Experimental investigation of the α-β transformation of San Carlos olivine single crystal     

T. Kubo  E. Ohtani  T. Kato  T. Shinmei  K. Fujino 《Physics and Chemistry of Minerals》1998,26(1):1-6

Single crystalline San Carlos olivine (1 mm cube) was transformed to (Mg,Fe)₂SiO₄β-phase at 13.5–15 GPa, 1030–1330 °C for 0–600 min using a multi-anvil high pressure apparatus. The α-β transformation occurred by incoherent surface nucleation and interface-controlled growth and recovered partially transformed samples showed sharply defined reaction rim. The growth rate of the β-phase rim significantly decreased with time and the growth eventually ceased. TEM observations revealed that many dislocations were created in both the relict olivine just near the α-β interface and the β-phase in the rim, which show evidence for deformation caused by interfacial stresses associated with the misfit elastic strain of the transformation. The observed tangled dislocation texture in β-phase suggested that the β-phase rim was hardened and relaxation of the interfacial stress was retarded. This probably caused a localized pressure drop in the relict olivine and decreased the growth rate. Time-dependent growth rates of β-phase is possibly controlled by the rheology of β-phase, which must be considered for the prediction of the olivine metastability in the subducting slabs. Received: 24 January 1997 / Revised, accepted: 24 July 1998  相似文献   

HONDURAS: WHEN THE SAINTS ARRIVE     

RICHARD SYMANSKI 《Geographical review》1998,88(4):571-579

ABSTRACT. Hurricane Mitch, the most deadly hurricane to strike the Western Hemisphere in two centuries, killed at least 10,000 people in Honduras, Nicaragua, and El Salvador and left tens of thousands homeless. Some needed food; others, medical attention. Americans, Europeans, Mexicans, and others almost immediately responded to the widespread devastation by sending large donations of food, clothing, and medicine. Six weeks after Mitch struck the Honduran mainland, the author traveled to Honduras with the aim of photographing the physical damage and its effect on humans. In San Pedro Sula he was sidetracked by the issue of where the refugees were being housed and whether they were receiving the donations that had been sent on their behalf. This essay narrates that search and what he found.  相似文献   

Climate sensitivity of water use by riparian woodlands at landscape scales     

Marc Mayes  Kelly K. Caylor  Michael Bliss Singer  John C. Stella  Dar Roberts  Pamela Nagler 《水文研究》2020,34(25):4884-4903

Semi-arid riparian woodlands face threats from increasing extractive water demand and climate change in dryland landscapes worldwide. Improved landscape-scale understanding of riparian woodland water use (evapotranspiration, ET) and its sensitivity to climate variables is needed to strategically manage water resources, as well as to create successful ecosystem conservation and restoration plans for potential climate futures. In this work, we assess the spatial and temporal variability of Cottonwood (Populus fremontii)-Willow (Salix gooddingii) riparian gallery woodland ET and its relationships to vegetation structure and climate variables for 80 km of the San Pedro River corridor in southeastern Arizona, USA, between 2014 and 2019. We use a novel combination of publicly available remote sensing, climate and hydrological datasets: cloud-based Landsat thermal remote sensing data products for ET (Google Earth Engine EEFlux), Landsat multispectral imagery and field data-based calibrations to vegetation structure (leaf-area index, LAI), and open-source climate and hydrological data. We show that at landscape scales, daily ET rates (6–10 mm day⁻¹) and growing season ET totals (400–1,400 mm) matched rates of published field data, and modelled reach-scale average LAI (0.80–1.70) matched lower ranges of published field data. Over 6 years, the spatial variability of total growing season ET (CV = 0.18) exceeded that of temporal variability (CV = 0.10), indicating the importance of reach-scale vegetation and hydrological conditions for controlling ET dynamics. Responses of ET to climate differed between perennial and intermittent-flow stream reaches. At perennial-flow reaches, ET correlated significantly with temperature, whilst at intermittent-flow sites ET correlated significantly with rainfall and stream discharge. Amongst reaches studied in detail, we found positive but differing logarithmic relationships between LAI and ET. By documenting patterns of high spatial variability of ET at basin scales, these results underscore the importance of accurately accounting for differences in woodland vegetation structure and hydrological conditions for assessing water-use requirements. Results also suggest that the climate sensitivity of ET may be used as a remote indicator of subsurface water resources relative to vegetation demand, and an indicator for informing conservation management priorities.  相似文献   

流空间视域下城市群功能协同发展研究     

郭杰  姜璐  张虹鸥  叶玉瑶  林浩曦 《热带地理》2022,42(2):195-205

文章基于流空间视角,提出了城市群协同发展双重内涵,从流动性、连通性与互补性3方面剖析了要素空间流动与区域功能协同的内在关联。并选取旧金山湾区为实证案例,梳理国际湾区城市群从单中心走向功能协同、一体化发展的路径及驱动机制。研究认为,要素流驱动下的旧金山湾区经历了从发育到成熟的3个阶段：1）要素流动强化旧金山与奥克兰双城联系,功能互补促进双城共生发展阶段;2）要素南汇促成三足鼎立,要素对流与功能互惠助推多中心联动发展阶段;3）要素分流促成泛网络空间结构,错位互补发展强化区域环流,湾区实现功能协同发展阶段。文章结论可为构建中国区域协调发展机制、推动国际湾区协同发展带来如下启示：应顺应要素流动的客观规律,推进体制机制创新,为要素流动破除障碍;发挥政府引导作用,提升区域软、硬件基础设施的连通性,为要素流动和实现区域优势互补奠定基础;探索区域协商机制,制定功能互补与联动的区域发展战略,引导要素合理分流与高效配置,实现区域功能联动发展。  相似文献   

Refining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique   总被引：2，自引：0，他引：2  

S. Roecker  C. Thurber  K. Roberts  L. Powell 《Tectonophysics》2006,426(1-2):189

The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20–25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L. , and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427–434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by  100 m. Tests using “virtual earthquakes” (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically.  相似文献   

What have natural and human changes wrought on the foraminifera of San Francisco Bay late Quaternary estuaries?     

Amy E. Lesen  Jere H. Lipps 《Quaternary Research》2011,76(2):211-219

In this study we compare the foraminifera of modern South San Francisco Bay with fossils from sediments of a previous estuary at 125 ka to provide a basis for interpreting the impact of natural and human change on the benthic ecosystem. All the species found in the Pleistocene sediments of this study are estuarine and/or shallow-water species occurring commonly in San Francisco Bay today, except for the introduced foraminifer Trochammina hadai, a native of Japan that was not found in samples taken in San Francisco Bay before 1983. The biodiversity and species composition of the fossil and modern assemblages before the introduction of T. hadai are nearly identical, suggesting that the environmental and physical changes in the 125,000-year-old and modern estuaries have not had a significant effect on the meiofauna of the Bay. In contrast, modern anthropogenic change in the form of species introductions has impacted the modern foraminiferal assemblage: T. hadai began to dominate the modern assemblage a decade after its introduction. Similar to the recorded impacts of introductions of marine metazoan invertebrate species, the dominance of T. hadai changed species proportions in the post-1980s foraminiferal assemblage, however no known extinctions in the native foraminiferal fauna occurred.  相似文献   

Post-glacial landscape response to climate variability in the southeastern San Juan Mountains of Colorado, USA     

Bradley G. Johnson  Martha Cary Eppes  John A. Diemer  Gonzalo Jiménez-Moreno  Anthony L. Layzell 《Quaternary Research》2011,76(3):352-362

Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that three distinct periods of aggradation have occurred since the end of the last glacial maximum (LGM). The first occurred during the Pleistocene–Holocene transition (~ 12.5–9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at ~ 5.5 ka. A third, more broadly identifiable period of sedimentation occurred in the late Holocene (~ 2.2–1 ka). The latest two periods of aggradation are concurrent with increases in the frequency of climate change in the region suggesting that Holocene alpine and sub-alpine landscapes respond more to rapid changes in climate than to large singular climatic swings. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused upslope of headwater streams, along tributary channels, or on ridge tops. This is in contrast to some models which assume equal erosion in headwater basins.  相似文献   

The Posthuman City: San Diego'S Dead Animal Removal Program     

David Lulka 《Urban geography》2013,34(8):1119-1143

Abstract

Nonhuman animals are part of the dynamism that typifies cities, yet prior research has obscured this condition by focusing upon a limited subset of animals and the privatized spaces that contain them. Consequently, previous research provides little guidance in developing a post-human conception of cities. This paper redresses this deficiency by redefining conceptions of “the public” and analyzing data from San Diego's Dead Animal Removal Program (DARP). Commensurate with modern political conceptions, “the public” is defined simultaneously as a spatial formation and (akin to democracies) a quantitative body of bodies. The DARP data indicate the pervasive presence of nonhuman animals in urban environments and the difficulties they face. As a nexus, the DARP illustrates how urban practices and bureaucratic systems reinstantiate “the public” in humanized form through the death and disposal of nonhumans. These hybrid relations challenge geographic methods.  相似文献   

Zero graffiti for a beautiful city: the cultural politics of urban space in San Francisco   总被引：1，自引：0，他引：1  

Hunter Shobe  David Banis 《Urban geography》2013,34(4):586-607

San Francisco, like many cities in the United States and across the world, has an official zero-tolerance policy on graffiti. In this article, we examine the academic literature concerned with graffiti and then present a case study of zero-tolerance abatement policies in San Francisco. Our analysis yields three main findings. First, zero-tolerance policy stimulates an anti-graffiti industry with vested interest in perpetuating an endless war for control of public space. Second, zero tolerance may generate an unintended result—the proliferation of tags and other forms of graffiti that people tend to dislike the most. Third, we find little evidence that the general public shares the same desire for zero tolerance as the San Francisco Government. Ultimately, we believe that more nuanced readings of graffiti allow greater numbers of people to make sensible, local, place-specific policies regarding graffiti.  相似文献   

The Ambiguous Roles of Suburbanization and Immigration in Ethnic Segregation: the Case of San Antonio1     

《Urban geography》2013,34(3):196-223

The traditional spatial assimilation model, though still operative, has proven inadequate to explain new trends in urban residential location in which, for example, disadvantaged and newly arrived groups move directly to the suburbs where they may re-segregate rather than disperse. Understanding residential patterns after 1990 often benefits from a micro-level approach, looking at specific cities and disaggregating traditional measures (e.g., the dissimilarity index) to examine changes within areas and neighborhoods of the city. This study takes such an approach. It analyzes segregation by means of the residential micro-patterns that give rise to it, and examines their relationship to suburbanization and immigration in greater San Antonio (Bexar County) during the 1990s for four ethnic groups: Hispanics, Blacks, non-Hispanic Whites, and Latino immigrants. The results reveal that segregation patterns in San Antonio have deep historic roots—the result of ongoing processes of urban job and housing availability, minority political power, and economic mobility. They show that the dissimilarity index declined for all groups in the 1990s, a decline chiefly attributable to deconcentration (lessening overrepresentation) in the inner city. By contrast, in the outer suburban city, incursion (moving into new areas) was offset by hyper-concentration (concentrating with fellow ethnics) such that the dissimilarity index neither increased nor declined. A typology is developed to explain the different pathways by which the dissimilarity index may increase or decrease in a metropolis. Finally, the results show that Latino immigration increased the overall dissimilarity index for Hispanics as well as for other groups in San Antonio.  相似文献