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Earthquake Mechanisms of the Adriatic Sea

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Earthquake Mechanisms of the Adriatic Sea Earthquake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction-continental collision transition Calum Baker, Denis Hatzfeld, Hélène Lyon-Caen, Elephteria Papadirnitriou, Alexis Rigo To cite this version: Calum Baker, Denis Hatzfeld, Hélène Lyon-Caen, Elephteria Papadirnitriou, Alexis Rigo. Earth- quake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction- continental collision transition. Geophysical Journal International, Oxford University Press (OUP), 1997, 131 (3), pp.559 - 594. 10.1111/j.1365-246X.1997.tb06600.x. hal-01415818 HAL Id: hal-01415818 https://hal.archives-ouvertes.fr/hal-01415818 Submitted on 13 Dec 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Geophys. J. Int. (1997) 131, 559-594 Earthquake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction-continental collision transition Calum Baker,'." Denis Hatzfeld,' H&ne Lyon-Caen,2 Elephteria Papadirnitrio~~ and Alexis Rigo2.T Luborutoire de Giophysique Interne et Tectonophysique, IRIGM, BP53, 3804 I Grenoble Cedev 9, Frunce. E-mail: [email protected] lnstitut de Physique du Globe, 4 place Jussieu, CP 89, 75252 Paris Cedex 05, France 'Geophysical Laboratory, Aristotelian University, PO Box 352-1, 540-06 Thessaloniki, Greece Downloaded from Accepted 1997 June 23. Received 1997 June 23; in original form 1996 November 20 SUMMARY http://gji.oxfordjournals.org/ We present 21 focal solutions (magnitude > 5.5) reliably computed by body-wave modelling for the western Hellenic arc from Yugoslavia to the southern Peloponnese. Mechanisms located within the Aegean show normal faulting, the T-axis trending N-S in the centre and parallel to the active boundary in the external part. Mechanisms associated with the Keffalinia fault are consistent with dextral strike-slip motion. Reverse mechanisms located along the active boundary are remarkably consistent and do not depend on the nature of the active boundary (continental collision or oceanic subduction). The consistency in azimuth of the slip vectors and of the GPS velocity at Observatoire de la Côte d'Azur - Geoazur on December 13, 2016 relative to Africa, all along the active boundary, suggests that the deformation is related to the same motion. The discrepancy between seismic-energy release and the amount of shortening confirms that the continental collision is achieved by seismic slip on faults but the oceanic subduction is partially aseismic. The northward decrease in velocity between continental collision and oceanic subduction suggests the continental collision to be a recent evolution of the active subduction. Key words: Adriatic Sea, continental collision, earthquake mechanisms, oceanic subduction. Western Greece. the depth and the seismic moment of the event are obtained. 1 INTRODUCTION Several recent studies have used this waveform-inversion tech- The Aegean region is one of the most rapidly deforming and nique to determine well-constrained source parameters in the seismically active continental regions, which makes it import- Aegean for different areas (e.g. Taymaz, Jackson & Westaway ant for the study of continental tectonics (Jackson 1994). Focal 1990; Taymaz, Jackson & McKenzie 1991; Papadimitriou mechanisms of strong earthquakes provide some information 1993) or for individual earthquakes (e.g. Kiratzi & Langston on this deformation. McKenzie (1972, 1978), Anderson & 1989, 1991; Lyon-Caen et al. 1988). These studies have concen- Jackson (1987) and Papazachos, Kiratzi & Papadimitriou trated on the central part of Greece, the Aegean Sea and the ( 1991), among others, have published focal mechanisms com- Hellenic Trench subduction zone, and have greatly added to puted from first-motion polarities for the Aegean, and the our understanding of the geometry of deformation and inferred geodynamics leads to models which are not without kinematic processes in these areas. controversy (e.g. McKenzie 1978; Le Pichon & Angelier 1979). In this paper we use waveform modelling to study the large The accuracy of some of the mechanisms was, however, earthquakes in Western Greece and the Adriatic Sea. This very sometimes questionable. The waveform-modelling inversion active region is of particular interest for several reasons: it is technique (Nabelek 1984) allows better constraint of the fault- located at the transition between the active subduction zone plane-solution parameters. In addition, the source function, that occurs beneath the Hellenic arc and the continental collision between Apulia and continental Greece. It is also *Now at: Department of Earth Sciences, University of Keele, located near the pole of rotation that Le Pichon & Angelier Staffordshire, UK. (1979) and Le Pichon et al. (1995) proposed for the Hellenic tNow at: OMP-GRGS, CNRS-UMR5562, 14 av. E. Belin, 31400 trench. Here we present 21 new solutions and discuss the Toulouse. France. source parameters for 32 large (magnitude > 5.5) earthquakes 0 1997 RAS 559 560 C. Baker et al. which have occurred in Western Greece and the Adriatic slip-vector directions are significantly different from those between 1959 and 1988. shown by events associated with African-Eurasian plate con- vergence. This is consistent with VLBI (Ward 1994) and SLR (Noomen et al. 1994) geodetic observations. 2 GEODYNAMIC SETTING To the north, the convergence between Apulia and the The Aegean is a region of intense deformation located between Hellenides, the Albanides and the Dinarides is a continental- two major lithospheric plates, the European plate and the type collision. To the south, the lithosphere of the Ionian basin African plate (Fig. l), whose relative convergent motion is is probably oceanic and the convergent motion results in N-S and of about 1 cm yr-' (Argus et ul. 1989). However, the subduction beneath the Hellenic trench. The transition between relative motion across the Hellenic trench as inferred by the active lithospheric subduction and the continental collision seismology (e.g. McKenzie 1978; Jackson 1994) or geodesy is located around the Ionian islands (McKenzie 1978; Anderson (e.g. Noomen et a/. 1994) is NE-SW and greater than 5 cm yr-I & Jackson 1987), where the Kefallinia strike-slip transform and is related to the lithospheric subduction of Africa beneath zone is located (Cushing 1985; Scordilis et a\. 1985; Kahle the Aegean. The difference in the relative motion is likely to et al. 1993). This termination is marked by a sharp bathymetric be due to the motion of the Turkish plate along the North slope, striking at N20". This dextral strike-slip transform zone Downloaded from Anatolian fault and to the important amount of internal was assumed to be the northern limit of the mobile Akarnania deformation that occurs within the Aegean. block. However, palaeomagnetic results show that the clock- The seismicity along the coast of the former Yugoslavia, wise rotation in northern, as in southern, Albania has remained Albania and NW Greece defines the eastern boundary of the the same as in the Ionian islands and in the Peloponnese since 'Adriatic Block' as a promontory of the African Plate. While the late Eocene (Speranza et al. 1995). it is generally agreed (eg. Channell, D'Argenio & Horvath Tectonic observations (Sore1 1989; Waters 1993) as well as http://gji.oxfordjournals.org/ 1979) that the structural complexity of the Adriatic orogenic focal mechanisms of microearthquakes (Hatzfeld et a/. 1995) belts indicates that this region was part of the African plate show predominant N-S extension in a more internal domain during Jurassic and Cretaceous times, geophysical as well as within Epirus and Akarnania. The Gulf of Corinth (Rigo et al. tectonic evidence suggests that this is no longer the case. 1996) and Macedonia (Soufleris & Stewart 1981) also exhibit Anderson & Jackson (1987) showed that the motions along N-S extension. The transition between extension and the boundaries of the Adriatic Block derived from earthquake compression in the western Hellenides is not well located. at Observatoire de la Côte d'Azur - Geoazur on December 13, 2016 45' 15 4O0 10 35" 35O 30; 30° Figure 1. General sketch of the area showing the main features and geographical names. The North Anatolian fault transfers the motion of Turkey towards the southwest. The Hellenic trench and the Scutari-Pec fault are indicated. Earthquakes of magnitude greater than 5 (NEIC file) are displayed. The white arrow is the direction of motion of Aegea relative to Europe deduced from SLR measurement (Noomen et al. 1994). 0 1997 RAS, GJI 131, 559-594 Earthquake mechanisms in western Greece 561 We shall investigate the focal mechanisms of strong authors (e.g. McCaffrey & Nabelek 1987; Nelson et ul. 1987; earthquakes in this zone in greater detail in this paper. Frederich, McCaffrey & Denham 1988; Molnar & Lyon-Caen 1989) where the source parameters are fixed at values close to, but different from, those of the minimum-misfit solution. We 3 DATA REDUCTION AND MODELLING then estimate whether the match
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