Megacities and natural disasters: a comparative analysis*     

James K. Mitchell 《GeoJournal》1999,49(2):137-142

Natural disasters are a worsening problem in many of the world's largest cities. Since an increasing majority of the world's population will soon live in cities, and mostly in large cities, the trend towards increasing urban hazard poses serious societal challenges for the future. Beginning in the mid-1990s, the International Geographical Union's Study Group on the Disaster Vulnerability of Megacities undertook a series of case studies of megacities that have been – and are – exposed to major natural disasters. These include: Tokyo, Seoul, Sydney, Lima, Mexico City, Los Angeles, San Francisco, Miami, London and Dhaka. Case studies focused on reviewing the historic record of major disasters in each city and on documenting recent changes in different components of hazard including, risk, exposure, vulnerability and response. It was found that exposure and vulnerability are the components of hazard that are changing fastest and with the gravest implications for urban populations. Because hazards are only one part of the typical urban management agenda, hazards management in large cities should be pursued with careful regard to the context of general urban policy manking and management. Sustainability has become a guiding principle of urban management but the relationship between hazard and sustainability is little understood and little explored. The way lies open for hazard researchers to develop alternative contextualized approaches to the analysis and resolution of urban hazard issues. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Multi-instrument probing of the polar ionosphere under steady northward IMF     

S. E. Pryse  A. M. Smith  L. Kersley  I. K. Walker  C. N. Mitchell  J. Moen  R. W. Smith 《Annales Geophysicae》1999,18(1):90-98

Observations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere.On sabbatical leave from Artic Geophysics, University Courses on Svalbard, N-9170 Longyearbyen, Norway  相似文献   

Single receiver phase ambiguity resolution with GPS data   总被引：26，自引：12，他引：14  

Willy Bertiger  Shailen D. Desai  Bruce Haines  Nate Harvey  Angelyn W. Moore  Susan Owen  Jan P. Weiss 《Journal of Geodesy》2010,84(5):327-337

Global positioning system (GPS) data processing algorithms typically improve positioning solution accuracy by fixing double-differenced phase bias ambiguities to integer values. These “double-difference ambiguity resolution” methods usually invoke linear combinations of GPS carrier phase bias estimates from pairs of transmitters and pairs of receivers, and traditionally require simultaneous measurements from at least two receivers. However, many GPS users point position a single local receiver, based on publicly available solutions for GPS orbits and clocks. These users cannot form double differences. We present an ambiguity resolution algorithm that improves solution accuracy for single receiver point-positioning users. The algorithm processes dual- frequency GPS data from a single receiver together with wide-lane and phase bias estimates from the global network of GPS receivers that were used to generate the orbit and clock solutions for the GPS satellites. We constrain (rather than fix) linear combinations of local phase biases to improve compatibility with global phase bias estimates. For this precise point positioning, no other receiver data are required. When tested, our algorithm significantly improved repeatability of daily estimates of ground receiver positions, most notably in the east component by approximately 30% with respect to the nominal case wherein the carrier biases are estimated as real values. In this “static” test for terrestrial receiver positions, we achieved daily repeatability of 1.9, 2.1 and 6.0 mm in the east, north and vertical (ENV) components, respectively. For kinematic solutions, ENV repeatability is 7.7, 8.4, and 11.7 mm, respectively, representing improvements of 22, 8, and 14% with respect to the nominal. Results from precise orbit determination of the twin GRACE satellites demonstrated that the inter-satellite baseline accuracy improved by a factor of three, from 6 to 2 mm up to a long-term bias. Jason-2/Ocean Surface Topography Mission precise orbit determination tests results implied radial orbit accuracy significantly below the 10 mm level. Stability of time transfer, in low-Earth orbit, improved from 40 to 7 ps. We produced these results by applying this algorithm within the Jet Propulsion Laboratory’s (JPL’s) GIPSY/OASIS software package and using JPL’s orbit and clock products for the GPS constellation. These products now include a record of the wide-lane and phase bias estimates from the underlying global network of GPS stations. This implies that all GIPSY–OASIS positioning users can now benefit from this capability to perform single-receiver ambiguity resolution.  相似文献   

Book review: Deep-sea Benthic Foraminifera from Cretaceous–Paleogene Boundary Strata in the South Atlantic—Taxonomy and Paleoecology by Joen G. V. Widmark. Fossils and Strata. 43, Scandinavian University Press,Oslo, 1997. No. of pages: 94. Price: $29.00. ISBN 82-00-37667-2.     

Simon Mitchell 《Geological Journal》1998,33(3):200-200

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Landslides control the spatial and temporal variation of channel width in southern Taiwan: Implications for landscape evolution and cascading hazards in steep,tectonically active landscapes          下载免费PDF全文

Brian J. Yanites  Nate A. Mitchell  Joshua C. Bregy  Grace A. Carlson  Kirstyn Cataldo  Margaret Holahan  Graham H. Johnston  Amelia Nelson  Jeffery Valenza  Matthew Wanker 《地球表面变化过程与地形》2018,43(9):1782-1797

Intense precipitation or seismic events can generate clustered mass movement processes across a landscape. These rare events have significant impacts on the landscape, however, the rarity of such events leads to uncertainty in how they impact the entire geomorphic system over a range of timescales. Taiwan is steep, tectonically active, and prone to landslide and debris flows, especially when exposed to heavy rainfall events. Typhoon Morakot made landfall in Taiwan in August of 2009, causing widespread landslides in southern Taiwan. The south to north trend in valley relief in southern Taiwan leads to spatial variability in landslide susceptibility providing an opportunity to infer the long‐term impact of such landslide events on channel morphology. We use pre‐ and post‐typhoon imagery to quantify the propagating impact of this event on channel width as the debris is routed through the landscape. The results show the importance of cascading hazards from landslides on landscape evolution based on patterns of channel width (both pre‐ and post‐typhoon) and hillslope gradients in 20 basins along strike in southern Taiwan. Prior to Typhoon Morakot, the river channels in the central part of the study area were about 3–10 times wider than the channels in the south. Following the typhoon, aggradation and widening was also a maximum in these central to northern basins where hillslope gradients and channel steepness is high, accentuating the pre‐typhoon pattern. The results further show that the narrowest channels are located where channel steepness is the lowest, an observation inconsistent with a detachment‐limited model for river evolution. We infer this pattern is indicative of a strong role of sediment supply, and associated landslide events, on long‐term channel evolution. These findings have implications across a range of spatial and temporal scales including understanding the cascade of hazards in steep landscapes and geomorphic interpretation of channel morphology. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

Local relief and the height of Mount Olympus     

David R. Montgomery  Harvey M. Greenberg 《地球表面变化过程与地形》2000,25(4):385-396

A three‐dimensional assessment of the net volume of rock differentially eroded from below mountain tops to form valleys yields a range‐wide constraint on feedback between valley development and the height of mountain peaks. The ‘superelevation’ of mountain peaks potentially attributable to differential removal of material from below peaks in the Olympic Mountains, Washington, was constrained by fitting a smoothed surface to the highest elevation points on a 30 m grid digital elevation model of the range. High elevation areas separate into two primary areas: one centred on Mount Olympus in the core of the range and the other at the eastern end of the range. The largest valleys, and hence areas with the greatest volume of differentially eroded material, surround Mount Olympus. In contrast, the highest mean elevations concentrate in the eastern end of the range. Calculation of the isostatic rebound at Mount Olympus attributable to valley development ranges from 500 to 750 m (21 to 32 per cent of its height) for a 5 to 10 km effective elastic thickness of the crust. Comparison of cross‐range trends in mean and maximum elevation reveals that this calculated rebound for Mount Olympus corresponds well with its ‘superelevation’ above the general cross‐range trend in mean elevation. It therefore appears that the location of the highest peak in the Olympics is controlled by the deep valleys excavated in the centre of the range. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation     

Alan E. Rubin  Tasha L. Dunn  Kyla Garner  Malena Cecchi  Mitchell Hernandez 《Meteoritics & planetary science》2024,59(4):670-684

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.  相似文献   

Sub-Micrometre Resolution FIB-SEM-based ToF-SIMS Used to Map Geochemical Zoning in Four Zircon Reference Materials     

Ryan North  Lloyd T. White  Mitchell Nancarrow  Anthony Dosseto  Dominique Tanner 《Geostandards and Geoanalytical Research》2023,47(1):125-142

Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials need to be well characterised before being used to calculate trace element mass fractions in unmeasured samples. Moreover, reference material homogeneity needs to be ensured with the accelerating rate of geoanalytical developments to map mineral chemistry at increasingly finer scales. Here, we investigate trace element zoning in four widely used zircon reference materials: 91500, Mud Tank, Temora and Plešovice, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex, Australia. Sub-micrometre resolution focused ion beam scanning electron microscope (FIB-SEM) based time-of-flight secondary ion mass spectrometry (ToF-SIMS) and 5 μm resolution LA-ICP-MS mapping show that trace elements are zoned in all reference materials, though 91500 exhibited the least zonation. We demonstrate that FIB-SEM-based ToF-SIMS can rapidly resolve variations in trace elements (e.g., U, Th, Sc, Y, Gd, Dy, Yb and Li) at sensitivities down to the μg g^-1 level with a spatial resolution of 195 nm for areas 100 × 85 μm to 959 × 828 μm. Zircon 91500 is recommended for future quantitative analyses provided that (1) the spatial distribution of elements is imaged before analysis of unknown samples and (2) it is used in conjunction with a doped glass as the primary reference material.  相似文献