Earth-Fissure Movements Associated 

with Fluctuations in Ground-Water 

Levels near the Picacho Mountains, 

South-Central Arizona, 1980-84 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-H 
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Ave. 
Earth-Fissure Movelllents Associated 

with Fluctuations in Ground-Water 

Levels near the Picacho Mountains, 

South-Central Arizona, 1980-84 

By M.C. CARPENTER 

MECHANICS OF AQUIFER SYSTEMS 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-H 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993 
Carpenter, Michael C. 

U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBI'IT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is 

for descriptive purposes only and does not imply endorsement 

by the U.S. Government 

Library of Congress Cataloging in Publication Data 

Earth-fissure movements associated with fluctuations in ground-water levels near the Picacho mountains, southcentral Arizona, 1980-84 I by M.C. Carpenter. 

p. cm.-(Mechanics of aquifer systems; H) (U.S. Geological Survey professional paper; 497) 

Includes bibliographical references. 

Supt. of Docs. no.: 119.16:497-H. 

1. Earth movements-Arizona-Picacho Mountains Region. 2. Water, Underground-Arizona-Picacho Mountains 

Region. 3. Faults (Geology)-Arizona-Picacho Mountains Region. I. Title. 

II. Series. III. Series: U.S. Geological Survey professional paper; 497. 

QE598.5.U6C36 1992 

551.8'7-dc20 92-33600 

For sale by the Book and Open-File Report Sales, U.S. Geological Survey, 

Federal Center, Box 25286, Denver, CO 80225 

CIP 
CONTENTS 

Page Page 

Abstract--------------------------------------------------------------------------- Hl Measurement of Deformation and Water-Level Fluctuations -

Introduction --------------------------------------------------------------------- 1 

Purpose and Scope------------------------------------------------------ 2 

Previous Investigations ----------------------------------------------- 2 

Acknowledgments------------------------------------------------------- 4 

Approach------------------------------------------------------------------- 4 

Physical Setting---------------------------------------------------------------- 4 

Hydrogeology ------------------------------------------------------------------- 5 

Picacho Basin------------------------------------------------------------- 5 

Fissure Study Site------------------------------------------------------ 5 

Earth-Fissure Development------------------------------------------------ 9 

Hypotheses of Earth-Fissure Formation------------------------ 10 

Measurement of Deformation and Water-Level 

Fluctuations------------------------------------------------------------ 12 

Surveying Stations ----------------------------------------------------- 12 

Surveying Procedures ------------------------------------------------- 15 

Continued 

Monitoring of Horizontal Movement and Water-Level 

Fluctuations ------------------------------------------------------------ Hl6 

Measurement Errors---------------------------------------------------- 17 

Correlation of Horizontal Strain with Water-Level 

Fluctuations------------------------------------------------------------- 18 

Surveyed Horizontal and Vertical Displacements------------------- 22 

Movements Along Nose Survey Line------------------------------ 22 

Movements Along TA-l Survey Line ------------------------------ 24 

Dislocation Modeling of Picacho Earth Fissure ---------------------- 25 

Comparison of Continuous Measurements with Survey 

Results -------------------------------------------------------------------- 36 

Comparison of Results with Previous Work--------------------------- 36 

Summary and Conclusions-------------------------------------------------- 3 7 

References Cited---------------------------------------------------------------- 38 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing location of study area, Eloy climatological station, selected earth fissures, and Eloy compaction recorder------- H3 

2. Map showing selected survey stations, test holes TA-l and TA-3, seismic-refraction line, and horizontal extensometer------- 6 

3. Diagram showing geologic section constructed from seismic-refraction profile and grain-size distribution for test holes 

TA-l and TA-3 ------------------------------------------------------------------------------------------------------------------------------------------------- 7 

4. Graph showing water levels in piezometers in test holes TA-l and TA-3 and in the Eloy observation well, horizontal 

movement at the TA-l horizontal extensometer, compaction at the Eloy compaction recorder, monthly rainfall at 

Eloy, and dates of surveys --------------------------------------------------------------------------------------------------------------------------------- 8 

5. Graph showing differences in altitudes of water level for selected pairs of piezometers in test holes TA-l and TA-3, 

horizontal movement at the nearby horizontal extensometer, and dates of surveys -------------------------------------------------- 9 

6. Diagrams showing possible earth-fissure mechanisms------------------------------------------------------------------------------------------------- 11 

7-13. Graphs showing: 

7. Relation between horizontal movement across earth fissure and water-level fluctuations in deep piezometer 

TA-1-l--------------------------------------------------------------------------------------------------------------------------------------------------- 19 

8. Correlation of horizontal movement with water-level fluctuations in piezometers-------------------------------------------------- 20 

9. Relation between water-level fluctuations and compaction at Eloy and correlation of horizontal movement at TA-l 

with compaction at Eloy -------------------------------------------------------------------------------------------------------------------------- 21 

10. Station trajectories along Nose survey line, 1980-84, with station AA as a reference frame----------------------------------- 23 

11. Station trajectories along TA-l survey line, stations F through Q, 1980-84, with station A as a reference frame------ 24 

12. Station trajectories along TA-l survey line, stations B through E, 1980-84, with station A as a reference frame------ 25 

13. Time series of horizontal and vertical displacements of selected stations along TA-l survey line----------------------------- 26 

14. Diagram showing dislocation model parameters-------------------------------------------------------------------------------------------------------- 27 

15-19. Graphs showing: 

15. Effects of fault width and dip on the pattern of vector displacements near a dip-slip fault--------------------~---------------- 28 

16. Station trajectories along TA-l survey line, stations F through Q, 1980-84, with station K as a reference frame------ 30 

17. Movement from May 31, 1980, to May 15, 1984, stations F through Q, with station K as a reference frame, 

compared with a vertical fault with width 300 meters, depth 0.1 meter, and slip 20 millimeters---------------------- 31 

18. Movement from May 31, 1980, to December 16, 1980, stations F through Q, with station K as a reference frame, 

compared with a vertical fault with width 300 meters, depth 0.1 meter, and slip 9 millimeters----------------------- 32 

19. Movement from December 16, 1980, to March 15, 1981, stations F through Q, with station K as a reference frame-- 33 

III 
IV CONTENTS 

Page 

20-23. Graphs showing: 

20. Movement from March 15, 1981, to November 17, 1981, stations F through Q, with station K as a reference frame, 

compared with a vertical fault with width 300 meters, depth 0.1 meter, and slip 9 millimeters------------------------ H33 

21. Movement from November 17, 1981, to March 17, 1982, stations F through Q, with station K as a reference frame-- 34 

22. Movement from March 17, 1982, to June 6, 1983, stations F through Q, with station K as a reference frame----------- 34 

23. Movement from June 6, 1983, to May 15, 1984, stations F through Q, with station K as a reference frame------------- 35 

TABLES 

Page 

TABLE 1. Horizontal distances from station A and altitudes of stations along TA-l survey line, 1980-84 ----------------------------------------- Hl3 

2. Horizontal distances from station AA and altitudes of stations along Nose survey line, 1980-84---------------------------------------- 14 

3. Altitude of water level in piezometer 1, test hole TA-l, 1980-84 ------------------------------------------------------------------------------------ 42 

4. Horizontal movement across Picacho earth fissure near test hole TA-l, 1980-84-------------------------------------------------------------- 46 

CONVERSION FACTORS AND VERTICAL DATUM 

Multiply By To obtain 

micrometer (J.Lm) 0.00003937 inch (in.) 

millimeter (tnm) 0.03937 inch (in.) 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi) 

hectare (ha) 2.471 acre 

cubic meter (m3) 0.0008107 acre-foot(acre-ft) 

liter per second 15.85 gallon per minute 

(L/s) (gal/min) 

kilometer per second 3,281 foot per second 

(km/s) (ft/s) 

kilogram (kg) 2.205 pound mass (Ibm) 

newton (N) 0.2248 pound force (lbf) 

kilopascal (kPa) 0.1450 pound per square inch 

(lb/in2) 

degree Celsius COC) °F=(9/5°C)+32 degree Fahrenheit COF) 

Normal strain (dimensionless) is obtained by dividing measured change in length between 

two points by the distan~ between the points. One microstrain is lx10"6 strain. 

SEA LEVEL 

In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 

1929)-a geodetic datum derived from a general adjustment of the first-order level nets of both 

the United States and Canada, formerly called Sea Level Datum of 1929. 
MECHANICS OF AQUIFER SYSTEMS 

EARTH-FISSURE MOVEMENTS ASSOCIATED WITH FLUCTUATIONS 

IN GROUND-WATER LEVELS NEAR THE PICACHO MOUNTAINS, 

SOUTH-CENTRAL ARIZONA, 1980-84 

By M.C. CARPENTER 

ABSTRACT 

The Picacho earth fissure transects subsiding alluvial sediments 

near the east periphery of the Picacho basin in south-central Arizona. The basin has undergone land subsidence of as much as 3.8 

meters since the 1930's owing to compaction of the aquifer system 

in response to ground-water-level declines that have exceeded 100 

meters. The fissure, which extends generally north-south for 15 

kilometers, exhibits horizontal tensile failure and as much as 0.6 

meter of normal dip-slip movement at the land surface. The west 

side of the fissure is down thrown. The fissure was observed as early as 1927 and is the longest earth fissure in Arizona. 

Vertical and horizontal displacements were monitored along a 

line normal to the fissure. The survey line extends from a bedrock 

outcrop in the Picacho Mountains on the east, past an observation 

well near the fissure, to a point 1,422 meters to the west. From 

May 1980 to May 1984, the downthrown west side of the fissure 

subsided 167±1.8 millimeters and moved 18±1.5 millimeters westward into the basin. Concurrently, the relatively upthrown east 

side subsided 148±1.8 millimeters and moved 14±1.5 millimeters 

westward. Dislocation modeling of deformation along the survey 

line near the fissure indicates that dip-slip movement has occurred 

along a vertical fault surface that extends from the land surface to 

a depth of about 300 meters. Slip was 9 millimeters from May to 

December 1980 and also 9 millimeters from March to November 

1981. 

Continuous measurements were made of horizontal movement 

across the fissure using a buried invar-wire horizontal extensometer, while water-level fluctuations were continuously monitored in 

four piezometers nested in two observation wells. The range of 

horizontal movement was 4.620 millimeters, and the range of 

water-level fluctuation in the nearest piezometer in the deep alluvium was 9.05 meters. The maximum annual opening of the fissure during the study period was 3. 7 40 millimeters from March to 

October 1981, while the water level declined 7.59 meters. The fissure closed 1.033 millimeters from October 1981 to March 1982, 

while the water level recovered 6.94 meters. Opening and closing 

of the fissure were smooth and were correlated with water-level 

decline and recovery, respectively, recorded in the nearby piezometers. Pearson correlation coefficients between the water-level fluctuations in the deeper piezometers and horizontal movement 

ranged from 0.913 to 0.925. The correlogram with water-level 

decline as ordinate and horizontal strain as abscissa exhibits 

hysteresis loops for annual cycles of water-level fluctuation as well 

as near-vertical excursions for shorter cycles of pumping and 

recovery. 

Vertical and horizontal displacements also were monitored along 

a second survey line 1 kilometer north of and nearly parallel to 

the first survey line. The north line extends from bedrock on the 

east across three fissures to a point 582 meters to the west but 

does not cross the Picacho earth fissure. From May 1980 to May 

1984, the fissure farthest from the mountain front along this line 

exhibited 20±1 millimeters of opening and 33.3±1.1 millimeters of 

vertical offset; the west side of the fissure was downthrown. During the same period, the zone between this fissure and the mountain front exhibited compression. 

The hypothesis of generalized differential compaction is supported by data taken at the study site for several reasons. First, 

the vertical offset across fissures and the fit of deformation to a 

dislocation model are consistent with an elastic model of differential vertical movement deep in the alluvium. Second, correlation is 

high between horizontal movement across the Picacho earth fissure and water-level fluctuations in the deeper local piezometers. 

Third, correlation is high between horizontal movement across the 

fissure and compaction farther west in the basin. The hypothesis 

of rotation of a rigid plate is not supported because (1) fissures 

sometimes opened by movements toward the mountain front, (2) 

fissures opened between existing fissures and the mountain front, 

and (3) the predominant movement of the Picacho earth fissure 

was vertical offset. The hypothesis of horizontal seepage stresses is 

not supported because of the low correlation between fissure movement and horizontal head gradients. 

INTRODUCTION 

Earth fissures are long, linear tensile cracks at the 

land surface, with or without vertical offset. In the 

arid Southwest, earth fissures have caused damage 

to manmade structures and pose additional future 

hazards. Structures damaged by fissures include 

highways, railroads, sewers, canals, buildings, and 

flood-control dikes. Potential damage from earth fissures forced a major rerouting of the Central Arizona 

Project aqueduct. Erosionally enlarged fissure gullies 

present a hazard to hikers, vehicles, grazing livestock, and wildlife. Aquifer contamination may occur 

through rupture of pipelines, surreptitious dumping 

of hazardous waste into fissures, and capture of surface flow containing agricultural chemicals. Fissures 

can be conduits for direct recharge of contaminants 

to deep aquifers. Surficial radon-gas emanations can 

Hl 
H2 MECHANICS OF AQUIFER SYSTEM 

be increased by stresses induced by fissure moveIlilent, and fissures can be conduits for radon gas that 

is released as a result of nearby pumping stresses 

(Fleischer, 1981; Harvey, 1981). Vertical offset presents the greatest potential for earth-fissure damage 

to structures such as large buildings, pipelines, and 

airport runways. 

The U.S. Geological Survey operated a recording 

horizontal extensometer across the Picacho earth fissure. near the Picacho Mountains in south-central Arizona (fig. 1) from November 14, 1980, to December 

13, 1984. In addition, from May 31, 1980, to May 15, 

1984, seven sets of precise horizontal-distance and 

\ leveling surveys were made along a line that crosses 

the fissure at nearly a right angle. 

PURPOSE AND SCOPE 

The purposes of this study were to test the hypothesis that the opening, closing, and vertical offset of a 

major earth fissure can be correlated with water-level 

fluctuations in nearby piezometers and to use dislocation modeling of land-surface deformation near an 

earth fissure to characterize the mechanism and geometry of fissure evolution. The study concentrated 

on characterizing and interpreting the horizontal and 

vertical movement of the Picacho earth fissure from 

May 1980 to December 1984. 

This report presents the hypotheses of earthfissure development, a description of the measurement techniques and instrumentation used in this study, 

and descriptive results. The correlation between horizontal movement of the fissure and water-level fluctuations in nearby piezometers and the results of 

dislocation modeling analysis are described. Analysis 

of the hydrologic data includes determination of seasonal and long-term interrelations among continuously monitored horizontal strain across the Picacho 

earth fissure, water-level fluctuations in nearby piezometers, and repeated surveys of displacements of 

stations near the fissure. This study provides the 

first continuous long-term measurements of earthfissure movements. 

PREVIOUS INVESTIGATIONS 

Leonard (1929) first reported a fissure about 5 km 

southeast of Picacho, Arizona, on September 12, 

1927, after a severe rainstorm. Leonard also reported 

that unusual cracks had occurred on September 11, 

1927, at the El Tiro Mine near Silver Bell, Arizona, 

about 13 km south of the study area. After considering several hypotheses for the formation of the fissure, Leonard tentatively concluded that an 

earthquake, which occurred on September 11, 1927, 

270 km from Tucson, Arizona, either caused the fissure or triggered release of accumulated strain at the 

site. Mountains that form the skyline in a photograph taken by Leonard do not line up at any point 

along the present 15-km-long Picacho earth fissure. 

The mountains form the proper skyline at an inactive fissure several hundred meters to the northwest 

(Holzer and others, 1979). This photographic evidence indicates that the fissure that Leonard observed in 1927 was not the present-day Picacho earth 

fissure. 

Photographic documentation indicates that ·the 

Picacho earth fissure existed as early as 1927. Archived photographs dated November 13, 1927 (University of Arizona Tree Ring Laboratory photographs 

GEOL 27-1 and GEOL 27-2), show Professor A.E. 

Douglas at the edge of an earth fissure. The mountain skyline to the south lines up at a point along the 

present-day Picacho earth fissure. This fissure was 

also attributed to the earthquake on September 11, 

1927. Some sections of the Picacho earth fissure are 

identifiable as lines of denser vegetation on aerial 

photographs taken in 1936 (U.S. Soil Conservation 

Service, 1936). 

Beginning in the early 1950's, several observers reported the occurrence of earth fissures. Feth (1951) 

attributed earth fissures west of the Picacho Mountains to induced differential compaction of poorly consolidated alluvium over the edge of a buried 

pediment. He noted the opening of fissures during 

and after storms and the ability of fissures to accept 

large quantities of runoff to great depth. Fletcher 

and others (1954) attributed earth fissures to piping 

erosion. Heindl and Feth (1955) rejected the piping 

hypothesis because earth fissures drain internally. 

Pashley (1961) suggested that hydrocompaction was 

a possible mechanism for earth fissuring near Casa 

Grande, Arizona. He documented a flow of water of 

about 20 L/s from a breached irrigation ditch into a 

fissure for 18 hours before the fissure filled. 

Robinson and Peterson (1962) noted the increased 

frequency of observation of fissures beginning about 

1949. They reported that fissures occurred in several 

areas in central, south-central, and southeastern Arizona, including areas with no ground-water pumping 

north of Phoenix and in south-central Arizona. North 

of Phoenix, some fissures are in semiconsolidated 

sedimentary rock rather than alluvium. Robinson 

and Peterson (1962) also described the correlation of 

water-level decline, subsidence, and earth fissuring 

in areas of significant ground-water withdrawal and 

presented subsidence profiles from 1905 to 1960 in 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA 

33' 00' 

45' 

32' 30' 

1 11 ' 45' 

Base from U.S. Geological Survey, 1:500,000, 

State base map 

0 

I 'r r' 

AREA OF MAP 

10 MILES 

I 

10 KILOMETERS 

D 

Fissure locations from 

Laney and others, 1978 

EXPlANATION 

ALLUVIUM 

CONSOUDATED ROCKS 

EARTH FISSURES 

FIGURE 1.-Location of study area, Eloy climatological station, selected earth fissures, and Eloy 

compaction recorder. 

H3 
H4 MECHANICS OF AQUIFER SYSTEM 

the Picacho-Toltec area. Peterson (1962) related the 

locations of earth fissures to gravity anomalies and 

supported the hypothesis that earth fissures are tensile failures caused by differential compaction over 

buried pediments or fault scarps. Schumann and Poland (1970) correlated ground-water pumping with 

land subsidence and earth fissuring in south-central 

Arizona. They presented subsidence profiles and 

maps of earth fissures and gravity anomalies. 

ACKNOWLEDGMENTS 

Many people contributed to the successful completion of the field work for this investigation. David C. 

Steinke, University of Arizona, designed electronic 

circuits used in the study. Caryn A. Carpenter and 

Eric R. Carpenter contributed surveying assistance. 

Test holes and piezometers were drilled and installed 

by personnel of the U.S. Bureau of Reclamation. 

Francis S. Riley provided valuable comments and 

criticisms. Bruce L. Massey aided in installing tapeextensometer monuments and provided rigorous instruction in techniques of precise surveying. Roger P. 

Denlinger assisted in dislocation modeling. Robert 0 . 

Burford furnished the mechanical components of the 

horizontal extensometer. James C. Savage designed 

the dislocation model MAIN113. 

APPROACH 

Continuously recorded measurements included hor-, 

izontal extension and compression across the fissure, 

water-level fluctuations in a nearby piezometer, and 

barometric pressure, as well as ancillary measurements of instrument-vault temperature and powersupply voltage. Correlation of horizontal strain with 

water-level fluctuations was tested statistically using 

Pearson correlation coefficients (Sokal and Rohlf, 

1969). 

Repeated horizontal and vertical surveys of nearby 

stations along a 1,422-m profile provided data on seasonal changes in the spatial patterns of deformation. 

Detailed data on a system of fissures closer to the 

mountain front were obtained from a second profile. 

The pattern of deformation across the Picacho earth 

fissure was compared with a fault-dislocation model 

(Savage and Hastie, 1966, 1969) to test the usefulness of dislocation modeling. 

The study site was selected to take advantage of 

data generated by the U.S. Geological Survey and 

the U.S. Bureau of Reclamation in a joint study of 

subsidence and fissure hazards along the route of the 

Central Arizona Project (CAP) aqueduct. The specific 

study site was selected to be near two piezometers 

that had been placed on opposite sides of the Picacho 

earth fissure to determine the effect of the fissure on 

water levels. Seismic-refraction profiles also were 

done near the site as part of the CAP subsidence 

study. These seismic profiles were used in the 

present study to determine basement geometry and 

to estimate material properties for preliminary finiteelement modeling of deformation. 

Hydrologic and geologic terms used in this report 

are defined by Poland and others (1972), Lohman 

and others (1972), and Bates and Jackson (1980). 

Holzer and others (1979) applied the term "Picacho 

fault" to the main fissure in this study because of the 

obvious vertical offset of the land surface. The more 

traditional term, earth fissure, used by Leonard 

(1929) for the tensile crack that he observed near 

Picacho, Arizona, in 1927, will be used in this report. 

This usage avoids confusion with structural features 

such as the Picacho Peak detachment fault (Brooks, 

1986). 

PHYSICAL SETTING 

The Picacho basin is a broad alluvial basin bounded by mountains of mostly igneous and metamorphic 

rocks within the Basin and Range lowlands province 

in south-central Arizona (fig. 1). The basin trends 

slightly east of north and is 70 km long north to 

south and 35 km wide east to west. The mountains 

range in altitude above sea level from 1,374 m at 

Newman Peak to 495 m at Toltec Buttes. The alluvial slope is gentle to the northwest, from an altitude 

of about 533 m at the base of the Picacho Mountains 

to 425 m near Casa Grande. The principal drainage 

is by the ephemeral Santa Cruz River, which enters 

the basin from the southeast and distributes into the 

Santa Cruz Flats in the southern part of the basin. 

The study area is in the Sonoran Desert. At the 

Eloy climatological station (fig. 1) for 1951-70, the 

January mean minimum temperature was 2.1 oc, the 

July mean maximum temperature was 40.6°C, and 

the annual mean temperature was 21.2°C. Mean annual rainfall at the Eloy station was 214 mm, with a 

maximum daily rainfall of 78 mm falling in January 

1956 (Sellers and Hill, 1974; National Oceanic and 

Atmospheric Administration, 1973-85). About half 

the annual rainfall occurs during July through September from storms associated with humid air masses from the Gulf of Mexico. The other half of the 

annual rainfall occurs during October to March from 

cyclonic storms originating in the Pacific Ocean (Sellers and Hill, 1974). From 1980 to 1985, the amounts 

of annual rainfall measured at the Eloy station were 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H5 

226 mm, 258 mm, 323 mm, 512 mm, 383 mm, and 

343 mm, respectively. These years were wetter than 

normal; 1983 had about 2.4 times the normal rainfall. Potential annual evapotranspiration is about 1.1 

m, or more than five times the mean annual rainfall 

(Hardt and Cattany, 1965). 

HYDROGEOLOGY 

PICACHO BASIN 

The water-bearing units in the Picacho basin are 

composed of alluvial deposits. These heterogeneous 

deposits are divided into a local gravel unit, lower 

sand and gravel unit, middle silt and clay unit, and 

upper sand and gravel unit. The local gravel unit, 

which ranges in thickness from 0 to 300 m and generally is a productive aquifer, is present only in the 

western part of the basin. The lower sand and gravel 

unit ranges from 0 to about 150 m thick. The middle 

silt and clay unit has the lowest permeability of the 

alluvial units, ranges from 0 to 600 m thick, and 

yields moderate quantities of water from sand and 

gravel lenses. The upper sand and gravel unit is the 

surface deposit in most of the area and ranges from 

less than 15 m to about 180 m thick (Hardt and Cattany, 1965). 

Before 1930, depth to water was 30 m or more in 

only about 12 wells in the Eloy area. During the 

1930's, development of ground water increased markedly. During 1940-45, depth to water increased from 

predevelopment levels of about 10 m to as much as 

55 m (Smith, 1940). Turner and others (194 7) estimated safe yield for the Eloy area at 3.lxl07 m3/yr; 

pumpage was estimated to be 3.38xl08 m3/yr, or 11 

times the safe yield. By 1985, total ground-water 

pumpage for the lower Santa Cruz basin was about 

8x108 m3/yr, and the depth to water was as much as 

. 133 m below land surface in a trough of depression 

between Eloy and the Picacho Mountains (U.S. Bureau of Reclamation, 1976; U.S. Geological Survey, 

1986). 

Subsidence was detected in the Picacho basin as 

early as 1948, when a level line run in 1905 was rerun. Between 1905 and 1948, 30 mm of subsidence 

had occurred near Eloy. From 1948 to 1967, subsidence was as much as 2.3 m northeast of Eloy; in the 

Picacho basin, 57,000 ha had subsided 150 mm or 

more and 15,500 ha had subsided 900 mm or more. 

From 1967 to 1973, subsidence of as much as 930 

mm occurred near Picacho (Schumann and Poland, 

1970; Winikka and Wold, 1976). By 1977, a total of 

3.8 m of subsidence had been measured at a bench 

mark 5 km south of Eloy (Laney and others, 1978). 

FISSURE STUDY SITE 

At the study site near test hole TA-l (figs. 2, 3), the 

land surface slopes gently about 1 percent to the 

northwest. Between TA-l and TA-3, the slope is also 

about 1 percent to the west. Slope of the potentiometric surface between the deep piezometers TA-1-1 and 

TA-3-1 (figs. 3, 4) is about 0.2 percent to the west. 

Slope of the contact between the alluvium and the 

crystalline basement rock (seismic velocity 4. 7-6.0 

km/s) is about 45 percent, or 25°, to the west with no 

indication of development of a pediment or bedrock 

bench. 

Alluvial deposits consist of three units that are 

poorly consolidated, poorly stratified, and commonly 

poorly sorted. Contacts between units are gradational 

over tens of meters. The upper unit is brown silty 

and clayey sand and gravel and extends from the 

land surface at an altitude of about 500 m above sea 

level to an altitude of about 400 m in each well. This 

unit has seismic velocities that range from 0.9 to 1.1 

km/s (fig. 3). The middle unit consists of brown to 

dark-brown sandy, clayey silt, which is cemented to 

mudstone or siltstone in some intervals. This unit 

contains tuff, reworked tuff, and many sand stringers. The middle unit extends from an altitude of 

about 400 m to about 170 m in TA-l and to about 

-20 m in TA-3. The middle unit has seismic velocities 

of 1.9 to 2.7 km/s. A slight increase in density gradient may characterize the vertical change in seismic 

velocity in the middle of this unit. The lower unit is a 

brown clayey, silty, sandy gravel and extends from an 

altitude of about 170 m to below the bottom of test 

hole TA-l and from an altitude of about -20 m to 

below the bottom of test hole TA-3. Clasts in this 

unit are as large as boulders and include granite, 

granodiorite, schist, quartz, and feldspar. The clasts 

commonly are severely weathered. The lower unit 

has seismic velocities of 3.3 to 3.6 km/s. The crystalline basement rocks probably are granodiorite and 

granite gneiss and have seismic velocities of 4. 7 to 

6.0 km/s (R.L. Laney, hydrologist, U.S. Geological 

Survey, written commun., 1979; H.H. Schumann, hydrologist, U.S. Geological Survey, written commun., 

1987). 

A discrepancy exiats between the contacts as determined from the seismic-refraction profiling and the 

core description in TA-l. Although the seismic lines 

were shot in both directions, some ambiguity remained and is shown as a triangular seismic shadow 

zone in figure 3. Neither the geophysical logs of the 

holes nor the lithologic information from the holes 

was used for the interpretation of the seismic lines 

(H.D. Ackerman, geophysicist, U.S. Geological Survey, oral commun., 1984). A hypothetical low-density, 
H6 MECHANICS OF AQUIFER SYSTEM 

111"27'30' 

32"46' 

li 

li / 

/ li '(}\~ II 

:t -' ::: i 

~ " =====~~~==~-~==;=~==-;~=~i 

" " :: :;: ;: II 

:' V\lilll :: 

17 1635 :: ==~~;::.;:;;~.--,~·===~="======;:;=~!! 

J; ,~ II : 

; / Ill 

: II' 

::: f 11 1 :/Q li Ji I~~ ~ 

'!l ti: :~ :s =::-..:=-=::::.:-J~-::-,-=""!::::= :; • .:.._;:;;;==:"-":''"""'==::.::i\j: I 

II • 1hi lj II l'l'o ;I !I 1iii; :1 

u!if-!1 

n Well oil!): n / ~5 -=•· ~-----=!~=~--"~ -- • wr. rn~ Bf~ ~ = = = 

m_" 

:u,~ 

/:::, 

--~====l-~--======-~~~ :; ,/ :: I! : !I 

I " :1 I 

~ 8 / 1647 :; 

::::; = ==~-~~ ,....,,.,.. = = = '~ ~.;;;. = ==-=-~::::- ·--·"""""""' ==~i u• n H I! 

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

Newman Peak, 1981; Picacho Reservoir, 1965 

0 

0 

{ 

N~ 

,-,\ 

[ 

c' r-' 

::.i 

-" \ 

/~ 

} 

r' 

i 

' i 

!I 

1: 

II 

---r·' 

I; 

,il

n ~ 

I! -. il 

I 

li 

•A 

1 MILE 

1 KILOMETER 

111"25' 

~=-

_c, 

-~ 9 (-':) 

,-e~O _ ___.-- - ~--· 

Fissures mapped by M.C. Carpenter 

and J.K. Boling, July 1979 

FIGURE 2.-Selected survey stations, test holes TA-l and TA-3, seismic-refraction line, and horizontal extensometer. Geologic section 

X-X' shown in figure 3. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H7 

low-velocity zone near TA-l caused by a fissure that 

extends nearly to bedrock could be the reason that 

the refracting horizon at the top of the basal gravel 

appears to be deeper than the contact determined 

from core description and well cuttings. The contact 

between the middle silt and the basal gravel is presented as three alternatives (fig. 3). The lower contact is the seismic interpretation by H.D. Ackerman 

and H.H. Schumann (U.S. Geological Survey, written 

commun., 1985). The upper contact is a linear inter600 

WEST 

X w 

0: 

:::::> 

en 

en 

u::: 

I 

I 0.9 

polation between the contacts determined in the 

wells. The middle contact is a speculative contact 

that qualitatively reflects the pattern of deformation 

measured along the TA-l survey line. 

Two piezometers were set in each of the two test 

holes-TA-l and TA-3 (figs. 2 and 3). The holes were 

200 mm in diameter and were drilled by normal rotary methods. Core was taken every 30 m. Deep and 

shallow piezometers, consisting of 30-mm black pipe, 

were nested in each hole and were designated with 

WLU 

0:0: 

:::::>:::::> 

encn 

C/)C/) 

u:::u::: B 

EAST 

X' 

I 

A 

0.9 ? SAND AND GRAVEL 

en 

0: 

w 

Iw 

::E 

z 

ui 

Cl 

:::::> 

!:::: 

I-

_J 

<( 

400 

200 

SEA 

LEVEL 

WELL SCREEN TA-3-2 -

1.9 

2.4 

1.9 

6.0 

CRYSTALLINE BASEMENT ROCK 

(GRANODIORITE, 

GRANITE GNEISS) 

-200~----~--~----~--~----~----~--~----~----~--~----~--~----~----~ 

A 

0.9 

2,800 2,400 2,000 1,600 1,200 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

VERTICAL EXAGGERATION x 2 

EXPLANATION 

400 

SELECTED SURVEY STATION ····? ···· SCHEMATIC INTERPRETATION TO ACCOUNT 

FOR DEFORMATION, QUERIED WHERE SEISMIC VELOCITY, IN KILOMETERS PER SECOND UNCERTAIN 

TA-1-1y MEAN ALTITUDE OF WATER SURFACE 

GRAIN-SIZE DISTRIBUTION 

SEISMIC 0 Silt and clay INTERPRETATION FROM H.D. 

ACKERMAN, GEOPHYSICIST, AND H.H. 

SCHUMANN, HYDROLOGIST, U.S. GEOLOGICAL 

SURVEY (written commun., 1985) 

STRATIGRAPHIC INTERPRETATION FROM 

CUTTINGS AND CORE 

0 Sand 

• Gravel 

0 

FIGURE a.-Geologic section constructed from seismic-refraction profile and grain-size distribution for test holes TA-l and TA-3. 

See figure 2 for location of section X-X'. 
HS MECHANICS OF AQUIFER SYSTEM 

the suffixes -1 and -2, respectively. Steel well screens 

in piezometers TA-1-1, TA-1-2, TA-3-1, and TA-3-2 

were set from 346 to 350 m, 186 to 190 m, 524 to 530 

m, and 224 to 229m, respectively. Piezometer TA-1-2 

r: 

z 

w 1 ~{I) 

wa: (!) 

>W <: 0 

~w 

01- ~ lU -1 ....J~ Q.. 

<t:....J 0 -2 1-::::::! 

z~ ! Oz -3 

N_ 

a: -4 

0 375 :c 

PIEZOMETERS: 

....J 

w 370 TA-3-2-----

> 

w ....J 

<t: 

w {/) 365 w 

> 

0 

IIl 

<t: 

{/) 

a: 

w 

1-

w 

~ 

z 

w 

(.) 

<t: 

u. 

a: 

:::::> 

{/) 

a: 

w 

1-

<t: 

~ 

LL 

0 

w 

Cl 

:::::> 

II-....J 

<t: 

{/) 

a: - w ....JI- ....Jw 

~~ 

Z::::i -....J <t:-

a:~ 

~ 

360 

355 

350 

345 

340 

335 

330 

120 

80 

40 

0 

airline leaks 

~ ........._ ELOY OBSERVATION WELL 

00 C\1 (<) '<t co 0) 0) co co 0) 0) 

....... - ....... - <s:i ..0 

c.) ::> r( UJ 

z >- UJ 0 <( ::::> <( 

0 z ::2 "'"') ::2 

(Includes data from Wrege and others, 1985b; Epstein, 1987; and 

National Oceanic and Atmospheric Administration, 1973-85) 

z 

0 

650 i=w 

Oa: 

700 <t:w 

0..1-

750 ~w 

0~ 

800 (.)::J 

_J....J 

850 <t:~ 

S2z 

900 1--

a: 

w 

> 

FIGURE 4.-Water levels in piezometers in test holes TA-l and TA-3 and in the Eloy observation well, horizontal movement at the TA-l horizontal extensometer, compaction at the Eloy compaction recorder, monthly rainfall at Eloy, 

and dates of surveys. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H9 

was vandalized in the fall of 1980 and provided only 

a brief record. 

The piezometer data show that as of 1985 none of 

the upper unit, most of the middle unit, and all of 

the lower unit were saturated. The middle unit has 

no well-defined confining unit but exhibits some aspects of confined behavior, including rapid drawdown 

in response to pumping in a well more than 1 km 

away from the piezometer. The middle unit consists 

largely of fine-grained, low-permeability beds that 

could act as laterally discontinuous aquitards. Thin 

sand lenses may enhance vertical hydraulic interconnection within the middle unit and hydraulic connection to the basal gravel. 

Water levels in the piezometers exhibited seasonal 

fluctuations of as much as 15 m with no apparent net 

decline from 1979 to 1985 (fig. 4). The annual high 

water level generally occurred in mid-March. The 

highest water level in TA-1-1 was 368.8 m on March 

14, 1980. In TA-3-2, the highest water level was 

about 373 m on February 22, 1982. The annual low 

water level generally occurred in late August to early 

September. In TA-3-1, the lowest water level was 

355.2 m on September 8, 1982. In TA-3-2, the lowest 

water level was 356.4 m on September 9, 1984 

(Wrege and others, 1985b). 

Water levels in the deep piezometers TA-1-1 and 

TA-3-1 are at nearly the same altitude and fluctuate 

en 

a: 

in a similar way. The maximum difference in water 

level between these piezometers was only 3 m and 

occurred in July 1980 (fig. 5). In contrast, the maximum difference in water level between the shallow 

piezometer TA-3-2 and the deep piezometer TA-3-1 

was as much as 10m. The water level in piezometer 

TA-3-2 w~s higher than in TA-3-1 except during May 

through September 1984. 

In the observation well at the Eloy compaction recorder, water levels fluctuated seasonally about 30 m 

(fig. 4) (Epstein, 1987). Water levels in the Eloy observation well were considerably lower than those in 

the piezometers at the study site until about November 1982. From 1983 to 1985, minimum water levels 

at Eloy were not as low as in previous years and seasonal recoveries were higher than in TA-3-2 (fig. 4). 

The water-level rise at Eloy after November 1982 

probably reflects increased rainfall during the latter 

years of the study and the consequent reduction in 

pumping. 

EARTH-FISSURE DEVELOPMENT 

Earth fissures are associated with land subsidence 

caused by aquifer compaction in Arizona, California, 

and Nevada. Earth fissures in south-central Arizona 

commonly occur as long, curvilinear cracks in the 

~ 15r-11--r-ll~l--rl-,l--r-III--TI-,,-,--,-,C\J,--,-III-,TI--,-I-II--,-I,I--TI-,I,-II--r-lll--,-,--,15 

~ ~ g a; a; 00 co ~ 

~ 

w 

() 

<( 

~ 10-

:::> 

en 

a: 

w 

I-

<( 

~ s~ 0 

w 

Cl 

:::> 

I-

~ 

~ 0 rz 

w 

() 

z 

w 

a: 

...,, 0> 0> 0> 0> 0> 0> 

c;; 

>-

<( 

:::2 

c) a: > a: 

~A~ ~ ~~ 

u:i 

UJ 

z 

::J 

--, >-

<( 

:::2 

d'rt ,~,~ \ 

TA-3-2 minus TA-3-1 I, , • ~ ~ .rlllf ~. 1

¥ ~ t: II ,J, ...-· \ 

I ll~ ~ I f'' } . I J. 1\ 

. " '-'· J'!~VtJ ,f ~f~ I'J. ~ ~~ I Jt~ "' ry -. ' '-J Ill f"'"'~ 

t.lf .. r;_ ~ ,,.t.·~J~~ , 

\ _. "'\-H minus TA-3-1 _ 1 ~ J \~ 

HORIZONTAL EXTENSOMETER 

~ .. 

'__/ 

~~~ \ 

- 10 

- 5 

- 0 

~ -sr-~19~7-9~-r-~~1~98~0~-+~~-1-9~81-L~~~L1_9L82~~~--L1-9~8-3~-~~~19~8-4~-~~~19.....J8_5_L~-s 

Cl 

en 

a: 

w 

1-

w 

~ 

.....J 

.....J 

~ 

~ 

~g Cl ~ 0 w 

~ 6 

~ 

.....J 

<( 

1-

z 

0 

~ 

a: 

0 

I 

FIGURE 5.-Differences in altitudes of water level for selected pairs of piezometers in test holes TA-l and TA-3, horizontal movement at the nearby horizontal extensometer, and dates of surveys. 
H10 MECHANICS OF AQUIFER SYSTEM 

land surface in alluvium near and subparallel to 

mountain fronts (Schumann and Poland, 1970). The 

ends of fissures may propagate as hairline cracks or 

as sequences of collapse holes that resemble rodent 

holes. The central parts of fissures generally are widened into fissure gullies by erosion and collapse of 

the opposing walls (Laney and others, 1978). Fissures vary in length from a few meters to as much as 

15 km but generally range from a few tens of meters 

to a few hundreds of meters. Fissures can be widened 

by erosion to as much as 10 m. Visible depths of fissures can be as much as 10m. One fissure northwest 

of Picacho Peak was taped to a depth of about 25 m 

(Johnson, 1980). Most fissures occur as long, linear 

features in the form of single, parallel, or anastomosing cracks. Fissures generally do not intersect at angles greater than about 30°, and polygonal patterns 

and en echelon cracks are rare. Vertical offset or dipslip movement is visually apparent at only a few fissures, and no strike-slip movement has been observed. Measurements of horizontal movement have 

been made across only a few fissures, and continuous 

measurements have been made for short periods of 

time. Measured movement rarely has been greater 

than 30 mm (Boling, 1987) and generally has been a 

few millimeters. 

Earth fissures have been reported to open during 

and after rainstorms (Boling, 1987). Because they 

commonly cross ephemeral stream channels at nearly 

right angles, fissures intercept storm runoff and redirect much or all of the flow downward into the crack. 

This stream capture lowers local base level, causing 

rapid headward erosion. Fissures can intercept all 

drainage for several years before through-flowing 

drainage is reestablished. From a quantitative study 

of the amount of sediment transported downward 

into cracks from headcutting, bedload, and suspendedsediment load of contributing ephemeral streams and 

from collapse of the walls, Johnson (1980) inferred 

that fissures exhibit large volumes of void space in 

the subsurface. He speculated that this large void 

space could represent subsurface extent beyond the 

ends of surface expression, widening of the crack at 

depth, or penetration of the fissure to great depth 

within the alluvium. 

HYPOTHESES OF EARTH-FISSURE FORMATION 

Several mechanisms have been proposed to account 

for earth fissures. One mechanism or a combination of 

mechanisms can produce fissures, and different mechanisms can operate in various geologic environments 

and under different induced stresses. Mechanisms 

that involve water-level declines and aquifer-system 

compaction include differential compaction, horizontal 

seepage stresses, and rotation of a rigid slab (fig. 6). 

Mechanisms that involve the unsaturated zone include hydrocompaction and increased moisture tension caused by evapotranspiration and water-table 

decline. Other proposed mechanisms include piping 

erosion, collapse of mines and caverns, soil rupture 

during an earthquake, new or renewed faulting, diapirism, and volume changes such as conversion of gypsum to anhydrite and oxidation of organic soils. 

Because of the unusual cracks noted at the nearby 

El Tiro Mine, the earthquake hypothesis proposed by 

Leonard (1929) cannot be discounted for the fissure 

that he observed. In general, however, earth fissures 

occur in areas of low seismicity (Holzer, 1986). In 

particular, the Basin and Range province in Arizona 

is an area of low seismicity (Sumner, 1976). Rare 

earthquakes can cause or trigger fissuring in an area 

undergoing strain from ground-water decline or other 

causes. 

Fletcher and others (1954) introduced the piping, 

or subsurface tunnel erosion, hypothesis. Erosional 

conduits or pipes typically develop in dispersive clayey soils where topographic relief permits seepage velocities high enough to transport fine-grained 

particles through primary or secondary porosity to a 

downgradient point of removal. Piping erosion commonly occurs near the edge of an arroyo or steepsided wash when subsurface tubes develop as conduits for drainage into the arroyo. Heindl and Feth 

(1955) argued against piping as a sole cause of fissuring because drainage of fissures is internal. Piping 

takes place within a fissure along its length during 

development of fissure gullies (Laney and others, 

1978), however, and can account for the presence of 

depressions at the ends of fissures. 

Hydrocompaction was proposed by Pashley (1961) 

as a mechanism for earth fissuring near Casa 

Grande, Arizona. Hydrocompaction, or near-surface 

subsidence (Lofgren, 1960), occurs when high-porosity, moisture-deficient sediments compact as they are 

wetted for the first time since burial (Poland and others, 1972). Hydrocompaction has been noted as a 

cause of fissuring in alluvial-fan deposits in California (Bull, 1964, 1972; Lofgren, 1969; Riley, 1970b). In 

Arizona, hydrocompaction has been observed northeast of the town of Marana near the alignment of the 

Central Arizona Project aqueduct (H.H. Schumann, 

hydrologist, U.S. Geological Survey, written commun., 1988). 

Lofgren (1971, 1972) proposed the hypothesis of 

horizontal seepage stresses directed toward the center of a deep cone of depression as a mechanism for 

earth fissuring. The seepage stress originates as a 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA Hll 

viscous drag of the flowing water on the aquifer skeleton and is either dissipated as heat or converted to 

work on the grains, translating them in the direction 

ZONE OF FISSURING 

~ SURFACE BEFORE 

COMPACTION 

SURFACE AFTER 

COMPACTION 

Earth fissuring caused by differential compaction of 

alluvium over a bedrock scarp 

ZONE OF 

~ 

FISSURING 

Earth fissuring caused by differential compaction of 

alluvium over a bedrock high 

Earth fissuring caused by rotation of alluvial slab 

about a buried bedrock scarp as a result of compaction 

of underlying sediments (modified from Bouwer, 1977) 

of flow. For this mechanism to cause earth fissuring, 

the accumulated centripetal translation of the aquifer must be distributed so that tensile strain in the 

t 

t t 

BEACH SAND WITH 15 PERCENT CLAY 

Faulting and fissuring 

'"""j 

t BEACH SAND 

Faulting only 

,.,... 

~ J 

\ 

\.. 

\ 

I 

FINE ST. PETER SAND 

Faulting only 

0 50 MILLIMETERS 

I I 

t t 

Results of sand-box experiments demonstrating fissuring 

and faulting phenomena consistent with shear and 

tensile failure of a body exhibiting differential offset at 

depth (generalized differential compaction) (modified from 

Sanford, 1959) 

FIGURE 6.-Possible earth-fissure mechanisms. 
Hl2 MECHANICS OF AQUIFER SYSTEM 

overlying sediments exceeds the strain at failure of 

those materials. 

Bouwer (1977) introduced the hypothesis of rotation of a rigid slab of alluvium around the edge of a 

buried fault (fig. 6). In Bouwer's model, the compacting alluvium is entirely below the buried fault edge 

and extends from the fault edge into the center of the 

basin. This model does not account for subsequent 

opening of earth fissures between existing fissures 

and the mountain front. 

The mechanism of horizontal shrinkage in the unsaturated zone includes near-surface desiccation and 

uptake by phreatophytes as well as shrinkage in the 

deep unsaturated zone by capillary forces. Largescale polygonal patterns of fissures occur in some playas and are attributed to near-surface desiccation 

(Neal and others, 1968). Quasi-polygonal patterns of 

fissures near Bowie and Willcox, Arizona (Holzer, 

1980), were attributed to water uptake by phreatophytes (Anderson, 1978). Holzer and Davis (1976) 

suggested that gravitational drainage in the deep unsaturated zone after dewatering of an unconfined aquifer could cause horizontal intergranular tensile 

stresses. Narasimhan (1979) demonstrated numerically that significant volumetric strain could occur 

from unsaturated drainage and argued that horizontal contraction could cause fissuring of soils. 

An early and widely accepted hypothesis of earthfissure formation is that of differential compaction 

over a bedrock structure (fig. 6) ('furner and others, 

1947; Feth, 1951; Robinson and Peterson, 1962; 

Schumann and Poland, 1970; Jachens and Holzer, 

1979). The bedrock structure could be a buried pediment, bedrock high, or steeply dipping fault. In addition to bedrock structures, sedimentary facies 

changes or pinchouts can cause lateral changes in 

compressibility of sediments or in thickness of compressible sediments. Differential compaction occurs 

when water levels decline and the resulting increase 

in effective stress is applied to compressible materials of varying thickness. A ground-water barrier, 

which could be a preexisting fault or other structure, 

can cause a differential water-level decline in sediments on opposite sides of the barrier. Greater compaction will occur on the side of the ground-water 

barrier that has the greater water-level decline. The 

alluvium overlying the compacting interval flexes as 

a beam and exhibits tension, which is greatest at the 

land surface, over a convex-upward structure. Fissuring occurs when the tensile strain exceeds the tensile 

strain at failure. 

In this report, the term "generalized differential 

compaction" is used for the hypothesis of structurally 

or geometrically controlled elastic deformation that 

includes the Mohr criterion of failure. This criterion 

depends on the inherent shear strength of the material and the angle of internal friction of the material 

(Jaeger and Cook, 1979). Failure is by shear, under 

the Mohr criterion, when the least principal stress is 

compressive and by tensile fracture when the least 

principal stress is tensile. This generalization links 

vertical offset and horizontal tensile opening as two 

types of failure resulting from structurally controlled 

differential compaction. 

Sanford (1959) provided an analytical and numerical treatment of the pattern of deformation, associated faults, and tensile fractures that would result 

from a vertical displacement at depth. His study provided a basis in elastic theory for the hypothesis of 

generalized differential compaction. Geometric properties, such as shape of the displacement and lengthwidth ratio of the deforming layer, affect the pattern 

of deformation more than do the material properties, 

such as Poisson's ratio. Both tensile fractures and 

faults occur in overlying sediments in response to the 

displacement at depth. Because the orientation of the 

principal stress axes varies with depth, the dip of a 

fracture varies with depth if the angle of internal 

friction is constant with depth. Scale models of the 

vertical displacements in clayey sand exhibited both 

tensile fractures and faults, whereas cohesionless 

sands exhibited only faults. 

Several mechanisms could contribute to initial and 

subsequent fissure formation and movement in an 

area. Combinations of effects could hasten fissuring if 

each of them cause tensile strains to occur at the 

same time and in the same region. Superposition of 

complex patterns of compressive and tensile strains 

in adjacent areas could reduce the magnitude of local 

tensile strains, thus delaying or preventing fissuring. 

MEASUREMENT OF DEFORMATION AND WATERLEVEL FLUCTUATIONS 

SURVEYING STATIONS 

Two survey lines, designated TA-l and Nose (fig. 2, 

tables 1, 2), were established in spring 1980 for 

measurement of horizontal and vertical deformation 

in subsiding alluvial sediments near the Picacho 

Mountains. The TA-l survey line consists of 17 stations extending from station A on an outcrop of granite gneiss, assumed to be stable, north 88° west, 

normal to and beyond the Picacho earth fissure, to 

station Q. Fissures cross the TA-l survey line east of 

station B, between stations B and C, between stations J and K, and between stations K and L. Station 

K is in an apparent graben block within the fissure 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H13 

TABLE !.-Horizontal distances from station A and altitudes of stations along TA-l survey line, 1980-84 

[Values in meters. Altitude datum is sea level. Values with underlined digits are interpolated. Underlined digits are not significant] 

1980 1981 1982 1983 1984 

Station May December March November March June May 

31 16 15 17 17 6 15 

Horizontal distances 

A 0.000 0.000 0.000 0.000 0.000 0.000 0.000 

B 216.178 216.173 216.176 216.173 216.173 216.177 216.179 

c 412.554 412.555 412.562 412.567 412.570 412.584 412.583 

D 569.514 569.516 569.521 569.525 569.529 569.539 569.539 

E 729.325 729.329 729.329 729.335 729.341 729.344 729.349 

F 909.038 909.036 909.039 909.038 909.042 909.051 909.049 

G 11008.273 11008.269 11008.275 11008.277 11008.276 11008.284 11008.284 

H 11027.015 11027.012 11027.017 11027.020 1,027.019 11027.028 1,027.028 

I 11045.769 11045.765 11045.772 11045.774 1 ,045. 774 1,045.783 1 ,045. 783 

J 11054.156 11054.149 11054.155 1 I 054.158 11054.157 11054.165 11054.167 

K 11060.059 11060.054 1,060.061 11060.067 1,060.066 1,060.075 1,060.077 

L 11065.770 11065.765 11065.771 11065.775 11065.774 1,065. 780 1,065.780 

M 1,075.059 11075.053 11075.060 11075.065 1,075.065 11075.075 1,075.077 

N 11096.566 1,096.559 11096.568 11096.572 1,096.571 1,096.582 11096.587 

0 11111.922 1,111.916 1,111.922 11111.923 1,111.925 1,111.934 11111.935 

p 1 I 272 • 271 1,272.272 11272.275 11272.284 1,272.283 1,272.292 11272.295 

Q 11422.361 11422.357 11422.365 11422.374 11422.368 1,422.379 1,422.376 

Altitude of stations 

A 527.4176 527.4176 527.4176 

B 512.7901 512.7897 512.7896 

c 509.8407 509.8304 509.8257 

D 507.1149 507.0992 507.0922 

E 504.8230 504.8036 504.7950 

F 502.3826 502.3622 502.3531 

G 501.9720 501.9517 501.9426 

H 501.9312 501.9110 501.9018 

I 501.8046 501.7849 501.7759 

J 501.6292 501.6086 501.5997 

K 501.6081 501.5866 501.5779 

L 501.1246 501.0942 501.0838 

M 501.0396 501.0105 501.0011 

N 500.5917 500.5630 500.5559 

0 500.7641 500.7346 500.7252 

p 498.1885 498.1576 498.1486 

Q 496.9990 496.9595 496.9508 

zone. The Nose survey line consists of 13 stations extending from station AA on an outcrop of granite 

gneiss, assumed to be stable, north 7 4 o west, approximately normal to and beyond three earth fissures, to 

station AL. The Nose survey line does not cross the 

Picacho earth fissure but does cross shorter fissures 

between stations AD and AE, AF and AG, and AH 

and AI. 

Station A is a 50-mm-diameter brass cap labeled 

"USGS A" that was center punched and cemented 

into a hole drilled into bedrock outcrop. The station 

is in a small saddle, near the west slope, on the end 

527.4176 527.4176 527.4176 527.4176 

512.7891 512.7889 512.7881 512.7875 

509.8129 509.8067 509.7835 509.7656 

507.0727 507.0632 507.0280 507.0009 

504.7709 504.7593 504.7159 504.6824 

502.3278 502.3155 502.2698 502.2346 

501.9179 501.9059 501.8597 501.8244 

501.8766 501.8649 501.8185 501.7834 

501.7506 501.7391 501.6927 501.6576 

501.5749 501.5635 501.5172 501.4821 

501.5522 501.5411 501.4950 501.4600 

501.0506 501.0386 500.9889 500.9536 

500.9680 500.9564 500.9075 500.8722 

500.5215 500.5110 500.4641 500.4294 

500.6923 500.6810 500.6319 500.5966 

498.1138 498.1037 498.0538 498.0186 

496.9112 496.9025 496.8508 496.8157 

of the southwest-trending protrusion of bedrock in 

the NWXNEXSWX sec. 4, T. 8 S., R. 9 E., Newman 

Peak 7.5' quadrangle (figs. 1, 2). The west, south, 

and southeast slopes of the outcrop are covered with 

petroglyphs. The station is about 14.6 m higher than 

the alluvial surface. 

Stations B through F, P, and Q are iron rods 1.5 m 

long and 20 mm in diameter. The rods were driven 

vertically into the ground through the bottoms of 

holes dug 400 mm deep and 200 mm in diameter. The 

rods were greased where exposed in the holes, and the 

holes were filled with concrete to the land surface, 
H14 MECHANICS OF AQUIFER SYSTEM 

TABLE 2.-Horizontal distances from station AA and altitudes of stations along Nose survey line, 1980-84 

[Values in meters. Altitude datum is sea level] 

1980 1981 1982 1983 1984 

Station May December March November March June May 

31 16 15 17 17 6 15 

Horizontal distances 

AA 0.000 0.000 0.000 0.000 0.000 0.000 0.000 

BA 10. 10. 10. 10. 10. 10. 10. 

AB 110.530 110.531 110.5-3+ 110.530 110.530 110.536 110.528 

AC 157.722 157.700 157.699 157.699 157.698 157.704 157.696 

AD 176.649 176.627 176.627 176.626 176.625 176.633 176.626 

"AC 194.967 194.946 194.945 194.945 194.944 194.952 194.945 

AF 211.584 211.563 211.562 211.562 211.561 211.570 211.564 

AG 231.123 231.102 231.101 231.100 231.099 231.111 231.106 

AH 245.076 245.055 245.054 245.054 245.053 245.064 245.060 

AI 263.368 263.352 263.350 263.353 263.352 263.372 263.372 

AJ 282.124 282.108 282.107 282.110 282.109 282.127 282.141 

AK 431.058 431.064 431.065 431.067 431.069 431.089 431.084 

AL 582.294 582.300 582.303 582.303 582.305 582.320 582.316 

Altitude of stations 

AA 505.9680 505.9680 505.9680 

BA -...... -.. - -------- ·------- AB 498.6557 498.6568 498.6576 

AC 499.1842 499.1854 499.1851 

AD 499.1481 499.1483 499.1492 

AE 498.9245 498.9251 498.9249 

AF 498.8333 498.8332 498.8330 

AG 498.7895 498.7884 498.7881 

AH 498.6457 498.6440 498.6442 

AI 498.3211 498.3142 498.3111 

AJ 498.2553 498.2488 498.2462 

AK 496.9218 496.9080 496.9053 

AL 496.0811 496.0608 496.0550 

leaving the rod tips exposed 5 mm or less above the 

land surface. The rods were greased to allow vertical 

decoupling between the concrete and the rods to prevent the stations from jacking from expansion and 

contraction caused by changes in moisture content of 

the soil surrounding the concrete. The tips of the rods 

were center punched and punched with letters to 

designate the stations. Horizontal and vertical instability of these stations was estimated to be less than 

±0.5 mm on the basis of the small relative horizontal 

movements across uncracked soil of dial-gage extensometers with similar rods and concrete and of small 

relative vertical movements among closely spaced stations in similar alluvium. 

Stations G through 0 are 3-m-long by 20-mmdiameter iron rods driven 1.05 minto the bottoms of 

1.5-m-deep by 300-mm-diameter holes that allow the 

rods to protrude 450 mm above the land surface. Iron 

505.9680 505.9680 505.9680 505.9680 

499.1516 499.1517 499.1519 499.1519 

498.6556 498.6561 498.6574 498.6579 

499.1842 499.1841 499.1846 499.1849 

499.1467 499.1472 499.1482 499.1481 

498.9232 498.9231 498.9237 498.9233 

498.8306 498.8317 498.8316 498.8318 

498.7846 498.7849 498.7835 498.7815 

498.6406 498.6405 498.6390 498.6368 

498.3020 498.3000 498.2873 498.2789 

498.2364 498.2344 498.2212 498.2129 

496.8882 496.8845 496.8642 496.8491 

496.0317 496.0246 495.9951 495.9735 

pipes 1.5 m long by 100 mm diameter were concreted 

in the holes so that the pipes projected 410 mm above 

the land surface, which allowed the rods to project an 

additional 40 mm above the pipes. The insides of the 

pipes were filled with concrete so that the iron rods 

were centered in the pipes. The iron rods were greased 

where they contacted concrete. Ball bearings 25.39 

mm in diameter, welded to 9.5-mm-diameter threaded 

rods, were screwed into threads previously drilled and 

tapped into the ends of the iron rods. The threaded 

rods were secured with lock nuts and thread-locking 

compound. This procedure produced stations that consisted of ball bearings about 450 mm above the land 

surface and that were stabilized by concrete and iron 

pipes. Horizontal and vertical instability of these stations was estimated to be less than ±0.5 mm on the 

basis of relative horizontal and vertical movements of 

closely spaced similar monuments in similar alluvium. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H15 

Station AA of the Nose survey line was a 50-mmdiameter brass cap labeled "USGS AA" set on a bedrock bench on a northwest-trending promontory in 

the NEXNEXNWX sec. 4, T. 8 S., R. 9 E., Newman 

Peak 7 .5' quadrangle (fig. 2). The station was 6.8 m 

higher than the alluvial surface. Stations AB, AK, 

and AL were set like stations B through F of the TAl survey line. Stations AC through AJ were set like 

stations G through 0. Station BA, set in November 

1981, was a 25-mm-long steel carriage bolt that was 

8 mm in diameter and was driven into a 7-mm-diameter hole drilled into bedrock at the contact between 

bedrock and the alluvial surface. Stations AA 

through AJ were destroyed by construction of the 

CAP aqueduct in the summer of 1986. 

SURVEYING PROCEDURFS 

Slope distances from A to B, C, D, E, F, G, 0, P, 

and Q and from AA to AB, AC, AJ, AK, and AL were 

measured using a Hewlett-Packard HP3808A Electronic Distance Meter (EDM), Hewlett-Packard retroprisms, and tripods with plumb rods that display 

instrument or target height. Setup inaccuracies were 

estimated to be less than 1 mm on the basis of repeated tests at the calibration base line at the University of Arizona. The instrument was calibrated in 

October 1980 and July 1984. Deviation from calibration over that period was less than 5xlo-5 percent. 

The instrument is specified for ±5 mm ±2xlo-4 percent accuracy and is capable of ±lx 10·4 percent accuracy if temperature and pressure corrections are 

made for differences between the refractive index of 

air for which the instrument is set and the refractive 

index of air along the line of sight at the time of 

measurement. In several tests for drift of instrument 

constant done at the University of Arizona calibration base line and at a National Geodetic Survey 

base line near Salt Lake City, Utah, the instrument 

constant did not depart from 0 mm or drift at a resolution of 1 mm. For this reason, the ±5-mm part of 

the instrument specification was discounted. Estimated contributions to error for horizontal distances 

were 1 mm for combination of setup and change in 

instrument constant and 1 mm for station instability. 

These contributions and the ±lxlo-4 percent accuracy 

give a root mean square error of 1.4 mm ±lxlo-4 percent. The maximum error was 3 mm or ±1.5 mm 

along the two survey lines. 

For the short measurements of 1.5 km or less along 

the two survey lines, end-point temperature and pressure measurements were made at the heights of the 

instrument and the retroprism. Temperature measurements were made using shaded, calibrated thermometers or thermistors. Pressure measurements 

were made using Wallace and Tiernan aneroid barometers that were calibrated against a U.S. Weather 

Service mercury barometer. The standard deviation 

and mean were determined from ten replications of 

each distance measurement. Temperature and pressure corrections were applied to the mean value of 

the replications to give a measured slope distance. 

The measured slope distance was then trigonometrically reduced to horizontal distance using the target 

height, the instrument height, and the altitudes of 

the stations as determined from leveling surveys. 

Slope distances between nearest pairs of stations 

from G through 0 and AC through AJ were periodically measured using a tape extensometer. The instrument incorporates two built-in dial gages and a 

surveying tape, which has registration pinholes 

punched at 50.8-mm intervals. One dial gage measures tape tension by the deflection of a proving ring. 

The second dial gage measures the distance from the 

punched hole in which the registration pin is locked 

to a reference point within the instrument. The instrument is mounted on a ball-bearing station, and 

the surveying tape is stretched and connected to another ball-bearing station. Because of random errors 

of ±0.07 mm in the locations of the registration pinholes and nonstandard conditions of tape stretch and 

sag, the error of tape-extensometer distance measurements may be as high as ±1.0 mm for some measurements. Because tape tension is constant at 146 

newtons (N) for all measurements, tape sag (as a 

function of length and tape tension) is constant for 

repeated measurements between pairs of stations. 

Thus, a constant error in the absolute distance between a pair of stations is eliminated as an error in 

the difference of distances between the pair for repeated measurements. The instrument measures 

changes in distance over intervals of as much as 30 

m with a repeatability of ±0.13 mm, or ±4xlo-4 percent. The temperature of the tape during ·a measurement was monitored at a point near the center of the 

tape using a calibrated surface-measuring thermistor. 

Five replications, each with its own temperature correction, were made for each pair of stations. Data 

were keyed into a hand-held data logger that stored 

the data for transfer to a computer. The data logger 

also used previously stored data to calculate and 

display in the field the displacement and strain since 

the last set of measurements, using the previous 

distance and the current distance between stations. 

The tape extensometer was calibrated before and 

after each set of field measurements on a frame 

that had been measured with a caliper with an accuracy of 0.03 mm and a calibration traceable to 
H16 MECHANICS OF AQUIFER SYSTEM 

the U.S. Bureau of Standards. Slope distances among 

tape-extensometer station pairs were trigonometrically reduced to horizontal distances using leveling 

data. 

Tape-extensometer data were proportionally adjusted to agree with the EDM data. The EDM is more 

accurate over long distances; tape extensometry is 

more accurate over short distances. The only tapeextensometer stations measured by the EDM were the 

end points of the tape-extensometer lines (points G 

and 0 and points AC and AJ) (tables 1, 2). The differences before adjustment between EDM distances and 

concatenated tape-extensometer distances from G to 

0 and from AC to AJ amounted to no more than 2 mm 

in more than 103m, or 2xlo-3 percent. 

Leveling was performed along the two survey lines 

using a Zeiss Ni-l level. Some Ni-l levels have exhibited errors caused by magnetic effects (Holdahl and 

others, 1986). These errors are considered to be insignificant for the TA-l and Nose survey lines because 

the lines are oriented approximately east to west, or 

orthogonal to the earth's magnetic field, and no power lines were nearby at the times of the surveys. In 

addition, the same instrument was used for all surveys, and the change in altitudes of a station is of 

interest in this study, not the absolute altitude itself. 

The datum for establishing stations ·A and AA is the 

bench mark near station B at an altitude of 514.19 

m. The leveling was double run and adhered to 1st 

Order, Class 1 standards (Federal Geodetic Control 

Committee, 197 4), with the exception that only one 

rod was used. Because maximum sight length was 30 

m, refraction errors were considered to be insignificant (Holdahl, 1981). Balance of sightings was assured by chaining and staking instrument locations 

for subsequent runs. The allowable closures for 1st 

Order, Class 1 standards are 3.6 mm for the TA-l 

survey line and 2.1 mm for the Nose survey line. 

Corresponding nominal accuracies are ±1.8 mm and 

±1.1 mm and apply to each line as a whole. Relative 

altitudes among closely spaced points should be more 

accurate than the nominal accuracy. For this reason, 

values of altitudes of stations in tables 1 and 2 are 

given to 0.1 mm. 

All horizontal distances by EDM and tape extensometer were measured for both Nose and TA-l survey lines for all seven observation times. All points 

on the Nose survey line and stations F through Q on 

the TA-l survey line were leveled for all observation 

times. Stations A through E on the TA-l survey line 

were leveled on May 31, 1980, and May 15, 1984, 

only. Altitudes of points B through Q (table 1) for December 16, 1980, through June 6, 1983, were interpolated by apportioning subsidence of stations B 

through F on the basis of proportional elapsed time 

between May 31, 1980, and May 15, 1984. Agreement 

between rates of subsidence for points AK and C, at 

about the same distance from the mountain front, is 

3.2 percent and for points AL and D, also at about 

the same distance from the mountain front, is 5. 7 

percent; therefore, the interpolation scheme seems 

reasonable. Altitudes of points B through F for intermediate intervals between surveys, however, are not 

significant beyond 0.01 m. Relative altitudes from 

point F to point Q are consistent on the basis of the 

leveling performed from F to Q for all observation 

times. 

MONITORING OF HORIZONTAL MOVEMENT AND 

WATER-LEVEL FLUCTUATIONS 

In the fall of 1980, a buried horizontal invar-wire 

extensometer (Duffield and Burford, 1973; Schulz 

and Burford, 1977) was installed spanning an interval of 29.61 m across the Picacho earth fissure along 

a line parallel to and 4 m north of the TA-l survey 

line (fig. 2). The piers of the extensometer were 

placed opposite stations I and M, which are 29.29 m 

apart, so that strains measured using the horizontal 

extensometer could be compared with strains measured from station I to M. 

The invar wire is strung through a 200-mm-diameter PVC pipe, buried at an average depth of 2 m. The 

extensometer piers and recording instrumentation 

are housed in two 1.2-m-diameter fiberglass vaults, 

which extend from a depth of 0.2 m to 2.4 m below 

the land surface. The vaults are divided into upper 

and lower sections and sealed against moisture and 

air leakage around the piers, pipe, and covers. The 

upper section of each vault is filled with polystyrene 

"peanuts" in plastic bags for thermal insulation. 

Piers consisting of 2-m-long by 100-mm-diameter iron 

pipe are set in concrete below the bottoms of the 

vaults so that the tops of the piers project about 450 

mm above the vault floors. The invar wire, which is 

attached to the west or anchor pier, 4 m north of station M, is suspended in catenary inside the pipe and 

extends to the east or instrument pier, 4 m north of 

station I. There, the wire passes over a low-friction 

pulley sector mounted on the pier and supports a 

freely suspended 2-kg counterweight. The core of a 

linear-variable-differential-transformer (LVDT) displacement transducer is strung on the invar wire 

above the counterweight. Thus, changes in horizontal 

distance between the piers are converted to vertical 

movement of a core inside a transducer. A micrometer head with a resolution of 1 !J.m attached to the 

instrument pier can be adjusted to contact a refer-
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA Hl7 

ence surface fixed to the invar wire for calibration of 

the transducer as well as for maintaining continuity 

of measurement during resets of the invar-wire anchor point on the west pier. 

A battery-powered data logger and audiocassette 

recorder were used to record the horizontal displacement data, water-level data in piezometer 1 at well 

TA-l, barometric pressure, power-supply voltage, and 

data from three thermistors measuring temperatures 

in the vaults and in the pipe. Resolution of the displacement transducer with the data logger was 1 f..tm. 

Resolution of the thermistors was 0.07°C. The waterlevel sensor in piezometer 1 was a continuous-purge 

''bubbler" system incorporating a 0-170 kPa-gage 

pressure transducer with a resolution of 6 mm of 

water-level fluctuation and a 6-mm copper-tube air 

line. The barometric transducer was a 0-100 kPaabsolute pressure transducer with a resolution of 4 

mm of water-level fluctuation. 

Values of altitude of water level and horizontal 

movement are noon values extracted from an array 

of data that was sampled 10 times a day for more 

than 4 years, for a total of more than 100,000 values 

(tables 3 and 4). Corrections to the horizontal-movement record include many resets of the invar-wire 

anchor and a temperature correction. The temperature correction used all three temperature records 

and was based on a thermal calibration done on September 10, 1981. Corrections to the water-level 

record include many resets of air-line length and periodic well taping. Test hole TA-l subsided from an 

altitude of 502.13 m in May 1980 to 501.98 m in May 

1984. This subsidence was applied as a correction to 

the water-level data by linear interpolation over the 

period of record. 

Several gaps exist in the recorded data. Most of 

these gaps were due to failures of the data logger, 

probably because of effects of nearby lightning 

strikes on long signal leads to thermistors and pressure transducers. Lightning protection was used on 

all leads into the data logger and is believed to have 

saved the data logger from serious damage on many 

occasions. Nevertheless, the data logger had to be 

repaired several times by the manufacturer. Other 

gaps were caused by pressure-transducer failures, 

thermistor failures, and premature battery-voltage 

dropoff. 

MEASUREMENT ERRORS 

Several factors, which are grouped under the term 

measurement error, contributed to difficulties in interpreting displacements measured along the survey 

lines and strains measured by the horizontal extensometer. These factors include bench-mark instability, tripod-setup error, EDM instrument error, 

leveling-closure error, vertical component of a nearly 

horizontal resultant vector, and local movement of a 

station. 

Several changes in procedure would reduce errors 

and establish error bands for some of these difficulties. Establishment of a survey array that consists of 

a series of quadrilaterals connected with common 

sides across critical areas would help in the determination of local movement of stations. Differences in 

movement for stations at about the same distance 

from the fissure would give an estimate of error for 

local movements. Sets of three closely spaced bench 

marks in representative sediments would provide a 

standard deviation and estimate of error for benchmark stability for both horizontal-distance measurements and leveling. For tape extensometry, diagonals 

of the quadrilaterals could not exceed 30 m. If 

lengths of both diagonals of a quadrilateral are measured as well as the lengths of all the sides and if the 

stations are leveled, the figure is overdetermined and 

measurement error can be estimated. 

Instrument offset for the EDM for a particular set 

of measurements could be calculated using a threepoint test at the measured line. The test consists of 

measuring three closely spaced collinear points A, B, 

and C, with the EDM set up on an end point, A, then 

remeasuring with the EDM set up on the middle 

point, B. The difference between the calculated distance BC from the first measurement and the measured distance BC from the second measurement is 

the instrument offset. The remainder of EDM instrument error would be determined at a calibration base 

line, as was done in the study. Tripod-setup error for 

EDM measurements consists simply of a determination of the standard deviation of replications of tripod 

setup at a station. 

Perhaps the greatest decrease in EDM error would 

be achieved by moving the instrument to a station 

near the fissure. This would result in shorter measurements across the area where the most accuracy is 

needed. The tie to bedrock would be made with a target station on bedrock. The instrument station near 

the fissure could be incorporated in a three-point test 

and in a comparison between the tape extensometer 

and EDM for each set of measurements. 

A trigonometric check was made to determine 

whether the displacements measured with the horizontal extensometer could actually be vector components of differential subsidence across the fissure and 

was done for several reasons. First, horizontal strain 

across the area from station G through 0, as measured by the EDM, was less than 20 microstrain. Sec-
H18 MECHANICS OF AQUIFER SYSTEM 

ond, the resolution of the horizontal extensometer 

was 0.03 microstrain, or 1.0 !J.ID, a small value compared with the net fault offset of about 20 mm. 

Third, the trigonometric effect, called the instrumentheight, target-height correction, is known to be significant in repeated strain measurements using EDM 

when the lines depart from horizontal. Using the values 29.29 m for the station spacing between stations 

I and M, 3.533 mm for horizontal displacement, and 

7.8 mm for vertical offset from March 17, 1981, to 

November 17, 1981, it was calculated that the piers 

of the horizontal extensometer would have to differ 

in altitude by 15 m for vertical offset to account for 

the strain measured by the horizontal extensometer. 

A similar calculation indicated that an initial difference in altitude of 10 mm would give an apparent 

extension of 3 !liD for 7.8 mm of differential subsidence. The conclusion was that the horizontal extensometer ~easured actual horizontal strain but may 

have included as much as 10 !J.ID of apparent horizontal movement caused by vertical slip across the 

fissure. 

The horizontal extensometer had few problems. 

Eliminating the problem of vertical component of almost horizontal strain would entail incorporating the 

extensometer piers in the repeated surveys and making the trigonometric correction. The consistent nature of the record from the horizontal extensometer 

suggests that horizontal station instability of a pier 

is considerably less than 1 mm and may be smaller 

than the 50 !J.ID/yr determined by Wyatt (1982) in 

weathered granodiorite. 

CORRELATION OF HORIZONTAL STRAIN WITH 

WATER-LEVEL FLUCTUATIONS 

The character of horizontal movement of the fissure was smooth, with no indication of sudden opening or closing at the level of resolution of the 

instrumentation, which was 1 !J.ID and 10 samples 

per day. The fissure exhibited seasonal opening from 

mid-March to August or September, followed by closing until mid-March (fig. 4). Maximum opening of the 

fissure during the study period was 3. 7 40 mm and 

occurred from March 14, 1981, to October 15, 1981. 

Water levels in the deep piezometers reached lows 

of 355 to 357 m during the summers of 1980, 1981, 

and 1982. In 1983, lows were 358 to 360 m; and, in 

1984, lows were about 358 m. Maximum opening of 

the fissure during the study period occurred with the 

minimum water levels in 1982 (fig. 4). Although the 

water-level record at the study site does not extend 

to before 1979, earlier records at Eloy indicate that 

annual minimum water levels in 1980 and 1981 were 

the lowest since record began in 1965 (Epstein, 

1987). 

The early part of the graph showing water-level 

fluctuation and horizontal movement (fig. 7) to about 

November 1981 exhibits mostly irreversible or inelastic behavior. This inelastic behavior, which has a fairly flat slope and specific horizontal strain of 17 

microstrain/m of water-level decline, may dominate 

when new low water levels are reached. Elastic or 

reversible behavior dominates during subsequent periods until new seasonal lows are reached. Steeper 

slope and lower specific horizontal strain of 5 microstrain/m of water-level decline are typical of elastic 

behavior. 

Small steep-sided or nearly vertical excursions are 

evident in the graphs showing water-level fluctuation 

and horizontal movement (figs. 7 and 8). These departures from the general trend of the relation were 

caused by short-term water-level fluctuations owing 

to brief cessation of nearby pumping during summer 

drawdown and brief pumping during winter recovery. 

The departures indicate that horizontal movement of 

the fissure is less responsive to short-term waterlevel fluctuations than to seasonal fluctuations. 

Horizontal movement of the fissure was highly correlated with water-level fluctuations in the deep piezometers, TA-1-1 and TA-3-1 (figs. 4 and 8), and 

moderately well correlated for the shallow piezometer, TA-3-2. Pearson correlation coefficients (Sokal 

and Rohlf, 1969) were calculated for sections of the 

graphs in figure 8. Because of the largely irreversible 

character of fissure opening during 1981, the correlograms were separated into two sections. The separation was made on November 1, 1981, during a 

transition period from largely inelastic behavior during 1981 to predominantly elastic behavior in subsequent years. Correlation coefficients between 

horizontal movement and water-level fluctuations in 

TA-1-1 were 0.913 for the early period (November 14, 

1980, to October 31, 1981) and 0.925 for the later period (November 1, 1981, to December 13, 1984). Corresponding values for TA-3-1 were 0.923 and 0.913, 

respectively. 

The water-level fluctuations in the deep piezometers are indicative of changes in effective stress that 

cause aquifer compaction. The high correlation 

between horizontal movement and deep water-level 

fluctuation is consistent with the hypothesis that differential compaction in the deep section of the aquifer is the forcing function for horizontal movement 

across the fissure. 

Horizontal movement of the fissure also was highly 

correlated with compaction of the aquifer system at 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H19 

Eloy, although no direct causal relation is inferred (fig. 

9). The correlation coefficient between horizontal movement and compaction at Eloy was -0.945 for November 

14, 1980, to October 31, 1981. The correlation coefficient was -0.284 for November 1, 1981, to December 13, 

1984. The graph of compaction and horizontal movement is linear during most periods of water-level decline, increasing compaction, and fissure opening. 

Successive hysteresis loops in the Eloy stress-compaction graph (fig. 9) are shifted to the right, indicating a 

component of irreversible compaction. This shift probably reflects delayed response of low-permeability units 

to sustained drawdown, superimposed on the elastic 

response to the cyclic loading caused by seasonal waterlevel fluctuations. This effect could account for the upward shift in segments of the horizontal-strain and 

compaction correlogram and the low correlation coefficient of -0.284 for November 1981 to December 1984. 

355 

NOV.1,1981 

DEC. 3, 1981 '-...,_ \ ....J 

MARaA~o~ w 

> 

w 360 ....J 

<( 0 w 

C/) 

0 

> 

w l" ~I MAY 4, 1981 

m 365 MAR. 8, 1982 ct 

C/) FEB. 16, 1981 

~t w(!) 

::2:~ 

z- _(/) 370 

1 0 -1 -2 -~ wa: 

(.)(.) 355 

;:t<: 

c:-

;::)(/) 

C/)~ 

C:ct: 

wt-: 

1-C/) 

<( 360 

~ 

u. 

0 

w \ c FEB. 4, 1984 '-...,_ ~\ SEPT. 5, 1983 

;::) 

1- 365 i= /J JAN. 19, 1983 ....J JAN. 8, 1984 - ct 

/ 

MAR. 8, 1984 

370 

0 -1 -2 -3 

Horizontal movement is marginally correlated with 

water-level fluctuations in shallow piezometer TA-3-2 

(fig. 8). The correlation coefficient between horizontal 

movement and water-level fluctuation in TA-3-2 was 

0.891 for November 14, 1980, to October 31, 1981, 

and 0.678 for November 1, 1981, to December 13, 

1984. The pore-pressure changes indicated by the 

water-level fluctuations in TA-3-2 probably are indicative of .changes in effective stresses in the upper 

part of the middle silt unit. The resulting compaction 

could be laterally uniform and thus might not contribute to the differential compaction that is the 

probable cause of horizontal strain across the fissure. 

Correlation coefficients between differences in water levels and horizontal strain were considerably 

lower than those between water-level fluctuations 

and horizontal strain (figs. 4, 5, and 8). Coefficients 

for the difference in water levels between TA-3-2 and 

MAY 19, 1982 

MAY 14, 1982\\ 

APR. 22, 1982 ~ 

!J JAN. 9, 1983 

-3 0 -1 -2 -3 -4 

~ 

DEC. 11, 1984 

MAR. 9, 1984 

-4 0 -1 -2 -3 -4 

OPENING --+ 

HORIZONTAL MOVEMENT, IN MILLIMETERS 

FIGURE 7.-Relation between horizontal movement across earth fissure and water-level fluctuations in deep piezometer TA-1-1. 

Arrows, letters A through L, and selected dates indicate time increasing along curve. 
H20 MECHANICS OF AQUIFER SYSTEM 

TA-3-1, which represent possible vertical seepage 

stresses, are 0.569 for the early period and 0.045 for 

the later period. Coefficients for the difference in 

water levels between TA-1-1 and TA-3-1, which 

represent possible horizontal seepage stresses, were 

-0.164 for the early period and -0.415 for the later 

period. 

The difference in altitude of water surfaces between two piezometers is equal to the seepage stress 

accumulated between the two points if a flow path 

exists between the points. The seepage energy is dis-

-10.------~,-----------.-,------,,.------~,----------~ 

TA-1-1 minus TA-3-1 

(Horizontal seepage stress) 

-5 - -

O- p1=-0.-1,, N=229 NOV. ~~81 _ 

w~~~ I 

5- p2=-0.415, N=779 _ 

10~-----~'-----------~-------~-----~~----------~ 

1 0 -1 -2 -3 -4 

OPENING ~ 

HORIZONTAL MOVEMENT, IN MILLIMETERS 

sipated as heat along the flow path or is converted to 

work on the grains by viscous drag of the flowing water on the grains. Low correlation coefficients indicate a lack of causal relation between the fissure 

movement and the two seepage stresses. 

A direct correlation does not appear to exist between horizontal movement and rainfall (fig. 4). Episodes of fissure opening at other fissures near the 

Picacho Mountains have been correlated with rainfall-runoff events (Boling, 1987); therefore, it is notable that the Picacho earth fissure did not exhibit any 

0 

5 

10 

TA-3-2 minus TA-3-1 

(Vertical seepage stress) 

15~-----~-------~----------~----------~-----------J 

1 0 -1 -2 -3 -4 

OPENING ~ 

HORIZONTAL MOVEMENT, IN MILLIMETERS 

EXPLANATION 

PEARSON CORRELATION COEFFICIENTS 

P1 Period before November 1, 1981 

P2 Period after November 1, 1981 

N NUMBER OF OBSERVATIONS FOR 

CALCULATING P 

FIGURE B.-Correlation of horizontal movement with water-level fluctuations in piezometers. Increasing values of horizontal 

movement indicate fissure closing; decreasing values indicate opening. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA 

>- 0 

..J 

w 

.... 

<( 

en 

a: 

w 

.... 

w 

~ 

:::i 

...J 

~ 

~ 

z 

0 

i= 

(.) 

< n. 

~ 

0 

(.) 

335 

340 

345 

350 

355 

360 

365 

370 

Possible 

air-line leaks 

: , ~ 

. r,, • J I 

:I 

. I 

.:r 

;( 

.~ 

u ("), J Q) ( 

I 0> I 

,-

375~------------~---------------~---------------~---------------~---------------~ 

650 

850 

800 

750 

700 

700 750 800 850 

COMPACTION, IN MILLIMETERS 

(From Epstein, 1987) 

900 

650~------------L-------L-----~~----~~----~ 

1 0 -1 -2 -3 -4 

OPENING ~ 

HORIZONTAL MOVEMENT, IN MILLIMETERS, AT TA-1 

FIGURE 9.-Relation between water-level fluctuations and compaction at Eloy 

and correlation of horizontal movement at TA-l with compaction at Eloy. 

Arrows, letters A through K, and years indicate time increasing along 

curve. 

H21 
H22 MECHANICS OF AQUIFER SYSTEM 

sudden openings during rainstorms during the study 

period. In a general way, rainfall was inversely correlated with fissure movement. During 1983-85, which 

were years of greater-than-average rainfall, a combination of increased recharge and reduced pumping 

probably caused the recovery of water levels. This recovery could have caused the reduction in the rate of 

fissure opening. 

Placement of piezometers in intervals in which 

pore-pressure changes reflect ongoing compaction is 

crucial to the correct understanding of the stressstrain relation involved in the deformation. When 

TA-1-2 was vandalized, data on shallow pore-pressure changes on the upthrown side of the fissure that 

would have contributed to the understanding of deformation at TA-l were lost. The additional piezometer would have enabled comparison of shallow water 

levels to determine whether the fissure is a hydrologic barrier in the silt unit. Also, the data would have 

enabled an analysis of shallow horizontal seepage 

stresses between TA-1-2 and TA-3-2 and vertical 

seepage stresses between TA-1-2 and TA-1-1 on the 

upthrown side of the fissure. 

In the correlograms showing water-level fluctuation 

and horizontal movement, the presence of nearly vertical excursions and hysteresis loops (figs. 7 and 8), 

on the time scales of short pumping cycles and annual water-level fluctuations, respectively, and of different slopes for the different sections of the curves can 

be explained by consolidation theory of an aquifer 

system that comprises interbedded fine- and coarsegrained sediments. The essential assumption is that 

flexuring caused by differential compaction drives the 

horizontal strains in the overlying sediments. Extrapolation from stress-strain analyses of vertical 

borehole-extensometer data (Riley, 1970a; Helm, 

1975, 1976; Epstein, 1987) can account for the phase 

lag and varying slopes exhibited in the correlograms. 

Alternatively, a quasi-black-box rheological model 

might be applied if the aquifer is considered to be a 

homogeneous material. Such a model might be a 

modified Kelvin substance with two coefficients relating stress or water-level fluctuation to strain magnitudes and one coefficient relating strain rate to stress 

(Jaeger and Cook, 1979). 

Placement of additional piezometers in the deeper 

part of the silt unit probably would have provided 

additional insight into the deformation process. A 

head gradient may exist from the lower part of the 

silt unit to the basal gravel. Pore-pressure changes 

measured in the lower part of the silt probably would 

be better correlated with horizontal strain at the 

land surface than are pore-pressure changes measured in the basal gravel. 

SURVEYED HORIZONTAL AND VERTICAL 

DISPLACEMENTS 

The origins of station trajectories along the Nose 

and TA-l survey lines are their topographic locations 

(figs. 10-12). The scales of horizontal and vertical 

movement are rake scales. The vertical and horizontal displacements are both exaggerated 1,000 times, 

preserving an undistorted vector representation of 

the trajectories. A single trajectory represents the 

path that the station followed in the vertical plane of 

the survey line, with the origin of a vector placed at 

the end point of the preceding vector. Because of the 

magnification of the displacements in relation to the 

scale of the profile, it appears that closely spaced stations have collided but, in fact, they have moved only 

slightly in relation to each other. The dates of the 

surveys are indicated on each graph as well as on the 

time-series plots of water-level fluctuations and horizontal movement (figs. 4 and 5). Combined trajectories were selected instead of separate time-series 

plots of all the horizontal and vertical displacements 

of stations because of the small number of surveys 

and the large number of stations and to present 

graphically the two-dimensional aspect of vector displacements. 

MOVEMENTS ALONG NOSE SURVEY LINE 

Seven surveys were done using station AA, which 

is set in bedrock, as the frame of reference for the 

Nose survey line (fig. 10). The horizontal measurement error is ±1 mm. Vertical measurement error is 

a maximum of ±1.1 mm over the whole survey line 

and probably is less than ±0.5 mm among closely 

spaced stations. Stations along the Nose line were located so that pairs of stations were in blocks of unfissured alluvium between the fissures. Comparison of 

fissure movement with strain in the unfissured 

blocks then is possible. 

Movement of stations AB through AF near the 

mountain front generally was horizontal during the 

study period. Stations AG and AF were transitional 

and had slight vertical movement. Stations AI and 

AJ exhibited slightly more vertical than horizontal 

movement. At stations AK and AL, vertical movements exceeded horizontal movements by factors of 

about 3 and 5, respectively. 

Stations AC through AF spanned two fissures and 

two unfissured blocks and exhibited similar behavior 

for all time intervals between surveys. Net vertical 

movement and average vertical movement were less 

than 2 mm for any interval between surveys. Uni-
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H23 

formity of movement from AC to AF indicates little 

movement on the fissure between AD and AE. From 

May 31, 1980, to December 16, 1980, stations AC 

through AF moved an average of 21 mm eastward 

toward the mountain front. This movement, which is 

bounded by the 1-mm basinward movement of AB, 

resulted in horizontal compression between AB and 

AC of 490 microstrain for the interval between surveys. Altitudes of stations AB and ACrose a marginally significant 1.2 mm in that period, which is 

consistent with horizontal compression in the area. 

Stations AC through AF moved little from December 

16, 1980, to March 17, 1982. From March 17, 1982, 

to June 6, 1983, these stations moved an average of 9 

mm westward into the basin without appreciable vertical movement. From March 17, 1982, to June 6, 

1983, the areas between stations AC and AD and between AF and AG exhibited horizontal extensions of 

100 and 150 micros train, respectively, in response to 

fissure opening of 9 mm between stations AH and AI. 

During the same period, the areas between stations 

AI and AJ exhibited compression of 110 microstrain. 

From June 6, 1983, to May 15, 1984, these stations 

reversed and moved an average of 7 mm toward the 

mountain front. Average net movement of these stations from May 31, 1980, to May 15, 1984, was 23 

mm toward the mountain front. Average net horizontal compression was 120 microstrain for this area. 

Between June 6, 1983, and May 15, 1984, station 

AI showed predominantly vertical movement while 

.....J 

w 

> 

w 

.....J 

<( 

w 

(f) 

w 

> 

0 

m 

<( 

600 

W:JST 100 <Czw ~~~ owlcn::::i!:w 

50 <i!~~ oo...J 

i=::::i!:::::! 

a:u.::::i!: 

o woz 0 50 100 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS .....J 

<( 

:::.::: 

<( g? 500 w 

1-

w 

~ 

~ 

MAY 31, 1980 

DEC. 16, 198 

MAR. 15, 1981 

NOV. 17, 198 

MAR. 17, 1982 ! ~ 400 :::::> 

It= 

.....J 

<( 

JUNE 6,1983 

MAY 15, 1984 

AJ continued to move basinward as well as downward. The intervening area, therefore, exhibited horizontal extension of 7 50 microstrain. The expected 

value of 200 to 600 microstrain of extension at failure as estimated by Jachens and Holzer (1982) was 

exceeded and may indicate a higher local value of 

strain at failure or strain-rate dependence of strain 

at failure. A concealed, incipient fissure, which could 

be verified using the techniques of seismic surfacewave attenuation (Wrege and others, 1985a), could be 

present between AI and AJ. 

The fissure between AH and AI exhibited net opening of 20 mm from May 31, 1980, to May 15, 1984. 

The fissure opened 5 mm from May to December 

1980 and 3 mm from March to November 1981. The 

fissure closed 1 mm from December 1980 to March 

1981 and had 0 mm of horizontal movement from 

November 1981 to March 1982. The fissure opened 9 

mm from March 1982 to June 1983 and 4 mm from 

June 1983 to May 1984. 

The fissure between stations AH and AI exhibited 

three styles of opening accompanied by vertical offset 

across the fissure. Fissure opening from May to December 1980 occurred while movement of stations AC 

through AJ was toward the mountain front. Fissure 

opening from March 1981 to November 1981 accompanied predominantly vertical movements in this 

area. Fissure opening from March 1982 to June 1983 

occurred while movement of nearby stations was toward the basin. 

w w w 

a: a: a: :::::> :::::> :::::> (f) (f) (f) 

(f) (f) (f) 

LL LL LL 

-,_ICJLLWO() 

<( <( <( <( <( <( <( <( /?" .. m 

<( 

. 

EAST 

HORIZONTAL DISTANCE, IN METERS 

FIGURE 10.-Station trajectories along Nose survey line, 1980-84, with station AA as a reference frame. 
H24 MECHANICS OF AQUIFER SYSTEM 

Fissures between stations AD and AE and between 

AF and AG exhibited movement that commonly was 

1 mm or less during the study period. An exception is 

the opening of the fissure between AF and AG of 3 

mm from March 17, 1982, to June 6, 1983, which correlated with the opening of 9 mm of the fissure between AH and AI. Movements during 1980 and 1981 

across the fissure between AD and AE were in the 

same sense as movements across the fissure between 

AH and AI but were below the estimated measurement error. 

For several reasons, no attempt was made to model 

the fissure system crossed by the Nose survey line 

with a dislocation model. First, several closely spaced 

fissures are nearby that could contribute to movement of stations. Second, stations were sparse beyond AJ, and only two stations were in each 

unfissured block between fissures. Third, because of 

the proximity of the fissures to the bedrock contact, 

the assumption of homogeneity in an elastic half 

space could be seriously violated. 

MOVEMENTS ALONG TA-l SURVEY LINE 

Seven surveys of the TA-l survey line were done; 

the frame of reference is station A, which is set in 

bedrock (figs. 11-13). Vertical displacements along 

this survey line were characterized by increasing 

subsidence from station B through E, extremely uniform subsidence from station F through K, fault off-

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w 

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::::) 

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i= 

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<( 

600 

500 

400 

W:JET 100 uw ~ ~ffi ..... CJ)~LU 

50 ~~~ uo..J 

i=~= a:u..~ o woz 

0 50 100 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS Q 

MAY 31, 1980 

MAR. 15, 1981 DEC. 16, 1980 

NOV. 17, 1981 

JUNE 6,1983 

p 

set between stations K and L, decreasing subsidence 

from station L through N, and increasing subsidence 

from station 0 through Q (figs. 11-13). Horizontal 

displacements along this survey line were characterized by movement toward the basin decreasing from 

station C through E, remaining generally constant 

from station F through M, and varying from station 

N through Q. 

From May 31, 1980, to May 15, 1984, station B 

moved only 1 mm toward the basin, horizontal strain 

between stations B and C was tensile and had a value of 140 microstrain, and station C moved 29 mm 

into the basin. Horizontal strain was compressive 

and had a value of 72 microstrain between stations E 

and F. Stations F through J moved an average of 12 

mm into the basin, station K moved 18 mm into the 

basin, and station L moved only 10 mm into the basin. Stations M through Q moved 18, 21, 13, 24, and 

14 mm, respectively, into the basin. The area from 

stations G through 0 exhibited 20 microstrain of extension, and the area from stations I through M had 

140 microstrain of extension. Between stations Land 

M, just west of the fissure, the accumulated extension amounted to 840 microstrain, which is a value 

higher than expected for strain at failure (Jachens 

and Holzer, 1979, 1982). The high strain is similar to 

the strain between stations AI and AJ on the Nose 

survey line and could indicate the presence of an incipient fissure, a higher local value for strain at failure, or strain-rate dependence of strain at failure. 

MEASURED MOVEMENT, MAY 1980 TO 

MAY 1984 

INTERPOLATED VERTICAL COMPONENT 

w 

a: 

::::) 

(f) 

(f) 

LL 

0 N MLKJ I H G F 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

EAST 

300~------~--------~------~--------~--------~------~--------~------~ 

1 ,600 1 ,500 1 ,400 1 ,300 1 ,200 1 '1 00 1 ,000 900 800 

HORIZONTAL DISTANCE, IN METERS 

FIGURE 11.-Station trajectories along TA-l survey line, stations F through Q, 1980-84, with station A as a reference frame. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H25 

Subsidence from May 31, 1980, to May 15, 1984, 

ranged from 2.6 mm at station B to 140.6 mm at station E. From station F through station K, subsidence 

varied from 147.0 to 148.1 mm. Net vertical offset 

between stations K and L was 22.9 mm. Subsidence 

decreased from 171 mm at station L to 162.3 mm at 

station N and increased to 183.3 mm at station Q. 

Deformation observed along the TA-l survey line 

was simpler than deformation observed along the 

Nose survey line. The mountainward movement of 

Nose line stations AB through AJ from May 31, 1980, 

to December 16, 1980, was accompanied by similar 

movement of stations F through Q. Otherwise, movement near the Picacho earth fissure along the TA-l 

survey line was predominantly vertical and did not 

exhibit the complexity of the area near the fissure 

between stations AH and AI on the Nose survey line 

(figs. 10-13). 

DISLOCATION MODELING OF PICACHO 

EARTH FISSURE 

A dislocation as used in this report is a rectangular 

tear within an isotropic, homogeneous, semi-infinite 

elastic body or half-space. The three types of dislocations are screw, edge, and tensile (Okada, 1985), 

which are analogous to a strike-slip fault, a dip-slip 

fault, and a tensile crack, dike, or sill injection, respectively. 

700 I I I 

.....J WEST 

w 

> 

Okada (1985) presented the displacement field 

ui (x1, x2, xg) caused by a Volterra dislocation Auj (;17 

; 2, ;g) across the surface I in a homogeneous, isotropic medium as 

u. == .!.lJIL\u.[ au? (au{ au~)~v di ' F J ~..~ik a; +,... ~ + a;. k n k J 

where 

bjk 

A. 

J.l 

u) 

' 

= 

= 

= 

= 

= 

the Kronecker delta; 

Lame's parameter; 

shear modulus; 

the ith component of displacement at coordinates (xl, x2, xg) 

caused by the jth direction point 

force with magnitude F at coordinates (;h ;2, ;g); 

the direction cosine normal to 

the surface di, and 

the summation convention holds. 

In a Cartesian coordinate system (fig. 14) in which 

elastic medium occupies the volume with z :s; 0 and 

the x-axis is parallel to the strike of the fault, the 

elementary dislocations U 17 U 2' U g are strike-slip, 

dip-slip, and tensile components of dislocation, respectively. The length of the fault is L, depth is D, 

width is W, and dip is e. For dip angles oo <8<90°, 

tensile fault opening, right lateral strike-slip 

I I I I 

EAST 

w 

.....J INTERPOLATED VERTICAL COMPONENT 

<( 600 r- w w w a: a: - en ::::> ::::> 

w (/) (/) 

> S£2 en 

0 u. u. 

co A 

<( E D c B (/) 500 r- I MAY 31, 1980 1 i . a: - w I MAR. 15, 1981 I DEC. 16, 1980 

1- I I I 

J ~100~~~ <Czw w I MAR. 17, 1982/ NOV. 17, 1981 ULUr- ~ I I (J):::iELU 

z I JUNE 6,1983 { 

I 50 :;;!~~ 

I uo..J 

u.i 400 r- I ~MAY 15, 1984 

i=:E::::! 

Cl I a:u.:E - ::::> o wo z 

!:: I 0 50 100 > -

1- HORIZONTAL SCALE .....J 

<( OF MOVEMENT, 

IN MILLIMETERS 

300 I I J I I I I 

800 700 600 500 400 300 200 100 0 

HORIZONTAL DISTANCE, IN METERS 

FIGURE 12.-Station trajectories along TA-l survey line, stations B through E, 1980-84, with station A as a reference frame. 
H26 MECHANICS OF AQUIFER SYSTEM 

faulting, and dip-slip thrust faulting are positive. If A. 

= Jl, then Poisson's ratio is 0.25. For vertical dip-slip 

faults and horizontal faults of all types, deformation 

is independent of Poisson's ratio. Singularities exist 

412.60 

~ 

I I I I 509.90 

... , .... ,, 

412.50 I- .. , ,, - 509.80 

en 

a: 

w 

'e.., 

' ... 

STATION C 

412.40 L...-_.J..I _ ___,.JI __ .L..._I_...J..I_---l 

1 ,054.20 I I I I ', 

~--·· ,, I --e 

tu 1,054.10 f-

.. ,,,', .. , -

~ 

z 

w 

() 

STATION J 

,, 

' ~ 1,054.00 I I I I 

len 1 ,065.80 .------r-----r---~--r-----, 

Cl 

.....J 

<( 

1-

z 

0 

N 

a: 

0 

I 

I 

1,065.70 

1 '112.00 .------.......-- 1--rl-----.1--1.-------, 

•, 

~ 

' 

509.70 

501.65 

501.55 

501.45 

501.15 

501.05 

500.95 

500.78 

1 '111.90 I- ',, - 500.68 

',, ', 

STAT ION 0 '',, 

1 111 80 l---+-1 -+1----41----11f----l 500.58 

' . 1980 11981 119821198311984 

909.10 

909.00 

908.90 .....J 

w 

> 1,060.10 w 

.....J 

<( 

w 

en en 

a: w w > 1- 1,060.00 0 w Im 

<( ~ 

en 

z 

a: w~ w 

1- () 

w z 1,059.90 ~ <( 

1- z en 1,096.60 

~ Cl 

z .....J 0 <( 

1- 1-

<( z 1- 0 

en N 1,096.50 LL a: I0 0 

w I 

Cl 

::> 

1-

1- 1,096.40 .....J 

<( 

1,422.40 

1,422.30 I-

,,_ 

.,. • • .. .....---- . ' 

•,,,, 

'• ',,, 

STATION F 

~ 

I I I I 

... ' ' ' .. '',, ,, STATION K ',, I I I I' 

' 

~ 

I I I I 

' ' ' "· '',, 

'• 

STATION N 

',,, 

I I I I 

~ 

•, I I I I 

•, 

' ' .. , 

',,, 

', 

502.40 

502.30 

502.20 

501.65 

- 501.55 

501.45 

500.60 

- 500.50 

500.40 

497.00 

- 496.90 

ST~TIO~ P ' '>• 

1 ,422.20 1---l~-~~--l---l-+-1---1 496.80 1980 1981 1982 1983 1984 

EXPLANATION 

HORIZONTAL MOVEMENT 

VERTICAL MOVEMENT 

FIGURE 13.-Time series of horizontal and vertical displacements of selected stations along TA-l survey line. 

.....J 

w 

> 

w 

.....J 

<( 

w 

en 

w 

> 

0 

m 

<( 

en 

a: 

w 

1-

w 

~ 

z 

~ 

z 

0 

1-

<( 

1-

en 

LL 

0 

w 

Cl 

::> 

1-

1-

.....J 

<( 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H27 

at the perimeter of the rectangular fault but have effects only in close proximity to the fault edges (Chinnery and Petrak, 1968). 

Complete equations for surface displacements, normal and shear strains in the horizontal plane, and tilts 

or shear strains in a vertical plane are given by Okada 

· (1985). Singularities at observation points above the 

perimeter of the fault in some earlier derivations are 

absent in Okada's equations. Field applications of dislocation theory to dip-slip fault movements include 

Savage and Hastie (1966, 1969) and Holdahl (1986). A 

tensile fault model has been applied to explain correlation of strains and water-level fluctuations in fractured rock (Evans and Wyatt, 1984). 

A suite of theoretical plots was generated using the 

program MAIN113, which calculates displacements 

and strains caused by dislocations (Savage and Hastie, 1966). The plots are of displacements of the stations along the survey line for dip-slip faults of 

various dips and widths (fig. 15). The plots were used 

for visual comparison with measured displacements 

at the survey stations. The frame of reference for the 

displacements shown in figure 15 is a point on the 

upthrown side of the fault and was selected for this 

z 

particular study to be compatible with the technique 

used to remove regional subsidence from measured 

displacements near the Picacho earth fissure. For 

other applications, a choice of reference frame at the 

land surface away from the fault would better illustrate the characteristic uplift near the edge on the 

upthrown side of the fault for theoretical plots and 

would avoid near-edge effects for real data. 

The theoretical plots show the effects of varying 

the width of the fault from 50 to 500 m and varying 

the dip of the fault from reverse 70° to normal 70°. 

The horizontal span of distinctive deformation is 

about four times the width of the fault. As the modeled fault deepens, the near downthrown side loses 

its cusp-shaped character and flattens. Similarly, the 

relative displacements on the upthrown side broaden 

and flatten. However, the relative displacements on 

the upthrown side are small and may not exceed 

measurement error or may be masked by local deformation. In these plots, which do not distort deformation with vertical exaggeration, the use of the vector 

combination of horizontal and vertical components of 

displacement allows better estimates of fault dip 

than the use of vertical displacements only. 

X 

EXPLANATION 

D = DEPTH (OKADA CONVENTION) 

L =LENGTH 

W =WIDTH 

8 =DIP ANGLE 

U1, U2, U3 =STRIKE SLIP, DIP SLIP, AND TENSILE 

COMPONENTS OF DISLOCATION 

X =COORDINATE ALONG LENGTH L IN 

PLANE OF LAND SURFACE 

Y =COORDINATE ORTHOGONAL TO X IN 

PLANE OF LAND SURFACE 

Z =VERTICAL COORDINATE 

FIGURE 14.-Dislocation model parameters (from Okada, 1985). 
H28 MECHANICS OF AQUIFER SYSTEM 

A dip-slip dislocation model was selected to characterize the pattern of displacement measured near the 

Picacho earth fissure for two reasons. First, the vertical offset across the fissure was five times greater 

than the tensile opening during the 4-year period of 

observation. Second, Sanford's (1959) sandbox model 

and theoretical results indicate that surface dip-slip 

faults and tensile cracks occur in response to vertical 

offset concentrated over a short horizontal distance 

at depth. 

An effort was made to remove the effect of differential subsidence from the measured movement using a 

finite-element, stress-strain model (Zienkiewicz, 1977) 

in order to produce a residual pattern of deformation 

700 ' I I 

- VERTICAL, WIDTH= 50 METERS, SLIP= 20 MILLIMETERS 600 -

Nr A 

:l p 

\~ ~ F E D c B 

\ I I I I I I 

-

I I 

500 

400 

300 

700 .---.---~.---~.---~r---lr-~r-~~~r-~1~~,~~~~~~~~~~~~~--

600 - VERTICAL, WIDTH = 100 METERS, SLIP = 20 MILLIMETERS 

~ 400 ._ I 

< 

E 

I 

D c 

I I 

-

B A 

I I 

-

~ soo ._ J p q~\\M~r~ 7j j 

~ 3QQ ~--~--~--~--~--L_I __ L_I __ L_ __ L_ __ L_ __ L_I __ L_ __ L_ __ L_ __ L_~ 

w 700 

> 

0 

m 600 

< 

~ 500 

w 

1-

w 400 ~ 

z 300 

w 700 

Cl 

:::::> 

II-

....J 

600 

< 500 

400 

300 

1-

1-

1-

1-

1-

rI I I T I I I I 

VERTICAL, WIDTH = 200 METERS, SLIP = 20 MILLIMETERS 

Nr Q p 

q~ ·~ F E D c 

I I \ I I I 

I 

I I 

VERTICAL, WIDTH = 400 METERS, SLIP = 20 MILLIMETERS Nr Q p o' I• ~ F E D c 

I \ \ \ I I I I 

I I I 

-

B A 

I I 

-

-

I 

-

B A 

I J 

-

700 r---~.---~.---.---,---.---r---r---r-~r-~r-~~~~~~~r~--

VERTICAL, WIDTH = 500 METERS, SLIP = 20 MILLIMETERS 600 r- -

Q p qr F E D c 

500 1- '\ \ I I \ 400 1-

300 I I I 

B A 

I J 

-

1,500 1,000 500 0 

HORIZONTAL DISTANCE, IN METERS 

FIGURE 15 . ...:_Effects of fault width and dip on the pattern of vector displacements near a dip-slip fault. 
....J 

w 

> 

w 

....J 

<( 

w 

en 

w 

> 

0 

ID 

<( 

en 

a: 

w 

1-

w 

~ 

z 

u.i 

c 

:::J 

1-

1-

....J 

<( 

FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA 

700 r--.---,--~--~--~~---r---~.--~.~.~-.1--~.--~.--~.---~.~ 

600 1- NORMAL, DIP = 70°, WIDTH = 300 METERS, SLIP = 20 MILLIMETERS 

a p F E D c 8 

500 r-/ I r

NLJH 

# ~ \ I I I I 

400 1-

300 

700 

600 

500 

400 

300 

700 

600 1-

500 1-

400 '-

300 

700 

600 ~ 

500 1-

400 1-

300 

700 

600 1-

500 1-

400 1-

300 

1,500 

I I 

NORMAL, DIP= 80°, WIDTH= 300 METERS, SLIP= 20 MILLIMETERS 

N L J H 

q\M,.~/,1/~ F 

\ 

E 

I 

D 

I 

c 

I 

8 

I 

p 

I 

a 

I 

I I I 

VERTICAL, WIDTH= 300 METERS, SLIP= 20 MILLIMETERS 

a p q\r F E D c 8 

j \ \ I I I I 

J 

I I 

REVERSE, DIP= 80°, WIDTH = 300 METERS, SLIP= 20 MILLIMETERS 

a p 

q~ F E D c 8 

\ \ \ \ I I I I 

I I 

REVERSE, DIP= 70°, WIDTH= 300 METERS, SLIP= 20 MILLIMETERS 

a p 

\ \ q~ F E D 

\ \ \ I I 

1,000 500 

HORIZONTAL DISTANCE, IN METERS 

20 ~~ffi uwt-

(/)~w 

..JW~ <C>-

uo..J 

i=~= 0 II: LL ~ 

0 20 ~0~ 

HORIZONTAL SCALE OF 

MOVEMENT, IN MILLIMETERS 

FIGURE 15.-Continued 

c 8 

I I 

A 

I 

-

A 

1 

-

-

A 

-I 

I 

-

-

A 

-

-

0 

H29 
H30 MECHANICS OF AQUIFER SYSTEM 

for dislocation modeling. The model mesh was based 

on the geometry of the seismic-refraction line (fig. 3). 

Ratios of Young's moduli were determined using seismic P-wave velocities and an assumed Poisson's ratio 

of 0.25 (Birch, 1966). The applied stress was the water-level fluctuation of TA-1-1 and TA-3-1. The resulting pattern of horizontal strain was one of 

compression across the area from station F through 0. 

Modeled subsidence approached a limiting value from 

station F through 0, which was the expected result 

for a concave-upward basement contact. The model 

gave a pattern of horizontal tension and subsidence 

increasing markedly toward the basin across the area 

centered at TA-3, which was the expected result for a 

convex-upward basement contact. Varying Poisson's 

ratio from 0.25 to 0.48 did not significantly alter the 

modeled results. The modeled pattern of surface deformation using the basement contact as determined 

by the seismic-refraction profile (fig. 3) differed markedly from the observed displacements along the TA-l 

survey line. In addition, the contact between basal 

gravel and silt differed between the lithologic description ofTA-1 and the seismic-refraction line. Therefore, 

the basement contact near TA-l was considered to be 

too poorly known to apply the stress-strain model. 

.....J 

~ 600 w 

.....J 

<( 

w 

(/) 

w 

> 

0 

al 

<( 

(/) 

a: 

w 

1-

w 

::!: 

~ 

w 

0 

::::> 

1-

i= 

.....J 

<( 

500 

400 

300 

WEST 

NOV. 17, 1981 

JUNE 6,1983 

Q p 

MAY 31, 1980 

f 

MAY 15, 1984 

As an alternative to the use of the finite-element 

model for removal of differential subsidence from the 

pattern of deformation, station K was selected as a 

reference frame (fig. 16). This change in frame of reference had the advantage of removing a central or 

average value of subsidence from the pattern of deformation, leaving a residue that could be fitted to a 

dislocation model. For example, total subsidence for 

station K was 148 mm compared with 148 mm for 

station F and 183 mm for station Q for the study 

period. Choice of reference frame did not affect dislocation-model results because the reference-frame adjustment was applied after the model run. The choice 

of reference frame does affect the pattern of deformation that is modeled, however, because each reference 

frame produces a different residual pattern of deformation. All stations from F to K were tried as reference 

frames before station K was selected as the representative station. Differences in patterns of deformation 

using station K or other stations on the upthrown side 

of the fault as reference frames did not exceed the 

measurement error for the modeled time periods. 

For selected intervals between surveys, patterns of 

movement relative to station K were fit visually to a 

suite of plots of modeled movement for faults with 

w 

a: 

::::> 

(/) 

EAST 

~ SEPARATED STATIONS 

~~ ~ ~ 

J I H G 

~)r F 

:: ~~: oo..J 

i=~= a::u.~ o LUoz 

0 10 20 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200~------~--------~------~---------L--------L-------~--------~------~ 

1,600 1,500 1 ,400 1 ,300 1 ,200 1 '1 00 1,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 16.-Station trajectories along TA-l survey line, stations F through Q, 1980-84, with station K as a reference frame. 

Because of overlap, the trajectories of stations G, H, I, and J are also shown separately. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H31 

various dips and widths (fig. 15). A two-dimensional 

fault model was used because of the great length of 

the Picacho earth fissure (15 km), the distance to 

other fissures, and the similarity in pattern of deformation between the Nose survey line and the TA-l 

survey line. The length of the modeled fault was 19.8 

km, and the depth to the top of the fault was 0.1 m. 

Although field evidence indicates that the fissure 

breaks the surface, an algorithm in MAIN113 requires a depth to the top of the fault of greater than 

0. The depth of 0.1 m was large enough for the algorithm to give valid results but small compared with 

the distance to the nearest station. 

A visual-fitting procedure was used because the 

statistical fitting provided by the program MAIN113 

was highly influenced by points P and Q. That program allows statistical weighting of stations by 

means of the standard deviation of the error of the 

station. A visual procedure was used, however, because the finite-element model could not be used to 

separate objectively total movement of stations P and 

Q into components caused by fault movement and 

components caused by differential subsidence. 

No single fault model gave a good fit for the net · 

movement from May 1980 to May 1984. The best fit 

...J 

~ 600 

UJ 

...J 

<( 

UJ 

(/) 

UJ 

~ 500 co 

<( 

(/) 

cc 

UJ 

fUJ 

~ 400 

z 

UJ 

0 

::J 

fi= 

...J 300 

<( 

WEST 

a p 

0 ERROR ELLIPSE 

was a vertical fault with a width of 300 m, depth of 

0.1 m, and slip of 20 mm (figs. 14, 15, and 17). On 

the upthrown side, movement measured relative to 

station K was essentially horizontal, whereas modeled movement exhibited the characteristic pattern of 

convex-upward movement. On the downthrown side 

near the fault, several stations exhibited movement 

similar to that modeled. Stations P and Q subsided 

two to three times as much as modeled movement. 

Because of the seasonal effects on the pattern of displacements that do not appear to be fault movement 

(fig. 16), probably no single fault model should be expected to fit the total measured displacement. 

Measured displacements from May 1980 to December 1980 provided a good fit to a vertical fault that 

had a width of 300 m, depth of 0.1 m, and a slip of 9 

mm (fig. 18). Stations on the upthrown side of the 

fault did not fit the modeled movement. Four stations 

on the downthrown side near the fissure had measurement-error ellipses that contain or nearly contain 

the modeled displacements. Model fit for stations P 

and Q was poor for this time period. The dip of the 

fault was well defined by the pattern of displacements on the downthrown side near the fissure. In 

the absence of stations between 0 and P and beyond 

UJ 

cc 

::J 

(/) 

(/) 

EAST 

F 

\ 

20 ~ .... ~ <Czw owlcn:::i:w 

10 ~~~ goj 

t-::2-

a:LL:::i: 

o woz 0 10 20 > -

MEASURED MOVEMENT, MAY 1980 TO MAY 1984 

MODELED VERTICAL FAULT-Width 300 meters, 

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

depth 0.1 meter, and slip 20 millimeters 

1,500 1,400 1 ,300 1 ,200 1 '1 00 1 ,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 17.-Movement from May 31, 1980, to May 15, 1984, stations F through Q, with station K as a reference frame, 

compared with a vertical fault with width 300m, depth 0.1 m, and slip 20 mm. 
H32 MECHANICS OF AQUIFER SYSTEM 

Q, the width cannot be more precisely determined 

than between 200 and 400 m. The measured displacements for this time period correspond with a period when water levels in piezometers TA-1-1 and 

TA-3-1 declined to annual lows (fig. 4). At the same 

time, the lowest water levels of record (since 1965) 

were observed in the Eloy extensometer well (Epstein, 1987). 

Displacements for December 16, 1980, to March 15, 

1981, are shown in figure 19. Few stations exhibited 

any movement greater than their error ellipses. No 

attempt was made to model movement for this time 

period during which water levels recovered to seasonal highs (fig. 4). Movement seemed to be locally random without any indication of systematic fault 

rebound or fissure closing. 

The best fit for movement from March 15, 1981, to 

November 17, 1981, was the same model as for May 

to December 1980 (fig. 20). The fit for this time period, however, was not as good as for the earlier one. 

As before, there was no fit on the upthrown side. On 

the downthrown side near the fault, the fit was good 

enough to define the dip of the fault but provided no 

definition of the width of the fault. This measured 

movement corresponded closely with a water-level 

decline from an annual high to an annual low (fig. 4). 

Water levels did not decline below previous lows during this time period. 

From November 17, 1981, to March 17, 1982 (fig. 

21), movements seemed to be locally random, as during the previous period of water-level recovery. Most 

movements were small, within the error ellipses, and 

did not appear to be fault reversal. The time period 

corresponded with a period of water-level recovery 

from an annual low to an annual high (fig. 4). 

From March 17, 1982, to June 6, 1983, the fault 

exhibited vertical offset of about 4 mm (fig. 22). The 

pattern of deformation did not appear to fit a particular dislocation and was not modeled. Movement during this time period corresponded with water-level 

fluctuation from an annual high water level to a midpoint in water-level decline during a year in which 

annual low water levels were higher than in previous 

years (fig. 4). 

Movement from June 6, 1983, to May 15, 1984, 

was essentially horizontal (fig. 23). Movement was toward the mountain front on the upthrown side of the 

fault and varied from toward the mountain front to 

toward the basin on the downthrown side of the 

fault. Because of the inconsistency of movement on 

700,-------,,--------.-,------,l--------.-,------,,--------.-,------,,~------. 

....1 

w 

> 

w 

....1 

<( 

w 

(j) 

w 

> 

0 

III 

<( 

(j) 

a: 

w 

..... 

w 

::2 

z 

w 

0 

::::> 

..... 

~ ....1 

<( 

600-

500-

400-

300 IWEST 0 ERROR ELLIPSE 

MEASURED MOVEMENT, MAY 1980 TO DEC. 1980 

MODELED VERTICAL FAULT-Width 300 meters, 

depth 0.1 meter, and slip 9 millimeters w 

a: 

::::> 

(j) 

(j) 

EAST 

-

Q p 

~~~ 

5

,m-

-

<(>-

(.)Q-' 

i=:::E::::! 

a:u.:::E 

o U.Joz - 0 5 10 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200~------~'~------~'~------~'--------~'--------~'--------~'-------~'------~ 

1 ,600 1 ,500 1 ,400 1 ,300 1 ,200 1 '1 00 1 ,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE lB.-Movement from May 31, 1980, to December 16, 1980, stations F through Q, with station K as a reference frame, 

compared with a vertical fault with width 300m, depth 0.1 m, and slip 9 mm. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA 

700,-------.-,------,,-------.,--------.-,------.-,------,,-------.,-------, 

__J 

~ 600-

w 

__J 

< 

w 

(J) 

w 

~ 500- [D 

< 

(J) 

a: 

w 

1--

w 

:::!! 400-

z 

w 

0 

:J 

1--

i= 

__J 390 - < 

WEST 

0 

w 

a: 

:J 

(J) 

(J) 

u.. 

EAST 

-

~ F 

-e -

ERROR ELLIPSE 

MEASURED MOVEMENT, DEC. 1980 TO MAR. 1981 

:omuo--' 

i=::2=' a:u_::2 

o UJoz - 0 5 10 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200L-_______ L_ I ______ __JIL_ ______ _,_ ______ ~I--------~I--------~-------L------___J 

1 ,600 1 ,500 1 ,400 1 ,300 1 ,200 1 '1 00 1 ,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 19.-Movement from December 16, 1980, to March 15, 1981, stations F through Q, with station K as a reference 

frame. 

__J 

~ 600 

w 

__J 

< 

w 

(J) 

w 

~ 500 [D 

< 

(J) 

a: 

w 

1--

w 

:::!! 400 

z 

w 

0 

:J 

1--

i= 

__J 300 

< 

WEST 

0 

w 

a: 

:J 

(J) 

(J) 

u::: 

EAST 

ONML~J~ 

~G 

Q p F 0 

ERROR ELLIPSE 

MEASURED MOVEMENT, MAR. 1981 TO NOV. 1981 

MODELED VERTICAL FAULT-Width 300 meters, 

depth 0.1 meter, and slip 9 millimeters 

:om 

uo...J 

i=::2=' a:u_::2 

0 UJ 0 z 0 5 10 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200L_ ______ _L ________ J_ ______ ___J ________ _L ________ L_ ______ _j ________ _,_ ______ ___J 

1,600 1,500 1 ,400 1 ,300 1,200 1 ' 1 00 1 ,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 20.-Movement from March 15, 1981, to November 17, 1981, stations F through Q, with station K as a reference 

frame, compared with a vertical fault with width 300 m, depth 0.1 m, and slip 9 mm. 

H33 
H34 MECHANICS OF AQUIFER SYSTEM 

700 I I I I I I I 

WEST EAST 

...J 

w 600 I- w - > 

w a: ...J ::J 

<( (j) 

w (j) 

(j) 

~ N LL 

w Q ~ F > 500 I- ~0 ~ - 0 

CD p <( M (j) 

a: 

w 

fw 

~"~"~ :::2: 400 f- -

z <(zUJ 

(.)UJfui Ul:::!:UJ 

5 ...JUJ::;; 0 <(>- ::J oo...J f- j::::;;:::! 

i= a::u.:::!: ...J 300 f- 0 0 UJ 0 z - <( ERROR ELLIPSE 0 5 10 > -

MEASURED MOVEMENT, NOV. 1981 TO MAR. 1982 HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200 I I I I I I 

1,600 1,500 1,400 1,300 1,200 1,100 1,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 21.-Movement from November 17, 1981, to March 17, 1982, stations F through Q, with station K as a reference frame. 

700.-------·~----------~------·~--------.-l------,l--------.-l------,l,-------, 

...J 

~ 6001-

w 

...J 

<( 

w 

(j) 

w 

~ 500-

CD 

<( 

(j) 

a: 

w 

1--

w 

:::2: 400-

z 

w 

0 

::J 

1--

i= 

...J 300-

<( 

WEST 

0 

UJ 

a: 

:::> 

(/) 

(/) 

u. 

EAST 

-

~~ Q p G) 

F -

ERROR ELLIPSE 

MEASURED MOVEMENT, MAR. 1982 TO JUNE 1983 

:, !U - ~o::J 

f-:::!:-

a::u.:::!: 

o UJoz - 0 5 10 > -

HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200L-______ _L ______ ~IL_ ______ ...JI ________ ~I--------~~--------~~--------L-~------~ 

1 ,600 1 ,500 1 ,400 1 ,300 1 ,200 1 ' 1 00 1 ,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 22.-Movement from March 17, 1982, to June 6, 1983, stations F through Q, with station K as a reference frame. 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H35 

the near downthrown side, no attempt was made to 

model this movement. At the beginning and end of 

this time period, water levels were approximately 

equal (fig. 4) before and after a period when waterlevel recoveries in the deep piezometers were higher 

than normal for the study period. 

The usefulness of dislocation modeling to estimate 

dip and width of faulting of the Picacho earth fissure 

depends on several factors. First, because differential 

compaction is inferred to cause the fault movement 

(Sanford, 1959), removal of differential subsidence to 

look for the pattern of displacements caused by the 

dislocation would require complex stress-strain modeling and could introduce spatial error. Whether it is 

done empirically or by modeling, removal of the pattern of deformation caused by differential subsidence 

over a sloping, irregular bedrock surface depends on 

the extent to which the geometry of the contact, the 

material properties of the alluvium, and the distribution of changes in effective stress are known. Second, 

the sloping bedrock contact (fig. 3) violates the assumption of a homogeneous, isotropic half-space. Dislocation modeling of an obliquely layered medium 

(Sato and Yamashita, 1975) suggests that vertical 

components of displacements on the upthrown side of 

the fault would be shifted upward because Young's 

700 I I I 

WEST 

_J 

w 600 - > 

w 

_J 

<( 

w 

(f) 

w 

~ p > 

0 500 tCD G 

<( 

(f) 

a: 

w 

Iw 

~ 400 1-

z 

ui 

0 

::J 

If= 300 1- _J 0 <( ERROR ELLIPSE 

modulus of the alluvium is lower than that of the 

basement rocks. Third, fault movement might be 

along several interrelated faults whose dips change 

with depth (fig. 6; Sanford, 1959). The superposition 

of patterns of deformation from a complex pattern of 

faults may defy interpretation or may appear to be a 

simpler fault than actually exists. Fourth, having 

only one station between 0 and Q and none west of 

Q limited the ability to estimate the width of the 

fault. 

In spite of these difficulties, dislocation modeling 

gave a good fit for fault movements for the periods 

May 1980 to December 1980 and March 1981 to November 1981. The lack of fit on the upthrown side 

may be caused by the effects of differential subsidence and the violation of homogeneous half-space. 

Lack of fit for stations P and Q is probably caused by 

the effects of differential subsidence. 

The fault model selected (figs. 18 and 20) with a 

depth of 0.1 m, a width of 300 m (fig. 14), and a dip 

of 90° is consistent with other arguments indicating 

that deformation extends from the land surface to a 

depth of about 300 m at the fissure. First, photographic evidence from 1936 (U.S. Soil Conservation 

Service, 1936) and 1927 indicates that the fissure 

extended to the surface as a feature that predates 

I I I I 

EAST 

UJ 

a: 

::J 

(/) 

(/) 

LL 

~ 

0 N MLKJ I H G F 

--e -

~1o~e~ - <~:zUJ 

(.)LJ.JtC/l::i:UJ 

5 ..JUJ::2; <1:>- oo..J 

i=::i:::::! 

a:LL::i: 

0 UJ 0 z - 0 5 10 > -

MEASURED MOVEMENT, JUNE 1983 TO MAY 1984 HORIZONTAL SCALE 

OF MOVEMENT, 

IN MILLIMETERS 

200 I I I I I I I 

1,600 1,500 1,400 1,300 1,200 1 ' 1 00 1,000 900 800 

HORIZONTAL DISTANCE FROM STATION A, IN METERS 

FIGURE 23.-Movement from June 6, 1983, to May 15, 1984, stations F through Q, with station K as a reference frame. 
H36 MECHANICS OF AQUIFER SYSTEM 

major water-level declines in the area. The fissure 

could be the surface expression of a tectonic fault 

that has been tectonically inactive since the gradation of the present alluvial surface. Another possibility is that the original fissure developed in response 

to prehistoric water-level declines. Second, assuming 

that the basal gravel generally has low compressibility, the width of the fault is about equal to the thickness of compressible material (fig. 3). Third, waterlevel fluctuations in the basal gravel, which probably 

reflect changes in effective stress in the deeper part 

of the silt unit, are correlated with fault movement 

and horizontal strain. Fourth, a possible bedrock irregularity directly below the fissure could be an indication of a tectonic fault that exhibits offset in the 

lower part of the alluvium and is consistent with a 

dip of about 90°. 

COMPARISON OF CONTINUOUS MEASUREMENTS 

WITH SURVEY RESULTS 

The continuous measurements of horizontal strain 

across a span of 30 m and the discrete surveying 

measurements over a much larger span supplement 

each other. Ideally, the combined sets of horizontal 

strain data would allow a comparison between the 

field measurements and the displacements from the 

selected dislocation model to determine whether the 

horizontal strains are completely accounted for by 

the model or whether modeling a superimposed tensile crack is warranted. The measurement error of 

±1.5 mm in the combined tape-extensometer and 

EDM measurements and the uncertainties in fit with 

the model preclude such an analysis. A comparison 

between horizontal strains measured by combined 

surveying techniques and by the horizontal extensometer was possible, however, and suggests nonuniformity of strain near the fissure. 

The fissure openings recorded from spring to summer during 1981 and 1982 (fig. 4) corresponded with 

fault offset (figs. 18 and 20). From March 15, 1981, to 

November 17, 1981, displacement of station M, relative to station I, was 3±1.5 mm extension, and the 

extension of the horizontal extensometer was 3.533 

mm. From March 17, 1982, to June 6, 1983, the values were 1±1.5 mm and 0.741 mm, respectively. 

During periods of fissure closing, such as December 

16, 1980, to March 15, 1981, and November 17, 1981, 

to March 17, 1982, the extensometer measurements 

also agreed well with relative displacement between 

stations I and M. Relative movements between I and 

M were 0±1.5 mm for both time periods, and therespective compression values for the extensometer 

were 0.704 mm and 0.760 mm. From June 6, 1983, to 

May 15, 1984, stations I and M exhibited an opening 

of 2±1.5 mm, and the extensometer closed 0.401 mm. 

Little can be inferred about the concentration of 

horizontal strain near the Picacho earth fissure for 

time intervals between surveys because almost all 

relative movements between nearest pairs of stations 

G through 0 were within the measurement error. 

The exceptions were stations L to M for March 17, 

1982, to June 6, 1983, and stations N to 0 for June 

6, 1983, to May 15, 1984. These stations exhibited 

4±1.5 mm relative opening and 4±1.5 mm closing, respectively. Notable strains within the measurement 

error were stations I to J, from May 15 to December 

16, 1980, with 350 microstrain of compression and 

stations J to K, for the same time period, with 330 

microstrain of extension. These strains probably reflect a local movement of station J toward the mountain front. A similar probable local movement of 

station K was reflected in 500 microstrain of extension between J and K and 340 microstrain of compression between K and L from March 15 to 

November 17, 1981. 

Some net horizontal displacements from May 31, 

1980, to May 15, 1984, for stations near the fissure 

exceeded the measurement error and suggested that 

strain is concentrated near the fissure. Values in microstrain between pairs of stations were as follows: 

110 extension between G and H, 40 extension between H and I, 350 compression between I and J, 

1,160 extension between J and K, 1,370 compression 

between K and L, 840 extension between L and M, 

140 extension between M and N, and 520 compression between N and 0. For comparison, strain across 

the larger span from G to 0 was 20 microstrain extension and across the intermediate span from I to M 

was 140 microstrain extension. The largest strains, 

which change sign between adjacent pairs of stations, 

probably reflect local movements of stations J, K, and 

L because of their proximity to the edges of the 

fissure. 

COMPARISON OF RESULTS WITH 

PREVIOUS WORK 

Two of the results of this study are consistent with 

the results of Holzer and others (1979) near the 

study site of Holzer (1978) and Pankratz and others 

(1978). First, movement of the fissure is concluded to 

be caused by changes in effective stress as represented by water-level fluctuations in TA-1-1 and TA-3-1. 

Second, seasonal effects of deformation caused by 

water-level fluctuations, as suggested by Holzer and 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H37 

others (1979) on scant evidence, were well substantiated in this study (figs. 4, 7, 11, and 16). Horizontal 

strain across the fissure was highly correlated with 

water-level fluctuations. Fault movement occurred 

during periods of water-level decline; however, instead of fault reversal during periods of water-level 

recovery, only slowing of subsidence and apparent local movements or adjustments occurred. 

The modeled fault differs between the two studies. 

Holzer and others (1979) obtained a normal fault 

with slip of about 260 mm, width of 190 m, and dip 

of 70° to the west. The modeled fault for this study 

has slip of 9 mm, width of 300 m, and dip of 90°. 

Several factors could have caused the differences in 

modeled faults. First, the study areas are 4.5 km 

apart, and the fault could differ in dip and width 

along its length. As indicated by Sanford (1959), several dips and complexities of faulting might result 

from deformation associated with a bedrock step. 

Second, Holzer and others (1979) and Boling (1987) 

found evidence of a hydrologic barrier to groundwater flow in the alluvium at their study site. The 

hydraulic gradient between TA-1-1 and TA-3-1 is 

very low, indicating a lack of any hydrologic barrier 

·at the TA-l site. Differences in material properties 

and in saturated thickness between the two sites 

could produce different angles of internal friction. 

These could, in turn, contribute to a difference in dip. 

Third, Holzer and others (1979) modeled movement 

spanning 14 years from 1963 to 1977. The total 

movement of about 260 mm during that period could 

represent a significant increase in signal-to-noise ratio so that the dip of 70° is correct, or it could represent a combination of movements along several faults 

of varying dips interspersed with deformations that 

are not fault movements (figs. 16--23). Fourth, Holzer 

and others (1979) modeled only vertical displacements. Holzer and Thatcher (1979) used horizontal 

displacements between a pair of stations on opposite 

sides of the fissure to substantiate their finding of a 

dip of 70° to the west. Measurements of horizontal 

displacements between a single pair of monuments 

on opposite sides of a fault or fissure cannot eliminate the possibility of superposition of a tensile crack 

and dip-slip fault. In this study, it appears that 

measurements of horizontal displacements of several 

stations on each side of the fissure contribute substantially to the ability to determine fault dip (figs. 

15, 18, and 20) and to whether a simple dip-slip fault 

model is sufficient or superposition of a tensile crack 

is warranted. Holzer and others (1979) argued that 

the fissure initially resulted from water-level declines 

caused by pumping. Photographic evidence for existence of the Picacho earth fissure in 1927 and 1936 

(U.S. Soil Conservation Service, 1936) is invoked to 

argue that the fissure predates major pumping and 

water-level declines. 

Holzer and others (1979) estimated that the average rate of slip on the Picacho earth fissure at their 

site was about 20 mm/yr from 1968 to 1978. The average rate of slip at the TA-l site was about 5 mm/yr 

from 1980 to 1984. The seasonal rate of slip at TA-l 

was at least as great as 16 mm/yr from May 1980 to 

December 1980. The lower average value of slip at 

TA-l may reflect differences along the trace of the 

fissure or a slowing of slip in recent years. 

SUMMARY AND CONCLUSIONS 

Vertical and horizontal displacements were monitored along two survey lines near the Picacho Mountains in south-central Arizona to determine 

correlation between water-level fluctuations and 

earth-fissure movement and to use dislocation modeling to characterize fissure development. Near the 

mountain front, movements were generally small, 

and horizontal displacements dominated over vertical 

displacements. With increasing distance into the basin, subsidence increased markedly and horizontal 

displacements decreased. No land-surface recovery, 

relative to stations on bedrock, was observed along 

the TA-l survey line. However, rates of subsidence 

were reduced in winter, and some stations along the 

TA-l survey line on the downthrown side of the fissure showed winter recovery relative to station K on 

the near upthrown side. Station C on the TA-l survey line exhibited the largest net horizontal movement of 29 mm toward the basin. 

Along the Nose survey line, stations between AB 

and AI exhibited movement toward the mountain 

front, which could have been caused by elastic rebound from the release of tensile stress as the fissure 

deepened between stations AH and AI. This fissure 

exhibited 20 mm of net opening and 33 mm of net 

vertical offset. The area between stations AB and AC 

exhibited horizontal compression of 490 microstrain 

from May 31 to December 16, 1980, in response to 

opening of the fissure between stations AH and AI of 

5 mm. The zone between stations AI and AJ exhibited horizontal extension of 750 microstrain from June 

6, 1983, to May 15, 1984, a value thought to be large 

enough to induce fissuring. 

Along the TA-l survey line, from May 31, 1980, to 

May 15, 1984, subsidence was uniform at 148±1.8 

mm from station F through K, while vertical offset of 

about 20 mm accumulated across the fissure between 

stations K and L. Horizontal extension between 
H38 MECHANICS OF AQUIFER SYSTEM 

stations L and M was 840 microstrain, the largest 

value measured during the study. The large value of 

strain could indicate the presence of an incipient or 

newly forming subsurface fissure between those stations or, perhaps, a dependence of strain at failure on 

strain rate. The maximum measured subsidence at 

station Q was 183± 1.8 mm. 

Four modes of deformation occurred at the fissures 

during the study period. Between stations AH and 

AI, opening and vertical offset occurred with predominantly horizontal movement toward the mountain 

front from May to December 1980, with predominantly vertical movement in the area from March to 

November 1981, and along with horizontal movement 

toward the basin from March 1982 to June 1983. Deformation across the Picacho earth fissure was predominantly vertical offset from May to December 

1980 and March to November 1981. 

Conti~uous measurements of horizontal movement 

across the Picacho earth fissure are the first longterm, continuous measurements of earth-fissure 

movement. Horizontal movement across the fissure 

was smooth without sudden opening or closing. The 

fissure generally opened from mid-March to August 

or September, followed by closing until mid-March. 

Horizontal movement of the fissure was highly correlated with water-level fluctuations in deep piezometers TA-1-1 and TA-3-1. 

High correlation between horizontal movement of 

the fissure and water-level fluctuations in the deep 

piezometers and lower correlation with shallow piezometer TA-3-2 supports the hypothesis of differential compaction for fissure movement. High 

correlation between horizontal movement and compaction at Eloy before November 1, 1981, also supports this hypothesis. Generalized differential 

compaction adds dip-slip faulting to horizontal tensile strain as a failure mechanism for fissure development. Vertical offsets measured across the Picacho 

earth fissure and between stations AH and AI support the hypothesis of generalized differential compaction for fissure movement. 

Observation of fissure opening by movement toward the mountain front suggests that the hypotheses of rotation of a rigid plate and seepage stresses 

are not the primary mechanisms of fissure formation 

at the study site. The rigid-plate hypothesis does not 

account for the predominant mode of failure of the 

Picacho earth fissure being dip-slip movement and 

for the opening of a fissure between an existing fissure and the mountain front. The seepage-stress hypothesis is also unsupported at the study site 

because of the low correlation between head gradients and horizontal movement. 

Dislocation modeling of a vertical fault within the 

alluvium provided a close fit to patterns of deformation observed at the Picacho earth fissure for periods 

from spring to winter. Periods from winter to spring 

did not show reversal of fault movement but did 

show local readjustment that was mostly horizontal. 

The modeled fault movement adds support to the hypothesis of generalized differential compaction as a 

mechanism for fissure formation and movement. 

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H40 MECHANICS OF AQUIFER SYSTEM 

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TABLES 3 AND 4 
H42 MECHANICS OF AQUIFER SYSTEM 

TABLE 3.-Altitude of water level in piezometer 1, test hole TA-l, 1980-84 

[Values in meters. Altitude datum is sea level. Location is in SEXSWl4NEl4 sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian. -----, not measured] 

1980 1981 

Day Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 361.36 361.09 363.06 364.62 361.51 361.25 362.02 358.78 357.89 357.97 360.06 

2 360.85 361.50 363.39 364.74 361.44 361.72 361.97 359.02 358.30 358.56 360.10 

3 360.75 361.18 363.53 364.85 361.29 362.16 362.10 359.62 357.54 358.16 358.47 360.16 

4 361.26 361.05 363.58 364.88 361.16 362.28 362.16 359.31 357.50 358.25 358.50 360.22 

5 361.52 361.02 363.63 365.00 361.15 361.84 362.02 359.33 357.79 358.68 358.50 360.38 

6 361.69 361.55 363.69 365.02 361.17 361.56 361.59 359.03 357.54 358.56 358.41 360.40 

7 361.87 361.73 363.62 365.04 361.07 361.34 361.39 358.96 357.92 358.55 358.28 360.44 

8 362.00 361.49 363.12 365.03 361.01 361.32 361.24 359.79 359.05 358.09 358.52 358.21 360.48 

9 362.12 361.36 363.22 365.00 360.98 361.25 361.22 359.65 359.18 358.25 358.50 358.17 360.53 

10 362.26 361.30 363.62 364.76 360.95 361.26 359.47 359.38 357.93 358.55 358.18 360.59 

11 362.42 361.58 363.81 364.38 360.85 361.21 359.34 359.33 357.91 358.50 358.40 360.77 

12 361.84 363.87 364.14 360.73 360.24 359.63 359.08 358.21 358.49 358.99 360.90 

13 362.27 363.88 364.00 360.75 361.08 360.27 358.27 358.44 359.21 360.98 

14 361.51 362.50 364.03 363.85 360.73 360.88 360.39 358.30 358.51 359.32 361.05 

15 361.58 362.69 364.20 360.67 360.66 359.98 358.41 358.08 359.43 361.06 

16 361.57 362.84 364.26 360.62 360.52 359.55 358.39 358.14 359.49 361.16 

17 361.57 362.95 360.56 360.37 359.41 358.44 358.28 358.94 361.23 

18 361.75 363.05 360.48 360.35 359.41 358.60 358.84 358.97 361.24 

19 361.90 363.18 360.34 360.95 359.36 358.69 359.13 358.94 361.29 

20 361.42 363.30 360.29 361.20 359.33 358.83 358.82 359.03 361.30 

21 360.78 363.36 360.36 361.45 359.32 358.83 359.08 359.06 361.41 

22 360.64 363.44 360.48 361.58 359.37 358.96 358.72 359.10 361.36 

23 360.41 361.03 363.55 363.81 362.53 361.21 361.73 359.25 358.90 358.49 359.15 361.37 

24 361.49 363.66 364.01 362.35 361.84 361.69 359.21 358.88 358.45 359.26 361.37 

25 361.91 363.77 364.12 362.22 361.85 361.96 359.08 358.39 358.27 359.39 361.54 

26 362.09 363.81 364.27 362.06 361.77 . 362.15 359.10 358.08 358.22 359.40 361.65 

27 361.66 363.76 364.43 361.88 361.78 362.07 359.04 357.80 358.27 359.68 361.77 

28 361.22 361.43 363.88 364.52 361.76 361.54 361.89 358.95 357.63 358.36 359.49 361.83 

29 361.32 361.28 363.42 361.68 361.38 361.90 358.89 357.51 358.05 359.73 361.77 

30 361.41 361.21 363.18 361.66 361.19 361.77 359.05 357.45 357.87 359.89 361.59 

31 361.15 363.10 361.55 361.72 358.87 357.79 361.06 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H43 

TABLE 3.-Altitude of water level in piezometer 1, test hole TA-1, 1980-84-Continued 

[Values in meters. Altitude datum is sea level. Location is in SEY.SWY.NEY. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian. -----, not measured) 

1982 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 360.76 363.31 364.30 361.76 359.94 358.93 359.31 360.55 

2 360.77 363.36 364.36 361.74 359.95 358.88 359.36 360.65 

3 361.15 363.45 364.39 361.67 359.80 358.85 359.33 360.73 

4 361.27 363.43 364.36 361.57 359.96 358.82 358.89 360.86 

5 361.62 363.07 364.30 361.58 360.10 358.83 358.68 360.96 

6 361.29 363.34 364.26 361.91 360.15 358.73 358.55 360.97 

7 361.18 363.37 364.27 362.15 360.23 359.11 358.47 360.55 

8 361.61 363.54 364.31 362.31 360.21 359.54 358.47 360.37 

9 361.85 363.49 363.98 362.29 360.26 359.49 358.89 360.65 

10 362.05 363.52 363.82 362.27 360.26 359.11 359.20 360.91 

11 362.18 363.53 363.75 362.31 360.26 358.92 359.36 361.02 

12 362.26 363.60 363.68 361.95 360.28 358.81 359.45 361.16 

13 362.17 363.64 363.65 361.65 360.30 358.79 359.12 361.28 

14 362.35 363.68 363.89 361.43 360.41 358.75 358.28 358.96 361.32 

15 362.48 363.67 364.04 360.60 358.97 358.14 358.14 359.00 361.37 

16 362.53 363.68 364.11 360.72 358.73 358.05 358.05 359.48 361.45 

17 362.33 363.78 364.04 360.81 358.57 357.99 357.99 359.72 361.55 

18 362.20 363.90 363.80 360.67 358.48 357.92 357.92 359.86 361.62 

19 362.10 363.96 363.88 360.60 360.21 358.39 357.87 359.96 361.69 

20 362.02 363.96 363.97 360.48 360.27 358.33 357.95 360.05 361.76 

21 362.00 364.02 363.98 360.41 360.26 358.35 357.97 360.14 361.75 

22 362.30 364.09 363.99 361.66 360.34 359.80 358.33 357.99 360.28 361.80 

23 362.49 364.12 364.02 361.55 360.22 359.51 358.58 358.12 360.35 361.93 

24 362.68 364.15 361.51 360.10 359.33 358.77 358.55 360.40 361.95 

25 362.81 364.18 361.44 360.01 359.18 358.44 358.84 360.37 361.97 

26 362.91 364.20 361.38 360.15 359.09 358.28 359.03 360.42 362.02 

27 362.97 364.25 361.33 360.11 358.97 358.73 359.08 359.97 362.06 

28 363.08 364.27 361.34 360.09 358.86 359.09 358.67 359.86 362.11 

29 363.15 361.79 360.15 358.84 359.28 358.98 360.29 362.19 

30 363.15 361.95 360.06 358.96 359.22 359.22 360.52 362.27 

31 363.24 360.00 359.27 361.83 
H44 MECHANICS OF AQUIFER SYSTEM 

TABLE 3.-Altitude of water level in piezometer 1, test hole TA-1, 1980-84-Continued 

[Values in meters. Altitude datum is sea level. Location is in SEY.SWY.NEY. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian.-----, not measured] 

1983 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 361.99 362.52 361.26 360.80 363.40 363.99 

2 362.16 362.43 360.90 360.74 363.08 364.12 

3 362.21 362.27 360.79 360.69 362.81 364.15 

4 361.77 362.40 360.75 360.66 362.63 364.26 

5 361.56 363.77 362.44 360.71 360.66 362.45 364.30 

6 362.00 363.67 362.15 360.69 360.63 362.34 364.36 

7 362.22 363.62 362.00 360.64 360.60 362.25 364.45 

8 362.34 363.55 361.94 360.55 360.57 362.18 364.55 

9 362.45 363.18 361.88 360.58 360.54 362.13 364.63 

10 363.02 361.83 360.79 360.51 362.22 364.65 

11 363.00 361.74 360.88 360.50 362.72 364.71 

12 363.15 361.68 360.91 360.53 362.97 364.80 

13 363.15 361.72 360.88 360.59 363.11 364.84 

14 363.13 361.88 360.87 360.61 362.68 364.92 

15 363.12 361.90 360.88 360.63 362.46 364.99 

16 363.08 361.97 360.95 360.65 362.35 365.01 

17 363.08 361.92 361.02 360.63 362.28 365.10 

18 363.09 362.04 361.04 360.61 362.28 365.13 

19 363.05 362.15 361.04 360.60 363.19 362.21 365.20 

20 363.05 362.39 361.08 360.58 363.18 362.21 365.30 

21 363.03 362.15 361.09 360.60 363.21 362.32 365.32 

22 362.99 361.99 361.02 360.55 363.33 362.88 365.36 

23 363.08 361.91 360.97 360.41 363.41 362.67 365.41 

24 363.15 361.58 360.94 363.53 362.59 365.48 

25 363.50 361.35 360.95 363.60 363.16 365.55 

26 363.19 361.20 360.91 363.68 363.40 365.60 

27 363.06 361.18 360.86 363.74 363.23 365.67 

28 363.01 361.37 360.85 363.78 363.54 365.69 

29 362.74 361.09 360.95 363.83 363.73 365.68 

30 362.58 361.02 360.86 363.90 363.85 365.74 

31 361.22 363.95 365.81 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H45 

TABLE 3.-Altitude of water level in piezometer 1, test hole TA-l, 1980-84-Continued 

[Values in meters. Altitude datum is sea level. Location is in SEY.SWY.NEY. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian.-----, not measured] 

1984 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 365.87 364.74 366.18 364.05 362.79 360.96 359.97 359.15 359.06 358.96 359.15 

2 365.93 364.66 365.73 363.97 362.74 360.91 360.06 359.17 358.99 358.94 359.20 

3 365.95 364.60 366.09 363.91 362.70 360.98 360.21 359.22 359.01 358.91 359.20 

4 366.01 364.57 366.21 363.85 362.73 360.87 359.91 359.24 359.11 358.97 359.13 

5 366.08 364.62 366.34 363.82 362.72 360.67 359.80 359.15 358.97 359.02 359.11 

6 366.11 364.71 366.46 363.81 362.64 360.39 359.75 359.08 358.94 358.98 359.18 

7 366.18 364.70 366.08 364.27 362.56 360.23 359.98 359.06 358.98 358.97 359.18 

8 366.21 364.69 365.68 364.62 362.52 360.10 359.71 359.15 359.02 358.95 359.22 

9 366.25 364.77 365.48 364.77 362.53 359.98 359.89 359.14 359.04 358.92 359.23 

10 366.27 365.36 365.22 364.27 362.51 359.98 359.63 359.18 359.04 358.97 359.23 

11 366.36 365.60 365.06 364.06 362.47 360.23 359.53 359.22 359.05 358.93 359.24 

12 366.36 365.77 364.99 363.91 362.45 360.40 359.21 359.25 359.06 358.90 359.27 

13 366.42 365.92 364.82 363.73 362.40 360.54 359.04 359.21 359.06 358.97 359.30 

14 366.43 366.08 364.67 363.57 362.38 360.51 359.22 359.07 359.04 

15 366.43 366.03 364.62 363.46 362.81 360.40 359.22 359.11 358.99 

16 366.50 365.59 364.56 363.45 362.53 360.34 359.13 359.09 358.96 

17 366.42 365.41 364.54 363.37 362.40 360.26 359.08 359.15 358.92 

18 366.36 365.37 364.53 363.25 362.32 360.12 359.11 359.11 358.99 

19 366.41 365.84 364.49 363.13 362.27 359.92 359.07 359.04 359.04 

20 366.39 365.45 364.45 363.01 362.23 359.72 359.04 359.02 359.08 

21 365.91 365.36 364.41 363.05 362.19 359.77 359.48 359.05 359.01 359.11 

22 365.68 365.83 364.48 363.07 362.11 359.63 359.49 358.95 358.99 359.04 

23 365.61 365.48 364.46 363.16 361.74 360.08 359.42 358.92 358.99 359.03 

24 365.90 365.35 364.46 363.13 361.55 359.77 359.35 358.93 358.97 359.08 

25 365.39 365.40 364.47 363.11 361.41 359.68 359.30 358.93 358.96 359.11 

26 365.37 365.83 364.40 363.05 361.24 359.61 359.26 358.94 358.96 359.07 

27 365.27 366.04 364.29 362.97 361.19 359.55 359.23 359.05 358.95 359.06 

28 364.95 366.13 364.16 362.94 361.31 359.49 359.20 359.15 358.95 359.09 

29 364.87 366.27 364.20 362.83 361.30 359.38 359.17 359.13 358.94 359.08 

30 365.08 364.24 362.81 ~61.27 359.78 359.15 359.09 358.96 359.11 

31 364.84 364.09 361.30 359.21 358.94 
H46 MECHANICS OF AQUIFER SYSTEM 

TABLE 4.-Horizontal movement across Picacho earth fissure near test hole TA-l, 1980-84 

[Values in millimeters are the noon daily measurements of the invar·wire extensometer, indicating change (positive = closing; negative = opening) from initial setting on 

starting date, November 14, 1980. Location is SEY.SWY.NEY. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian.-----, not measured] 

1980 1981 

Day Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 0.032 0.233 0.642 0.764 0.564 -0.176 -0.570 -1.149 -1.631 -2.777 -2.791 -2.670 

2 .037 .236 .656 .774 .543 -.180 -.585 -1.184 -1.647 -2.n8 ·2.782 ·2.650 

3 .042 .241 .662 .788 .528 .. 181 ·.590 ·1.208 ·1.659 ·2.162 ·2.772 ·2.764 ·2.634 

4 .045 .244 .663 .800 .499 .• 178 .. 593 ·1.219 ·1.673 ·2.168 ·2.762 ·2.756 ·2.619 

5 .052 .247 .666 .816 .471 .• 177 .. 599 ·1.223 ·1. 709 ·2.191 ·2.757 ·2.748 ·2.604 

6 .060 .254 .668 .819 .436 .. 187 .. 606 ·1.232 -1.741 ·2.215 ·2.764 ·2.741 ·2.587 

7 .072 .261 .664 .827 .404 -.204 .. 614 ·1.262 ·1.773 ·2.240 ·2.774 ·2.734 ·2.572 

8 .082 .259 .662 .837 .372 -.204 -.628 ·1.296 -1.803 -2.263 -2.789 ·2.728 ·2.557 

9 .099 .264 .668 .840 .345 -.207 -.640 -1.329 -1.832 -2.285 ·2.804 -2.723 ·2.541 

10 .116 .266 .673 .878 .320 -.204 .. 654 ·1.365 ·1.867 -2.294 ·2.816 ·2.717 ·2.526 

11 .133 .264 .687 .880 .293 .• 208 .• 667 -1.395 -1.893 ·2.299 ·2.829 ·2.710 -2.510 

12 .282 .697 .875 .275 .• 237 -.6n ·1.415 ·1.897 -2.305 ·2.837 ·2.700 ·2.492 

13 .312 .707 .873 .254 .. 260 ·.684 ·1.415 ·2.331 ·2.847 ·2.689 ·2.471 

14 0.000 .336 .718 .862 .232 .• 282 .• 693 ·1.409 ·2.358 ·2.847 ·2.681 ·2.451 

15 .006 .730 .207 -.314 -.707 ·1.408 ·2.383 ·2.860 ·2.672 ·2.432 

16 .008 .730 .186 -.344 .• 722 ·1.424 ·2.410 ·2.852 ·2.666 ·2.422 

17 .016 .165 .. 373 .• 735 ·1.439 ·2.439 ·2.845 ·2.671 ·2.408 

18 .026 .142 .• 393 .• 752 ·1.443 ·2.469 ·2.835 ·2.673 ·2.390 

19 .034 .119 ·.415 .• 766 ·1.448 ·2.495 ·2.829 ·2.682 ·2.382 

20 .037 .080 ·.443 .. 784 ·1.455 ·2.517 ·2.820 ·2.691 ·2.373 

21 .022 .070 .• 473 .• 800 ·1.462 ·2.539 ·2.812 ·2.700 ·2.360 

22 .015 .048 .. 469 .• 817 ·1.471 ·2.553 ·2.807 ·2.707 ·2.357 

23 .000 .194 .731 .725 .019 ·.469 .• 836 ·1.502 ·2.575 ·2.800 ·2.715 ·2.348 

24 .200 .736 .703 .• 004 .• 484 -.872 ·1.504 ·2.599 ·2.792 ·2.721 ·2.339 

25 .209 .729 .680 .• 027 .• 488 ·.936 ·1.518 ·2.624 ·2.790 ·2.724 ·2.331 

26 .209 .740 .656 .• 053 .• 488 .. 960 ·1.529 ·2.647 ·2.784 ·2.725 ·2.323 

27 .222 .752 .645 .. 072 .• 503 ·1.000 ·1.546 ·2.671 ·2.777 ·2.717 ·2.315 

28 .016 .225 .758 .643 .• 096 .. 518 ·1.045 ·1.560 ·2.698 ·2.769 ·2.719 ·2.307 

29 .022 .227 .612 .638 ·.124 ·.530 ·1.080 ·1.581 ·2.726 ·2.780 ·2.714 ·2.303 

30 .029 .229 .625 .629 ·.157 ·.552 ·1.115 ·1.598 ·2.755 ·2.791 ·2.691 ·2.316 

31 .230 .634 .595 .• 570 ·1.615 ·2.799 ·2.328 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H47 

TABLE 4.-Horizontal movement across Picacho earth fissure near test hole TA-l, 1980-84-Continued 

[Values in millimeters are the noon daily measurements of the invar-wire extensometer, indicating change (positive = closing; negative = opening) from initial setting on 

starting date, November 14, 1980. Location is SEV.SWV.NEV. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian. -----, not measured] 

1982 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 -2.335 ·2.080 ·1.839 ·2.198 ·2.733 -3.386 ·3.389 -3.426 ·3.496 ·3.175 

2 ·2.335 ·2.068 ·1.832 ·2.203 ·2.744 -3.396 ·3.586 ·3.389 ·3.428 ·3.498 ·3.158 

3 ·2.328 ·2.054 ·1.831 ·2.206 ·2.765 ·3.409 ·3.555 ·3.403 ·3.432 ·3.500 ·3.134 

4 ·2.308 ·2.049 ·1.827 ·2.213 ·2.788 -3.423 ·3.542 -3.420 ·3.437 ·3.498 -3.116 

5 ·2.303 ·2.045 ·1.830 ·2.218 -2.812 ·3.438 ·3.548 ·3.434 ·3.442 ·3.503 ·3 .105 

6 ·2.298 ·2.038 -1.828 ·2.222 ·2.836 ·3.453 -3.544 -3.447 ·3.441 ·3.507 -3.093 

7 -2.292 ·2.029 ·1.827 ·2.226 ·2.856 ·3.459 ·3.540 ·3.449 -3.441 ·3.513 -3.082 

8 ·2.280 ·2.025 ·1.829 -2.228 ·2.879 ·3.463 -3.540 ·3.448 ·3.443 -3.519 ·3.081 

9 -2.270 ·2.013 -1.853 ·2.231 ·2.906 ·3.466 ·3.541 ·3.448 ·3.441 ·3.518 ·3.065 

10 ·2.265 ·2.007 ·1.875 ·2.234 ·2.933 ·3.473 ·3.544 ·3.448 ·3.438 -3.512 ·3.046 

11 ·2.254 ·2.000 -1.900 ·2.239 ·2.962 ·3.486 ·3.546 ·3.444 ·3.441 ·3.513 ·3.010 

12 ·2.250 ·1.988 ·1.922 -2.251 ·2.988 -3.499 ·3.542 ·3.439 ·3.443 ·3.499 ·2.986 

13 -2.243 -1.975 ·1.928 ·2.263 ·3.014 ·3.513 ·3.536 -3.437 ·3.447 ·3.498 ·2.965 

14 ·2.227 ·1.966 ·1.921 -2.280 -3.039 ·3.529 ·3.529 -3.434 ·3.460 ·3.498 -2.951 

15 ·2.223 ·1.959 ·1.918 ·3.049 ·3.545 ·3.525 -3.430 ·3.468 ·3.494 ·2.932 

16 ·2.217 ·1.951 ·1.912 ·3.056 ·3.562 ·3.524 ·3.426 ·3.476 ·3.495 ·2.913 

17 ·2.210 ·1.936 ·1. 911 ·3.064 ·3.579 -3.521 ·3.424 ·3.484 ·3.490 ·2.901 

18 ·2.210 -1.934 ·1.927 -3.083 ·3.594 ·3.523 ·3.415 ·3.491 ·3.470 ·2.890 

19 ·2.217 ·1. 918 ·1.930 ·2.444 ·3.090 ·3.611 ·3.527 ·3.416 ·3.495 ·3.441 ·2.875 

20 ·2.212 -1.906 -1.925 ·2.470 ·3.118 ·3.645 ·3.535 ·3.418 ·3.499 ·3.414 -2.861 

21 ·2.207 ·1.893 ·1.929 ·2.500 ·3.148 ·3.670 ·3.542 ·3.419 ·3.502 ·3.384 ·2.855 

22 -2.191 ·1.886 ·1.929 ·2.123 ·2.533 ·3.173 ·3.677 -3.545 ·3.423 ·3.502 ·3.349 ·2.844 

23 -2.179 ·1.881 ·1.932 ·2.130 -2.565 ·3.198 ·3.690 ·3.549 ·3.413 ·3.505 ·3.327 ·2.823 

24 -2.169 ·1.877 ·2.138 ·2.599 ·3.226 ·3.707 ·3.546 ·3.412 ·3.505 ·3.304 ·2.807 

25 -2.159 -1.867 ·2.144 -2.631 ·3.254 ·3.723 ·3.494 ·3.412 ·3.508 ·3.282 -2.793 

26 ·2.150 ·1.855 ·2.152 ·2.646 ·3.278 ·3.740 -3.474 ·3.411 ·3.507 ·3.267 -2.784 

27 ·2.142 ·1.849 ·2.159 ·2.662 -3.308 ·3.730 ·3.456 ·3.419 ·3.506 -3.249 ·2.775 

28 ·2.125 ·1.843 ·2.167 ·2.674 ·3.339 -3.720 ·3.442 ·3.419 -3.501 ·3.231 -2.768 

29 ·2.117 ·2.181 ·2.688 ·3.362 -3.694 ·3.427 ·3.417 ·3.498 ·3.216 ·2.756 

30 . 2.105 ·2.191 ·2.708 ·3.374 -3.412 ·3.424 ·3.497 ·3.197 ·2.749 

31 -2.089 ·2.721 ·3.399 -3.498 ·2.744 
H48 MECHANICS OF AQUIFER SYSTEM 

TABLE 4.-Horizontal movement across Picacho earth fissure near test hole TA-l, 1980-84-Continued 

[Values in millimeters are the noon daily measurements of the invar-wire extensometer, indicating change (positive = closing; negative = opening) from initial setting on 

starting date, November 14, 1980. Location is SEY.SWY.NEY. sec. 5, T. 8 S., R. 9 E., Gila and Salt River base line and Meridian.-----, not measured] 

1983 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 -2.732 -2.216 -2.578 ·2.797· -3.014 -2.920 -2.202 -1.982 

2 -2.724 -2.234 -2.570 -2.803 -3.035 -2.926 -2.194 -1.973 

3 -2.718 -2.242 -2.594 -2.815 ·3.056 -2.934 -2.185 -1.964 

4 -2.713 -2.248 -2.615 -2.825 -3.080 -2.946 -2.180 -1.955 

5 -2.709 -2.179 -2.256 -2.635 -2.833 ·3.106 ·2.956 ·2.173 -1.945 

6 -2.706 -2.168 -2.275 -2.652 -2.843 -3.132 -2.967 -2.167 -1.933 

7 -2.700 -2.167 -2.294 ·2.668 -2.853 -3.157 -2.976 -2.160 ·1.923 

8 ·2.693 -2.167 ·2.314 ·2.690 -2.858 ·3.179 ·2.989 ·2.160 -1 • 911 

9 -2.687 ·2.183 -2.335 ·2.722 -2.868 ·3. 188 ·3.000 -2.155 -1.906 

10 -2.195 -2.355 ·2.727 ·2.877 -3.183 ·3.011 -2.148 -1.904 

11 -2.207 ·2.376 ·2.724 -2.887 -3.174 ·3.038 ·2.140 -1.893 

12 ·2.203 ·2.394 ·2.726 -2.899 -3.166 -3.068 -2.134 ·1.893 

13 -2.201 ·2.408 ·2.724 ·2.906 -3.161 -3.097 -2.127 ·1.883 

14 -2.195 -2.408 ·2.725 ·2.914 ·3.159 ·3.120 -2.123 ·1.877 

15 ·2.193 -2.405 ·2.728 ·2.921 -3.143 ·3.135 -2.115 -1.871 

16 ·2.191 ·2.404 ·2.732 -2.927 -3.112 -2.109 ·1.867 

17 ·2.187 -2.405 ·2.733 -2.934 ·3.078 -2.102 -1.859 

18 -2.187 -2.405 -2.733 ·2.941 -3.040 ·2.097 ·1.856 

19 -2.185 ·2.408 ·2.733 -2.949 -3.013 -2.391 ·2.092 ·1.847 

20 ·2.188 -2.407 ·2.733 -2.957 ·2.990 ·2.378 ·2.080 ·1.844 

21 -2.185 ·2.406 -2.737 ·2.965 -2.973 ·2.359 -2.076 -1.840 

22 -2.185 ·2.406 ·2.738 ·2.970 ·2.957 -2.342 -2.063 ·1.835 

23 -2.182 ·2.407 ·2.742 ·2.967 -2.947 ·2.324 -2.051 -1.829 

24 -2.179 -2.426 -2.745 -2.960 -2.938 ·2.310 -2.040 ·1.824 

25 ·2.178 ·2.450 ·2.751 -2.950 ·2.930 -2.294 -2.033 -1.818 

26 -2.178 ·2.480 ·2.760 ·2.941 ·2.925 ·2.275 ·2.026 -1.806 

27 -2.169 -2.501 -2.767 -2.934 ·2.923 -2.261 -2.014 -1.800 

28 ·2.175 ·2.507 -2.773 -2.934 -2.919 -2.249 -2.006 -1.799 

29 -2.186 ·2.529 ·2.779 -2.953 -2.916 -2.235 -1.997 ·1. 789 

30 ·2.201 -2.550 -2.788 ·2.975 -2.915 ·2.223 -1.987 ·1. 781 

31 -2.554 ·2.994 -2.916 ·2.212 -1.777 
FLUCTUATIONS IN GROUND-WATER LEVELS NEAR PICACHO MOUNTAINS, ARIZONA H49 

TABLE 4.-Horizontal movement across Picacho earth fissure near test hole TA-l, 1980-84-Continued 

[Values in millimeters are the noon daily measurements of the invar-wire extensometer, indicating change (positive "' closing; negative "' opening) from initial setting on 

starting date, November 14, 1980. Location is SEl4SWl4NEY. sec. 5, T. 8 8., R. 9 E., Gila and Salt River base line and Meridian. -----, not measured] 

1984 

Day Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

1 ·1. 772 -1.719 -1.676 ·1.894 -2.115 ·2.623 -3.486 ·2.952 ·3.241 ·3.041 ·2.938 

2 ·1. 767 ·1.723 ·1.676 ·1.906 ·2.123 -2.654 ·3.491 ·2.964 ·3.237 ·3.039 -2.929 

3 ·1.767 ·1.722 ·1.677 ·1.916 ·2.133 ·2.672 ·3.485 ·2.979 ·3.236 ·3.038 ·2.916 

4 ·1. 763 ·1.723 ·1.678 ·1.928 ·2.141 ·2.696 ·3.478 ·2.997 ·3.219 ·3.036 -2.912 

5 ·1.762 ·1.723 ·1.678 ·1.942 ·2.148 ·2.732 ·3.479 ·3.009 ·3.217 ·3.034 ·2.888 

6 ·1. 753 ·1. 721 ·1.672 ·1.955 ·2.160 ·2.772 ·3.485 ·3.025 ·3.218 ·3.031 ·2.884 

7 ·1. 749 ·1.718 ·1.671 ·1.964 ·2.167 ·2.818 ·3.493 ·3.045 ·3.207 ·3.027 ·2.851 

8 ·1. 743 ·1. 717 ·1.672 ·1.962 ·2.175 ·2.864 ·3.503 ·3.060 ·3.189 ·3.027 ·2.851 

9 ·1. 737 ·1. 712 ·1.698 ·1.969 ·2.182 ·2.910 ·3.513 ·3.075 ·3.182 ·3.026 ·2.845 

10 ·1.735 -1.712 ·1.688 ·1.969 . 2.191 ·2.949 ·3.529 ·3.092 ·3.164 ·3.024 ·2.829 

11 ·1.727 ·1.708 ·1.695 ·1.975 ·2.202 ·2.970 ·3.559 -3.097 ·3.151 ·3.023 ·2.819 

12 ·1. 718 ·1. 705 ·1.703 ·1.983 ·2.214 ·2.986 ·3.593 ·3. 102 ·3.143 ·3.022 -2.805 

13 ·1. 718 ·1.699 ·1.714 ·1.988 ·2.227 ·3.010 ·3.626 ·3.115 ·3.127 ·3.023 ·2.793 

14 ·1.720 -1.697 ·1.723 ·1.995 ·2.240 ·3.023 ·3.130 ·3.114 ·3.022 

15 ·1.714 ·1.695 ·1.735 ·2.003 ·2.251 ·3.037 ·3.147 ·3.105 ·3.022 

16 -1.709 ·1.689 ·1.745 ·2.011 ·2.267 ·3.053 ·3.151 ·3.096 ·3.022 

17 ·1.711 ·1.694 ·1. 752 ·2.017 ·2.279 ·3.073 ·3.160 ·3.090 ·3.018 

18 ·1.708 -1.688 ·1. 762 ·2.026 ·2.296 ·3.095 ·3.172 ·3.085 ·3.017 

19 ·1.703 ·1.687 ·1. 769 ·2.037 ·2.310 ·3.126 ·3.183 ·3.052 -3.016 

20 ·1. 701 -1.684 -1.n8 ·2.058 ·2.322 ·3.162 -3.203 ·3.057 ·3.014 

21 ·1.696 ·1.679 ·1.787 -2.064 ·2.335 ·3.187 ·2.992 ·3.227 ·3.056 ·3.014 

22 ·1. 701 ·1.680 ·1.798 ·2.068 ·2.349 ·3.222 ·2.973 ·3.254 ·3.056 ·3.011 

23 ·1.700 ·1.681 ·1.803 ·2.072 ·2.381 ·3.250 ·2.961 ·3.271 ·3.055 ·3.009 

24 ·1. 701 ·1.677 ·1.808 ·2.078 ·2.417 ·3.285 ·2.951 ·3.279 ·3.052 ·3.004 

25 ·1. 711 ·1.679 ·1.818 ·2.083 ·2.458 ·3.314 ·2.943 -3.287 ·3.053 ·2.994 

26 ·1.712 ·1.679 ·1.823 ·2.088 ·2.492 ·3.344 ·2.938 ·3.295 ·3.051 ·2.979 

27 ·1. 716 ·1.677 ·1.842 ·2.090 ·2.519 ·3.378 ·2.938 ·3.283 ·3.051 ·2.964 

28 ·1.715 ·1.674 ·1.848 ·2.092 ·2.536 ·3.412 ·2.938 ·3.273 ·3.050 ·2.944 

29 ·1. 718 ·1.676 ·1.857 ·2.095 ·2.551 ·3.441 -2.939 ·3.261 ·3.047 ·2.945 

30 ·1. 721 ·1.874 ·2.101 ·2.568 ·3.467 ·2.942 ·3.249 ·3.043 ·2.942 

31 -1.720 ·1.881 ·2.585 ·2.946 -3.042 



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booklet form only. 

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications of the U.S. Geological Survey" should write to the U.S. 

Geological Survey, 582 National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect. Therefore, 

the prices charged may differ from the prices in catalogs, announcements, and publications.