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International Journal of Applied Earth Observation and Geoinformation 13 (2011) 801–811

Contents lists available at ScienceDirect

International Journal of Applied Earth Observation andGeoinformation

jo u r n al hom epage: www.elsev ier .com/ locate / jag

urface deformation caused by April 6th 2009 earthquake in L’Aquila (Italy): Aomparative analysis from ENVISAT ASAR, ALOS PALSAR and ASTER

ohammad Ali Goudarzi ∗, Tsehaie Woldai, Valentyn A. Tolpekinaculty of Geo-Information Science and Earth Observation (ITC), University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands

r t i c l e i n f o

rticle history:eceived 18 November 2010ccepted 19 May 2011

eywords:atellite geodesynSARub-pixel Correlation Technique (SCT)

a b s t r a c t

Surface deformations in L’Aquila (centre of Italy) caused by the April 6th, 2009 earthquake were studiedfrom space geodesy and remote sensing points of view using Synthetic Aperture Radar Interferom-etry (InSAR) and Sub-pixel Correlation Technique (SCT). InSAR was used to measure ground surfacedeformation in the satellite line of sight (LOS) direction and the deformation was determined usingtwo separate interferometric pairs of ENVISAT ASAR and ALOS PALSAR data sets. Furthermore, SCT wasemployed to investigate the horizontal displacements in the area. Two separate pairs of ENVISAT ASARand ASTER optical image data sets were employed, and horizontal displacements in Range/Azimuth and

’Aquila earthquakearth surface deformation

in west–east/south–north directions were investigated, respectively.Maximum 28.1 cm subsidence in LOS direction was observed on the descending interferogram of the

ENVISAT ASAR data set, to the north-west of Onna village. The initial coseismic surface deformationstarted from the north of the main event epicentre. Most of the post-seismic relaxation occurred afterApril 7th, toward the south and south-east of the main shock epicentre, and results from SCT showedthat the horizontal displacement was not significant on the surface (not more than a few centimetres).

. Introduction

Interferometric Synthetic Aperture Radar (InSAR) is a powerfulechnique that uses phase components of two (or more) SAR imagescquired from relatively close positions to distinguish and study thenterference patterns between the two images. InSAR has been useduccessfully to investigate deformation resulting from land subsi-ence (Hoffmann et al., 2001; Crosetto et al., 2002; Woldai et al.,009), earthquakes (Massonnet et al., 1993) and volcanic activi-ies (Massonnet et al., 1995; Hanssen, 2001). While InSAR is moreensitive to deformation in the satellite line of sight (LOS) direc-ion, Sub-pixel Correlation Technique (SCT) is applied on a pairf remotely sensed data sets (SAR or optical images) to measureorizontal surface deformation where large deformation gradientsomplicate phase identification and unwrapping by InSAR (Sartit al., 2006).

The L’Aquila area is historically known as one of the most seismi-

ally active areas in Italy. Since the end of December 2008, the areaas experienced prolonged seismic events (Anzidei et al., 2009) andn Monday April 6th, 2009 at 01:32:39 UTC, this culminated in an

∗ Corresponding author at: Département des sciences géomatiques, Pavillonouis-Jacques-Casault, Université Laval, Québec (QC), G1V 0A6 Canada.el.: +1 418 656 2131x6526; fax: +1 418 656 7411.

E-mail address: mohammad-ali.goudarzi.1@ulaval.ca (M.A. Goudarzi).

303-2434/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jag.2011.05.014

© 2011 Elsevier B.V. All rights reserved.

earthquake of Mw 6.3 that shocked the city and the surroundingarea. According to USGS (2009), the earthquake occurred at coor-dinate of (42.334◦N, 13.334◦E) and at depth of 8.8 km in the centralregion of Abruzzi province (Fig. 1A).

Before the last major earthquake of 1980 in Popoli, about 40 kmto the south-east of L’Aquila city (Gruppo di lavoro CPTI, 2004), theL’Aquila basin and its active faults system had not been studiedwith the advanced geodetic methods. This study (1) investigatedand measured actual surface deformation signals associated withthe Mw 6.3 April 6th 2009 earthquake in the satellite LOS and inthe horizontal directions, and (2) to compare the displacementsderived from various remotely sensed data sets (SAR and opticalimages).

1.1. Geological and tectonic settings

The L’Aquila area is located within the central part of the Apen-nines mountain chain which passes most of the length of the Italianpeninsula. It is the result of the convergence between the Africanand European tectonic plates and the subsequent collision of thetwo continental margins (Fig. 1B). The bedrock of the affected

area comprises Jurassic to the Miocene limestone formations thatlargely outcrop along the valley flanks and on the ridges locatedwithin the Aterno Valley. The latter is partly filled by Pleistocenelacustrine deposits formed by a complex sequence of pelitic and

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F al., 20L

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ig. 1. (A) Geodynamic model for central Mediterranean (modified from Devoti et

’Aquila area (modified from Blumetti et al., 2009).

oarse grained units, with frequent lateral variations overlying theedrock (Fig. 1A) (GEER, 2009).

Quaternary deposits are found in morphological depressionsnside the uplifted and emergent hill and mountain chains. Theseedimentary basins are primarily delimited by high-angle (70◦)ormal faults with Apennines trends that vertically break up, at dif-

erent elevations, the orogenic structure, thereby forming a horstnd graben structure.

The elongation in the north-west to south-east direction (par-llel to many of the active normal faults) is notable in thesentra-Apennines basins, and occurs in localities such as L’Aquila,he city affected by the April 6th, 2009 earthquake. The extensionalystem comprising normal faults (master faults, synthetic and anti-hetic faults) is active as demonstrated by the L’Aquila earthquakend the regional geomorphology. Extension is also demonstratedy the normal fault focal mechanism of the present earthquakeGEER, 2009). The existing faults are mostly normal with variableength that predominantly strike in a north-west to south-eastirection (Galadini and Galli, 2000; Galadini and Messina, 2001;oncio et al., 2004). To the south-east of L’Aquila, the active faultsave a strongly rectilinear trend, dipping either toward the south-est or the north-east.

. Image data sets and analysis

.1. Displacement in the satellite LOS direction

Various interferometric and coseismic ENVISAT ASAR and ALOSALSAR image pairs were selected to study the area. With the

xception of the third ENVISAT ASAR (see Table 1), all image pairsere acquired with the smallest available acquisition time dif-

erence. Since ALOS PALSAR interferograms made from the Fineeam Dual-polarization mode (FBD, 14 MHz) to Fine Beam Single-

08). (B) Geological-structural map of the central Apennines and active faults of the

polarization mode (FBS, 28 MHz) conversion have lower noise thanthe interferograms made from the FBS to FBD conversion (Fielding,2008), the raw data in FBD was converted to FBS and subsequentlyto SLC format using the precise orbits information available in theALOS PALSAR image leader files to remove orbital phase signature.

The ENVISAT ASAR and ALOS PALSAR interferograms were cor-rected for orbital and topographic phase signatures, georeferencedusing ASTER GDEM (ASTER G-DEM, 2007) and processed withROI PAC 3.0 (Rosen et al., 2004; Julea et al., 2006).

The interferograms generated from both data sets (seeFigs. 2 and 3) show high quality closed fringes. In comparing theresults however, it is evident that the coherence maps of ALOS PAL-SAR interferograms show higher values relative to those of ENVISATASAR. Although some residual fringes which might result fromatmospheric artefacts are seen in the EVISAT ASAR interferograms(Fig. 2), particularly in the eastern and south-eastern parts of Pog-gio Picenze village, their amplitudes are estimated to be less thana fringe, which should not affect the deformation fringe patternsignificantly.

The fringes have relatively simple structures, and display a onelobe pattern for the ascending interferogram on the hanging-wallfor both data sets, and an asymmetric two lobe pattern for theENVISAT ASAR descending interferogram (Fig. 2B) on the hangingand foot-wall of the causative fault. The gradients of deformationin the satellite LOS direction on the hanging-wall are asymmetricin both interferograms with the largest gradient in the north-eastof the fringe centres. This fringe system corresponds to elongationon the hanging-wall and shortening on the foot-wall of the faultin the satellite LOS direction. With respect to the geometry of the

fault, they are interpreted as subsidence and uplift in the satelliteLOS direction, respectively. The result is consistent with a down dipmotion on a north-west to south-east strike trending normal fault,dipping to the south-west. This fault (known as “Paganica Fault”) is

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Table 1ENVISAT ASAR and ALOS PALSAR image pairs for interferometry.

Sensor Image pairs Orbit Track Pass B⊥ (m) Span (day)

ENVISAT ASAR 2009/04/15–2009/03/11 37254–36756 129 Asc. 197 352009/04/12–2009/02/01 37207–36205 079 Des. 168 702009/04/12–2008/04/27 37207–32197 079 Des. 36 350

ALOS PALSAR 2009/05/21 (FBS)–2008/07/03 (FBD) 17706–13009 840 Asc. 355 3222009/04/22 (FBS)–2008/07/20 (FBD) 17283–13257 830 Asc. 202 276

F 009/04c

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ig. 2. ENVISAT ASAR interferograms of the L’Aquila area for (A) the ascending (2oordinate system (zone 33). Major faults are shown in black lines.

ocated about 6.5 km to the east of L’Aquila city, lies near the eastide of Paganica village, and is considered responsible for the mainarthquake event.

The fringe systems show variations in trend at many points and

long different lines. They are associated with active tectonic move-ents that have had a coseismic displacement of modest extent,

.e. Pettino fault (located to the north-west of L’Aquila) and Baz-

ig. 3. ALOS PALSAR interferograms of the L’Aquila area for (A) the first (2009/05/21–20ystem (zone 33).

/15–2009/03/11) and (B) descending (2009/04/12–2009/02/01) pairs in the UTM

zano fault (located between L’Aquila and Paganica) (Fig. 1B) (GEER,2009).

The ENVISAT ASAR interferograms (Fig. 2) show decorrela-tion in the south and south-western parts of the fringe systems

centre, where a hill of higher than 2100 m elevation is located.The NDVI map in this part of the study area shows low vegeta-tion index, suggesting the decorrelation is probably due to snow

08/07/03) and (B) second (2009/04/22–2008/07/20) pairs in the UTM coordinate

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Fig. 4. The ENVISAT ASAR interferogram in descending pass(2009/04/12–2008/04/27) in the UTM coordinate system (zone 33). The areawithin the rectangle box corresponds with the farm lands in the Aterno river valley.Ip

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2.3 km (374,680 mE, 4,691,650 mN) to the north-east of Paganica

n spite of good quality of fringes over the hill between Fossa and L’Aquila, a clearattern is not present in this area (cf. Fig. 2) due to the temporal variation of crops.

overage at summits during the acquisition of the first scenes ofhe ENVISAT ASAR pairs. This hypothesis was tested using the thirdNVISAT ASAR image pair in Table 1 taken in nearly the same dateut one year later when snow was absent on the hill. Although theew ENVISAT ASAR interferogram (Fig. 4) shows high quality of

ringes over the hill, it was inferior to the one shown in Fig. 2 in therea between Paganica and Bazzano faults (the red rectangle boxn Fig. 4). Farmlands at Paganica, Onna and Poggio Picenze villagesre located here. At the image acquisition date (April), rapid growthf vegetation in farmlands affects interferometric processing of Cand ENVISAT ASAR images, but longer wavelength ALOS PALSARata are not affected, and fringes are closed (Fig. 3).

Shadow originating from steep slopes and Radar imaging geom-try is another hypothesis for the decorrelation. The latter wasnvestigated by making classified slope maps for 23◦ and 35◦ inci-ence angles (Fig. 5) and superimposing the main ENVISAT ASAR

nterferograms on the slope maps. A meaningful relation betweenhe uncorrelated parts and the area with slope more than incidencengle of ENVISAT ASAR (23◦) and ALOS PALSAR (35◦) was observed.hese areas (blue patches in the slope maps) make strong layovernd shadow effects (slope > incidence angle) on the SAR images andead to the decorrelation. As decorrelation caused by shadow andayover phenomena is less expected in the ALOS PALSAR interfer-grams, less shadow and layover prone area are seen in the ALOSALSAR slope map in comparison to the ENVISAT ASAR one (Fig. 5).

The interferograms were unwrapped using SNAPHU softwareChen and Zebker, 2002) and converted to displacement in theatellite LOS direction (Figs. 6 and 7). Three profiles and six crossections were drawn roughly along (parallel to the causative fault)nd across (perpendicular to the causative fault) the deformed area,espectively. The measured deformations along the profiles and theross sections are shown in Figs. 8 and 9. Profiles from the ENVISATSAR interferograms are in agreement in terms of measured val-

es for subsidence, except at the beginning of the descending pair,here more uplift is seen on the foot-wall of the fault. Profile 1

nd profile 2 of ENVISAT ASAR and ALOS PALSAR interferograms

Observation and Geoinformation 13 (2011) 801–811

are numerically closer together rather than profile 2 and profile 3,and therefore, the area between the profile 1 and 2 shows moredisplacement gradient (in the satellite LOS direction) in compar-ison to the area between profile 2 and 3. The inflection points ofall cross-sections correspond to the common trace of Paganica andAterno-Poggio Picenze faults. This shows that the subsidence hasstarted about 0.5 km before the common trace of these faults (tothe northwest).

High frequency variations were seen in the interferograms ofboth data sets due to ground temporal variations, unwrappingerrors and other contributing factors to noise in the data. ALOSPALSAR unwrapped interferograms however showed more localvariations considered as artefacts (cf. Figs. 8 and 9). As a generalfact, images obtained using coherent sensors such as SAR systemsalways suffer from strong speckle noise: a spatially random multi-plicative noise with signal and noise statistically independent ofeach other. This is a great obstacle to coherence (or correlationcoefficient) estimation and interferometric phase, and it further-more hinders the accuracy of phase unwrapping and informationextraction with InSAR techniques (Chen et al., 2008).

The “enhanced Lee filter” was applied on ALOS PALSARunwrapped interferograms to reduce the effect of local extremes(maxima and minima). Speckle effect in Radar imagery is reducedby this filter, while texture information is preserved simultane-ously. Lee filtering is a standard deviation based (sigma) filter thatremoves data based on statistics calculated within individual filterwindows. Unlike a typical low-pass smoothing filter, the Lee filterand other similar sigma filters preserve image sharpness and detailwhile suppressing noise. The pixel being filtered is replaced by avalue calculated using the surrounding pixels (Lee, 1980). The filterhas been adapted from Lee filter and uses local statistics (coefficientof variation) within individual filter windows (Lopes et al., 1990).

Noise variation coefficient (Cu) and its maximum threshold(Cmax) for the enhanced Lee filter were calculated using the fol-lowing formulae:

Cu = 0.523√L

(1)

and

Cmax =√

1 + 2L

(2)

where L corresponds to the looks number of the Radar image (Lopeset al., 1990). For this case, the enhanced Lee filter was appliedwith kernel size of 13, Cu = 0.234 and Cmax = 1.183 for L = 5. Fig. 10compares the results from the filtered and unfiltered unwrappedinterferograms along the second profiles in Fig. 7, which passalmost along the centre of the fringes. The profiles from the fil-tered unwrapped interferograms (the green graph in Fig. 10) aresmooth (without local variations) and fit to the profiles from unfil-tered ones (the purple graph in the same figure). The new profilesgive more realistic values for measuring the deformation.

Spatial analysis of the ENVISAT ASAR unwrapped interfer-ograms showed maximum of 26.1 cm (for the ascending) and28.1 cm subsidence (for the descending) in the satellite LOS direc-tion in two separate points; the first at (371,950 mE, 4,686,590 mN)about 2.3 km, and the latter at (372,950 mE, 4,687,120 mN) about1.5 km to the north-west of Onna village (the nearest habitat centreto the point of maximum subsidence which received the strongestshakes during the earthquake). In addition, an uplift of maximum1.3 cm and 5.5 cm in the satellite LOS direction was detected intwo separate points about 2.7 km (375,640 mE, 4,690,520 mN) and

village on the foot-wall of the Paganica fault in the ascending anddescending interferograms, respectively. The same spatial analysisfor ALOS PALSAR unwrapped interferograms showed a maximum

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SAR I

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Fig. 5. The classified slope map based on the incidence angle of (A) ENVISAT A

f 25.6 cm and 25.1 cm subsidence in the satellite LOS direction inwo separate points, the first at (372,360 mE, 4,686,357 mN) andhe latter at (370,720 mE, 4,687,050 mN) about 1.8 km and 4.0 kmo the north-west of Onna village. ALOS PALSAR interferograms didot show uplift on the foot-wall of the fault (Fig. 9).

The results from the ALOS PALSAR data set strongly comple-ent the results obtained from ENVISAT ASAR data set in terms of

he spatial distribution pattern and the extend of deformation. Theescending ENVISAT ASAR interferogram, however, shows largeralues for subsidence in the satellite LOS direction than the ascend-ng one. We attribute this to a combination of the looking directionnd the incidence angle of the ENVISAT ASAR and the geometryf the fault. The InSAR technique measures earth surface defor-ations along the satellite LOS direction (slant rage), and it is only

artially sensitive to variations perpendicular to the azimuth direc-ion. Following the notation used in Sarti et al. (2006), assume

= (dx, dy, dz)T is vector of east, north and vertical displacement of a

ig. 6. The unwrapped ENVISAT ASAR interferograms for (A) the ascending (2009/04/oordinate system (zone 33).

S2 (23◦) and (B) ALOS PALSAR (35◦) in the UTM coordinate system (zone 33).

point at the earth surface (displacement vector), and v =(vx, vy, vz)T

is the unit satellite LOS vector (sensitivity vector) expressed in thesame local reference system, pointing from ground to the satellite.The observed change in range with InSAR is a projection of the dis-placement vector (d) on the sensitivity vector (v) and is representedas follows:

r = −vT · d (3)

Components of the sensitivity vector are calculated using thefollowing formula set:

vx = sin � cos(ϕ − �/2)v = sin � sin(ϕ − �/2) (4)

vz = cos �

where � is the Radar sensor look angle and ϕ is the satellite headingangle measured from the east direction anticlockwise (Goudarzi,

15–2009/03/11) and (B) descending (2009/04/12–2009/02/01) pairs in the UTM

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F 008/0(

2

r

A

r

Fp

ig. 7. The unwrapped ALOS PALSAR interferograms for (A) the first (2009/05/21–2zone 33).

010). If Eq. (3) is developed, then:

= vx · dx + vy · dy + vz · dz (5)

By substituting the values of sensitivity vector (v) for ENVISATSAR, it can be written as:

= (0.377) · dx + (0.101) · dy + (−0.921) · dz (6)

ig. 8. Displacement in the satellite LOS direction along profiles (A and B) and along crossairs, respectively.

7/03) and (B) second (2009/04/22–2008/07/20) pairs in the UTM coordinate system

for the ascending pass, and:

r = (−0.377) · dx + (0.101) · dy + (−0.921) · dz (7)

for the descending pass. In the case of the Paganica fault andthe April 6th 2009 earthquake, components of the displacementvectors (v) are all equal, negative, and are estimated smallerthan dz. Subtracting the common terms in Eqs. (6) and (7)

sections (C and D) obtained from the ascending and the descending ENVISAT ASAR

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F ng crr

r

r

a

|

swfp(s

ig. 9. Displacement in the satellite LOS direction along profiles (A and B) and aloespectively.

esults in:

Des. > rAsc. (8)

nd the absolute value of difference is equal to:

2vx · dx| (9)

Eq. (9) emphasizes that the difference between maximum sub-idence points in the satellite LOS direction should be more thanhat was observed from the interferograms. This is due to the dif-

erent spanning days of the ascending and descending pairs and theost-seismic relaxation. While the ascending pair spans 35 daysincluding nine days after the main event), the descending pairpans 70 days (including six days after the main event). There-

Fig. 10. Comparison between profiles from filtered and unfiltered unwrapp

oss sections (C and D) obtained from the first and the second ALOS PALSAR pairs,

fore, although mathematically the descending pair should showmore subsidence in the satellite LOS direction in comparison to theascending pair, the ascending pair spans three days more duringthe post-seismic relaxation time, and consequently the differencebetween the ascending and descending pairs is decreased.

The centres of fringes are also slightly different in the ascendingand descending ENVISAT ASAR and both ALOS PALSAR ascendinginterferograms. For ENVISAT ASAR interferograms, the separationis about 1 km and the centre of fringes in the ascending pair is

located to the south-east of the centre of fringes in the descendingpair. For ALOS PALSAR interferograms, the centre of the first inter-ferogram is placed about 2 km farther south-east than the secondinterferogram. The separation is due to different temporal baselines

ed interferograms for the first (A) and second (B) ALOS PALSAR pairs.

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Fig. 11. COSMO-SkyMed interferogram (2009/02/19–2009/04/09) modified fromS

Fs

alvi (2009) in the UTM coordinate system (zone 33).

ig. 12. The offsets in azimuth (A and C) and in range (B and D) directions obtained frohown in the UTM coordinate system (zone 33).

Observation and Geoinformation 13 (2011) 801–811

of interferograms and the considerable post-seismic relaxation ofthe affected area.

The post-seismic relaxation was investigated further by com-paring ENVISAT ASAR and ALOS PALSAR interferograms with thestudy of Salvi (2009). The author has processed one pair of COSMO-SkyMed SAR images based on data sets acquired on February 19thand April 9th, 2009 with B⊥ = 480 m and 36◦ incidence angle. Theresulting interferogram (Fig. 11) shows that the coseismic surfacedeformation was initially concentrated to the north of the mainevent epicentre along a line passing L’Aquila and Poggio Picenze.However, the ENVISAT ASAR and ALOS PALSAR coseismic interfer-ograms showed that the extent of the displacement is increasedconsiderably toward the south and south-east of the main shockepicentre after April 9th, and has become much more homoge-neous around it; i.e., the deformation has had a movement fromnorth-west to south-east along the L’Aquila – Poggio Picenze axis.

The ENVISAT ASAR data set therefore represents more move-

ment, and can reveal its shift to the south-east, because theascending interferogram spans three days after the descendingpair. During the time interval between the end of the ascending

m the ascending and the descending ENVISAT ASAR pairs, respectively. Maps are

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F m the

aceTsmlodmi

scAmIah

2

pEoadsa

epwwliTctac

ig. 13. The offsets in (A) west–east and in (B) south–north directions obtained fro

nd the descending pairs (April 12th–15th), the movement wasurrent with a calculated rate of ∼0.3 km/day toward the south-ast. The ALOS PALSAR interferograms also confirms this result.he first ALOS PALSAR interferogram spans 29 days more than theecond interferogram, and during this time span, the point withaximum deformation has still moved toward south-east paral-

el to the L’Aquila – Poggio Picenze axis with a calculated ratef ∼0.07 km/day. This shows that the post-seismic relaxation wasecreasing significantly, in comparison to the calculated rate of theovement between April 9th and April 15th from ENVISAT ASAR

nterferograms.According to Cheloni et al. (2010), contribution of the post-

eismic deformation in the time span of the ENVISAT ASARoseismic interferograms, including the two main after-shocks ofpril 7th and 9th, amounted to approximately 7% of the releasedain shock moment, which is equivalent to less than one fringe.

t can therefore be inferred that this movement is not due to thefter-shocks, especially when the April 9th after-shock is known toave occurred to the north of the main event epicentre.

.2. The horizontal surface displacement

To investigate horizontal displacements on the surface, Sub-ixel Correlation Technique (SCT) (Goudarzi, 2010) was applied onNVISAT ASAR and ASTER images. Table 2 shows the coseismic pairsf ENVISAT ASAR images chosen for SCT which had the minimumvailable perpendicular base line. In this condition (i.e. same inci-ence angle and short baselines), the speckle signature remainsimilar on both images and stereoscopic effect is avoided as muchs possible.

Both the phase and amplitude of SAR images were used to gen-rate offsets with azimuth and range window size of 100 and 20ixels, respectively (see Fig. 12). The SCT outputs seen in this figureere quite noisy, the pixel values were dispersed, and the valuesere extremely high (e.g. 200 times larger than length of the corre-

ation window). Moreover no displacement pattern was observedn either the azimuth direction (with finer resolution) or in range.his might be due to temporal changes (e.g. variation in vegetation

overage and soil moisture) that affect SAR imagery. To supporthis idea, it was observed that a concentration of pixels on thescending pair around L’Aquila (which extended toward the east)orresponds to the urban part of the city (the red rectangle box in

second ASTER pair. Maps are shown in the UTM coordinate system (zone 33).

Fig. 12A). Moreover, the concentrated pixels in the south-west partof Paganica fault are also located on the farmlands of Paganica vil-lage (the black rectangle box in Fig. 12A), and might have resultedfrom temporal decorrelation during acquisitions of the SAR images.

The other reason for the noisy results could be the length ofspatial baselines which are still long, and this might lead to dom-ination of speckles. The optimum baseline is a function of a radarwavelength and the desired measurement. For topography, thereis a trade-off between sensitivity to surface height at shorter base-lines, and speckle effects that affect coherence and decorrelation atlonger baselines. In mountainous terrain, a smaller baseline can beused; otherwise a larger baseline is necessary.

The result of SCT from the ascending pair, with a shorter tem-poral baseline (175 days in comparison to 910 days) and a longerspatial baseline (44 m in comparison to 28 m) shows more pixels incomparison to the descending pair (compare Fig. 12A with Fig. 12C).This shows SCT using SAR images is more sensitive to the spatialbaseline than the temporal. However, the longer the temporal andspatial baselines are in SAR SCT, the stronger the noise is.

The horizontal displacement was also studied using two ASTERoptical images taken at 2003/09/19 (scene ID 2017273494) and2009/07/17 (scene ID 2075058903). The pair has minimum baselineto height ratio (0.0038) to avoid stereoscopic effect with the sameday time acquisition for reducing the solar illumination differenceeffects. The SCT was done by COSI-corr software (Leprince et al.,2007) using the band VNIR 3N. The pre-earthquake image was co-registered and ortho-rectified using ancillary data provided withinthe image files and ASTER GDEM, respectively. Sinus cardinal (Sinc)re-sampling kernel was used to reconstruct the images accordingto the mapping matrices defined in ortho-rectification step withthe kernel size of 25. Fig. 13 shows the results.

The offsets in this figure do not show any meaningful pattern forhorizontal displacements, neither in west–east nor in south–northdirections. The patch of displaced points (the white rectangle boxin Fig. 13) corresponds to the large farmlands of Paganica andOnna villages. Even though they are distributed between Paganicaand Pettino faults and to the south-eastern part of Pettino faults,where the maximum displacements are expected, they resulted

from the temporal variations of vegetation in the area insteadof a real horizontal displacement. Regardless of the noisy aggre-gated points, other parts of the offset maps show zero horizontaldisplacements. This outcome is supported by the results obtained

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Table 2ENVISAT ASAR image pairs for SCT.

Sensor Image pairs Orbit Track Pass B⊥ (m) Span (day)

ENVISAT ASAR 2009/07/13–2009/01/19 38531–36026 401 Asc. 44 1752009/08/30–2007/03/04 39211–26185

F(r

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ig. 14. Coseismic horizontal displacement of permanent GPS stations in L’Aquilamodified from Anzidei et al., 2009), in the UTM coordinate system (zone 33). Theed triangles show position of GPS stations.

rom the ENVISAT ASAR data set. While coarse resolution of thismage data set is the main barrier against detection of small hor-zontal displacements, temporal variation (mainly due to changesn vegetation) is another important obstacle for optical SCT.

Anzidei et al. (2009) have studied the earthquake of April 6thsing GPS observations. The GPS data set for this study includesbservations of forty permanent sites, ten days before and two daysfter the main shock of April 6th (i.e. until the first significant after-hock of April 7th, 17:47 UTC). Even though contribution of theost-seismic deformation (until April 15th) was measured about% of the main shock moment release, this selection minimizeshe effects of cumulated deformation caused by the after-shocksAnzidei et al., 2009). Fig. 14 shows the measured coseismic hor-zontal displacements in nearby field GPS stations. The figurehows that the density of GPS stations located near L’Aquila isnsufficient to give a clear spatial pattern of the coseismic displace-

ents. According to Anzidei’s study, the horizontal displacement of.03 ± 0.36 cm toward the north-east direction has been observedt INFN GPS station, which is aligned to the foot-wall motion ofhe fault. The maximum coseismic horizontal surface displacementetected at CADO GPS station equals to 10.39 ± 0.45 cm toward theorth-west direction. The other sites around the epicentre showinor (a few centimetres) displacements. While the horizontal dis-

lacement vector at INFN is aligned in the direction of the foot-wallotion, the displacement vector in CADO is toward the north-west

nd is not aligned with the expected direction of the hanging-wallotion (toward the south-west). Other stations are oriented nearly

o the south, except in the SELL GPS station which is toward theouth-west.

. Discussion and conclusion

Different techniques of InSAR and SCT were employed to studyurface deformations resulting from the April 6th, 2009 earthquakeear L’Aquila, Italy. While InSAR was used to reveal the ground sur-

ace displacement in the satellite LOS direction, SCT was employed

079 Des. 28 910

to detect the horizontal surface displacements. The study was alsodone using different Radar and optical data sets to achieve morecomprehensive results.

An elongation in the satellite LOS direction on the hanging-wallof the north-west to south-east striking normal fault (dipping tosouth-west) was observed by using the InSAR technique, which isconsidered as subsidence and down dip motion with respect to thegeometry of the fault. A shortening in the satellite LOS directionwas also observed on the foot-wall of the fault, considered as upliftin the satellite LOS direction, from the ENVISAT ASAR data set.

While different image data sets gave slightly different values forthe elongation, a maximum of 28.1 cm subsidence along the satel-lite LOS direction was measured from the descending pair of theENVISAT ASAR interferogram to the north-west of the Onna vil-lage (located about 7 km to the south-east of L’Aquila city), and theresults are still strongly supportive and consistent. The differencesare mainly due to the sensitivity vector of the corresponding dataset and the satellite pass, which can be justified by Eq. (3), the dif-ferent time span of the interferometric pairs and the post-seismicrelaxation.

As hypothesized by other researchers (e.g. Walters et al., 2009;Valensise, 2009 in GEER, 2009) the causative fault would have dislo-cated a portion of the crust without rupturing the ground surface.This is inferred from the pattern of the fringe systems which donot suddenly close on the trace of the fault (Figs. 2 and 3), and thetransition between the foot-wall and the hanging-wall occurredsmoothly and is not considerable (cf. Figs. 8 and 9).

The difference between the extent of negative and positive dis-placements along the satellite LOS direction indicates the activationof a gravitational shaking mechanism, especially in the followingconditions: (1) when the movement occurs along a normal faultwithout rupturing along the fault (Figs. 8 and 9), (2) while no sig-nificant uplift along the satellite LOS direction was observed onthe foot-wall, (3) the points of maximum subsidence do not coin-cide with the surface expression of the Paganica fault, and (4)none of the fringe systems are closed unexpectedly on the planeof the causative fault but display two trends at different gradients(Valensise, 2009 in GEER, 2009).

Comparison of the interferograms obtained from the ENVISATASAR (Fig. 2) and COSMO-SkyMed data sets (Fig. 11) revealed thedeformation originated from north of the epicentre and graduallyextended to the south and south-east after April 9th during thepost-seismic relaxation process. The similarity of the deformationpattern between the ENVISAT ASAR and the ALOS PALSAR inter-ferograms (in comparison to the COSMO-SkyMed interferogram)indicates that the largest coseismic deformation occurred betweenApril 9th and April 12th. This behaviour and the interaction amongthe multiple fault segments provide an excellent opportunity tostudy interaction of segmented fault blocks during coseismic defor-mations.

Although the exact amount and pattern of horizontal displace-ment could not be determined using the ENVISAT ASAR and ASTERdata sets, it was shown that the horizontal displacement was notsignificant (no more than a few centimetres). The absence of any

horizontal surface displacement pattern is either due to the coarsespatial resolution of ENVISAT SAR images which are inadequateto detect small horizontal displacements, or insignificancy of theinduced horizontal displacements by the earthquake with respect

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o the normal structure of the causative fault. It can also be inferredhat the causative fault was either not ruptured or the rupture wasot significant enough to be detected by the ENVISAT SAR data setthis is supported by InSAR fringe systems).

This study showed that the SCT using Radar images is more sen-itive to the spatial baseline than the temporal, and the longer theemporal and spatial baselines are in SAR SCT, the stronger the noises. Furthermore, while detection of small horizontal displacements difficult from coarse resolution images, temporal variation dueo changes in vegetation is another important obstacle for opticalCT.

The activation or reactivation movement of Paganica, Pettinond Bazzano faults confirms that these faults are still active. Amonghem, the Paganica fault shows no significant geomorphologicxpression, and had been given less importance in relation to otherearby active faults. This point highlights the existing problem inhe identification of the potentially dangerous seismogenic faultsn this and similar tectonic regions

cknowledgements

Authors would like to sincerely thank the anonymous review-rs, M. Peyret from Université de Montpellier II and A. Farshadrom University of Twente for their constructive comments andeviews, B.G. Merkle and J.-R. Hikey for editing the text, Europeanpace Agency (ESA) for providing ENVISAT ASAR and ALOS PALSARadar images, and U.S. Geological Survey (USGS) Land Processesistributed Active Archive Centre (LP DAAC) for providing ASTER

mages free of charge.

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