ISARS-NOA

DISPLACEMENT FIELD MAPPING AND FAULT MODELING OF THE Mw=5.9, SEPTEMBER 7, 1999 ATHENS EARTHQUAKE BASED ON ERS-2 SATELLITE RADAR INTERFEROMETRY

C. Kontoes⁽¹⁾, P. Elias⁽¹⁾, O. Sykioti⁽¹⁾,
P. Briole⁽²⁾, D. Remy⁽²⁾, M. Sachpazi⁽³⁾ , G. Veis(4), I. Kotsis⁽⁴⁾

Geophysical Research Letters, Vol. 27, No. 24, 3989-3992, December 15, 2000

On September 7^th, 1999 at 11^h 56^m 50^s UT a magnitude M_w=5.9 earthquake struck the area of Attica. It was strongly felt by the Athenian population, caused heavy damages and made several buildings collapse killing 143 people. Its epicenter is located at 38.10^oN; 23.56^oE which is about 20km NNW from the Athens center, between the main rifts of Evia and Corinth, at the oriental extremity of the latter.

In the area around the epicenter, several normal faults dipping northward or southward are clearly expressed on the seismotectonic map (Plate 1a). However, the absence of any seismic rupture at the surface after the earthquake does not allow a direct field identification of the seismogenic fault. Seismological operations for the purpose of recording the aftershock activity were launched immediately by several state and research organizations. At the same time geodetic data were collected with terrestrial and satellite methods. These data are currently at the stage of processing.

Soon after the earthquake a parallel operation using radar interferometry, was initiated at the Institute for Space Applications and Remote Sensing of the National Observatory of Athens (ISARS/NOA) in order to measure the surface deformation and its characteristics. The study was done in collaboration with the National Technical University of Athens/Department of Surveying Engineering, Institut de Physique du Globe de Paris (IPGP) and the Geodynamics Institute of NOA (NOAGI).

To capture the deformations caused by the main shock and aftershocks, interferograms were constructed from the phase difference recorded on ERS-2 satellite images. Interferometric analysis of SAR images has demonstrated potential to monitor and measure surface deformations associated with earthquakes [Massonet et al., 1993, Zebker et al., 1994, Murakami et al., 1996]. In Greece the same technique has been used for the study of Kozani (13/5/1995. Mw=6.6) and Aigion (15/6/1995, Mw=6.1) earthquakes [Meyer et al., 1996; Bernard et al., 1997].

In order to isolate surface deformations from the interferograms, it was necessary to remove the topography. The elimination of the topography was done by subtracting a synthetic fringe pattern produced by using the Digital Elevation Model (DEM) of the area of interest from the interference pattern resulted by the two SAR images. The DEM was produced by digitizing contour lines and spot height data shown on existing 1 :5000-scale topographic maps. The accuracy of the DEM was tested with a set of independent control points of known elevation and the estimated RMS error was of the order of ±10m.

The selection of the interferometric pairs to process was based on their sensitivity to the topography, expressed by the altitude of ambiguity (h_a). Only image pairs with h_a values greater than 5 times the expected DEM error, were processed in order to avoid residual fringes due to topography. The magnitude of the expected topographic artifact expressed in phase cycles is of the order of 0.2 phase cycles or 5.6mm in range, for the worst co-seismic pair (h_a = 50 m) and 0.075 cycles or 2.1 mm in range for the best co-seismic pair (h_a =133m).

Table 1 illustrates the time spanning of the interferometric pairs used and their corresponding h_a values. The images collected for interferometric calculations were acquired at the descending pass of the ERS-2 satellite.

From the interferograms it is apparent that the main shock and aftershocks have induced a co-seismic surface deformation, which appears with at least two concentric, but not symmetric, fringes centered at 38.10^oN; 23.60^oE. This point is located in a distance of less than 3 km away from the main shock epicenter. The fringes indicate a change of 56mm in range. The concentric fringes are apparent in all co-seismic interferograms spanning the periods [December 28^th, 1995 to September 23^rd, 1999], [November 27^th, 1997 to September 23^rd, 1999], [September 19^th, 1998 to October 9^th, 1999].

Plate 2a shows one of the calculated interferograms, spanning the period [September 19^th, 1998 to October 9^th, 1999].

It is important to notice that the interferograms composed from images acquired before the seismic event (Table 1), do not show fringes in the affected by the earthquake area. Unfortunately we could not find appropriate SAR images just before the Athens earthquake to create interferograms, in order to investigate for any pre-seismic deformation signals.

The calculated co-seismic interferograms were used to sample slant-range displacements along the two observed fringes. The sampled slant range data were used to fit a simple inversion model, assuming for the earthquake a dislocation of a rectangular fault in homogeneous elastic half-space. The forward algorithm used is that developed by Okada, [1985]. The inverse algorithm is that developed by Briole et al., [1986] using a least squares approach proposed by Tarantola and Valette, [1982].

The model predicted two faults. A main one which is responsible for more than the 90% of the energy released and a secondary smaller fault, which was added to fit the fringe asymmetry to the east. Due to the large predominance of the main fault segment, the parameters of this secondary structure are not well constrained by the data and are less reliable than those relative to the main segment.

Based on the model results the main fault segment has experienced a slip of 50cm at the depth of 8km resulting in the observed surface deformations. No slip above the depth of 8km is predicted for the main fault segment. The fault segments intersect the Earth surface at points 38.17^oN; 23.53^oE and 38.14^oN; 23.76^oE, defining a trace, which lies about 5 km northern to the Thriassion depression, crosses the Fili mountain and shows a WNW-ESE trending.

The resulted fringes are bounded by the Fili mountain in the NE and the Aegaleo mountain in the SE (Plate 2a). They define an active zone of more than 20km in the E-W and 10km in the N-S directions. The area defined by the fringes encompasses the vast majority of the located epicenters, extending S-W to the Fili fault with a striking along a WNW-ESE axis (Plate 1b) (Papadopoulos et al., 1999). However, a high concentration of aftershocks is also observed at the eastern border of the Thriassion depression along the Aegaleo mountain. If this is not due to the geometry of the seismograph array, it indicates the activation of other faults parallel to the direction of the Aegaleo mountain. This is consistent with the results obtained from the focal mechanism study done by Tselentis and Zahradnik, [1999]. It should be noted that the observed fringes follow the same direction in the south. Also the fact that no fringes are observed beyond the root of the Aegaleo mountain, it corroborates the opinion of other researchersthat this mountain acted as a boundary, preventing further activation to the south-east direction (V. Papazachos, oral communication).

The absence of shallow seismic activity prevented the rupture propagation towards the Earth surface and hindered the direct identification of the seismogenic fault. N.N. Ambraseys (internal field report) after his visit at the earthquake site confirms that the two faults located in the epicentral area, that is the Aspropyrgos and the Fili faults (Plate 1a), nowhere showed any field evidence of reactivation. This makes Ambraseys to draw the conclusion that the fault associated with the Athens earthquake it could well has been one of the many not mapped, not clearly visible, or most probably blind faults. Currently several studies dealing with aftershock recordings are in progress (Papanastasiou et al., 1999). However, the first results of this study suggest that the activation of a fracture zone located at the Fili mountain, is more likely associated with the Athens earthquake resulting in the observed surface deformations.

Thanks are due to prof. N.N. Ambraseys for making available to us his internal field report on the Athens earthquake and for his comments on the paper. We are grateful to the European Space Agency for providing ERS SAR data. Sincere thanks to Dr. George Stavrakakis, director of the Geodynamics Institute of NOA (NOAGI) for providing the aftershock data recorded by the local seismograph array in the period September 8^th to October 5^th 1999. Thanks are also due to Helene Vadon and Kurt Feigl for their support in using DIAPASON sw tool (CNES), and Antonio Avallone (IPGP) for his contribution to the fault modeling.

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