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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 a
Laboratoire 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 b Geodynamics 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
* Corresponding author. Tel.: +33 14427 3914; fax: +33 14427 4783. E-mail address: [email protected] (M. Laigle). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.009
The active seismicity and active crustal deformation in the Eastern Mediterranean region (Fig. 1) have been discussed by many people since the pioneering plate tectonic studies by McKenzie (1970, 1972),
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Fig. 1. Sketch of the regional situation of the western Hellenic subduction zone, at the boundary between Aegea and Anatolia. Arrows are representative GPS velocity vectors with respect to stable Europe after Kahle et al. (2000). These indicate convergence at the subduction boundary: northward motion of Africa at 6 mm/ year and southwestward motion of the Aegean domain at 35 mm/ year.
Papazachos and Delibasis (1969), Papazachos and Comninakis (1971), and others. Subduction of the African plate is clearly expressed along the western Hellenic Arc, where intermediate-depth seismicity reveals a subducting slab and shallower flat thrustfaulting seismicity reveals an active seismogenic subduction interface (Fig. 2). In comparison with other subduction zones worldwide, the western Hellenic convergence of the Greek landmass with the Ionian Sea basin appears as an end-member case, as its overriding upper plate has a much faster absolute velocity toward the plate boundary than the subducting lower plate does (Fig. 1), as is inferred from geology and GPS measurements (e.g. Kahle et al., 2000). The upper plate is thus actively overriding the lower one. 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; Papazachos and Kiratzi, 1996; Tselentis et al., 1988; Baker et al., 1997), indicating that this subduction is occurring largely aseismically. Three possible reasons can explain such an apparent deficit of seismic slip. First, there may be only partial seismic coupling across the seismogenic part of the interface, the rest of the motion on it occurring as aseismic creep. Second,
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 plate in the model of Scholz and Campos (1995). This revised assessment resulted from estimating appropriate source models for the subduction earthquakes and dimensions of the seismogenic interface and other parameters, based on reinterpretations of the seismicity and analyses of the structure using seismic reflection profiling (Hirn et al., 1996; Sachpazi et al., 2000; Cle´ment et al., 2000). This new view on the seismic coupling of the Ionian Islands leads us to extend the discussion to the part of the Hellenic Arc farther south, to include its parts adjoining the Peloponnese (Messenia) region, the straits of Kithira, and western Crete (Fig. 2). We will suggest that in spite of a similar seismic moment rate, characteristics such as the spatial- and magnitude-distributions of interplate seismicity, as well as the deep structure, vary along the Hellenic arc. Structures and mechanisms clearly contrast between these Ionian Islands and western Crete, 300 km apart. However, at localities in between, the lack of reliable data leave the question open regarding seismic or aseismic behavior, although there are indications of very large earthquakes in the past.
2. Fast trenchward motion of the upper plate in the Ionian Islands region: upper plate rheology and slab breaking-off, complete seismic coupling and moderate seismicity We recall briefly the main results of our previous study of the Ionian Islands region (Laigle et al., 2002) and analyze the aspects of plate structure and
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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 investigate their variation along the arc. 2.1. Complete seismic coupling on a seismogenic zone limited to shallow depth As previously noted, the western Hellenic Arc has been commonly considered as a largely aseismic
subduction zone, from the comparison of a small value of the rate of shortening as derived from the rate of seismic moment release, with a large value of the rate of convergence inferred from geology. For its Ionian Islands part, we have shown that there may instead be complete seismic coupling (Laigle et al., 2002). This is consistent with the absolute velocity of the upper plate being directed toward
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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 Campos (1995). This situation can be explained because the slab pull force, which enters into the balance of tectonic forces, is missing here, as the slab has broken off (according to seismic tomography; e.g., Spakman et al., 1988). In the vicinity of Zante island (Fig. 2) and Cephalonia (farther north), we imaged the subduction interface and constrained its seismogenic downdip width to be moderate, of the order of 50 km, and restricted to shallow depths
of 10–15 km, from reflection seismic profiling and local earthquake tomography (Hirn et al., 1996; Cle´ ment et al., 2000; Sachpazi et al., 2000). Calculations, which take into account these observations, and assume a localized, subduction interplate fault (Brune, 1968; Scholz, 1998), indicate that although seismic moment release rate is moderate, it is consistent with the complete seismic coupling indicated by the fast trenchward motion of the upper plate. This result is also consistent with the
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occurrence of major earthquakes in the region, which are known to have maximum magnitudes of ~7.2, depths of 10–15 km, and recurrence times of ~50 years. 2.2. Downdip limit of seismogenic interplate due to ductility of the lower part of the upper plate, which also eases overriding We have interpreted the shallow downdip limit of this seismogenic zone as a consequence of rheological layering of the upper plate (Fig. 3a). At shallow depth, the interface is overlain by the ductile lower continental crust, which forms the root of the Hellenides mountain range. Crustal shortening in the region has caused crustal thickening, increasing the temperature of the deepest crust, from which it can be expected to be particularly mobile. Such ductile rheology allows free slip on the interface at shallow depth, causing the small downdip width and moderate seismic moment rate. Oleskevich et al. (1999) suggested that serpentinization may induce ductility in uppermost mantle. However, we propose that here the ductility is instead caused by the lower crust, and that this rheology also allows the de´collement of the upper crustal layer above its ductile base, thus easing the motion of the upper crust over the subducting plate. Thus, both the moderate seismic moment rate and the fast trenchward motion of the upper plate appear to have a common cause. The ductile lower crust of the upper plate, first, allows the de´collement above it, and thus the seaward relative motion of this upper crust, resulting in an active overriding with full seismic coupling; then it allows free-gliding to resume on the interplate beneath it, resulting in only a shallow downdip extent of the seismogenic zone and correspondingly moderate seismic moment release. 2.3. Additional upper plate force, and lower plate trigger, for overriding We presume that this inferred rheological layering of the upper plate is due to past orogenic thickening of the crust. However, recent studies (e.g., Westaway, 2002), suggest that much of the topography and excess crustal thickness of the
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Peloponnese has developed since the Early Pleistocene. Regardless of the mechanism, such elevated crust can be expected to spread laterally, to try to reduce its excess gravitational potential energy (cf. Clarke et al., 1998), and this mechanism may thus also contribute to the general southwestward motion of Anatolia and Aegea. A further supporting factor may be the recent onset of fast clockwise rotation of the Ionian islands less than 1 Ma ago (Duermeijer et al., 2000). They suggested that this rotation was a consequence of break off of the slab by lateral tearing, propagating from the north. They also suggested that this process may be coeval with a major change in the evolution of the Gulf of Corinth after ~1 Ma (cf. Armijo et al., 1996; Westaway, 2002). Armijo et al. (1996) suggested that this event marked an increase in the rate of rifting across this Corinth, resulting from result from the westward propagation of the North Anatolian fault into central Greece. Westaway (2002) suggested instead that this change resulted from coupling between surface processes and flow in the ductile lower crust, which caused the dramatic Middle–Late Pleistocene uplift of the Peloponnese already noted. We thus suggest that this complete seismic coupling, due to the active overriding of the upper plate, may be transient. When a slab breaks off (as shown in this case by seismic tomography; Spakman et al., 1988), the cessation of the slab pull causes the plate above it to rebound (e.g. Westaway, 1993; Yoshioka and Wortel, 1995; Buiter et al., 2001). The modeling results of Buiter et al. (2002) may provide a first-order indication of this behaviour. Following slab break-off, the excess rebound of the upper plate over the lower plate that unflexes from its bulge, favours overriding of the upper plate, or at least here its upper part over an intracrustal de´collement.
3. Western Crete: same upper plate velocity and moment release rate as the Ionian islands, but wider seismogenic zone and less seismic coupling 3.1. Deeper downdip limit of seismogenic interplate and different upper plate structure Offshore of western Crete, the spatial distribution of earthquakes is significantly different than in the
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Ionian Islands. Earthquakes large enough to have been instrumentally well-recorded and their focal mechanisms constrained involve thrust-faulting mechanisms down to ~40 km depth (Taymaz et al., 1990; Fig. 2), significantly deeper than their ~10–15 km depth limit in the Ionian Islands. This suggests that the seismogenic part of the interface has a strong variation in its downdip limit (Fig. 3). We suggest that this variation may be governed by a corresponding variation in the mechanical properties of the upper plate (cf. Oleskevich et al., 1999). However, it is difficult to test this suggestion, because the deep structure in and around Crete is not wellresolved and conflicting interpretations exist. For instance, Bohnhoff et al. (2001), from an OBS and land receiver refraction survey interpret the slab to be under a continental crust just west of Crete. However, Knapmeyer and Harjes (2000) interpret from their teleseismic receiver-function that the slab is overlain by sediments of a former accretionary wedge and is located much deeper than is interpreted by Bohnhoff et al. (2001). Gravity investigations (Makris and Stobbe, 1984; Tsokas and Hansen, 1997) indicate that the crustal thickness is less than 30 km beneath Crete, compared with more than 40 beneath western Peloponnese and the Ionian Islands. However, one clear difference is that (except for the localised high topography on Crete itself, which reaches ~2 km) the typical altitude of the EarthTs surfaace in this region is much lower than in western Greece. Furthermore, a crustal low-velocity zone, revealed in western Greece by seismic tomography (Papazachos and Nolet, 1997) also fades out southward. This evidence and the lower typical crustal thickness in this region suggest the possibility that the lower crust around Crete is less ductile than beneath the Ionian Islands, and the lower typical surface altitude indicates that any role of excess gravitational potential energy in affecting the mechanics of subduction will also be reduced. 3.2. Same seismic moment rate For the part of the arc from south of the Ionian Islands to west of Crete, we consider here the estimates of seismic moment rates of Papazachos and Kiratzi (1996). Other studies (e.g., North, 1974; Jackson and McKenzie, 1988a,b; Papazachos and Papaioannou, 1993) have been mainly based on
instrumental seismicity. However, Wyss and Baer (1981a) discussed historical reports of an earthquake in 1886 offshore the Messenia peninsula of southwestern Peloponnese (Fig. 2), and another in 1903 between the southern Peloponnese and Kithira Island, representing possible evidence of Mz8 subduction interplate earthquakes on this segment. Papazachos and Kiratzi (1996) took into account such high values of Mz8 for the maximum magnitude of earthquakes and, using the approach of Molnar (1979), estimated the seismic moment rate. Their results are much higher than previous ones: on average ~21013 Nm year 1 per meter of arc length is being released in the segments of the arc offshore of the Messenia peninsula (their zones 3a and 3A), between the southern Peloponnese and the Kithira straits (their zones 3b, 3c, and 3B) and in western Crete (their zones 4a and 4A). These estimates are similar to those for the Cephalonia and Zante segments of the Ionian Islands region of Papazachos and Kiratzi (1996). 3.3. Larger maximum magnitude, incomplete seismic coupling In the Ionian Islands, the maximum magnitude of 7.2 assumed by Papazachos and Kiratzi (1996) matches the magnitude of the largest instrumentally documented earthquake (in 1953, in Cephalonia), which probably ruptured a ~60 km length of the arc. Such large earthquakes are expected to have a ~50 year recurrence time in this region, are also expected to rupture the full downdip seismogenic width of the interface, and can account for the observed convergence rate (Laigle et al., 2002). In western Crete, the ~40 km depth limit of the seismogenic part of the interface (compared with ~15 km) provides the potential rupture area for much larger earthquakes, easily sufficient to account for events of Mz8. However, even using the increased seismic moment rate estimates from Papazachos and Kiratzi (1996) only part of the plate interface that could give the large earthquakes can be completely seismically coupled at the Kahle et al. (2000) 40 mm/year convergence rate. With the same seismic moment release rate here as for the Ionian islands, complete seismic coupling can then not be achieved on the greater downdip width
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which is needed here for the Mz8 rupture assumed for the regions south of the Ionian islands, if other parameters scale. 3.4. Interplate normal force reduced by increased slab-pull force: incomplete seismic coupling despite trenchward motion of the upper plate In the Ionian Islands, we proposed that the seismicity permits complete seismic coupling across the subduction interface, and that this suggestion is consistent with the observed tranchward motion of the upper plate, given the model of Scholz and Campos (1995). However, although the trenchward motion and the seismic moment rate remain the same further south, the seismicity indicates a low degree of seismic coupling across the subduction interface. In terms of the Scholz and Campos (1995) model, we interpret this incomplete seismic coupling around Crete as a consequence of the strong slab pull force, which will reduce the compressive normal stress across the subduction interface at shallow depths. A strong slab pull force is suggested from the length of slab inferred from seismic tomography. In addition, the part of the slab that has broken off farther north seems to have remained attached to the southern part of the slab beneath Crete (Wortel and Spakman, 2000), so its weight adds to the slab pull force in this area (cf. Dvorkin et al., 1993). We suggest that this enhanced slab pull force may bring shallow parts of the subduction interface, which would normally be expected to slip seismically, into the field of the conditionally stable gliding, although rupture in large earthquakes could propagate upward into this region (Fig. 2b). As a result, we suggest that seismic coupling may be confined to a relatively narrow depth range of the subduction interface, and that within this depth range seismic coupling may be complete (Fig. 2b).
4. Possible lateral variation between the SW Peloponnese and Crete We now look for evidence of lateral variations in the mechanical properties of the Hellenic subduction zone between the contrasting end-member localities identified in the Ionian Islands and Crete.
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4.1. Possible variation in maximum magnitude and return time from Peloponnese to Crete As already noted, the moment release rate and maximum magnitude of earthquakes are assumed similar in the regions west of the Peloponnese, around Kithira Strait, and west of Crete (Papazachos and Kiratzi, 1996). Large earthquakes in the western part of this region at the turn of the 19th and 20th centuries region have been interpreted as subduction events, and used to forecast that a similar event as imminent in the eastern part of this region (Wyss and Baer, 1981a,b). The historical earthquake with the largest estimated magnitude in Greece of 8.3, the Gortyna event in AD 365, which caused destruction in Crete and damage as far as Patras (Papazachos and Papazachou, 1997), could also have occurred in the western Crete-Kithira Strait region. This earthquake has been recently attributed to a thrust fault 100 km offshore in this region, from a study of the paleoseismic record of western Crete by Stiros and Papageorgiou (2001). These authors also stress that this interpretation requires that no similar earthquake has occurred in this area in the subsequent 16 centuries, in contrast with the recurrence interval of a few centuries estimated for the largest earthquakes off Messenia by Wyss and Baer (1981a,b). This contrast suggests the possibility of a change in the mechanical properties of the arc between southern Peloponnese and western Crete. 4.2. Variation in slab geometry along the arc According to Papazachos and Nolet (1997), the slab bends abruptly to a steeper dip over the depth range of ~70 to ~120 km, between its part under the Peloponnese and that under Crete. Earlier tomographic studies, by Spakman (1990) and Spakman et al. (1988, 1993), mapped the seismic velocity anomalies caused by this slab to several hundred kilometers depth in the upper mantle beneath Crete, significantly deeper than its ~150 km depth limit of seismicity. They resolved a lower velocity as separating the lithosphere at the surface from subducted lithosphere deeper under the Peloponnese, whereas it remains unresolved in a more global tomography (Bijwaard et al., 1998). As a result, they interpreted the slab to be broken off
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beneath the Peloponnese and farther north. Tomography thus provides several lines of evidence suggesting lateral variations in slab geometry in this region, although they do not provide precise locations for details such as the present tip in the interpreted tear in the slab. 4.3. Spatial variation in earthquakes locations along the arc Earthquake hypocenters have been used to delineate the shallow part of the slab (e.g. Hatzfeld and Martin, 1992; Papazachos et al., 2000). Knapmeyer (1999) proposed that shallower than 150 km depth it dips inward toward the center of the Aegean, and reported a steepening from north to south, between the Peloponnese and Kithira. However, many of the routinely located hypocenters used in this study were not well-constrained, as shown by Papadopoulos et al. (1988) comparing locations obtained using the temporary addition of extra seismograph stations on the outer arc. This addition of stations caused many epicenters to shift by tens of kilometers landward, from apparent positions within the African plate to the plate boundary or possibly even the slab. They attributed the previous mislocation to the effect of the uneven distribution of permanent stations, and to the seismic velocity anomaly in the backarc region where they are located. Such heterogeneity of structure was established by Taymaz (1996) using the spatial variation of the S–P traveltime residuals to teleseismic stations, for large earthquakes in this region. The most reliable hypocentral depth estimates for earthquakes in this region are obtained by teleseismic waveform modeling or moment tensor inversion (e.g., Taymaz et al., 1990; Fig. 2). Papazachos et al. (2000) discussed the focal mechanisms of earthquakes in the vicinity of Kithira as associated with the sinking and rollback of the slab. These events have occurred at depths of up to ~60 km just north of Matapan Trough (Fig. 2) and show downdip tension and arc-parallel compression. Farther west there are significant shallower events, some with thrust mechanisms, which occurred on or near the interplate boundary. These events all contribute to the recognition by Papazachos et al. (2002) that this region will be the probable source of a future large earthquake.
4.4. Shallow seismicity and interplate: respective locations Earthquake locations (Fig. 2) indicate that seismicity persists well offshore of the Peloponnese and SW of Crete. However, as already noted, these epicenters may well be mislocated. A clear bathymetric deep, the deepest part of the Eastern Mediterranean basin, is evident on the sea floor in this region (Fig. 2) and has been thought by many people to mark the active subduction trench at the plate boundary. However, Lallemant et al. (1994) suggested that this feature, which they called the South Matapan Trough, is instead an active normal fault zone within the upper plate. The true subduction interface could instead be indicated farther south, ~100 km beyond the South Matapan Trough, beneath the Mediterranean Ridge (itself usually interpreted as an accretionary wedge), on the seismic reflection profiles of Le Meur (1997). Their coverage, reaching ~9 km depth, shows what is interpreted as the edge of the upper-plate backstop to this accretionary wedge (dashed dark gray line in Fig. 2), although it is difficult to resolve due to very strong sea-bottom multiples. The region ~50 km SW of the South Matapan Trough was also studied by multichannel seismic refraction expanded spread profile ESP 9 of Truffert et al. (1992). They interpret the subduction interface at 11 km depth, beneath 3.5 km of water, under a thin wedge of upper-plate crust and mantle. This implies that Aegean upper plate material persists more than 50 km southward of the normally accepted location of the subduction trench, and forms the backstop to the Mediterranean ridge. Multi-channel seismic reflection profile STREAMERS AEG-01 (Fig. 4), which runs from the vicinity of profile ESP 9 northeastward across the site of profile ESP 11 and the South Matapan Trough (Fig. 2), provides evidence in support of this view. It images a northward upsloping reflector beneath the young seafloor sediment, which we interpret as the top of the wedge of upper-plate crust, located south of the South Matapan Trough. This confirms the suggestion by Lallemant et al. (1994) that the South Matapan Trough is not the subduction trench, but may instead be an actively extending graben.
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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 to the presumed tip of the tear in the slab, between its attached and broken-off parts, suggesting a possible contribution of the lower plate slab mechanics to its formation. We indeed note that excess subsidence just above the tip of a propagating tear dissecting subducted lithosphere has been proposed by Van der Meulen et al. (2000) as an explanation of the migration of the Pliocene depocenters they observed along the Italian Apennines. The lower plate may also change structure between the Ionian Islands and Crete, possibly because of a transition from thinned continental crust resembling the Apulian continental margin to fully oceanic crust, although its structure remains to be surveyed. Such a change may contribute to the variation in seismic behaviour.
5. Conclusions 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 corre-
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spondingly 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 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.
Acknowledgements We thank Tuncay Taymaz for the opportunity of presenting this work at the symposium, journal reviewers and Rob Westaway for constructive criticism and editing.
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Spakman, W., Wortel, M.J.R., Vlaar, N.J., 1988. The Hellenic subduction zone: a tomographic image and its geodynamic implications. Geophys. Res. Lett. 15, 60 – 63. Spakman, W., van der Lee, S., van der Hilst, R.D., 1993. Traveltime tomography of the European-Mediterranean mantle down to 1400 km. Phys. Earth Planet. Inter. 79, 3 – 74. Stiros, S.C., Papageorgiou, S., 2001. Seismicity of Western Crete and the destruction of the town of Kisamos at AD 365: archaeological evidence. J. Seismol. 5, 381 – 397. Taymaz, T., 1996. S-P traveltime residuals from earthquakes and lateral inhomogeneity in the upper mantle beneath the Aegean and the Hellenic Trench near Crete. Geophys. J. Int. 127, 545 – 558. Taymaz, T., Jackson, J., Westaway, R., 1990. Earthquake mechanisms in the Hellenic Trench near Crete. Geophys. J. Int. 102, 695 – 731. Truffert, C., Chamot-Rooke, N., Lallemant, S., de Voogd, B., Huchon, P., Le Pichon, X., 1992. A crustal-scale cross-section of the western Mediterranean Ridge from deep seismic data and gravity modelling. Geophys. J. Int. 114, 360 – 372. Tselentis, G.-A., Stavrakakis, G., Makropoulos, K., Latousakis, J., Drakopoulos, J., 1988. Seismic moments of earthquakes at the western Hellenic arc and their application to the seismic hazard in the area. Tectonophysics 148, 73 – 82. Tsokas, G., Hansen, R., 1997. Study of the crustal thickness and subducting lithosphere in Greece from gravity data. J. Geophys. Res. 102, 20585 – 20597. Van der Meulen, M.J., Buiter, S.J.H., Meulenkamp, J.E., Wortel, M.J.R., 2000. An early Pliocene uplift of the central Apenninic foredeep and its geodynamical significance. Tectonics 19, 300 – 313. Westaway, R., 1993. Quaternary uplift of southern Italy. J. Geophys. Res. 98, 21741 – 21772. Westaway, R., 2002. The Quaternary evolution of the Gulf of Corinth, central Greece: coupling between surface processes and flow in the lower continental crust. Tectonophysics 348, 269 – 318. Wortel, M.J.R., Spakman, W., 2000. Subduction and slab detachment in the Mediterranean-Carpathian region. Science 290, 1910 – 1917. Wyss, M., Baer, M., 1981a. Earthquake hazard in the Hellenic Arc. In: Simpson, D.W., Richards, P.G. (Eds.), Earthquake Prediction, An International Review, AGU Maurice Ewing Series, vol. 4, pp. 153 – 172. Wyss, M., Baer, M., 1981b. Seismic quiescence in the Western Hellenic Arc may foreshadow large earthquakes. Nature 289, 785 – 787. Yoshioka, S., Wortel, M.J.R., 1995. Three-dimensional numerical modelling of detachment of subducted lithosphere. J. Geophys. Res. 100, 20233 – 20244.
Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone Mireille Laiglea,*, Maria Sachpazib, Alfred Hirna a
Laboratoire 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 b Geodynamics 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
* Corresponding author. Tel.: +33 14427 3914; fax: +33 14427 4783. E-mail address: [email protected] (M. Laigle). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.009
The active seismicity and active crustal deformation in the Eastern Mediterranean region (Fig. 1) have been discussed by many people since the pioneering plate tectonic studies by McKenzie (1970, 1972),
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Fig. 1. Sketch of the regional situation of the western Hellenic subduction zone, at the boundary between Aegea and Anatolia. Arrows are representative GPS velocity vectors with respect to stable Europe after Kahle et al. (2000). These indicate convergence at the subduction boundary: northward motion of Africa at 6 mm/ year and southwestward motion of the Aegean domain at 35 mm/ year.
Papazachos and Delibasis (1969), Papazachos and Comninakis (1971), and others. Subduction of the African plate is clearly expressed along the western Hellenic Arc, where intermediate-depth seismicity reveals a subducting slab and shallower flat thrustfaulting seismicity reveals an active seismogenic subduction interface (Fig. 2). In comparison with other subduction zones worldwide, the western Hellenic convergence of the Greek landmass with the Ionian Sea basin appears as an end-member case, as its overriding upper plate has a much faster absolute velocity toward the plate boundary than the subducting lower plate does (Fig. 1), as is inferred from geology and GPS measurements (e.g. Kahle et al., 2000). The upper plate is thus actively overriding the lower one. 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; Papazachos and Kiratzi, 1996; Tselentis et al., 1988; Baker et al., 1997), indicating that this subduction is occurring largely aseismically. Three possible reasons can explain such an apparent deficit of seismic slip. First, there may be only partial seismic coupling across the seismogenic part of the interface, the rest of the motion on it occurring as aseismic creep. Second,
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 plate in the model of Scholz and Campos (1995). This revised assessment resulted from estimating appropriate source models for the subduction earthquakes and dimensions of the seismogenic interface and other parameters, based on reinterpretations of the seismicity and analyses of the structure using seismic reflection profiling (Hirn et al., 1996; Sachpazi et al., 2000; Cle´ment et al., 2000). This new view on the seismic coupling of the Ionian Islands leads us to extend the discussion to the part of the Hellenic Arc farther south, to include its parts adjoining the Peloponnese (Messenia) region, the straits of Kithira, and western Crete (Fig. 2). We will suggest that in spite of a similar seismic moment rate, characteristics such as the spatial- and magnitude-distributions of interplate seismicity, as well as the deep structure, vary along the Hellenic arc. Structures and mechanisms clearly contrast between these Ionian Islands and western Crete, 300 km apart. However, at localities in between, the lack of reliable data leave the question open regarding seismic or aseismic behavior, although there are indications of very large earthquakes in the past.
2. Fast trenchward motion of the upper plate in the Ionian Islands region: upper plate rheology and slab breaking-off, complete seismic coupling and moderate seismicity We recall briefly the main results of our previous study of the Ionian Islands region (Laigle et al., 2002) and analyze the aspects of plate structure and
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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 investigate their variation along the arc. 2.1. Complete seismic coupling on a seismogenic zone limited to shallow depth As previously noted, the western Hellenic Arc has been commonly considered as a largely aseismic
subduction zone, from the comparison of a small value of the rate of shortening as derived from the rate of seismic moment release, with a large value of the rate of convergence inferred from geology. For its Ionian Islands part, we have shown that there may instead be complete seismic coupling (Laigle et al., 2002). This is consistent with the absolute velocity of the upper plate being directed toward
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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 Campos (1995). This situation can be explained because the slab pull force, which enters into the balance of tectonic forces, is missing here, as the slab has broken off (according to seismic tomography; e.g., Spakman et al., 1988). In the vicinity of Zante island (Fig. 2) and Cephalonia (farther north), we imaged the subduction interface and constrained its seismogenic downdip width to be moderate, of the order of 50 km, and restricted to shallow depths
of 10–15 km, from reflection seismic profiling and local earthquake tomography (Hirn et al., 1996; Cle´ ment et al., 2000; Sachpazi et al., 2000). Calculations, which take into account these observations, and assume a localized, subduction interplate fault (Brune, 1968; Scholz, 1998), indicate that although seismic moment release rate is moderate, it is consistent with the complete seismic coupling indicated by the fast trenchward motion of the upper plate. This result is also consistent with the
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occurrence of major earthquakes in the region, which are known to have maximum magnitudes of ~7.2, depths of 10–15 km, and recurrence times of ~50 years. 2.2. Downdip limit of seismogenic interplate due to ductility of the lower part of the upper plate, which also eases overriding We have interpreted the shallow downdip limit of this seismogenic zone as a consequence of rheological layering of the upper plate (Fig. 3a). At shallow depth, the interface is overlain by the ductile lower continental crust, which forms the root of the Hellenides mountain range. Crustal shortening in the region has caused crustal thickening, increasing the temperature of the deepest crust, from which it can be expected to be particularly mobile. Such ductile rheology allows free slip on the interface at shallow depth, causing the small downdip width and moderate seismic moment rate. Oleskevich et al. (1999) suggested that serpentinization may induce ductility in uppermost mantle. However, we propose that here the ductility is instead caused by the lower crust, and that this rheology also allows the de´collement of the upper crustal layer above its ductile base, thus easing the motion of the upper crust over the subducting plate. Thus, both the moderate seismic moment rate and the fast trenchward motion of the upper plate appear to have a common cause. The ductile lower crust of the upper plate, first, allows the de´collement above it, and thus the seaward relative motion of this upper crust, resulting in an active overriding with full seismic coupling; then it allows free-gliding to resume on the interplate beneath it, resulting in only a shallow downdip extent of the seismogenic zone and correspondingly moderate seismic moment release. 2.3. Additional upper plate force, and lower plate trigger, for overriding We presume that this inferred rheological layering of the upper plate is due to past orogenic thickening of the crust. However, recent studies (e.g., Westaway, 2002), suggest that much of the topography and excess crustal thickness of the
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Peloponnese has developed since the Early Pleistocene. Regardless of the mechanism, such elevated crust can be expected to spread laterally, to try to reduce its excess gravitational potential energy (cf. Clarke et al., 1998), and this mechanism may thus also contribute to the general southwestward motion of Anatolia and Aegea. A further supporting factor may be the recent onset of fast clockwise rotation of the Ionian islands less than 1 Ma ago (Duermeijer et al., 2000). They suggested that this rotation was a consequence of break off of the slab by lateral tearing, propagating from the north. They also suggested that this process may be coeval with a major change in the evolution of the Gulf of Corinth after ~1 Ma (cf. Armijo et al., 1996; Westaway, 2002). Armijo et al. (1996) suggested that this event marked an increase in the rate of rifting across this Corinth, resulting from result from the westward propagation of the North Anatolian fault into central Greece. Westaway (2002) suggested instead that this change resulted from coupling between surface processes and flow in the ductile lower crust, which caused the dramatic Middle–Late Pleistocene uplift of the Peloponnese already noted. We thus suggest that this complete seismic coupling, due to the active overriding of the upper plate, may be transient. When a slab breaks off (as shown in this case by seismic tomography; Spakman et al., 1988), the cessation of the slab pull causes the plate above it to rebound (e.g. Westaway, 1993; Yoshioka and Wortel, 1995; Buiter et al., 2001). The modeling results of Buiter et al. (2002) may provide a first-order indication of this behaviour. Following slab break-off, the excess rebound of the upper plate over the lower plate that unflexes from its bulge, favours overriding of the upper plate, or at least here its upper part over an intracrustal de´collement.
3. Western Crete: same upper plate velocity and moment release rate as the Ionian islands, but wider seismogenic zone and less seismic coupling 3.1. Deeper downdip limit of seismogenic interplate and different upper plate structure Offshore of western Crete, the spatial distribution of earthquakes is significantly different than in the
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Ionian Islands. Earthquakes large enough to have been instrumentally well-recorded and their focal mechanisms constrained involve thrust-faulting mechanisms down to ~40 km depth (Taymaz et al., 1990; Fig. 2), significantly deeper than their ~10–15 km depth limit in the Ionian Islands. This suggests that the seismogenic part of the interface has a strong variation in its downdip limit (Fig. 3). We suggest that this variation may be governed by a corresponding variation in the mechanical properties of the upper plate (cf. Oleskevich et al., 1999). However, it is difficult to test this suggestion, because the deep structure in and around Crete is not wellresolved and conflicting interpretations exist. For instance, Bohnhoff et al. (2001), from an OBS and land receiver refraction survey interpret the slab to be under a continental crust just west of Crete. However, Knapmeyer and Harjes (2000) interpret from their teleseismic receiver-function that the slab is overlain by sediments of a former accretionary wedge and is located much deeper than is interpreted by Bohnhoff et al. (2001). Gravity investigations (Makris and Stobbe, 1984; Tsokas and Hansen, 1997) indicate that the crustal thickness is less than 30 km beneath Crete, compared with more than 40 beneath western Peloponnese and the Ionian Islands. However, one clear difference is that (except for the localised high topography on Crete itself, which reaches ~2 km) the typical altitude of the EarthTs surfaace in this region is much lower than in western Greece. Furthermore, a crustal low-velocity zone, revealed in western Greece by seismic tomography (Papazachos and Nolet, 1997) also fades out southward. This evidence and the lower typical crustal thickness in this region suggest the possibility that the lower crust around Crete is less ductile than beneath the Ionian Islands, and the lower typical surface altitude indicates that any role of excess gravitational potential energy in affecting the mechanics of subduction will also be reduced. 3.2. Same seismic moment rate For the part of the arc from south of the Ionian Islands to west of Crete, we consider here the estimates of seismic moment rates of Papazachos and Kiratzi (1996). Other studies (e.g., North, 1974; Jackson and McKenzie, 1988a,b; Papazachos and Papaioannou, 1993) have been mainly based on
instrumental seismicity. However, Wyss and Baer (1981a) discussed historical reports of an earthquake in 1886 offshore the Messenia peninsula of southwestern Peloponnese (Fig. 2), and another in 1903 between the southern Peloponnese and Kithira Island, representing possible evidence of Mz8 subduction interplate earthquakes on this segment. Papazachos and Kiratzi (1996) took into account such high values of Mz8 for the maximum magnitude of earthquakes and, using the approach of Molnar (1979), estimated the seismic moment rate. Their results are much higher than previous ones: on average ~21013 Nm year 1 per meter of arc length is being released in the segments of the arc offshore of the Messenia peninsula (their zones 3a and 3A), between the southern Peloponnese and the Kithira straits (their zones 3b, 3c, and 3B) and in western Crete (their zones 4a and 4A). These estimates are similar to those for the Cephalonia and Zante segments of the Ionian Islands region of Papazachos and Kiratzi (1996). 3.3. Larger maximum magnitude, incomplete seismic coupling In the Ionian Islands, the maximum magnitude of 7.2 assumed by Papazachos and Kiratzi (1996) matches the magnitude of the largest instrumentally documented earthquake (in 1953, in Cephalonia), which probably ruptured a ~60 km length of the arc. Such large earthquakes are expected to have a ~50 year recurrence time in this region, are also expected to rupture the full downdip seismogenic width of the interface, and can account for the observed convergence rate (Laigle et al., 2002). In western Crete, the ~40 km depth limit of the seismogenic part of the interface (compared with ~15 km) provides the potential rupture area for much larger earthquakes, easily sufficient to account for events of Mz8. However, even using the increased seismic moment rate estimates from Papazachos and Kiratzi (1996) only part of the plate interface that could give the large earthquakes can be completely seismically coupled at the Kahle et al. (2000) 40 mm/year convergence rate. With the same seismic moment release rate here as for the Ionian islands, complete seismic coupling can then not be achieved on the greater downdip width
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which is needed here for the Mz8 rupture assumed for the regions south of the Ionian islands, if other parameters scale. 3.4. Interplate normal force reduced by increased slab-pull force: incomplete seismic coupling despite trenchward motion of the upper plate In the Ionian Islands, we proposed that the seismicity permits complete seismic coupling across the subduction interface, and that this suggestion is consistent with the observed tranchward motion of the upper plate, given the model of Scholz and Campos (1995). However, although the trenchward motion and the seismic moment rate remain the same further south, the seismicity indicates a low degree of seismic coupling across the subduction interface. In terms of the Scholz and Campos (1995) model, we interpret this incomplete seismic coupling around Crete as a consequence of the strong slab pull force, which will reduce the compressive normal stress across the subduction interface at shallow depths. A strong slab pull force is suggested from the length of slab inferred from seismic tomography. In addition, the part of the slab that has broken off farther north seems to have remained attached to the southern part of the slab beneath Crete (Wortel and Spakman, 2000), so its weight adds to the slab pull force in this area (cf. Dvorkin et al., 1993). We suggest that this enhanced slab pull force may bring shallow parts of the subduction interface, which would normally be expected to slip seismically, into the field of the conditionally stable gliding, although rupture in large earthquakes could propagate upward into this region (Fig. 2b). As a result, we suggest that seismic coupling may be confined to a relatively narrow depth range of the subduction interface, and that within this depth range seismic coupling may be complete (Fig. 2b).
4. Possible lateral variation between the SW Peloponnese and Crete We now look for evidence of lateral variations in the mechanical properties of the Hellenic subduction zone between the contrasting end-member localities identified in the Ionian Islands and Crete.
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4.1. Possible variation in maximum magnitude and return time from Peloponnese to Crete As already noted, the moment release rate and maximum magnitude of earthquakes are assumed similar in the regions west of the Peloponnese, around Kithira Strait, and west of Crete (Papazachos and Kiratzi, 1996). Large earthquakes in the western part of this region at the turn of the 19th and 20th centuries region have been interpreted as subduction events, and used to forecast that a similar event as imminent in the eastern part of this region (Wyss and Baer, 1981a,b). The historical earthquake with the largest estimated magnitude in Greece of 8.3, the Gortyna event in AD 365, which caused destruction in Crete and damage as far as Patras (Papazachos and Papazachou, 1997), could also have occurred in the western Crete-Kithira Strait region. This earthquake has been recently attributed to a thrust fault 100 km offshore in this region, from a study of the paleoseismic record of western Crete by Stiros and Papageorgiou (2001). These authors also stress that this interpretation requires that no similar earthquake has occurred in this area in the subsequent 16 centuries, in contrast with the recurrence interval of a few centuries estimated for the largest earthquakes off Messenia by Wyss and Baer (1981a,b). This contrast suggests the possibility of a change in the mechanical properties of the arc between southern Peloponnese and western Crete. 4.2. Variation in slab geometry along the arc According to Papazachos and Nolet (1997), the slab bends abruptly to a steeper dip over the depth range of ~70 to ~120 km, between its part under the Peloponnese and that under Crete. Earlier tomographic studies, by Spakman (1990) and Spakman et al. (1988, 1993), mapped the seismic velocity anomalies caused by this slab to several hundred kilometers depth in the upper mantle beneath Crete, significantly deeper than its ~150 km depth limit of seismicity. They resolved a lower velocity as separating the lithosphere at the surface from subducted lithosphere deeper under the Peloponnese, whereas it remains unresolved in a more global tomography (Bijwaard et al., 1998). As a result, they interpreted the slab to be broken off
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beneath the Peloponnese and farther north. Tomography thus provides several lines of evidence suggesting lateral variations in slab geometry in this region, although they do not provide precise locations for details such as the present tip in the interpreted tear in the slab. 4.3. Spatial variation in earthquakes locations along the arc Earthquake hypocenters have been used to delineate the shallow part of the slab (e.g. Hatzfeld and Martin, 1992; Papazachos et al., 2000). Knapmeyer (1999) proposed that shallower than 150 km depth it dips inward toward the center of the Aegean, and reported a steepening from north to south, between the Peloponnese and Kithira. However, many of the routinely located hypocenters used in this study were not well-constrained, as shown by Papadopoulos et al. (1988) comparing locations obtained using the temporary addition of extra seismograph stations on the outer arc. This addition of stations caused many epicenters to shift by tens of kilometers landward, from apparent positions within the African plate to the plate boundary or possibly even the slab. They attributed the previous mislocation to the effect of the uneven distribution of permanent stations, and to the seismic velocity anomaly in the backarc region where they are located. Such heterogeneity of structure was established by Taymaz (1996) using the spatial variation of the S–P traveltime residuals to teleseismic stations, for large earthquakes in this region. The most reliable hypocentral depth estimates for earthquakes in this region are obtained by teleseismic waveform modeling or moment tensor inversion (e.g., Taymaz et al., 1990; Fig. 2). Papazachos et al. (2000) discussed the focal mechanisms of earthquakes in the vicinity of Kithira as associated with the sinking and rollback of the slab. These events have occurred at depths of up to ~60 km just north of Matapan Trough (Fig. 2) and show downdip tension and arc-parallel compression. Farther west there are significant shallower events, some with thrust mechanisms, which occurred on or near the interplate boundary. These events all contribute to the recognition by Papazachos et al. (2002) that this region will be the probable source of a future large earthquake.
4.4. Shallow seismicity and interplate: respective locations Earthquake locations (Fig. 2) indicate that seismicity persists well offshore of the Peloponnese and SW of Crete. However, as already noted, these epicenters may well be mislocated. A clear bathymetric deep, the deepest part of the Eastern Mediterranean basin, is evident on the sea floor in this region (Fig. 2) and has been thought by many people to mark the active subduction trench at the plate boundary. However, Lallemant et al. (1994) suggested that this feature, which they called the South Matapan Trough, is instead an active normal fault zone within the upper plate. The true subduction interface could instead be indicated farther south, ~100 km beyond the South Matapan Trough, beneath the Mediterranean Ridge (itself usually interpreted as an accretionary wedge), on the seismic reflection profiles of Le Meur (1997). Their coverage, reaching ~9 km depth, shows what is interpreted as the edge of the upper-plate backstop to this accretionary wedge (dashed dark gray line in Fig. 2), although it is difficult to resolve due to very strong sea-bottom multiples. The region ~50 km SW of the South Matapan Trough was also studied by multichannel seismic refraction expanded spread profile ESP 9 of Truffert et al. (1992). They interpret the subduction interface at 11 km depth, beneath 3.5 km of water, under a thin wedge of upper-plate crust and mantle. This implies that Aegean upper plate material persists more than 50 km southward of the normally accepted location of the subduction trench, and forms the backstop to the Mediterranean ridge. Multi-channel seismic reflection profile STREAMERS AEG-01 (Fig. 4), which runs from the vicinity of profile ESP 9 northeastward across the site of profile ESP 11 and the South Matapan Trough (Fig. 2), provides evidence in support of this view. It images a northward upsloping reflector beneath the young seafloor sediment, which we interpret as the top of the wedge of upper-plate crust, located south of the South Matapan Trough. This confirms the suggestion by Lallemant et al. (1994) that the South Matapan Trough is not the subduction trench, but may instead be an actively extending graben.
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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 to the presumed tip of the tear in the slab, between its attached and broken-off parts, suggesting a possible contribution of the lower plate slab mechanics to its formation. We indeed note that excess subsidence just above the tip of a propagating tear dissecting subducted lithosphere has been proposed by Van der Meulen et al. (2000) as an explanation of the migration of the Pliocene depocenters they observed along the Italian Apennines. The lower plate may also change structure between the Ionian Islands and Crete, possibly because of a transition from thinned continental crust resembling the Apulian continental margin to fully oceanic crust, although its structure remains to be surveyed. Such a change may contribute to the variation in seismic behaviour.
5. Conclusions 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 corre-
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spondingly 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 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.
Acknowledgements We thank Tuncay Taymaz for the opportunity of presenting this work at the symposium, journal reviewers and Rob Westaway for constructive criticism and editing.
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