Three-Dimensional Modeling of Shallow Shear-Wave Velocities for Las Vegas, Nevada, Using Sediment Type     

Barbara Luke  Helena Murvosh  Wanda Taylor  Jeff Wagoner 《中国地质大学学报(英文版)》2009,20(3)

A three-dimensional model of near-surface shear-wave velocity in the deep alluvial basin underlying the metropolitan area of Las Vegas, Nevada (USA), is being developed for earthquake site response projections. The velocity dataset, which includes 230 measurements, is interpolated across the model using depth-dependent correlations of velocity with sediment type. The sediment-type database contains more than 1 400 well and borehole logs. Sediment sequences reported in logs are assigned to one of four units. A characteristic shear-wave velocity profile b developed for each unit by analyzing closely spaced pairs of velocity profiles and well or borehole logs. The resulting velocity model exhibits reasonable values and patterns, although it does not explicitly honor the measured shear-wave velocity profiles. Site response investigations that applied a preliminary version of the velocity model support a two-zone ground-shaking hazard model for the valley. Areas in which clay predominates in the upper 30 m are predicted to have stronger ground motions than the rest of the basin.  相似文献   

Tsunami sediments and their foraminiferal assemblages     

Briony Mamo  Luke Strotz  Dale Dominey-Howes 《Earth》2009,96(4):263-278

Tsunami hazard assessment begins with a compilation of past events that have affected a specific location. Given the inherent limitations of historical archives, the geological record has the potential to provide an independent dataset useful for establishing a richer, chronologically deeper time series of past events. Recent geological studies of tsunami are helping to improve our understanding of the nature and character of tsunami sediments. Wherever possible, geologists should be working to improve the research ‘tool kit’ available to identify past tsunami events. Marine foraminifera (single celled heterotrophic protists) have often been reported as present within tsunami-deposited sediments but in reality, little information about environmental conditions, and by analogy, the tsunami that deposited them, has been reported even though foraminifera have an enormous capacity to provide meaningful palaeo-environmental data. Here, we review what foraminifera are, describe their basic form and significance, summarise where they have been reported in tsunami sediments and identify what can be learnt from them. We review the gaps in our understanding and make recommendations to assist researchers who examine foraminiferal assemblages in order to enhance their use within tsunami geology.  相似文献   

等离子体片中能量离子的空间分布--Cluster/RAPID观测数据分析          下载免费PDF全文

杨剑  傅绥燕  刘文龙  阮鹏  濮祖荫  P. Daly  王永福 《地球物理学报》2005,48(6):1226-1232

本文根据Cluster卫星上的粒子成像质谱仪（RAPID）探测器在穿越地球等离子体片过程中的观测数据,统计研究了等离子体片中能量离子能量密度的空间分布（氢离子能量范围从40keV到1500keV,氦离子和氧离子从10keV到1500keV）,并且给出了离子能量密度在不同地磁活动时期随GSE Z向分布的剖面.研究表明能量离子的能量密度以及能量密度的梯度与地磁活动指数Kp之间存在近似线性的关系.观测结果表明形成这种分布变化的主要原因是在地磁活动期间在电流片附近离子能量密度的增加,特别是其中的重离子成分增加更为显著.本文通过一个简化的电流片模型的数值计算,定性地研究了形成能量离子空间分布的机理.计算表明重离子在电流片中可以获得更多的能量,电流片加速可能是形成能量密度分布变化的一种可能的机制.  相似文献   

Timing and thermochemical constraints on multi-element mineralisation at the Nori/RA Cu–Mo–U prospect, Great Bear magmatic zone, Northwest Territories, Canada     

Luke Ootes  Steve Goff  Valerie A. Jackson  Sarah A. Gleeson  Robert A. Creaser  Iain M. Samson  Norman Evensen  Louise Corriveau  A. Hamid Mumin 《Mineralium Deposita》2010,45(6):549-566

The timing of Cu–Mo–U mineralisation at the Nori/RA prospect in the Paleoproterozoic Great Bear magmatic zone has been investigated using Re–Os molybdenite and ⁴⁰Ar–³⁹Ar biotite geochronology. The Re–Os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the Great Bear magmatic zone. Cu–Mo–U mineralisation is hosted in early to syn-deformational hydrothermal veins consisting of quartz and K-feldspar or more commonly tourmaline-biotite-quartz-K-feldspar, with associated wall-rock alteration assemblages being predominantly biotite. Sulphide and oxide minerals consist of chalcopyrite, molybdenite and uraninite with lesser pyrite and magnetite. Elevated light rare earth elements and tungsten concentrations associated with the Cu–Mo–U mineralisation have also been reported at the prospect by previous workers. Molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins, attesting to their syngenetic nature (with respect to hydrothermal veining). Two molybdenite separates yield Re–Os model ages of 1,874.4 ± 8.7 (2σ) and 1,872.4 ± 8.8 Ma (2σ) with a weighted average model age of 1,873.4 ± 6.1 Ma (2σ). Laser step heating of biotite from the marginal alteration of the wall-rock adjacent to the veins yields a ⁴⁰Ar–³⁹Ar maximum cooling age of 1,875 ± 8 Ma (MSWD = 3.8; 2σ), indistinguishable from the Re–Os molybdenite model age and a previously dated ‘syn-tectonic’ aplitic dyke in the region. Dravitic tourmaline hosts abundant primary liquid–vapour–solid-bearing fluid inclusions. Analytical results indicate liquid–vapour homogenisation at >260°C constraining the minimum temperature of mineralisation. The solids, which are possibly trapped, did not homogenise with the liquid–vapour by 400°C. Salinities in the inclusions are variable. Raman spectra identify that at least some of the solids are calcite and anhydrite. Raman spectra also confirm the vapour phases contain some CO₂; whereas clathrates or CH₄ was not observed or detected. Quartz grains only host secondary fluid inclusions, which fluoresce under ultraviolet light, indicating trapped hydrocarbons. We speculate that these resulted from Phanerozoic fluid circulation through the Proterozoic basement. The collective interpretation of the age, hydrothermal character and associated metals, high temperature and variable salinity suggests that the Nori/RA Cu–Mo–U mineralisation can be linked with the earliest stages of plutonism in the Great Bear magmatic zone. From a regional perspective, the mineralisation may pre-date the extensive multi-element mineralisation now recognised as part of the iron oxide copper–gold (IOCG) spectrum of deposits. As IOCG provinces generally contain a variety of mineralisation styles, we interpret this as the earliest phase of the extensive mineralising system.  相似文献   

The structure and timing of lateral escape during the Scandian Orogeny: A combined strain and geochronological investigation in Finnmark, Arctic Norwegian Caledonides   总被引：4，自引：0，他引：4  

C.L. Kirkland  J.S. Daly  E.A. Eide  M.J. Whitehouse 《Tectonophysics》2006,425(1-4):159-189

Determining the timing, duration and mechanism of tectonic events within an orogenic cycle, such as ocean subduction, continent–continent collision or gravitational collapse, is challenging, especially in ancient orogenic belts. Variations in the tectonic transport direction, however, can be used as a guide to these stages of orogeny. While thrust sheets within the Caledonian allochthon in north Norway were emplaced broadly eastwards perpendicular to the trend of the orogen, many features indicate material transport in other orientations. One dominant feature of the Magerøy Nappe, sitting above and infolded with the Kalak Nappe Complex, is the development of a strong N–S lineation orthogonal to the main transport direction. Strain measurements, in part determined by a new method, are used, in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen-parallel lateral escape. Quantitative 3-D strain estimation in the Magerøy Nappe indicates prolate deformation with c. 50% horizontal shortening parallel to the thrusting direction (E–W) and c. 200% extension along the orogenic strike (N–S) with c. 30% vertical shortening. Temporal constraint on this fabric is provided by Ar–Ar isotopic analysis of undeformed white mica in cross-cutting granitic pegmatites. These data show that prolate deformation occurred before the white mica cooling age of 416 ± 4 Ma, while the previously determined depositional age of the Hellefjord Schist indicates that it occurred after 438 ± 4 Ma. A granitic pegmatite that intruded the Hellefjord Schist after an initial deformation phase but during or prior to a later deformation, has been dated at 431 ± 2 Ma by U–Pb zircon. A previous lower age constraint on this deformation of 428 ± 5 Ma is given by metamorphic zircon overgrowths on fractured grains. These results constrain the continental collision between Baltica and Laurentia in Finnmark to the interval c. 431–428 Ma. Placed in a regional context, these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent–continent collision stage in the Scandian Orogeny before gravitationally driven collapse.  相似文献   

Multiple Flux Rope Events at the High-Latitude Magnetopause: Cluster/Rapid Observation on 26 January, 2001     

Z. Y. Pu  Q. -G. Zong  T. A. Fritz  C. J. Xiao  Z. Y. Huang  S. Y. Fu  Q. Q. Shi  M. W. Dunlop  K. -H. Glassmeier  A. Balogh  P. Daly  H. Reme  J. Dandouras  J. B. Cao  Z. X. Liu  C. Shen  J. K. Shi 《Surveys in Geophysics》2005,26(1-3):193-214

Cluster measurements of the cusp and high latitude magnetopause boundary on 26 January, 2001 confirm that the cusp is a dynamic region full of energetic charged particles and turbulence. An energetic ion layer at high-latitudes beyond and adjacent to the duskside magnetopause exists when the Interplanetary Magnetic Field (IMF) has a southward orientation. Multiple energetic ion flux bursts were observed in the energetic ion layer. Each energetic ion flux burst was closely related to a magnetic flux rope. The axes of the flux ropes lie in the direction pointing duskward/tailward and somewhat upward. An intense axis-aligned current flows inside the ropes, with the current density reaching ∼10⁻⁸ A/m². The main components of the energetic ions are protons, helium and CNO ions, which originate from the magnetosphere, flowing out into the magnetosheath along the axis of the flux ropes. The velocity of the magnetosheath thermal plasma relative to the deHoffman-Teller (DHT) frame is found to be basically along the axis of the flux ropes also, but towards the magnetosphere. These flux ropes seem to be produced somewhere away via magnetic reconnection and move at similar DHT velocities passing over the spacecraft. These observations further confirm that the high-latitude magnetopause boundary region plays an important role in the solar wind-magnetopause coupling.  相似文献   

Understanding ship-grounding impacts on a coral reef: potential effects of anti-foulant paint contamination on coral recruitment     

Negri AP  Smith LD  Webster NS  Heyward AJ 《Marine pollution bulletin》2002,44(2):111-117

The 184 m cargo ship Bunga Teratai Satu collided with Sudbury Reef, part of the Great Barrier Reef and remained grounded for 12 days. The ship was re-floated only 3 days prior to the November 2000 mass coral spawning. No cargo or fuel was lost but the impact resulted in significant contamination of the reef with anti-foulant paint containing tributyltin (TBT), copper (Cu) and zinc (Zn). Larvae of the reef-building scleractinian coral Acropora microphthalma were exposed to various concentrations of sediment collected from the grounding site in replicated laboratory experiments. Two experiments were performed, both of which used varying ratios of contaminated and control site sediment in seawater as treatments. In the first experiment, the influence of contaminated sediment on larval competency was examined using metamorphosis bioassays. In the second, the effect of contaminated sediment upon larval recruitment on pre-conditioned terracotta tiles was assessed. In both experiments, sediment containing 8.0 mg kg(-1) TBT, 72 mg kg(-1) Cu and 92 mg kg(-1) Zn significantly inhibited larval settlement and metamorphosis. At this level of contamination larvae survived but contracted to a spherical shape and swimming and searching behaviour ceased. At higher contamination levels, 100% mortality was recorded. These results indicate that the contamination of sediment by anti-fouling paint at Sudbury Reef has the potential to significantly reduce coral recruitment in the immediate vicinity of the site and that this contamination may threaten the recovery of the resident coral community unless the paint is removed.  相似文献   

Calcite-filled borings in the most recently deposited skeleton in live-collected Porites (Scleractinia): Implications for trace element archives     

Luke D. Nothdurft  Gregory E. Webb  Llew Rintoul 《Geochimica et cosmochimica acta》2007,71(22):5423-5438

Skeletons of the scleractinian coral Porites are widely utilized as archives of geochemical proxies for, among other things, sea surface temperature in paleoclimate studies. Here, we document live-collected Porites lobata specimens wherein as much as 60% of the most recently deposited skeletal aragonite, i.e., the part of the skeleton that projects into the layer of living polyps and thus is still in direct contact with living coral tissue, has been bored and replaced by calcite cement. Calcite and aragonite were identified in situ using Raman microspectroscopy. The boring-filling calcite cement has significantly different trace element ratios (Sr/Ca_(mmol/mol) = 6.3 ± 1.4; Mg/Ca_(mmol/mol) = 12.0 ± 5.1) than the host coral skeletal aragonite (Sr/Ca_(mmol/mol) = 9.9 ± 1.3; Mg/Ca_(mmol/mol) = 4.5 ± 2.3). The borings appear to have been excavated by a coccoid cyanobacterium that dissolved aragonite at one end and induced calcite precipitation at the other end as it migrated through the coral skeleton. Boring activity and cement precipitation occurred concomitantly with coral skeleton growth, thus replacing skeletal aragonite that was only days to weeks old in some cases. Although the cement-filled borings were observed in only ∼20% of sampled corals, their occurrence in some of the most recently produced coral skeleton suggests that any corallum could contain such cements, irrespective of the coral’s subsequent diagenetic history. In other words, pristine skeletal aragonite was not preserved in parts of some corals for even a few weeks. Although not well documented in coral skeletons, microbes that concomitantly excavate carbonate while inducing cement precipitation in their borings may be common in the ubiquitous communities that carry out micritization of carbonate grains in shallow carbonate settings. Thus, such phenomena may be widespread, and failure to recognize even very small quantities of early cement-filled borings in corals used for paleoclimate studies could compromise high resolution paleotemperature reconstructions. The inability to predict the occurrence of cement-filled borings in coralla combined with the difficulty in recognizing them on polished blocks highlights the great care that must be taken in vetting samples both for bulk and microanalysis of geochemistry.  相似文献   

On the penetration of a hot diapir through a strongly temperature-dependent viscosity medium     

Stephen F. Daly  Arthur Raefsky 《Geophysical Journal International》1985,83(3):657-681

Summary. The ascent of a hot spherical body through a fluid with a strongly temperature-dependent viscosity has been studied using an axisymmetric finite element method. Numerical solutions range over Peclet numbers of 10⁻¹– 10³ from constant viscosity up to viscosity variations of 10⁵. Both rigid and stress-free boundary conditions were applied at the surface of the sphere. The dependence of drag on viscosity variation was shown to have no dependence on the stress boundary condition except for a Stokes flow scaling factor. A Nusselt number parameterization based on the stress-free constant viscosity functional dependence on the Peclet number scaled by a parameter depending on the viscosity structure fits both stress-free and rigid boundary condition data above viscosity variations of 100. The temperature scale height was determined as a function of sphere radius. For the simple physical model studied in this paper pre-heating is required to reduce the ambient viscosity of the country rock to less than 10²² cm² s⁻¹ in order for a 10 km diapir to penetrate a distance of several radii.  相似文献   

Evolution of lava flow-fields at Mount Etna, 27–28 October 1999, observed by Landsat 7 ETM+     

Robert Wright  Luke P. Flynn  Andrew J. Harris 《Bulletin of Volcanology》2001,63(1):1-7

Landsat 7 Enhanced Thematic Mapper + (ETM+) data are presented which document the thermal characteristics of a series of lava flows emplaced at Mount Etna volcano, Sicily, during 27-28 October 1999. By examining the composition of the short-wave infrared (SWIR) signal emitted from the flow surface, we identified distinctive flow units. The first unit appears to comprise recently active lava flows with relatively cool crusts which, by virtue of the integrity of this crust as determined from the ETM+ data, we infer are stationary or barely moving. The second unit is characterized by much higher levels of SWIR radiance, consistent with a channel-fed active flow unit. Analysis of the SWIR data confirm that this is fed by a lava channel, the properties of which are consistent with vigorously active channels observed on Kilauea, Hawaii. Model predictions of the maximum length that such flows could attain compare favorably with the actual flow lengths observed in the ETM+ data, indicating that the cooler flows had indeed stopped advancing, and may have attained a cooling-limited, rather than volume-limited, maximum length. Our observations and modeling provide a physical corroboration for the supposition made by Wadge (1978) in his analysis of the shape of lava flow fields on Mount Etna, which in the cooling-limited case principal flows are active one after the other and not at the same time.  相似文献