Variation of Seismic Coupling with Slab Detachment and Upper Plate Structure Along the Western Hellenic Subduction Zone

Tectonophysics 391 (2004) 85–95 www.elsevier.com/locate/tecto Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone Mireille Laiglea,*, Maria Sachpazib, Alfred Hirna aLaboratoire de Sismologie Expe´rimentale, De´partement de Sismologie, UMR 7580 CNRS, Institut de Physique du Globe de Paris, case 89, 4 Place Jussieu, 75252 Paris Cedex 05, France bGeodynamics Laboratory, National Observatory of Athens, Lofos Nymfon, Athens, Greece Accepted 3 June 2004 Available online 11 September 2004 Abstract The western Hellenic subduction zone is characterized by a trenchward velocity of the upper plate. In the Ionian islands segment, complete seismic coupling is achieved, as is predicted by standard plate-tectonic models in which there is no slab pull force because the slab has broken off. The moderate local seismic moment rate relates to a shallow downdip limit for the seismogenic interface. This characteristic may be attributed to the ductility of the lower crust of the upper plate, which allows a de´collement between the upper crust of the overriding plate and the subducting plate. Farther south, a deeper downdip limit of the seismogenic interface is indicated by thrust-faulting earthquakes, which persist much deeper in western Crete. A correspondingly larger downdip width of this seismogenic zone is consistent with the suggested larger maximum magnitude of earthquakes here. However, since the seismic moment release rate seems to be moderate in the Peloponnese and western Crete, like in in the Ionian islands, this seismically active interface cannot maintain complete seismic coupling across its larger downdip width. A cause may be the lateral addition of overweight to the part of the slab still attached in Crete, by the free fall of its part that has broken off from the surface further north. This increased slab pull reduces the compressive normal stress across the seismogenic interface and thus causes partial seismic coupling in its shallower part. However, the width of this part may provide an additional area contributing to slip in large earthquakes, which may nucleate deeper on stick-slip parts of the interface. Hints at anomalies in structure and seismicity, which need to be resolved, may relate to the present location of the edge of the tear in the slab. D 2004 Elsevier B.V. All rights reserved. Keywords: Subduction; Hellenic arc; Earthquakes; Seismic coupling; Seismic structure 1. Introduction The active seismicity and active crustal deforma- * Corresponding author. Tel.: +33 14427 3914; fax: +33 14427 tion in the Eastern Mediterranean region (Fig. 1) have 4783. been discussed by many people since the pioneering E-mail address: [email protected] (M. Laigle). plate tectonic studies by McKenzie (1970, 1972), 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.009 86 M. Laigle et al. / Tectonophysics 391 (2004) 85–95 the dimensions of the seismogenic zone at the interface, or other parameters used in the analyses of the seismic contribution, such as rigidity (shear modulus) and relative plate velocity, may have been overestimated. Third, the seismicity catalogs may be missing significant earthquakes, for instance because they cover only a limited time-span and the largest earthquakes, which are rare, may not be represented. For the western Hellenic subduction, the apparent deficit in seismic moment has commonly been regarded as due to aseismic slip. In the Ionian islands (Fig. 3a), we have showed instead that full seismic coupling could be achieved (Laigle et al., 2002), as is implied from the trenchward velocity of the upper Fig. 1. Sketch of the regional situation of the western Hellenic plate in the model of Scholz and Campos (1995). This subduction zone, at the boundary between Aegea and Anatolia. revised assessment resulted from estimating appro- Arrows are representative GPS velocity vectors with respect to priate source models for the subduction earthquakes stable Europe after Kahle et al. (2000). These indicate convergence and dimensions of the seismogenic interface and other at the subduction boundary: northward motion of Africa at 6 mm/ parameters, based on reinterpretations of the seismic- year and southwestward motion of the Aegean domain at 35 mm/ year. ity and analyses of the structure using seismic reflection profiling (Hirn et al., 1996; Sachpazi et al., 2000; Cle´ment et al., 2000). Papazachos and Delibasis (1969), Papazachos and This new view on the seismic coupling of the Comninakis (1971), and others. Subduction of the Ionian Islands leads us to extend the discussion to African plate is clearly expressed along the western the part of the Hellenic Arc farther south, to include Hellenic Arc, where intermediate-depth seismicity its parts adjoining the Peloponnese (Messenia) reveals a subducting slab and shallower flat thrust- region, the straits of Kithira, and western Crete faulting seismicity reveals an active seismogenic (Fig. 2). We will suggest that in spite of a similar subduction interface (Fig. 2). In comparison with seismic moment rate, characteristics such as the other subduction zones worldwide, the western spatial- and magnitude-distributions of interplate Hellenic convergence of the Greek landmass with seismicity, as well as the deep structure, vary along the Ionian Sea basin appears as an end-member case, the Hellenic arc. Structures and mechanisms clearly as its overriding upper plate has a much faster contrast between these Ionian Islands and western absolute velocity toward the plate boundary than the Crete, 300 km apart. However, at localities in subducting lower plate does (Fig. 1), as is inferred between, the lack of reliable data leave the question from geology and GPS measurements (e.g. Kahle et open regarding seismic or aseismic behavior, al., 2000). The upper plate is thus actively overriding although there are indications of very large earth- the lower one. quakes in the past. The contribution of seismicity to the convergence between these plates has been found to be small (e.g., North, 1974; Jackson and McKenzie, 1988a,b; Papa- 2. Fast trenchward motion of the upper plate in the zachos and Kiratzi, 1996; Tselentis et al., 1988; Baker Ionian Islands region: upper plate rheology and et al., 1997), indicating that this subduction is slab breaking-off, complete seismic coupling and occurring largely aseismically. Three possible reasons moderate seismicity can explain such an apparent deficit of seismic slip. First, there may be only partial seismic coupling We recall briefly the main results of our previous across the seismogenic part of the interface, the rest of study of the Ionian Islands region (Laigle et al., 2002) the motion on it occurring as aseismic creep. Second, and analyze the aspects of plate structure and M. Laigle et al. / Tectonophysics 391 (2004) 85–95 87 Fig. 2. Map of the SW Aegean region. Epicenters of all earthquakes with Mz4.8 (from Engdahl et al. (1998) are plotted, as circles for depth less than 50 km, gray triangles for 50–70 km depth, and black triangles deeper. Earthquake focal mechanisms are labelled with centroRd depths from the Harvard CMT catalog, and are located at the epicenters of Engdahl et al. (1998). These are equal area projections with compressional quadrants shaded. Dark gray shading indicates Harvard CMT solutions, light gray shading indicates solutions by Papazachos et al. (2000), and black shading indicates solutions by Taymaz et al. (1990) around Crete and by Baker et al. (1997) in the Ionian Islands. Solid line locates the reflection profile in Fig. 3, with the midpoints of the ESPs of Truffert et al. (1992) also shown. Note the South Matapan depression, or South Matapan Trough, which is interpreted as due to extension in the upper plate that, we suggest, reaches as far SW as the dashed dark grey line marked, and forms the backstop to the Mediterranean Ridge accretionary wedge further southwest, as suggested by Lallemant et al. (1994). dynamics that determine these results, in order to later subduction zone, from the comparison of a small investigate their variation along the arc. value of the rate of shortening as derived from the rate of seismic moment release, with a large value 2.1. Complete seismic coupling on a seismogenic zone of the rate of convergence inferred from geology. limited to shallow depth For its Ionian Islands part, we have shown that there may instead be complete seismic coupling (Laigle et As previously noted, the western Hellenic Arc al., 2002). This is consistent with the absolute has been commonly considered as a largely aseismic velocity of the upper plate being directed toward 88 M. Laigle et al. / Tectonophysics 391 (2004) 85–95 Fig. 3. Sketch sections through the Ionian Islands and western Crete. (a) In the Ionian region, there is presumed to be complete seismic coupling (black line) at the interface, and there is also presumed to be no slab pull force as the slab is thought to have broken off (cf. Scholz and Campos, 1995). This full seismic coupling applies on only a moderate downdip width of the interface, which is thought to be seismogenic from evidence of seismicity and structure. The shallow position of its downdip limit is interpreted as a consequence of the lower crust in the adjacent thick continental crust being ductile (cf. Laigle et al., 2002). (b) Western Crete. Flat thrust-fault earthquakes are documented to 40 km depth, suggesting that the seismogenic interface reaches as deep as this. Given the same moderate seismic moment release rate as in the Ionian islands, it is deduced that there is only partial seismic coupling (dashed black line) on parts of the interplate boundary, because of a reduced compressive stress due to an abnormally strong slab pull force. However, slip in rare large-magnitude earthquakes may propagate into the upper zone of presumed conditionally stable gliding.

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