Receiver function images of the Hellenic subduction zone and comparison to microseismicity

New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23–27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati–Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago

Focal mechanisms in the southern Aegean from temporary seismic networks – implications for the regional stress field and ongoing deformation processes

The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located microearthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini–Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW–SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE–SSW. Intermediate-depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east–west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW–SE compression. Tensional principal stresses in the crust exhibit very good alignment with extensional strain rate principal axes derived from GPS velocities except in volcanic areas, where both appear to be unrelated, and in the forearc where compressional principal stresses are very well aligned with compressional principal strain rates. This finding indicates that, except for volcanic areas, microseismic activity in the southern Aegean is not controlled by small-scale local stresses but rather reflects the regional stress field. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the southeast, segmentation of the slab, arc volcanism and extension of the Aegean crust

Das OBS-Netzwerk des EGELADOS-Projektes

Das OBS-Netzwerk des EGELADOS-ProjektesSchmidt, A. (1); Brüstle, A. (1); Friederich, W. (1); Meier, T. (1);Schmidt-Aursch, M. (2) & EGELADOS working group(1) Institut für Geologie, Mineralogie und Geophysik der Ruhr-Universität Bochum(2) Alfred-Wegener-Institut für Polar- und Meeresforschung, BremerhavenStichworte: EGELADOS-Netzwerk, Ozeanbodenstationen, hochauflösende seismologische UntersuchungenDas EGELADOS-Projekt umfasst ein Netzwerk von 89 Breitband-Stationen, verteilt über das gesamte Gebiet der Hellenischen Subduktionszone. Aufgrund der geographischen Gegebenheiten wurde es notwendig, neben den auf den Inseln gelegenen Landstationen, auch eine Reihe von seismischen Stationen am Meeresboden zu installieren, um eine annähernd gleichmäßige Verteilung zu gewährleisten. Durch die zusätzliche Verwendung von Ozeanbodenstationen (OBS) des neuen deutschen Pools amphibischer Seismometer (DEPAS) ergibt sich die Möglichkeit, einen mittleren Stationsabstand von 60 km im EGELADOS-Netzwerk zu erreichen. Im Mai 2006 wurden zu diesem Zweck 24 OBS mit dem Forschungsschiff Poseidon in der südlichen Ägäis ausgesetzt.Ziel dieses Projektes ist es, hochauflösende seismologische Untersuchungen im Gebiet der Hellenischen Subduktionszone durchführen zu können. Die hohe Dichte des Netzwerkes ist für die flächendeckende Abbildung der Seismizität von enormer Bedeutung. Ebenfalls soll für die Scher- und Oberflächenwellentomographie einemöglichst lückenlose Überdeckung erreicht werden, wodurch kleinere Strukturen im Mantelkeil und am Plattenkontakt untersucht werden können.Die OBS bestehen aus einem breitbandigen Seismometer (60 s - 50 Hz) und einem Hydrophon (100 s - 8 kHz). Die Einsatztiefe der ausgeliehenen Geräte wird mit bis zu 6000 m angegeben, womit diese für die Erforschung der Hellenischen Subduktionszone mit einer maximalen Meerestiefe von 4400 m geeignet sind. Die Stromversorgung erfolgt durch die Verwendung von Lithium-Zellen, die eine Einsatzdauer von bis zu 16 Monaten ermöglichen. Nach einer Laufzeit von 10 Monaten werden die OBS im März 2007 wieder geborgen