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421.
Anomalous seismic crustal structure of oceanic fracture zones 总被引:2,自引:0,他引:2
Robert S. White Robert S. Detrick Martin C. Sinha Marie H. Cormier 《Geophysical Journal International》1984,79(3):779-798
Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment. 相似文献
422.
Marie K. McCabe 《Solar physics》1971,19(2):451-462
High velocity H ejections were observed in association with an important solar flare on March 12, 1969, and simultaneously with Type II followed by Type IV radio emission detectable to 3–4 solar radii (R
) from the center of the Sun. From a sequence of H coronagraph photographs, trajectories and velocity determinations were made for fragments of the flare spray which was visible to a distance of 2 R
. The temporal and spatial relationship between the optical and radio events is discussed. The mass motions appear to be controlled by the gravitational field while the fragments move in the direction of the open magnetic field lines. 相似文献
423.
424.
Geochemical methods (major elements and Sr, Nd isotopes) have been used to (1) characterize Lake Le Bourget sediments in the French Alps, (2) identify the current sources of the clastic sediments and estimate the source variability over the last 600 years. Major element results indicate that Lake Le Bourget sediments consist of 45% clastic component and 55% endogenic calcite. In addition, several individual flood levels have been identified during the Little Ice Age (LIA) on the basis of their higher clastic content (> 70%).Potential sources of Lake Le Bourget clastic sediments have been investigated from Sr and Nd isotope compositions. The sediments from the Sierroz River and Leysse River which are mainly derived from the Mesozoic Calcareous Massifs are characterised by lower 87Sr/86Sr ratios and slightly lower ?Nd(0) ratios than the Arve River sediments which are derived from the Palaeozoic Mont-Blanc External Crystalline Massifs. The Rhône River appears to have been the main source of clastic sediments into the lake for the last 600 years, as evidenced by a similar Sr and Nd isotopic compositions analyzed in core B16 sediments (87Sr/86Sr = 0.719, ?Nd(0) = − 10) and in the sediments of the Rhône River (87Sr/86Sr = 0.719, ?Nd(0) = − 9.6).The isotopic signatures of flood events and background samples from core B16 in Lake Le Bourget are also similar. This indicates that prior to ∼ 1800, the inputs into the lake have remained relatively homogeneous with the proportion of clastic component mainly being a function of the palaeohydrology of the Rhone River. Early human modification (deforestation and agriculture) of the lake catchment before the 1800s appears to have had little influence on the source of clastic sediments. 相似文献
425.
The management of tidal inlets requires the accurate prediction of equilibrium morphologies. In areas where the flow from
rivers is highly regulated, it is important to give decision makers the ability to determine optimal flow management schemes,
in order to allow tidal inlets to function as naturally as possible, and minimise the risk of inlet closure. The River Murray
Mouth in South Australia is one such problem area. Drought and the retention of water for irrigation and urban water consumption
have limited the amount of water entering the estuary. As a result, sediment from the coastal environment is being deposited
in the mouth of the estuary, reducing the effect of further coastal interactions. Currently, situations such as this are modelled
using traditional process-based methods, where wave, current, sediment transport and sediment balance modules are linked together
in a time-stepping process. The modules are reapplied and assessed until a stable morphology is formed. In this paper, new
options for modelling equilibrium morphologies of tidal inlets are detailed, which alleviate some of the shortfalls of traditional
process-based models, such as the amplification of small errors and reliance on initial conditions. The modelling problem
is approached in this paper from a different angle and involves the use of entropy based objective functions, which are optimised
in order to find equilibrium morphologies. In this way, characteristics of a system at equilibrium can be recognised and a
stable system predicted without having to step through time. This paper also details the use of self-organisation based modelling
methods, another non-traditional model application, where local laws and feedback result in the formation of a global stable
equilibrium morphology. These methods represent a different approach to traditional models, without some of the characteristics
that may add to their limitations.
Responsible Editor: Alejandro Souza 相似文献
426.
427.
Anne Marie de Grosbois Philip E. LaMoreaux Jörg Matschullat 《Environmental Geology》2006,50(7):1105-1106
428.
Changes in wind speed and sediment transport are evaluated at a gap and adjacent crest of a 2 to 3 m high, 40 m wide foredune built by sand fences and vegetation plantings on a wide, nourished fine sand beach at Ocean City, New Jersey. Anemometer masts, cylindrical sand traps and erosion pins were placed on the beach and dune during two obliquely onshore wind events in February and March 2003. Results reveal that: (1) changes in the alongshore continuity of the beach and dune system can act as boundaries to aeolian transport when winds blow at an angle to the shoreline; (2) oblique winds blowing across poorly vegetated patches in the dune increase the potential for creating an irregular crest elevation; (3) transport rates and deflation rates can be greater within the foredune than on the beach, if the dune surface is poorly vegetated and the beach has not had time to dry following tidal inundation; (4) frozen ground does not prevent surface deflation; and (5) remnant sand fences and fresh storm wrack have great local but temporary effect on transport rates. Temporal and spatial differences due to sand fences and wrack, changes in sediment availability due to time‐dependent differences in surface moisture and frozen ground, combined with complex topography and patchy vegetation make it difficult to specify cause–effect relationships. Effects of individual roughness elements on the beach and dune on wind flow and sediment transport can be quantified at specific locations at the event scale, but extrapolation of each event to longer temporal and spatial scales remains qualitative. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
429.
Evaluating rainfall patterns using physics scheme ensembles from a regional atmospheric model 总被引:2,自引:0,他引:2
Fei Ji Marie Ekström Jason P. Evans Jin Teng 《Theoretical and Applied Climatology》2014,115(1-2):297-304
This study evaluated the ability of Weather Research and Forecasting (WRF) multi-physics ensembles to simulate storm systems known as East Coast Lows (ECLs). ECLs are intense low-pressure systems that develop off the eastern coast of Australia. These systems can cause significant damage to the region. On the other hand, the systems are also beneficial as they generate the majority of high inflow to coastal reservoirs. It is the common interest of both hazard control and water management to correctly capture the ECL features in modeling, in particular, to reproduce the observed spatial rainfall patterns. We simulated eight ECL events using WRF with 36 model configurations, each comprising physics scheme combinations of two planetary boundary layer (pbl), two cumulus (cu), three microphysics (mp), and three radiation (ra) schemes. The performance of each physics scheme combination and the ensembles of multiple physics scheme combinations were evaluated separately. Results show that using the ensemble average gives higher skill than the median performer within the ensemble. More importantly, choosing a composite average of the better performing pbl and cu schemes can substantially improve the representation of high rainfall both spatially and quantitatively. 相似文献
430.
Jade Petersen Bruno Wilhelm Marie Revel Yann Rolland Christian Crouzet Fabien Arnaud Elodie Brisset Eric Chaumillon Olivier Magand 《Journal of Paleolimnology》2014,51(3):343-355
We studied sediment cores from Lake Vens (2,327 m asl), in the Tinée Valley of the SW Alps, to test the paleoseismic archive potential of the lake sediments in this particularly earthquake-sensitive area. The historical earthquake catalogue shows that moderate to strong earthquakes, with intensities of IX–X, have impacted the Southern Alps during the last millennium. Sedimentological (X-ray images, grain size distribution) and geochemical (major elements and organic matter) analyses show that Lake Vens sediments consist of a terrigenous, silty material (minerals and organic matter) sourced from the watershed and diatom frustules. A combination of X-ray images, grain-size distribution, major elements and magnetic properties shows the presence of six homogenite-type deposits interbedded in the sedimentary background. These sedimentological features are ascribed to sediment reworking and grain sorting caused by earthquake-generated seiches. The presence of microfaults that cross-cut the sediment supports the hypothesis of seismic deposits in this system. A preliminary sediment chronology is provided by 210Pb measurement and AMS 14C ages. According to the chronology, the most recent homogenite events are attributable to damaging historic earthquakes in AD 1887 (Ligure) and 1564 (Roquebillière). Hence, the Lake Vens sediment recorded large-magnitude earthquakes in the region and permits a preliminary estimate of recurrence time for such events of ~400 years. 相似文献