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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Journal of algebraic combinatorics 4 (1995), S. 145-164 
    ISSN: 1572-9192
    Keywords: Tits building ; generalized octagon ; group of projectivities ; coordinatization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We coordinatize the Moufang generalized octagons arising from the Ree groups of type 2 F 4. In this way, we obtain a very concrete and explicit description of these octagons. We use this to prove some results on suboctagons, generalized homologies, Suzuki-Tits ovoids and groups of projectivities of the Ree octagons. All our results hold for arbitrary Ree octagons, finite or not.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Geometriae dedicata 49 (1994), S. 143-154 
    ISSN: 1572-9168
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract By ‘slanting’ symplectic quadrangles W(F) over fieldsF, we obtain very simple examples of non-classical generalized quadrangles. We determine the collineation groups of these slanted quadrangles and their groups of projectivities. No slanted quadrangle is a topological quadrangle.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Discrete & computational geometry 24 (2000), S. 325-344 
    ISSN: 1432-0444
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract. Neighborly cubical polytopes exist: for any n≥ d≥ 2r+2 , there is a cubical convex d -polytope C d n whose r -skeleton is combinatorially equivalent to that of the n -dimensional cube. This solves a problem of Babson, Billera, and Chan. Kalai conjectured that the boundary $\partial C_d^n$ of a neighborly cubical polytope C d n maximizes the f -vector among all cubical (d-1) -spheres with 2 n vertices. While we show that this is true for polytopal spheres if n≤ d+1 , we also give a counterexample for d=4 and n=6 . Further, the existence of neighborly cubical polytopes shows that the graph of the n -dimensional cube, where n\ge5 , is ``dimensionally ambiguous'' in the sense of Grünbaum. We also show that the graph of the 5 -cube is ``strongly 4 -ambiguous.'' In the special case d=4 , neighborly cubical polytopes have f 3 =(f 0 /4) log 2 (f 0 /4) vertices, so the facet—vertex ratio f 3 /f 0 is not bounded; this solves a problem of Kalai, Perles, and Stanley studied by Jockusch.
    Type of Medium: Electronic Resource
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  • 4
  • 5
    Publication Date: 2011-05-01
    Description: Applying the method of nanoseismic monitoring in field campaigns between 2005 and 2008 at the Heumoes landslide in the Austrian Alps, we discovered and located fracture processes with seismic local magnitudes (ML) between -2.2 and -0.7. The creeping Heumoes slope consists of weak sediments (loamy scree and glacial till) and moves an average of a few centimeters per year at the surface. The seismic detection of single fracture processes or initial stress relief resolves the creeping movement of the slope into discrete rupture episodes. The spatial distribution of fractures is concentrated in parts of the slope with higher deformation rates at the surface. The temporal occurrence correlates with rainfall events and reinforces the assumption of a rainfall-triggered slope movement. In addition, we observed weak local earthquakes (ML [~] 2) with a distance of [~]10 km to have an influence on fracture generation at the slope. Furthermore, an average thickness of [~]20 m (and a maximum of 〉30 m) for the unstable sediment cover of Heumoes slope was determined by refraction seismic techniques along several seismic profiles. The slope's total volume is estimated to be 107 m3; its mass is approximately 1.6 x 107 Mg. The determination of the landslide volume is essential for further hazard assessment of the unstable Heumoes slope.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 6
    Publication Date: 2015-04-17
    Description: Automatic classification of local seismic events which are only recorded at single stations poses great challenges because of weak hypocentre constraints. This study investigates how single-station event clusters relate to geographic hypocentre regions and common source processes. Typical applications arise in local seismic networks where reliable ground truth by a dense temporal network precedes or follows a sparse (permanent) installation. The seismic signals for this study comprise a 3-month subset from a field campaign to map subduction below northern Chile (PISCO ’94). Due to favourable ground noise conditions in the Atacama desert, the data set contains an abundance of shallow and deeper earthquakes, and many quarry explosions. Often event signatures overlap, posing a challenge to any signal processing scheme. Pattern recognition must work on reduced seismograms to restrict parameter dimensionality. Continuous parameter extraction based on noise-adapted spectrograms was chosen instead of discrete representation by, for example, amplitudes, onset times or spectral ratios to ensure consideration of potentially hidden features. Visualization of the derived feature vectors for human inspection and template matching algorithms was hereby possible. Because event classes shall comprise earthquake regions regardless of magnitude, clustering based on amplitudes is prevented by proper normalization of feature vectors. Principal component analysis is applied to further reduce the number of features used to train a self-organizing map (SOM). The SOM will topologically arrange prototypes of each event class in a 2-D map. Overcoming the restrictions of this black-box approach, the arranged prototypes could be transformed back to spectrograms to allow for visualization and interpretation of event classes. The final step relates prototypes to ground-truth information, confirming the potential of automated, coarse-grain hypocentre clustering based on single-station seismograms. The approach was tested by a twofold cross-validation whereby multiple sets of feature vectors from half the events are compared by a one-nearest neighbour classifier in combination with an Euclidean distance measure resulting in an overall correct geographic separation rate of 80.5 per cent.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
    Publication Date: 2017-07-04
    Description: Acquisition of unmanned aerial vehicle (UAV)-based imagery with resolution down to a few centimeters is challenging in alpine conditions. In recent years, UAV-based images have been used in an increasing number of case studies to monitor landslides. Processing of multitemporal high-resolution aerial images provides multitemporal 3D point clouds and multitemporal orthomosaics, which provide substantial information about slope dynamics. Surface processes deciphered from 3D point clouds and orthomosaic temporal sequences supplement on-site geophysical measurements of subsurface structures and dynamics by bringing key spatial and temporal constraints for process interpretation. However, accurate spatial mapping and successful analysis of surface dynamics are functions of the original optical image quality, which in turn relies on the flight system and configuration, camera characteristics and settings, as well as weather conditions and object texture. We evaluate the capabilities and limitations of UAV-based landslide dynamic mapping using illustrative examples gathered from a pioneering low-cost UAV-based imagery data set acquired at the slow-moving (mm-cm/d) Super-Sauze landslide (southeastern France) in 2008 and 2010, which included surface deformations (fissure development and kinematics) and surface conditions (soil moisture, erosion, and sedimentary processes) with very different certainty levels. In a few promising instances, processes detected in the 3D point clouds and orthomosaics could be correlated to geophysical data (passive seismic monitoring of landslide endogenous seismicity and electrical resistivity tomography for water content mapping), thereby validating the UAV analytical approach while providing constraints that improve interpretation of landslide dynamics.
    Print ISSN: 1070-485X
    Electronic ISSN: 1938-3789
    Topics: Geosciences
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  • 8
    Publication Date: 2013-05-09
    Description: This work focuses on the characterization of seismic sources observed in clay–shale landslides. Two landslides are considered: Super-Sauze (France) and Valoria (Italy). The two landslides are developed in reworked clay–shales but differ in terms of dimensions and displacement rates. Thousands of seismic signals have been identified by a small seismic array in spite of the high-seismic attenuation of the material. Several detection methods are tested. A semi-automatic detection method is validated by the comparison with a manual detection. Seismic signals are classified in three groups based on the frequency content, the apparent velocity and the differentiation of P and S waves. It is supposed that the first group of seismic signals is associated to shearing or fracture events within the landslide bodies, while the second group may correspond to rockfalls or debris flows. A last group corresponds to external earthquakes. Seismic sources are located with an automatic beam-forming location method. Sources are clustered in several parts of the landslide in agreement with geomorphological observations. We found that the rate of rockfall and fracture events increases after periods of heavy rainfall or snowmelt. The rate of microseismicity and rockfall activity is also positively correlated with landslide displacement rates. External earthquakes did not influence the microseismic activity or the landslide movement, probably because the earthquake ground motion was too weak to trigger landslide events during the observation periods.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
    Publication Date: 2013-11-14
    Description: In this study, we describe conditions for slidequake generation at three different creeping softrock landslides: the Slumgullion landslide in the San Juan Mountains, Colorado, U.S., the Heumoes slope in the Austrian Alps, and the mudslide in Super-Sauze, French Alps. From a geomorphologic point of view, all three landslides are classified as creeping landslides with average velocities between centimeters to meters per year. Associating creep with viscous flow, and considering the largely saturated, clayey consistency of the slope body, one would not expect any brittle behavior. Thus, it came as a surprise that impulsive seismic signals indicative of shear fracture could be discovered by sensitive passive monitoring methods at all three slopes. These fracture signals occur in episodes, have similar signatures as small earthquakes, and could be located within the slide bodies, i.e. , are evidence of slidequakes. Our investigations identified seismic and aseismic slip in each slide, with slidequakes focusing at significant bedrock structures or at lateral boundaries. Synoptic comparison of three scenarios underlines the importance of landslide-bedrock and landslide-lateral boundary interactions under gravitational loading and Mohr-Coulomb-type failure. Comparison to frictional and asperity models of crustal- and plate-scale boundaries may pave the way to a comprehensive understanding of slidequake generation, and future slope failure prediction.
    Print ISSN: 1083-1363
    Electronic ISSN: 1943-2658
    Topics: Geosciences
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  • 10
    Publication Date: 2001-06-30
    Print ISSN: 0036-0279
    Electronic ISSN: 1468-4829
    Topics: Mathematics
    Published by Institute of Physics
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