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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.
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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
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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
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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
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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
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r
o
C
S
v
B
N
L
O
A
G
E
s
n
W
L
D
M
M
E
/
4
O
5
M
L
E
1
2
3
4
5
6
K
I
L
O
M
E
F
g
-
M
a
s
w
i
n
p
n
p
a
t
s
n
s
u
h
n
C
f
o
n
a
a
n
a
e
a
c
e
e
b
d
a
e
m
a
a
o
n
E
s
n
e
f
a
t
F
g
2
4
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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).
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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 ) .
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G
U
C
L
I
E
N
T
E
S
N
B
r
a
n
c
h
E
l
s
l
n
o
l
e
F
V
L
l
C
I
T
O
V
L
l
Y
L
C
M
E
A
O
F
s
w
n
d
p
d
e
n
w
e
c
e
q
e
w
e
n
e
e
E
o
s
e
o
r
e
y
a
v
s
n
o
E
s
n
e
a
4
K
o
m
e
e
s
I
F
i
g
.
2
-
M
a
p
s
h
o
w
i
n
g
E
l
s
i
n
o
r
e
f
a
u
l
t
z
o
n
e
b
e
t
w
e
e
n
B
a
n
n
e
r
C
a
n
y
o
n
a
n
d
A
q
u
a
C
a
l
i
e
n
t
e
S
p
r
i
n
g
s
.
A
b
b
r
e
v
i
a
t
i
o
n
:
C
G
:
C
a
m
p
b
e
l
l
G
r
a
d
e
.
F
r
o
m
L
a
m
a
r
a
n
d
M
e
r
i
f
i
e
l
d
(
1
9
7
6
)
.
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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).
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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
7/24/2019 los perritos son tontitos
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.
7/24/2019 los perritos son tontitos
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
7/24/2019 los perritos son tontitos
14/35
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r
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(
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7/24/2019 los perritos son tontitos
15/35
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(
1
9
6
5
)
w
i
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(
1
9
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,
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(
1
9
7
7
)
a
n
d
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n
v
i
c
o
m
(
1
9
7
6
)
.
7/24/2019 los perritos son tontitos
16/35
T
a
b
l
e
1
-
D
a
t
a
o
n
p
o
s
s
i
b
l
e
e
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c
a
v
a
t
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o
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r
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a
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t
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t
r
a
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l
s
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f
a
u
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t
;
l
o
c
a
t
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s
a
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s
h
o
w
n
o
n
F
i
g
s
.
2
-
4
.
R
i
v
e
r
s
i
d
e
C
o
u
n
t
y
G
e
o
l
o
g
y
R
e
p
o
r
t
s
(
G
P
-
)
a
r
e
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i
s
t
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d
i
n
A
p
p
e
n
d
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x
A
.
N
u
m
b
e
r
_
1
R
e
c
e
n
t
s
c
a
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p
a
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d
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p
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d
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d
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t
r
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s
a
n
d
b
u
s
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e
s
a
l
o
n
g
G
l
e
n
I
v
y
N
o
r
t
h
f
a
u
l
t
(
W
e
b
e
r
,
1
9
7
7
,
p
.
9
0
;
S
h
a
r
p
,
1
9
7
8
,
p
.
1
9
4
)
.
P
i
o
n
e
e
r
C
o
n
s
u
l
t
a
n
t
s
(
G
P
-
4
5
)
r
e
p
o
r
t
:
(
1
)
F
a
u
l
t
g
o
u
g
e
a
n
d
s
l
i
c
k
e
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s
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d
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s
i
n
o
l
d
e
r
f
a
n
d
e
p
o
s
i
t
s
(
Q
o
f
,
K
e
n
n
e
d
y
,
1
9
7
7
)
i
n
t
h
r
e
e
t
r
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n
c
h
e
s
o
n
p
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b
u
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d
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a
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t
h
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n
o
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p
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t
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n
o
f
t
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e
s
a
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p
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d
a
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a
.
(
2
)
W
a
t
e
r
-
l
i
n
e
i
n
v
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c
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n
i
t
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o
f
H
u
n
t
R
o
a
d
(
n
o
r
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t
o
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m
a
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)
r
e
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p
a
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p
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a
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e
p
a
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t
h
e
f
a
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t
.
(
3
)
c
r
a
c
k
s
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n
a
s
p
h
a
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t
o
f
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a
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p
.
2
W
i
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d
o
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a
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a
u
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a
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s
o
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w
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a
k
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n
t
s
(
W
e
b
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r
,
1
9
7
7
,
p
.
9
3
)
.
3
G
l
e
n
I
v
y
N
o
r
t
h
f
a
u
l
t
m
a
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f
o
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a
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s
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e
(
W
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,
1
9
7
7
,
p
.
9
3
)
.
4
W
i
l
d
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m
a
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f
a
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l
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s
o
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a
s
t
s
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o
f
L
a
k
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l
s
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n
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m
a
y
d
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s
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a
k
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s
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d
i
m
e
n
t
s
(
W
e
b
e
r
,
M
1
9
7
7
,
P
.
9
3
)
.
5
P
o
s
s
i
b
l
e
s
c
a
r
p
s
i
n
y
o
u
t
h
f
u
l
l
a
k
e
s
e
d
i
m
e
n
t
s
a
n
d
s
t
e
e
p
n
o
r
t
h
s
l
o
p
e
o
f
R
o
m
e
H
i
l
l
s
u
g
g
e
s
t
r
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c
e
n
t
m
o
v
e
m
e
n
t
o
n
W
i
l
d
o
m
a
r
f
a
u
l
t
(
W
e
b
e
r
,
1
9
7
7
,
p
.
9
4
)
.
6
C
l
o
s
e
d
d
e
p
r
e
s
s
i
o
n
s
a
n
d
s
m
a
l
l
s
a
g
p
o
n
d
s
a
l
o
n
g
W
i
l
d
o
m
a
r
f
a
u
l
t
(
W
e
b
e
r
,
1
9
7
7
,
p
.
9
4
;
K
e
n
n
e
d
y
,
1
9
7
7
,
p
.
9
)
.
L
o
h
r
(
G
P
-
8
4
)
r
e
p
o
r
t
s
N
o
r
t
h
a
n
d
S
o
u
t
h
b
r
a
n
c
h
e
s
o
f
W
i
l
d
o
m
a
r
f
a
u
l
t
d
i
s
p
l
a
c
e
P
a
u
b
a
F
o
r
m
a
t
i
o
n
t
o
w
i
t
h
i
n
1
-
3
f
e
e
t
o
f
s
u
r
f
a
c
e
;
t
r
e
n
c
h
e
s
a
r
e
l
o
c
a
t
e
d
d
i
r
e
c
t
l
y
s
o
u
t
h
e
a
s
t
o
f
l
a
r
g
e
s
t
d
e
p
r
e
s
s
i
o
n
.
7
G
r
o
u
n
d
-
w
a
t
e
r
b
a
r
r
i
e
r
a
l
o
n
g
s
t
r
a
n
d
o
f
W
o
l
f
V
a
l
l
e
y
f
a
u
l
t
(
K
e
n
n
e
d
y
,
1
9
7
7
,
P
l
a
t
e
I
)
.
8
M
i
d
d
l
e
o
f
t
h
r
e
e
c
l
o
s
e
d
d
e
p
r
e
s
s
i
o
n
s
a
l
o
n
g
W
o
l
f
V
a
l
l
e
y
f
a
u
l
t
z
o
n
e
(
K
e
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r
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9
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p
.
5
9
)
.
7/24/2019 los perritos son tontitos
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
7/24/2019 los perritos son tontitos
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
7/24/2019 los perritos son tontitos
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
7/24/2019 los perritos son tontitos
20/35
c
c
I
1
I
S
1
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s
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f
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p
l
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n
.
7/24/2019 los perritos son tontitos
21/35
w
o
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:
.
-
V
.
A
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p
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b
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a
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s
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t
l
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m
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p
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/
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p
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m
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p
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/
2
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r
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Y
3
/
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.
V
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a
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y
(
f
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t
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v
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c
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)
s
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a
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b
r
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.
_
/
/
*
X
X
X
K
V
>
S
a
n
d
y
(
f
i
n
e
t
o
m
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d
i
u
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)
c
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a
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s
i
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.
M
a
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M
a
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f
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r
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s
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g
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7
-
E
x
p
l
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t
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o
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s
o
n
F
i
g
s
.
6
,
8
a
n
d
9
.
7/24/2019 los perritos son tontitos
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
7/24/2019 los perritos son tontitos
23/35
3m
14 m
m
6m
^ r^-'>
^
^ 1
?N
i ^ - f p p ' '
^ - . - - 3 1
i - ^ ^ v j -
' .
S^
I I M i ^ . : V - ; . - i [
^ s s \
Mm^
1.0
.0
3.0
40
50
ao
7 0
Meters
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
7/24/2019 los perritos son tontitos
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
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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
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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
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