DEFORMATION IN THE GRANADA BASIN (SOUTHERN BETIC CORDILLERA) STUDIED BY PS-INSAR: PRELIMINARY RESULTS

Ruiz, A.M.(1), Sousa, J.J. (2), Hanssen, R.F. (3), Perski, Z.(3), Bastos, L.(2), Gil, A.J.(1)

(1) Departamento de Ingeniería Cartográfica, Geodésica y Fotogrametría, Universidad de Jaén Escuela Politécnica Superior de Jaén, Campus Las Lagunillas s/n, Edif. A3, 23071 Jaén, Spain

amruiz@ujaen.es; ajgil@ujaen.es

(2) Observatório Astronómico da Universidade do Porto Alameda do Monte da Virgem, 4430 Vila Nova de Gaia, Portugal

jjsousa@gmail.pt; lcbastos@fc.up.pt

(3) Delft Institute of Earth Observation and Space Systems, Delft University of Technology Kluyverweg 1, 2629 HS, Delft, The Netherlands R.F.Hanssen@tudelft.nl; Z.A.Perski@tudelft.nl

ABSTRACT

Recently, two CAT-1 projects have been initiated related to Granada Basin area (Betic Cordillera, Southern Spain), aimed at the detection of deformation in the region applying time series InSAR methodology. Due to the outstanding availability of ERS-1/2 and Envisat acquisitions, a time span of more than 12 years is covered and time series can be produced, enabling us to assess the feasibility of monitoring deformation with millimetre precision. The ultimate goal of these projects is the quantification of displacements and determination of their mean directions, relating them to dynamic changes and stress accumulation in order to identify potential seismic hazard locations. For the moment, not all available scenes have been received, so a preliminary data processing has been carried out at the Padul fault area. In this paper, we present a status report of both projects.

Keywords: SAR Interferometry, Permanent Scatterer Interferometry, Betic cordillera, Ground deformation, Padul fault

1. INTRODUCTION

The study area, the Southern Betic Cordillera (Southern Spain) which is located in the western part of the Mediterranean Sea (Fig.1) was formed during the Alpine orogeny as a consequence of the NNW–SSE continental collision between the African and the Eurasian plates. The Betic chain has been classically divided into external and internal zones. The internal zones of the Betic Cordillera form part of an Alpine collisional orogen, known as the Alboran Domain that underwent late orogenic extensional collapse in the Miocene to form the present-day Alboran Sea in the

Western Mediterranean. The Alboran Domain is superimposed over the hypothetical limit between African and Iberian plates, which are converging in the region at an estimated rate of about 4-5 mm/y. During the Alpine orogeny, the internal zones shifted to the west, colliding with Iberia at the end of the Middle Miocene. Given this relative right lateral movement between internal and external zones, strike-slip faults played an important role in the configuration of the Betic orogeny.

At present, the Betic Cordillera can be regarded as one of the most tectonically active zones in the Iberian Peninsula. It is characterised by a moderate to high seismicity and has been affected by I-VIII (MSK) earthquakes in historical times. The Andalusia earthquake (1884) was the most recent catastrophic event recorded in the Iberian Peninsula, with an estimated Ms magnitude of 6.5-6.7. Four areas with high seismic activity can be distinguished: the Enguera–Alcoi–Tavernes de Valldigna zone located in the northeastern corner; the Murcia–Torrevieja zone in the eastern area; the Almería–Adra zone located at the southern edge and the Granada area (see Fig. 1) in the western part.

The seismotectonic studies done in the central sector of this Alpine orogen, based on the focal mechanism, reveal that nowadays, the region is subject to a NW–SE compressive stress field with a NE–SW linked extension. This NE-SW extension is accommodated by normal faults in various orientations, amongst which the NW-SE faults stand out. The NW–SE and NE–SW normal faults with a strike-slip component are the most common faults in the southernmost part of the study area, being the Padul fault one of them. These normal faults show geological evidence of recent activity (fault scarps, triangular facets, deformed alluvial fans, etc.).

_____________________________________________________

Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

Figure 1. Location and simplified geological map of the Betic Cordillera (Southern Spain) with location of the Padul fault area (red box) and the ascending /descending frames

used in the study (black boxes). The Padul fault area is enlarged in Fig.2. From the different heights of Neogene and Quaternary materials displaced by these faults, some authors have estimated an average rate of uplift for the Sierra Nevada western sector from 0.4 to 0.6 mm/y, and occasionally of 0.8 mm/y [1, 2]. However, it is not known whether localized deformations along faults can exhibit larger displacements.

Recently, two CAT-1 projects have been initiated, directed towards detecting displacements in the region applying time series InSAR methodology (e.g., PSI, SBAS). Due to the outstanding availability of ERS and Envisat images, a time span of more than 12 years is covered and time series can be produced, enabling us to assess the feasibility of monitoring deformation with millimeter precision. The ultimate goal of these projects is the quantification of displacements and determination of their mean directions and relate this to dynamic changes and stress accumulation in order to identify potential seismic hazard locations. First, the Padul fault area (Fig. 1, 2) has been selected as test area for processing following classical interferometry InSAR and Persistent Scatterer Interferometry (PSI). PSI techniques have been introduced in the late 1990s by POLIMI to overcome the major limitations of repeat pass SAR interferometry, temporal and geometrical decorrelation, and variations in atmospheric conditions. Since the introduction of PSI, the applicability of radar interferometry has increased considerably. For data

processing, we used the Integer Least Squares (ILSQ) PSI concept developed at TU Delft.

In this paper we present the status of these CAT-1 projects and the preliminary results obtained at the moment, after processing some PSI stacks at the Padul fault area.

In the following section, we briefly present the Padul fault area, followed by the description of our datasets. Then, the data processing with InSAR and PSI is covered. Finally, a summary is given to conclude the paper.

2. THE PADUL FAULT

The Padul normal fault (NW–SE locally E–W) is approximately 15 km long (Fig. 2) and has various segments which dip towards the SW and S. There is both geomorphologic and tectonic evidence to indicate that this fault continues to be active at present (fault scarps, deformed Holocene alluvial fans, triangular facets, strong downcutting in the drainage network, Holocene peat deposits dipping against the fault plane, etc). From the different elevations of the Red Formation Plio-Pleistocene deposits displaced by the Padul fault, some authors deduce an average rate of uplift of 0.35 mm/y over the last million years [3]. Analysis of the slickensides (striations, corrugations,…) present on the fault planes show that the principal displacement has a marked left-lateral horizontal component.

Figure 2. Topographic map of the Padul fault area (enlarged red box from Fig. 1). The Padul fault track is represented as the white dot line. The map shows the height differences in the area (more than 3000 m).

The western sector of the Sierra Nevada mountain is shown in the right part of the image.

The level of seismic activity along the Padul fault is very low. No evidence of moderate or high magnitude historic earthquakes exists. In the period 1980–1996 only one earthquake of magnitude higher than 3.5 has occurred. In spite of this low recent seismic activity, there is geological evidence of moderate to high-magnitude earthquakes during the Quaternary [4].

3. InSAR: DATA AND METHOD

Analysis of ERS and Envisat archives shows that the Padul fault area is covered by two frames. One of them is from the ascending track 187 (frame 737) and the other from the descending track 280 (2859) (see locations on Fig. 1). Different acquisitions of these frames are available from October 1992 to July 2006. Tab.1 details the number of them from ERS-1/2 and the Envisat satellites in the ascending and descending passes selected within CAT-1 3963 and 3858 projects. By now, only 64 scenes have been delivered, so preliminary processing has been done from incomplete stacks.

For the first stage of this study we applied the InSAR technique in the 2-pass approach which combines a pair of images (ERS-1/2 or Envisat satellites) of the same area acquired at different times. For that, we used the public-domain Delft Object-Oriented Radar Interferometric Software (DORIS) [5, 6]. From the delivered scenes, several interferograms have been generated and analyzed with different combinations of temporal and perpendicular baselines. Nine of them are summarized in Tab. 2 and Fig. 3: 5 long-term and 4 relative short-term (combinations with temporal baseline less than 1 year). The DEM correction has been applied using the SRTM data. The potential precision of InSAR depends on many factors but in principle the surface displacement measurement can have a precision of 2%-5% of the SAR wavelength (millimetre precision for ESA satellites). Line-of-sight deformation between two image acquisitions is measured as a fraction of the (semi) wavelength. InSAR is a powerful technique for measuring changes in the Earth’s surface, however it

H = 123 m

H = 1100 m

H = 3190 m

0 10 km

Padul Fault

No. Scenes Sensor Pass Track Frame

Ordered Received (March / 2007) From To

asc 187 737 6 2 03/06/1993 06/04/1996 ERS 1

des 280 2859 10 8 08/10/1992 30/10/1999 asc 187 737 26 0 06/08/1995 09/07/2006

ERS 2 des 280 2859 29 14 13/08/1995 28/04/2002 asc 187 737 22 22 13/10/2002 09/07/2006

ENVISAT des 280 2859 18 18 13/04/2003 22/01/2006

Table 1. Datasets from ERS-1/2 and Envisat satellites selected for this study.

No. Orbits numbers Data acquisitions Satellites Track BTemp [days] BPerp[m] 1 20675_21677 1999/04/04 – 1999/06/13 ERS-1/2 280 70 673 2 21677_17669 1999/06/13 – 1998/09/06 ERS-1/2 280 280 642 3 21677_11657 1999/06/13 – 1997/07/13 ERS-1/2 280 700 462 4 24724_41758 1996/04/06 – 1999/07/10 ERS-1/2 280 1190 601 5 22312_02639 1995/10/21 – 1995/10/22 ERS-1/2 280 1 129 6 20675_17669 1999/04/04 - 1998/09/06 ERS-1/2 280 210 32 7 20675_11657 1999/04/04 -1997/07/13 ERS-1/2 280 630 213 8 20675_25318 1999/04/04 – 1996/05/18 ERS-1/2 280 -1051 68 9 20675_21310 1999/04/04 – 1995/08/12 ERS-1/2 280 1331 68

Table 2. Data from interferograms calculated as test at the Padul fault area for this study.

Figure 3. Interferograms calculated as test at the Padul fault area for this study. does have limitations. These include temporal and geometrical decorrelations and variable tropospheric water vapour, which can generate variable phase delay due to the impact of water vapour on the propagation speed of microwave signals. The data processing carried out show the main limitations of conventional DInSAR applied to measurement of small displacements. Because of the small deformation value of the expected signal and the decorrelation factors, none of the interferograms (even the long-term ones) provide fringes able to be interpreted as terrain deformation.

4. PS-InSAR

The traditional InSAR techniques have been extended in last years to pixel-based approaches like e.g. Permanent Scatterer method [7, 8, 9]. PSI, introduced in the late 1990s by POLIMI, is the latest development in radar interferometric processing, which offers a practical way to overcome the major limitations of repeat pass SAR interferometry; temporal and geometrical decorrelations, and atmospheric artifacts. This is achieved by the analysis of the inteferometric phase of individuals long time-coherent scatterers in a stack of tens of differential intererograms with one master image. Since the introduction of PSI, the applicability of radar interferometry has increased considerably. For

1 2 3 4 5 6 7 8 9

data processing, we used the Integer Least Squares (ILSQ) PSI concept developed at TU Delft.

Permanent Scatterers InSAR exploits several characteristics of radar scattering and atmospheric decorrelation to measure surface displacement in non-optimum conditions. The PS technique has been developed to detect isolated coherent pixels and tackle the problem of atmospheric delay errors at the expense of a large number of required images (>30) and a sparse, pixel-by-pixel based evaluation [8,9]. Point targets, not affected by temporal decorrelation, are recognized by means of a statistical analysis of their amplitude in all available SAR images. The contribution of topography, deformation, and atmosphere can be estimated by carefully exploiting their different time-space behaviour. Topography is not dependent of time, but linearly correlated with the perpendicular baseline length and spatially correlated dependent on terrain roughness. Deformation is independent of baseline, but correlated in time and space. Atmosphere is independent of baseline, uncorrelated over time intervals of one day or more, but spatially correlated per interferogram [10]. Thus, atmospheric effects can be estimated and removed

by combining data from long time series of SAR images, averaging out the temporal fluctuations.

Radar scatterers that are affected by temporal and geometrical decorrelation are used, allowing exploitation of all available images regardless of imaging geometry. In this sense the scatterers are “permanent”, i.e., persistent over many satellite revolutions (the technique is also known as Persistent Scatterer InSAR). 5. RESULTS AND DISCUSSION The initial PSI processing has been carried out exploring 22 ERS-1/2 SAR which were received from ESA. The temporal coverage and spatial baselines were sub-optimal for that processing but the analysis will be further extended to the PSI processing of the full stack. Finally we identified 300. PS points covering southern part of Granada city and few PS in Padul and Durcal towns. Obtained initial set of PS does not allow drawing any conclusion about deformations within the area.

Fig. 4. Preliminary results of PSI processing based on 22 ERS-1/2 images

ACKNOWLEDGEMENTS ERS-1/2 and Envisat data were kindly provided by the European Space Agency (ESA) within the CAT-1 projects 3963 and 3858. This work was conducted in the framework of the project SWITNAME PDCT/CTE-GIN/59244/2004 supported by Fundação da Ciência e Tecnologia (Portugal) and the project PR2006-0330 supported by Ministerio de Educación y Ciencia (Spain) REFERENCES 1. Sanz de Galdeano, C. (1996). Neotectónica y

tectónica activa en el sector Padul-Dúrcal (Borde SW de Sierra Nevada, España). In Proc. 1st Conferencia Internacional Sierra Nevada, Granada, March, 1996, pp. 219–231.

2. Keller, E.A., Sanz de Galdeano, C. & Chacón, J. (1996). Tectonic geomorphology and earthquake hazard of Sierra Nevada, South Spain. In Proc. 1st Conferencia Internacional Sierra Nevada, Granada, Spain, March 1996, pp. 201–218.

3. Sanz de Galdeano, C., Peláez, J.A. & López-Casado, C. (2003) Seismic potential of the main active faults in the Granada Basin (South of Spain). Pure and Applied Geophysics, 160, 1537-1556.

4. Alfaro, P., Galindo-Zaldívar, J., Jabaloy, A., López-Garrido, A. & Sanz de Galdeano, C. (2001).

Evidence on the activity and paleoseismicity of the Padul fault (Betic Cordillera, southern Spain). Acta Geológica Hispánica 36, 283–295.

5. Kampes, B. & Usai, E. (1999) Doris: the Delft Object-Oriented Radar Interferometric Software. In 2nd International Symposium on Operationalization of Remote Sensing, Enschede, The Netherlands, 16-20 August, 1999.

6. Kampes, B. (1999) Delft Object-Oriented Radar Interferometric Software: Users manual and technical documentation. Delft University of Technology, Delft, 1.2 ed.

7. Ferretti, A., Rocca, F. & Prati, C. (1999) Permanent scatterers in SAR Interferometry. In Proc. of the International Geoscience and Remote Sensing Symposium. Hamburg, Germany, 28 June – 2 July, 1999, pp. 1-3.

8. Ferretti, A., Rocca, F. & Prati, C. (2000) Nonlinear subsidence rate estimation using permanent scatterers in differential SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 38 (5), 2202-2212.

9. Ferretti, A., Rocca, F. & Prati, C. (2001) Permanent scatterers in SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 39 (1), 8-20.

10. Hanssen, R.F. (2001) Radar Interferometry: Data Interpretation and Error Analysis. Kluwer Academic Publishers. Dordrecht. The Netherlands.