US Department of the Interior 

US Geological Survey

Fact Sheet-051-00

April 2000

San Jose

Sunnyvale

Mountain View

Santa Clara

Palo Alto

San

 Francisco

 Bay

0 5 Miles

N

0.8

0

1.2

Uplift

Subsidence

Change in 

land-surface

altitude 

(inches)

Satellite flight

path

L

D L ≈ 32.81 feet (10 m)

D ≈ 3.28 feet (1 m)

Side-looking, 

slanted antenna

Radar “line-of-sight”

Nadir track

Azimuth

Range

Swath

60 miles

3–4 miles

450 miles

Range distance

to center of 

swath ≈ 528 miles

Footprint

θ

θ = 23°

Not to scale

Simplified geometry for a European Remote Sensing (ERS) Satellite

Wavelengths ≈ 2.2 inches

Frequency ≈ 5.3 billion

cycles per second

Radar waves travel at the 

speed of light

Land subsidence is a gradual settling 

or sudden sinking of the Earth‘s surface 

owing to subsurface movement of 

earth materials. Subsidence in the 

United States has directly affected 

more than 17,000 square miles in 45 

states, and associated annual costs are 

estimated to be approximately $125 

million. The principal causes of subsidence are aquifer-system compaction, 

drainage of organic soils, underground 

mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991).

A powerful new mapping tool (InSAR) 

is a critical element in the assessment 

and mitigation of subsidence. InSAR 

is capable of remotely sensing small 

changes in land surface elevation at an 

unprecedented level of spatial detail. 

The new displacement maps enhance 

our capabilities to monitor and manage 

subsidence caused by the compaction 

of susceptible aquifer systems, and 

reveal new insights into the controlling 

physical processes.

Radar is an active sensor, transmitting a signal of electromagnetic energy. Satelliteborne radar using one antenna transmits a pulsed train of microwaves. The waves 

reflect off the ground surface, and echoes are received by the moving antenna, 

producing a recorded image of the scanned ground that is continuous along the 

track of the satellite and about 60 miles wide.

The restricted size of the satellite antenna limits the spatial resolution to 3 to 4 miles 

on the ground. Synthetic Aperture Radar (SAR) imaging “synthesizes” an effectively 

larger antenna (about 3 miles long) with a spatial resolution on the order of 16 feet by 

pulsing the microwaves every 16 feet of satellite travel.

The 3- to 4-mile-wide footprints overlapped at 16-foot intervals along the ground 

track are processed through a technique similar to medical x-ray imaging. Averaging 

of the signal is done to improve signal coherence, and the actual spatial resolution is 

typically on the order of 260 feet or better.

Interferograms are made by differencing successive SAR images taken from the 

same orbital position but at different times. Under favorable radiometric conditions 

0.4-inch to 0.2-inch resolution is possible in the line-of-sight (range) of the radar.

Measuring Land Subsidence from Space

Interferogram of Santa Clara Valley shows patterns of subsidence and some uplift occurring 

over 7 months in 1997, from January 4 to August 2. A central subsidence zone, (red) the 

result of seasonal ground-water pumping, is 

fully recoverable.

I

nterferometric Synthetic Aperture 

Radar (InSAR) is a powerful new 

tool that uses radar signals to measure deformation of the Earth’s crust at 

an unprecedented level of spatial detail 

and high degree of measurement resolution. InSAR is now being used by the 

USGS and others to map and monitor 

subsidence caused by the compaction 

of aquifer systems. Geophysical applications of radar interferometry take advantage of the phase component of refl ected 

radar signals to measure apparent changes 

in the range distance of the land surface 

(Gabriel and others, 1989; Massonnet and 

Feigl, 1998). Ordinary radar on a typical 

Earth-orbiting satellite has a very poor 

ground resolution of about 3 to 4 miles 

because of the restricted size of the antenna 

on the satellite. Synthetic Aperture Radar 

(SAR) takes advantage of the motion 

of the spacecraft along its orbital track 

to mathematically reconstruct (synthesize) 

an operationally larger antenna and yield 

high-spatial-resolution imaging capability 

on the order of hundreds of feet. The 

size of a picture element (pixel) on a typical SAR image made from satellite-borne 

radar may be as small as tens of thousands 

of square feet, depending how the image 

is processed. 

For landscapes with more or less stable 

radar reflectors (such as buildings or 

other engineered structures, or undisturbed rocks and ground surfaces) over a 

period of time, it is possible to make highprecision measurements of the change in 

the position of the reflectors by subtracting or “interfering” two radar scans made 

of the same area at different times. This is 

the principle behind InSAR. 

Under ideal conditions, it is possible to 

resolve changes in elevation on the order 

of 0.2 to 0.4 inches (5 to 10 mm) at 

Displacement map (interferogram) of Santa Clara 

Valley, California.

Shaded relief map of above 

interferogram 

(greatly exaggerated 

in the vertical dimension)
Different methods of measuring land subsidence

Spirit level vertical 0.1–1 10–100 line-network

Geodimeter horizontal 1 10–100 line-network

Borehole vertical 0.01–0.1 1–3 point extensometer 

Horizontal

extensometer: 

 Tape horizontal 0.3 1–10 line-array

 Invar wire horizontal 0.0001 1 line

 Quartz tube horizontal 0.00001 1 line

GPS vertical 20 10–100 network horizontal 5

InSAR range 5–10 100,000– map pixel3

10,000,000 

METHOD Component Resolution1 Spatial density2 Spatial scale

displacement (millimeters) (samples/survey) (elements)

1

Measurement resolution attainable under optimum conditions. Values are given in metric units to 

conform with standard geodetic guidelines. (One inch is equal to 25.4 millimeters and 1 foot is equal

to 304.8 millimeters.)

2

Number of measurements generally attainable under good conditions to define the spatial extent of 

land subsidence at the scale of the survey.

3

A pixel on an InSAR displacement map is typically 40 to 80 meters square on the ground. 

10

20

20

20

30

30

30

40

0 10 Miles

0 10 Kilometers

Subsidence (millimeters)

(1 inch =25.4 millimeters)

Lancaster

Contours (millimeters) 

showing computer-simulated land subsidence 

for the same period as 

the interferogram.

Beige-colored regions depict areas 

where there was poor correlation between the two radar scans used to 

form the interferogram; black regions 

are areas of small uplift.

Borehole

extensometer

N

0 20 30 40 50 mm 10

Map

area

CALIFORNIA

Antelope

Valley

the scale of one pixel. Interferograms, 

formed from patterns of interference 

between the phase components of two 

radar scans made from nearly the same 

antenna position (viewing angle) but 

at different times, have demonstrated 

dramatic potential for high-density spatial mapping of ground-surface displacements associated with tectonic (Massonnet and others, 1993; Zebker and others, 

1994) and volcanic strains (Massonnet 

and others, 1995; Rosen and others, 

1996; Wicks and others, 1998). InSAR 

has also recently been used to map 

localized crustal deformation and land 

subsidence associated with geothermal 

fields in Imperial Valley, California 

(Massonnet and others, 1997), Long 

Valley, California (W. Thatcher, USGS, 

written communication, 1997), and Iceland (Vadon and Sigmundsson, 1997), 

and with oil and gas fields in the 

Central Valley, California (Fielding and 

others, 1998). InSAR has also been used 

to map regional-scale land subsidence 

caused by aquifer-system compaction in 

the Antelope Valley, California (Galloway and others, 1998), Las Vegas Valley, 

Nevada (Amelung and others, 1999), 

and Santa Clara Valley, California (Ikehara and others, 1998).

InSAR maps subsidence at high spatial detail and reveals new insights

In the Antelope Valley, Mojave Desert, 

California, an interferogram for the period 

October 20, 1993 to December 22, 1995 

Simulated and InSAR-detected land subsidence 

in the Antelope Valley are compared for the 

time period October 1993–December 1995.

revealed up to 2 inches of additional 

subsidence in areas previously affected 

by as much as 6 feet of subsidence 

between 1930-92 (Galloway and others, 

1998). The regions of maximum subsidence detected during the 26-month 

period correlated well with declining 

ground-water levels. In another part of 

Antelope Valley formerly affected by 

ground-water depletion and subsidence, 

about 1 inch of subsidence was detected 

on the interferogram despite the fact that 

ground-water levels had been recovering since 1990. This suggests residual 

(time-delayed) compaction due to the 

presence of thick aquitards. An independent computer simulation of aquifer-system compaction for the same 

period as the interferogram showed that 

though the general patterns of subsidence detected by the interferogram 

could be explained by the simulation, 

the detailed spatial variations in the 

interferogram could not. These results 

highlight the potential use of spatially 

detailed InSAR measurements to pro-
This InSAR-derived surface-deformation map 

shows subsidence in the Las Vegas Valley 

between April 1992 and December 1997. 

It was obtained by summing three timesequential interferograms. The subsidence is 

caused by aquifer-system compaction and 

controlled in part by the surface faults, which 

have also been the focal point of earth-fissure 

formation (Amelung and others, 1999). 

0 5 Miles

0 5 Kilometers

Las Vegas

L

a

s

V

e

g

a

s

V

a

l l e

y

The northeast subsidence bowl

is bounded on the southeast by 

the Eglington fault. 

This central subsidence zone

follows the general trend of

several surface faults.

Eglington fault

Faults

15

15

95

95

N

NEVADA

Map area

One color cycle represents

about 4 inches of subsidence.

Increasing

relative subsidence

San Jose and surrounding communities 

sprawl across Santa Clara Valley (“Silicon 

Valley”) where a maximum detected 

subsidence of 14 feet occurred in downtown 

San Jose between 1910 and 1995. Since the 

late 1980s subsidence has been arrested due 

largely to the importation of surface water. 

Urban settings are good candidates for radar 

interferometry because the structures make 

good radar reflectors.

vide better constraints for computer simulations of land subsidence.

Interferograms for Las Vegas Valley demonstrate the intimate connection between 

ground-water withdrawal, faults and subsidence. An interferogram for the period 

April 21, 1992 to December 5, 1997 

delineates two main features—a subsidence bowl in the northwest and an elongated subsiding zone in the central part 

of the valley. The northwest subsidence 

bowl includes the area of maximum subsidence of nearly 7.5 inches. Its southeastern boundary is aligned with the 

Eglington fault, one of several Quaternary faults cutting the valley-floor alluvium. Little subsidence is detected immediately southeast of the fault. Similarly, 

the shape of the central subsidence zone 

follows the general trends of several 

mapped faults. The map suggests that the 

spatial distribution of land subsidence in 

Las Vegas Valley is controlled by Quaternary faults, to a much greater degree than 

previously suspected. The faults may juxtapose sediments of differing time-consolidation properties, or they may act as 

barriers to ground-water flow, impeding 

the horizontal propagation of fluid-pressure changes and creating ground-waterlevel differences across the faults. 

In Santa Clara Valley, California, subsidence has been arrested by importation 

of surface water and careful management of the aquifer system. However, 

the potential for renewed subsidence is a 

concern for the Santa Clara Valley Water 

District. One of the District’s objectives 

is to limit ground-water extractions 

that would cause inelastic (irreversible) 

compaction of the valley’s aquifer system. Seasonal and longer-term elevation 

changes were measured from successive 

satellite radar passes during 1992–1997. 

The longer-term (~5 year) interferogram 

indicates no change for most of the 

southwestern Santa Clara Valley and 

land-surface uplift of up to about 1 inch 

in the northern and eastern parts of the 

valley. This uplift is correlated to the 

recovery of ground-water levels that has 

been occurring for several years as a 

result of reduced pumpage and increased 

recharge. In contrast, the seasonal (8 

month) interferogram reveals a large 

region in San Jose undergoing elastic 

deformation caused by seasonal groundwater-level fluctuations. The eastern 
InSAR imagery reveals seasonal and long-term land-surfaceelevation changes influenced by ground-water levels and 

fault alignment in the Santa Clara Valley.

CALIFORNIA

Map

area

Guadalupe R.

Coyo te Cr.

0 5 Miles

0 5 Kilometers

San Francisco Bay

Between 1992 and 1997 some uplift occurred 

due to recovering ground-water levels.

Between January and August 1997 seasonal, recoverable subsidence occurred due to seasonal ground-water-level declines. 

Subsidence patterns during this period are similar 

to historical subsidence patterns (1934–1967).

San Jose

Alviso

Sunnyvale

Mountain View

Santa Clara

Cupertino

Palo Alto

2

4

6

8

1

Gu adalupe R.

Coyote Cr.

San Francisco Bay

-1.2

Uplift

Range (inches)

Subsidence

0

-0.6

0.6

Uplift

Range (inches)

Subsidence

0

-0.6

0.6

1.2

Hayward

fault

Silver Creek

fault

San Andreas 

fault

Subsidence contours,

1934–1967 (feet)

A A' B B'

B

A

A'

B'

(Ikehara and others, 1998)

0 4 8 12

0

0.8

-0.8

1.6

0 4 8 12 16

Uplift

Silver Creek fault

Subsidence

San Jose

Alviso

Sunnyvale

Mountain View

Santa Clara

Palo Alto

Cupertino

Distance (miles)

Ground

displacement

(inches)

N

REFERENCES

Amelung, F., Galloway, D.L., Bell, J.W., Zebker, H.A., 

and Laczniak, R.J., 1999, Sensing the ups and downs 

of Las Vegas—InSAR reveals structural control of land 

subsidence and aquifer-system deformation: Geology, 

v. 27, p. 483-486.

Fielding, E.J., Blom, R.G., and Goldstein, R.M., 1998, 

Rapid subsidence over oil fi elds measured by SAR 

interferometry: Geophysical Research Letters, v. 27, p. 

3,215–3,218.

Gabriel, A.K., Goldstein, R.M., and Zebker, H.A., 

1989, Mapping small elevation changes over large 

areas—Differential radar interferometry: Journal of 

Geophysical Research, v. 94, p. 9,183–9,191.

Galloway, D.L., Hudnut, K.W., Ingebritsen, S.E., 

Phillips, S.P., Peltzer, G., Rogez, F., and Rosen, 

P.A., 1998, Detection of aquifer system compaction 

and land subsidence using interferometric synthetic 

aperture radar, Antelope Valley, Mojave Desert, 

California: Water Resources Research, v. 34, p. 

2,573–2,585.

Ikehara, M.E., Galloway, D.L., Fielding, E., Bürgmann, 

R., Lewis, A.S., and Ahmadi, B., 1998, InSAR 

imagery reveals seasonal and longer-term land-surface 

elevation changes infl uenced by ground-water levels 

and fault alignment in Santa Clara Valley, California 

[abs.]: EOS (supplement) Transactions, American 

Geophysical Union, no. 45, November 10, 1998, p. F37.

Massonnet, D., Briole, P., and Arnaud, A., 1995, 

Defl ation of Mount Etna monitored by spaceborne 

radar interferometry: Nature, v. 375, p. 567–570.

Massonnet, D., and Feigl, K.L., 1998, Radar 

interferometry and its application to changes in the 

earth's surface: Reviews of Geophysics, v. 36, p. 

441–500.

Massonnet, D., Holzer, T., and Vadon, H., 1997, Land 

subsidence caused by the East Mesa geothermal 

fi eld, California, observed using SAR interferometry: 

Geophysical Research Letters, v. 24, p. 901–904.

Massonnet, D., Rossi, M., Carmona, C., Adragna, F., 

Peltzer, G., Feigl, K., and Rabaute, T., 1993, The 

displacement fi eld of the Landers earthquake mapped 

by radar interferometry: Nature, v. 364, p. 138–142.

National Research Council, 1991, Mitigating losses 

from land subsidence in the United States: 

Washington, D. C., National Academy Press, 58 p.

Rosen, P.A., Hensley, S., Zebker, H.A., Webb, F.H., and 

Fielding, E., 1996, Surface deformation and coherence 

measurements of Kilauea volcano, Hawaii, from SIR-C 

radar interferometry: Journal of Geophysical Research, 

v. 101, p. 23,109–23,125.

Vadon, H., and Sigmundsson, F., 1997, 1992–1995 crustal 

deformation at Mid-Atlantic ridge, SW Iceland, mapped 

by radar interferometry: Science, v. 275, p. 193–197.

Wicks, C. Jr., Thatcher, W., and Dzurisin, D., 1998, 

Migration of fl uids beneath Yellowstone Caldera 

inferred from satellite radar interferometry: Science, v. 

282, p. 458-462.

Zebker, H.A., Rosen, P.A., Goldstein, M., Gabriel, 

A., and Werner, C.L., 1994, On the derivation of 

coseismic displacement fi elds, using differential radar 

interferometry—The Landers earthquake: Journal of 

Geophysical Research, v. 99, p. 19,617–19,634.

For additional information contact:

Devin Galloway (email: dlgallow@usgs.gov)

U.S. Geological Survey

Placer Hall/California State University, Sacramento

6000 J Street

Sacramento, CA 95819-6129

Profiles of ground displacements sampled from cross sections A-A’ and B-B’

on the seasonal interferogram reveal steep displacement gradients near the 

trace of an inferred northwesterly extension of the buried Silver Creek fault 

zone. Recent geophysical surveys (seismic reflection/refraction), undertaken as a 

result of the interferogram, confirm the presence of buried faults at this boundary 

(R. Catchings, USGS, written communication, 2000). 

—D.L. Galloway, D.R. Jones, 

and S.E. Ingebritsen

extent of this deformation appears to be 

aligned with a Quaternary fault, the Silver Creek fault, several miles west of 

and roughly parallel to the tectonically 

active Hayward Fault. The InSARderived displacements are consistent with 

historical patterns of subsidence and vertical displacements measured in deepseated borehole extensometers in Sunnyvale and San Jose. 

(Radar data used to produce the interferograms shown in 

this Fact Sheet were obtained from the European Space 

Agency, distributed through Eurimage Corporation and Radarsat International for purposes of research and development.)