Uncovering spatiotemporal micromobility patterns through the lens of space–time cubes and GIS tools

Journal of Geographical Systems - In the past ten years, cities have experienced a burst of micromobility services as they offer a flexible transport option that allows users to cover short trips...     

Compression mechanisms of coesite

 The structure of coesite has been determined at ten pressures up to a maximum of 8.68 GPa by single-crystal X-ray diffraction. The dominant mechanism of compression is the reduction of four of the five independent Si–O–Si angles within the structure. There is no evidence of the fifth linkage, Si1–O1–Si1, deviating from 180°. Some Si–O bond distances also decrease by up to 1.6% over the pressure range studied. The pattern of Si–O–Si angle reduction amounts to a rotation of the Si2 tetrahedron around the [001] direction. This rotation induces significant internal deformation of the Si1 tetrahedron. Comparison of the experimental data with rigid-unit distance least-squares simulations of coesite suggests that this pattern of compression, the anomalous positive values of both s₂₃ and K′′ in the equation of state of coesite, its high elastic anisotropy and the unusual straight Si1–O1–Si1 linkage within the structure are all consequences of the connectivity of the tetrahedral framework. Received: 11 July 2002 / Accepted: 14 January 2003 Acknowledgements The help of Christian Baerlocher of ETH Zurich in providing both the DLS-76 software and advice in its use is gratefully acknowledged, as are discussions with Paul Ribbe of Virginia Tech and the comments of two anonymous reviewers. The data analysis was supported by the National Science Foundation under grant EAR-0105864 to N.L. Ross and R.J. Angel.     

A depth average SPH model including μ(I) rheology and crushing for rock avalanches

Classical depth-integrated smoothed particle hydrodynamics (SPH) models for avalanches are extended in the present work to include a μ(I)− rheological model enriched with a fragmentation law. With this improvement, the basal friction becomes grain distribution dependent. Rock avalanches, where grain distribution tends to change with time while propagating, are the appropriate type of landslide to apply the new numerical proposal. The μ(I)− rheological models considered in the present work are those of Hatano and Gray, combined with two different fragmentation laws, a hyperbolic and a fractal-based law. As an application, Frank avalanche, which took place in Canada in 1903, is analyzed under the scope of the present approach, focusing in the influence of the rheological and fragmentation laws in the evolution of the avalanche.     

Ab initio study of MgSiO3 C2/c enstatite

We have used a newly developed ab initio constant-pressure molecular dynamics with variable cell shape technique to investigate the zero temperature behaviour of high pressure clinoenstatite (MgSiO₃-C2/c) from 0 up to 30 GPa. The optimum structure at 8 GPa, as well as structural trends under pressure, compare very well with experimental data. At this pressure, we find noticeable “fluctuations” in the chain configuration which suggests the structure is on the verge of a mechanical instability. Two distinct compressive behaviours then appear: one below and another above 8 GPa. This phenomenon may be related to the observed transition to a lower symmetry P2₁/c phase which involves a reconfiguration of the silicate chains, and suggests that the C2/c structure at low pressures found here, may be an artifact of the dynamical algorithm which preserves space group in the absence of symmetry breaking fluctuations. Comparison with calculations in other magnesium silicate phases, indicates that the size and shape of the silicate units (isolated and/or linked tetrahedra and octahedra) are generally well described by the local density approximation; however, the weaker linkages provided by the O-Mg-O bonds, are not as well described. This trend suggests that, as in the recently studied case of H₂O-ice, the structural properties of more inhomogeneous systems, like enstatite, may be improved by using gradientcorrected density functionals.     

Thermochronology and exhumation rates of granitic intrusions at Mesa Central,Mexico

ABSTRACT

The Mesa Central of Mexico (MC) is an elevated plateau located 2000 m above sea level in central Mexico, where intrusions outcrop that register the history of exhumation-erosion occurring during the Late Cretaceous-Paleogene. The tectonic history of the region records formation of the Late Cretaceous-Paleogene ‘Mexican orogen’; this was followed by extension of the entire region and several plutons were then exhumed. The age and magnitude of the crustal uplift and erosion occurring during exhumation has not been addressed to date. Therefore, this study reports the crystallization and cooling ages of two plutons, the Tesorera Granodiorite and the Comanja Granite, and estimates their emplacement depths. Based on these data, the exhumation age of the Tesorera Granodiorite is estimated to be between ~73 Ma and ~63 Ma at an exhumation rate of ~528 m/m. y. and that of the Comanja Granite is 52 Ma and 48 Ma at an exhumation rate of ~2500 m/m. y. Exhumation-erosion event of the Tesorera Granodiorite was located on the trace of the San Luis-Tepehuanes Fault System and that of the Comanja Granite on the a trace of the El Bajío Fault System. Furthermore, the high exhumation rate in the Comanja Granite suggests that gravitational collapse played an important role during exhumation.     

Volcanic ash forecast during the June 2011 Cordón Caulle eruption

We modelled the transport and deposition of ash from the June 2011 eruption from Cordón Caulle volcanic complex, Chile. The modelling strategy, currently under development at the Argentinean Naval Hydrographic Service and National Meteorological Service, couples the weather research and forecasting (WRF/ARW) meteorological model with the FALL3D ash dispersal model. The strategy uses volcanological inputs inferred from satellite imagery, eruption reports and preliminary grain-size data obtained during the first days of the eruption from an Argentinean ash sample collection network. In this sense, the results shown here can be regarded as a quasi-syn-eruptive forecast for the first 16 days of the eruption. Although this article describes the modelling process in the aftermath of the crisis, the strategy was implemented from the beginning of the eruption, and results were made available to the Buenos Aires Volcanic Ash Advisory Centers and other end users. The model predicts ash cloud trajectories, concentration of ash at relevant flight levels, expected deposit thickness and ash accumulation rates at relevant localities. Here, we validate the modelling strategy by comparing results with satellite retrievals and syn-eruptive ground deposit measurements. Results highlight the goodness of the combined WRF/ARW-FALL3D forecasting system and point out the usefulness of coupling both models for short-term forecast of volcanic ash clouds.     

The Juchatengo complex: an upper-level ophiolite assemblage of late Paleozoic age in Oaxaca,southern Mexico

International Journal of Earth Sciences - The Juchatengo complex (JC) suite is located between the Proterozoic Oaxacan complex to the north and the Xolapa complex to the south, and was amalgamated...     

Calibration of Al/Si order variations in anorthite

New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q _OD , for the transition. Numerical values ofQ _OD obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain, _s , as _s Q _OD ² , and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections asI _b/I _a Q _OD ² . An empirical calibration for pure anorthite is obtained giving varies between 0.92 and 0.87 in samples equilibrated at T1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ _OD withQ of the transition is suspeeted.     

Neoproterozoic events in the Tcholliré area: Pan-African crustal growth and geodynamics in central-northern Cameroon (Adamawa and North Provinces)

The Rei Bouba Group is a sedimentary and volcanic sequence (750–?650 Ma), regarded as a remnant of a Pan-African (back-arc?) orogenic basin that separated a remobilized Paleoproterozoic crust from an accretionary area (Poli Complex: 800-650 Ma). The latter was subjected to early deformation (D1) and intruded by calc-alkalic plutons (670 Ma). Transpressive tectonics and major thrusting, with emplacement of synkinematic granite, occurred at ca 630 Ma (D2-3) and a late compression (D4), with emplacement of calc-alkalic granite, occurred at ca 570 Ma. The Tcholliré Shear Zone is regarded as part of the major boundary between a recently accreted crust and the remobilized margin of the Congo Craton.