(Greece) Earthquake of 7 September 1999

Journal of Seismology 8: 381–394, 2004. 381 C 2004 Kluwer Academic Publishers. Printed in the Netherlands.

Deformation patterns associated with the M5.9 Athens (Greece) earthquake of 7 September 1999

Gerassimos A. Papadopoulos1,∗, Hiroyuki Matsumoto2, Athanassios Ganas1, Vassilis Karastathis1 &Spyros Pavlides3 1Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece; 2Deep Sea Research Department, Japan Marine Science and Technology Center, Yokosuka, Japan; 3Department of Geology, University of Thessaloniki, Thessaloniki, Greece; ∗Author for correspondence: e-mail: [email protected]

Received 10 November 2003; accepted in revised form 21 January 2004

Key words: Athens earthquake, normal faulting, ground deformation, numerical simulation, SAR interferometry, macroseismic ﬁeld

Abstract The static displacement field of the Athens 1999 earthquake has been numerically modeled by a BEM method and analysed from SAR interferometry images with compatible results: (a) for a fault that reaches the surface the subsidence field coincides with the hangingwall domain of the Fili neotectonic normal fault with maximum amplitude, dmax, 5.5–7 cm, which is consistent with the possibly co-seismic displacement of 6–10 cm observed in the field, the average fault dislocation of 5–8 cm found by the application of circular source models, and the displacement up to ∼6cmpredicted by empirical relations between magnitude and displacement; the field of uplift covers the footwall domain of the fault with dmax ∼ 1.5 cm; d gradually decreases with distance from the fault at a gradient of ∼0.4 cm/km, (b) for a blind fault dmax is only 1.8 and 0.3 cm in the hangingwall and footwall, respectively, and the decay gradient becomes ∼0.15 cm/km, (c) the total deformation area is ∼15 km × 15 km and the Fili fault, with a preferred mean dip of 60◦, constitutes the natural boundary between the subsidence and uplift areas. The macroseismic field pattern is similar with that of the static ground deformation. The majority of intensity values ≥VI (MM and EMS-98 scales), are distributed within the hangingwall of the Fili fault, while the highest intensities (VIII and IX) concentrate very close to the Fili fault within its hangingwall domain. A gradual decrease of the intensities with the distance from the Fili fault is evident. Because of the similarity between the intensity distribution pattern and the static ground deformation pattern, we make the hypothesis that the latter predicts well enough the main characteristics of the former although the ground displacement is dominated by relatively low frequency as compared to the ground acceleration.

Introduction low seismic potential given that such a strong earthquake was never reported from that region in the past The earthquake of 7 September 1999 (Mw = 5.9), here- (Papadopoulos et al., 2002). after referred to as “the Athens earthquake,” was the The Athens 1999 earthquake is well studied as re- largest seismic event ever reported in the vicinity of gards its source properties, seismotectonics and macro- the city of Athens, central Greece, occurring at an epi- seismic ﬁeld. This makes a good case to study the central distance of only 18 km from the historical center ground deformation pattern associated with the earth- of the city (Figure 1). It caused 143 casualties and a tan- quake with three independent approaches: (a) 3-D nu- gible loss on the order of US$ 3 billion (Papadopoulos merical modeling of the static ground displacement et al., 2000a). The earthquake was unexpected in on the basis of the boundary element method (BEM), the sense that it occurred in a region considered of (b) analysis of SAR interferometry images, and (c) 382

Figure 1. Aftershocks (dots) occurring up to 11 September 1999 after the mainshock of 7 September 1999 and focal mechanism of the mainshock (beach ball) (data from Papadopoulos et al., 2000a, 2000b). The line represents the surface position of the neotectonic Fili fault as mapped by Pavlides et al. (2002). Triangles show seismic stations. ATH, Athens station. examination of higher degree intensity distribution as indicating a restriction to the dip range from 56 to 60◦. it results from macroseismic ﬁeld observations. The rake is stably around −80◦ and a left-lateral component is recognized in most of the focal mechanisms The seismic source and faulting process proposed. The focal depth of the mainshock as well as the Approaches of the focal and source parameters, kine- dimensions of the seismogenic fault are parameters matic rupture process and fault plane solutions of strongly dependent on the method of calculation. Pre- the Athens 1999 earthquake can be found in sev- liminary focal depth was calculated at 15 km by the eral papers, based on various techniques and datasets CMT solution of Harvard, 10 km by USGS and 30 km (Papadopoulos et al., 1999, 2000a, 2000b; Kontoes et by National Observatory of Athens, Institute of Geody- al., 2000; Tselentis and Zahradnik, 2000; Louvari and namics (NOAGI). After relocations of the mainshock Kiratzi, 2001; Papazachos et al., 2001; Baumont et al., the focal depth ranges between 8 and 16.8 km, while the 2002; Papadimitriou et al., 2002; Sargeant et al., 2002; centroidal source depth was calculated at 10 km. The Stavrakakis et al., 2002; Roumelioti et al., 2003), as several source models tried indicate that the faulting well as on routine determinations of seismological cen- extended from minimum depth z to maximum depth ters such as Harvard, USGS and MedNet. Since a va- Z, with z ranging from 0 to 6 km and Z ranging from riety of values have been obtained for the several pa- 12 to 16.8 km. rameters, depending on the method applied and dataset The seismic moment was found to range between used, we summarize them in Table 1. For a rectangular 5.7 × 10E17 Nm and 1.1 × 10E18 Nm while the stress fault the along-strike length calculated ranges from 10 drop is relatively low given that several values calcu- to 20 km while the along-dip width ranges again from lated do not exceed 9 bars. The rise time of the rupture 10 to 20 km. It is generally accepted that the fault- is estimated to vary between 0.1 and 0.3 s. ing was normal with the fault plane striking between For the average fault dislocation some diverse re- 105 and 117◦, that is from WNW to ESE, and dip- sults were obtained. Rectangular fault models yielded ping to SW between 47 and 60◦ with most solutions avarage dislocation ranging from 25 to 30 cm (Kontoes 383

Table 1. Summary of the fault and slip parameters determined for the Athens 1999 earthquake

Parameter BKLK Pd Pdm Pz Sr St T

Fault strike 115 113 105 115 117 Fault dip 57 56 55 47 60 52 Fault rake −80 −80 −80 Fault length (km) 10 18 20‡ 15 18 20 10 or 20 Fault width (km) 20 15‡ 10 11‡ 16 10‡ Focal depth (km) 13 ± 4 16.8 8 14.5 Centroidal source 10 10 depth (km) Energy centroid 10.3 8 depth (km) Fault extents 6/16 0/16.8 4/12 0/14 from/to (km) Average dislocation 25 30 30 (or from 8* 6.81* or 5.33* (cm) 5 to 22*) Fracture initiation 10 ± 2 depth (km) Rise time (s) 0.1–0.3 0.1–0.3 Average source 5–6 4.2 5 5–6 duration (s) Stress drop (bars) 10 9 5.4 Scalar seismic 9.22 × 10E17 1.01 × 10E18 6.014 × 10E17 5.7 × 10E17 moment (Nm) Rupture speed (km/s) 2.1

Authors key: B, Baumont et al. (2002); K ,Kontoes et al. (2000); LK, Louvari and Kiratzi (2001); Pd,Papadopoulos et al. (2000); Pdm, Papadimitriou et al. (2002); Pz,Papazachos et al. (2001); Sr, Sargeant et al. (2002); St, Stavrakakis et al. (2002); TZ, Tselentis and Zahradnik (2000). ‡Calculated from the dimensions of the aftershock area. s tCircular model. et al., 2000; Tselentis and Zahradnik, 2000; Louvari western part of the fault and propagated to the east and and Kiratzi, 2001; Baumont et al., 2002; Papadimitriou upwards (e.g. Papadopoulos et al., 2000; Papazachos et et al., 2002) while significantly lower values, rang- al., 2001; Papadimitriou et al., 2002). On the contrary, ing from 5 to 8 cm, were found from the application the modelling of Roumelioti et al. (2003) indicated a of circular source models (Louvari and Kiratzi, 2001; downward propagation of the rupture. Based on a value Sargeant et al., 2002; Stavrakakis et al., 2002). The lat- of 6.5 cm and assuming that the shear modulus equals ter are more compatible not only with the amplitude of 3.3 × 10E4 N/m2 a seismic moment of the order of 6.5 the possibly co-seismic fault dispalacement observed × 10E17 Nm results, which is quite consistent with the in the field by Pavlides et al. (2002) but also with dis- seismic moment value calculated by several research placement up to ∼6cmpredicted by empirical relations groups. Papadopoulos (2002) showed that at a high among earthquake magnitude and co-seismic displace- probability level the Athens earthquake was triggered ment (Wells and Coppersmith, 1994; Ambraseys and by the 17 August 1999 large shock in Izmit, Turkey. Jackson, 1998; Pavlides et al., 2000). In the nonuni- In a geological field campaign conducted the form slip distribution found by Roumelioti et al. (2003) first days after the mainshock, Pavlides et al. (1999, most of the slip (∼50% of the total slip) occurred at a 2002) observed ground dislocations like open fissures depth greater than the assumed hypocentral depth of 11 (1–2 cm) with variable vertical displacements (6–10 km, while it seems that maximum slip reached at about cm), affecting mainly the basement rock and some very 70 cm. There is evidence that the rupture started at the thin loose deposits along the neotectonic Fili fault, and 384 argued that they could be of co-seismic origin. They evaluated as the Cauchy principal value, and ψ¯ i on the also concluded that the seismogenic structure should right-hand side represents effects of an incident wave. be associated with the neotectonic normal fault of Fili Forapoint dislocation source located at xF, ψ¯ i is ◦ ◦ that strikes N110–133 , dips 64–85 SW and bounds expressed as part of the aftershock zone at its north side (Figure 1), ψ¯ = , ¯ that is the main part of the aftershock zone covers the i (x) Tij(x xF)d j (xF) (2) hangingwall of the seismogenic fault. Seismological ¯ = + − − and interferometric observations are in favour of such a where d j u j u j denotes the x j component of conclusion (Kontoes et al., 2000; Tselentis and Zahrad- the dislocation. When it comes to the response to the nik, 2000; Louvari and Kiratzi, 2001; Papazachos et al., rupturing of a plane fault, the entire history of incident 2001; Sargeant et al., 2002). waves may be considered to consist of a succession of those from point sources, each producing its own Numerical modeling differential effects of the form of equation 2. For the linearly elastic system under consideration, the total re- The method sponse can be obtained by summing all the differential effects. Static coseismic displacements have radiation pat- By discretizing the boundary B into a set of bound- terns analogous to the propagating wave displacements ary elements, equation 1 can be written in matrix form and, therefore, can also provide important informa- as (Brebbia et al., 1984) tion about the fault geometry and slip. However, since [H ] {u }={ψ¯ } (3) these displacements contain 1/r 2 terms, compared to B B the 1/r terms for the propagating waves, static dis- { } {ψ¯ } placements decay more rapidly with distance from the where u B is the nodal displacement and ψ¯ earthquake. The field of static ground displacement is a vector that consists of i . Isoparametric lin- of the Athens earthquake was studied with the ap- ear elements are adopted to discretize boundaries. plication of a 3-D numerical modeling based on the Strongly singular integrals, which are required to cal- boundary element method. The detailed formulation culate diagonal components of [H], including the of BEM is that described by Kataoka and Ohmachi free terms cij, are directly evaluated referring to the (1997) and proved to be of high accuracy in calcu- studies of Guiggiani and Gigante (1990) and Mantiˇc lating the static deformation due to seismic faulting (1993). The ground motion is simulated by solving (Ohmachi et al., 2001). Here we outline only the main equation 3. × points of the method as described by Ohmachi et al. The simulation area covers 50 km 50 km in (2001). N-S and E-W directions and its center coincides with the mainshock epicentre which is taken at 38.08◦ N, A direct BEM in frequency domain is used to sim- ◦ ulate seismic waves propagated in irregular half-space 23.58 E according to the relocation of Papadopoulos et al. (2000a, 2000b). This epicentre is almost coin- media. A boundary integral equation in x1 − x2 − x3 Cartesian coordinates can be written as (Niwa and cident with that found by the relocation performed Hirose, 1983) by Papadimitriou et al. (2002) from independent data. The grid size of the boundary element used for the modeling is 1 km and the frequency range for the c ij(x)u j (x) + Tij(x, x0)u j (x0)d(x0) computation goes up to 0.5 Hz. It is assumed that B the dislocation occurs within a rectangular fault in a

− Uij(x, x0)τ j (x0) d(x0) =ψ¯i (x) (1) homogeneous half-space and that the rupture started B at the lower southwestern corner of the fault plane and was directed upward and eastward. It is of importance where ui and τ i denote xi component of displacement to note that the modeling was executed by keeping and traction, respectively, B is the boundary of the both the earthquake epicentre and focal depth fixed. layer, Uij is the fundamental solution, Tij is its traction, What has been varying is the dip of the fault plane x0 and x source and field points, respectively, and cij a which as we will see later was considered to range free term that is determined from the boundary shape at between 56◦ and 63◦. This makes the along-dip width x. The second term on the left-hand side of equation 3 is of the fault to vary. As a consequence the intersection 385 of the fault plane with the free surface shifts with are listed in Table 2. The parameters associated with the respect to any constant reference point. medium are the P-wave and S-wave velocities, taken equal to 6.0 and 3.4 km/s, respectively, and the material density taken as 2.6 g/cm3. Selection of parameters Results From the summary of the fault parameters presented in Table 1 and discussed in the previous section, a real- The results obtained incorporate absolute values of the istic integration of the several estimations was finally vertical static displacement, d, measured at surface as adopted as follows. The dimensions we initially intro- either uplift or subsidence. Several modeling trials per- duced for a rectangular fault are 16.6 km for the along- formed for fault plane dip ranging from 56 to 63◦ repro- strike length and 17.3 km for the along-dip width with duce similar deformation patterns. The total expansion ◦ the fault plane striking 115 and dipping 60 S-W. Deci- of the deformed area is about 15 km × 15 km and as mal approximation was taken for computation reasons. one may expect the field of ground subsidence falls For reasons of simplicity the strike-slip component was in the hangingwall domain of the fault and the field omitted from the fault’s kinematics and, therefore, the of ground uplift covers the footwall domain. However, ◦ rake was fixed at −90 .Weinitially assumed that the the deformation pattern is dependent on the faulting rupture started at a depth equal to 15 km and reached geometry considered. For a nonburied fault that rup- the surface. An alternative was to adopt a burried fault tured the entire layer from the focus at 15 km to the extending from 5 to 15 km, which implies that the up- surface and a fault dip ranging from 56 to 60◦, the max- permost crustal layers remained unruptured. These two imum displacement amplitude, dmax,is∼6 and 1.2 cm fault geometries not only compensates the discrepan- in the fields of hangingwall and footwall, respectively cies between the different results reached at by different (Figures 2 and 3), which makes a ground deformation research groups as mainly regards the fault dimensions, quite consistent with the source dislocation of ∼6.5 cm but also is consistent with the preferred solutions of independently adopted earlier. In both fields d grad- 8 and 9.39 km found by Sargeant et al. (2002) and ually decreases with distance at a gradient of ∼0.4 Stavrakakis et al. (2002), respectively, for the radius of a cm/km. On the contrary, for a fault plane dip larger ◦ circular fault in the Brune’s (1970, 1971) model. Some than 60 dmax drastically decreases. The boundary be- additional alternatives were considered by varying the tween the subsidence and uplift fields takes its min- along-dip width from 16.8 to 18.1 km which corre- imum distance of ∼2kmfrom the neotectonic Fili ◦ spond to a range of the fault plane dip from 63 to 56 , fault for 60◦ fault plane dip while at 56◦ the distance is respectively. ∼4 km. The rupture velocity was taken equal to 3.0 km/s Foraburied fault extending from depth Z = 15 while the rise time considered was 0.1 s. A summary of km to depth z = 5kmthe deformation is reduced the fault parameters inserted in the numerical modeling in amplitude and varies more smoothly with distance (Figures 4 and 5). In fact, the maximum deformation amplitude is on the order of only 1.8 in the hangingwall Table 2. Summary of the fault parameters introduced in and localised at a distance ∼6kmaway from the fault. the BEM numerical modeling for fault plane dip of 56◦ (left column) and 60◦ (right column) Displacement reached up to only 0.2 cm in the footwall. The amplitude gradually decreases with distance at a Length (along-strike) 16.6 km 16.6 km gradient of ∼0.15 cm/km. Width (along-dip) 18.0 km (13.0 km*) 17.3 km We see that the surface deformation field is quan- Depth 15.0 km 15.0 km titatively different in the case of a buried fault with Fault strike 125◦ 125◦ respect to that of a fault extending to the surface. In a Fault dip 56◦ 60◦ model that assumes that the slip is uniform in a verti- Rake −90◦ −90◦ cal fault this difference is well explained by that while in the case of the nonburied fault the across the fault Fault dislocation 6.8 cm 6.8 cm displacement is Rupture velocity 3.0 km/s 3.0 km/s Rise time 0.1 s 0.1 s D D l d =± − tan−1 (4) ∗Width for buried fault extending from 5 to 15 km. 2 π H 386

Figure 2. The ﬁeld of static ground deformation (in cm) caused by the Athens earthquake for fault dip 56◦ (up) and 60◦ (down) produced from the BEM numerical modeling. Negative and positive contours indicate subsidence and uplift, respectively. Star, 7 September 1999 mainshock epicenter; bar, Fili fault; AA axis of sections shown in Figure 3. for a buried fault it becomes l < 0. From (4) it results that near the fault, that is for l → 0, d =±D/2. The deformation decays away D − l − l from the fault and at distance l/H = 1, that is equal d = tan 1 − tan 1 (5) π h H to the fault width, the inverse tangent is π/4 and the displacement is D/4, or half that at the fault. In the where D is the slip across the fault and l is the dis- case of a buried fault, however, the maximum surface tance from the fault (see review in Stein and Wysession, displacement is less than half the fault slip and occurs 2003). The ±D term is positive for l > 0, negative for a distance from the fault equal to the mean depth, that 387

behavior of active faults in various parts of the world including the Athens 1999 earthquake area (Kontoes et al., 2000; Ganas et al., 2001; Papadimitriou et al., 2002). The technique works much better in flat and dry areas because of constraints in the imaging geometry, such that of the ERS satellite which has a high incidence angle of 23◦, and the climatic influ- ence on phase coherence since wet climatic conditions cause quick, temporal decorrelation of the phase returns. This is of importance for the performance of the technique we applied in the case of the Athens 1999 earthquake. More precisely, coherence in the study area is maintained better on the central and southern parts of the hangingwall domain, where geomorpho- logical flat areas prevail, than on the Mount Parnitha which occupies the footwall domain and the northern part of the hangingwall domain. In this study we used the ATLANTIS software for Windows NT (version 1.2.1) where we applied the 2-pass method involving, (a) resampling of the 20-m external Digital Elevation Model (DEM) to the master SAR image (ERS-2 or- bit 23136—23 September, 1999), (b) distortion of the re-sampled DEM creating foreshortening and layover distortions, (c) fine, interactive co-registration of the DEM to the master SAR image using more than 20 tie points with residuals less than a pixel, (d) calculation and subtraction of the topographic phase from the interferogram. The phase unwrapping is done by the use of the iterative disk-masking algorithm (copyright of ATLANTIS) with controlled error propagation and er- Figure 3. Cross section of the static ground deformation along axis ror correction. The algorithm uses multiple tiling and AA Figure 2) for fault dip 56◦ (up) and 60◦ (down). Distance is mea- seeding techniques. The best pair has an altitude of am- sured from point A. Vertical dashed line shows the surface position biguity of 67 m and a time difference of 68 days. Neither of the Fili fault. the remaining topographic phase nor the atmospheric

1/2 effects were detected on the interferogram. With re- is (hH) . These differences occur because a buried spect to the DEM, we used a grid size of 20-m which fault is further away from each point on the surface, was constructed by digitizing the 1:50,000 topographic and the higher spatial frequencies in the displacement map sheets Elefsis and Kiﬁssia, Geographical Survey decay faster with distance, making the displacement of the Greek Army. This elevation dataset is rather smoother. If the fault is not vertical, as it is the Athens ideal for interferometry. For example, the recent NASA earthquake case, the displacement varies in magnitude Shuttle Radar Topography Mission (SRTM) will only as well as sign across the fault. produce global DEMs of 30-m in order to compute differential displacements. The use of a 20-m external SAR interferometry DEM makes it more denser than that of Papadimitriou et al. (2002) and of the same resolution as Kontoes et The method al. (2000) although these authors do not explicitly state so. This means that our topographic phase removal is The Synthetic Aperture Radar (SAR) interferometry more accurate at least from that of Papadimitriou et al. technique has been applied to study the earthquake (2002). 388

Figure 4.AsinFigure 2 for a buried fault extending in depth from 5 to 15 km. The ﬁeld of the static ground deformation of the Athens earthquake for fault dip 56◦ as produced from the BEM numerical modeling.

Results least two concentric, but not symmetric, fringes cen- tered at 38.10◦ N, 23.60◦ E. This point is located at a Two phase cycles (fringes) can be seen from south distance of less than 3 km away from the mainshock to north, that is from the coastline to the center of epicenter. The fringes indicate a change of 56 mm sub- the image, with the following color succession: red- sidence in the central part of the hangingwall domain. blue-green-red (Figure 6). This succession shows a Also, the pattern of extracted fringes shows an N 110◦– negative phase cycle which tectonically corresponds 120◦ axis of the deformation ellipse, parallel to the to ground subsidence. From the interferogram it is Fili neotectonic fault in agreement with geological and apparent that the earthquake sequence induced a co- seismological data. The calculated height-change map, seismic surface deformation, which appears with at based on the vertical component of the line-of-sight 389

the 0-fringe contour as the b/2 axis of the ellipse, where b is the minor axis, then it is evident that the less steep the fault plane is the larger the b/2 axis in the hangingwall domain will be. However, what we notice is that our 0-fringe contour is at the coastline (Figure 4) or at about the same area as the 0-deformation contour from the BEM modeling (Figures 2–5). In other words the InSAR data point at a plane steeper than 56◦. The dip of the fault plane is more likely to be around 60◦ or a little bit more which is more compatible with the results of the numerical modelling. As for the precision of our remote sensing measurements, we believe that it is more than satisfactory and establishes the validity of the Athens earthquake deformation field. In fact, it is well established (e.g. Zebker et al., 2000; Mora et al., 2001) that the precision of the slant range difference is better than 1 cm. In our case we measured surface deformations ranging from 1 to 7 cm, i.e. seven times our precision capability. With respect to the fringe pattern we can clearly discrim- inate two fringes associated with surface subsidence, at –28 and –56 mm. We can also measure more subsidence in the interior of the –56 mm fringe at –70 mm at the maximum. One may also note that the outer fringe asymmetry is mostly confined to the eastern part of the deformation field and is not an artifact of the DEM. This asymmetry is probably associated with the loose sediment structure of the Ano Liosia area, that is it is a surface effect. The InSAR results obtained are also significant in ◦ Figure 5.AsinFigure 3 for a buried fault extending in depth from that they validate the average fault strike, N 120 ,as 5to15km. inferred from source modeling and field geological observations, as well as a maximum hangingwall subsi- vector, shows that the amplitude of subsidence ranges dence of 7 cm, which is independently consistent with from 2 to 7 cm with the maximum observed in the area the BEM modeling results and the field observations. of the ancient Fili Fort located very close to the north Although we did not invert our results to constrain fault part of the neotectonic Fili fault mapped by Pavlides plane geometry, no doubts may be raised against our et al. (1999, 2002). Remarkable subsidence from 2 to interpretations on the fault strike and the hangingwall 4cmalso occurred in the Acharnes and Ano Liosia subsidence. area of the Athens basin, in the hangingwall domain close to the south part of the Fili fault, where extensive destruction and loss of life occurred. An area of uplift The macroseismic field at the bottom center of the field near the coastline is so marginal, ranging from 3 to 8 mm, that lies within the One common way of describing a ground motion is limits of error. with one or more of amplitude parameters: ground ac- Our InSAR results compare better with the surface celeration, velocity or displacement, any two of them location of the Fili neotectonic fault. The ellipticity of computed from the third by integration and/or differen- our fringes is greater than that in Kontoes et al. (2000). tiation provided that it is directly measured from strong This means that if we take the major axis of the ellipse motion instruments. On the other hand, empirical rela- (a-axis) as the surface projection of the neotectonic tions have been established between the ground accel- fault, that is as the location of zero deformation, and eration, a, and the macroseismic intensity, I , observed 390

Figure 6. Image of the interferogram of the 7 September 1999 Athens earthquake, from the phase difference (in slant range) of two ERS-2 satellites, showing the amount of ground subsidence in the hangingwall domain of the neotectonic Fili fault. The major axis of the deformation ellipse is nearly identical with the surface position of the fault (see also Figure 1). at a particular site: log a = A + BI, where A and (2002)), 10 of them were installed in a very narrow B are parameters. Then, although the displacement is zone of the Athens metropolitan area to the southeast dominated by relatively low frequency as compared of the epicenter. to the acceleration, an indirect relation should asso- ciate the ground displacement field with the intensity field. Then, the general pattern of the seismic intensity Pattern of intensity distribution distribution is expected to correlate well with the general pattern of the ground displacement. This indirect We performed macroseismic observations in the approach, which is suitable if no adequate strong mo- Athens earthquake area the first days after its tion records exist, was tested in the case of the Athens occurrence. Maximum intensity in many observation 1999 earthquake by comparing its macroseismic field points of several residential zones were estimated by with the ground displacement field. In fact, the ground both the modified Mercalli–Sieberg and the EMS-98 acceleration field is not adequately represented in the scales. No important difference was found between largest part of the affected by the earthquake area be- the two estimations. Additional information was taken cause although the earthquake triggered 18 strong mo- from the standard questionnaires of macroseismic tion instruments, located at epicentral distances ranging effects collected by the Institute of Geodynamics, Na- from about 10 to 47 km (see information summaries tional Observatory of Athens. Moreover, our intensity in Papadopoulos et al. (2000a) and Bouckovalas et al. estimations were compared with those published only 391

Figure 7. Macroseismic ﬁeld of the 7 September 1999 Athens earthquake. Only intensities equal to or larger than degree 6 in EMS scale are mapped. Star and line show the mainshock epicenter and the surface position of the neotectonic Fili fault, respectively. for some locations by Lekkas (2001) and Pomonis of the fault, that is within the footwall domain. Besides, (2002) and no remarkable deviation was found. the majority of observation points of VII degree as well The earthquake caused remarkable destruction and as the most important ground failures, like rockfalls the maximum seismic intensity reached up to degree and small-scale landslides, caused by the earthquake IX. As already noted by Papadopoulos et al. (2000a), (Pavlides et al., 2002) fall in the hangingwall domain the heaviest destruction and, therefore, the highest in- of the fault. In addition, it is obvious that the intensity tensities occurred in the hangingwall domain of the gradually decreases toward SW, that is away from the neotectonic Fili fault and very close to it. In fact, a pat- fault zone within the hangingwall. tern of the high-degree intensities to concentrate in the hangingwall domain is evident in Figure 7, which illus- trates the geographical distribution of seismic intensity Conclusions and discussion of degree VI and over. All of the observation points assigning seismic intensity of degrees VIII and IX are Both the numerical modeling of the static displacement located at distances less than 10 km from the neotec- ﬁeld for a purely normal fault with uniform slip and tonic Fili fault, while only one of them falls to the north the analysis of the SAR interferometric image of the 392

Athens 1999 earthquake show the same general pattern fault, while the highest intensity values, estimated to of static ground deformation: (a) the field of subsidence reach VIII and IX degrees, tend to concentrate very is coincident with the hangingwall domain of the neo- close to the Fili fault within its hangingwall domain. tectonic normal fault of Fili with estimated maximum In addition, a gradual decrease of the intensities with amplitude of vertical component on the order of 7 cm distance from the Fili fault is evident. Results indicat- from SAR interferometry and 7.5 cm for a nonburied or ing local site effects (Bouckovalas et al., 2002; Gazetas 1.8 for a buried fault from numerical modeling, (b) the et al., 2002), do not contradict the general pattern for displacement amplitude gradually decreases with dis- the seismic intensity distribution. Because of the simi- tance at a gradient of 0.4 and 0.15 cm/km as an average larity between the intensity distribution pattern and the for a nonburied and a buried fault, respectively, (c) the static ground deformation pattern, we suggest that the total area of deformation is about 15 km × 15 km and latter predicts well enough the main characteristics of the Fili fault with a preferred fault plane dip of 60◦ the former although the ground displacement is dom- seems to constitute the natural boundary between the inated by relatively low frequency as compared to the subsidence and uplift fields. The numerical modeling ground acceleration. As a follow-up for the verifica- also indicates maximum uplift of only 1 and 0.2 cm in tion of the hypothesis validity we started with a new the footwall domain of the Fili fault for a nonburied research initiative by examining several cases of strong and a buried fault, respectively, while deformation is shocks associated with normal faulting. not possible to be reliably detected by SAR interferometry because the footwall field covers the area of Mount Parnitha where wet climatic conditions cause Acknowledgements quick, temporal decorrelation of the phase returns. The maximum ground deformation estimated from Dr. H. Matsumoto was a post-doc researcher at the numerical modeling and SAR interferometry is quite Institute of Geodynamics, National Observatory consistent with the possibly co-seismic displacement of Athens, from October 2001 to March 2002, of 6–10 cm observed in the field by Pavlides et al. supported by a Grant-in-Aid for young scientists of (2002), the average fault dislocation of 5–8 cm found the Japanese Society for the Promotion of Science. from the application of circular source models (Louvari Field observations were performed after the Athens and Kiratzi,s 2001; Sargeant et al., 2002; Stavrakakis 1999 earthquake within the frame of the research et al., 2002) as well as with displacement up to ∼6cm project Assessment of Seismic Potential in European predicted by empirical relations among earthquake Large Earthquake Areas (ASPELEA), supported by magnitude and co-seismic displacement (Wells and the Commission of European Communities—DG Coppersmith, 1994; Ambraseys and Jackson, 1998; XII, INCO-COPERNICUS Program, contr. No. Pavlides et al., 2000). However, source models for IC-15CT-97-0200 and partly by the General Secretary a rectangular fault provided dislocation values rang- of Research and Technology, Greece. We thank Prof. ing from 15 to 30 cm (Kontoes et al., 2000; Tselentis E. Lagios, Division of Geophysics and Geothermics, and Zahradnik, 2000; Louvari and Kiratzi, 2001; Bau- University of Athens, for his permission to use the mont et al., 2002; Papadimitriou et al., 2002) which ATLANTIS software. Thanks are extended to two are not only one order of magnitude larger than those reviewers for their constructive comments that helped the other approaches indicate but also are rather incon- in improving the paper’s original version. sistent each other. 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