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(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 earth- quake 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 com- ponent 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.
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