GEOLOGICAL SURVEY CIRCULAR 691-A, B 

The Van Norman Reservoirs Area, 

Northern San Fernando Valley, . 

California 

Geologic Environment of the 

Van Norman Reservoirs Area 

Expectable Earthquakes and 

Their Ground Motions in the 

Van Norman Reservoirs Area 

Prepared on behalf of the Federal Disaster Assistance Administration 

The Van Norman Reservoirs Area, 

Northern San Fernando Valley, 

California 

Geologic Environment of the 

Van Norman Reservoirs Area 

By R. F. Yerkes, M. G. Bonilla, T. L. Youd, and J. D. Sims 

Expectable Earthquakes and 

Their Ground Motions in the 

Van Norman Reservoirs Area 

By R. L. Wesson, R. A. Page, D. M. Boore, and R. F. Yerkes 

GEOLOGICAL SURVEY CIRCULAR 691-A, 8 

Prepared on behalf of the Federal 

Disaster Assistance Administration 

Washington 1974 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to U.S. Geological Survey, National Center, Reston, Va 22092 
Geologic Environment of the 

Van Norman Reservoirs Area 

By R. F. YERKES, M. G. BONILLA, 

T. L. YOUD, and J. D. SIMS 

THE VAN NORMAN RESERVOIRS AREA, 

NORTHERN SAN FERNANDO VALLEY, CALIFORNIA 

GEOLOGICAL SURVEY CIRCULAR 691-A 

Prepared on behalf of the Federal 

Disaster Assistance Administration 

CONTENTS 

Page Page 

Abstract____________________________________________ A1 Geology of the Van Norman reservoirs area-Continued 

Introduction---------------------------------------- 1 Structure -------------------------------------- A20 

Purpose and scope------------------------------

Previous investigations and sources _____________ _ 

Geologic setting ___________________________________ _ 

Faults in the San Fernando Valley _____________ _ 

Historic seismicity and faulting _________________ _ 

San Fernando earthquake _____________________ _ 

Transitory effects _________________________ _ 

Permanent effects _________________________ _ 

Tectonic ruptures _____________________ _ 

Vertical displacements _________________ _ 

Horizontal displacements _______________ _ 

Summary ------------------------------

Geology ofthe Van Norman reservoirs area _________ _ 

Stratigraphy ___________________________________ _ 

2 

2 

2 

4 

6 

7 

7 

7 

8 

8 

11 

15 

15 

16 

Lower dam site__________________________________ 20 

Upper damsite ---------------------------------- 22 

Proposed damsite ------------------------------ 23 

Surficial deposits____________________________ 23 

Bedrock faults------------------------------ 27 

Expectable earthquakes and their effects ____________ 29 

Transitory effects ------------------------------ 29 

Permanent effects ------------------------------ 29 

Lower damsite ------------------------------ 30 

Upper and proposed damsites ________________ 32 

Other effects________________________________ 33 

Conclusions ---------------------------------------- 33 

References__________________________________________ 34 

ILLUSTRATIONS 

Page 

FIGURE 1. Map of part of southern California showing major faults and epicenters of large historic 

earthquakes ---------------------------------------------------------------------------------- A3 

2. Generalized geologic map of northern San Fernando Valley and western San Gabriel Mountains________ 5 

3. Map showing 1971 ruptures and ancient faults, northern San Fernando Valley ------------------------ 9 

4. Map showing 1971 elevation changes determined from preearthquake and postearthquake surveys______ 10 

5. Profiles of 1971 elevation changes determined from preearthquake and postearthquake surveys________ 12 

6. Map showing 1971 horizontal displacements determined from preearthquake and postearthquake 

surveys_______________________________________________________________________________________ 14 

7. Graph showing 1971 displacements along the San Fernando zone and its westward extension ---------- 16 

8. Map showing orientation and magnitude of dilatational strains -------------------------------------- 17 

9. Map showing distribution and magnitudes of maximum shear strains -------------------------------- 18 

10. Map showing orientation and magnitude of principal strains ---------------------------------------- 19 

11. Generalized vertical section, north wall of proposed damsite ------------------------------------------ 24 

12. Graph showing surface displacement relative to earthquake magnitude for selected events from the 

worldwide record ----------------------------------------------------------------------------- 31 

TABLES 

Page 

TABLE 1. Data on displacements along the San Fernando zone and its westward extension ---------------------- A16 

2. Radiocarbon dates from surficial deposits, proposed damsite ------------------------------------------ 25 

3. Summary of credible fault and fault-related displacements, M 7.7 local earthquake -------------------- 30 

v 

THE VAN NORMAN RESERVOIRS AREA, 

NORTHERN SAN FERNANDO VALLEY, CALIFORNIA 

Geologic Environment of the Van Norman Reservoirs Area 

By R. F. Yerkes, M. G. Bonilla, T. L. Youd, and J. D. Sims 

ABSTRACT 

The upper and lower Van Norman dams, in northwesternmost San Fernando Valley about 20 mi (32 km) northwest of 

downtown Los Angeles, were severely damaged during the 

1971 San Fernando earthquake. An investigation of the 

geologic-seismologic setting ofthe Van Norman area indicates 

that an earthquake of at least M 7. 7 may be expected in the Van 

Norman area. The expectable transitory effects in the Van 

Norman area of such an earthquake are as follows: peak horizontal acceleration of at least 1.15 g, peak velocity of displacement of 4.43 ft/sec (135 em/sec), peak displacement of2.3 ft (70 

em), and duration of shaking at accelerations greater than 0.05 

g, 40 sec. A great earthquake (M 8+) on the San Andreas fault, 

25 mi distant, also is expectable. Transitory effects in the Van 

Norman area from such an earthquake are estimated as follows: peak horizontal acceleration of0.5g, peak velocity ofl.97 

ftJsec (60 em/sec), displacement ofl.31 ft (40 em), and duration 

of shaking at accelerations greater than 0.05 g, 80 sec. 

The permanent effects of the expectable local earthquake 

could include simultaneous fault movement at the lower damsite, the upper damsite, and the site proposed for a replacement 

dam halfway between the upper and lower dams. The maximum differential displacements due to such movements are 

estimated at 16.4 ft (5 m) at the lower damsite and about 9.6 ft 

(2.93 m) at the upper and proposed damsites. 

The 1971 San Fernando earthquake (M 6lh) was accompanied by the most intense ground motions ever recorded instrumentally for a natural earthquake. At the lower Van Norman dam, horizontal accelerations exceeded 0.6 g, and shaking 

greater than 0.25 g lasted for about 13 sec; at Pacoima dam, 6 

mi (10 km) northeast of the lower dam, high-frequency peak 

horizontal accelerations of 1.25 g were recorded in two directions, and shaking greater than 0.25 g lasted for about 7 sec. 

Permanent effects of the earthquake include slope failures in 

the embankments of the upper and lower Van Norman dams, 

rupturing of the ground surface by faulting along parts of the 

zone of old faults that extends easterly through the reservoir 

area and across the northern part of the valley, folding or 

arching of the ground surface, and differential horizontal displacement of the terrane north and south of the fault zone. 

Although a zone of old faults extends through the reservoir 

area, the 1971 surface ruptures apparently did not; however, 

arching and horizontal displacements caused small relative 

displacements of the abutment areas of each of the three damsites. The 1971 arching coincided with preexisting topographic 

highs, and the surface ruptures coincided with eroded fault 

scarps and a buried ground-water impediment formed by 

pre-1971 faulting in young valley fill. This coincidence with 

evidence of past deformation indicates that the 1971 deformations were the result of a continuing geologic process that is 

expected to produce similar deformations during future events. 

The 1971 San Fernando earthquake probably was not the 

largest that has occurred in this area during the last approximately 200 years, as indicated by a buried fault like scarp 

about 200 years old that is higher than, and alined with, 1971 

fault scarps. In addition, the San Fernando zone of 1971 ruptures is part of a regional tectonic system that includes the San 

Andreas and associated faults; one of these, the White Wolf 

fault north of the San Andreas, is symmetrical in structural 

attitude with the San Fernando zone and ruptured the ground 

surface during the 1952 Kern County earthquake (M 7.7). 

Other large earthquakes associated with surface rupturing on 

faults ofthis system include the 1857 Fort Tejon earthquake (M 

8+) and possibly the 1852 Big Pine earthquake. Several other 

historic earthquakes in this general area are not known to be 

associated with surface ruptures, but were large enough to 

cause damage in the northern San Fernando Valley. 

The Van Norman reservoirs are in an area of high seismicity 

that was permanently deformed during the 1971 earthquake; 

this area could be subjected to even larger earthquakes. Such 

earthquakes could be accompanied by transitory and permanent effects that would impose severe deformations on an earth 

dam. Because the Van Norman area overlooks the densely 

populated central San Fernando Valley, the presence of the 

high seismic and fault risk justifies great conservatism in the 

selection of critical geologic and seismologic parameters to be 

used in the planning, design, review, and construction of dams 

in that area. 

A1 

INTRODUCTION 

The Van Norman reservoir complex is at the 

west margin of the northern San Fernando Valley 

about 20 mi (32 km) northwest of downtown Los 

Angeles. The complex consists of an upper dam 

and reservoir and a lower dam; the dams were 

built between 1912 and 1921 of hydraulic fill 

placed on young valley alluvium as thick as 50ft 
(15m) at the upper dam and 20ft (6 m) at the lower 

dam. A small bypass reservoir and dam, built of 

compacted fill and completed in 1970, is also part 

of the complex. Both the upper and lower Van 

Norman dams were severely damaged by slope 

failures in the embankments during the 1971 San 

Fernando earthquakes. The lower reservoir was 

immediately drained and the lower dam taken out 

of service, whereas the upper dam still impounds a 

reservoir operated at reduced capacity. A proposed 

new dam would be built of compacted earthfill 

placed on bedrock at a site midway between the 

upper and lower dams; the proposed dam would 

impound about 10,000 acre-feet (12,300,000 m3) of 

water, about half the maximum capacity of the 

former lower reservoir. 

PURPOSE AND SCOPE 

This report contains the results of an investigation, made on behalf of the Federal Disaster 

Assistance Administration, of the geologic environment of the Van Norman area with respect to 

repair or replacement of the dams. The investigation included several phases, the first of which was 

a brief review of the regional geologic setting, 

structural framework, and seismic history. A second phase consisted of mapping (scale 1:12,000) 

the permanent deformation of the ground surface 

in the northern San Fernando Valley associated 

with the 1971 San Fernando earthquake, in part 

because a complete, detailed record of the effects of 

the 1971 earthquake will serve as an example of 

the effects to be expected from similar future 

events. The third phase was to map in detail 

(1:4,800 and 1:1,200) the distribution and attitude 

of faults in the reservoir area and to determine 

their history and degree of activity, with emphasis 

on their relation to the 1971 deformation and the 

regional structure. It was then necessary to determine whether these relations would persist 

and, finally, to estimate the effects that future 

earthquakes might have in the reservoir area. 

PREVIOUS INVESTIGATIONS AND SOURCES 

The present work is a continuation and extension of investigations ofthe February 9, 1971, San 

Fernando earthquake sequence. Early results of 

that multiagency investigation were reported in 

U.S. Geological Survey Professional Paper 733, 

published about 10 weeks after the earthquake, 

which includes numerous basic sources in geology, 

seismology, and effects ofthe earthquakes. A very 

useful product of similar investigations by the 

California Division of Mines and Geology is their 

map of surface effects of the San Fernando earthquake (Barrows and others, 1974, pl. III). The 

standard geologic reference for the area and for the 

western San Gabriel Mountains in general is 

Oakeshott's (1958) comprehensive report on the 

San Fernando quadrangle, which contains a 

geologic map of the western San Gabriel Mountains at a scale of1:62,500 and a very useful review 

of the nomenclature of the geologic units. Other 

published sources valuable to the present study 

include a report by the California State Water 

Rights Board (1962) on the ground-water geology 

and regime of the San Fernando Valley and the 

Los Angeles Sheet of the Geologic Map of California by Jennings and Strand (1969). 

We especially appreciate the cooperation of the 

Los Angeles Department of Water and Power in 

dressing and cleaning the cut slopes in the embankment area of the proposed damsite and surveying geologic control points, and in supplying 

numerous reports, maps, and data, all of which 

contributed significantly to the investigation. 

Most of the fieldwork for this report was done during the period April to August 1972; office work 

and report preparation extended into 1973. 

GEOLOGIC SETTING 

The San Fernando Valley is in the central part 

of the Transverse Ranges structural province of 

southern California (across center of fig. 1), 

athwart the northwesterly regional trend of the 

Coast Ranges and Peninsular Ranges provinces 

and faults of the San Andreas system such as San 

Jacinto, Whittier-Elsinore, and NewportInglewood. The Transverse Ranges province consists of numerous east-trending mountain ranges 

and valleys, characterized by late Cenozoic compressive structural features such as tight folds and 

reverse-slip and thrust faults of similar trend. One 

of the most impressive products of this late 

Cenozoic tectonism is the east-trending Ventura 

basin, northwest of San Fernando Valley, which is 

a syncline of Cenozoic sediments that include 

about 5,000 ft (1,500 m) of marine Pleistocene sediments. Both limbs of the syncline are overturned 

inward and overridden by thrust faults. A more 

spectacular feature of the province is the bold 

southern front of the San Gabriel Mountains, 

north and east of the San Fernando Valley, which 

A2 
35° 

34° ~0-~ - -~--

./> SANTA ROSA SANTA CRUZ 

-<'~c ./.P 

./c 

N 

1 

0 

I I 0 50 

FIGURE 1.-Part of southern California showing western 

Transverse Ranges (shaded area between Coast Ranges 

and Peninsular Ranges), major faults, known or inferred 

epicenters oflarger historic earthquakes (stars and dates), 

and associated tectonic ruptures (sawtooth lines) related to 

has been elevated thousands of feet above the San 

Fernando Valley and the Los Angeles basin on the 

south along reverse or thrust faults such as the 

San Fernando and Sierra Madre. 

Unlike most of the major features in California, 

which trend northwest-southeast, the mountains 

and valleys of the Transverse Ranges all trend 

east-west. The earthquakes, faulting, and 

mountain-building activity in the western Transverse Ranges reflect the influence of the San Andreas fault and the ttgreat bend" that it forms as it 

passes through the province. North of the Transverse Ranges the San Andreas fault has a long, 

straight, south-southeasterly trend, but as it enters the Transverse Ranges, it swings more eastward to the ttgreat bend." As the fault leaves the 

Transverse Ranges east of Riverside, it again assumes a more south-southeasterly path. 

50 

I 

I 

100 MILES 

I 

100 KILOMETERS 

' 

the San Andreas stress system. Arrows indicate relative 

horizontal movement on strike-slip faults; barbed lines indicate reverse faults, barbs on upper plate. Modified from 

Hill (1954, fig. U. 

The parts of California northeast ofthe San Andreas fault, along with the rest of North America, 

are moving southeast relative to the part of 

California southwest ofthe fault and the adjacent 

Pacific Ocean floor. The direction of movement is 

parallel to the northern and southern segments of 

the fault. Because of the ttgreat bend," however, 

the two major blocks are in part moving toward 

each other in that section, causing strong northsouth compression over a fairly large area. The 

compression is relieved in part by other faults that 

move in a reverse and (or) left-lateral fashion. 

These faults generally have an east-west orientation and are responsible for much of the uplift of 

the mountainous terrain in the Transverse 

Ranges. The San Fernando fault is one of these 

reverse/left-lateral faults, as are the Santa 

Susana and Sierra Madre faults south of the San 

A3 
Andreas and the Pleito and White Wolf faults 

north of the San Andreas. 

The 1857 Fort Tejon earthquake (M 8±) ruptured a 220-mi (350-km) segment of the San Andreas fault between Cholame Valley and San Bernardino, essentially spanning the Transverse 

Ranges. Since then, this section of the San Andreas system has been relatively quiet, although 

stress has continued to accumulate across the area 

for the last 115 years. Some of the stress can be 

released on the left-lateral/reverse faults, as apparently happened during the 1952 Kern County 

earthquake (M 7.7), which occurred on the White 

Wolf fault, and during the 1971 San Fernando 

earthquake (M 6¥2). Because these stresses continue to accumulate throughout the region, additional fault movements are expected. 

Movements on left-lateral/reverse faults of the 

Transverse Ranges, such as the San Fernando 

'fault, are thus attributable to effects of stresses 

related to the San Andreas system of faults. 

Geologic evidence at all scales indicates that the 

faults have been active for several million years; 

there is no reason to expect significant changes in 

the system. 

FAULTS IN THE SAN FERNANDO VALLEY 

The San Fernando Valley is an asymmetric 

basin filled chieflly by Miocene and younger (less 

than 20 million years old) sedimentary rocks. The 

rocks have been deformed by late Cenozoic folding 

and, especially at the north margin of the valley, 

by thrusting along the Santa Susana fault and its 

eastward equivalents (fig. 2). It is chiefly this sequence of late Cenozoic sedimentary rocks that 

forms the foothills around the northern part of the 

valley. 

The structure of the San Fernando Valley area 

is dominated by two intersecting regional fault 

systems: the northwest-trending San Andreas system of right-lateral strike-slip faults, including 

the San Gabriel fault, and the east-trending system of north-dipping reverse and thrust faults, 

including the Santa Susana-San Fernando-Sierra 

Madre zone, along which the mountain ranges 

have been elevated. Although the geologic history 

of the area formerly was dominated by the San 

Gabriel fault zone, the present structure of the 

north margin of the valley has been greatly 

modified by overthrusting along the mountainfront faults. 

The Van Norman reservoirs occupy a southeastdraining stream course where it crosses elements 

of the Santa Susana-San Fernando fault zone. The 

Santa Susana is a zone of east-trending gently 

north-dipping thrust faults characterized by a 

sinuous trace. The zone extends from the Santa 

Clara River valley about 18 mi (29 km) west of the 

reservoirs to the foothills north of the reservoirs. 

Horizontal shortening across the zone exceeds 1.5 

mi (2.4 km) (Hazzard, 1944). In the Santa Susana 

Mountains (west part of fig. 2), the fault includes 

two strands about 0.3 mi (0.5 km) apart. The 

northern or upper strand is entirely within Tertiary rocks, whereas the lower strand involves 

sedimentary deposits of inferred Quaternary age. 

Northeast of the reservoirs the two traces merge to 

form a series of east-trending reverse faults on 

which rocks of the basement complex have been 

thrust over sedimentary units as young as Pleistocene. Rupturing and reverse faulting occurred at 

the ground surface along or near the lower strand 

during the 1971 earthquake both northwest and 

northeast of the northern San Fernando Valley; 

the zone is therefore active. 1 

The Granada Hills fault extends westward from 

the southwest corner of the lower reservoir; in this 

area the trace is based on its expression in aerial 

photographs taken in 1952. The westward continuation and age relations of this fault are poorly 

known; eastward the trend merges with elements 

of the San Fernando fault zone. 

The San Fernando fault zone, a zone of tectonic 

ruptures associated with the 1971 San Fernando 

earthquake (Wentworth and Yerkes, 1971, p. 13), 

extends eastward across the northern San Fernando Valley from the east shore of the lower 

reservoir. It coincides with young fault-induced 

topographic features, which attest to its continuing activity, and also coincides with the previously 

mapped trace of a buried ground-water impediment, which results in a 50-ft (15-m) fall in 

ground-water level from north to south (California 

State Water Rights Board, 1962). The groundwater impediment is attributed to prehistoric, but 

geologically young, faulting along the trend of the 

San Fernando zone, which has elevated bedrock on 

the north against alluvial gravels on the south 

1 As used here an active fault is one which, on the basis of its geologic history, is 

considered to have potential for causing vibratory ground motion and (or) differential displacement of the ground surface. Such faults commonly exhibit one or more of 

the following: ( 1) movement at or near the ground surface at least once in the past 

35,000 years or more than once in the past 500,000 years, (2) instrumentally 

well-determined macroseismicity, (3) a relationship to a fault with characteristics 

(1) or (2l such that movement on one could be accompanied by movement on the 

other. These criteria are essentially those used by the U.S. Atomic Energy Commission for siting nuclear power plants (Atomic Energy Commission, 1971, p. 22602). 

A4 
0 2 

0 2 4 

EXPLANATION 

D Alluvial deposits 

118° 22'30" 

-

4 MILES 

6 KILOMETERS 

Pliocene and Pleistocene sedimentary rocks 

Upper Cretaceous to upper Miocene sedimentary 

rocks and minor Miocene volcanic rocks 

Mesozoic and older rocks of the basement complex 

Tectonic rupture ---- associated with the 1971 

San Fernando earthquake 

~......_......_ 

Thrust or reverse fault 

0 

Epicenter of 1971 aftershock greater than M3.9 ; 

FIGURE 2.-Northern San Fernando Valley and western San Gabriel Mountains. Unpatterned areas with half arrows represent 

landslide deposits. Dotted line north of Van Norman reservoirs indicates buried axis of Mission Hills syncline. Modified 

from Wentworth and Yerkes (1971, fig. 2). 

(Wentworth and Yerkes, 1971, p. 13). Near the 

east margin of the valley, the zone coincides with a 

topographic escarpment along the south margin of 

a low mesa of bedrock and stream terrace deposits. 

Between this mesa and the hills just east of the 

reservoirs, the ground-water impediment coincides with a very gentle south-facing topographic 

ramp on the valley floor, which may be due to 

faulting. Faults alined with the zone extend westward through the reservoir to merge with the 

Granada Hills fault. 

During investigation of the 1971 ruptures in 

Lopez Canyon, 0.6 mi (1 km) east of the northern 

San Fernando Valley, ancient wood was discovered in rubble beneath a buried fault scarp, the top 

of which was eroded away and the lower part of 

which was more than 3.3 ft (1m) high and alined 

with the 1971 ruptures (Bonilla, 1974). Displacement at the ground surface on the main fault and 

magnitude of the associated earthquake vary directly as based on correlation of these parameters 

for 50 historic events known from the world record 

(Bonilla and Buchanan, 1970). Thus, if the buried 

scarp at Lopez Canyon was formed during a single 

event, it is probable that a larger-than-1971 

earthquake occurred along this zone some 

100-300 years ago (the radiometric age of the 

wood; see Bonilla, 1974), since the vertical separation on the 1971 ruptures in Lopez Canyon is about 

3.1 ft (0.95 m). 

A5 
HISTORIC SEISMICITY AND FAULTING 

Although the 1971 surface ruptures were the 

first to be recorded in historic time in the Transverse Ranges southwest of the San Andreas fault 

the following historic record does include ~ number of large earthquakes capable of causing 

substantial damage in the valley area: 

1700's (1769?). Magnitude and location unknown. Associated with buried faultlike 

scarp 3.3 + ft (1 + m) high in Lopez Canyon, 

dated by means of ancient wood at about 

200 years before present (Bonilla, 1974). 

1852, Big Pine. (See fig. 1.) Magnitude unknown. Probably associated with surface 

ruptures along east-trending left-obliqueslip fault opposite the great bend of the San 

Andreas. 

1857, Fort Tejon. (See fig. 1.) About M 8. Associated with more than 220 mi (350 km) of 

surface rupture along the San Andreas in 

and north of the Transverse Ranges. 

1893, ~~Pico Canyon". Location unknown. 

About M6 (Allen, 1971). 

1916, Tejon Pass. About M 6 (Richter, 1958, p. 

520). 

1930, Location unknown. M 5.2. Intensity 

VIII (RF)2 at Santa Monica, near VII at 

Olive View, 2 mi (3 km) northeast of the 

reservoirs. Shaking caused settlement (0.25 

ft, 7.6 em) and upstream displacement (0.06 

ft, 1.8 em) of the parapet of the lower dam; 

minor damage also was sustained by the 

Chatsworth dam, a hydraulic-fill dam about 

9 mi (15 km) southwest of lower Van Norman dam. 

1952, Kern County. M 7.7. (See fig. 1.) Three 

aftershocks greater than M 6. Associated 

with about 31 mi (50 km) of surface rupture 

along the White Wolf fault, an easttrending left-oblique reverse-slip fault opposite the great bend of the San Andreas. 

1971, San Fernando. (See fig. 1.) M 61h. Associated with about 9.3 mi (15 km) of surface rupture along an east-trending left-oblique reverseslip fault opposite the great bend of the San Andreas. 

Local effects of historic earthquakes are also 

. 

2 Intensity ~cale ofRossi-Forel (1883). (See Richter, 1958, p. 650--651 for explanation.) Intensity VIII (RF) is equivalent to intensity VII--VIII on the Modified Mercalli 

scale. (See footnote 3.) 

A6 

preserved as soft-sediment deformation of the deposits that accumulated in lower Van Norman reservoir after it formed in 1915. After the reservoir 

was drained following the 1971 San Fernando 

earthquake, evidence of soft-sediment deformation, attributed to effects of the 1971 earthquake, 

was found in the upper llh-2 in. ( 4--5 em) of the 

lake sediments (Sims, 1973). The deformation included low-amplitude folds, load-type structures, 

pseudonodules, and upward-penetrating heave 

structures. Two zones of similar deformation, 

buried at lower levels in the lake deposits, are 

truncated and overlain by underformed parallellaminated or massive sediment. On the basis of the 

estimated rate of sediment accumulation, the deformed zones correlate in time with, and thus 

could have been produced by, the following earthquakes, otherwise known to have produced intensities of VI (MM)3 or greater in the northern San 

Fernando Valley area: 

February 9, 1971; San Fernando, Intensity 

VIII-XI 

July 21, 1952; Kern County, Intensity VI-VII 

August 30, 1930--March 10, 1933, ~~Los 

Angeles"-Long Beach. Intensity VI 

3 Intensity refers to the degree of shaking at a given place; it is based on a 

qualitative judgment of the effects of shaking, chiefly on structures, assigned by an 

experienced observer. Excerpt from the 1956 version of the Modified Mercalli scale 

(Richter, 1958, p. 137) follows. 

VI: Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and poor-quality masonry cracked. Small 

bells ring. Trees, bushes shaken visibly or heard to rustle. 

VII: Difficult to stand. Noticed by drivers of motorcars. Hanging objects quiver. 

Furniture broken. Damage to poor-quality masonry. Weak chimneys broken at 

roofline. Fall of plaster, loose bricks, stones, tiles, cornices, and unbraced parapets 

and architectural ornaments. Some cracks in unreinforced ordinary-quality 

masonry. Waves on ponds; water turbid with mud. Small slides and caving-in 

along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. 

VIII: Steering of motorcars affected. Damage to ordinary-quality masonry; partial 

collapse. Some damage to reinforced good-quality masonry, but none to welldesigned masonry reinforced against lateral forces. Fall of stucco and some 

masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, 

elevated tanks. Frame houses moved on foundations if not bolted down; loose 

panels thrown out. Decayed piling broken off. Branches broken from trees. 

Changes in flow or temperature of springs and wells. Cracks in wet ground and on 

steep slopes. 

IX: General panic. Poor-quality masonry destroyed, ordinary-quality masonry heavily damaged, sometimes with complete collapse; good-quality masonry seriously 

damaged; general damage to foundations. Frame structures, if not bolted, shifted 

off foundations. Frames racked. Serious damage to reservoirs. Underground pipes 

broken. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, 

earthquake fountains [ejection of fluids], sand craters. 

X. Most masonry and frame structures destroyed with their foundations. Some 

well-built wooden structures and bridges destroyed. Serious damage to dams, 

dikes, embankments. Large landslides. Water thrown on banks of canals rivers 

lakes, etc. Sand and mud shifted horizontally on beaches and flat land. R~ils ben; 

slightly. 

XI. Rails bent greatly. Underground pipelines completely out of service . 

XII: Damage nearly total. Large rock masses displaced. Lines of sight and level 

(permanently] distorted [by deformation of the ground surface]. Objects thrown 

into the air. 
SAN FERNANDO EARTHQUAKE 

The epicenter of the main shock of February 9, 

1971 (M 61h) was at 34°24.7' N. and 118°24.0' W., 

about 7 mi (12 km) northeast of the Van Norman 

reservoirs in the westernmost San Gabriel Mountains (fig. 2). About 39 large aftershocks, 4 of 

which were greater than M 5 and 17 of which were 

greater than M 4.0 (Allen and others, 197 4, table 

2), occurred within 2 hours of the main shock. By 

the end of 1971, 55 aftershocks ofM 4 and greater 

had occurred. The focal depth of the main shock 

was about 5 mi (8 km) (Allen and others, 197 4). 

Tectonic ruptures associated with the earthquake 

formed discontinuously for about 9 mi (15 km) 

along a narrow zone trending N. 70--75° E. across 

the northern San Fernando Valley from immediately east of the lower reservoir. The 

U-shaped pattern formed by the epicenters of the 

aftershocks opens to the south and extends well 

north of the main shock epicenter; the south 

boundary of the epicenters essentially coincides 

with the zone of surface ruptures, although the 

aftershocks extended westward several miles 

beyond the rupture zone. (See fig. 2.) The focal 

depths of the aftershocks generally increase 

northward from near surface along the trend of the 

rupture zone to more than 6 mi (10 km) in the area 

north of the main-shock epicenter (Wesson and 

others, 1971, fig. 3). 

Fault-plane solutions for the main shock indicate that the basic mechanism of the initial faulting was that of a thrust or reverse fault striking 

about N. 70° W., dipping about 50° NE, and including a significant component of left-lateral slip 

(Allen and others, 197 4). These data, combined 

with the distribution and fault-plane solutions of 

the aftershocks, describe a thrust fault that strikes 

about N. 70° W. and is inclined about 36°in the 

direction N. 20° E. away from the rupture zone 

where it intersected the earth's surface as a leftoblique thrust fault, north block upthrown. 

This general model is complicated by an alinement, trending about N. 20° E. along the west edge 

of the uu," of a series of aftershocks with focal 

mechanisms indicating chiefly left-lateral strike 

slip, in contrast to the common reverse-slip solution. These solutions may be explained by the 

hypothesis that the inclined fault surface is not 

planar, but rather that it is flexed or stepped down 

to the west along that trend, the trend being that of 

the basement rock-sedimentary rock contact 

north of the reservoirs and south of the San Gabriel fault (fig. 2; Allen and others, 197 4). 

The geologic and seismic evidence thus indicates that the main shock and tectonic surface 

ruptures were caused by displacement on a 

northeast-dipping thrust fault; the upper block of 

the thrust, north of the surface ruptures, was 

thrust southwestward up and over the main San 

Fernando Valley block south of the ruptures. The 

west-facing flexure or step postulated for the west 

margin of the fault surface is probably parallel to 

the trend of slip on the fault and may have controlled the west limit of the zone of surface ruptures. 

TRANSITORY EFFECTS 

The transitory effects of the main shock were 

unique in several ways. Most significantly, they 

included the most intense ground motions ever 

recorded instrumentally for a natural earthquake: 

peak accelerations of 1.25 g for both horizontal 

components and vertical acceleration of 0. 7 g, velocity of displacement 3. 75 ft (114 em) per second, 

and magnitude of displacement about 1.25 ft (38 

em), measured at Pacoima dam, about 51h mi (9 

km) northeast of the reservoirs; duration of shaking at accelerations greater than 0.25 g was about 

7 sec at Pacoima dam. 

Records from seismoscopes (an inverted pendulum whose movement is recorded on a smoked 

glass) were obtained from the Van Norman reservoirs area. Prof. R. F. Scott of the California Institute of Technology (Scott, 1973) developed a time 

history of the ground movement from a seismoscope located on bedrock of the east abutment of 

lower Van Norman dam. His analysis indicates 

that maximum horizontal acceleration there was 

at least 0.6 g and that the duration of shaking at 

accelerations greater than 0.25g was about 13 sec. 

PERMANENT EFFECTS 

As a basis for comparing the permanent effects 

of the earthquake in the Van Norman reservoir 

area, the maximum dimensions of differential 

ground surface deformation in the northern San 

Fernando Valley are summarized. These include 

the first record of tectonic surface rupture in the 

Los Angeles area in historic time, as well as prominent differential elevation of the block north of 

the rupture zone and minor depression of the south 

block, and horizontal displacements of the two 

blocks relative to each other. 

A7 
TECTONIC RVPTVRES 

The main zone of surface ruptures extended discontinuously eastward for about 9 mi (15 km) from 

the east margin of the lower reservoir across the 

valley through San Fernando, thence eastward 

along the base of the foothills to Big Tujunga Canyon (fig. 2). The ruptures dip generally northward, 

from about 30° on the Tujunga segment east of San 

Fernando to about 70° on the Sylmar segment in 

the San Fernando area, 74° on the Mission Wells 

segment near the west margin of the northern 

valley, and 57° on the Freeway segment just east of 

the lower reservoir. (See fig. 3.) Relative displacement on all segments commonly included components of both reverse dip slip (upper or north block 

relatively up) and left-lateral strike slip (north 

block moved relatively westward). The average 

ratio of these components was about 1:1, but varied considerably from segment to segment. In the 

San Fernando area, where the maximum known 

displacements were measured across the Sylmar 

segment, left-lateral strike slip was 6.2 ft (1.9 m), 

reverse dip slip was 4.9 ft (1.5 m), vertical separation was 4.6 ft (1.39 m), and horizontal shortening 

normal to the trend of the zone was 1.8 ft (0.55 m), 

for a net left-oblique-reverse slip of 7.9 ft (2.4 m). 

(Data from U.S. Geological Survey, 1971, p. 62.) 

Equivalent data for the Mission Wells segment, 

about 1 mi east of the lower reservoir, are as follows: left-lateral strike slip 1.5 ft (0.46 m) and 

reverse dip slip 0.52 ft (0.16 m), for an inferred net 

left-oblique-reverse slip of about 1.59 ft (0.49 m). 

Data for the Freeway segment just east of the 

lower reservoir are as follows: left-lateral strike 

slip 0.53 ft (0.16 m) and reverse dip slip about 1.25 

ft (0.38 m), for net left-oblique-reverse slip of about 

1.36· ft (0.42 m). Thus, net slip along the San Fernando zone decreased progressively westward by 

about 83 percent over a distance of about 2.4 mi 

(3.9 km), from 7.9 ft (2.39 m) near the east margin 

of the valley to about 1.3 ft (0.40 m) just east of the 

reservoir. 

The displacements on the Sylmar segment during the 1971 event occurred across a zone about 

350 ft (107 m) wide, but nearly all the lateral 

displacement and about half the vertical displacement occurred in a narrow zone about 100ft 

(30m) wide (U.S. Geological Survey, 1971, p. 57, 

figs. 2, 4). Within the broader zone individual ruptures were spaced as closely as about 50 ft (15 m); 

this zone in general contained about five subparallel ruptures, on which vertical displacements as 

much as 1.6 ft (0.5 m) occurred. Alternatively, 

displacements of 3.4 ft (1.05 m) of reverse dip slip 

and 2.6 ft (0.8 m) of left-lateral slip occurred on a 

single rupture that affected an area about 3ft (1m) 

wide in the natural ground of Lopez Canyon, 

where steeply dipping bedrock is at or near the 

surface (U.S. Geological Survey, 1971, p. 68--69). 

A 1-mi (1.6-km) segment, subparallel to the 

lower trace of the Santa Susana fault about 1.5 mi 

(2.4 km) northwest of the upper reservoir, ruptured discontinuously during the 1971 earthquake 

sequence. Relative displacement of about 1 ft (0.3 

m) left-lateral strike slip occurred near the south 

end of the rupture, under circumstances perhaps 

attributable to slope failure. The thick soil on 

ridgetops above all the ruptures was intensely 

churned, and the ruptures locally traverse 

ridgetops as well as slopes. The lower Santa 

Susana fault is intersected at depth by several 

active oil wells about 0.3 mi (0.5 km) northwest of 

the fault trace. None of the wells appear to have 

been damaged by movement of the fault. An extension of this zone of ruptures just northeast of 

the freeway showed left-lateral reverse displacement of about 0.25 ft (0.08 m). 

VERTICAL DISPLACEMENTS 

Comparison of leveling surveys before 

(1968--70) and after the earthquake (fig. 4) shows 

that changes in elevation formed an arch that affected much of the northern San Fernando Valley. 

The surveys have been referred to a common 

datum at Los Angeles Harbor; the elevation 

changes are accurate to about ±0.05 ft (0.015 m). 

The axis of the arch trends generally east-west and 

plunges westward; thus, the greatest changes 

were in the foothills east of the valley, where maximum uplift exceeds 6:3 ft (1.9 m). (See Burford 

and others, 1971; Savage and others, 1974.) The 

arch extends westward into the. northern San Fernando Valley, where maximum uplift decreases 

progressively westward from more than 5 ft (1.5 

m) just north of the Sylmar segment in the San 

Fernando area to more than 2.5 ft (0.8 m) just east 

of the lower reservoir and 1ft (0.3 m) or less along 

the west margin of the reservoir. Maximums in the 

pattern of uplift coincide in general with topographic highs, indicating that the 1971 event continued a long-established pattern of uplift. 

Profiles of elevation change (profiles B-B 1

, C--C 1, 

D-D1

, fig. 5) show abrupt changes in gradient 

AS 
118° 30; 

> 

~ 

118° 27'30" 118° 25' 

ij~'\(~ ~ EXPLANATION ~~: 

0 1 MILE 

0 1 KILOMETER 

FIGURE 3.-Northern San Fernando Valley showing 1971 ruptures and ancient faults. 

1971 tectonic rupture 

F, Free-y aegment of San 

Fernando fault zone 

MW, Miuton Wells segment S, Sylmar segment 

SS, ruptures along Santa 

Susana fault zone 

Ancient fault 

GH , Granada Hills fault 

Geology from U.S. Geol. Survey, 1971 

and supplementary mapping by 

R. F. Yerkes and M. G. Bonilla, 1972 
,_... 

> 

0 

118° 30' 

~ 

118° 27'30" 118° 25' .-:- ;:: .. if!-,1,~ 20 !X ; , · 9 . $ .. , \.?\ 7 • · l ur W )ll9.i\. ) ,· .- 134°20' 

34°17'30" 

Data collected and reduced by J. N. Alt and T. L. Youd; 

interpretation by J. N. Alt and R. F. Yerkes · 

FIGURE 4.-Northern San Fernando Valley showing 1971 elevation changes determined from preearthquake and postearthquake surveys. 
Cli = Cl) 

Cl) 

.j..~ 

s:: ~ ~ Cl) 

~ .0 

.., "' ... "' Cl) >. 

~ ,Q~ 

e~ I e :; 

0 

z .a 

2 Q., .... 

s:: 

0 .... s:: 

~ (~ ~~ E=: t) .... ~~ co:sC" 

< ·- ~ ~ ..=..= 

z § ·. ~ ~t)~ 

< u ~ ~ = Cl) .5! -;; ~ ~ ~ .s ~8. ~ - ,j.;~ r- ~"CC ~ 0'1 s:: 

~ Cl) = 

- ~ -; co:s :sco .., C"I.O 

CI)O'\ 

~ .... 

- ~ 0 

:; ~ 

~ 0 

tt = 0 11.. u 

~· .g~ ..:. 

.., 

~'0- 0 

~ 

~ ~ ~ 

~~ s:: .., ~ 

'13 oo--.. 

~5 '8_11) 

~~ .... ~ 

~ s:: ;:s 0~ Cl) ~~ <:,) <:,) E ~ 

~~ g t ~ 1: Q.,oC)~ 

%~ .... .., .., ~s:: 0 ~~ s::' .. Cl) s:: ~ 

co .s ~a ':!:~ ,_;j~ 

~-<:,) 0 ~ s::~ s~~ ~ 0 ;:s ~~ ~t:r 

~<:,)~ ~~ 

~t ~~~ ~ ~ 

~.~ ~ bo~ 

~'ts~ ·~ ~·5 ~ 0 

t; 

§ 

"CC 

... 

Cl) 

f3: 

,..g 

0 

...J 

6 

co:s 

"CC 

"CC 

~ 0 

Q., 

0 

Q. 

a" 

a.. 

6~ 

co:s 

"CC 

... 

Cl) 

Q., 

Q., 

:s 

';). 

where they cross the trend of the rupture zone just 

north of the lower dam, indicating that vertical 

deformation was concentrated along that trend in 

the lower reservoir, even though surface ruptures 

were not detected there. However, the degree of 

gradient change along the trend of the rupture 

zone clearly decreases westward (from profiles 

D-D' to B-B'), and no equivalent change in gradient was detected along Balboa Boulevard west of 

the lower reservoir (profile A-A'). 

The profiles also show that differential changes 

in elevation affected the damsites. Although only 

minor changes occurred between the abutments of 

the lower dam (less than 0.05 ft or 0.015 m), the 

east abutment of the upper dam was elevated 

about 0.34 ft (0.1 m) relative to the west abutment 

(profile E-E'), and the east abutment area of the 

proposed damsite was elevated about 0.76 ft (0.23 

m) relative to the west abutment (profile F-F'). 

HORIZONTAL DISPLACEMENTS 

Deformation of the ground surface involved 

permanent horizontal displacements as well as 

vertical changes. The horizontal displacements of 

about 90 control points in the northern San Fernando Valley were mapped by means of vectors 

(fig. 6) that show the differences in station coordinates between 1940 to 1970 and postearthquake 

surveys. Free adjustments of the preearthquake 

and postearthquake coordinates of two survey 

points (PAC E2-ECC and Reservoir) were computed by B. K. Meade, U.S. National Geodetic 

Survey (written commun., Aug. 17, 1972). Preearthquake and postearthquake coordinates for 

the remaining stations were provided by the 

California Division of Highways, City of Los 

Angeles Bureau of Engineering and Department 

of Water and Power, Los Angeles County Engineer, Metropolitan Water District of Southern 

California, and the U.S. Geological Survey. The 

coordinates of these stations were then transformed to a common system, using the two freely 

adjusted National Geodetic Survey points for reference. Stations held fixed for the NGS free adjustments are in the eastern Santa Monica Mountains about 13.7 mi (22 km) S. 38° E. of the reservoirs and in the Verdugo Mountains about 18.6 mi 

(30 km) S. 62° E. of the reservoirs. The line between the fixed stations is about 8.1 mi (13 km) 

long and trends N. 76° E. The displacements indicated by the vectors are accurate to about ±0.2 ft 

(±0.06 m). 

All 
SOUTH NORTH 

FT CM 

] ... 

"' 3! 

g 

iZ 

+ 

~ea of ground failure 

~~ /~-r 20 

---·-/ 10 ------ 0 

'·--

FT ui 1.0 ,. 

j 

·;_!: :i: • "' CM 

"§ ~ I•Bypas~~ .___....____. 30 

ji 

0 • ~.!'! ~~· J20 

._!__-/ PROFILE B-B', WEST SHORE 10 

~=====-~~==--~-~~~--------------------~---------------------------- 0 ti 

FT 

2.0 

1.0 

FT 

2.5 

2.0 

1.0 

0 

·=IE ... 

en -:.: ti= ·S. = .t-

-~= CM 

/·~~ ·-...____ ~f Proposed reservoir •I a : ./ -...... / ~ -~i \ ---------~ . • 50 "/ ~--------------=! ::0 40 

30 

20 J 

' t 10 

~~====~===-~~----------------------------------------------------~0 

PROFILE C-C', EAST SHORE 

en 

CM 

80 

10 

WEST EAST 

FT 

2.0 

~ 

iii 

g 

1- Axis, upper dam •I 

1.0 ~ --------~ ' ~· ·-- PROFILE E-E', UPPER DAM 

CM 

60 

50 

40 

30 

20 

-10 

L---------------------------------------------------------------~0 

~is, proposed dam •I CM 

60 

~~"'- ·---· 50 ----- '~ ~ 40 

-~· --------------- ·---------

" PROFILE F-F', PROPOSED DAM 

: ~========~==============~----------------~~~----~----------------------~0 

1000 2000 

I 

1000 

3000 4000 5000 l'eel 

I 

2000 Meters 

FIGURE 5.-Profiles of 1971 elevation changes determined from preearthquake and postearthquake surveys. 

An abrupt discontinuity in the pattern and 

amount of horizontal displacements coincides with 

the rupture zone and its westward extension. 

North ·of the rupture zone displacement was generally westward; south of the zone it was northward. Distances between stations on opposite sides 

of the zone were shortened, consistent with thrust 

faulting. Northwest-trending lines that cross the 

zone at low angles were lengthened, whereas 

northeast-trending lines across the zone were 

shortened, consistent with left-lateral strike slip. 

North-south shortening and left-lateral displacement decreased progressively westward along the 

zone. 

A12 
The discontinuity passes through the lower reservoir just north of stations CROFTS, WP 10, and 

ZEL L-3B. Although poorly defined because of 

widely spaced stations, the discontinuity appears 

to be subparallel to, but about 0.25 mi (0.4 km) 

north of, the Granada Hills fault and may coincide 

with an unnamed fault (fig. 6). Although this latter fault displaces surficial deposits, there is no 

evidence that it moved during the 1971 earthquake. 

Plots of displacement versus distance along the 

Sylmar, Mission Wells, and Freeway segments of 

the rupture zone form fairly systematic curves (fig. 

7), which indicate that displacement across the 

rupture zone decreases westward at rates of about 

1 in 3,000. Alternatively, the rate across the westward extension of the zone in the area of the lower 

reservoir is less than one-third as much, approximately 1 part in 10,000. 

The horizontal and vertical displacements of the 

east abutment area of each damsite relative to the 

west abutment, attributed to the 1971 deformation, are as follows: 

Upper dam, (stations 1,550 ft or 472 m apart). 

Horizontal, 0.30 ft (0.09 m) S. 18° W; vertical, 0.34 ft (0.10 m) up. 

Proposed dam, (stations 2,260 ft or 689 m 

apart). Horizontal, no significant relative 

displacement; vertical, 0. 76ft (0.23 m) up. 

Lower dam, (stations 2,651 ft or 808 m apart). 

Horizontal, 0.476 ft (0.145 m) N. 44° E.; 

vertical, no significant relative displacement. 

The horizontal deformations (strains) in the 

Van Norman reservoir area were analyzed 

through the use of finite element methods. The 

area west of San Fernando Road was divided into 

250 triangular elements with corners at horizontal control points whose preearthquake and postearthquake positions are known. For each element 

the following surficial strains were determined: 

1. The dilatational strain (a measure of the 

amount and sense of change in area); 

2. The maximum shear strain (a measure of 

the change in shape); 

3. The principal strains and their orientation. 

The accuracy of the strain values depends on the 

accuracy of the horizontal surveys (estimated at 

±0.01 ft or 0.003 m) and the size of the particular 

triangular element. Using an average-size element this accuracy is estimated at ±0.01 percent 

strain, the value used as contour interval. In addition, it was assumed that the strains within each 

element were homogeneous. Because of these uncertainties and assumptions, the mapped strains 

are approximate; however, the general orientation 

and relative intensities shown are generally valid. 

The dilatational strain (percent change in 

element area) is contoured and shown as positive 

(expansion) or negative (contraction) in figure 8. 

The most conspicuous feature on the map is the 

narrow band of contraction that coincides with the 

rupture zone, the greatest values (as large as 

-0.33 percent) being concentrated around the 

west end of the rupture zone just east of the lower 

reservoir. Westward across the reservoir along the 

trend, the values of contraction decrease by a factor of 5-10, although the general trend continues 

as far west as Balboa Boulevard. A broad area of 

weak expansion (values less than 0.02 percent) 

extends generally east-westacross the map south 

of the rupture zone, coincident with areas of slight 

depression. Areas of greatest expansion (values as 

great as 0.15 percent) coincide with areas of 

greatest positive elevation change. The most conspicuous of these coincides with the low hills about 

0.6 mi (1 km) northeast of the lower dam; this area 

of deformation extends northwestward toward the 

east abutment of the proposed dam. An easttrending band of milder expansion is present in 

the west abutment area of the upper dam, and a 

third is roughly coincident with the area of differential elevation 0.5 mi (0.8 km) east of the upper 

dam. 

Angular distortion of the elements is mapped as 

(1) maximum shear strain (the maximum angular 

change of previously orthogonal lines within each 

element, in percent; fig. 9) and (2) axes of principal 

strain showing the magnitude and orientation of 

the most extensional and most contractional normal strains in each element (fig. 10). The map of 

maximum shear strain is similar in some aspects 

to the dilatation map. A narrow band of relatively 

high shear strain (values greater than 0.03 percent) coincides with the west end of the rupture 

zone. The band broadens and decreases in intensity westward across the lower reservoir. High 

shear strain and relatively large expansion also 

characterize the areas along the east margin of the 

lower reservoir that were elevated, most notably 

the area just southeast of the proposed dam. 

The pattern of principal strains (fig. 10) also 

clearly delineates a discontinuity coincident with 

A13 
> 

..... 

""'" 

118° 30' 

Base from U.S. Geological Survey, 1:24,000 

Oat Mountain, 1952; San Fernando, 1966 

118° 27'30" 

0 1 MILE 

0 1 KILOMETER 

118° 25' 

Data collected and reduced by T. L. Youd 

and Sammy Shaler; interpretation by 

T. L. Youd and R. F. Yerkes 

34° 20' 

34° 17'30" 

FIGURE 6.-Northern San Fernando Valley showing 1971 horizontal displacements determined from comparison ofpreearthquake and postearthquake surveys. 
(/). 

1.: 

s:: 

(II 

., 

~ 

~ 

z ~ ~ 0 E §~ 

~ 0.. .... (II 

:::1 ~t < .~~(II ~ c: ., 

3 s:· ~ ....:I C) 

~ 2 ~.s 

>< ...... ·-~ 

~ r- ~f:l:l 

0\ (II 

...... t 

~ 

~ 

~ (II 

(II 

~ 

~-

~~ -'1::! 

a§ 

s::c 

C)'t 

·!::! 0\ 

5"" ~ 

CD ..... ~ ~ ~ 

~ .8'0-~t)' C) -s ~ ~ ~~ ~ > "c:: .2 (II j a c: ~l~ o::s (II ..... 

.9 E t (II 

E'l::S;;-s 

"' s::., a.. 

~ o::s.5~ > s:: C) o::s ;:; .g 1:).'1::! 

Vl '-~'0-~ 

.~ ~ ~ 't!S::a... 

boE;; .5 (II~ ~g'l::S 

o- s:: ~ t:l.o::s f:l:l 

s 

C<:S 

"c:: 

..... 

a) 

~ 

.9 

c:i 

...J 

ef 

C<:S 

"c:: 

"c:: 

~ 0 

0.. 

0 

0. 

c:l 

Q.. 

s 

C<:S 

"c:: 

..... 

a) 

0.. 

0.. 

:::1 

:Jthe rupture zone and its westward extension; 

strains north of the rupture zone and its westward 

extension are, in general, considerably larger than 

those to the south. The major principal strains in 

the vicinities of the upper and proposed dams are 

generally parallel to the axes of the dams; they 

range in magnitude from about 0.01 percent to 

0.02 percent in extension. The minor principal 

strain is oriented generally normal to the axes of 

the dams, with values of about 0.03 percent contraction. 

SUMMARY 

In general, the intensity of surface deformation 

associated with the 1971 earthquake decreased 

westward across the northern San Fernando Valley and northward away from the rupture zone and 

its westward extension across the lower reservoir; 

the Van Norman area is well within the deformed 

area. Deformation was concentrated along the 

rupture zone and its westward extension; but 

another prominent area of deformation is the ridge 

of low hills east of the lower reservoir, where relatively large vertical uplift, areal expansion, and 

shear strain were combined. The map of principal 

strains emphasizes the general north to northwest 

trend of the most contractive strain, the correlation of discontinuities in the principal strain field 

with areas of relatively large shear strain and 

mapped faults, and the slight extensional strains 

( <0.02 percent) parallel to the axes of the upper 

and proposed dams. 

The deformations of the ground surface that accompanied the 1971 earthquakes are integral 

parts of an established regional pattern of tectonic 

deformation that includes left-oblique-reverse 

displacements along the San Fernando fault zone 

and differential arching and horizontal displacements that extend across the northern San Fernando Valley into the reservoir area just north of 

the rupture zone. 

GEOLOGY OF THE VAN NORMAN 

RESERVOIRS AREA 

The reservoirs occupy a southeast-draining 

stream course at the west margin of the northern 

San Fernando Valley; the bedrock flanks of the 

reservoirs area are formed by irregular ranges of 

low hills that rise about 200ft (60 m) above the 

adjoining lowlands. The hills were formed in late 

geologic time by arching and faulting of the sort 

that occurred in 1971 and subsequently were 

A15 
DISTANCE ALONG ZONE (MI) 

0 I 2 3 4 5 9r------------.-.---------,~.r-----------~~------------,-----------~------~ I I 

i= 

u.. 

1-

z 

LU 

~ 

LU 

(.) 

<( 

....J 

0... 

(fJ 

0 

e Total displacement of cultural features 

8 _ across rupture zone (Sharp, 1974) 

7 -

6-

5 -

4-

3r 

• Horizontal displacement of cultural 

features across rupture zone (Sharp, 

1974; this report) 

0 

A 

Relative horizontal displacement of 

survey pointe mora than 0.2 km from 

rupture zona (tabla 1) 

Vertical displacement of cultural 

features across rupture zona 

(U.S. Gaol. Survey, 1971; this report) 

Fraallf--k _L_owe--:--r ~>I a r" rasarvo i r "71 s g ant 

Mission 

Walls 

~agmen?l 

I 

5 

I 

6 

.w 

I 

7 

tV 

1

v r··" T 

j_ 

8 

- 2.5 

- 2.0 

1-

1.5 ffi 

~ 

LU 

(.) 

<( 

....J 

0... 

- 1.0 (fJ 

----f 

- 0.5 

0 

9 

0 

DISTANCE ALONG ZONE (KM) 

FIGURE 7.-Horizontal displacements, 1971, including 

total displacements and vertical components where 

known, across the several segments of the San Fernando zone between Foothill Boulevard on the east and 

Balboa Boulevard on the west. Displacements based on 

TABLE 1.-Data on displacements along the San Fernando zone 

and its westward extension 

A. Total displacements and horizontal components across segments of rupture zone. based on offset of cultural features ,(from Sharp, 1974; U.S. Geological Survey, 

1971; this report! 

Total Horizontal Vertical RVS No. (mJ (m) <ml 

R 2.2 s 2.1 2.1 T 2.4 2.0 v 2.5 2.0 1.4 w 2.0 1.1 

X,Y ========== .5 .4 .15 z ---------- .41 .16 .32 

B. Relative horizontal dzsplacements across rupture zone and its westward extension based on displacement of survey points more than 0.2 km from zone <this report) 

Vector pair 

ZEL L-1/ZEL K-3B ---------------------------- ZEL L-1/ZEL L-3D ---------------------------- Avg ofZEL L-1+LITTLER/ 

PAC A 3-B ---------------------------------- GOTHIC/PAC B-3B ---------------------------- WP 9/WP 1L __________________________________ _ 

LF 9/avg ofWP ll+TOWER ___________________ _ LF9/TOWER _________________________________ _ NOSE/TOWER _______________________________ _ 

5-41.7/405-4 7.8 -------------------------------- 5-41.5/405-4 7.8 --------------------------------

Pr~r.~o~~!1kTf 118 __ ----------------------- _ Proportioned avg of 

t!t8!~~d G':~8~- --------------------------

Vector 

difference (ft) 

0.8 

. 8 

1.0 

1.4 

1.3 

1.5 

1.7 

1.7 

1.5 

1.4 

1.8 

2.6 

Orientation of difference1 

s. 66·w. S.6o·w . 

s. 5o·w. s. 42·w. s. 12·w. s. 44•w. s. 61·w. s. 52•w. s.45·w. 

s.48·w. 

s. 5o•w. 

offset of cultural features (from Sharp, 1974; U.S. Geol. 

Survey, 1971; this report); relative horizontal displacements across the zone and its westward extension determined from displacement of survey points more than 

0.2 km from the zone (from table 1, this report). 

TABLE 1.-Data on displacements along the San Fernando zone 

and its westward extension-Continued 

B. Relative horizontal displacements across rupture zone, etc.-Continued 

Vector Orientation difference of Vector pair (ft) difference1 

SF-1/avg of 5-41.0+PAC G--2B ------------------ 2.7 s. 32•w. 

PAC Fl-B/PAC G--2B -------------------------- 3.0 s. 3s•w. Avg of PAC F-lB+SYL H-12B/ 

i>1c ~~}tl~j~1/3 ~~---------------------- 3.2 s. 57•w. 

SYL H 12-B/avg of PAC G--2C+PAC J-lD ------------------------ 3.9 s. 7o•w. SYL 1-12/proportioned avg of 

PAC G--2C+PAC J-lD ------------------------ 4.3 s. 75•w. SYL J-12B/PAC J-lD -------------------------- 5.4 s. 76·w. 

modified by erosion. As a consequence of the structural deformation, the bedrock strata throughout 

the reservoirs area are steeply tilted and faulted, 

and flat-lying stream terrace deposits that cap the 

bedrock hills, as well as younger valley fill deposits, are locally faulted. 

STRATIGRAPHY 

The oldest bedrock exposed in the reservoirs 

area is siltstone and shale of the upper Miocene 

A16 
·- --- ~~- ;.a.rtt:'~:-,'- --

Base from U.S. Geological Survey, 1:24,000 

Oat Mountain, 1952; San Fernando, 1966 

11~0 27'30" 

EXPLANATION 

1971 

.... tectonic ~ 

rupture 

F, Freeway segment; MW, Mission 

Wells segment 

--+3-- --·2--

Contour of approximately equal dilatation 

Number shows change in horizontal com- ponent, in hundredths of percent. Positive 

(+) values indicate that a pre-1971 unit of area is now larger; negative (-) values, that a unit of area ts now smaller. Hachures on side of most negative dilatation 

Area of positive ~ dilatation (expansion) 

greater than 0.03 percent 

UD, upper dam; PD, proposed dam; 

LD,lower dam 

Data preparation and programing by 

T.L.Youd,A.T.F.Chen,and 

D. Lichtenstein; interpretation 

by T. L. Youd and R. F. Yerkes 

FIGURE B.-Northern Sal1 Fernando Valley showing orientation and magnitude of dilatational strains derived from 

horizontal displacements based on 194~ 70 and postearthquake surveys (fig. 6; see text for explanation). 

Modelo Formation. This unit forms the east abutment area of the lower dam and, together with 

younger rocks, is folded into a west-plunging anticline that places strata of the Pliocene Pico Formation under the west abutment. (See Yerkes and 

others, 1973, sheet 1.) An intervening 400ft (120 

m) of marine sandstone and thin sandy shale of the 

Towsley Formation, exposed north of the east 

abutment on the north flank of the anticline, is 

missing in the west abutment area because of 

faulting, gradational variations, or erosion prior 

to deposition of the Pico. 

The Pico Formation is about 400ft (120m) thick 

and consists of friable coarse-grained marine 

sandstone that contains lenses of calcareous sandstone, locally having abundant mollusks representative ofthe middle Pliocene of Pacific Coast 

usage. (See Oakeshott, 1958, p. 81.) The Pico is 

A17 
118"30' 

- ... 

... _, =-r-- ~~ .. q{,. = 

rl'-JiL 

_.., 

_k.:. .. - _I!. -WL~'= ,;, ~~=~'- -~-' .:_ -- ! '~ 

. Ju.~~,-" 

Base from U.S. Geological Survey, 1:24,000 

Oat Mountain, 1952; San Fernando, 1966 

118° 27'30" 

' - 1 -

EXPLANATION 

1971 ~ tectonic rupture 

F, Freeway segment; MW, Mission 

Wells segment 

---3---

Contour of approximately equal shear 

strain, in hundredths of percent 

* Area of maximum shear 

strain greater than 

0.04 percent 

Area of maximum 

~ shear 

strain greater than 

0.1 percent 

UD, upper dam 

PO, proposed dam 

LD,lower dam 

Data preparation and programing by 

T.L.Youd,A.T.F.Chen,and 

D. Lichtenstein; interpretation 

by T. L. Youd and R. F. Yerkes 

FIGURE 9.-Northern San Fernando Valley showing distribution and magnitudes of maximum shear strains derived from 

horizontal displacements based on 1940-70 and postearthquake surveys (fig. 6; see text for explanation). 

overlain unconformably by nonmarine gravels, 

mudstone, pebbly sandstone, sandstone, and 

fresh-water limestone, the Sunshine Ranch 

Member of the Saugus Formation. This unit underlies much of the reservoirs area north of the 

lower dam. On the west margin of the reservoir, 

the Sunshine Ranch is about 3,000 ft (910 m) thick 

and on the east less than 2,000 ft (610 m) thick; the 

lower part of the sequence is missing on the east, 

perhaps because of faulting. The Sunshine Ranch 

Member was assigned a probable late Pliocene age 

on the basis of correlation with marine equivalents to the west and general stratigraphic position (Oakeshott, 1958, p. 82). Fossil horse teeth of 

middle or late Pliocene age, referred toPliohippus 

mexicanus or P. osborni, were collected from the 

Sunshine Ranch Member in an exploratory trench 

along the axis of the proposed dam. 

The Sunshine Ranch Member is unconformably 

overlain by the upper member of the Saugus 

A18 
Base from U.S. Geological Survey, 1:24,000 

Oat Mountain, 1952; San Fernando, 1966 

118°27'30" 

~ ~ # He~ek A~ "" e-.. ~ 

:•J.S":~::·,. 

EXPLANATION .,.___,.~....,._ 

1971 tectonic rupture 

F, Freewcy ,.,.ent; 

MW, MiMon welll N6'"mt 

-+- '« Axes of principal strain 

~~- Heavy line 8how• oiWntt~tlon tJnd ~..,.ltude 

of eontrtu:tioMl principal nrwln; light' line 

6howl .rame for ex•n.toMl principal •train. 

SCIIIe, 0.3 Inch (7.6 mm) = 0.06 percent; 

symbol In circle, 0.31nch = 0.02 perctmt 

Generalized trajectory of minor 

principal strains 

These linea and their aptSCing have no quan -

tltative value, but indicate only the geneNl 

orientation of the mOlt contractional principal mains 

Data preparation and programing by 

T. L. Youd, A. T. F. Chen, and Sammy Shaler; 

interpretation by T. L. Youd and R. F. Yerkes 

FIGURE 10.-Northern San Fernando Valley showing axes of principal strains and trajectories of most contractional principal strains derived from horizontal displacements based on 1940-70 and postearthquake surveys (fig. 6; see text for 

explanation). 

Formation-fluvial and alluvial deposits consisting of poorly sorted loosely consolidated sandy 

conglomerate and crossbedded pebbly sand. These 

deposits form the bedrock margins of the reservoirs area north of the proposed damsite, where 

they are about 3,000 ft (910 m) thick. Oakeshott 

(1958, p. 85) assigned them to the lower Pleistocene on the basis of a tooth of the primitive horse 

Equus, found in Saugus beds on the east margin of 

the reservoir. He noted, however, that some strata 

lower in the unit could be older. 

In some parts of the reservoirs area, the bedrock 

is overlain by generally subhorizontal surficial 

deposits of varying ages. The older and more intensely faulted and weathered surficial deposits 

are preserved on the relatively high margins of the 

reservoirs. Oakeshott (1958, pl. 1, p. 86) mapped a 

small exposure of distinctly tilted and faulted 

A19 
reddish-brown fanglomerate that caps the hills 

immediately south of the west abutment of the 

lower dam and tentatively correlated it with his 

Pacoima Formation, of inferred middle Pleistocene age. This is probably the oldest surficial 

deposit in the reservoirs area. 

Stream terrace and alluvial deposits of late 

Pleistocene and Holocene age form erosional remnants that overlie bedrock of the low hills northeast of the reservoirs, as well as that in the foundation area of the proposed dam. The basal contact of 

these flat-lying deposits forms a prominent reference horizon against which to measure the tectonic deformation of the area. 

STRUCTURE 

The Van Norman reservoirs area overlies the 

steep south limb of a broad syncline (Mission Hills 

syncline) that extends under the entire western 

part of the northern San Fernando Valley (fig. 2). 

The buried axis of the syncline, as inferred from its 

exposed limbs, trends about N. 60° E. north of the 

upper reservoir. The attitude of the south limb in 

the reservoirs area is such that the bedrock there 

dips northward between 50° and 80°, with steeper 

and locally overturned dips toward the south. 

Immediately north of the lower dam, the south 

limb of the syncline intersects the east-trending 

zone of generally north-dipping faults that extends westward along the trend of the San Fernando zone. This zone includes an element of the 

Granada Hills fault (fig. 3) which extends westward from immediately west of the lower dam 

within the Sunshine Ranch Member. East of the 

lower dam, faults of the zone form boundaries of 

the stratigraphic units. The faults generally dip 

northward subparallel to stratification in the bedrock, and the compressive nature of the faulting 

has locally caused overturning of the bedrock and 

greatly decreased the exposed thickness of the 

bedrock units. The westernmost (Freeway) segment of the main zone of 1971 tectonic ruptures is 

about 1,200 feet (360 m) long and parallels the 

zone of older faults immediately east of the lower 

reservoir. The 1971 displacement on this segment 

and on the San Fernando zone in general was 

left-oblique-reverse, which is compatible with 

north-south compressive deformation. The similarity in attitude and compressive nature of deformation along the older faults suggests that 

movement on them was similar to that of 1971. 

Faults and ruptures in the proposed damsite 

have been mapped at a scale of 1:1,200 (Yerkes and 

others, 1973, sheet 2) and those in the Van Norman reservoirs area have been mapped at a scale 

of 1:4,800 (Yerkes and others, 1973, sheet 1). Bedrock faults that displace the basal contact of the 

surficial deposits are differentiated and shown by 

special symbols on those maps. Many faults were 

recognized on the basis of bedrock exposures only; 

and some of these probably would displace 

surficial deposits if such deposits were present at 

the particular localities. The distribution and density of faults shown on the maps are chiefly functions of exposure; virtually all the faults were first 

detected in artificial cuts, and areas of bedrock 

shown as unfaulted generally have no artificial 

cuts. Thus, faults shown as discontinuous in the 

reservoirs area because they have not been continuously exposed in artificial cuts should be considered as segments of continuous, throughgoing 

zones. On this basis the Granada Hills fault would 

be an integral element of the zone that merges 

eastward with the San Fernando zone of 1971 tectonic ruptures. 

A similar but narrower zone of generally 

north-dipping faults extends eastward through 

the bedrock abutments of the upper dam, generally on trend with the 1971 ruptures on the east 

margin of the reservoir area near the proposed 

damsite. A large number of short fault segments 

having the same general trend are present at the 

site of the proposed dam. These fault segments are 

inferred to be parts of a relatively broad zone that 

extends northeastward through the reservoirs 

area from the main San Fernando zone. Lateral 

displacement on these faults cannot be determined 

from present evidence. Maximum measured vertical separation at the proposed damsite is about 

0.33 ft (0.1 m) on faults that clearly displace the 

bedrock-surficial deposits contact; steplike discontinuities of the contact as much as several feet 

high are alined with bedrock faults in a number of 

places where evidence for fault displacement of the 

contact is ambiguous (Yerkes and others, 1973, 

sheet 2). The latest movement is as young as late 

Pleistocene for at least some of the faults in each of 

the damsites. On the basis of the distribution and 

age of faulting, each of the damsites is intersected 

by elements of fault zones that are active or capable of movement. 

LOWER DAMSITE 

The abutment areas of the lower dam have been 

extensively exposed by exploratory trenches cut 

after the 1971 earthquake: 2,550 ft (777 m) of 

A20 
trench north of the east abutment, several 

trenches totalling 1,850 ft (564 m) north of the 

west abutment, and 1,850 ft (564 m) of trench 

extending south of the west abutment. (See Yerkes 

and others, 1973, sheet 1, for locations.) The bedrock geology exposed in the trenches has been logged graphically at a scale of 1 in. equals 10ft (1 

cm=l.2 m) by the Los Angeles Department of 

Water and Power; we have reviewed copies of the 

logs in the field. 

Trench 1 extends northward across the main 

elements of the zone of faults north of the east 

abutment. Bedrock there dips steeply north or is 

overturned to the south. A number of easttrending folds are exposed near the trend of the 

1971 rupture zone. The limbs of some of the folds 

have been thrust over adjoining strata along 

gently dipping faults. The bedrock is also cut by 

numerous closely spaced steeply dipping faults 

that transect bedding at low angles; these faults 

are common in the Pico Formation north of both 

abutments and may be related to the position of 

the Pico as a sliver between two of the more continuous faults of the zone. 

The most likely spatial correlative of the Freeway segment of the 1971 ruptures is an easttrending older fault that dips 68° N. near the south 

margin of the small valley that is alined with the 

rupture. Reverse separation of about 3 ft (1 m) is 

indicated by bedrock relations on either side of this 

older fault, which thus correlates with the 1971 

fault in alinement, attitude, and apparent displacement. However, no surficial deposits cap 

these exposures, nor is there any evidence of 1971 

movement on the fault. Where surficial deposits 

are preserved, as in the central part of this small 

valley, they apparently are not disturbed by the 

bedrock faults. 

Bedrock north of the west abutment of the dam, 

as exposed in trenches 2, 3, 4, and 5 (numbered 

from east to west), dips steeply north; although the 

stratification is only locally tightly folded or overturned in this area, numerous closely spaced faults 

transect bedding in the upper part of the Pico Formation. 

Trench 2, north of west abutment, exposed a 

narrow zone of east-trending faults that traverses 

the former island just south of survey station 

CROFTS (Yerkes and others, 1973, sheet 1). The 

northernmost of the faults dips 70-80° S. and the 

southern fault dips 73° N.; the two faults intersect 

or merge near the floor of the trench. Both faults 

are subparallel to adjoining bedrock strata, which 

are overturned in the area just north of the zone. 

The base of the surficial deposits apparently is 

stepped down to the south about 2ft (0.6 m) over 

the south-dipping fault, whereas the bedrock surface over the north-dipping fault extends down 

about 5 ft (1.5 m) into the clayey gouge of the fault 

but apparently is not offset appreciably. The faults 

were not well exposed in the overlying surficial 

deposits, which are so very poorly sorted and 

poorly bedded that no strata could be traced with 

confidence across the fault trace. The ground surface and the modern lake deposits that formed on 

the reservoir floor apparently were undisturbed 

over the upward extension of the fault traces. 

A 1971 surface rupture, generally alined with 

the faults in trench 2, extends across the top of the 

former island (Barrows and others, 1974, pl. Ill). 

The surface rupture had an apparent vertical separation of 0.8 in. (2 em) down on the north in 

bedrock on the upper part of the east slope of the 

island, in opposition to the separation as exposed 

in the trench wall. The surface rupture occupied 

the steeply north-dipping bedding surface between relatively hard pebbly sandstone and soft 

mudstone. On the west slope of the island, the 

rupture appeared as a somewhat discontinuous, 

but linear, narrow fissure in the loose sand and 

gravel that covers the surface there. 

Although the surface rupture may be attributable to local effects of shaking rather than to tectonic faulting, the fact that it coincides with a fault 

directly alined with the San Fernando zone east of 

the lower reservoir indicates that it should be considered a significant element of that zone. A fault 

exposed 400ft (120m) to the west in the wall and 

floor of trench 3 is alined with the island faults; in 

plan view the trench 3 fault is offset about 9ft (2. 7 

m) by a north-trending cross-fault. 

Trenches 3, 4, and 5 expose north-dipping sandstone and pebbly sandstone of the Sunshine Ranch 

Member, which is cut by numerous faults and 

shear zones, generally subparallel to stratification 

in bedrock. None of the faults exposed in these 

trenches disturb the contact at the base of surficial 

deposits. 

Trench 6 extends southward along the east edge 

of the hills south of the west abutment; it exposes 

northwest-dipping strata of the Pico Formation 

and Sunshine Ranch Member of the Saugus Formation. A number of bedrock faults in the Sunshine Ranch exposed in this trench trend west to 

northwest, generally at a considerable angle to the 

trend of the Granada Hills-San Fernando zone. 

A21 
Most of these faults dip southward, thus transecting bedrock, and show relative movement down on 

the south with dip separation of locally more than 

25ft (7.6 m) measurable at the bedrock-surficial 

deposits contact. Many of these faults extend well 

up into the stream terrace deposits that cap the 

hills; although some are truncated by beds within 

the surficial deposits, others extend up into or near 

the soil zone near the ground surface. 

In summary, some of the faults in the area of the 

lower dam have moved since deposition of the 

middle Pleistocene(?) terrace gravels that cap the 

hills southwest of the dam. Unambiguous evidence that the modern reservoir sediments were 

displaced by 1971 faulting was not found, but such 

evidence could have been obliterated by drawdown 

of the reservoir. Even though such evidence is 

lacking, the west abutment area of the lower dam 

is cut by a number of bedrock faults that show late 

Pleistocene movement and that are elements of 

the active San Fernando zone. 

UPPER DAMSITE 

The bedrock abutments of the upper dam are 

also cut by faults. The bedrock consists of steeply 

dipping strata of the upper member ofthe Saugus 

Formation, the youngest bedrock unit in the area. 

The west abutment was exposed in 1968 by a 

trench about 250 ft (76 m) long that trended S. 30° 

E. from a point about 90ft (27m) west of the west 

end of the dam parapet; bedrock was exposed to a 

depth of about 40ft (12m) below the ground surface. A map and graphic log of the trench were 

furnished by the Los Angeles Department of 

Water and Power (DWP drawings D-3936-G-5, L 

955-G-1). The bedrock consists of interbedded 

silty to pebbly sandstone, siltstone, and shale, dipping 70°-80° N.; the sandstone is firm and well 

consolidated but not cemented. The shale is very 

stiff and contains slickensides parallel to bedding 

surfaces. The trench exposed a fault zone that 

forms the boundary between two sequences of 

shale that have discordant attitudes; the zone 

trends N. 80° E. and dips 67° N. (Yerkes and 

others, 1973 sheet 1) and extends westward more 

than 1,000 ft (300m) into the bedrock ridge west of 

the reservoir. There is no evidence for sense of slip 

or age of last movement on the fault. 

An east-trending rupture associated with the 

1971 earthquake, on the southwest slope of the 

west abutment (DWP drawing D-4222--G-5), is 

approximately alined with a north-dipping fault 

mapped during the present investigation. This 

alinement extends eastward toward a pressure 

ridge on the valley bottom at the toe of the dam 

embankment. The pressure ridge probably was 

formed by a slide in the dam embankment during 

the 1971 earthquake; the slide moved downstream 

and involved the south slope, crest, and upstream 

face of the embankment. The pressure ridge extends almost continuously across the toe of the 

embankment (Yerkes and others, 1973, sheet 1) 

and is as much as 1. 7ft (0.52 m) high (DWP drawing D-4222--G-5). The pressure ridge was exposed 

at three localities along its length by exploratory 

trenches. In each trench the ridge overlies small 

north-dipping thrust faults in the fill material; the 

trenches were not deep enough to determine 

whether the thrust faults are related only to the 

slope failure within the embankment or, possibly, 

to deformation of the underlying alluvium. Seed, 

Lee, Idriss, and Makdi~i (1973, p. 141) found no 

evidence of slide movements in the foundation alluvium. The alinement of the pressure ridge coincides with a vertical fault in the bedrock of the east 

abutment. 

The east abutment of the upper dam was exposed by a pipeline trench opened after the 1971 

earthquake. The trench trends N. 13° W. along the 

crest of the ridge south of the east abutment and 

was cut deep enough to expose bedrock only discontinuously along the southernmost 185ft (56 m) 

of the ridge. The trench was not examined by us; a' 

graphic log of the west wall of the trench, at a scale 

of 1 in. equals 10ft (1 cm=1.2 m) (DWP drawing 

N-762--G-1), forms the basis of this description. 

Bedrock exposed in the trench consists of steeply 

north-dipping silty to pebbly sandstone of the 

upper member of the Saugus Formation. Near the 

south end of the bedrock ridge, there is a zone 25 ft 

(7.6 m) wide of east-trending north-dipping reverse faults and gouge. About 65 ft (20 m) north of 

this fault zone, there is a south-facing step in the 

bedrock surface at least 13ft ( 4 m) high. The upper 

4ft (1.2 m) of this step is erosional; the bottom part 

is not exposed but the step dips northward so that 

the upper part overhangs, and thus it could coincide with a north-dipping bedrock fault. 

Exploratory drilling in the area southeast of the 

east abutment of the upper dam indicates that the 

south-facing step in the bedrock surface extends at 

least 400ft (120m) S. 65° E. from the trench; in 

that area it also coincides with a south-facing topographic scarp. Additional drilling another 150 ft 

A22 
( 45 m) southeast indicates that the bedrock step is 

not present there on the same trend, although its 

absence does not preclude a fault. 

About 28 ft (8.5 m) north of the upper lip of the 

bedrock step in the trench, a vertical east-trending 

fault cuts the bedrock and apparently displaces 

the base of the surficial deposits about 1ft (0.3 m) 

down on the north. North of this fault the bedrock 

surface slopes about 10° N.; thus, the vertical fault 

marks a pronounced change in slope of the bedrock 

surface, from 10° N. to 12° S. About 122ft (37m) 

north of the vertical fault an east-trending northdipping reverse fault displaces surficial deposits 

with a vertical separation (up on the north) of more 

than 1 ft (0.3 m). 

The several east-trending faults in the abutments of the upper dam, including their probable 

correlatives in the bedrock ridge west of the dam, 

constitute a zone about 300 ft (90 m) wide and at 

least 2,600 ft (790 m) long. The trend of these 

faults extends eastward across the northern part 

of the proposed reservoir toward the 1971 ruptures 

on the bedrock ridge to the east. It is reasonable to 

consider that all these faults and ruptures are 

segments of a zone, the total length of which is at 

least 6,000 ft (1,830 m) from the ridge west of the 

upper dam to the ridge east of the proposed reservoir. Such a zone may be part of a yet broader and 

longer zone, other elements of which, subparallel 

in general attitude, are exposed in the bedrock of 

the proposed damsite. This distribution of subparallel bedrock faults is considered to be part of a 

broad throughgoing zone that extends northwestward, perhaps from the San Fernando zone east of 

the lower reservoir, through the site of the proposed dam and the abutments of the upper dam, 

through the ridge west of the upper dam, and on 

toward the Santa Susana fault. This zone contains 

1971 ruptures as well as older faults that displace 

surficial deposits and is thus considered active. 

Displacement may have occurred in 1971 on an 

inferred, buried fault approximately coincident 

with an extensive zone of ground failure that 

trends northeastward across the valley floor north 

of the upper reservoir. The trend of this zone of 

damage overlies the inferred axis of the Mission 

Hills syncline; it also coincides with a prominent 

1971 rupture near the Santa Susana fault about 

1.25 mi (2 km) west of the upper dam and with the 

older faults that extend northeastward across the 

lowland area northeast of the San Fernando Valley Juvenile Hall (fig. 3). Postearthquake resurveys of horizontal control stations on bedrock well 

north and south of the zone of ground failure indicate that there was about 0.4 ft (12 em) of rightlateral displacement across the zone during the 

earthquake. (See Y oud, 197 4, for details.) In addition, the lower part of Grapevine Canyon bends in 

a right-lateral sense near the north margin of the 

zone of failure just west of San Fernando Valley 

Juvenile Hall. The bend of the stream course suggests that similar movements may have occurred 

in the geologic past; consequently, such ground 

failures as occurred along this zone in 1971 should 

be expected to accompany future large earthquakes. 

PROPOSED DAMSITE 

Bedrock at the site of the proposed dam consists 

of steeply north-dipping sandstone, pebbly sandstone, and siltstone of the Sunshine Ranch 

Member of the Saugus Formation. These rocks 

have been exposed throughout most of the embankment area by excavation and removal of the 

subhorizontal surficial deposits, which are as thick 

as 70ft (21 m). The basal contact of the surficial 

deposits is a sharp angular unconformity, representing the time interval during which the bedrock was consolidated, steeply tilted and faulted, 

and eroded. The age control for dating these 

events, and for dating the faults that displace or 

are truncated by the unconformity, varies considerably, depending on the ages of the materials 

above and below the unconformity. The upper 

limit or minimum age is determined by the age of 

the surficial deposits, which were examined and 

mapped in detail in order to determine their absolute and relative ages. 

SURFICIAL DEPOSITS 

The surficial deposits exposed on the north wall 

of the perimeter cut of the embankment area were 

differentiated and mapped on Polaroid photographs at various scales (between approximately 1 

in.=25 ft or 1 cm=3 m and 1 in.=70 ft or 1 cm=8.4 

m). The lithologies of the mapped units were described in the field with emphasis on the contact 

relations between lithologic subdivisions and 

buried soil profiles. A vertical section of the 

surficial sediments (fig. 11) was constructed from 

coordinates and altitudes of surveyed markers 

(U-222, U-214, and so on) placed on the contact 

between the bedrock and surficial deposits and 

A23 
> 

~ 

~ 

1140 

t-til 1120 LL 

z 

- 1100 z· 

0 

j:: 1080 <( 

> 

~ 1060 w 

D 

Soils; late Pleistocene and Holocene 

EIJ] 

Unit 1; gravel and pebbly sand 

interbedded with sand, silt, 

and "peat"; 290-785 years 

D 

Unit 2; sandy silt, pebbly sand, 

and gravel;contains root 

tubules; 670 (?)years 

~ Unit 3; sand and sandy gravel; iron 

oxide stain; 1,650 (?)years 

D 

Unit 4; carbonaceous sand and 

silt; 8,820-9,270 years at base, 

910 years at top 

U-70 U-219 

0 100 FEET 

VERTICAL EXAGGERATION X 2.5 

EXPLANATION 

Unit 5; pebbly 

-

sand and sandy 

gravel; iron oxide stain 

Unit 6; fine-grained 

-

silty sand 

[[[[]] 

Unit 7; argillaceous pebbly sand 

grading up into massive mudstone 

[[]] 

Unit 8; Interbedded sandy gravel and 

argillaceous pebbly sand; fossil 

teeth (late Pleistocene) 

~ Unit 9; silty to pebbly massive sand; 

iron oxide stain; >40,000 years 

U-55 U-82 U-80 U-221A U-49 

~ Unit 10; sandy gravel, partly calcite 

cemented; fossil teeth (middle and 

late Pleistocene) 

~ Unit 11; conglomerate, calcite-cemented 

~ Unit 12; sand, silty and pebbly, crossbedded 

~ 

Unit 13; conglomerate, calcite-cemented 

e505 

14 C age, in years before present 

347 en 

a: 

w 

tw 337 :E 

z 

i. 

0 

327 j:: 

<( 

> w 

...J 

w 

FIGURE H.-Generalized vertical section, north wall of proposed damsite, showing inferred ages of units. Modified from Yerkes, Bonilla, Sims, Wallace, and 

Frizzell (1973, sheet 2). 
within the surficial deposits. The mapped association of the markers and surficial deposits was then 

projected onto the vertical section. 

The surficial deposits consist of a complex gradationally interfingering association of noncemented gravel, sand and silt, and subordinate 

amounts of calcite-cemented gravel and peaty 

sand and silt. They rest unconformably on an irregular surface that was eroded on the steeply 

dipping bedrock. The thickness of the surficial deposits ranges from 3 to 70 ft (1-21 m). 

The deposits have been grouped into 13 units on 

the basis of lithology, buried soils, erosionaldepositional features at contacts, and radiocarbon 

(

14C) age determinations of wood and carbonaceous sediments. The units are discussed in 

chronologie order from youngest to oldest. The 14C 

age dates are corrected for temporal variations of 

14C flux (table 2). 

TABLE 2.-Radiocarbon dates from surficial deposits, 

proposed damsite 

Location No. Material 

U-222 ________ Carbonaceous 

sediments. U-22L _____________ do _____________ _ 

U-214 ______________ do -------------- U-220 ______________ do _____________ _ U-16A ______________ do _____________ _ 

U-91-EQ-5VN __ Peat #3 ___________ _ 

U-91 _________ Carbonaceous 

sediments. 

U-91-43VN ____ Peat #1 ------------ U-91-39VN __ Peat #2 wood U-210 ________ Carbonized 

wood. U-13-37VN __ Carbonaceous 

sediments. 

it~l-!6vN-======~~:: ============= U-13A __________ do---------------- U-13-41VN ______ do---------------- U-13B ____________ do _______________ _ 

U-11 ____________ do----------------

Conventional 

radiocarbon date (radiocarbon 

years B.P.) 

2 •3 285±85 

3 285±85 

3355±90 

3430±85 

3660±95 3 1615±85 

3 830±90 

3 1,350±90 

3 1,595±95 

3 1,700±90 

3945±90 

3 2,715±90 

34,890±255 

69,580±300 

3 8,665±120 6 10,000±300 6 >40,000 

1Corrected for variation in 14C flux with time. 

Corrected radiocarbon date (in calendar years B.P.)1 

29~440 

29~440 4465 

4 505 

4670 

4 1630 

75~785 

4 1,300 

4 1,530 

4 1,650 

4910 

42,840 

4 5,650 

78,820 

78,130 

79,270 

2j:figures are a measure of the precision of the counting statistics for the specific 

sample. 3Age determination by James Buckley, Teledyne, Isotopes. 4Corrected to bristlecone pine chronology ofSeuss (1970). Range in age is due to secular 14C fluctuations in the atmosphere with time. Such fluctuations yield non unique solutions to bristlecone pine corrections of conventional 14C age determi- nations. 

:s'fotal organic carbon used because humic material and plant debris were in- separable. 6Age determination by Meyer Rubin, U.S. Geological Survey. 7Corrected to varve chronology of Lake-of-the-Clouds (Stuiver, 1970). 

Unit 1. Gravel with subordinate amounts of interbedded sand and peaty sand and silt in three 

beds, and silt and sand interbedded with gravel 

and pebbly silt in the upper one-third to onehalf. Massive sandy gravel rests on the bedrock 

surface. The peaty sand and silt units occupy 

abandoned stream channels cut into underlying 

gravel units. Five 14C age dates were determined on wood and peaty sand and silt from this 

unit (U-222, U-221, U-214, U-220, U-91) ranging from 1,630 to 290-440 years B.P. (before 

present) (table 2). 14C ages for U-214, U-221, 

and U-220 represent the time of growth of the 

wood that constitutes the samples and indicate 

the maximum ages of the adjacent sediments. 

The deposits between U-91 and U-93 represent 

swampy conditions following a period of channel 

cutting and filling that ended about 1,630 years 

B.P. A similar deposit of carbonaceous silt and 

sand between U-94 and U-96 has a channellike 

cross section and is between 290 and 440 years 

old. Sample U-91 probably represents a contaminated sample. 

Unit 2. Silt and pebbly sand containing root 

tubules. Unit is correlated on the basis of lithology and stratigraphic position with the carbonaceous sandy silt ofU-16A (southeast wall). 

He age determination on rootlets at U-16A 

yielded a date of 670 years B.P. This unit is 

channellike in cross section and is under lain by 

carbonaceous silty sand. 

Unit 3. Crossbedded and wavy-laminated iron 

oxide-stained medium sand and locally crossbedded iron oxide-stained sandy gravel; gravel 

overlain by a thin ( <20 in. or 50 em) irregular 

ashy white friable sand overlain in turn by a 

thin ( <8 in. or 20 em) carbonaceous sand. These 

sediments in part overlie calcite-cemented 

coarse sandy gravel of unit 11 between localities 

U-88 and U-89. This unconsolidated sand and 

gravel is correlated with the deposits in the vicinity of U-210 (southwest wall) on the basis of 

lithology. 14C age determination on carbonized 

wood at U-210 yielded a radiocarbon date of 

1,650 years _B.P. (The peat in the vicinity of 

U-91 from unit 1 can be traced into the soil zone 

mapped on top of unit 3 and thus yields a 

minimum age for the soil and underlying deposits of 1300-1630 years B.P. (samples 

U-91-43VN, U-91-39VN, and U-91-EQ5VN). 

Unit 4. Very carbonaceous sand and silt with a 

peaty bed at the base. This unit is channellike in 

cross section, as is the overlying unit 2. Bedding 

is indistinct, and the upper three-fourths is massive and contains rootlets in apparent growth 

position. This unit represents a swampy or 

nearly perennially wet condition that promoted 

abundant plant growth in an abandoned channel of the depositing stream system. Two samples (U-13A, U-13B) from near the base of this 

A25 
unit gave 14C ages of8,820 and 9,270 years B.P. 

One sample (U-13-37VN) gave a 14C age of910 

years B.P. These dates indicate that the wet 

plant-promoting environment persisted for at 

least 8,000 years. Other samples at intermediate stratigraphic positions are of intermediate ages (table 2, samples U-13-40VN, 

U-13--41 VN, U-13 Top). 

Unit 5. Iron oxide-stained pebbly sand and sandy 

gravel; gravel concentrated near the base of the 

unit. This unit truncates units 6- 8 and is thus 

younger than those units. No material datable 

by the 14C method was found in unit 5, and so its 

absolute age relative to unit 4 is not known. 

Unit 6. Buff-colored fine to very fine silty sand. 

Upper and lower contacts are erosional, and the 

unit partly fills channels cut into units 7 and 8 

and is overlain unconformably by unit 5. Thus, 

unit 6 is older than unit 5 and younger than 

units 7 and 8. No material suitable for 14C age 

dating was found. 

Unit 7. Argillaceous pebbly sand with iron oxide 

stain grading up into massive sandy pebbly 

well-consolidated mudstone. Calcareous root 

tubules locally common. The bottom contact of 

the unit is indistinctly channellike between 

U-55 and U-50-5. Eastward of U-50-5 an indistinct soil zone forms the base of the unit, 

beyond which it is truncated by unit 6. Nomaterial for 14C age determination was found; the 

unit underlies unit 4, and so it is more than 

9,270 years old. 

Unit 8. Complexly interfingering and intergrading lithologies, dominantly sandy iron oxidestained gravel and massive argillaceous pebbly 

sand, locally iron oxide stained and cross bedded. 

The unit contains subordinate amounts of massive silty sand that is locally iron oxide stained 

or pebbly. Bone fragments were found at U-218, 

U-219, and U-221A. The bone material from 

U-221A was insufficient for 14C age determination. A complete fossil first(?) molar of 

Paramylodon sp. was found with other bone 

fragments at U-218. The tooth, identified by C. 

A. Repenning, U.S. Geological Survey, is of 

Rancholabrean (late Pleistocene) age. 

Unit 9. Massive sand, locally silty or pebbly and 

locally iron oxide-stained, crossbedding, and 

wavy lenticular lamination common. A lenticular unit of carbonaceous silty sand contained a 

carbonized tree stump in apparent growth position, from which a 14C age determination of 

>40,000 years B.P. was obtained. Parts of the 

calcite-cemented coarse gravels at the contact 

between units 9 and 11 were eroded at the eastern end prior to deposition of unit 9. The contact 

with the underlying unit 10 is poorly exposed, 

undulatory, and may also be an erosional unconformity. 

Unit 10. Sandy coarse gravel, locally indistinctly 

crossbedded in the lower two-thirds and slightly 

cemented by calcite in the upper one-third. This 

unit probably represents a point bar deposit. No 

material for 14C age determination was found. 

This unit is correlated with nonindurated deposits in the vicinity of U-14 (center of south 

wall) where a large fragment of a fossil tooth 

was found. The tooth was identified by C. A. 

Repenning of the U.S. Geological Survey as 

questionably from Equus (Hemionus) sp. or 

Equus ( Amerhippus) sp. of early to late and 

middle to late Pleistocene age, respectively. 

This tooth fragment may have been reworked 

from older materials because the fractured surfaces are abraded and slightly rounded. 

Unit 11. Calcite-cemented conglomerate; coarse 

grained with a sand matrix. This unit rests on 

the eroded surface of the bedrock east of U-80 

and is in erosional contact with unit 12 west of 

U-80. Unit 11 is truncated by units 8-10. A 

small eroded remnant of cemented gravel, 

which fills a depression eroded in bedrock at 

U-88, is correlated with unit 11 on the basis of 

lithology. This remnant is overlain by unit 3 at 

U-88. Thus, at station U-88 unit 11 is more than 

1,650 years old and between stations U-50-5 

and U-80 is more than 40,000 years. This latter 

age is probably also the age of the conglomerate 

at U-88, on the basis of its lithologic similarity 

and stratigraphic position. 

Unit 12. Silty to pebbly poorly crossbedded sand; 

gravel concentrated near contact with unit 11. 

Upper contact of the eastern one-third of this 

unit is erosional and partly stained by iron 

oxide. The age of this unit, based on strati,; 

graphic criteria and the 14C date at U-1i, is 

greater than 40,000 years. 

Unit 13. Calcite-cemented conglomerate; coarse 

grained with sand matrix. This unit rests ~n the 

eroded surface of the bedrock between U-82 and 

U-50-5. The upper contact is undulatory and 

may be truncated py units 10 and 12. This gravel 

is not contiguous with unit 11, although the 

lithologies are similar, unit 11 having slightly 

A26 
more sand matrix. Because of the similarity of 

lithologies the eroded remnant at U-88 may be 

correlative with unit 13, but correlation with 

unit 11 is preferred. 

Soils. Soils developed on the surficial deposits are 

divisible into A, B, and C zones, of variable but 

nearly equal thickness. The A zone is dark olive 

gray, sandy, and carbonaceous. The B zone is 

dark yellow brown, commonly calcareous, 

breaks into peds, and is hard owing to relative 

enrichment of clay derived from the A zone. The 

C zone is yellow gray, with a variable development of caliche. 

The surficial deposits represent a continental 

fluvial environment. The sediment was deposited 

intermittently by streams of dominantly high-flow 

regime, as shown by the coarse gravels, and 

swampy or low-flow regimes at other times, as 

shown by the peaty units. 

The surficial deposits are separable for purposes 

of discussion into two areas on either side ofU-70, 

where the uppermost soil horizon is in contact with 

weathered bedrock. The depositional history east 

of U-70 was about as follows: Unit 13 was deposited in the bedrock depression between U-50-5 

and U-80 and was at least partly cemented and 

partly eroded before units 12 to 10 were deposited. 

The contacts of units 10, 11, and 12 with unit 13 

are irregular; each records a period of erosion prior 

to deposition ofthe next younger unit. The gravel. 

of unit 11, however, was at least partly cemented 

and partly eroded prior to deposition of units 10 to 

8. After unit 8 was deposited, the deposition of 

units 7, 4, and 2 records the almost uninterrupted 

fillings of an ancient stream channel. A smaller 

channel was eroded at the easternmost end of the 

embankment, where units 5 and 6 were deposited. 

Unit 4 evidently required about 8,000 years to 

accumulate. Unit 2 is correlated with deposits at 

U-16A (on the southeast wall) and is thus inferred 

to be about 670 years old, an age substantiated by 

the upper age limit of unit 4. There may be a hiatus 

of about 240 years between units 4 and 2. 

The early history west of U-70 has been largely 

destroyed by erosion down into bedrock (fig. 11). 

After unit 3 was deposited and a soil formed on it, 

the modern stream deposits of unit 1 buried the 

soil and filled the channel to the present level (fig. 

11). The modern deposits (less than 800 years old) 

filled a channel cut to an elevation of 1,042 ft (317 

m), whereas the earlier erosion before deposition 

of unit 13 cut to 1,055 ft (321m). The significance 

of the differing depths of erosion cannot be assessed on the basis oflithologic and sedimentologic 

properties of the sediments. 

The unconformity at the base of the surficial 

deposits may be as old as 2-3 million years, the 

approximate age of the beginning of the Pleistocene epoch in southern California. The oldest 

surficial deposits that overlie the unconformity 

(unit 13 between U-80 and U-50-5, northeastern 

part of the embankment area) are probably about 

60,000-100,000 years old. The unconformity 

throughout the site area probably once was overlain by surficial deposits of this general age, but 

these deposits in part have been eroded and replaced by units that become successively younger 

westward toward the axis of the modern stream. In 

the north-central part of the site, where the unconformity is overlain by unit 8, the upper age limit of 

the time interval represented by the unconformity 

is about 10,000 years. In the western part of the 

site, where the unconformity is overlain by unit 1, 

the upper limit, excepting the deposits between 

U-91 and U-95, is about 500 years. More 

significantly, the oldest surficial deposits in the 

southeastern part of the site, which locally are cut 

by faults (at U-1, U-6, U-7, U-72-C, U-72-M, 

Yerkes and others, 1973, sheet 2), are correlated 

with unit 8, estimated to be 10,000 to about 40,000 

years old. The faulting is therefore younger than 

about 40,000 years. 

BEDROCK FAULTS 

Bedrock faults are widely distributed throughout the area of the proposed damsite; some displace 

the unconformity between the surficial deposits 

and bedrock and are inferred to have moved within 

the last 40,000 years. Throughout the general area 

of the proposed damsite, the unconformity is an 

erosional surface of locally steep relief with steps 

as high as 24 ft (7.3 m) and locally overhanging. 

Some steps were formed by erosion along old bedrock faults, others by faulting that displaced the 

erosional surface, and still others by a combination of these. Since this erosional surface is the 

basal contact of the surficial deposits, bedrock 

faults that displace it must be younger than the 

deposits that are displaced. For this reason the 

unconformable contact was carefully cleaned, examined for evidence of fault displacement, and 

mapped at a scale of1 in. equals 100ft (1 em= 12m) 

around most of the 6,200-ft (1890-m) perimeter of 

the embankment area (Yerkes and others, 1973, 

A27 
sheet 2); the contact also was examined for evidence of faults in about 2,225 ft (678 m) of exploratory trenches east of the embankment area. 

All bedrock faults that are alined with steps in 

the bedrock surface around the perimeter of the 

embankment excavation (except in the southwest 

part) were identified by station number and differentiated on the basis of whether or not they 

displace surficial deposits; the amount of vertical 

separation in feet, based on displacement of the 

contact across the fault, is opposite the station 

number (Yerkes and others, 1973, sheet 2). The 

relative age of last movement on many of the 

faults cannot be established, especially on the floor 

of the embankment, where surficial deposits have 

been stripped off, and around the periphery of the 

embankment site, where the surficial deposits are 

so poorly sorted, poorly bedded, and poorly consolidated that evidence of faulting can be detected 

only where the fault has been mineralized. 

Wherever the relative age of last movement cannot be established, it is assumed that movement 

could have been relatively recent. 

Bedrock faults are expressed as bands of sheared 

and slickensided, finely brecciated, or even minutely comminuted (gouged) siltstone and shale a 

few millimeters to a few centimeters thick. About 

half are north-dipping ((bedding plane" faults subparallel to stratification of the bedrock; the rest dip 

south and cut across the bedrock strata. Beddingplane faults displace the base of the surficial deposits a few tenths of a foot at many places. 

Cumulative displacement on them equals or exceeds that on the south-dipping faults. 

The fine-grained material along the fault surfaces commonly is striated or slickensided in the 

dip direction, and so the sense of movement during 

the last event was parallel to the dip of the faults. 

The most prominent, and almost the sole, exception is the northeast-trending fault at the south 

end of trench 2 east of the embankment area, 

where slickensides on the fault plunge 48° NE. 

Geometric reconstruction of bedrock relations 

there shows that displacement in bedrock was at 

least 40ft (12.2 m) on this fault, although there is 

no vertical separation at the base of the surficial 

deposits. The only prominent correlation between 

attitude and apparent separation on the faults is 

that south-dipping faults are predominantly 

ttnormal"-the south side is downfaulted relative 

to the north side. The south-dipping faults are 

commonly truncated by north-dipping faults. Of 

the many north-dipping faults mapped, about half 

are ureverse" (north side relatively upthrown) and 

half are ((normal" (north side relatively 

downthrown). Furthermore, the total amounts of 

vertical separation recorded for the two types of 

north-dipping faults are practically equal. However, if the displacement of the north-dipping reverse faults is added to that of the south-dipping 

faults (on which the displacement is almost all 

relatively up on the north side), the overall effect is 

one of relative upthrow on the north. 

Bedrock faults in the perimeter of the embankment area could not be traced upward through the 

surficial deposits to the ground surface, although 

in several places they could be traced upward for a 

few feet. In some of these places, the fault was 

clearly truncated by an undisturbed bed in the 

surficial deposits, but in most places a clear truncation could not be discerned. There is no evidence 

of 1971 movement on the bedrock faults within the 

embankment perimeter. 

There are several1971 surface ruptures on the 

ridge east of the damsite; the longest, just northeast of the proposed site, was explored by trenches 

cut through the base of the surficial deposits into 

bedrock. This rupture coincides with a buried step 

in the bedrock surface that is alined with a bedrock 

fault, but the 1971 surface rupture can be attributed to a slope failure that caused a pressure 

ridge in the freeway to the northeast. Other ruptures, such as the pair 700ft (213m) northwest on 

the same ridge, were downthrown on the upslope 

side and therefore probably are not due to slope 

failure; the rupture at the south end of the same 

ridge showed compressive effects, also probably 

not due to slope failure. All these ruptures are 

considered to be evidence of buried bedrock faults 

that are elements of the zone of active faults that 

traverses the reservoir area. 

Although the immediate site of the proposed 

dam is not known to have been affected by tectonic 

ruptures during the 1971 earthquake, it was subjected to differential uplift and horizontal displacement, and the embankment area is traversed 

by numerous closely spaced east-trending faults. 

A numbe;r of these displace surficial deposits and, 

with similar faults to the northwest and southeast, 

may form a zone that extends entirely across the 

reservoirs area, probably to merge with the 

known-active San Fernando zone to the southeast. 

Therefore, the proposed damsite is considered to be 

within a zone of faults capable of movement. 

A28 
EXPECTABLE EARTHQUAKES AND 

THEIR EFFECTS 

We assume that failure of a dam in the Van 

Norman reservoirs area is intolerable because 

such a failure could cause inundation of the central San Fernando Valley downstream, an area 

inhabited (in 1970) by about 80,000 people (Seed, 

1972, p. 18). To prevent failure, dams built in the 

Van Norman area must be designed to survive 

without serious damage the most severe effects of 

expectable large earthquakes. 

Available evidence does not permit precise 

statements of the frequency, magnitude, or effects 

of the largest earthquakes to be expected in the 

Van Norman area. Hence, we derive estimates, 

based chiefly on the structural-seismologic setting 

of the area, the relation between the effects of the 

1971 San Fernando earthquake and that setting, 

and extrapolation of the 1971 effects. 

Lower Van Norman damsite is in a zone of 

north-dipping faults alined with, and correlative 

in general attitude and history with, the active 

San Fernando fault zone. These faults combine to 

form an important element of the throughgoing 

Santa Susana-San Fernand~Sierra Madre zone 

of frontal faults, along which the Santa Susana 

and San Gabriel Mountains have been uplifted. 

The upper Van Norman and proposed damsites are 

traversed by numerous east-trending steeply dipping faults, some of which displace surficial deposits, and all of which probably are elements of a 

broad zone related to the active San Fernando 

zone. 

The 1971 ground-surface deformations are integral parts of an established regional pattern of 

deformation that includes reverse faulting along 

the San Fernando zone and arching that extends 

westward from the foothills east of San Fernando, 

across the valley, and into the reservoir area north 

of the rupture zone. These deformations were similar to, if not identical with, those that formed the 

geologic features that underlie these areas: (1) The 

1971 arching uplifted the steeply dipping bedrock 

that forms the foothills east of the valley and the 

topographic highs north of the rupture zone in the 

valley and reservoirs area; (2) the Sylmar and 

Mission Wells segments of the 1971 rupture zone 

formed along the base of old south-facing topographic scarps; (3) those two segments also coincide with a continuous fault-formed ground-water 

impediment in the young valley alluvium; (4) evidence of geologically young, pre-1971 reverse 

faulting has been found by trenching and drilling 

at several localities along the Tujunga segment of 

the San Fernando zone (Barrows and others, 1971; 

Bonilla, 1974); and (5) the pre-1971 buried faultlike scarp in Lopez Canyon east of San Fernando is 

alined with the 1971 ruptures there and reflects 

the same relative movement as formed the 1971 

ruptures. 

The 1971 earthquake and fault movements apparently were smaller than those that apparently 

occurred on the same zone about 200 years ago. 

The 1971 San Fernando earthquake was 

significantly smaller than the 1952 Kern County 

earthquake, which is attributed to the same tectonic stress system; thus, the 1971 events were not 

the largest that can be produced by this system. 

This stress system has produced at least six strong 

earthquakes in the last approximately 200 years, 

at least one and probably two of which involved 

tectonic rupturing at the surface along the San 

Fernando zone; therefore, the reservoirs area is 

expected to experience significant earthquake effects in the foreseeable future. These considerations indicate that earthquake effects from two 

different earthquake sources should be expected in 

the Van Norman reservoirs area: an M 8+ earthquake on the San Andreas fault 25 mi (40 km) 

northeast of the reservoirs and a local earthquake 

of M 7. 7 on the Santa Susana-San Fernando 

-Sierra Madre fault system. (See part B of this 

report for details.) 

TRANSITORY EFFECTS 

The transitory ground motions expected in the 

Van Norman reservoirs area from these earthquakes are as follows (see part B of this report for 

derivation): San Andreas Local 

M 8+ M 7.71 

Peak horizontal acceleration (g) ______ ~0.5 1.15 

Peak velocity of displacements, ft/sec (em/sec) ____________________ 2 (60) 4.4 (135) 

Peak dynamic displacement, 

ft (em) -------------------------- 1.3 (40) 2.3 (70) 

Duration of acceleration ~0.05g (sec) _____________________ 80 40 

1The values are for a single horizontal component of motion within 2-3 mi (3--5 

km) ofthe causative fault; they are for sites at which ground motion is not strongly 

altered by extreme contrasts in the elastic properties within the local geologic 

section or by the presence of structures; they contain no factor relating to the nature 

or importance of the structure being designed. 

PERMANENT EFFECTS 

Estimated fault or fault-related displacements 

believed to be credible effects of an M 7. 7 local 

A29 
TABLE 3.-Summary of credible fault and fault-related displacements, M 7.7 local earthquake 

Feet Meters Lower Van Norman damsite: 1. Total displacement, reverse-oblique slip on main fault (upper-bound curve of fig. 12) __________________________________ 16.4 

Vertical component (upper-bound curve) __________________ 12.6 

Vertical component (1971 ratio on Sylmar segment) ________ 9.5 

Vertical component (1971 ratio on Freeway segment) ______ 13.3 

2. Resolved components, east abutment · relative to west: Horizontal convergence parallel to 

mrr::e~ti~f~hi~g~-in-ele,;;;tiO~(;~g~)-== gtri3 Total component in vertical plane along axis (range) ____________________ 12.2-13.7 

Horizontal component normal to axis 

To~:ifJer~~ti;~-~~~~~e~t--============1~:~ 1. 0 

Upper Van Norman and proposed damsites: 1. Vertical displacement, dip slip on subsidiary fault (range)_ ___________________ 3-9 

2. Resolved components, east abutment relative to west: 

5.0 

3.84 

2.9 

4.05 

0.98-2.35 

2.9-4.05 

3.72-4.18 

2.74-3.35 

5.0 

1-2.74 

Horizontal separation parallel to axis ____ 1.7 0.52 Differential change in elevation (range) __ 13-9 11.0 to 2. 7 Horizontal component normal to axis ____ 3.3 1.0 Total differential movement (range) ______ 2 3.3-9.6 2 1.0-2.93 1

These values could be increased as much as 75 percent by the effects of regional tilting. 2These values could be increased as much as 69 percent by the effects of regional tilting. 

mated displacements are derived from the known 

effects of the 1971 San Fernando earthquake, 

scaled up on the basis of magnitude-displacement 

correlations from the worldwide record of historic 

displacements on similar faults. Although the estimated displacements are described separately 

for two areas (lower Van Norman damsite, upper 

Van Norman and proposed damsites), we emphasize that these and other deformations should 

be expected to occur simultaneously at all the 

sites. The estimated displacements are derived in 

the following two sections. 

LOWER DAMSITE 

Tectonic rupturing of the ground surface along 

faults in the area of lower Van Norman dam could 

accompany any strong local earthquake. The particular faults on which displacement could occur 

cannot be identified; therefore, we assume that 

displacement could occur any place near, beneath, 

or between the abutments of the present dam. As a 

means of quantifying the expected displacements, 

those associated with the 1971 earthquake on the 

Sylmar segment of the San Fernando zone (U.S. 

Geological Survey, 1971, p. 62) are scaled up on the 

basis of the worldwide record of historic displacements on reverse-slip or reverse-oblique slip faults 

associated with earthquakes of M 6¥2-8.4. 

The maximum displacement on the Sylmar 

segment of the San Fernando zone during the 1971 

earthquake was 7.9 ft (2.4 m), which, for an M 6¥2 

event, is large compared with the least-squares 

curve of the magnitude-displacement plot for 50 

cases, including all types of faults, from the 

worldwide record of historic events (Bonilla and 

Buchanan, 1970, table 1, fig. 2). Alternatively, the 

maximum displacement (3.9 ft or 1.2 m) associated 

with the 1952 Kern County, Calif., earthquake on 

the White Wolf fault is anomalously low for an 

event of its magnitude (7. 7). The least-squares 

curve for the 50 cases passes midway between the 

points for the 1971 and 1952 events and indicates a 

displacement of11.5 ft (3.51 m) for an M 7.7 earthquake. 

Of the 50 cases used by Bonilla and Buchanan 

(1970) for the magnitude-displacement plot, however, only 12 pertain to shallow-focus events associated with reverse-slip and reverse-oblique slip 

faults such as the Santa Susana-San Fernando 

and White Wolf; the rest pertain to normal-slip 

and strike-slip faults. When considered separately 

the magnitude-displacement plots for these 12 

(plus the 1971 San Fernando data) form a widely 

scattered pattern to which no significant curve can 

be fitted; however, an upper limit for displacement 

apparently is defined by four of these events: 

M Total displacement Vertical component 

1964, Patton Bay, Alaska ____________ 8.4 26 ft (7.9 m) 23 ft (7 m) 

1929, West Nelson, 

New Zealand ______ 7 .8 17 ft (5.2 m) 14.75 ft (4.5 m) 

1968, Meckering, Australia __________ 6.9 10 ft (3.05 m) 6.1 ft (1.86 m) 

1971, San Fernando, California __________ 6.5 7.9 ft (2.4 m) 4.6 ft (1.39 m) 

The curves (fig. 12) indicate a possible maximum 

total displacement of 16.4 ft (5 m) and a possible 

maximum vertical component of about 12.6 ft 

(3.84 m) for an M 7.7 event on a reverse-slip or 

reverse-oblique slip fault. The physical basis of the 

apparent correlation between magnitude and displacement, and especially the apparent systematic 

variation in the ratio of vertical component to total 

displacement (fig. 12), is not understood. In the 

absence of that knowledge, and being unable to 

justify lower limits, we (1) must assume that the 

indicated maximum displacements could occur 

and (2) cannot guarantee on the basis of so small a 

sample that those maximums could not be exceeded. 

The data. for the upper-bound events on 

reverse-slip faults indicate that the ratio of maximum vertical component to total displacement 

varies directly with magnitude of the associated 

earthquake (fig. 12). However, the ratio on the 

Sylmar segment in 1971 (58 percent) was at the 

low end of the range shown by figure 12, whereas 

the ratio on the Freeway segment (81 percent) was 

A30 
6 

10 

8 

6 

5 

4 

~ 

~ 3 

z 

w 

~ 

w 

u 

<( 2 

...J 

c.. 

(/) 

Q 

6 

0 

Total displacement 

13 

Vertical component 

/ 

/ 

0 

'0 

c: 

CQ 

c: 

... 

Q) 

LL 

c: 

CQ 

(/) 

..... 

,..... 

0) 

7 

/ 

/ 

/ 

/ 

/ '0 

c: 

CQ 

co 

Q) 

N 

~ Q) 

z 

c:' 

0 

en 

Q) 

z 

..... 

en 

Q) 

~ 

0) 

N 

0) 

8 

30 

25 

20 

15 

10 i= 

LL 

9 ~ 

8 z 

w 

7 ~ 

w 

u 

6 <( 

...J 

c.. 

5 5Q 

Q 

4 

3 

2 

1.5 

MAGNITUDE 

FIGURE 12.-Plot of total displacement and vertical component of displacement relative to magnitude of upper-bound 

events on reverse-slip and reverse-oblique-slip faults. Based on worldwide data of Bonilla and Buchanan (1970) and 

supplementary data. 

nearly the same as that indicated by the curve for 

an M 7. 7 earthquake (77 percent). Alternative estimates of the expectable vertical component for 

an M 7. 7 event are derived as follows by applying 

ratios from the 1971 San Fernando displacements 

to the upper-bound total displacement of 16.4 ft: 

Displacement 

1971 San Fernando (M61h): Total 

Sylmar segment 

(vertical=58 percent of total) 7.9 ft 

Freeway segment (2.4 m) 

(vertical=81 percent of total) ____ 1.36 ft 

Estimates forM 7.7: (0.42 m) 

Vertical=58 percent of total ________ 16.4 ft 

(5.0 m) 

Vertical =81 percent of total ________ 16.4 ft 

(5.0 m) 

Vertical 

4.6 ft 

(1.39 m) 

1.1 ft 

(0.34 m) 

9.5 ft 

(2.9 m) 

13.3 ft 

(4.05 m) 

To determine the effects of the estimated displacements on a dam at the lower Van Norman 

site, the displacements are resolved into components of the east abutment area relative to the 

west (in effect holding the west abutment fixed). 

The dam axis trends N. 70°W. and the activated 

fault is assumed to trend N 70°E. and dip 70°N., 

essentially as in the 1971 case. First, the displacements are resolved on the basis of the 1971 

Sylmar ratio (maximum vertical=0.58 of total); 

then the displacements based on the 1971 Freeway 

segment ratio (maximum vertical=0.81 of total) 

are resolved. The smaller ratio yields greater lateral separation of the abutments, whereas the 

larger ratio yields greater vertical separations, as 

follows: 

A31 
Components based on maximum 

vertical= 0.58 of total: 

Horizontal convergence parallel 

Feet Meters 

to axis -------------------------- 7.7 2.35 

Differential change in elevatign ____ -9.5 -2.89 

Total component in vertical 

plane along axis ________________ 12.2 3.72 

Horizontal component normal 

to axis -------------------------- 11.0 3.35 

Total differential movement 

(12.9 ft N. 56°E. and 9.5 ft 

down at about 36° to 

horizontal) ---------------------- 16.4 5.0 

Components based on maximum 

vertical=0.81 of total: 

Horizontal convergence parallel 

to axis -------------------------- 3.2 0.98 

Differential change in elevation ____ -13.3 -4.05 

Total component in vertical 

plane along axis ---------------- 13.7 4.18 

Horizontal component normal 

to axis -------------------------- 9.0 2.74 

Total differential movement 

(9.6 ft N. 40°E. and 13.3 ft 

down at about 54° to 

horizontal) ---------------------- 16.4 5.0 

Thus, any dam at the lower Van Norman site 

should be able to withstand the strong earthquake 

motions cited, plus a differential movement of the 

abutments of an estimated 16.4 ft (5 m), about 0.8 

percent of the 1971 axis length of 2,100 feet. 

Whether this movement would occur as a result of 

displacement on one fault, or on several faults, 

cannot be determined; either possibility should be 

assumed likely. 

UPPER AND PROPOSED DAMSITES 

In considering the effects of a local M 7. 7 earth· 

quake on the upper Van Norman and proposed 

damsites, we assume the most critical case-that 

the sites are less than 1 mi (1.6 km) north of the 

main zone of surface ruptures on the upper plate of 

the thrust fault and that they are traversed by 

active subsidiary faults related to the main zone. 

One or more of the faults in the sites should be 

expected to move during a strong local earth· 

quake. The minimum expectable vertical compo· 

nent of displacement is the 3 ft (1 m) that occurred 

in 1971 on the subsidiary fault in Lopez Canyon. 

(See following discussion.) 

One method for estimating the maximum credi· 

ble displacements for a subsidiary fault is to com· 

pare displacements on such a fault with those on 

the main fault for the same event; however, this is 

difficult because of the scarcity of data. Bonilla 

(1970, fig. 3.7) summarized the known record for 

North America; at that time only one example of 

historic subsidiary faulting associated with areverse fault event was available-the 1952 Kern 

County event, for which the maximum ratio was 

0.25. Nevertheless, displacements occurred on a 

subsidiary fault in Lopez Canyon, 1 mi (1.6 km) 

east of the Sylmar segment of the main rupture 

zone, during the 1971 San Fernando earthquake. 

The displacements occurred in natural ground underlain by steeply north-dipping bedrock similar 

to that at the proposed damsite. The total displacement was 4.35 ft (1.32 m); the vertical component was 3.1 ft or 0.95 m (U.S. Geol. Survey, 

1971, p. 68-69). The ratios of these displacements 

to those on the Sylmar segment of the main zone 

are 0.55 for total displacement and 0.68 for the 

vertical component. 

Thus, taking the vertical displacement to be expected on the main zone from an M 7. 7 earthquake 

(13.3 ft or 4.05 m), we would expect that a vertical 

displacement of 13.3x0.68=9.04 ft (2.76 m) could 

occur on a subsidiary fault associated with that 

event. Because the last movements (as indicated 

by striations and grooves) on the east-trending 

faults in the proposed site were essentially parallel to the dip of the faults, we expect the next 

movements to be dip slip in nature; in this case all 

displacement would be expressed as dip slip. As in 

the case- of the lower Van Norman damsite, the 

fault or faults that would move cannot be determined, nor can the distribution of the displacement among one or more faults be determined. 

The axis of the proposed dam trends N. 64° E. 

(Los Angeles Dept. Water and Power drawing 

D4244-G-1, Oct. 1973), and the faults in the area 

trend about N. 85° W.; therefore, the effects of such 

a displacement can be resolved into component 

displacements of the east abutment area relative 

to the west abutment. If vertical displacement of 

about 9 ft (2. 7 m) were to occur on a normal fault 

dipping 70° N., horizontal separation of the abutments of about 1. 7 ft (0.52 m) parallel to the axis, 

and horizontal offset normal to the axis of about 

3.3 ft (1.0 m) would occur, for a total separation of 

about 9.6 ft (3.3 ft or 1m N. 5° E. and 9.0 ft or 2. 7 m 

down at 70° to the horizontal). If normal slip were 

to occur on a fault dipping less steeply, more horizontal extension would be expected. Conversely, if 

reverse slip were to occur, slight convergence of 

the abutments would be expected. 

A32 
Although the zone of disturbance associated 

with reverse-slip ruptures may be relatively wide 

in soft rocks and alluvial cover, it may be very 

narrow where it is confined to bedrock. Examples 

of the former situation from the worldwide record 

include the 1968 Meckering, Australia, event (M 

6.9), which had a vertical displacement of about 

4. 7ft (1.44 m) over a zone about 5 ft (1.5 m) wide, 

and the subsidiary Hanning Bay faulting on Montague Island during the 1964 Alaska earthquake 

(M 8.4), where vertical displacement as great as 

16.33 ft (5 m) occurred within a zone about 12.5 ft 

(4 m) wide. 

A directly applicable example of a rupture 

confined to bedrock occurring during the San Fernando earthquake is presented by a scarp about 1 

m high formed in Lopez Canyon. The circumstances there are comparable with those at the 

proposed damsite: The rupture was subparallel to 

bedding in steeply north-dipping sandstone and 

shale that was exposed at the surface. Even 

though a monoclinal warp formed where bedrock 

was covered by alluvium, a sharp overhanging 

scarp formed where bedrock was exposed (U.S. 

Geol. Survey, 1971, p. 67-71). Where the rupture 

was confined to bedrock, the principal displacement, as expressed both at the surface and exposed 

in a postearthquake trench, occurred within a zone 

a few centimeters wide (U.S. Geol. Survey, 1971, 

figs. 18--20; Bonilla, 197 4). Thus, it is clear that a 

meter of vertical displacement can occur within a 

very narrow zone; we know of no reason why much 

larger displacements cannot occur within equally 

narrow zones, especially where confined to bedrock. 

OTHER EFFECTS 

In addition to discrete faulting, other expectable 

permanent surface deformations include differential uplift or tilting. Differential uplift in 1971, 

along a profile having the same trend as the proposed dam, was greatest just north of the Sylmar 

segment and immediately west of the San Fernando city boundary (along a line extending S. 64° 

W. from the intersection of Fellows Avenue and 

Hubbard Street). There the westward gradient of 

the 1971 arch averaged 0.15 percent over a 2250-ft 

(686 m) distance but was as much as 0.3 percent 

within a distance of 1,000 ft (300m). If the steeper 

of these gradients were to occur in the proposed 

damsite, the differential change in elevation of the 

abutments (2260 ft or 689 m apart) could approach 

6.8 ft (2.07 m); if the same sense of uplift were to 

occur, the east abutment would be elevated relative to the west. 

Tilting or subsidence of the dam relative to the 

reservoir could lead to overtopping the dam and 

possible erosion of the crown and downstream face. 

This effect occurred at Hebgen Dam during the 

1959 Hebgen Lake earthquake (M 7 .1) in southwestern Montana. Regional deformation during 

the earthquake resulted in subsidence of the 

20-mile-long Hebgen reservoir by an average of 

more than 10ft (3.05 m), whereas the dam subsided less than 10 feet. Even so, warping of the 

lake basin produced damaging surges of water 

that overtopped the dam at least three times and 

eroded the downstream face (Myers and Hamilton, 

1964, p. 70). 

A dam built at the upper or proposed sites should 

be able to withstand the transitory ground motions of a strong local earthquake cited, as well as 

permanent displacement of the ground surface beneath the embankment. Vertical displacement 

due to fault movement could be as much as 9ft (2. 7 

m) within a very narrow zone. The estimated effects of the inferred faulting might be compensated in part by the effects of differential tilting 

and horizontal displacement as based on their 

1971 pattern in the northern San Fernando Valley. However, if the effects were additive, a differential change in elevation of the abutments of as 

much as 15.8 ft (4.8 m) and a small extension could 

be expected. 

CONCLUSIONS 

The Van Norman reservoirs are in an area of 

high seismicity that was permanently deformed 

by the M 6¥.l San Fernando earthquake-an area 

that could be subjected to an even larger earthquake. Such an earthquake would be accompanied 

by very severe shaking and could result in large 

permanent displacements due to faulting and 

tilting-effects that would impose severe strains 

on an earth dam. In addition, the Van Norman 

area overlooks the central San Fernando Valley, 

where a district inhabited (in 1970) by about 

80,000 people could be inundated by a dam failure 

(Seed, 1972, p. 18). Thus, the Van Norman reservoirs area must be considered as one of high risk 

for earth dams. Because of the likelihood of seismicity and rupture by faulting and the severe consequences of a dam failure, we believe that great 

conservatism is justified in the selection of critical 

A33 
seismologic and geologic parameters used in the 

planning, design, review, and construction of 

dams in the Van Norman reservoirs area. Estimated values of the critical parameters, directly 

applicable to the Van Norman area, are given in 

this report (table 3) and in part B of this report. 

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A34 
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A35