomar

Fault, Site

6

Field

work was

accomplished

at t h i s site during

September

1 9 8 0 .

Trenches

were

dug with a

John Deere

Model 540C

extendahoe

backhoe equipped with a

two-foot wide bucket. A single trench

trending

N41E was excavated at right

angles t o t h e

N50

W trend o f t h e Wildomar fault and

was

located s o a s t o

intersect the

projected traces

o f the

fault

( F i g . 5 ) . The trench was ex

cavated

t o

a

depth

o f 2 . 5

meters

over

i t s

entire 96-meter

l e n g t h .

Nails

and

string were used

t o

establish a one meter grid on the northern trench f a c e ;

t h e

southwestern

end o f the trench

was defined

a s the 0.0

reference.

Strati-

graphic details

were

plotted relative

t o the g r i d .

The

trench was

deepened

t o 5

meters

between 1 2 and 1 6

meters

t o

expose

more details o f the faulting.

A

layered section

o f c l a y , silt, sand and

gravel

was exposed

in

the

trench;

this sequence

i s faulted

and deformed between

7 and 1 4

meters

( F i g s .

6 - 9 ) near the southwest end

o f the trench.

The sediments between 1 4

and

6 5 meters consist

o f

horizontally layered sandy clay

which

overlies dense

silty clay and

appeared

t o

be unfaulted and undeformed. Beyond

6 5

meters,

t h e

upper

sandy clay unit graded into a slightly

clayey coarse

sand

eroded

and washed

down

from the hill ( s c a r p ) t o the northeast. Groundwater

was

perched

on the silty clay horizon a t a depth of approximately 1 . 5

meters.

Continuous

seepage

o f

groundwater

caused

filling

o f the trench and

contrib

uted t o the

complete collapse

o f t h e

entire central

portion o f t h e

trench

between

2 7

and 7 5

meters.

A second trench was

excavated

parallel

t o

and 1 0

meters

north o f t h e

collapsed main trench

( F i g .

5 ) in order

t o

insure complete cross-section

coverage.

No evidence

o f

faulting

was observed a t t h e

northeast

end

o f

either trench. Projection

o f

the north branch o f the Wildomar fault north

westward from

i t s

mapped location

in Lohr ( G P - 8 4 , Appendix

A ) Trench

N o . B

at a trend

parallel

t o t h e

south

branch i s

shown

on

Figure 5 . This projected

trend

l i e s beyond t h e northeastern end o f our

exploration

trenches

on

a

slope which i s

t o o

steep

for t h e

operation o f a backhoe.

1 5


20/35

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22/35

7m

0 0 I

1.0

2.0

3.0

4.0

Meters

1.0

2 0

3.0

Sv 4.0

50

Meters

F i g . 8

-

D e t a i l e d t r e n c h l o g

b e t w e e n

6 . 6

a n d 1 2 . 0 m e t e r s .

S e e

F i g . 6

f o r l o c a t i o n

a n d F i g . 7 f o r e x p l a n a t i o n.

1 8


23/35

3m

14 m

m

6m

^ r^-'>

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F i g .

9 -

D e t a i l e d t r e n c h l o g b e t w e e n

1 2 . 0

a n d 1 6 . 5 m e t e r s .

S e e F i g .

6 o r

l o c a t i o n a n d F i g .

7 o r

e x p l a n a t i o n .

C 1 4 : l a y e r d a t e d b y

r a d i o c a r b o n

m e t h o d ( A p p e n d i x B

1 9


24/35

Details

o f

the structure

within

the fault

z o n e

between

6 . 6

and 16.5

meters

are

illustrated

in

Figures

8 and 9 .

Completely different sedimen

t a r y

sections occur on opposite sides

o f t h e

principal structural break

a t

1 3 . 5

meters, which i s interpreted t o be

t h e

south branch o f t h e Wildomar

f a u l t . The gouge

zone

i s 5-10

cm wide, near vertical and

truncates all

sediments

except

t h e

uppermost massive sandy

c l a y

within

one

meter

o f

the

surface. Sediments on both sides

o f the

fault

are

dragged

d o w n . A

gravel

horizon on the southwest side o f the fault ( F i g . 9 )

shows

approximately

one meter o f vertical d r a g . The lack o f

correlation

o f

the

sediments on

opposite sides

o f t h e

fault and

t h e

fact that beds are dragged down on both

sides

o f t h e

fault suggest a significant,

but

unknown, component

o f

hori

zontal

displacement.

The attitude

symbols

northeast

o f t h e

main

fault ( F i g .

6 )

suggest a

southeast plunging anticlinal axis trending

about

N75

W , a t

an

angle

o f

about 2 3

degrees t o

t h e f a u l t .

Such an orientation

i s

consistent

with

drag

folding

caused by right-slip

(Wilcox et a l , 1 9 7 3 ) .

Sediments northeast

o f

t h e fault consist

o f

gently dipping relatively undeformed clayey

silts,

silty clays, sandy silts and fine silty sands. Abundant

slickensides

were

observed and

measured in t h e massive

dusky

brown silty clay horizon between

depths

o f

4.2 and

4 . 6

m

( F i g . 6 ,

plan view). Striations on most of the

slickensided surfaces appear

t o

be roughly parallel

t o t h e

northwest trend

o f t h e

breaks observed

between and

13.5

meters, thus

strengthening

t h e

interpretation

that

the major

displacement along

the main

fault was hori

zontal,

rather than

vertical.

Sediments southwest

o f

t h e main fault break consist

o f

a sequence

o f

sandy clays

and

sandy silts with a prominent sand and gravel layer which

occurs a t a depth

o f

1 t o 2 meters below the

present

ground surface.

This

unit

provided a marker horizon for

determining deformation between

the

and 13.5-meter interval in

the trench.

Faults were identified within the

sediments

as

steeply dipping

or vertical

light-green

clay

gouge

zones rang

ing between 1 and 1 0 cm in width.

These faults are

divided

into the

following

two classes: ( 1 )

rela

tively older

faults which

are truncated by

t h e base o f the

gravel; strikes:

N6AW

t o

N75W;

d i p s :

77SW t o

77NE, and ( 2 )

relatively younger faults

which displace the base o f the gravel

layer

(denoted by C on F i g . 6 ) and

appear

t o die out within the gravel;

strikes:

N47W t o N55 W ; d i p s :

2 0


25/35

72SW

t o 73NE.

No

fault

except

the main break a t

13.5 meters displaces

t h e t o p o f the gravel. However, the upper contact o f the gravel shows minor

deformation associated

with

faults located

a t 1 2 . 5

m ,

1 1 m , 8 . 5 m

and possi

bly

7 . 5 m ( F i g s .

8

and 9 ) .

The

deformation

may

be t h e result o f a

smooth

i n g over o f t h e upper contact o f the

gravel

subsequent t o rupture.

Redis

tribution

of t h e clasts in t h e upper

portion

o f t h e gravel

could

be accom

plished

i f these sediments were at or

near

t h e ground

surface

a t the time o f

rupture. A small (10-20 cm

h i g h )

scarp

produced during an earthquake

could

be

eroded

down after

or

at t h e time o f

deposition

o f

the overlying sandy

c l a y .

The

main

fault ( 1 3 . 5

m ,

F i g . 9 ) may have had t h e most

recent

displace

ment because

a small

gravel wedge projects upward

from the

gravel

layer

into

t h e

overlying

clay

within

t h e

fault zone.

Above

t h i s wedge, the fault i s

not

discernible within

t h e

massive sandy

c l a y ,

which apparently continued

t o

be deposited

after displacement. I t i s also

possible

that the

most recent

displacement on

t h e main

fault was contemporaneous with formation o f

the

youngest

secondary faults

described

above and t h a t , because o f

smaller

dis

placement, the effects are not a s

obvious

a t and

above

t h e t o p o f the

gravel

layer. It i s concluded

that

t h e deformation was t h e

result

of t w o ,

or less

likely three, earthquakes which originated on

t h e

south branch

o f t h e

Wildomar

f a u l t .

Clearly identifiable

organic

material

was

not

observed

in

any

o f

the

sediments exposed within

t h e

trench. However, t h e dense, dark gray, slicken-

1 4

sided clay northeast

o f the main

fault

was sampled for C

dating

because its

dark color

suggested

i t

might contain sufficient organic

material

for analysis

This material was sent t o Teledyne Isotopes (Appendix B ) and the following

results were

obtained:

age

in years

(B.P.),

humic acids: 4330 +

4 0 0 ;

carbon

residue:

4120

+

2 6 0 .

The

clay

unit sampled

i s truncated

by

the main fault

and must predate a t least one o f t h e seismic events. Its

relationship

t o

t h e

event which occurred prior

t o

the

deposition

o f

the

gravel

i s

not

clear,

because units cannot be correlated across

the

main

fault.

Thus, we have no

basis for deciding whether

t h e

clay

predates

one o r both earthquakes caused

by movement on t h e

south branch

o f

the

Wildomar f a u l t .

We

also

have

no

basis

for estimating the time which elapsed

between

deposition

o f

the clay

and its

truncation

a s a result o f fault displacement.

21


26/35

CONCLUSIONS

Radiocarbon dating o f disrupted sediment indicates

t h a t t h e

south

branch o f t h e Wildomar fault, southeast of

Lake

Elsinore, has been active

within

t h e

past 4120

+ 260 years.

The relationship

o f t w o

distinct periods

o f

secondary faulting

t o t h e

dated sediments indicates

t h a t

one or

t w o

seismic events occurred

during t h i s

period;

a n

additional

and more

recent

displacement event on

t h i s

fault i s

also

possible but

i s

considered less

likely.

Because

o f

a lack o f correlation

o f

sediments on opposite sides

o f t h e main break, t h e

amount

and

sense

of

displacement

are unknown.

This

lack o f correlation and secondary

structures suggest

an important,

but

unknown, component

o f

right-slip. A

vertical component o f

displacement,

northeast side d o w n , would explain

t h e

topographic relief between

the

south

edge and center o f

a depression

which i s bounded on the southwest

by

the

fault.

FUTURE

PLANS

Permission has been

obtained

t o trench Sites 1 and 8 (Table 1 ) . It

i s planned t o begin work on these sites in

t h e

near future. I t i s probable

t h a t

study o f these t w o sites will exhaust

t h e

funds

currently

available

on this

project.

ACKNOWLEDGMENT S

Permission t o trench on their property given

by

t h e

following

indi

viduals

i s

gratefully

acknowledged:

Site 1 :

M r .

John B .

Hoeger,

president,

Temescal Properties,

Inc., Corona;

Site 6 :

M r .

Timothy

W .

Archer, Lake

Elsinore;

Site 8 : D r . Edward H . Boseker, Santa

A n a . M r .

Anthony

B .

Brown,

Engineering Geologist, Riverside County, was generous in t h e loan o f

reports

on

file

in his

office and

provided

helpful discussions. D r . Mason

Hill,

M r .

David Douglass

and

M r . Blake Schow assisted in t h e field work. The

manuscript was typed and

reviewed by

Mrs.

Ruth

Merifield. The manuscript

was also

reviewed

by

D r s .

Paul Merifield

and

Mason

Hill.

Miss Sandra Petit-

jean helped with

the

illustrations.

2 2


27/35

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