Geoscience Journal Vol. 2, No. 2, p. 78--87, June 1998

Feasibility of applying space-borne SAR interferometry for earthquake tectonic investigation

Wooil M. Moon John Ristau Paris Vachon

Geophysics, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada (e-mail: wmoon@cc.umanitoba.ca) Applications Division, Canada Center for Remote Sensing, Ottawa, Ontario K1A OY7, Canada

ABSTRACT: Space-borne Synthetic Aperture Radar (SAR) techniques have recently become one of the most flexible and cost-effective Earth-observation tools for monitoring surface processes, including natural hazard monitoring and man- agement tasks such as landslides, volcanic activities, and earth- quake-related problems. This study investigates the feasibility of applying space-borne SAR interferometry to the monitoring of earthquake hazards and investigation of the earthquake- related tectonic processes, focusing on the investigation of the geological setting and associated tectonic processes in the Na- hanni Earthquake area, NWT, Canada.

Investigation of seismotectonics in the Nahanni area was carried out in two stages: traditional analysis of remote-sensing data, including both optical and microwave data, for static aspects of tectonic processes (Moon et al., 1991); and new SAR interferometry using RADARSAT, ERS-1/2, and JERS-1 SAR data to study relative movements of several active geological tectonic blocks.

Preliminary results indicate that (1) conventional geological remote-sensing methods provide us with important basic infor- mation on the tectonic setting associated with local earthquake activities, including several newly discovered structural fea- tures, and remain important as Earth-observation tools, (2) the newly discovered tectonic features correlate well with the earthquake focal plane solutions obtained from teleseismic data, (3) multi-temporal SAR interferometric (or differential InSAR) analysis can provide us with detailed tectonic movements and understanding of the earthquake processes in the study area, but (4) availability of suitable space-borne SAR data suitable for differential SAR interferometry may pose more problems than the technical development.

Key words: SAR interferometry, earthquake tectonics, RADARSAT, geological remote sensing

1. INTRODUCTION

Theoretical and laboratory simulation of optical inter- ferometry and holographic intefferometry techniques were proposed and developed in the 1960"s (Gabor et al., 1965; Hilderbrand and Haines, 1967). However, it was the pio- neering work by Graham (1974) that demonstrated that microwave synthetic aperture radar (SAR) interferometry was a viable technique for the derivation of digital terrain models and related applications. More recent works have greatly extended the technological basis and application

capabilities of both airborne and space-bome interferome- tric SAR (Zebker and Goldstein, 1986; Goldstein et al., 1988; Prati et al., 1989; Li and Goldstein, 1990; Zebker and Villasenor, 1992; Massonnet and Rabaute, 1993; Gray et al., 1994; Ishikawa et al., 1994; Massonnet et al., 1994; Stevens et al., 1994).

The application of radar interferometry to topographic mapping was first successfully implemented by Graham (1974), and was followed by Zebker and Goldstein (1986), Gabriel and Goldstein (1988), Goldstein et al. (1988), and Gabriel et al. (1989). Airborne SAR interferometric tech- niques were studied and successfully tested by Gray and Farris-Manning (1993), Stevens et al. (1994), Gray et al. (1994), and his group at the Canada Center for Remote Sensing (CCRS). Airborne interferometric SAR (or inSAR) techniques were also developed by several Japanese scientists including Ishikawa et al. (1994). Realistic multi- baseline SAR interferometry was successfully developed and tested using SEASAT (L-Band) SAR data over Death Valley, California by Li and Goldstein (1990). Their work is important because they tested several para- meters for space-borne repeat orbit interferometric tech- niques. They also defined an "optimal baseline separation" for the maximum surface-height accuracy for a given sys- tem configuration.

Massonnet and Rabaute (1993) and Massonnet et al. (1994) reported the first successful differential interfero- metric SAR imaging of the displacement field of the earthquake in Landers, California using the ERS-1 SAR system. According to Massonnet and his group (personal communication), sub-centimeter accuracy can now be achieved using intefferometric SAR if optimum con- ditions can be met. Even though the basic principle of SAR intefferometry is the invariant, the actual approach and implementation of obtaining topographic interfer- ograms are different between the JPL (e.g., Zebker and Goldstein, 1986; Zebker and Villasenor, 1992) group and the Massonnet (CNES, France) group. In this research, we initially planned to thoroughly review both approach- es and develop an optimum algorithm for a selected modes of RADARSAT and compare the results with

Space-borne SAR interferometry 79

ERS-1/2 and JERS-1 SAR results (Moon et al., 1997; Ristau and Moon, 1998).

The north Nahanni River area, which was chosen as the test area, has been known for many years to be seis- mically active, however, there has been very little infor- mation on the tectonic processes associated with these earthquakes. Some of the large earthquakes occurred within a small undeformed plateau area of the Mackenzie Plain in the Northwest Territories. The large ones include those in October 5, 1985 (6.5 Ms), December 23, 1985 (6.9 Ms), and March 25, 1988 (6.2 Ms). The geological block in which these earthquakes occurred forms part of the Foreland Fold Belt along the northeastern Cordillera (Wetmiller et al., 1988). Detailed studies of the two earthquakes which occurred in 1985 were conducted by Wetmiller et al. (1988) and Homer et al. (1989), and an evaluation of earthquakes prior to 1973 was undertaken by Leblanc and Wetmiller (1974). These studies describe earthquakes occurring mainly along the Mackenzie Valley northwest of the current study area.

The epicentral area encompassing the earthquakes is logistically difficult to access, and as a result, no geo- physical data, other than earthquake epicenter data and a short reflection-seismic line (Lamontagne and Milkereit, 1988), are available. Recently, a Lithoprobe SNORE tran- sect team carried out deep crustal refraction surveys on the southwestern and eastern portions of the study area (Lithoprobe, 1993). Because of the logistic difficulties of studying the tectonics of the epicentral area in detail,

remote-sensing techniques were initially proposed and carried out utilizing several sets of data, including CCRS airborne C-SAR, SPOT, Landsat MSS, ERS-1, and JERS- 1 data sets (Moon et al., 1991; Li, 1993). Preliminary as- sessment of the remote-sensing data was made with Landsat MSS data, but the final enhanced and integrated image also utilized SPOT, CCRS airborne C-SAR, and JERS-1 data (Li, 1993). The preliminary results of the study include revision of the old inaccurate geologic map of the study area, and imaging of major geological blocks which are closely associated with the local earthquakes (Moon et al., 1991; Li, 1993).

In this paper, we extend the previous studies (Moon et al., 1991; Li, 1993), including new remote-sensing data sets such as ERS-1 and ERS-2 SAR data, RADARSAT SAR data, and the DEM model from Geomatics Canada. The feasibility of using space-borne SAR technology for active tectonic investigation, particularly for investigation of earthquakes and earthquake-related tectonics, is evaluat- ed based on the available information.

2. G E O L O G I C A L BACKGROUND

The geological evolution of northwestern North Amer- ica began with the formation of continental nuclei between 4 and 3 GA (Bowering et al., 1989). Subsequent westward growth of the continent took place through a series of extension, ocean basin formation, and accretion episodes, i.e., Wilson cycles, that resulted in approxi-

124ow

520N

Fig. 1. Schematic geology map of the study area (after Wetmiller et al., 1988, modified from Wheeler et al., 1972). Some of the geological structures must now be modified after Moon et al., (1991).

80 Wooil M. Moon, John Ristau, and Paris Vachon

mately N--S-striking orogenic belts between the oldest rocks and the Pacific Ocean.

In the epicentral area, the Mackenzie Fold Belt is subdivided into four major tectonic units: the Interior Platform, the Franklin Mountains, the Mackenzie Plain, and the Mackenzie Mountains from east to west (Wheeler et al., 1972) (Fig. 1). Magnetic and subsurface data indicate that the Interior Platform, in which the Mac- kenzie Fold Belt is located, is underlain by Precambrian rocks of the Canadian Shield (de Wit et al., 1973). The Mackenzie Plain represents a relatively undefomaed zone between the Battlement and Iverson Thrust Faults. The Mackenzie Mountains, west of the Iverson Thrust Fault, contain concentric folds and long thrust faults, which in addition to the conventional lineaments, are target fea- tures for this study.

Geological information is not available due to the lack of detailed geological mapping. However, recent earth- quake data indicate that most large-magnitude earthquakes (> 6.0 Ms) occur in the same epicentral area, east of the Iverson Thrust Fault which has an approximate strike of NNW--SSE. The geological setting of the area was discussed by Homer et al. (1989). More recently, it has been discovered that the geology and tectonic setting of the North Nahanni epicentral area is more complicated, with at least two sets of ENE--WSW-trending active

Interpreted

Tectonic Blocks ~6_ wi th

Focal Plane

i t Solutions

iJw ; , i , J

[

Fig. 2. Schematic map of geological blocks in the study area (after Moon et al., 1991 and Li, 1993). Also see Figure 4 for the photograph of the lower ENE--WSW-trending fault.

faults intersecting the NNW--SSE-trending Iverson Thrust Fault (Fig. 2; Moon et al., 1991; Li, 1993).

3. EARTHQUAKE DATA

Earthquakes in this area have had magnitudes ranging from 3 to 6.9 Ms and the digital data sets were obtained from the Geological Survey of Canada. The focal-depth information of each main- and after-shock event provides important criteria for building tectonic models for the associated earthquake processes. Even though focal-plane solutions of a limited number of main- and after-shock events could be determined, the focal depth of each event could not be determined accurately (Li, 1993); however, most earthquake events in the study area, including both main- and after-shocks, are of shallow origin (Wet- miller et al., 1988). For digital integration, the epicenter coordinates of main- and after-shocks for the January 1960--January 1997 period were first input onto a base map and then coordinates were transformed for the later geocoding steps.

4. DIGITAL DATA SETS

4.1. Geological Data

There has been no new geological work done since Moon et al., (1991) and Li, (1993) published the prelimi- nary results of regional tectonic investigation. For a new differential InSAR study, we have used the revised local tectonic map obtained by Moon et al. (1991). Limited field work was carried out during the summer of 1997; further detailed fieldwork and major geological mapping are still required for ground truthing purposes.

4.2. Geophysical Data

Regional magnetic and gravity data for the study area were obtained from the GMT software packages. Even though the resolution ( l~ 1 ~ of the data is not high, the general trend, specifically the ENE--WSW direction, fits well with the newly identified series of fault systems which intersect the Iverson Thrust Fault in the study area. For detailed study of the deep structures, further inversion and modeling are required.

Two refraction-seismic transect experiments (Litho- probe, 1993) were planned as a part of the SNORCLE transect in the vicinity of the North Nahanni. The eastern corridor (# l) from the east reaches and ends at Nahanni Butte, and the western corridor (# 2) from northern British Columbia ends further south at Fort Nelson. Refraction- seismic surveys along these corridors were carried out dur- ing the summer of 1997 by the Lithoprobe-member teams and the data sets are currently being processed and inter-

Space-borne SAR interferometry 81

preted by the participating scientists. The final interpretation from these profiles would provide us with important crustal- velocity image data on a regional scale.

4.3. Satellite Data

Earlier studies have been based on Landsat MSS, SPOT, and CCRS airborne C-SAR data (Moon et al., 1991)2 In this study, we have included ERS-1, ERS-2, JERS-1, and RADARSAT data for both traditional geo- logical remote-sensing studies and new differential inter- ferometric SAR applications. For conventional applica- tions of space-borne SAR data for structural investigation, repeated temporal coverage was not too important, as long as one could process, filter for removing speckle, and transform for geocoding (Moon et al., 1991; Li, 1993). For interferometric SAR applications, however, pairs of SAR images are required from exactly the same repeat orbit separated by a preset time interval. This imposes a further limitation on data availability of SAR data for multi-temporal geological applications. Because of the temporal decorrelation and baseline decorrelation, actual data windows for the proposed study are very limited (Fig. 3).

In the case of RADARSAT, there are a number of different data-acquisition modes available. There are no such choices with ERS-1, ERS-2, and JERS-1 SAR data because each of these satellites has only one data-acqui- sition geometry. For the differential SAR interferometry, it was judged that the fine mode would be most suitable and fine-mode SLC (Single Look Complex) data pairs were ordered and received. There was, however, another problem with the RADARSAT data. RADARSAT does not have a GPS navigation system on-board and the RADARSAT orbit cannot be determined accurately. To overcome this problem, the RADARSAT data for this experiment was ordered during the specific time windows of each orbit boosting cycle. This imposed further limits on the data availability.

5. DATA PROCESSING

The ERS-1 and JERS-1 SAR data sets received for conventional geological remote-sensing applications were already fully processed and we carried out only the basic steps such as the speckle filtering (Ristau and Moon, 1997). PCA (Principal Component Analysis) processing and geocoding steps were performed before interpretation

DATA AVAILABILITY WINDOW FOR DIFFERENTIAL SAIl INTEIIFEIIOMETRY OVER NAHANNI AREA, NWT, CANADA

1993 1994 1995 1996 1997 1998 1999 I I I I I I I

RADARSAT

2000 I

JERS-, I

Diff. InSAR D a t a ~ Maybe available available in future

Fig. 3. Data availability window of the interferometric SAR data for three space-borne SAR systems: ERS-1/2, JERS-1, and RA-

82 Wooil M. Moon, John Ristau, and Paris Vachon

(Moon et al., 1991; Li, 1993). The SAR data sets, which were obtained for the SAR

interferometry to measure the relative movements of the tectonic blocks, were either raw data or in SLC format, so that we could digitally correlate and form the fringe patterns. The theory of interferometry is well known. To form an interferogram, we require two SAR images of the same scene from slightly different positions. If we use only one satellite, the InSAR data pair will be repeat orbit data pairs recorded over the same ground target. In the case of using more than one space-borne platform, data processing is complicated unless the space-borne SAR systems have identical or very similar radar and orbit parameters. The first step of SAR data processing is to process the raw data to SLC form and register the images. If necessary, the data can be bandpass filtered at this stage. The next step involves multiplication of the first image by the complex conjugate of the second image. Given the two images:

First image, S 1 (x, y) = a 1 (X, y) exp {i~1 (x, y)} Second image, Sz (x, y) = a2 (x; y) exp {i~2 (x, y)},

the interferogram is formed by the complex multiplica- tion:

I(x, y) = Sl(x, y) S2(x ~ y)* = al(x, y) a2( X, y) exp {i[r y) - ~(x; y ) ] }

where Sl(x, y) and Sz(x; y) are the InSAR pair images, a~(x, y) and a2(x, y) are the amplitude terms and ~l(X, y) - ~2(x, y) is the interferogram phase term. The coherence of the interferogram formed is

7(x, y )= (Sl(x, y)S2(x, y)*)

N f (ISl(X, y)l 2) ( IS2(x, y)l 2 )

changes obtained as a 2-D expression of the 3-D surface displacements can then be utilized.

Generation of DEMs and differential interferograms from SLC data sets involves a number of steps. The first step is co-registration analysis of the master and slave images. This step verifies that the master and slave images overlap both spatially and spectrally, and then generates parameterization of slave-image resampling and removal of flat earth phase. If the orbit state vector is not very accurate, there can be a large difference between the predicted and refined image tie points. This may cause the co-registration step to fail. To correct this problem, a manual bias is used so that the row and column positions of the same pixel in both images are forced to co-register. If this does not work, then manual tie-pointing can also be used to generate a set of tie points. The next step involves co-registration of the master and slave images where the slave SAR images are resampled and co- registered with the master image in both the range and azimuth directions. The following steps involve genera- tion of an interferogram, filtering for baseline decorrela- tion in the range direction, and azimuth spectral overlap in the azimuth direction. During the enhancement of the interferogram, a phase coherence map is calculated and saved for later classification applications. The phase information associated with the interferogram generated can also be saved at this stage for later steps of phase- unwrapping, generation of phase unwrapping control masks, and data reduction. Phase unwrapping using un- wrapping control masks can then be performed at this stage. The DEM generated from the phase product is mapped to the corresponding height values. The resulting DEM and coherence image is then geocoded for tectonic interpretation and for other later applications.

In the above equation, y(x, y) approaches 1.0, the maximum, when there is little change in the ground- surface scatters between the data acquisitions, ff there are significant changes in the surface scatters at a scale of 7 (wavelength of SAR signal) between the satellite passes, ~(x, y) will be very small approaching 0. In addition to the changes associated with the ground scatters with time, there are many other factors such as system noise, inadequate baseline length, point scatterers, and proces- sing errors, which can degrade the interferogram. The interferogram formed above can be used for topographic applications with appropriate further processing such as systematic phase unwrapping techniques.

For earthquakes and other tectonic applications which involve surface changes, including surface displacements, we have to estimate the vector displacements. These 3-D vectors can sometimes be represented in terms of changes in phase as a function of time between each acquisition of the two InSAR data pairs. The interferogram phase

6. D I S C U S S I O N

Generation of coherence images and interferograms from repeat orbit RADARSAT and ERS-1/2 tandem data were focused first on imaging of surface topographic features and subsequently on confirming the geological blocks interpreted by conventional geological remote- sensing techniques. Although relative movement of each block is unknown, the spatial distribution of the focal- plane solutions in relation to each geological block indicates that there is considerable interaction between the blocks. The photograph taken over the study area clearly shows the ENE--WSW-trending faults in Figure 2 (Fig. 4a). Figure 4b also shows landslides along the ENE --WSW-trending fault, indicating that this fault has been active recently.

Both RADARSAT and ERS-1/2 tandem SAR data (Table 1) were centered approximately at the junction of the Iverson Thrust Fault and the newly mapped ENE--

Space-borne SAR interferometry 83

Fig. 4. Aerial photographs of the study area. (a) The newly identified E W-trending fault intersecting the Iverson Thrust Fault. (b) Recent landslides along the newly mapped E W-trending faults east of Iverson Thrust Fault. The Iverson Thrust Fault is shown in the upper left-hand corner of a.

WSW-trending fault. The data sets received were in SLC format and the coherence images and interferograms were obtained as shown in Figure 5. Coherence of the RADARSAT data pair over the study area is very poor, probably for several reasons (Fig. 5a). The InSAR data sets were acquired on February 15 and March 11, 1997, which is the middle of winter in northern Canada. The surface is usually covered with thick snow and con- siderable amount of drifting snow between the two acqui- sition dates was expected, resulting in a serious temporal decorrelation. The corresponding interferogram is very spotty and the fringe patterns are scattered over the study area (Fig. 5b).

The ERS-I/2 tandem data pairs were collected on January 8 and January 9, 1996 (Table 1), again in the

middle of Canadian winter. The time inte~al between the data acquisitions was, however, only one day and the coherence between the two data pairs is excellent (Fig. 5c). The resultant interferogram is almost complete (Fig. 5d). We do not have compatible accompanying pairs of either RADARSAT, or ERS-1 or ERS-2 data for dif- ferential interferometry.

Further acquisitions ef data pairs compatible for differential SAR interferometry are required, even though the future prospects of widely accepted application of differential InSAR are very limited due to the con- straints on the data acquisition with respect to baseline requirements, problem of temporal decorrelation, and orbit boosting problem in the case of RADARSAT, etc. The study area is located in a stable continental region in

84 Woofl M. Moon, John Ristau, and Paris Vachon

Table 1. Data characteristics of the ERS-1/2 tandem and RADARSAT repeat-orbit data pairs.

ERS-1/2 tandem data pair* RADARSAT fine mode pair

Platform altitude Noise equiv. ~~ Across track orbit maintenance D Orbit repeat period Orbit Incidence angle Nominal swath width Ground resolution

Azimuth resolution Range resolution

Range bandwidth Polarization Dates of data acquisition

Baseline length Time interval between the image data pair

785 km 798 km - 2 7 dB -25 dB l k m 5 k m 1"*, 3, 35, or 168 days 24 days descending orbit ascending orbit 20 ~ < 0 < 26 ~ 20 ~ < ot < 49 ~ 100 km --50 km

4.49 m 9.0 m 9.65 m 15.0 m 15.54 MHz 11.6, 17.3, or 30.3 MHz VV HH January 8, 1996 February 15, 1997 January 9, 1996 March 11, 1997 23.44 m 766.7 m 1 day 24 days

*Most ERS-1 and ERS-2 parameters are essentially identical. ** 1 day repeat period represents the ERS-1/ERS-2 tandem mode.

the middle of the North American Plate and the earth- quakes in this area are of intra-plate. In order to under- stand the causal relationships between earthquakes and relative movements of each geological block mapped (Moon et al., 1991; Li, 1993), we will have to understand the regional stress pattern, including the postglacial rebound effects on the Hudson Bay area, east of the study area. The stress build-up in the lithosphere and mantle by glacial loading, melting, and postglacial rebound is nu- merically computed and failure potential is estimated by Wu and Hasegawa (1996).

Results of this study and the follow-up studies indicate that under all combinations of tectonic-stress magnitude and parameters, crustal loading promotes fault stability directly underneath the load. This may explain why the interiors of the ice-covered Antarctic and Greenland are virtually non-seismic at present. Additionally upon remov- al of the ice load, thrust faulting occurs within the ice margin. Even though the study area is at some distance from the center of the Hudson Bay ice sheet, it is in the western marginal zone of the ice sheet. Actual application of the theoretical models requires further detailed study of stress patterns associated with the earthquake processes in the North Nahanni River area (Wu and Hasegawa, 1996; Bell and Wu, 1997; Wu, 1997).

One of the main objectives of this study was the estima- tion of the actual surface displacement of each geological block in the North Nahanni earthquake area, using dif- ferential SAR interferometry. If one can estimate the hor- izontal stress (tectonic stress) from the detailed displace- ment fields, the overburden stress associated with the earthquake processes may provide the relatively unknown cause of the intra-plate earthquakes in the North Nahanni River area (Wu and Hasegawa, 1996).

Even though the theory on differential interferometric SAR techniques has been well developed, the availability

of suitable SAR data pairs puts considerable limitations on wide acceptance of the technology. Figure 3 shows availability of the space-borne SAR data for investigation of earthquake tectonics in the North Nahanni River area. The ERS-1, ERS-2, and RADARSAT SAR systems all collect data in C-band while the JERS-1 SAR system operates with L-band.

For conventional geological application of SAR data, L-band JERS-1 SAR provides us with better penetra- tion capability through vegetation canopies and probably through snow cover. Furthermore, L-band SAR of JERS-1 has a longer wavelength and the JERS-1 SAR data are expected to provide us with data sets with better coher- ence with respect to interferometry. As mentioned above, even without the usual limitations due to temporal and baseline-related problems, the problem of data availabil- ity alone puts serious limitations on applications of inter- ferometric SAR technology to earthquake studies. This can pose further problems when one use differential SAR interferometry as an earthquake-monitoring tool, unless we specifically design our satellite-borne SAR systems for this purpose.

7. CONCLUSIONS

This paper is an interim report on the on-going re- search on the effectiveness of applying differential inter- ferometric SAR technique to the investigation of earth- quake tectonics, for which the North Nahanni River earthquake area, Northwest Territories, Canada was chosen as the test site. The advantages of using space- borne SAR technology in geological science have been widely recognized and SAR interferometry, in particular, is becoming an important tool for studying and under- standing near-surface changes in Earth one of the most important earthquake-related problems. The results of this

Space-borne SAR interferometry

62~ 124~23 ] I

85

62~ 7/, 124o23 , I

62~ ], 124o33 , I

, / ~ 62~ ], 124033 ] ~!2

~"00 3~12

62~ ', 124o40 ' 1 62+23 ~. 124"40'

I

I 62~ ~, 124+131

~2

J~/2

62010', 124~

Fig. 5. Coherence and interferogram images generated over the study area. Coherence (a) and interferogram (b) generated from RADARSAT (February March, 1996) data pair (Table 1). Coherence (c) and interferogram (d) computed from the ERS-1/2 tandem (January, 1966) data pair (Table 1).

86 Wooil M. Moon, John Ristau, and Paris Vachon

study indicate that (1) conventional geological remote- sensing techniques such as image enhancement, principal component analysis, and classification techniques, are still valuable for monitoring the relatively slow changes occurring over large areas, (2) interferometric techniques using space-borne SAR data can provide us with an efficient and economical way of mapping and producing DEMs and topographic maps of remote study areas, (3) differential SAR interferometry can provide us with the capability to monitor surface changes associated with earthquakes and other geodynamic processes in the frac- tional range of the wavelength of the radar beams used, (4) temporal decorrelation appears to be at least as serious a problem for the Nahanni study area as the repeat orbit baseline requirements, and (5) the most serious problem at this time is the availability of adequate SAR data with an optimum baseline but without temporal decorrelation.

Availability of timely and suitable data sets is identi- fied as the most critical problem for applying the differen- tial SAR interferometry technique to the investigation of earthquake-related surface faulting. Although differential SAR interferometry has been promoted as the most impor- tant research tool for studying shallow earthquakes, severely limited availability of space-borne SAR data for specific time windows puts considerable limitations on the wide public acceptance of the technique for earth- quake monitoring and related applications.

ACKNOWLEDGMENTS: This research is supported by the Natural Science and Engineering Research Council (NSERC) of Canada Operating Grant (A-#7400) to W.M. Moon and Canadian Space Agency ADRO Program (#495). The authors would like to thank Canada Center for Remote Sensing (CCRS) and NASDA (Japan) for various technical and digital data support. J. Ristau would like to gratefully acknowledge partial support by the Cana- dian Society of Exploration Geophysicists (CSEG) in the form of a CSEG Graduate Scholarship.

R E F E R E N C E S

Bell, J.S. and Wu, P., 1997, High horizontal stresses in Hudson Bay, Canada. Canadian Journal of Earth Sciences, 34, 949-- 957.

Bowering, S.A., Williams, I.S. and Comston, W., 1989, 3.96 Ga gneisses from the Slave Province, Northwest Territories, Ca- nada. Geology, 17, 971--975.

de Wit, R. Gronberg, E.C., Richards, W.B. and Richmond, W.O., 1973, Tathlina area, District of Mackenzie. In: McCrosson, T. G. (ed.), The Future Petroleum Provinces of Canada. Cana- dian Society of Petroleum Geologists, Memoir, 1,187--212.

Gabor, D., Stroke, G.W., Restrick, R., Funkhouser, A. and Brumm, D., 1965, Optical image synthesis (complex am- plitude addition and subtraction) by holographic Fourier transform. Physics Letter, 18, 116--118.

Gabriel, A.K. and Goldstein, R.M., 1988, Crossed orbit in- terferometry. International Journal of Remote Sensing, 9, 857--872.

Gabriel, A.K., Goldstein, R.M. and Zebker, H.A., 1989, Map- ping of small elevation changes over large areas: differential radar interferometry. Journal of Geophysical Research, 94, 10~17.

Goldstein, R.M., Zebker, H.A. and Werner, C.L., 1988, Satellite radar interferometry: two dimensional phase unwrapping. Ra- dio Science, 23, 713--720.

Graham, L.G., 1974, Synthetic interferometer for topographic mapping. IEEE, 62, 763--768.

Gray, A.L. and Farris-Manning, P.J., 1993, Repeat-pass in- terferometry with airborne synthetic aperture radar. IEEE Transaction of Geoscience and Remote Sensing, 31, 180-- 191.

Gray, A.L., Marco, W.A., van der Kooij, Mattar, K.E. and Farris- Manning, P.J., 1994, Progress in the development of the CCRS along-track interferometer. IGARSS'94 (Proceedings), Pasadena, p. 2285--2287.

Hilderbrand, B.P. and Haines, K.A., 1967, Multiple wavelength and multiple source holography applied to contour gen- eration. Journal of Optical Society of America, 57, 155--162.

Horner, R.B., Wetmiller, R.J., Lamontagne, M. and Plouff, M., 1989, A fault model for the Nahanni Earthquakes from after- shock studies. Bulletin of Seismological Society of America, 80, 1553--1570.

Ishikawa, Y., Yazawa, A., Osanai, N.N. and Yajima, S., 1994, Measurement of topography used by synthetic aperture radar at Mt. Unzen, Japan. The First Workshop on SAR In- terferometry (Proceedings), Tokyo, December, p. 17--24.

Lamontagne, M. and Milkereit, B., 1988, Reprocessing of Industry Data of the North Nahanni Region: Implications for the In- terpretation of the Nahanni Earthquake Sequence. Open File Report 1728, Geological Survey of Canada, p. 1--32.

Leblanc, G. and Wetmiller, R.J., 1974, An evaluation of seismo- logical data available for the Yukon Territory and the Mack- enzie Valley. Canadian Journal of Earth Sciences, 11, 1435-- 1454.

Li, B., 1993, Application of Remote Sensing Techniques for the Seismo-Tectonic Study of Nahanni Earthquake Area, Northwest Territory, Canada. M.S. thesis, University of Mani- toba, Winnipeg (Canada), 213 p.

Li, K.F. and Goldstein, R.M., 1990, Studies of multibaseline space-borne intefferometric synthetic aperture radars. IEEE Transaction of Geoscience and Remote Sensing, 28, 88--97.

Lithoprobe, 1993, Slave-Northern Cordillera Lithospheric Evo- lution (SNORCLE) Transect, Lithoprobe Phase IV Proposal. LITHOPROBE (Canada), 4-21--4-42.

Massonnet, D. and Rabaute, T., 1993, Radar interferometry: lim- its and potential. IEEE Transaction of Geoscience and Re- mote Sensing, 31,455--464.

Massonnet, D., Feigl, K., Rossi, M. and Adragna, F., 1994, Ra- dar interferometric mapping of deformation in the year after Landers Earthquake. Nature, 369, 227--230.

Moon, W.M., Won, J.S., Li, B., Slaney, R. and Lamontagne, M., 1991, Application of airborne C-SAR and SPOT image data to the geological setting of the Nahanni earthquake area. Canadian Journal of Remote Sensing, 17, 272--278.

Moon, W.M., Ristau, J. and Paris, V., 1997, Application of dif- ferential InSAR for earthquake tectonic investigation in Nahan- ni, NWT, Canada. Annual Meeting of Canadian Geophysical Union (Proceedings), Banff (Canada), May, p. 27-- 28.

Prati, C., Rocca, F. and Monti Guarnieri, A., 1989, Effects of

Space-borne SAR interferometry 87

speckle and additive noise of the altimetric resolution of interferometric SAR survey. IGARSS'89 (Proceedings), p. 2469 2472.

Ristau, J. and Moon, W.M., 1997, Adaptive filtering of random noise in 2-D geophysical data, Geophysics. (in press)

Ristau, J. and Moon, W.M., 1998, Extraction of DEM and topo- graphic features from space-borne SAR interferometry. An- nual Meeting of Canadian Geophysical Union (Proceedings), Quebec, p. 36--37.

Stevens, D.R., Cumming, I.G. and Gray, A.L., 1994, Motion compensation for airborne interferometric SAR. IGARSS'94 (Proceedings), Pasadena, p. 1967--1970.

Wetmiller, R.J., Homer, R.B., Hasegawa, H.S., North, R.G., Lamontagne, M., Weichert, D. and Evans, S.G., 1988, An analysis of the 1985 Nahanni Earthquakes. Bulletin of Seismological Society of America, 78, 590--616.

Wheeler, J.O, Aitken, J.D., Berry, M.J., Gabrielse, H., Hutchin- son, W.W., Jacoby, W.R., Monger, E.R., Niblet, E.R., Norris, D.K., Price, R.A. and Stacey, R.A., 1972, The Cordilleran

Structural Province. In: Price, R.A. and Douglas, R.J.W. (eds.), Variations in Tectonic Styles in Canada. Geological Survey of Canada, Special Paper, 11,325 378.

Wu, P., 1997, Effect of viscosity structure on fault potential and stress orientations in eastern Canada. Geophysical Journal In- ternational, 130, 365 382.

Wu, P. and Hasegawa, H., 1996, Induced stress and fault po- tential in eastern Canada due to a disc load: a preliminary analysis. Geophysical Journal International, 125,415--430.

Zebker, H.A. and Goldstein, R.M., 1986, Topographic mapping from interferometric synthetic aperture radar observations. Journal of Geophysical Research, 91, 4993 4999.

Zebker, H.A. and Villasenor, J., 1992, Decorrelation in inter- ferometric radar echos. IEEE Transaction of Geoscience and Remote Sensing, 30, 9 5 ~ 9 5 9 .

Manuscript received February 9, 1998 Manuscript accepted June 15, 1998