U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3025

July 2005

Monitoring Ground Deformation from Space

Printed on recycled paper

Figure 1. Diagram showing how radar interferometry detects uplift of the Earth’s surface. 

Both the distance between the satellite positions and the amount of uplift (red layer) are 

exaggerated for clarity. Modified from Zhong 

Lu, USGS, written commun., 2004.

Introduction

Scientists at the U.S. Geological 

Survey (USGS) are using an exciting new 

tool to detect, measure, and monitor subtle 

changes in the shape or relative position of 

the Earth’s surface. By bouncing signals 

from a radar satellite off the ground in 

successive orbits and looking at the differences between the images, interferometric 

synthetic aperture radar (InSAR) can 

detect small differences in the distance 

between its position and the ground as the 

land surface moves—whether up, down, 

or sideways. InSAR shows spatial patterns of deformation in remarkable detail 

and, in combination with ground-based 

monitoring, gives USGS scientists unprecedented insight into a wide range of earth 

science processes.

How InSAR Works

Detecting changes in the position of 

the Earth’s surface requires two radar 

images of a selected area taken from 

approximately the same position in space 

but at two different times. In figure 1, the 

radar signal is shown as a wave, and different segments are marked with different 

colors to emphasize the shift in position 

of the reflected wave. Uplift of the ground 

surface has occurred between the first 

and second pass of the satellite, and so 

the total length of the return signal for the 

second pass is very slightly shorter than 

for the first pass.

Interferometry is based on processing 

the pair of images to map out the differences in the reflected signals over the 

area. Because the area covered by the 

two radar images is typically a square 

50 or 100 kilometers (km) on a side, this 

method allows scientists to look at deformation over large areas.

Reading an Interferogram

The amount and pattern of deformation in an interferogram are shown by 

using the range of colors in the spectrum 

from red to violet. Figure 2 shows an 

interferogram documenting subsidence 

in the Santa Clara Valley of California 

(upper image) and a shaded-relief map 

(lower image) correlating the color bands 

with the deformation pattern.

This and many other interferograms 

use the convention where moving from 

red to violet in the pattern indicates that 

the ground at the center has moved closer 

to the satellite (uplift). In the Santa Clara 

example, the color bands change in the 

reverse order, indicating that the center 

has subsided. This shift corresponds to 

3 centimeters (cm) of subsidence over a 

distance of roughly 8 km. Interferometry 

thus detects very small changes in the elevation of the Earth’s surface and resolves 

the exact shape of those changes.

The subsidence seen in figure 2 is a 

consequence of seasonal ground-water 

pumping that occurred from January to 

August 1997. This kind of surface-displacement map enhances the ability to 

monitor and manage subsidence caused 

by the compaction of aquifer systems (see 

Galloway and others, 2000).

Deformation at Volcanoes

Decades of experience in monitoring active volcanoes, such as Kilauea in 

Hawaii, have led scientists to realize that 

the movement of molten rock (magma) 

underground can be tracked by monitoring deformation of the surface of the 

volcano. An unexpected result of doing 

systematic InSAR analysis for volcanoes 

in Alaska and in the Western United 

States is the discovery that some longdormant volcanoes are actively deforming 

and may erupt eventually, although they 

currently exhibit little activity.

Figure 3 shows an InSAR-defined area 

of uplift near the cluster of volcanoes 

known as the Three Sisters, in central 

Oregon. This deformation, which does 

not lie directly beneath any volcano, is in 

an area where the most recent eruption 

occurred 1,500 years ago. Uplift of the 

ground’s surface, which began in 1997, 

reached 15 cm at the center of the bull’s 

eye pattern in 2001. Subsequent Global 

Positioning System (GPS) monitoring 

shows that uplift continues at a steady 

pace, suggesting that the uplift is produced by upward movement of magma 

(intrusion). The InSAR pattern places the 

depth of intrusion at 6–7 km (Wicks and 

others, 2002).

Because of the observed uplift, the 

USGS installed seismometers, GPS stations, and gas-monitoring equipment in 

the area to check for other signs of volcanic unrest. At first, the new seismometers 

detected no earthquakes in the area of 

uplift. In March 2004, however, a swarm 

Figure 2. Interferogram (upper image) of the Santa 

Clara Valley, Calif., showing patterns of subsidence (with local uplift) that occurred from January 

to August 1997. The shaded-relief map (lower 

image) translates the color-zonation pattern of the 

interferogram into three-dimensional topography. 

Vertical dimension is greatly exaggerated. From 

Galloway and others, 2000.

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Figure 3. Interferogram showing the area of uplift (1997–2001) at the Three Sisters volcanoes (red 

triangles), in the Cascade Range in central Oregon, based on European Remote Sensing Satellite 

imagery. In March 2004, a swarm of more 300 small earthquakes (yellow circles) occurred near 

this center of uplift. White circles, Global Positioning System stations. From Charles Wicks, USGS, 

written commun., 2004.

of 300 small earthquakes occurred, having epicenters just to the northeast of the 

center of the uplift shown in figure 3. 

This example shows how using InSAR 

to prospect for deformation at otherwise 

unmonitored volcanoes helps USGS scientists design more efficient and effective 

monitoring networks for volcanoes in the 

early stages of unrest.

Other U.S. volcanoes where there 

is InSAR-documented deformation 

include several volcanoes in the Aleutians (Okmok, Akutan, Westdahl, Fisher, 

Kiska, Makushin, Seguam, and Aniakchak), plus Peulik Volcano on the Alaskan 

Peninsula, Long Valley caldera in California, and Yellowstone caldera in Wyoming. 

InSAR results for Yellowstone were given 

by Brantley and others (2004). The most 

complete set of data exists for Westdahl, 

where a series of interferograms records 

a decade of continuous deformation, with 

deflation during the 1991–92 eruption, 

followed by steady reinflation (Lu, Masterlark, and others, 2003). Ideally, volcanologists would like to use this technique to 

look at all volcanoes worldwide to see if 

they are deforming.

InSAR’s greatest strengths are that (1) 

it can be applied to extremely remote and 

otherwise unmonitored volcanoes and (2) 

it gives scientists an areal image of the 

deformation field, not just deformation 

at a series of points on a map. The main 

limitation, aside from the limited stream 

of imagery (currently only two satellites 

are operating), is the difficulty of seeing 

through vegetation to the ground beneath. 

This problem is especially acute for the 

many active volcanoes in the tropics.

A Different Wrinkle—Deformation 

Associated with Earthquakes

InSAR-detected deformation at volcanoes is considered to be predominantly 

vertical, as it involves either (1) swelling 

as magma intrudes or a hydrothermal system pressurizes or (2) subsidence caused 

by magma withdrawal during an eruption, 

depressurization, or thermal decay of a 

hydrothermal system. For many earthquakes, however, most of the deformation 

observed is horizontal and happens along 

the fault on which the earthquake occurs. 

Because InSAR can detect movement 

only toward or away from the satellite, 

other sources of information are needed 

to characterize the offset if both horizontal and vertical movements occur.

Figure 4 shows deformation fields 

associated with two large earthquakes 

that occurred in Alaska in October and 

November 2002. In each case, the interferogram is based on a pair of images—

one taken before and one soon after each 

event. In each case, there are large paired 

lobes showing deformation across the 

fault. The symmetry of the paired lobes 

suggests that the motion was predominantly horizontal, a hypothesis consistent 

with ground observations.

The most important new information 

conveyed by InSAR is that deformation occurred over a much larger area 

than the swath of land along the faults. 

The epicenter for the Nenana Mountain 

earthquake (fig. 4A) lies at the west end 

of its strain centers, suggesting that strain 

was propagating to the east. This strain 

propagation helped trigger the much 

larger Denali earthquake (fig. 4B), farther 

east along the same fault (Lu, Wright, 

and Wicks, 2003). The strain lobes for 

the Denali earthquake lie almost entirely 

to the east of its epicenter, extending far 

out of the range of the interferogram. This 

pattern is consistent with the direction of 

propagation of ground breaking (red lines 

in fig. 4B) during the earthquake.
Figure 4. Interferograms showing deformation fields associated with the Nenana Mountain and 

Denali earthquakes (epicenters at small and large stars, respectively). A, Magnitude 6.7 Nenana 

Mountain earthquake (October 23, 2002) and paired deformation lobes north and south of the 

fault. Based on Radarsat images taken on August 16, 2002, and October 27, 2002. B, Magnitude 

7.9 Denali earthquake (November 3, 2002) and extensive deformation. Note that each color band 

(fringe) has twice the value assigned in figure 4A. Based on Radarsat images taken on October 29, 

2002, and November 22, 2002. Yellow lines, faults; red lines, ground breaks. From Lu, Wright, and 

Wicks, 2003.

Much of the off-fault deformation 

probably occurred after the earthquakes, 

as the Earth’s surface caught up with 

deformation that had begun at depth and 

propagated very rapidly along the fault 

surfaces involved. If scientists are able to 

demonstrate, from sufficiently frequent 

InSAR analysis, that some of this strain 

actually occurs before an earthquake, then 

this new technique may eventually lead 

to a means of forecasting earthquakes for 

some fault systems.

InSAR and GPS

The Los Angeles, Calif., area is an 

important target for USGS research 

because this heavily populated and highly 

developed metropolis is very active 

tectonically. The geology of the Los 

Angeles area is complicated, with many 

active faults and other structures. For 

these reasons, the USGS and its partners 

(including the National Aeronautics and 

Space Administration (NASA) and the 

National Science Foundation (NSF)) 

have installed an extensive network of 

GPS stations called SCIGN (Southern 

California Integrated GPS Network) 

to help monitor and understand ongoing tectonic activity in the area (see the 

USGS Earthquake Hazards Program Web 

site http://earthquake.usgs.gov/products/

factsheets.html#west).

InSAR data for the Los Angeles basin 

cover at least part of the area monitored 

by the SCIGN array (fig. 5). There are 

few other places on Earth where both 

InSAR and time-continuous GPS deformation data overlap, allowing a direct 

comparison of the data. The comparisons 

possible for the Los Angeles basin demonstrate that the two methods produce 

quantitatively similar results. The data 

also complement each other; continuous 

GPS monitoring provides continuity in 

time, whereas InSAR shows the pattern of 

spatial deformation for the area.

What has been learned from monitoring deformation in the Los Angeles area? 

First, SCIGN and InSAR data reveal 

seasonal fluctuations in ground-water levels caused by ground-water pumping, as 

well as deformation caused by pumping 

of oil and gas (Bawden and others, 2001). 

To decipher ongoing tectonic activity, 

scientists must be able to remove the 

signal of these more surficial processes 

from the overall deformation signals. A 

further benefit, especially of InSAR, is 

that monitoring allows scientists to see 

previously unrecognized faults or other 

potentially seismogenic subsurface structures and to better define the subsurface 

position of known structures. A more 

detailed analysis of the information in 

figure 5 was given by Bawden and others 

(2003).

What Kind of Synthetic Aperture 

Radar (SAR) Is Best?

The interferograms shown in figures 

2–4 of this Fact Sheet were generated 

from pairs of images from the European Remote Sensing Satellites (ERS; 

launched by the European Space Agency) 

or from Radarsat (launched by the 

Canadian Space Agency). The sensor on 

these satellites is C-band radar, which has 

a wavelength of 5.66 cm. Unfortunately 

this relatively short wavelength is not 

ideal for looking at many kinds of natural 

surfaces and is hindered by vegetation.

L-band SAR, which has a significantly 

longer wavelength of 23.53 cm, is better 

suited for interferometry, especially in 

vegetated areas; however, there is no 

L-band satellite operating at present. The 

only significant L-band archive is the 

imagery obtained by the Japanese Earth 

Resources Satellite JERS–1, which was 

operated by Japan from 1992 to 1998. 

USGS scientists, drawing on this archival 

imagery, as well as C-band ERS imagery, 

created two interferograms for Akutan 

Volcano in the Aleutians (figs. 6 and 7). 

Figure 5. Interferogram showing deformation in the Los Angeles basin, from April 1998 to May 1999, 

draped over shaded-relief map. Based on European Remote Sensing Satellite imagery. Triangles, 

Global Positioning System stations. Modified from Bawden and others, 2001.

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This pair of interferograms, covering 

very nearly the same time period, allows 

scientists to compare InSAR-determined 

deformation fields for C- and L-band 

radar over the same target volcano (Lu 

and others, 2005).

The interferogram based on C-band 

radar (fig. 6) shows only limited details 

of the deformation of Akutan Island. 

Although vegetation can hinder the 

effectiveness of C-band radar, this is not 

the case for much of the Akutan image, as 

the vegetation on the island is sparse (fig. 

8) or tundra. Much of the limited coherence results from the nature of the surface 

material, which in many areas consists 

primarily of unconsolidated volcanic 

deposits. This material is unstable on the 

scale of a few centimeters over a period 

of years. Only young lava flows and other 

rock surfaces were stable enough to show 

coherent deformation (rainbow areas in 

fig. 6) at the shorter C-band wavelength 

over 3 years’ time.

By contrast, at the longer L-band 

wavelength, almost the entire surface 

of the island appears stable enough for 

the deformation field to be seen (fig. 7). 

The complexity of the deformation that 

occurred during the 1996 seismic event is 

clearly defined. There has been subsidence at the summit and to the northwest 

of the volcano, consistent with the formation of ground cracks and grabens like 

the one shown in figure 8, with broader 

inflation of the flanks on either side. To 

the east of the volcano is an area of subsidence, which is clearly shown only in 

the L-band interferogram (fig. 7). USGS 

scientists have inferred that the inflation was caused by high-level intrusion 

of a dike, while the subsidence reflects 

contraction of a deeper source (Lu and 

others, 2005).

Data Acquisition—A Cooperative 

Interagency Effort

SAR satellites are few in number, 

and the requirements for collection of 

repeat images that can be used to generate interferograms are stringent. As a 

result, useful image pairs, even in large 

archives of imagery, are not common. 

The successful applications of InSAR 

shown here reflect seamless cooperation among several agencies. Some of 

the ERS data were acquired through the 

WInSAR consortium, a joint project that 

includes scientists in both government 

and academia and is funded by the NSF, 

NASA, and USGS. Other ERS data, plus 

the JERS and Radarsat imagery, were 

acquired with NASA funding through the 

Alaska Satellite Facility, under cooperative agreements between NASA and the 

European, Canadian, and Japanese satellite agencies. Further development of this 

promising technique depends on expanding and strengthening interagency and 

international coordination in the acquisition of SAR data, including the launching 

of additional L-band or multiband SAR 

satellites.

References Cited

Bawden, G.W., Sneed, Michelle, Stork, S.V., 

and Galloway, D.L., 2003, Measuring 

human-induced land subsidence from 

space: U.S. Geological Survey Fact Sheet 

069–03, 4 p. (Available on the Web at 

http://water.usgs.gov/pubs/fs/fs06903/.)

Bawden, G.W., Thatcher, Wayne, Stein, R.S., 

Hudnut, K.W., and Peltzer, Gilles, 2001, 

Tectonic contraction across Los Angeles 

after removal of groundwater pumping effects: Nature, v. 412, no. 6849, p. 

812–815.

Brantley, S.R., Lowenstern, J.B., Christiansen, R.L., Smith, R.B., Heasler, Henry, 

Waite, Greg, and Wicks, Charles, 2004, 

Tracking changes in Yellowstone’s restless 

volcanic system: U.S. Geological Survey 

Fact Sheet 100–03, 4 p. (Available on the 

Web at http://pubs.usgs.gov/fs/fs100-03/.)

Galloway, D.L., Jones, D.R., and Ingebritsen, S.E., 2000, Measuring land subsidence from space: U.S. Geological Survey 

Fact Sheet 51–00, 4 p. (Available on the 

Web at http://water.usgs.gov/pubs/FS/fs051-00/.)

Lu, Zhong, Masterlark, Timothy, Dzurisin, 

Daniel, Rykhus, Russell, and Wicks, 

Charles, Jr., 2003, Magma supply dynamics at Westdahl volcano, Alaska, modeled from satellite radar interferometry: 

Journal of Geophysical Research, v. 108, 

no. 7, p. 9-1 to 9-17.

Lu, Zhong, Wicks, Charles, Jr., Kwoun, 

Ohig, Power, J.A., and Dzurisin, Daniel, 

2005, Surface deformation associated 

with the March 1996 earthquake swarm 

at Akutan Island, Alaska, revealed by Cband ERS and L-band JERS radar interferometry: Canadian Journal of Remote 

Sensing, v. 31, no. 1, p. 7–20.

Lu, Zhong, Wright, Tim, and Wicks, Charles, 

Jr., 2003, Deformation of the 2002 Denali 

fault earthquakes, mapped by RadarSat–1 

interferometry: Eos, v. 84, no. 41, p. 425, 

430–431.

Wicks, C.W., Jr., Dzurisin, Daniel, Ingebritsen, Steven, Thatcher, Wayne, Lu, Zhong, 

and Iverson, Justin, 2002, Magmatic 

activity beneath the quiescent Three Sisters volcanic center, central Oregon Cascade Range, USA: Geophysical Research 

Letters, v. 29, no. 7, p. 26-1 to 26-4.

By Rosalind L. Helz

For Additional Information

Rosalind L. Helz

U.S. Geological Survey

926A National Center

12201 Sunrise Valley Drive

Reston, VA 20192

Telephone: 703–648–6086

E-mail: rhelz@usgs.gov

Figure 6. Interferogram showing deformation 

of Akutan Island (August 1993 to October 1996), 

draped over shaded-relief map of the island. 

Based on European Remote Sensing Satellite 

imagery (C-band radar). From Lu and others, 

2005.

Figure 7. Interferogram showing deformation 

of Akutan Island (October 1994 to June 1997), 

draped over shaded-relief map of the island. 

Based on Japanese Earth Resources Satellite 

imagery (L-band radar). From Lu and others, 

2005.

Figure 8. Photograph showing a small graben 

west of the summit of Akutan Volcano. Note 

unconsolidated surficial material and sparse 

vegetation. Photograph by John Power, USGS, 

August 2001.

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