# Fault plane modelling of the 2003 August 14 Lefkada Island (Greece) earthquake based on the analysis of ENVISAT SAR interferograms

M. Ilieva, Pierre Briole, Athanassios Ganas, D. Dimitrov, Panagiotis Elias, A.

Mouratidis, R. Charara

► To cite this version:

M. Ilieva, Pierre Briole, Athanassios Ganas, D. Dimitrov, Panagiotis Elias, et al.. Fault plane modelling of the 2003 August 14 Lefkada Island (Greece) earthquake based on the analysis of ENVISAT SAR interferograms. Tectonophysics, 2016, 693, pp.47-65. 10.1016/J.TECTO.2016.10.021. hal-02411712

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# Fault plane modelling of the 2003 August 14 Lefkada Island (Greece) earthquake based on the analysis of ENVISAT SAR interferograms

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M. Ilieva,ª,ᵇ P. Briole,ᶜ,*, A. Ganas,ᵈ D. Dimitrov,ª P. Eliasᶜ, A. Mouratidisf, R. Chararaᶜ

a. Bulgarian Academy of Sciences, National Institute of Geophysics, Geodesy and Geography, Department of Geodesy, Acad. G. Bonchev str, bl. 3, 1113 Sofia, Bulgaria

b. University of Architecture, Civil Engineering and Geodesy, Faculty of Geodesy, Department of Geodesy and Geoinformatics, 1, Hristo Smirnenski Blvd., 1046 Sofia, Bulgaria

c. Ecole Normale Supérieure, PSL Research University, Laboratoire de Géologie - UMR CNRS 8538, 24 Rue Lhomond, 75005 Paris, France

d. National Observatory of Athens, Institute of Geodynamics, Lofos Nymfon, Thission, 11810 Athens, Greece

e. National Observatory of Athens, Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, Vas. Pavlou & I. Metaxa, GR-15 236 Penteli, Greece

f. Aristotle University of Thessaloniki, School of Geology, Department of Physical and Environmental Geography, 54124 Thessaloniki, Greece

E-mail addresses: m_b_ilieva@abv.bg (M. Ilieva), pierre.briole@ens.fr (P. Briole), clgdimi@bas.bg (D. Dimitrov), aganas@noa.gr (A. Ganas), pelias@noa.gr (P. Elias), amourati@auth.gr (A. Mouratidis), rana_charara@trimble.com (R. Charara)

## Abstract

On 2003 August 14, a M<sub>w</sub> = 6.2 earthquake occurred offshore the Lefkada Island in the eastern Ionian Sea, one of the most seismically active areas in Europe. The earthquake caused extended damages in the island, and a number of ground failures, especially along the north-western coast. Seven ascending ENVISAT/ASAR images are used to process six co-seismic interferograms. The ROI-PAC package is used for interferogram generation with the SRTM DEM applied in a two-pass method. The formation of the co-seismic pairs is limited due to the existence of one pre-seismic image only. Dense vegetation is covering the island, which is an obstacle in getting good coherence, since C-band images are used. Nevertheless, ground deformation, of more than 56 mm (two fringes) in the line of sight of the satellite, is detected in all six co-seismic interferograms. By inversion of the data from the observed fringes, a best fitting model of the activated fault is calculated assuming a dislocation in an elastic half space. The inferred fault is a pure dextral strike-slip fault, dipping 59 ± 5° eastward, 16 ± 2 km long and 10 ± 2 km wide. It is located north of the fault of the M<sub>w</sub> = 6.5 2015 November 17 earthquake, and a 10-15 km gap remains between the two faults. The 2003 fault does not reach the surface and its upper edge is at a depth of 3.5 ± 1 km. No evidence is found of slip south of the Lefkada Island as suggested by some seismological studies.

**Keywords:** Earthquake dynamics, Seismicity and tectonics, Radar interferometry, Lefkada

## 1. Introduction

Greece is the country with the highest seismicity in Europe. The main crustal deformations occur along the Hellenic Arc, as a result of the subduction of the African Nubian plate

<footnote>* Corresponding author.</footnote>
beneath the Aegean plate (Le Pichon and Angelier, 1979; Ganas and Parsons, 2009a; Pérouse et al., 2012). The Central Ionian region (Fig. 1) is one of the most active areas of shallow seismicity in Greece (Benetatos et al., 2007) with an estimated full seismic coupling according to Chousianitis et al. (2015). The Central Ionian Islands, Lefkada, Cephalonia, Zakynthos, are located on the Aegean Plate between the Apulia – Eurasia continental collision to the north and oceanic subduction to the south, with intensive surface deformations due to ENE-WSW shortening (Ganas et al., 2013) with earthquake than can reach or exceed magnitude 7 like in 1953 or 1983 (Papazachos and Papazachou, 1997). The plate boundary in the Central Ionian Sea is defined by a major, right-lateral, strike-slip fault, the Cephalonia Transform Fault (CTF) which is located at the western termination of the Hellenic subduction zone, and is characterised by a bathymetric trough exceeding 3000 m depth. Strike-slip faulting prevails in a large area to the east of the CTF reaching the western boundaries of the Gulf of Corinth (Kiratzi et al., 2008; Ganas et al., 2009b; Serpetsidaki et al., 2014; Briole et al., 2015; Chousianitis et al., 2015).

In terms of geology, the Ionian Islands are built with sedimentary rocks, mainly carbonates, belonging to the so-called external Hellenides (Jacobshagen 1979; Underhill 1989). In particular, the boundary between the two different geological zones, called Ionian and Paxos, runs through this region and outcrops onshore Lefkada, forming the noteworthy Ionian Thrust which is assumed to be the westernmost limit of External Hellenides chain (Lagios et al., 2007). The main part of the Lefkada Island is made of a carbonate sequence of the Ionian zone, while the SW part of the island consists of limestone of Paxos geological zone (Bornovas 1964; Pavlides et al., 2004).

This tectonic regime predetermines the high seismic activity in the area of Lefkada Island. The island was affected indeed by several significant events in the past (Table 1). The two most recent strong earthquakes are the M<sub>w</sub> = 6.2 2003 August 14 earthquake analyzed in this paper and the recent M<sub>w</sub> = 6.5 2015 November 17 earthquake (Ganas et al., 2016; Chousianitis et al., 2016, Sokos et al., 2016). Both earthquakes occurred offshore the western coast of Lefkada Island, causing severe damages around the whole island.

## 2. Seismicity in the area and Lefkada fault location

According to the historical documentation of the last centuries several strong earthquakes occurred since the 17th century in the area of Lefkada Island. During the instrumental era five events with magnitude higher than 6 were recorded: one in 1914, two in 1948 (Fig. 2, Table 1), the 2003 event studied here, and the recent earthquake of 2015.

Taking into account 23 microseismic events recorded with portable seismographs by Hatzfeld et al. (1995) during a 7-week experiment in the summer of 1989, available macroseismic data, as well as the bathymetry change to a NNE direction northern of Cephalonia Island, Louvari et al. (1999) identify an active seismic segment of the CTF offshore west of the coast of Lefkada. The authors suggest that this northern segment strikes in a NNE-SSW direction, dips to ESE, has strike-slip motion combined with a small thrust component (Table 2), and has a length of ~40 km starting from the northern part of the Cephalonia Island. They interpret the thrust component of the M = 5.8 1973 November 4 earthquake (Baker et al., 1997) as an indication of the junction of the Albanian thrust belt with the Lefkada segment at the northern end of the CTF. Pavlides et al. (2004) assumes a division of Lefkada segment into two smaller segments separated by Sesoula Islet, west of Lefkada, striking at different directions, NNE-
SSW and NE-SW respectively for northern and southern sub-segment and behaving independently.

### 3. The 2003 August 14 earthquake

On 2003 August 14 at 05:15 GMT a $M_w = 6.2$ earthquake occurred offshore the western coast of Lefkada Island causing considerable damages (Fig. 3) in several settlements, all harbours and the road network, as well as landslides, rock falls, soil liquefaction, subsidence, ground cracks, especially along the north-western part of the island (Katopodi and Iosifidou, 2004; Papathanassiou et al., 2005; Pavlides et al., 2004).

A maximum intensity of VIII in the town of Lefkada was observed, while elsewhere in the island the intensity ranged from V to VII (Papadopoulos et al., 2003; Papathanassiou et al., 2005). The character and location of the damages are similar to those related to the previous strong earthquakes of similar magnitude in the area. Compared to the past events, the intensity decreased from X-XI, possibly due to the strict construction rules established after the last destructive earthquake of 1948. Nevertheless, ~120 houses were destroyed almost half of the buildings of the island were damaged in different degree (Pavlides et al., 2004).

Up to 2003 August 31 more than 470 aftershocks ($M > 1.5$), concentrated in two clusters, were recorded by the Patras Seismological Laboratory (Fig. 4), the strongest of which ($M_s = 5.25$) occurred eleven hours after the main shock. Papadopoulos et al. (2003), Karakostas et al. (2004) and Papadimitriou et al. (2006) consider the southern cluster, located close to the north-western coast of Cephalonia Island, rather representing a triggered seismicity on the Cephalonia segment, due to stress increase from the main shock, than typical aftershock activity, and it has to be noted that the same area was also activated after the 2014 Cephalonia sequence. On the contrary, Benetatos et al. (2005; 2007), Zahradník et al. (2005), Saltogianni and Stiros (2015), Sokos et al. (2016) suggest a complicated fault geometry of Lefkada segment, with two main sub-events and segmentation of the rupture in a double cluster. According to this latter group of authors, the multiple source character of the main shock is represented by two main sub-events along a N12°E to N20°E (depending on the authors) line that ruptured Lefkada segment from north to south (Table 2, Fig. 4). The two sub-events are separated in time and space at 14 s and ~40 km, respectively, have strike-slip focal mechanisms and induced concentration of aftershocks in two clusters with mainly strike-slip character along the NE-SW trending fault segment (Fig. 4). Thrust and reverse faulting mechanisms also detected, mostly in the north and mainland parts of the island, indicate possible activation of faults in adjacent areas, as the Ionian thrust zone.

The present investigation is concentrated on the deformations in the area of the main shock (or first sub-event) and the following sequence of aftershocks on the Lefkada Island (Fig. 4).

### 4. InSAR processing and analysis

In the catalogue of the European Space Agency (ESA), for the period between January 2003 and January 2008, we found 13 ascending and 20 descending ENVISAT/ASAR (C-band) passes from track 458 and track 322 respectively, over the area of interest (Fig. 5). In both cases of satellite viewing, only one of these scenes is pre-seismic: 2003 March 21 for the
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ascending and 2003 June 25 for descending (Table 5). This limits the generation of co-seismic
pairs with only one master image.

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In spite of this limitation, the choice of the scenes used in our investigation is made in such a
way that the image pairs formed have perpendicular baselines less than 200 m with respect to
the pre-seismic image. This requirement is valid for six ascending images, which form 6
ascending co-seismic and 15 ascending post-seismic interferometric pairs (Fig. 6, Table 4 and
Table 5), and 8 descending scenes, forming 8 co-seismic and 23 post-seismic interferograms
(Table 4 and Table 5).

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Standard processing flow of differential (2-pass) method was followed to generate the interferograms, using the Jet Propulsion Laboratory/California Institute of Technology open source package ROI_PAC 3.0 (Rosen et al., 2004). The raw images (level 0) were provided by ESA as well as the Doris precise orbit files, used for removing the orbital effects. To eliminate the topographic effect, a simulated interferogram is generated from the SRTM digital elevation model (DEM) version 4 (Jarvis et al., 2008). This DEM has 90 m resolution and a nominal vertical error of less than 16 m. Nevertheless, thorough studies conducted by Mouratidis et al. (2010) indicate that the actual absolute and relative errors (standard deviation) are well below 10 m. Multilooking (4 looks in range and 20 looks in azimuth) was applied to minimize the loss of coherence and reduce the speckle noise in the produced interferogram.

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With this operation, the original pixel size of 7.8 m in range and 4.6 m in azimuth was averaged to 31.2 m and 91.4 m respectively. The Goldstein-Werner filter (Goldstein and Werner, 1998), optional in ROI_PAC package, was applied in the final stage of processing to improve the quality of the interferogram. The results are geo-referenced to the coordinate system of the used DEM (GCS/WGS84). Tacking into account the nominal accuracy of the DEM ($h_{DEM} = 16$ m) and the altitude of ambiguity ($h_a$) for each interferogram, the maximum expected error due to the topography is calculated (Table 4).

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A main problem in our interferogram processing is the fact that the ENVISAT/ASAR images
are C-band, so the presence of vegetation has a strong influence on the coherence. The whole
island is indeed dense vegetated mainly with olive groves and pine forests, especially to the
east and south. This is probably the reason for coherence loss in all produced interferograms
(see the Appendixes). To illustrate better the regions where the received signal is more
reliable, a map of coherence is produced (Fig. 7) on a basis of all processed ascending
interferograms. The value of the coherence for every pixel from the map is calculated first as
a mean between the values of its neighbouring in the respective image and second the results
of the corresponding pixels from all interferogram are averaged. In this way the reoccurrence
in time and space of stable backscattered signal for each pixel is characterized. The areas with
better scattering capabilities on the produced map correspond to the not overgrown Stavrota
mountain (in the centre of the island), Kalavros to the north, big settlements like town of
Lefkada and Preveza, and plain areas around Voulkaria Lake in mainland north-eastern from
the island.

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Examining the ascending data set, the interferogram which comprises the shortest time span
(174 days) and has only 35 m perpendicular baseline, namely 21.03.2003-12.09.2003
(interferogram No 1 – *ifg1* in Table 4), is expected to have the best quality. Lagios et al.
(2007) analyse exactly this interferometric pair and describe 1 fringe in the northernmost part
of the island and 2 fringes in the western part. However, in the current investigation a wider
set of data is studied and it is found that the interferogram *ifg5* (21.03.2003-5.11.2004), which covers a longer period (592 days) and has a larger perpendicular baseline (88 m), shows much better quality (Fig. 9 left) than the interferogram *ifg1* (APPENDIX A). The fringes of the 21.03.2003-5.11.2004 pair are more sharpened, while the interferogram pattern of pair *ifg1* is strongly deformed. A possible explanation about this difference could be the lower percentage of humidity and cloudiness during the acquisition of the slave image in the pair *ifg5* (Table 5), than the same weather characteristics in the day of the slave image of pair *ifg1*. Although, the local disturbances and the local temporal differences in humidity level have more important role in the phase de-correlation, even the homogeneous change in the atmosphere (pressure, humidity, and temperature) can be revealed by a contrasted relief, which modulates the thickness of the troposphere that the signal must cross (Massonnet and Feigl, 1998) since the topography induces tropospheric delays. In the case of Lefkada Island the elevation varies from sea level to 1160 m in the central part of the island, which is roughly the place where the strongest distortions occur. From the data of this study and other recent data over Lefkada Island, we estimate that tropospheric disturbances could be of ~1 C_band fringe (28 mm of delay) over the island.

No post-seismic deformation is observed in our interferograms (APPENDIX B) but we cannot exclude the presence of a small amount of such deformation as it has been observed elsewhere for strike-slip events of similar magnitude (e.g. Bürgmann et al., 1997).

The ascending co-seismic interferogram with the best quality (*ifg5*), covering the time period between 2003 March 21 and 2004 November 5 (Fig. 9 left), shows at least 2 fringes in the western part of the island. Since one fringe is equal to half the wavelength, which for ENVISAT C-band is 28 mm, then these two fringes are equal to 56 mm of deformation of the crust in the satellite line of sight (LoS). A verification in Table 4 shows that a very small part of detected fringes could be attributed to errors due to the topography – 4 mm, which is only 1/7 of the fringe.

In the case of descending co-seismic set of data, the deformations after the occurrence of the strong earthquake are detected with significantly lower quality, mainly because of the look direction of the satellite towards the highest mountains, so that the deformation pattern appeared distorted. The descending co-seismic interferogram with the best quality is the one with the smallest temporal separation (*ifg22* – Fig. 8) and has a perpendicular baseline of 118 m (Table 4).

## 5. Modelling

The modelling and interpretation of the interferograms is made in three successive phases: first we model the best ascending wrapped interferogram only, then we analyze the stacks of unwrapped ascending and descending interferograms, and finally we perform a joint inversion of ascending and descending data.

The fringes in the best ascending interferogram are sampled to 34 points which we found sufficient to account for the level of quality of the data (using more sample points leads to the same model). One value of slant range displacement is sampled every ~1 km (Fig. 9 left - inset). The descending interferograms are not used in this first analysis, due to their lower quality. The Harvard CMT solution is used as initial parameters in inversion procedure (Table 2) fixing the strike angle to 18° and dip angle to 59°. The applied inversion algorithm was
249 developed by Briole et al. (1986), it is based on the least-square minimization approach
250 proposed by Tarantola and Valette (1982). The Okada (1992) formulation for rectangular-
251 plane source in uniform elastic half-space is adopted for the dislocation source. The
252 parameters of the best fitting model (Fig. 9 right) are listed in Table 3. The epicentre is
253 located at the SW end of our modelled fault plane. If this is so the geodetic epicentre is offset
254 by about 6 km to the NE of the NOA epicentre.
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256 The fringes obtained in the synthetic interferogram are in good agreement with the observed
257 ones, evident also by the residuals between the phase values of the fringe points from the
258 model and the same points in the real interferogram. The value of the mean is -0.9 mm and
259 the root-mean-square (r.m.s.) residual is 2.3 mm i.e. only 1/12 part of a fringe. The
260 concordance of the proposed modelled fringe points with the observed ones is manifested by
261 the distribution of the residual (Fig. 10 left), which is close to the normal one. Additionally,
262 since the number of the points is relatively small, a normal probability plot is presented (Fig.
263 10 right). This plot shows some outliers in both ends of the data set but still confirms the
264 normal distribution of the residuals.
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265 In order to confirm the reliability of the proposed model of the fault, a series of inversions are
266 conducted with varying dip and length of the fault, taking into account the values proposed by
267 previous studies (Fig. 11). In both cases, while the different values for the dip and length are
268 tested, all the other parameters are left fixed. This investigation of the confidence of these
269 parameters shows that the best fitting model would be achieved at dip of 60° and length of the
270 faulting segment of 16 km. However, the r.m.s. difference for a model with dip of 60° and
271 59°, like the value proposed here, on the basis of Harvard CMT solution, is only 0.4 mm (1/70
272 of the fringe), which is not significant.
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274 The sensitivity of the model to the dip alteration is expressed by “pressing” of the fringe’s
275 shape. In the case investigated here, the mean part of the outer fringe is not clearly outlined.
276 To increase the quality of the interferometric pattern and to improve the fringe shape, an
277 interferogram averaged from all the six co-seismic ascending interferograms is produced. As a
278 first step all of them are unwrapped using the SNAPHU software (Chen and Zebker, 2002)
279 (APPENDIX A). The stacking of all co-seismic ascending interferograms and the backward
280 wrapping (Fig. 12d) of the phases increased in some aspect the quality of the single wrapped
281 picture (Fig. 12a). The resulting field of deformation of the single interferogram (Fig. 12b)
282 shows maximum displacement toward the satellite in the range of 60 mm. The stacking of the
283 available interferograms (Fig. 12c) narrows the area of deformations and decreases its value
284 to ~50 mm.
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286 When the re-wrapped interferogram is superimposed over the topography (Fig. 12e), high
287 dependency of the local relief is revealed. In spite of the more homogeneous field of
288 deformation which is represented by the unwrapped stacked interferogram compared to the
289 interferogram with the best quality, the averaged unwrapped pattern also combines all defects
290 of the interferograms used in the stacking. This is illustrated by cross-sections through the
291 unwrapped interferograms in Fig. 13. The A'-A" profile is traced parallel to the strike of the
292 fault and is presented in the upper part of the figure, while the B'-B" profile passes through
293 the island and represents the area with the most pronounced crust deformation (bottom of the
294 figure). The corresponding local relief is plotted to the bottom of the profiles.
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296 It is seen how a processing error during the unwrapping procedure (APPENDIX A) caused
297 distortion of the field of deformation of *igf4* showed in the profile by dotted line (UWRI4 –
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299 unwrapped real interferogram *ifg4*). If this interferogram is excluded of the averaging, the
300 middle part of the interferogram becomes more uniform (red line, UWSRI – unwrapped
301 stacked real interferogram), but the level of deformation in the outlying areas vary
302 significantly in the A’A” profile. The abrupt change of the deformation line appeared in a
303 place of a steep topographic depression at the boundary between the Ionian and Paxos
304 geological zones, which could influence strongly the unwrapping process. Nevertheless, the
305 averaged interferogram shows high level of concordance to the modelled one (MI in Fig. 13),
306 even better that the single interferogram used for the modelling (UWRI5). The received field
307 of crustal deformations, excluding the wrong *ifg4*, shows a maximum in the range of ~60 mm
308 towards the satellite (Fig. 12f). The shift between the real and the modelled data in the right
309 part of the A’A” profile is again a consequence of the unwrapping defect in the area of the
310 Ionian Thrust like in the case of *ifg4* but in the opposite direction. If the real data in this part
311 of the interferograms are shifted with the amount of exactly one fringe downward they will
312 correspond to the model. On the other hand, the left part of the B’B” profile shows a big
313 difference between the data and the model. Since the images used for producing these
314 interferograms are ascending, this area follows in the shadow of the high mountain in the
315 centre of the island looked from the satellite. Therefore, the signal in this area is in fact an
316 incoherent noise and the unwrapping procedure performs false information about the
317 deformation in this area. None conclusions can be made in this situation about the reliability
318 of the model in this area.
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320 In order to find another confirmation about the agreement between the fields of deformation
321 revealed by the interferograms and predicted by the model, a comparison on a pixel level is
322 performed. The examination of the residuals between the real and the synthetic deformation
323 interferograms (Fig. 14) displays a high degree of correspondence between both patterns,
324 which is confirmed also by the histogram of the residuals (Fig. 15). It manifests very close to
325 the normal distribution, shifted by the mean value of 3.4 mm. The r.m.s. for the entire
326 examined pattern is 10.5 mm.
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328 The stacking procedure is applied also to the descending set of data, so that the field of
329 deformation from the other point of satellite view could be gained. The stacked re-wrapped
330 interferogram shows one clear fringe (Fig. 16), but in the rest of the interferogram still high
331 level of noise presents (APPENDIX C).
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333 Nevertheless, the averaged descending unwrapped interferogram will be used, together with
334 the ascending result, in a theoretical determination of the horizontal and vertical components
335 of the field of deformations. To define two of the three components of the LoS displacement,
336 an assumption for the third one should be adopted. Here, the northern component is equalised
337 for both satellite viewings, thus the displacement in up and east direction (Fig. 17) is extracted
338 using the parameters of the satellite geometry (Hanssen, 2001). The resulting pattern shows a
339 maximum vertical displacement of the Earth’s crust in the range of ~50 mm in the western-
340 central part of the island and eastward displacements in the western coast in the maximum
341 range of ~60 mm.
342

343 Finally we make a model by jointly inverting ascending and descending data. The
344 combination of ascending and descending observations is potentially powerful to constrain
345 some of the fault parameters, in particular depth, location of the fault in the east-west axis,
346 and dip-slip component of motion. We use the two best interferograms (ascending *ifg5* and
347 descending *ifg22*, see Table 4), with the 34 picked values on two fringes visible on the
348 ascending interferogram (those already used before) and 20 picked values on the only clear
fringe of the descending one. The ratio 20/34 gives less weight to the descending data in the inversion. This is consistent with the fact that, given its lower coherence, we are less confident in this data than in the ascending one. Starting with the parameters of our preferred model (IL16, Table 3) we first leave free all fault parameters in the inversion. Only two of the fault parameters show significant changes: the depth of the upper edge of the fault that is estimated deeper, at 4.5 km instead of 2.5 km, and the east coordinate of the fault that moves westward in a correlated manner with the fault depth. The other parameters are not significantly affected by the addition of the descending data. Another benefit of the joint inversion of ascending and descending data is the fact that, because of the existence of two opposite LoS, it is possible to seek for an optimum in the common mode offset to apply to the picked value of the fringes. We tune the picked value so as to minimize the post-fit residuals at a minimum value of 9 mm, and Table 7 and Table 8 contain the location and observed and modelled value of our picked fringe points. In all inversions the component of dip-slip on the fault plane remains close to zero, ranging from -18 mm (reverse) when the upper fault depth was assigned to be 5 km to +21 mm (normal) when the upper fault depth was assigned to be 4 km. Table 9 contains the value of the two preferred models. In summary, the two main constraints provided by the combination of the ascending and descending data are the following: no significant component of dip-slip on the fault, an upper limit of the fault possible deeper (4.5 km instead of 2.5 km with ascending only).

## 6. Discussion and conclusions

We examine fault size, geometry and its relation to earthquake magnitude and how this relates to the historical seismicity record (Table 1). Our geodetic solution (Fig. 9) indicates that the rupture length is ~16 km, a value close to that predicted by the empirical relationship of Wells and Coppersmith (1994) (see Table 6), and much less than those predicted by the equations of Papazachos and Papazachou (1997) and Konstantinou et al. (2005). The 2003 Lefkada event occurred along one fault segment plane dipping to the east as suggested by the linear inversion of the InSAR fringes. The earthquake was followed by numerous aftershocks on neighbouring fault planes that are characterised as capable to produce moderate events (Karakostas and Papadimitriou, 2010). The geodetically determined fault size is about half the length of the Lefkada plate boundary segment defined by seismological data by Louvari et al. (1999). In addition, Papadopoulos et al. (2003) have suggested that offshore earthquakes near Lefkada follow the characteristic earthquake model, i.e. slip distribution and earthquake magnitude are roughly constant. Our results indicate that, the 2003 earthquake ruptured only the northern portion of the 50-km Lefkada segment of CTF, which is about 16 km long. Assuming that the Lefkada segment of CTF comprises two seismic segments then, this segmentation model predicts rupture lengths of the order of 15-20 km and maximum earthquake magnitudes in the range 6.2-6.5. This suggestion has important consequences for the seismic hazard of the area, as the earthquakes expected offshore Lefkada may reach $M_w = 6.5$ like the 2015 November 17 southern Lefkada earthquake (Ganas et al., 2016). For both the 2003 earthquake (this study; Fig. 9 right) and the 2015 one (Ganas et al. 2016; Fig. 4), InSAR constrains well the spatial extend of the rupture along the axis of the CTF. We infer from the two studies that a 10-15 gap remains between the 2003 and the 2015 fault planes. A gap of such length corresponds to a potential earthquake of $M_w \sim 5.7-6$. Some authors (Saltogianni and Stiros, 2015; Sokos et al., 2016) have suggested that the 2003 event also broke a fault segment south of the 2015 fault, in front of the northern Cephalonia, in a very unusual manner. We found no evidence of such southern segment in our 2003 interferograms. Moreover, this hypothetical jump of the 2003 rupture over the 2015 fault should also jump
over the 10-15 gap located between the 2003 and 2015 faults. This corresponds to a distance of ~40 km between the second hypothetical sub-event and the main shock of 2003, itself well modelled from the InSAR data and with a good agreement between geodetic and seismological moments.

The fault model we propose has a geodetic moment $M_o = 3.6 \ 10^{18}$ N m which corresponds to $M_w = 6.3$, greater than the seismic moment reported by the Harvard CMT solution which is $M_o = 2.98 \ 10^{18}$ N m or $M_w = 6.2$. The difference might be due to the influence of the two larger shallow aftershocks, that occurred the same day as the main shock, with $M_w = 5.1$ and $M_w = 5.5$ respectively.

The interferometric pattern, produced by ENVISAR/ASAR images, defines field of deformations caused by the 2003 Lefkada earthquake, in the range of 60-70 mm in the line of sight of the satellite. An attempt to determine the horizontal and vertical component of displacement is performed. An uplift of about 50 mm in the central area of deformation is calculated and deformations with eastward direction in the range of 60 mm in the western coast of the island are estimated.

The 2003 fault does not reach the surface. Its upper tip is located at a depth comprised between 2.5 km (using ascending data only) and 4.5 km (using a combination of ascending and descending). This is much deeper that the depth of the upper tip of the 2015 fault, estimated to be ~1 km around the northern end of the fault according to Ganas et al. (2016). The mode and timing of the deformation between that depth and the surface requires further analysis. Is the uppermost part of the crust deforming continuously during the seismic cycle in that segment or is there a possibility of very shallow and strong earthquakes? And why is the 2015 rupture shallower? In addition to SAR interferometry (which is insufficient for sampling long time periods in areas of poor coherence), dense and permanent GPS will be important to decipher the spatio-temporal behaviour of the uppermost crust along the Cephalonia Transform Fault. The combination of ascending and descending InSAR observations indicates that there is no significant component of dip-slip on the fault of this 2003 earthquake.

## Acknowledgements

The ENVISAT/ASAR images used in the present study were provided by the European Space Agency in the framework of the Category-1 project ID 4484: "Investigation of the south-eastern part of the Sofia complex graben (Bulgaria) by InSAR and GPS methods with comparative analysis with the Lefkada Island (Greece)". The study was realised in the framework of the bilateral project RILA (3/29, 2007-2008) between the Ministry of Education, Youth and Science of Bulgaria and Ministry of National Education, France. The Generic Mapping Tools system (Wessel and Smith, 1998) was used to plot some of the figures. The authors would like to thank Hélène Lyon-Caen, Marie-Pierre Doin, Efthimios Sokos, Krassimir Matev and Dragan Chobanov for their invaluable help. We thank Ziyadin Çakir for his review and comments.

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<figure><img src="image_2.png" /></figure>

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<caption>Fig. 1. Left: Seismicity (M>4) in the Eastern Mediterranean since 1973 according to USGS. Rectangle – the region of Central Ionian Islands. Right: Seismicity in the area of the Central Ionian Islands. Fault geometry is adapted from Barrier et al. (2004).</caption>

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<figure><img src="image_3.png" /></figure>

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<caption>Fig. 2. Historical seismicity in the area of Lefkada Island according to Table 1. In inset the location of the Lefkada Island in Eastern Mediterranean. In grey, the mechanisms of strong historical events are marked, while in black – the mechanisms of most recent earthquakes even though they were of smaller magnitudes. The mechanisms of the 2003 and 2015 earthquakes are plotted in red.</caption>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Map of ground failures caused by the 14th August 2003 earthquake (after Pavlides et al., 2004; Papathanassiou and Pavlides, 2007). On the background simplified geological map of the Lefkada Island (Geological map of Greece 1963).</figcaption></figure>
<figure><img src="image_5.png" /></figure>

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Fig. 4. The 14th August 2003 Lefkada earthquake from different sources (Table 2) – the notation is the same as the references notations in the table. Other fault plane solutions (Table 3) are also shown (the notation is the same as the references notations in the table): LOU99 – typical focal mechanism obtained by waveform modelling of micro-earthquakes; PD02 – stress evolutionary model; LAG07 – InSAR forward modelling; HOL08 – Okada model based on co-seismic GPS measurements. Aftershock sequence from PSL is introduced with coloured circles according to the depth. In the present investigation the Harvard solution for the 2003 event (in red – HRV03) is taken as a basic in inversion process. The final solution of our study is shown in brown (IL16) referred to the centre of the fault predicted by the fault model. The Harvard solution for the 2015 event is shown (HRV15) with relocated epicentre and modelled fault plane (red rectangle) from Ganas et al. (2016).
<figure><img src="image_6.png" /><figcaption>Fig. 5. ENVISAT/ASAR ascending footprint, track 458, frame 774, represented by red dashed rectangle, descending footprint, track 322, frame 2821 – by blue dashed rectangle. Location of 14th August 2003 earthquake from NOA is shown with red star and aftershock sequence by PSL with red dots. The long red arrow represents the satellite flight direction and short arrow is the satellite looking direction (LOS); black rectangle – the area of investigation presented in the next figures; orange dashed line – generalized view of Cephalonia Fault Zone; isobaths – generated from ETOPO1 (Amante and Eakins, 2009); topography – SRTM v4.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

Fig. 6. Available ascending scenes covering the Lefkada Island: with the red line – time of the 14th August 2003 earthquake, with grey lines – the generated interferograms, with blue line – couple with the best quality (ifg5: 21.03.2003 – 05.11.2004).

<figure><img src="image_8.png" /></figure>

Fig. 7. Coherence map generated from 21 ascending interferograms, showing the mean level of coherence on the territory of the island. The parts which return better signals are in the central and northern part of the area as well as in the closest mainland.
Lefkada earthquake: descending scenes, Track 322, ENVISAT

<figure><img src="image_9.png" /></figure>

Fig. 8. Available descending scenes covering the Lefkada Island: with the red line – time of the 14th August 2003 earthquake, with grey lines – the generated interferograms, with blue line – couple with the best quality (ifg22: 25.06.2003 – 14.07.2004).

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<figure><img src="image_10.png" /></figure>

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Fig. 9. SAR interferometry results for the Lefkada earthquake. Left: Differential ascending interferogram – *ifg5* (BL 88m). The star shows the main shock according to NOA; the arrows – direction of ascending flight and side of looking of the satellite. An approximation of the Cephalonia Fault Zone is presented by white dashed line. Inset: The observed fringes. Right: Synthetic ascending interferogram obtained from inversion of the received observed fringes (left). The rectangle represents the approximate model of the fault segment.

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<figure><img src="image_11.png" /></figure>

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Fig. 10. Left: Residual distribution for the 34 points chosen from the observed fringes. Right: Normal probability plot for the same residual values.
<figure><img src="image_12.png" /><figcaption>Fig. 11. Dip and length confidence study by comparison with the values proposed by previous studies. The notations are the same as for Table 2 and Table 3. To the right: rms misfits plot for the dip and the length respectively, inverted by the InSAR data. All other parameters are fixed (see the left of the figure)</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 12. Stacking of ascending interferograms. (a) The ascending interferogram with the best quality (ifg5). (b) The same but unwrapped. (c) Average of the six ascending co-seismic unwrapped interferograms. (d) Re-wrapped stacked interferogram and draped (e) over the *Google Earth* topography of the island. (f) Stacking of five unwrapped interferograms (without ifg4).</figcaption></figure>

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<figure><img src="image_14.png" /><figcaption>Fig. 13. Unwrapped phases over the profiles A' – A" (up) and B' – B" (down) (see Fig. 12).</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 14. Residual interferometric pattern (right) compared to the real (left) and synthetic (centre) ascending interferograms</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 15. Pixel residuals distribution</figcaption></figure>
<figure><img src="image_17.png" /></figure>

a)

d)

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

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<figure><img src="image_20.png" /></figure>

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<caption>Fig. 16. Stacking of the descending interferograms. (a) The descending interferogram with the best quality (ifg22). (b) The same but unwrapped. (c) Average of the eight descending co-seismic unwrapped interferograms. (d) Re-wrapped stacked descending interferogram.</caption>
<figure><img src="image_21.png" /><figcaption>Fig. 17. Theoretical displacements in Up and East directions.</figcaption></figure>
<table><thead><tr><th>Date (Y.M.D)</th><th>lat (°)</th><th>long (°)</th><th>M</th><th>depth (km)</th><th>Io</th><th>strike (°)</th><th>dip (°)</th><th>rake (°)</th><th>Mo (* 10<sup>18</sup> Nm)</th><th>length (km)</th><th>slip (cm)</th><th>Ref.*</th></tr></thead><tbody><tr><td>1612.05.26</td><td>38.8</td><td>20.6</td><td>6.6</td><td></td><td>VIII</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1613.10.12</td><td>38.8</td><td>20.8</td><td>6.4</td><td></td><td>VIII</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1625.06.28</td><td>38.8</td><td>20.7</td><td>6.6</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1630.07.02</td><td>38.8</td><td>20.8</td><td>6.7</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>4 in 3</td></tr><tr><td>1704.11.22</td><td>38.8</td><td>20.7</td><td>6.3</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1722.06.05</td><td>38.6</td><td>20.7</td><td>6.4</td><td></td><td>VIII</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1723.02.22</td><td>38.6</td><td>20.7</td><td>6.7</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1769.10.12</td><td>38.8</td><td>20.6</td><td>6.7</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1783.03.23</td><td>38.7</td><td>20.61</td><td>6.7</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1815.03.17</td><td>38.8</td><td>20.7</td><td>6.3</td><td></td><td>VIII</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1820.02.21</td><td>38.8</td><td>20.6</td><td>6.4</td><td></td><td>IX</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1825.01.19</td><td>38.7</td><td>20.6</td><td>6.5</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2 in 1</td></tr><tr><td>1869.12.28</td><td>38.80</td><td>20.65</td><td>6.4</td><td>3-15</td><td>X</td><td>1</td><td>60</td><td>165</td><td></td><td>26</td><td>35</td><td>4 in 5</td></tr><tr><td>1914.11.27</td><td>38.65</td><td>20.62</td><td>6.3</td><td>3-15</td><td>IX</td><td>16</td><td>60</td><td>165</td><td></td><td>23</td><td>29</td><td>6 in 5</td></tr><tr><td>1948.04.22</td><td>38.68</td><td>20.57</td><td>6.5</td><td>3-15</td><td>IX</td><td>1</td><td>60</td><td>165</td><td></td><td>30</td><td>42</td><td>4 in 5</td></tr><tr><td>1948.06.30</td><td>38.80</td><td>20.60</td><td>6.4</td><td>3-15</td><td>IX</td><td>1</td><td>60</td><td>165</td><td></td><td>26</td><td>35</td><td>4 in 5</td></tr><tr><td>1973.11.04</td><td>38.89</td><td>20.44</td><td>5.8</td><td>23</td><td>VII+</td><td>324</td><td>50</td><td>81</td><td>0.61</td><td></td><td></td><td>8 in 7</td></tr><tr><td>1994.02.25</td><td>38.76</td><td>20.54</td><td>5.5</td><td>9</td><td></td><td>22</td><td>58</td><td>168</td><td>0.215</td><td></td><td></td><td>7</td></tr></tbody></table>

<caption>**Table 1.** Historical and instrumental seismicity review. References: 1 – Papathanassiou et al. (2005); 2 – Papazachos et al. (2000); 3 – Fokaefs and Papadopoulos (2004); 4 – Papazachos and Papazachou (1997); 5 – Papadimitriou (2002); 6 – Papazachos et al. (1998); 7 – Louvari et al. (1999); 8 – Baker et al. (1997). The intensity values are according to Papathanassiou et al. (2005) except the event of 1630, whose value is taken from Fokaefs and Papadopoulos (2004).</caption>
<table><caption>**Table 2.** Published source parameters for the 2003 August 14 Lefkada earthquake (compared also in Fig. 4). ms – main shock, s1 – sub-event 1, s2 – sub-event 2, av – averaged.</caption><thead><tr><th>Ref.*</th><th>lat<br>(°)</th><th>long<br>(°)</th><th>Mw</th><th>dept<br>h<br>(km)</th><th>strike<br>(°)</th><th>dip<br>(°)</th><th>rake<br>(°)</th><th>lengt<br>h<br>(km)</th><th>width<br>(km)</th><th>Mo (*<br>10<sup>18</sup><br>Nm)</th><th>Slip<br>(cm)</th></tr></thead><tbody><tr><td>HRV</td><td>38.70</td><td>20.67</td><td>6.2</td><td>15</td><td>18</td><td>59</td><td>-174</td><td>-</td><td>-</td><td>2.98</td><td>-</td></tr><tr><td>USGS</td><td>39.16</td><td>20.60</td><td>6.1</td><td>21</td><td>13</td><td>84</td><td>172</td><td>-</td><td>-</td><td>2.0</td><td>-</td></tr><tr><td>MN</td><td>38.88</td><td>20.62</td><td>6.2</td><td>24</td><td>196</td><td>85</td><td>-166</td><td>-</td><td>-</td><td>2.6</td><td>-</td></tr><tr><td>ETHZ</td><td>39.24</td><td>20.49</td><td>6.32</td><td>31</td><td>97</td><td>63</td><td>-30</td><td>-</td><td>-</td><td>3.72</td><td>-</td></tr><tr><td>NOA</td><td>38.79</td><td>20.56</td><td>6.4</td><td>12</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>PSL</td><td>38.83</td><td>20.62</td><td>ML<br>5.85</td><td>15.5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>AUT</td><td>38.74</td><td>20.54</td><td>6.3</td><td>11</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>ZH05: ms</td><td>38.84</td><td>20.60</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ZH05_1 s1</td><td>38.78</td><td>20.57</td><td>5.7</td><td>13</td><td>17</td><td>88</td><td>-177</td><td>18</td><td>9</td><td>0.47</td><td>-</td></tr><tr><td>ZH05_2 s2</td><td>38.46</td><td>20.43</td><td>5.8</td><td>13</td><td>24</td><td>74</td><td>164</td><td>-</td><td>-</td><td>0.49</td><td>-</td></tr><tr><td>BT07: av</td><td></td><td></td><td>6.2</td><td>13</td><td>16</td><td>72</td><td>178</td><td>23</td><td>-</td><td>2.23</td><td>42</td></tr><tr><td>BT07_1 s1</td><td>38.78</td><td>20.57</td><td>6.0</td><td>11</td><td>12</td><td>81</td><td>174</td><td>17</td><td>10</td><td>1.3</td><td>34</td></tr><tr><td>BT07_2 s2</td><td>38.50</td><td>20.44</td><td>5.8</td><td>15</td><td>20</td><td>63</td><td>-179</td><td>13</td><td>10</td><td>0.65</td><td>23</td></tr><tr><td>PD06</td><td>38.86</td><td>20.56</td><td>6.3</td><td>9</td><td>15</td><td>80</td><td>170</td><td>24</td><td>2-12</td><td>2.9</td><td>40</td></tr></tbody></table>

* HRV – CMT Harvard; USGS – U.S. Geological Survey; MN – MedNet (Mediterranean Very Broadband Seismographic Network – INGV) in Zahradník et al. (2005); ETHZ – Swiss Federal Institute of Technology Zurich; NOA – National Observatory of Athens, Institute of Geodynamics; PSL – Patras Seismological Laboratory; AUT – Aristotle University of Thessaloniki; ZH05_1 and ZH05_2 – 1 and 2 sub-events in Zahradník et al. (2005); BT07_1 and BT07_2 – 1 and 2 sub-events in Benetatos et al. (2007); PD06 – Papadimitriou et al. (2006).

<table><caption>**Table 3.** Other fault plane solutions for Lefkada Segment of Cephalonia Fault Zone (compared also in Fig. 4). The coordinates are for the centre of the fault. * estimated here; ⚠ - after Papazachos et al. (1998); # - centre of the fault; L – length; W – width; LOU99 – Louvari et al. (1999); PD02 – Papadimitriou (2002); KK04 – Karakostas et al. (2004); LAG07 – Lagios et al. (2007); HOL08 – Hollenstein et al. (2008).</caption><thead><tr><th>Ref.</th><th>lat<br>(°)</th><th>long<br>(°)</th><th>Mw</th><th>dept<br>h<br>(km)</th><th>strik<br>e (°)</th><th>dip<br>(°)</th><th>rake<br>(°)</th><th>L<br>(km)</th><th>W<br>(k<br>m)</th><th>Mo (*<br>10<sup>18</sup><br>Nm)</th><th>slip<br>(cm)</th></tr></thead><tbody><tr><td>LOU99</td><td>-</td><td>-</td><td>-</td><td>-</td><td>14</td><td>65</td><td>167</td><td>40</td><td>15</td><td>-</td><td>-</td></tr><tr><td>PD02 ⚠</td><td>38.71</td><td>20.58</td><td>-</td><td>15</td><td>11</td><td>60</td><td>+165</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>KK04</td><td>-</td><td>-</td><td>-</td><td>3-12</td><td>-</td><td>-</td><td>-</td><td>16</td><td>-</td><td>-</td><td>~60</td></tr><tr><td>LAG07</td><td>-</td><td>-</td><td>-</td><td>13</td><td>15</td><td>80</td><td>174*</td><td>12</td><td>11</td><td>-</td><td>-45</td></tr><tr><td>HOL08</td><td>38.60</td><td>20.50</td><td>6.3*</td><td>7</td><td>15</td><td>80</td><td>180*</td><td>60</td><td>10</td><td>2.97</td><td>16.5</td></tr><tr><td><strong>model in this<br>study (IL16)</strong></td><td><strong>38.83</strong></td><td><strong>20.64#</strong></td><td><strong>6.3</strong></td><td><strong>6.8 #</strong></td><td><strong>18</strong></td><td><strong>59</strong></td><td><strong>180</strong></td><td><strong>16</strong></td><td><strong>10</strong></td><td><strong>3.60</strong></td><td><strong>75</strong></td></tr><tr><td><strong>Uncertainties<br>of our model</strong></td><td><strong>0.01</strong></td><td><strong>0.01</strong></td><td></td><td><strong>1.</strong></td><td><strong>3.</strong></td><td><strong>5.</strong></td><td><strong>8.</strong></td><td><strong>2.</strong></td><td><strong>2.</strong></td><td></td><td><strong>15</strong></td></tr></tbody></table>
<table><caption>**Table 4.** Details for the ascending (A) and descending (D) interferograms processed for this study, where Ni – number of interferogram, B – perpendicular baseline (m), Days – offset between acquisition of the two images; h<sub>a</sub> – altitude of ambiguity; e<sup>h</sup><sub>max</sub> – maximum error due to the topography. The bold lines show the best ascending (A) and descending (D) interferograms.</caption><thead><tr><th>Ni</th><th>Pass</th><th>Date 1</th><th>Date 2</th><th>Days</th><th>B<sub>⊥</sub> (mm)</th><th>h<sub>a</sub> (m)</th><th>e<sup>h</sup><sub>max</sub>(mm)</th></tr></thead><tbody><tr><td>ifg 1</td><td>A</td><td>21.03.2003</td><td>12.09.2003</td><td>174</td><td>-35</td><td>268</td><td>2</td></tr><tr><td>ifg 2</td><td>A</td><td>21.03.2003</td><td>05.03.2004</td><td>349</td><td>-209</td><td>45</td><td>10</td></tr><tr><td>ifg 3</td><td>A</td><td>21.03.2003</td><td>18.06.2004</td><td>453</td><td>-87</td><td>108</td><td>4</td></tr><tr><td>ifg 4</td><td>A</td><td>21.03.2003</td><td>23.07.2004</td><td>489</td><td>-104</td><td>90</td><td>5</td></tr><tr><td><b>ifg 5</b></td><td><b>A</b></td><td><b>21.03.2003</b></td><td><b>05.11.2004</b></td><td><b>592</b></td><td><b>-88</b></td><td><b>107</b></td><td><b>4</b></td></tr><tr><td>ifg 6</td><td>A</td><td>21.03.2003</td><td>23.02.2007</td><td>1432</td><td>-177</td><td>53</td><td>8</td></tr><tr><td>ifg 7</td><td>A</td><td>12.09.2003</td><td>05.03.2004</td><td>175</td><td>-179</td><td>52</td><td>9</td></tr><tr><td>ifg 8</td><td>A</td><td>12.09.2003</td><td>18.06.2004</td><td>280</td><td>-56</td><td>167</td><td>3</td></tr><tr><td>ifg 9</td><td>A</td><td>12.09.2003</td><td>23.07.2004</td><td>315</td><td>-70</td><td>134</td><td>3</td></tr><tr><td>ifg 10</td><td>A</td><td>12.09.2003</td><td>05.11.2004</td><td>419</td><td>-57</td><td>165</td><td>3</td></tr><tr><td>ifg 11</td><td>A</td><td>12.09.2003</td><td>23.02.2007</td><td>1258</td><td>-146</td><td>64</td><td>7</td></tr><tr><td>ifg 12</td><td>A</td><td>05.03.2004</td><td>18.06.2004</td><td>104</td><td>122</td><td>77</td><td>6</td></tr><tr><td>ifg 13</td><td>A</td><td>05.03.2004</td><td>23.07.2004</td><td>140</td><td>109</td><td>86</td><td>5</td></tr><tr><td>ifg 14</td><td>A</td><td>05.03.2004</td><td>05.11.2004</td><td>243</td><td>121</td><td>77</td><td>6</td></tr><tr><td>ifg 15</td><td>A</td><td>05.03.2004</td><td>23.02.2007</td><td>1082</td><td>32</td><td>293</td><td>2</td></tr><tr><td>ifg 16</td><td>A</td><td>18.06.2004</td><td>23.07.2004</td><td>35</td><td>-18</td><td>521</td><td>1</td></tr><tr><td>ifg 17</td><td>A</td><td>18.06.2004</td><td>05.11.2004</td><td>139</td><td>-6</td><td>1563</td><td>0</td></tr><tr><td>ifg 18</td><td>A</td><td>18.06.2004</td><td>23.02.2007</td><td>978</td><td>-92</td><td>102</td><td>4</td></tr><tr><td>ifg 19</td><td>A</td><td>23.07.2004</td><td>05.11.2004</td><td>104</td><td>18</td><td>521</td><td>1</td></tr><tr><td>ifg 20</td><td>A</td><td>23.07.2004</td><td>23.02.2007</td><td>943</td><td>-77</td><td>122</td><td>4</td></tr><tr><td>ifg 21</td><td>A</td><td>05.11.2004</td><td>23.02.2007</td><td>839</td><td>-92</td><td>102</td><td>4</td></tr><tr><td><b>ifg 22</b></td><td><b>D</b></td><td><b>25.06.2003</b></td><td><b>14.07.2004</b></td><td><b>385</b></td><td><b>-118</b></td><td><b>79</b></td><td><b>6</b></td></tr><tr><td>ifg 23</td><td>D</td><td>25.06.2003</td><td>05.01.2005</td><td>560</td><td>-300</td><td>31</td><td>14</td></tr><tr><td>ifg 24</td><td>D</td><td>25.06.2003</td><td>25.05.2005</td><td>700</td><td>-201</td><td>47</td><td>9</td></tr><tr><td>ifg 25</td><td>D</td><td>25.06.2003</td><td>01.03.2006</td><td>980</td><td>-48</td><td>195</td><td>2</td></tr><tr><td>ifg 26</td><td>D</td><td>25.06.2003</td><td>14.06.2006</td><td>1085</td><td>-154</td><td>61</td><td>7</td></tr><tr><td>ifg 27</td><td>D</td><td>25.06.2003</td><td>14.02.2007</td><td>1330</td><td>79</td><td>119</td><td>4</td></tr><tr><td>ifg 28</td><td>D</td><td>25.06.2003</td><td>08.08.2007</td><td>1505</td><td>-45</td><td>208</td><td>2</td></tr><tr><td>ifg 29</td><td>D</td><td>25.06.2003</td><td>26.12.2007</td><td>1645</td><td>212</td><td>44</td><td>10</td></tr></tbody></table>
<table><caption>**Table 5.** ENVISAT images used in the research and the weather conditions during their acquisitions according to the weather archives for Aktion Airport (38°37′N 20°46′E) (http://meteo.infospace.ru; http://www.tutiempo.net). The bold lines correspond to the dates of acquisition of the images used for the best ascending and descending interferograms.</caption><thead><tr><th>Scene date</th><th>Pass</th><th>Mean temperature (°C)</th><th>Mean pressure at sea level (hPa)</th><th>Mean humidity (%)</th><th>Wind (m/s)</th><th>Cloudiness (%)</th><th>Cloud base altitude (m)</th><th>Precipitation</th></tr></thead><tbody><tr><td><strong>21.03.2003</strong></td><td><strong>A</strong></td><td><strong>+10.6</strong></td><td><strong>1016</strong></td><td><strong>74</strong></td><td><strong>4</strong></td><td><strong>20</strong></td><td><strong>1750-3000</strong></td><td><strong>None</strong></td></tr><tr><td>12.09.2003</td><td>A</td><td>+22.6</td><td>1013</td><td>75</td><td>5</td><td>40</td><td>800</td><td>None</td></tr><tr><td>05.03.2004</td><td>A</td><td>+9.4</td><td>1014</td><td>54</td><td>6</td><td>80</td><td>800-2250</td><td>None</td></tr><tr><td>18.06.2004</td><td>A</td><td>+22.8</td><td>1004</td><td>72</td><td>3</td><td>50</td><td>800</td><td>Rain or Drizzle</td></tr><tr><td>23.07.2004</td><td>A</td><td>+27.3</td><td>1014</td><td>71</td><td>3</td><td>10</td><td>3000</td><td>None</td></tr><tr><td><strong>05.11.2004</strong></td><td><strong>A</strong></td><td><strong>+18.1</strong></td><td><strong>1018</strong></td><td><strong>54</strong></td><td><strong>8</strong></td><td><strong>10</strong></td><td><strong>800-2250</strong></td><td><strong>None</strong></td></tr><tr><td>23.02.2007</td><td>A</td><td>+11.8</td><td>1011</td><td>90</td><td>3</td><td>30</td><td>450-2250</td><td>None</td></tr><tr><td><strong>25.06.2003</strong></td><td><strong>D</strong></td><td><strong>+26.6</strong></td><td><strong>1014</strong></td><td><strong>78</strong></td><td><strong>3</strong></td><td><strong>20</strong></td><td><strong>1750-3000</strong></td><td><strong>None</strong></td></tr><tr><td><strong>14.07.2004</strong></td><td><strong>D</strong></td><td><strong>+23.3</strong></td><td><strong>1011</strong></td><td><strong>57</strong></td><td><strong>6</strong></td><td><strong>20</strong></td><td><strong>800</strong></td><td><strong>None</strong></td></tr><tr><td>05.01.2005</td><td>D</td><td>+7.4</td><td>1028</td><td>75</td><td>1</td><td>30</td><td>3000</td><td>None</td></tr><tr><td>25.05.2005</td><td>D</td><td>+20.4</td><td>1014</td><td>85</td><td>1</td><td>30</td><td>800</td><td>None</td></tr><tr><td>01.03.2006</td><td>D</td><td>+11.6</td><td>1013</td><td>75</td><td>0</td><td>70</td><td>800</td><td>Mist</td></tr><tr><td>14.06.2006</td><td>D</td><td>+19.3</td><td>1019</td><td>80</td><td>5</td><td>20</td><td>800</td><td>None</td></tr><tr><td>14.02.2007</td><td>D</td><td>+12.1</td><td>1009</td><td>72</td><td>6</td><td>40</td><td>800</td><td>27 mm</td></tr><tr><td>08.08.2007</td><td>D</td><td>+24.3</td><td>1013</td><td>82</td><td>3</td><td>20</td><td>450-3000</td><td>None</td></tr><tr><td>26.12.2007</td><td>D</td><td>+10.0</td><td>1025</td><td>94</td><td>3</td><td>50</td><td>250-800</td><td>15 mm, Fog, Rain and Drizzle</td></tr></tbody></table>

<table><thead><tr><th>Source of empirical relationship</th><th>L (Surface rupture length) - km</th><th>W (Down-dip rupture width) - km</th><th>U (Average displacement) - cm</th></tr></thead><tbody><tr><td>Konstantinou et al. (2005)</td><td>log L = -1.49 + 0.47 M<sub>w</sub><br>L = 29</td><td>log W = -1.07 + 0.34 M<sub>w</sub><br>W = 11</td><td></td></tr><tr><td>Papazachos & Papazachou (1997)</td><td>log L = -1.85 + 0.51 M<sub>w</sub><br>L = 23</td><td>log W = -0.13 + 0.19 M<sub>w</sub><br>W = 12</td><td>log u = -3.71 + 0.82 M<sub>w</sub><br>u = 24</td></tr><tr><td>Well & Coppersmith (1994)</td><td>log L = -3.55 + 0.74 M<sub>w</sub><br>L = 13</td><td>log W = -0.76 + 0.27 M<sub>w</sub><br>W = 9</td><td></td></tr><tr><td><strong>this study</strong></td><td><strong>L = 16</strong></td><td><strong>W = 10</strong></td><td><strong>u = 75</strong></td></tr></tbody></table>
<table><caption>Table 7. The 34 picked values on the two visible ascending fringes in the best ascending interferogram. The origin for the LoS was estimated during the joint inversion process. The coordinates are also given in UTM 34.</caption><thead><tr><th>Long (°)</th><th>Lat (°)</th><th>UTM_E (km)</th><th>UTM_N (km)</th><th>LoS observed (mm)</th><th>LoS modelled (mm)</th></tr></thead><tbody><tr><td>20.5765</td><td>38.6244</td><td>463.16</td><td>4275.20</td><td>24.4</td><td>7.2</td></tr><tr><td>20.5793</td><td>38.6332</td><td>463.41</td><td>4276.17</td><td>24.4</td><td>8.4</td></tr><tr><td>20.5853</td><td>38.6428</td><td>463.93</td><td>4277.24</td><td>24.4</td><td>10.2</td></tr><tr><td>20.5874</td><td>38.6558</td><td>464.11</td><td>4278.67</td><td>24.4</td><td>12.8</td></tr><tr><td>20.5884</td><td>38.6646</td><td>464.20</td><td>4279.64</td><td>24.4</td><td>14.9</td></tr><tr><td>20.5891</td><td>38.6709</td><td>464.26</td><td>4280.34</td><td>24.4</td><td>16.7</td></tr><tr><td>20.5909</td><td>38.6794</td><td>464.42</td><td>4281.29</td><td>24.4</td><td>19.6</td></tr><tr><td>20.5916</td><td>38.6844</td><td>464.48</td><td>4281.83</td><td>24.4</td><td>21.4</td></tr><tr><td>20.5926</td><td>38.6929</td><td>464.57</td><td>4282.78</td><td>24.4</td><td>24.9</td></tr><tr><td>20.5975</td><td>38.6984</td><td>465.00</td><td>4283.39</td><td>24.4</td><td>27.8</td></tr><tr><td>20.6084</td><td>38.7001</td><td>465.96</td><td>4283.57</td><td>24.4</td><td>28.9</td></tr><tr><td>20.6236</td><td>38.7042</td><td>467.28</td><td>4284.03</td><td>24.4</td><td>29.9</td></tr><tr><td>20.6335</td><td>38.7097</td><td>468.15</td><td>4284.64</td><td>24.4</td><td>30.8</td></tr><tr><td>20.6415</td><td>38.7166</td><td>468.85</td><td>4285.40</td><td>24.4</td><td>31.8</td></tr><tr><td>20.6493</td><td>38.7246</td><td>469.53</td><td>4286.28</td><td>24.4</td><td>32.5</td></tr><tr><td>20.6524</td><td>38.7345</td><td>469.81</td><td>4287.38</td><td>24.4</td><td>34.5</td></tr><tr><td>20.6588</td><td>38.7433</td><td>470.36</td><td>4288.35</td><td>24.4</td><td>33.8</td></tr><tr><td>20.6602</td><td>38.7524</td><td>470.49</td><td>4289.35</td><td>24.4</td><td>34.5</td></tr><tr><td>20.6630</td><td>38.7601</td><td>470.73</td><td>4290.21</td><td>24.4</td><td>33.6</td></tr><tr><td>20.6637</td><td>38.7692</td><td>470.79</td><td>4291.21</td><td>24.4</td><td>32.9</td></tr><tr><td>20.6608</td><td>38.7758</td><td>470.54</td><td>4291.94</td><td>24.4</td><td>33.5</td></tr><tr><td>20.6584</td><td>38.7857</td><td>470.33</td><td>4293.04</td><td>24.4</td><td>32.4</td></tr><tr><td>20.6516</td><td>38.7934</td><td>469.74</td><td>4293.89</td><td>24.4</td><td>32.9</td></tr><tr><td>20.5889</td><td>38.7416</td><td>464.25</td><td>4288.16</td><td>52.4</td><td>53.1</td></tr><tr><td>20.5971</td><td>38.7367</td><td>464.96</td><td>4287.61</td><td>52.4</td><td>50.9</td></tr><tr><td>20.6084</td><td>38.7347</td><td>465.95</td><td>4287.40</td><td>52.4</td><td>49.3</td></tr><tr><td>20.6193</td><td>38.7356</td><td>466.91</td><td>4287.50</td><td>52.4</td><td>47.6</td></tr><tr><td>20.6316</td><td>38.7370</td><td>467.99</td><td>4287.65</td><td>52.4</td><td>44.1</td></tr><tr><td>20.6351</td><td>38.7419</td><td>468.29</td><td>4288.20</td><td>52.4</td><td>44.7</td></tr><tr><td>20.6394</td><td>38.7469</td><td>468.66</td><td>4288.74</td><td>52.4</td><td>44.2</td></tr><tr><td>20.6397</td><td>38.7521</td><td>468.69</td><td>4289.32</td><td>52.4</td><td>45.3</td></tr><tr><td>20.6383</td><td>38.7593</td><td>468.57</td><td>4290.11</td><td>52.4</td><td>47.2</td></tr><tr><td>20.6354</td><td>38.7686</td><td>468.32</td><td>4291.15</td><td>52.4</td><td>48.8</td></tr><tr><td>20.6294</td><td>38.7799</td><td>467.79</td><td>4292.39</td><td>52.4</td><td>49.7</td></tr></tbody></table>

754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
<table><thead><tr><th>Long (°)</th><th>Lat (°)</th><th>UTM_E (km)</th><th>UTM_N (km)</th><th>LoS observed (mm)</th><th>LoS modelled (mm)</th></tr></thead><tbody><tr><td>20.6485</td><td>38.7945</td><td>469.46</td><td>4294.01</td><td>54.4</td><td>41.8</td></tr><tr><td>20.6358</td><td>38.7901</td><td>468.35</td><td>4293.52</td><td>54.4</td><td>51.4</td></tr><tr><td>20.6227</td><td>38.7804</td><td>467.21</td><td>4292.45</td><td>54.4</td><td>65.3</td></tr><tr><td>20.6136</td><td>38.7738</td><td>466.40</td><td>4291.72</td><td>54.4</td><td>71.7</td></tr><tr><td>20.6072</td><td>38.7625</td><td>465.85</td><td>4290.47</td><td>54.4</td><td>74.9</td></tr><tr><td>20.5998</td><td>38.7523</td><td>465.20</td><td>4289.35</td><td>54.4</td><td>70.0</td></tr><tr><td>20.5978</td><td>38.7375</td><td>465.02</td><td>4287.70</td><td>54.4</td><td>59.6</td></tr><tr><td>20.6003</td><td>38.7262</td><td>465.24</td><td>4286.46</td><td>54.4</td><td>52.4</td></tr><tr><td>20.6042</td><td>38.7144</td><td>465.58</td><td>4285.15</td><td>54.4</td><td>45.4</td></tr><tr><td>20.6151</td><td>38.7078</td><td>466.54</td><td>4284.42</td><td>54.4</td><td>43.9</td></tr><tr><td>20.6268</td><td>38.7084</td><td>467.56</td><td>4284.49</td><td>54.4</td><td>46.2</td></tr><tr><td>20.6366</td><td>38.7119</td><td>468.42</td><td>4284.88</td><td>54.4</td><td>49.1</td></tr><tr><td>20.6458</td><td>38.7205</td><td>469.22</td><td>4285.83</td><td>54.4</td><td>54.1</td></tr><tr><td>20.6574</td><td>38.7331</td><td>470.24</td><td>4287.23</td><td>54.4</td><td>58.2</td></tr><tr><td>20.6630</td><td>38.7453</td><td>470.73</td><td>4288.56</td><td>54.4</td><td>59.6</td></tr><tr><td>20.6665</td><td>38.7587</td><td>471.04</td><td>4290.05</td><td>54.4</td><td>57.8</td></tr><tr><td>20.6686</td><td>38.7689</td><td>471.22</td><td>4291.18</td><td>54.4</td><td>53.5</td></tr><tr><td>20.6651</td><td>38.7813</td><td>470.91</td><td>4292.55</td><td>54.4</td><td>47.5</td></tr><tr><td>20.6615</td><td>38.7907</td><td>470.60</td><td>4293.58</td><td>54.4</td><td>40.7</td></tr><tr><td>20.6545</td><td>38.7945</td><td>469.98</td><td>4294.01</td><td>54.4</td><td>39.5</td></tr></tbody></table>

<table><caption>Table 8. The 20 picked values on the visible fringe in the best descending interferogram. The origin for the LoS was estimated during the joint inversion process. The coordinates are also given in UTM 34.</caption><thead><tr><th>Ref.</th><th>lat (°)</th><th>long (°)</th><th>depth (km)</th><th>strike (°)</th><th>dip (°)</th><th>rake (°)</th><th>L (km)</th><th>W (km)</th><th>Mo (* 10<sup>18</sup> Nm)</th><th>slip (cm)</th></tr></thead><tbody><tr><td>model in this study (IL16)</td><td>38.83</td><td>20.64</td><td>6.8</td><td>18</td><td>59</td><td>180</td><td>16</td><td>10</td><td>3.60</td><td>75</td></tr><tr><td>alternative model using the combination of ascending and descending interferograms</td><td>38.83</td><td>20.63</td><td>8.8</td><td>18</td><td>59</td><td>180</td><td>16</td><td>10</td><td>3.50</td><td>73</td></tr></tbody></table>

<caption>**Table 9.** Comparison of the fault models from ascending interferograms only (IL16, see Table 3) and from a combination ascending of descending interferograms. The coordinates are for the centre of the fault.</caption>
785 APPENDIX A. ENVISAT ascending co-seismic interferograms

786

787 Wrapped (top) and unwrapped (bottom) ascending co-seismic interferograms (numbers
788 correspond to the interferogram number in Table 4)

789

790

791

792

<figure><img src="image_22.png" /></figure>

793

794
<figure><img src="image_23.png" /><figcaption>APPENDIX B. ENVISAT ascending post-seismic interferograms</figcaption></figure>
816

817

APPENDIX C. ENVISAT descending co-seismic interferograms

818

819

Wrapped (top) and unwrapped (bottom) descending co-seismic interferograms (numbers correspond to the interferogram number in Table 4)

820

821

822

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

823