89 research outputs found
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
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
Torsion pairs and rigid objects in tubes
We classify the torsion pairs in a tube category and show that they are in
bijection with maximal rigid objects in the extension of the tube category
containing the Pruefer and adic modules. We show that the annulus geometric
model for the tube category can be extended to the larger category and
interpret torsion pairs, maximal rigid objects and the bijection between them
geometrically. We also give a similar geometric description in the case of the
linear orientation of a Dynkin quiver of type A.Comment: 25 pages, 13 figures. Paper shortened. Minor errors correcte
Enhanced TCR-induced Apoptosis in Interferon Regulatory Factor 4–deficient CD4+ Th Cells
Transcription factors of the interferon regulatory factor (IRF) family contribute to the regulation of cell proliferation and apoptosis. Here, we show that CD4+ T helper (Th) cells lacking IRF4 (IRF4−/−) are highly sensitive to apoptosis. After infection of IRF4−/− mice with the protozoan parasite Leishmania major, the lesion-draining lymph nodes developed the prototypic lymphadenopathy of wild-type mice after 4 wk, but demonstrated almost total loss of cellularity and enhanced apoptosis after 7 wk. In vitro, activation of IRF4−/− CD4+ Th cells led to greatly increased apoptosis compared with wild-type cells. Coculture of IRF4−/− and IRF4+/+ CD4+ cells did not increase survival of IRF4−/− CD4+ cells, indicating that the enhanced rate of IRF4−/− Th cell apoptosis was neither transferable nor due to lack of a cytokine. Enhanced CD4+ cell apoptosis was also observed after anti-CD95 mAb treatment, despite normal CD95 expression. Removal of endogenous cytokines, notably interleukin (IL)-4, led to increased and equally high levels of IRF4−/− and IRF4+/+ cell apoptosis, whereas the protective activity of exogenous IL-4 was reduced in IRF4−/− CD4+ cells despite normal expression of the IL-4 receptor. Therefore, IRF4 is central in protecting CD4+ cells against proapoptotic stimuli
On quiver Grassmannians and orbit closures for representation-finite algebras
We show that Auslander algebras have a unique tilting and cotilting module which is generated and cogenerated by a projective-injective; its endomorphism ring is called the projective quotient algebra. For any representation- nite algebra, we use the projective quotient algebra to construct desingularizations of quiver Grassmannians, orbit closures in representation varieties, and their desingularizations. This generalizes results of Cerulli Irelli, Feigin and Reineke
Cycle-finite module categories
We describe the structure of module categories of finite dimensional algebras
over an algebraically closed field for which the cycles of nonzero
nonisomorphisms between indecomposable finite dimensional modules are finite
(do not belong to the infinite Jacobson radical of the module category).
Moreover, geometric and homological properties of these module categories are
exhibited
Hirnorganoide – Modellsysteme des menschlichen Gehirns
This is the final version. Available from Deutsche Akademie der Naturforscher Leopoldina via the DOI in this record. Hirnorganoide sind Gewebestrukturen aus dem Labor, die Teile der Hirnfunktion imitieren. Sie eröffnen als vereinfachtes Modellsystem einen experimentellen Zugang zu Fragen rund um die Entwicklung und die Funktion des menschlichen Gehirns. Während die Forschung an menschlichen lebenden Gehirnen aus ethischen Gründen enge Grenzen hat und Tiermodelle viele Fragen nur bedingt beantworten können, bieten Hirnorganoide neue Forschungsmöglichkeiten. In der Stellungnahme „Hirnorganoide ‒ Modellsysteme des menschlichen Gehirns“ der Nationalen Akademie der Wissenschaften Leopoldina beschreiben Wissenschaftlerinnen und Wissenschaftler die Möglichkeiten dieses Forschungsgebietes und erörtern, ob es aus ethischen oder juristischen Gründen stärker reguliert werden sollte
Capture of Neuroepithelial-Like Stem Cells from Pluripotent Stem Cells Provides a Versatile System for In Vitro Production of Human Neurons
Human embryonic stem cells (hESC) and induced pluripotent stem cells (iPSC) provide new prospects for studying human neurodevelopment and modeling neurological disease. In particular, iPSC-derived neural cells permit a direct comparison of disease-relevant molecular pathways in neurons and glia derived from patients and healthy individuals. A prerequisite for such comparative studies are robust protocols that efficiently yield standardized populations of neural cell types. Here we show that long-term self-renewing neuroepithelial-like stem cells (lt-NES cells) derived from 3 hESC and 6 iPSC lines in two independent laboratories exhibit consistent characteristics including i) continuous expandability in the presence of FGF2 and EGF; ii) stable neuronal and glial differentiation competence; iii) characteristic transcription factor profile; iv) hindbrain specification amenable to regional patterning; v) capacity to generate functionally mature human neurons. We further show that lt-NES cells are developmentally distinct from fetal tissue-derived radial glia-like stem cells. We propose that lt-NES cells provide an interesting tool for studying human neurodevelopment and may serve as a standard system to facilitate comparative analyses of hESC and hiPSC-derived neural cells from control and diseased genetic backgrounds
Cancer Genes Hypermethylated in Human Embryonic Stem Cells
Developmental genes are silenced in embryonic stem cells by a bivalent histone-based chromatin mark. It has been proposed that this mark also confers a predisposition to aberrant DNA promoter hypermethylation of tumor suppressor genes (TSGs) in cancer. We report here that silencing of a significant proportion of these TSGs in human embryonic and adult stem cells is associated with promoter DNA hypermethylation. Our results indicate a role for DNA methylation in the control of gene expression in human stem cells and suggest that, for genes repressed by promoter hypermethylation in stem cells in vivo, the aberrant process in cancer could be understood as a defect in establishing an unmethylated promoter during differentiation, rather than as an anomalous process of de novo hypermethylation
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