Subsidence in the

Santa Clara Valley

California

A Progress Report

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1619-C
.AYW*'-*-

Subsidence in the

Santa Clara Valley

California

A Progress Report

By J. F. POLAND and J. H. GREEN

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1619-C

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.G.
CONTENTS

	Page 

Abstract__ ______________________________________________________ G-l

Introduction._____________________________________________________ 1

Geology._________________________________________________________ 2

History of leveling control.___--_-____________-_______-_--__-_-_____ 4

Magnitude and distribution of subsidence.__________-__---__--___-___ 5

Conclusions_ ____________________________________________________ 15

Ref erences _-_-____---_--____--_-__._____________-_.--___----__-___ 16

ILLUSTEATIONS

Page 

FIGURE 1. Geologic section from Alviso to Edenvale.__________________ C-3

2. Geologic section from Los Altos to Milpitas__________-._.__ 4

3. Map showing network of level lines in the San Jose subsidence

area, Santa Clara Valley____________-___-_--___-___--__ 5

4. Map of Santa Clara Valley, showing land subsidence, 1934-54_ 6

5. Map of Santa Clara Valley, showing land subsidence, 1948-54._ 8

6. Map of Santa Clara Valley, showing land subsidence, 1940-54- 9

7. Profiles of land subsidence, San Carlos to Coyote, l912-59____ 10

8. Profiles of land subsidence, Mountain View to Milpitas,

1934-54.____________________._______.____-__-_- 11

9. Profiles of land subsidence, Los Gatos to Alum Rock Park,

1934-54_____._______________ _.____-.-- 12

10. Change in altitude at bench mark P 7 in San Jose and change

in artesian pressure at nearby well---_--_---_-------__--- 13

11. Change in altitude at bench marks F 111 and J 111 Reset near 

Mountain View and change in artesian pressure at nearby 

well________-__-___-__________--_-_-_._--------_. 14

12. Change in altitude at bench mark 35 (USGS) near Campbell

and change in water level in nearby well. ___.____--_--_____ 15

m

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

SUBSIDENCE IN THE SANTA CLARA VALLEY, 

CALIFORNIA A PROGRESS REPORT

By J. F. POLAND and J. H. GREEN

ABSTRACT

Subsidence of the land surface in the Santa Clara Valley, Santa Clara County, 

Calif., has been observed since 1933. A network covering the area of subsidence 

was first surveyed in the spring of 1934. That network was releveled seven times 

between November 1934 and April 1940, again in 1948 and 1954, and in part in 

1956 and 1959, by the U.S. Coast and Geodetic Survey.

The releveling of the network in 1954 indicated that the land surface had 

subsided 7.75 feet at bench mark P 7 at San Jose. Subsidence near Sunnyvale 

from 1934 to 1954 was greater than at San Jose. The maximum subsidence from 

1948 to 1954 occurred at San Jose and was on the order of 2.4 feet. By December 

1959, total subsidence at bench mark P 7 since 1912 was 9.04 feet.

The area of land-surface subsidence extends southward about 25 miles from 

Redwood City to and beyond San Jose, has a maximum width of about 13 miles, 

and includes at least 230 square miles.

Plots of subsidence at individual bench marks compared to fluctuation in 

artesian pressure indicate that decline in artesian pressure is a major cause of 

subsidence. The rate of subsidence is generally in accord with the rate of 

fluctuation of artesian pressure. Subsidence virtually ceased during a period 

of increasing artesian pressure from 1937 to 1940, but resumed at an accelerated 

rate during a period of rapidly decreasing artesian pressure from 1944 to 1950.

INTRODUCTION

The Santa Clara Valley is a large structural trough that extends 

from Hollister to San Francisco, Calif. It is bounded on the southwest 

by the Santa Cruz Mountains and on the northeast by the Mount 

Hamilton Range (inset map, fig. 4). The San Andreas fault system 

is a few miles southwest of the valley plain and the Hayward fault 

borders the valley on the northeast.

Land subsidence occurs in the central reach of the valley in an area 

of intensive ground-water development. Discussion in this report is 

limited to this central reach of the valley, which extends southeastward about 30 miles from Eedwood City and Niles to Coyote (fig. 4).

The purpose of this report is to present a brief summary of the

C-l
C-2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATESsubsidence that lias occurred in the central part of the Santa Clara 

Valley, as defined by repeated leveling by the U.S. Coast and Geodetic 

Survey; to compare the subsidence with change in artesian pressure 

observed at the two principal centers of subsidence; and to draw preliminary conclusions concerning the principal cause of the subsidence.

GEOLOGY

A generalized geologic map of the central part of the Santa Clara 

Valley is shown on figure 4 and repeated on figures 5 and 6. This map 

was prepared chiefly from county geologic maps of the State of 

California Division of Mines (Davis and Jennings, 1954; Davis, 1955). 

The bedrock, shown as a single unit on the map, ranges in age from 

Jurassic to Pliocene and consists principally of consolidated sedimentary rocks but includes minor areas of metamorphic and igneous 

rock. Overlying this consolidated bedrock is the Santa Clara formation of Pliocene and Pleistocene age. Where exposed the Santa Clara 

consists of semiconsolidated conglomerate, sandstone, siltstone, and 

claystone. The conglomerate and sandstone are poorly sorted and have 

a fine-grained matrix; thus the formation has a low permeability and 

yields only small to moderate quantities of water to wells, rarely 

enough for irrigation purposes. Along the western margin of the 

valley the Santa Clara was warped and folded during the last uplift 

of the Santa Cruz Mountains. Beneath the valley the formation may 

be undisturbed and in part conformable with overlying beds.

Unconsolidated alluvial and bay deposits of clay, sand, and gravel 

overlie the Santa Clara formation and form the valley floor. As shown 

by well logs, the alluvial and bay deposits reach a thickness of 1,000 feet 

or more in the valley trough. However, the lower parts of these wells 

may be in the Santa Clara formation, which to date has not been differentiated from the overlying unconsolidated alluvium in drill holes.

The geology of the bedrock has been described in some detail in 

earlier reports (Davis and Jennings, 1954; Davis, 1955), and the 

geology of the water-yielding deposits has been described by Clark 

(1924), Tolman and Poland (1940), and the California State Water 

Resources Board (1955).

Geologic sections A-A' (fig. 1) and B-B' (fig. 2) show the general 

nature of the alluvial and bay deposits that underlie the valley floor. 

Fine-grained materials such as clay, silt, and sandy clay, which retard 

the vertical movement of confined ground water, constitute the major 

part of the valley fill. Sand and gravel occur in lesser amounts but 

are more abundant near the valley margins and in old stream channels. 

None of the deposits can be traced laterally for more than a short 

distance and possible correlations are not shown in figures 1 and 2.
SUBSIDENCE IN THE SANTA CLARA VALLEY C-3

SE

EDENVALE

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S^ f^ EXPLANATION

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Silt

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Gravel

aClay

Clay, sand, and 

gravel

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200'

-100'

SEA LEVEL

-100'

-200 

-30O'

-4OO'

-5OO'

-600'

-700'

--son'

FIGURE 1. Geologic section from Alviso to Edenvale.

Along section A-A' (fig. 1) the deposits become increasingly coarse 

grained from Alviso toward Edenvale. Gravel beds at the north end 

of the section, particularly in wells 6S/1W-9J2,1 10P2, and 15J1, 

probably are relatively recent deposits of the Guadalupe Eiver (fig. 

5). In the southern part of the section between San Jose and Edenvale, the coarse material most probably was deposited by Coyote Creek.

Beds logged as gravel at the west end of section B-B' (fig. 2) near 

Los Altos represent the subsurface extension of the Santa Clara formation. Between Alviso and Milpitas, the coarse materials are deposits 

of the Guadalupe Eiver and Coyote Creek.

1 The "well-numbering system used shows the locations of wells 

according to the rectangular system for the subdivision of public 

land. That part of the number preceding the slash (as in 6S/1W9J2) indicates the township (T. 6 S.) ; the number following the 

slash is the range (R. 1 W.) ; the digit following the hyphen is the 

section (sec. 9) ; and the letter following the section number indicates the 40-acre subdivision of the section according to the 

accompanying diagram. Within each 40-acre tract the wells are 

numbered serially, as indicated by the final digit.
C-4 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

-SRM-SEA LEVEL

J

FIGURE 2. Geologic section from Los Altos to Milpitas. 

HISTORY OF LEVELING CONTROL

Interest in the subsidence of Santa Clara Valley first developed when 

the 1932 releveling of a 1912 first-order level line indicated that several 

feet of settlement had occurred in the San Jose area. More extensive 

releveling in 1933 confirmed the subsidence and indicated that it was 

continuing.

In order to determine the extent and magnitude of subsidence, the 

U.S. Coast and Geodetic Survey established an extensive level network 

in 1933 (fig. 3). The network was completely leveled for the first time 

in the spring of 1934; releveled 7 times from November 1934 to April 

1940 (lines 1 and 2 were releveled 9 times in this period); once in 1948; 

and again in 1954.

As shown in figure 3, main level line 1 extends from Morgan Hill 

northwest to San Jose and north along the east side of San Francisco 

Bay to Hay ward. Main line 2 extends along the west side of the bay 

from San Jose northwest to San Mateo. Three transverse lines (3,18, 

and 9) extend southwest across the San Andreas fault and three (1, 4,
SUBSIDENCE IN THE SANTA CLARA VALLEY C-5

EXPLANATION

Bedrock

Fault

Dashed where approximate, 

dotted where concealed

FIGURE 3. Map showing network of level lines In the San Jose subsidence area, Santa 

Clara Valley, Calif. (Modified after U.S. Coast and Geodetic Survey.)

and 6) extend east across the Hayward fault. Additional cross lines 

bring the length of the level network in the area to about 190 miles. 

The U.S. Coast and Geodetic Survey (1956) published a compilation 

of adjusted elevations for all bench marks in the level network. The 

elevations of certain bench marks at the valley margins were found to 

have remained stable (within the limit of accuracy of precise leveling) 

since they were established; these bench marks were assumed to be 

fixed, and elevations of the valley-floor bench marks were adjusted 

accordingly by the least-square method. The adjusted elevations were 

used in the preparation of this report.

MAGNITUDE AND DISTRIBUTION OF SUBSIDENCE

Figure 4 shows lines of equal subsidence from 1934 to 1954 superimposed on the simplified geologic map. The line of 0.1-foot subsidence partly encloses an area of at least 230 square miles, is open at

621995 62 , 2
C-6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

-in I
SUBSIDENCE IN THE SANTA CLARA VALLEY C-7

both the north and the south ends, and would surround a still larger 

area if sufficient level control existed to permit closure. Control is 

not available to define the line of zero subsidence.

Maximum subsidence during the 20-year period was 4.06 feet at 

bench mark J 111 Reset near Sunnyvale at the intersection of the 

Alviso-Mountain View Road and the Bayshore Highway (figs. 7, 8). 

Subsidence in and around San Jose during the same period exceeded 

3 feet in many places. At the southeast end of San Francisco Bay, 

the tidelands subsided as much as 2.5 feet.

The area of subsidence lies wholly within the area of valley fill and 

corresponds in general with the area of confined ground water as 

defined by the State (California State Water Resources Board, 1955, 

pi. 1). No subsidence has occurred in areas underlain exclusively by 

the Santa Clara formation, nor has any change in altitude of the land 

surface occurred across the San Andreas fault.

Figure 5 shows subsidence from the spring of 1948 to the summer of 

1954. Subsidence in and near San Jose and near Sunnyvale exceeded 

2 feet during the period. Maxium subsidence was 2.40 feet at bench 

mark E 886, about 2 miles north of downtown San Jose; the average 

rate of subsidence at this bench mark was 0.4 foot per year.

Figure 6 shows subsidence from 1940 to 1954 and the locations of 

various bench marks, wells, and cross sections which are discussed in 

this report. Subsidence in excess of 2.5 feet occurred in part of San 

Jose and in the area near Sunnyvale. The map differs only slightly 

from that of 1948-54 (fig. 5) because subsidence from 1940 to 1948 was 

small.

Figure 7 shows land-subsidence profiles A-A' from San Carlos to 

Coyote between 1912 and 1956. The May-July 1934 leveling was used 

as a reference line because leveling before that time is incomplete for 

the profile reach. Also, the 1934 leveling is the earliest included in 

all three of the profiles shown in figure 6. Subsidence before 1934 is 

shown above the 1934 reference base; subsidence after 1934 is shown 

below it.

Subsidence from 1912 to 1954 increased southeastward from about 

0.1 foot at Redwood City to about 2.2 feet at Palo Alto, 6 feet at 

Sunnyvale, and 7.8 feet at San Jose. At a knob of bedrock (BM 

152.36), 4 miles south of San Jose no subsidence occurred.

Maximum known subsidence prior to the spring of 1934 was 4.66 

feet at bench mark P 7 in San Jose. From 1934 to 1954, maximum 

known subsidence was 4.06 feet at bench mark J 111 Reset in Sunnyvale. 

However, the maximum known subsidence from 1912 to 1959 was 9.04 

feet at bench mark P 7 in San Jose.
C-8 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES
Bedrock

Undiftertntioted igneous, metomorphic, and consolidated ffdimentory roe/is _ I22°I5' - -- - I22°00'

FIGUEB 6. Map of Santa Clara Valley, Calif., showing land subsidence, 1940-54.
C-10 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

133d Nl '3DN3QIS8nS
SUBSIDENCE IN THE SANTA CLARA VALLEY C-ll

Figure 8 shows land-subsidence profiles B-B' from Mountain View 

to Milpitas between 1933 and 1954. Along the entire line, subsidence 

from 1948 to 1954 exceeded that from 1934 to 1948.

Apparent increases in altitude, such as that at bench mark Y 180, 

between 1937 and 1938, probably are due to adjustments of fieldobserved altitudes.

Figure 9 shows land-subsidence profiles C-G' from Los Gatos 

through San Jose to Alum Eock Park between 1934 and 1954. Noappreciable subsidence occurred between Los Gatos and bench mark 

35 (USGS). In this area the ground water is unconfined and coarsegrained deposits constitute about 75 percent of the underlying 

materials.

The edge of the area of artesian confinement, as shown in Bulletin 

7 of the California State Water Eesources Board (1955, pi. 1), lies 

approximately between bench marks T 176 and S 176 and marks the 

point at which subsidence increases sharply. Maximum subsidence 

along line C-Cr , 3.49 feet, occurred at bench mark E 176 in San Jose. 

Along this line, as along profile B-B' (fig. 8), subsidence from 1948 

to 1954 exceeded that from 1934 to 1948.

Mountoin 

View Alviso

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1934 BASE

1938 \9*

NOTE A!! leveling by the U. S. 

Coast and Geodetic Survey

2 MILES

 Profiles of land subsidence, Mountain View to Milpitas, Calif., 1934-54.
C-12 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

eo cvi 10 

133dNl'33N3GISanS
SUBSIDENCE IN THE SANTA CLARA VALLEY CMS

Figure 10 shows the change in altitude at bench mark P 7 in San 

Jose and the change in artesian pressure at a nearby well. The artesian 

pressure head at well 7S/1E-7K1, which was above the land surface 

in 1915, declined 100 feet more or less continuously from 1916 to 1934, 

rose 74 feet to a peak in 1943, and then declined 95 feet by 1951. In 

1958 the water level was about the same as in 1951. The water-level 

fluctuation in this well is reasonably representative of wells in the 

artesian area between San Jose and San Francisco Bay.

FIGURE 10. Change in altitude at bench mark P 7 in San Jose and change in artesian

pressure at nearby well.

Subsidence at bench mark P 7 was 0.39 foot from 1912 to 1919 and 

4.56 feet from 1919 to November 1934, the first period of water-level 

decline. From November 1934 to November 1937 the rate of subsidence gradually decreased and from 1937 to 1940 there was a decrease 

of only 0.03 foot. The net was not leveled between 1940 and 1948; 

it is not known whether the land surface recovered in response to the 

water-level recovery. However, the trend from 1938 to 1940 suggests 

that recovery, if any, was small. The subsidence of 2.72 feet from 

1948 to 1956 probably was a result of the rapid decline of artesian 

pressure from 1943 to 1951.

Subsidence was most rapid, averaging 0.5 foot a year, from April 

1932 to November 1934. From 1948 to 1954 the average rate was 0.32 

foot a year. The ratio of subsidence to artesian-pressure decline from 

1920 to 1935, when both were occurring at nearly uniform rates, was 

about 1 to 13.

The close correlation between the subsidence plot and the hydrograph
C-14 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

strongly suggests that subsidence of the land surface is caused by the 

decline in artesian pressure in the confined aquifers. If the hydrostatic 

pressure, which helps support the fine-grained clay and silt beds overlying the confined aquifers, decreases, the result is a like increase in 

effective stress (grain-to-grain load) on the aquifer skeleton. This 

effect is instantaneous in the coarse-grained permeable aquifers. However, permeability and head differential determine the rate at which 

water will escape from the fine-grained beds, and the rate of escape 

determines the rate of compaction. Therefore, compaction occurs 

more slowly in the fine-grained beds than in the coarse-grained beds.

The cessation of subsidence in 1938, when the artesian pressure had 

recovered to about 60 feet below the land surface, suggests that all 

compaction attributable to the net pressure decline from 1916 to 1938 

had taken place. Ignoring possible land-surface recovery as a result 

of artesian pressure recovery, it would be expected that no additional 

subsidence would occur until the artesian pressure declined again below 

the 1938 level, at which time the effective stress due to overburden load 

would begin to exceed the preconsolidation condition. Such a decline 

occurred in 1947 and renewed subsidence probably began at that time.

Figure 11 shows the changes in altitude at two bench marks near 

Mountain View and the change in artesian pressure at a nearby well 

The pattern of change in artesian pressure is similar to that at San

FIGURE 11. Change in altitude at bench marks F 111 and J 111 Reset near Mountain View 

and change in artesian pressure at nearby well.
SUBSIDENCE IN THE SANTA CLARA VALLEY C-15

Jose, although the magnitude of the long-term fluctuation is less. Both 

bench marks subsided markedly from the beginning of record to the 

end of 1937 and again after 1948, each time presumably in response 

to the decline of artesian pressure. Neither bench mark had reached 

temporary stability by 1940, although the last two levelings of 1939-40 

show little change.

Figure 12 shows the change in altitude at bench mark 35 (USGS) 

near Campbell, and the change in water level in a nearby well. Both 

bench mark and well are in the intake area where the ground water 

occurs under unconfined conditions and the alluvial deposits contain 

about 75 percent sand and gravel. Although clay beds are not numerous, the large annual water-level fluctuation suggests that the well 

taps confined deposits at depth. The long-term fluctuation of water 

level at Campbell is nearly the same as in the artesian area near San 

Jose and Sunny vale. The plot of bench mark 35 (USGS), however, 

shows subsidence of only 0.06 foot from 1934 to 1954, indicating that a 

substantial decline of the \vater table may cause only a small amount 

of subsidence.

FIGURE 12. Change in altitude at bench mark 35 (USGS) near Campbell and change ia

water level in nearby well.

CONCLUSIONS

From available evidenec it is concluded that decline of artesian pressure is the principal cause of land subsidence in the Santa Clara Valley. 

The area of intensive subsidence is limited to the area of confined 

water; subsidence is not appreciable in the area of unconfined water. 

Also, the rate of subsidence generally has corresponded with the rate
C-16 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

of decline of artesian pressure; the ratio between the two was about 1 

to 13 when the rates were relatively steady.

Tectonic movement may be a minor contributing cause of the subsidence. However, the level net is tied to consolidated bedrock at six 

widely spaced points around the valley margin, and these points remained stable from 1934 to 1954. Thus, there was no warping or displacement of any of the bedrock points relative to the others. For this 

reason it is concluded that tectonic movement could have occurred 

in only a minor amount, if at all.

The Geological Survey and the Coast and Geodetic Survey are continuing subsidence investigations. The Geological Survey has established bench marks at depth in nine wells, selected to span the vertical 

section of water-bearing sediments. The purpose of the installations 

is to determine whether compaction of the fresh-water-bearing deposits tapped by water wells is occurring and, if so, how much and 

at what depth intervals. The physical, hydrologic, and consolidation 

characteristics of the sediments being compacted are being tested in 

cores from holes drilled near Sunnyvale and in San Jose at the 

principal centers of subsidence.

REFERENCES

California State Water Resources Board, 1955, Santa Clara Valley Investigation : Bull. no. 7,154 p. 

Clark, W. O., 1924, Ground water in Santa Clara Valley, Calif.: U.S. GeoL

Survey Water-Supply Paper 519,209 p. 

Davis, F. F., 1955, Mines and mineral resources of San Mateo County, Calif, r

California Jour. Mines and Geology, v. 51, p. 401-458. 

Davis, F. F., and Jennings, C. W., 1954, Mines and mineral resources of Santa

Clara County, Calif.: California Jour. Mines and Geology, v. 50, p. 321-430. 

Tolman, C. F., and Poland, J. F., 1940, Ground-water, salt-water infiltration,.

and ground-surface recession in Santa Clara Valley, Santa Clara County,.

Calif.: Am. Geophys. Union Trans., pt. 1, p. 23-35. 

U.S. Coast and Geodetic Survey, 1956, San Jose area, California, 1912-1954

leveling (190A Calif.) : 42 p.

U.S. GOVERNMENT PRINTING OFFICE: 1962!