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),

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. the trench (Fig. 1), given the model of Scholz and of 10–15 km, from reflection seismic profiling and Campos (1995). This situation can be explained local earthquake tomography (Hirn et al., 1996; because the slab pull force, which enters into the Cle´ment et al., 2000; Sachpazi et al., 2000). balance of tectonic forces, is missing here, as the Calculations, which take into account these obser- slab has broken off (according to seismic tomog- vations, and assume a localized, subduction inter- raphy; e.g., Spakman et al., 1988). In the vicinity of plate fault (Brune, 1968; Scholz, 1998), indicate that Zante island (Fig. 2) and Cephalonia (farther north), although seismic moment release rate is moderate, it we imaged the subduction interface and constrained is consistent with the complete seismic coupling its seismogenic downdip width to be moderate, of indicated by the fast trenchward motion of the upper the order of 50 km, and restricted to shallow depths plate. This result is also consistent with the M. Laigle et al. / Tectonophysics 391 (2004) 85–95 89 occurrence of major earthquakes in the region, Peloponnese has developed since the Early Pleisto- which are known to have maximum magnitudes of cene. Regardless of the mechanism, such elevated ~7.2, depths of 10–15 km, and recurrence times of crust can be expected to spread laterally, to try to ~50 years. reduce its excess gravitational potential energy (cf. Clarke et al., 1998), and this mechanism may thus 2.2. Downdip limit of seismogenic interplate due to also contribute to the general southwestward motion ductility of the lower part of the upper plate, which of Anatolia and Aegea. also eases overriding A further supporting factor may be the recent onset of fast clockwise rotation of the Ionian islands less We have interpreted the shallow downdip limit of than 1 Ma ago (Duermeijer et al., 2000). They this seismogenic zone as a consequence of rheo- suggested that this rotation was a consequence of logical layering of the upper plate (Fig. 3a). At break off of the slab by lateral tearing, propagating shallow depth, the interface is overlain by the from the north. They also suggested that this process ductile lower continental crust, which forms the may be coeval with a major change in the evolution of root of the Hellenides mountain range. Crustal the Gulf of Corinth after ~1 Ma (cf. Armijo et al., shortening in the region has caused crustal thicken- 1996; Westaway, 2002). Armijo et al. (1996) sug- ing, increasing the temperature of the deepest crust, gested that this event marked an increase in the rate of from which it can be expected to be particularly rifting across this Corinth, resulting from result from mobile. Such ductile rheology allows free slip on the westward propagation of the North Anatolian fault the interface at shallow depth, causing the small into central Greece. Westaway (2002) suggested downdip width and moderate seismic moment rate. instead that this change resulted from coupling Oleskevich et al. (1999) suggested that serpentiniza- between surface processes and flow in the ductile tion may induce ductility in uppermost mantle. lower crust, which caused the dramatic Middle–Late However, we propose that here the ductility is Pleistocene uplift of the Peloponnese already noted. instead caused by the lower crust, and that this We thus suggest that this complete seismic rheology also allows the dećollement of the upper coupling, due to the active overriding of the upper crustal layer above its ductile base, thus easing the plate, may be transient. When a slab breaks off (as motion of the upper crust over the subducting plate. shown in this case by seismic tomography; Spakman Thus, both the moderate seismic moment rate and et al., 1988), the cessation of the slab pull causes the the fast trenchward motion of the upper plate appear plate above it to rebound (e.g. Westaway, 1993; to have a common cause. The ductile lower crust of Yoshioka and Wortel, 1995; Buiter et al., 2001). The the upper plate, first, allows the dećollement above modeling results of Buiter et al. (2002) may provide a it, and thus the seaward relative motion of this first-order indication of this behaviour. Following slab upper crust, resulting in an active overriding with break-off, the excess rebound of the upper plate over full seismic coupling; then it allows free-gliding to the lower plate that unflexes from its bulge, favours resume on the interplate beneath it, resulting in only overriding of the upper plate, or at least here its upper a shallow downdip extent of the seismogenic zone part over an intracrustal dećollement. and correspondingly moderate seismic moment release. 3. Western Crete: same upper plate velocity and 2.3. Additional upper plate force, and lower plate moment release rate as the Ionian islands, but trigger, for overriding wider seismogenic zone and less seismic coupling

We presume that this inferred rheological layer- 3.1. Deeper downdip limit of seismogenic interplate ing of the upper plate is due to past orogenic and different upper plate structure thickening of the crust. However, recent studies (e.g., Westaway, 2002), suggest that much of the Offshore of western Crete, the spatial distribution topography and excess crustal thickness of the of earthquakes is significantly different than in the 90 M. Laigle et al. / Tectonophysics 391 (2004) 85–95

Ionian Islands. Earthquakes large enough to have been instrumental seismicity. However, Wyss and Baer instrumentally well-recorded and their focal mecha- (1981a) discussed historical reports of an earthquake nisms constrained involve thrust-faulting mechanisms in 1886 offshore the Messenia peninsula of south- down to ~40 km depth (Taymaz et al., 1990; Fig. 2), western Peloponnese (Fig. 2), and another in 1903 significantly deeper than their ~10–15 km depth limit between the southern Peloponnese and Kithira Island, in the Ionian Islands. This suggests that the seismo- representing possible evidence of Mz8 subduction genic part of the interface has a strong variation in its interplate earthquakes on this segment. Papazachos downdip limit (Fig. 3). and Kiratzi (1996) took into account such high We suggest that this variation may be governed by values of Mz8 for the maximum magnitude of a corresponding variation in the mechanical properties earthquakes and, using the approach of Molnar of the upper plate (cf. Oleskevich et al., 1999). (1979), estimated the seismic moment rate. Their However, it is difficult to test this suggestion, because results are much higher than previous ones: on the deep structure in and around Crete is not well- average ~2Â1013 Nm yearÀ1 per meter of arc length resolved and conflicting interpretations exist. For is being released in the segments of the arc offshore instance, Bohnhoff et al. (2001), from an OBS and of the Messenia peninsula (their zones 3a and 3A), land receiver refraction survey interpret the slab to be between the southern Peloponnese and the Kithira under a continental crust just west of Crete. However, straits (their zones 3b, 3c, and 3B) and in western Knapmeyer and Harjes (2000) interpret from their Crete (their zones 4a and 4A). These estimates are teleseismic receiver-function that the slab is overlain similar to those for the Cephalonia and Zante by sediments of a former accretionary wedge and is segments of the Ionian Islands region of Papazachos located much deeper than is interpreted by Bohnhoff and Kiratzi (1996). et al. (2001). Gravity investigations (Makris and Stobbe, 1984; Tsokas and Hansen, 1997) indicate that 3.3. Larger maximum magnitude, incomplete seismic the crustal thickness is less than 30 km beneath Crete, coupling compared with more than 40 beneath western Peloponnese and the Ionian Islands. However, one In the Ionian Islands, the maximum magnitude of clear difference is that (except for the localised high 7.2 assumed by Papazachos and Kiratzi (1996) topography on Crete itself, which reaches ~2 km) the matches the magnitude of the largest instrumentally typical altitude of the EarthTs surfaace in this region is documented earthquake (in 1953, in Cephalonia), much lower than in western Greece. Furthermore, a which probably ruptured a ~60 km length of the arc. crustal low-velocity zone, revealed in western Greece Such large earthquakes are expected to have a ~50 by seismic tomography (Papazachos and Nolet, 1997) year recurrence time in this region, are also expected also fades out southward. This evidence and the lower to rupture the full downdip seismogenic width of the typical crustal thickness in this region suggest the interface, and can account for the observed conver- possibility that the lower crust around Crete is less gence rate (Laigle et al., 2002). In western Crete, the ductile than beneath the Ionian Islands, and the lower ~40 km depth limit of the seismogenic part of the typical surface altitude indicates that any role of interface (compared with ~15 km) provides the excess gravitational potential energy in affecting the potential rupture area for much larger earthquakes, mechanics of subduction will also be reduced. easily sufficient to account for events of Mz8. However, even using the increased seismic moment 3.2. Same seismic moment rate rate estimates from Papazachos and Kiratzi (1996) only part of the plate interface that could give the For the part of the arc from south of the Ionian large earthquakes can be completely seismically Islands to west of Crete, we consider here the coupled at the Kahle et al. (2000) 40 mm/year estimates of seismic moment rates of Papazachos convergence rate. and Kiratzi (1996). Other studies (e.g., North, 1974; With the same seismic moment release rate here as Jackson and McKenzie, 1988a,b; Papazachos and for the Ionian islands, complete seismic coupling can Papaioannou, 1993) have been mainly based on then not be achieved on the greater downdip width M. Laigle et al. / Tectonophysics 391 (2004) 85–95 91 which is needed here for the Mz8 rupture assumed 4.1. Possible variation in maximum magnitude and for the regions south of the Ionian islands, if other return time from Peloponnese to Crete parameters scale. As already noted, the moment release rate and 3.4. Interplate normal force reduced by increased maximum magnitude of earthquakes are assumed slab-pull force: incomplete seismic coupling despite similar in the regions west of the Peloponnese, around trenchward motion of the upper plate Kithira Strait, and west of Crete (Papazachos and Kiratzi, 1996). Large earthquakes in the western part In the Ionian Islands, we proposed that the of this region at the turn of the 19th and 20th centuries seismicity permits complete seismic coupling across region have been interpreted as subduction events, the subduction interface, and that this suggestion is and used to forecast that a similar event as imminent consistent with the observed tranchward motion of the in the eastern part of this region (Wyss and Baer, upper plate, given the model of Scholz and Campos 1981a,b). The historical earthquake with the largest (1995). However, although the trenchward motion and estimated magnitude in Greece of 8.3, the Gortyna the seismic moment rate remain the same further event in AD 365, which caused destruction in Crete south, the seismicity indicates a low degree of seismic and damage as far as Patras (Papazachos and coupling across the subduction interface. Papazachou, 1997), could also have occurred in the In terms of the Scholz and Campos (1995) model, western Crete-Kithira Strait region. This earthquake we interpret this incomplete seismic coupling around has been recently attributed to a thrust fault 100 km Crete as a consequence of the strong slab pull force, offshore in this region, from a study of the paleo- which will reduce the compressive normal stress seismic record of western Crete by Stiros and across the subduction interface at shallow depths. A Papageorgiou (2001). These authors also stress that strong slab pull force is suggested from the length of this interpretation requires that no similar earthquake slab inferred from seismic tomography. In addition, has occurred in this area in the subsequent 16 the part of the slab that has broken off farther north centuries, in contrast with the recurrence interval of seems to have remained attached to the southern part a few centuries estimated for the largest earthquakes of the slab beneath Crete (Wortel and Spakman, off Messenia by Wyss and Baer (1981a,b). This 2000), so its weight adds to the slab pull force in this contrast suggests the possibility of a change in the area (cf. Dvorkin et al., 1993). We suggest that this mechanical properties of the arc between southern enhanced slab pull force may bring shallow parts of Peloponnese and western Crete. the subduction interface, which would normally be expected to slip seismically, into the field of the 4.2. Variation in slab geometry along the arc conditionally stable gliding, although rupture in large earthquakes could propagate upward into this region According to Papazachos and Nolet (1997), the (Fig. 2b). As a result, we suggest that seismic slab bends abruptly to a steeper dip over the depth coupling may be confined to a relatively narrow range of ~70 to ~120 km, between its part under depth range of the subduction interface, and that the Peloponnese and that under Crete. Earlier within this depth range seismic coupling may be tomographic studies, by Spakman (1990) and Spak- complete (Fig. 2b). man et al. (1988, 1993), mapped the seismic velocity anomalies caused by this slab to several hundred kilometers depth in the upper mantle 4. Possible lateral variation between the SW beneath Crete, significantly deeper than its ~150 Peloponnese and Crete km depth limit of seismicity. They resolved a lower velocity as separating the lithosphere at the surface We now look for evidence of lateral variations in from subducted lithosphere deeper under the Pelo- the mechanical properties of the Hellenic subduction ponnese, whereas it remains unresolved in a more zone between the contrasting end-member localities global tomography (Bijwaard et al., 1998). As a identified in the Ionian Islands and Crete. result, they interpreted the slab to be broken off 92 M. Laigle et al. / Tectonophysics 391 (2004) 85–95 beneath the Peloponnese and farther north. Tomog- 4.4. Shallow seismicity and interplate: respective raphy thus provides several lines of evidence locations suggesting lateral variations in slab geometry in this region, although they do not provide precise Earthquake locations (Fig. 2) indicate that seis- locations for details such as the present tip in the micity persists well offshore of the Peloponnese and interpreted tear in the slab. SW of Crete. However, as already noted, these epicenters may well be mislocated. A clear bathy- 4.3. Spatial variation in earthquakes locations along metric deep, the deepest part of the Eastern Medi- the arc terranean basin, is evident on the sea floor in this region (Fig. 2) and has been thought by many people Earthquake hypocenters have been used to delin- to mark the active subduction trench at the plate eate the shallow part of the slab (e.g. Hatzfeld and boundary. However, Lallemant et al. (1994) suggested Martin, 1992; Papazachos et al., 2000). Knapmeyer that this feature, which they called the South Matapan (1999) proposed that shallower than 150 km depth it Trough, is instead an active normal fault zone within dips inward toward the center of the Aegean, and the upper plate. reported a steepening from north to south, between the The true subduction interface could instead be Peloponnese and Kithira. However, many of the indicated farther south, ~100 km beyond the South routinely located hypocenters used in this study were Matapan Trough, beneath the Mediterranean Ridge not well-constrained, as shown by Papadopoulos et al. (itself usually interpreted as an accretionary wedge), (1988) comparing locations obtained using the tem- on the seismic reflection profiles of Le Meur porary addition of extra seismograph stations on the (1997). Their coverage, reaching ~9 km depth, outer arc. This addition of stations caused many shows what is interpreted as the edge of the epicenters to shift by tens of kilometers landward, upper-plate backstop to this accretionary wedge from apparent positions within the African plate to the (dashed dark gray line in Fig. 2), although it is plate boundary or possibly even the slab. They difficult to resolve due to very strong sea-bottom attributed the previous mislocation to the effect of multiples. The region ~50 km SW of the South the uneven distribution of permanent stations, and to Matapan Trough was also studied by multichannel the seismic velocity anomaly in the backarc region seismic refraction expanded spread profile ESP 9 of where they are located. Such heterogeneity of Truffert et al. (1992). They interpret the subduction structure was established by Taymaz (1996) using interface at 11 km depth, beneath 3.5 km of water, the spatial variation of the S–P traveltime residuals to under a thin wedge of upper-plate crust and mantle. teleseismic stations, for large earthquakes in this This implies that Aegean upper plate material region. The most reliable hypocentral depth estimates persists more than 50 km southward of the for earthquakes in this region are obtained by normally accepted location of the subduction trench, teleseismic waveform modeling or moment tensor and forms the backstop to the Mediterranean inversion (e.g., Taymaz et al., 1990; Fig. 2). ridge. Papazachos et al. (2000) discussed the focal Multi-channel seismic reflection profile STREAM- mechanisms of earthquakes in the vicinity of Kithira ERS AEG-01 (Fig. 4), which runs from the vicinity of as associated with the sinking and rollback of the profile ESP 9 northeastward across the site of profile slab. These events have occurred at depths of up to ESP 11 and the South Matapan Trough (Fig. 2), ~60 km just north of Matapan Trough (Fig. 2) and provides evidence in support of this view. It images a show downdip tension and arc-parallel compression. northward upsloping reflector beneath the young sea- Farther west there are significant shallower events, floor sediment, which we interpret as the top of the some with thrust mechanisms, which occurred on or wedge of upper-plate crust, located south of the South near the interplate boundary. These events all Matapan Trough. This confirms the suggestion by contribute to the recognition by Papazachos et al. Lallemant et al. (1994) that the South Matapan (2002) that this region will be the probable source Trough is not the subduction trench, but may instead of a future large earthquake. be an actively extending graben. M. Laigle et al. / Tectonophysics 391 (2004) 85–95 93

Fig. 4. Time-section of a normal-incidence reflection profile (see Fig. 1b for location) shot by N/O Nadir during the STREAMERS survey (Avedik et al., 1996), at 24-fold coverage with a 96-channel, 2.4 km streamer and a 1480 in.3 source with eight Generator-Injector airguns shot in single-bubble mode (Avedik et al., 1996). Two-way travel time in seconds is labelled at the right-hand edge of line. Label ESP 11 is at the approximate midpoint of the corresponding expanded spread profile of Truffert et al. (1992), and their ESP 9 midpoint is 20 km south of the left end of the line displayed. M is the sea-bottom multiple. Note the clear reflection under recent cover, shallowing northward, which has been highlighted with black dashes. It can be interpreted as top of the upper plate basement which further north, in the South Matapan Trough, appears dissected by normal-faulted half-graben structure, consistent with interpretation of the depression not as a subduction trench, but an extensional feature in the upper plate (Lallemant et al., 1994), analogous to a fore-arc basin.

We note also that this structure is located close 5. Conclusions to the presumed tip of the tear in the slab, between its attached and broken-off parts, suggesting a The western Hellenic subduction zone is charac- possible contribution of the lower plate slab terized by a trenchward velocity of the upper plate. mechanics to its formation. We indeed note that In the Ionian Islands segment, complete seismic excess subsidence just above the tip of a propagat- coupling is achieved, as is predicted by standard ing tear dissecting subducted lithosphere has been plate-tectonic models in which there is no slab pull proposed by Van der Meulen et al. (2000) as an force because the slab has broken off. The moderate explanation of the migration of the Pliocene depo- local seismic moment rate relates to a shallow centers they observed along the Italian Apennines. downdip limit for the seismogenic interface. This The lower plate may also change structure between characteristic may be attributed to the ductility of the Ionian Islands and Crete, possibly because of a the lower crust of the upper plate, which allows a transition from thinned continental crust resembling dećollement between the upper crust of the over- the Apulian continental margin to fully oceanic riding plate and the subducting plate. Farther south, crust, although its structure remains to be surveyed. a deeper downdip limit of the seismogenic interface Such a change may contribute to the variation in is indicated by thrust-faulting earthquakes, which seismic behaviour. persist much deeper in western Crete. A corre- 94 M. Laigle et al. / Tectonophysics 391 (2004) 85–95 spondingly larger downdip width of this seismo- Buiter, S.J.H., Govers, R., Wortel, M.J.R., 2001. A modelling study genic zone is consistent with the suggested larger of vertical surface displacements at convergent plate margins. Geophys. J. Int. 147, 415–427. maximum magnitude of earthquakes here. However, Buiter, S.J.H., Govers, R., Wortel, M.J.R., 2002. Two-dimensional since the seismic moment release rate seems to be simulations of surface deformation caused by slab detachment. moderate in the Peloponnese and western Crete, like in Tectonophysics 354, 195–210. the Ionian Islands, this seismically active interface Clarke, P., Davies, R., England, P., Parsons, B., Billiris, H., cannot maintain complete seismic coupling across its Paradissis, D., Veis, G., Cross, P., Denys, P., Ashkenazi, V., Bingley, R., Kahle, H.G., Briole, P., 1998. Crustal strain in larger downdip width. A cause may be the lateral central Greece from repeated GPS measurements in the interval addition of overweight to the part of the slab still 1989–1997. Geophys. J. 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Tectonophy- sics 227, 63–79. at anomalies in structure and seismicity, which need to Engdahl, E.R., van der Hilst, R., Buland, R., 1998. Global be resolved, may relate to the present location of the teleseismic earthquake relocation with improved travel times edge of the tear in the slab. and procedures for depth determination. Bull. Seismol. Soc. Am. 88, 722–743. Hatzfeld, D., Martin, C., 1992. Intermediate depth seismicity in the Acknowledgements Aegean defined by teleseismic data. Earth Planet. Sci. Lett. 113, 267–275. Hirn, A., Sachpazi, M., Siliqi, R., Mc Bride, J., Marnelis, F., the We thank Tuncay Taymaz for the opportunity of STREAMERS-PROFILES Group, 1996. A traverse of the presenting this work at the symposium, journal Ionian Islands front with coincident normal incidence and reviewers and Rob Westaway for constructive wide-angle seismics. Tectonophysics 264, 35–49. criticism and editing. Jackson, J., McKenzie, D., 1988. 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