Shear-wave anisotropy: spatial and temporal variations in time delays at Parkfield, Central California   总被引：4，自引：0，他引：4  

Yun Liu  Stuart Crampin  Ian Main 《Geophysical Journal International》1997,130(3):771-785

Shear-wave splitting is analysed on data recorded by the High Resolution Seismic Network (HRSN) at Parkfield on the San Andreas fault, Central California, during the three-year period 1988-1990. Shear-wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress, at some 70° to the fault strike, whereas at station MM in the immediate fault zone, shear-wave polarizations are aligned approximately parallel to the fault. Normalized time delays at this station are found to be about twice as large as those in the rock mass either side. This suggests that fluid-filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks, and that the alignment of fault-parallel shear-wave polarizations are associated with some fault-specific phenomenon.
Temporal variations in time delays between the two split shear-waves before and after a M_L = 4 earthquake can be identified at two stations with sufficient data: MM within the fault zone and VC outside the immediate fault zone. Time delays between faster and slower split shear waves increase before the M_L = 4 earthquake and decrease near the time of the event. The temporal variations are statistically significant at 68 per cent confidence levels. Earthquake doublets and multiplets also show similar temporal variations, consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass. This study is broadly consistent with the behaviour observed before three other earthquakes, suggesting that the build-up of stress before earthquakes may be monitored and interpreted by the analysis of shear-wave splitting.  相似文献   

A bivariate meta-Gaussian density for use in hydrology   总被引：3，自引：0，他引：3  

K. S. Kelly  R. Krzysztofowicz 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(1):17-31

Convenient bivariate densities found in the literature are often unsuitable for modeling hydrologic variates. They either constrain the range of association between variates, or fix the form of the marginal distributions. The bivariate meta-Gaussian density is constructed by embedding the normal quantile transform of each variate into the Gaussian law. The density can represent a full range of association between variates and admits arbitrarily specified marginal distributions. Modeling and estimation can be decomposed into i) independent analyses of the marginal distributions, and ii) investigation of the dependence structure. Both statistical and judgmental estimation procedures are possible. Some comparisons to recent applications of bivariate densities in the hydrologic literature motivate and illustrate the model.  相似文献   

Radiative Exchange in an Urban Street Canyon   总被引：1，自引：4，他引：1  

Ian N. Harman  Martin J. Best  Stephen E. Belcher 《Boundary-Layer Meteorology》2004,110(2):301-316

The influence of building geometry on the radiation terms ofthe surface energy balance is a principal reason for surfacetemperature differences between rural and urban areas.Methods exist to calculate the radiation balance in an urban area,but their validity across the range of urban geometries andmaterials has not been carefully considered.Here the exchange of diffuse radiation in an urban street canyon isinvestigated using a method incorporating all reflections of radiation.This exact solution is compared to two commonly used approximationsthat retain either no reflections, or just one reflection of radiation.The area-averaged net radiative flux density from the facets of the canyondecreases in magnitude monotonically as the canyon aspect ratio increases.The two approximate solutions possess unphysical differences from thismonotonic decrease for high canyon aspect ratios or low materialemissivities/high material albedos.The errors of the two approximate solutions are small for near blackbodymaterials and small canyon aspect ratios but can be an order ofmagnitude for intermediate material properties and deep street canyons.Urban street canyon models need to consider at least one reflectionof radiation and multiple reflections are desirable for full applicability.  相似文献   

International and National Aspects of a Legislative Framework to Manage Soil Carbon Sequestration   总被引：1，自引：0，他引：1  

Ian Hannam 《Climatic change》2004,65(3):365-387

This article discusses the international and national environmental law framework for the management of soil carbon sequestration. Aspects of the legislative framework important to this process include its ability to recognise carbon sinks, expand existing sinks, and the procedures available to return and store carbon in soil reservoirs. International law provides global standards and guidelines and national legislative systems provide the substantive and procedural legal mechanisms to manage soil carbon. The United Nations Framework Convention on Climate Change and the Kyoto Protocol are the primary international legislative instruments but other international instruments and strategies have a significant synergistic role. Various approaches are presented for framing new legislation or to reform existing legislative frameworks to improve the procedures to manage soil carbon.  相似文献   

Human exploration of the Moon and Mars: implications for Aurora   总被引：1，自引：0，他引：1  

Ian A Crawford 《Astronomy& Geophysics》2004,45(2):2.28-2.29

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Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica     

Ian J. Basson  Samantha Perritt  Michael K. Watkeys  Andrew H. Menzies   《Gondwana Research》2004,7(1):57-73

The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions.  相似文献   

Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial environments in Georgia Basin and Puget Lowland, British Columbia, Canada and Washington, USA     

Ian Hutchinson  Thomas S. James  Brian D. Bornhold 《Quaternary Research》2004,61(2):193-203

Models of late-glacial environmental change in coastal areas are commonly based on radiocarbon ages on marine shell and basal lake sediments, both of which may be compromised by reservoir effects. The magnitude of the oceanic reservoir age in the inland waters of the Georgia Basin and Puget Lowland of northwestern North America is inferred from radiocarbon ages on shell-wood pairs in Saanich Inlet and previously published estimates. The weighted mean oceanic reservoir correction in the early and mid Holocene is −720±90 yr, slightly smaller than, but not significantly different from, the modern value. The correction in late-glacial time is −950±50 yr. Valley-head sites yield higher reservoir values (−1200±130 yr) immediately after deglaciation. The magnitude of the gyttja reservoir effect is inferred from pairs of bulk gyttja and plant macrofossil ages from four lakes in the region. Incorporation of old carbon into basal gyttja yields ages from bulk samples that are initially about 600 yr too old. The reservoir age declines to less than 100 yr after the first millennium of lake development. When these corrections are accounted for, dates of deglaciation and late-glacial sea-level change in the study area are pushed forward in time by more than 500 yr.  相似文献   

A German catalogue of archaeomagnetic data     

Elisabeth Schnepp  Rudolf Pucher  Jan Reinders  Ulrich Hambach  Heinrich Soffel  Ian Hedley 《Geophysical Journal International》2004,157(1):64-78

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Frequency dependence of elastic wave speeds at high temperature: a direct experimental demonstration     

Ian Jackson 《Physics of the Earth and Planetary Interiors》2005,148(1):85-96

At sufficiently high temperatures and/or long periods, the elastic behaviour of crystalline material gives way progressively to viscoelastic behaviour associated with the stress-induced migration of crystal defects. This transition is marked by the onset of appreciable strain energy dissipation accompanied by frequency dependence (dispersion) of the shear modulus and elastic wave speeds. Ultrasonic interferometry and torsional forced-oscillation techniques can be used to probe the low-amplitude stress-strain behaviour of fine-grained polycrystalline material in two very different frequency ranges, respectively 10-100 MHz and 1 mHz-1 Hz. Here we demonstrate and apply these two complementary methods in a study of the high-temperature mechanical behaviour of a fine-grained synthetic olivine polycrystal. At the high frequencies of ultrasonic interferometry, the shear wave speed varies linearly with temperature between room temperature and the highest experimentally accessible temperature (1300 °C) in close accord with expectations based on similarly high-frequency studies of the elastic behaviour of single-crystal olivine. However, at teleseismic frequencies (<1 Hz) and temperatures >900 °C, the shear wave speed becomes much more strongly temperature-sensitive reflecting markedly viscoelastic behaviour. Newly emerging laboratory-derived constraints on this viscoelastic enhancement of the temperature sensitivity of seismic wave speeds and its grain-size dependence will provide a more robust interpretation of seismological models for the variation of wave speeds and attenuation within the Earth's interior.  相似文献   

‘Committing to Place’ at the Local Scale: the potential of youth education programs for promoting community participation in regional natural resource management     

Ruth Lane  Damian Lucas  Frank Vanclay  Sophie Henry  Ian Coates 《The Australian geographer》2005,36(3):351-367

Emerging approaches to environmental governance require a greater level of community participation than did previous approaches in which these responsibilities largely rested with government agencies. There is consequently a need for increased engagement with NRM among a broad community sector. This paper examines initiatives by two prominent government agencies, the Murray–Darling Basin Commission (MDBC) and the National Museum of Australia (NMA), to engage school children from regional communities using education programs that focus on place and environmental health. We focus on the MDBC's International Riverhealth Conference held in Mildura in 2003 and the associated Murray–Darling Basin TalkBack Classroom sponsored by the NMA and the Parliamentary Education Office (PEO). We explore how key themes of local scale, place-based identities, youth voice and critical engagement are developed in these programs and consider how they relate to the environmental agency of children. We then reflect on the potential for the kinds of environmental agency promoted through these programs to help build the capacity of local communities to progress larger goals of environmental restoration and sustainability in the Murray–Darling Basin. The evaluation research reported here forms part of the Committing to Place research project, an Australian Research Council Linkage grant involving the University of Tasmania, the National Museum of Australia and the Murray–Darling Basin Commission.  相似文献