Effets de la charge des édifices volcaniques sur la propagation de structures régionales compressives : exemples naturels et modèles expérimentaux

Nous présentons ici des exemples naturels d'édifices volcaniques coniques reposant sur un substratum fragile, soumis à une compression régionale ainsi que des résultats expérimentaux. Nous montrons que la charge de l'édifice induit une perturbation de la déformation régionale se traduisant par une déflexion et une horizontalisation des structures compressives régionales. Le contrôle tectonique est de nature topographique. Nous discutons ensuite certaines conséquences, en particulier concernant l'étalement gravitaire des volcans. We present natural examples and experimental models of volcanic cones located above brittle substratum undergoing regional compressive deformation. The volcanic loading induces a strain partitioning involving deflection and flattening of regional compressive structures. The main control is the topographic load. Anticlinal thrust ridges, observed around many volcanoes, have generally been interpreted as being due to gravitational spreading; however, this study shows that this is not necessarily the case, as they can also be a symptom of regional compression

Field evidence for summit subsidence, flank instability and basal spreading at Mt Cameroon volcano, West Africa

Mt Cameroon is a steep lava-dominated volcano located on the coast of the Gulf of Guinea. This 1400 km3 edifice is one of two active centres in the Cameroon Volcanic Line. Despite recent lava eruptions along its rift zones in 1999 and 2000, little geological or monitoring data are available to understand the structure of this large volcanic system. Here we report results from a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mount Cameroon. Eruptive fissures and open fractures’ orientation, vents’ location and alignment above 3500 m a.s.l were systematically surveyed. In addition to the tectonically-controlled N40°E orientation of eruptive fissures along the rift zones, other dominant orientations were identified such as N60°E (summit vents alignment), N20°E and N90° (extension related structures). These were attributed to local instability around the summit, stress field re-orientation around the head of a deep valley cutting through the NW flank and radial pattern around the summit. Inward-dipping structures were also observed to border the relatively flat upper part of the rift zones. Geological profiles were also measured along rivers cutting through a topographic bulge at the SE base of Mt Cameroon. This topographic step was seen to be associated with deformed Miocene sediments from the Douala basin overlain by volcanic products.Weak sediments of this area are deformed by N50- 60°E trending asymmetrical folds verging toward the SE and by N10-30°E trending symmetrical folds and thrusts. Initial NE-SW trending structures formed following the sliding of sediments on the flank of a NE-SW elongated uplift dome. Later, the same area has been deformed by NNE-SSW trending compressive structures linked to the spreading of Mt Cameroon southern flank toward the SE. Combined with the interpretation of a 30 m Digital Elevation Models and multispectral satellite data, the field observations suggest that Mt Cameroon is affected by major instabilities. Both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak sedimentary substratum and tectonic structures

Information Retrieval and User-Centric Recommender System Evaluation

Traditional recommender system evaluation focuses on raising the accuracy, or lowering the rating prediction error of the recommendation algorithm. Recently, however, discrepancies between commonly used metrics (e.g. precision, recall, root-mean-square error) and the experienced quality from the users' have been brought to light. This project aims to address these discrepancies by attempting to develop novel means of recommender systems evaluation which encompasses qualities identified through traditional evaluation metrics and user-centric factors, e.g. diversity, serendipity, novelty, etc., as well as bringing further insights in the topic by analyzing and translating the problem of evaluation from an Information Retrieval perspective

Granular fingering as a mechanism for ridge formation in debris avalanche deposits: Laboratory experiments and implications for Tutupaca volcano, Peru

The origin of subparallel, regularly-spaced longitudinal ridges often observed at the surface of volcanic and other rock avalanche deposits remains unclear. We addressed this issue through analogue laboratory experiments on flows of bi-disperse granular mixtures, because this type of flow is known to exhibit granular fingering that causes elongated structures resembling the ridges observed in nature. We considered four different mixtures of fine (300–400 µm) glass beads and coarse (600–710 µm to 900–1000 µm) angular crushed fruit stones, with particle size ratios of 1.9–2.7 and mass fractions of the coarse component of 5–50 wt%. The coarse particles segregated at the flow surface and accumulated at the front where flow instabilities with a well-defined wavelength grew. These formed granular fingers made of coarse-rich static margins delimiting fines-rich central channels. Coalescence of adjacent finger margins created regular spaced longitudinal ridges, which became topographic highs as finger channels drained at final emplacement stages. Three distinct deposit morphologies were observed: 1) Joined fingers with ridges were formed at low (= 1.9) size ratio and moderate (10–20 wt%) coarse fraction whereas 2) separate fingers or 3) poorly developed fingers, forming series of frontal lobes, were created at larger size ratios and/or higher coarse contents. Similar ridges and lobes are observed at the debris avalanche deposits of Tutupaca volcano, Peru, suggesting that the processes operating in the experiments can also occur in nature. This implies that volcanic (and non-volcanic) debris avalanches can behave as granular flows, which has important implications for interpretation of deposits and for modeling. Such behaviour may be acquired as the collapsing material disaggregates and forms a granular mixture composed by a right grain size distribution in which particle segregation can occur. Limited fragmentation and block sliding, or grain size distributions inappropriate for promoting granular fingering can explain why ridges are absent in many deposits

Contrasting catastrophic eruptions predicted by different intrusion and collapse scenarios

Catastrophic volcanic eruptions triggered by landslide collapses can jet upwards or blast sideways. Magma intrusion is related to both landslide-triggered eruptive scenarios (lateral or vertical), but it is not clear how such different responses are produced, nor if any precursor can be used for forecasting them. We approach this problem with physical analogue modelling enhanced with X-ray Multiple Detector Computed Tomography scanning, used to track evolution of internal intrusion, and its related faulting and surface deformation. We find that intrusions produce three different volcano deformation patterns, one of them involving asymmetric intrusion and deformation, with the early development of a listric slump fault producing pronounced slippage of one sector. This previously undescribed early deep potential slip surface provides a unified explanation for the two different eruptive scenarios (lateral vs. vertical). Lateral blast only occurs in flank collapse when the intrusion has risen into the sliding block. Otherwise, vertical rather than lateral expansion of magma is promoted by summit dilatation and flank buttressing. The distinctive surface deformation evolution detected opens the possibility to forecast the possible eruptive scenarios: laterally directed blast should only be expected when surface deformation begins to develop oblique to the first major fault

Shoulder Pain Is Associated With Rate of Rise and Jerk of the Applied Forces During Wheelchair Propulsion in Individuals With Paraplegic Spinal Cord Injury

OBJECTIVE: To investigate the association between propulsion biomechanics, including variables that describe smoothness of the applied forces, and shoulder pain in persons with SCI. DESIGN: Cross-sectional, observational study. SETTING: Non-university research institution. PARTICIPANTS: 30 (age: 48.6±9.3 years, 83% males) community dwelling, wheelchair dependent participants with a chronic paraplegia between T2 and L1, with and without shoulder pain. INTERVENTIONS: Not applicable MAIN OUTCOME MEASURE: Rate of rise and jerk of applied forces during wheelchair propulsion. Participants were stratified in low, moderate and high pain groups based on their Wheelchair User Shoulder Pain Index (WUSPI) score at the day of measurement. RESULTS: A mixed-effect multilevel analysis showed that wheelchair users in the high pain group propelled with significantly greater rate of rise and jerk - measures that describe smoothness of the applied forces - as compared to persons with less or no pain, when controlling for all co-variables. CONCLUSIONS: Persons with severe shoulder pain propelled with less smooth strokes as compared to persons with less or no pain. This supports a possible association between shoulder pain and rate of rise and jerk of the applied forces during wheelchair propulsion

Atmospheric PSF Interpolation for Weak Lensing in Short Exposure Imaging Data

A main science goal for the Large Synoptic Survey Telescope (LSST) is to measure the cosmic shear signal from weak lensing to extreme accuracy. One difficulty, however, is that with the short exposure time ($\simeq$15 seconds) proposed, the spatial variation of the Point Spread Function (PSF) shapes may be dominated by the atmosphere, in addition to optics errors. While optics errors mainly cause the PSF to vary on angular scales similar or larger than a single CCD sensor, the atmosphere generates stochastic structures on a wide range of angular scales. It thus becomes a challenge to infer the multi-scale, complex atmospheric PSF patterns by interpolating the sparsely sampled stars in the field. In this paper we present a new method, PSFent, for interpolating the PSF shape parameters, based on reconstructing underlying shape parameter maps with a multi-scale maximum entropy algorithm. We demonstrate, using images from the LSST Photon Simulator, the performance of our approach relative to a 5th-order polynomial fit (representing the current standard) and a simple boxcar smoothing technique. Quantitatively, PSFent predicts more accurate PSF models in all scenarios and the residual PSF errors are spatially less correlated. This improvement in PSF interpolation leads to a factor of 3.5 lower systematic errors in the shear power spectrum on scales smaller than $\sim13'$, compared to polynomial fitting. We estimate that with PSFent and for stellar densities greater than $\simeq1/{\rm arcmin}^{2}$, the spurious shear correlation from PSF interpolation, after combining a complete 10-year dataset from LSST, is lower than the corresponding statistical uncertainties on the cosmic shear power spectrum, even under a conservative scenario.Comment: 18 pages,12 figures, accepted by MNRA