5 research outputs found
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