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871.
We present numerical modeling of SH-wave propagation for the recently proposed whole Moon model and try to improve our understanding of lunar seismic wave propagation. We use a hybrid PSM/FDM method on staggered grids to solve the wave equations and implement the calculation on a parallel PC cluster to improve the computing efficiency. Features of global SH-wave propagation are firstly discussed for a 100-km shallow and 900-km deep moonquakes, respectively. Effects of frequency range and lateral variation of crust thickness are then investigated with various models. Our synthetic waveforms are finally compared with observed Apollo data to show the features of wave propagation that were produced by our model and those not reproduced by our models. Our numerical modeling show that the low-velocity upper crust plays significant role in the development of reverberating wave trains. Increasing frequency enhances the strength and duration of the reverberations. Surface multiples dominate wavefields for shallow event. Core–mantle reflections can be clearly identified for deep event at low frequency. The layered whole Moon model and the low-velocity upper crust produce the reverberating wave trains following each phases consistent with observation. However, more realistic Moon model should be considered in order to explain the strong and slow decay scattering between various phases shown on observation data. 相似文献
872.
873.
The undrained shear behaviour of sands has been a key topic after the devastating geo-disasters during the 1964 Niigata Earthquake
in Japan. Extensive geo-technical soil tests, especially undrained triaxial compression tests, have revealed that the liquefaction
phenomenon was the major cause for the disaster expansions. To numerically reproduce the liquefaction phenomenon, the pore-water
pressure was coupled with a distinct element method. In this model, the dynamic changes in pore-water pressure were taken
into consideration by the changes in volumetric strain and modulus of compressibility of water in the respective measurement
spheres. Fluid-flows among the measurement spheres were controlled by Darcy’s law. The effective stress paths and steady state
strengths in undrained triaxial compression tests associated with the wide ranges of initial void ratio were investigated.
The effective mean stresses of medium-dense to dense numerical specimens at the steady state were negatively proportional
to the initial void ratio. Loose numerical specimens reproduced quasi-liquefaction with the effective mean stresses that were
less than 25% of the initial value. The medium-dense numerical specimens reproduced the phase transformation that was a typical
characteristic of granular materials. The rolling restraints did not much influence of the effective angle of internal friction
but strongly affected pore-water pressure behaviour within a certain range of initial void ratio. 相似文献
874.
Interpretation of landslide distribution triggered by the 2005 Northern Pakistan earthquake using SPOT 5 imagery 总被引:7,自引:4,他引:7
Hiroshi P. Sato Hiroyuki Hasegawa Satoshi Fujiwara Mikio Tobita Mamoru Koarai Hiroshi Une Junko Iwahashi 《Landslides》2007,4(2):113-122
The 2005 northern Pakistan earthquake (magnitude 7.6) of 8 October 2005 occurred in the northwestern part of the Himalayas.
We interpreted landslides triggered by the earthquake using black-and-white 2.5-m-resolution System Pour l’Observation de la Terre 5 (SPOT 5) stereo images. As a result, the counts of 2,424 landslides were identified in the study area of 55 by 51 km. About
79% or 1,925 of the landslides were small (less than 0.5 ha in area), whereas 207 of the landslides (about 9%) were large
(1 ha and more in area). Judging from our field survey, most of the small landslides are shallow rock falls and slides. However,
the resolution and whitish image in the photos prevented interpreting the movement type and geomorphologic features of the
landslide sites in detail. It is known that this earthquake took place along preexisting active reverse faults. The landslide
distribution was mapped and superimposed on the crustal deformation detected by the environmental satellite/synthetic aperture
radar (SAR) data, active faults map, geological map, and shuttle radar topography mission data. The landslide distribution
showed the following characteristics: (1) Most of the landslides occurred on the hanging-wall side of the Balakot–Garhi fault;
(2) greater than one third of the landslides occurred within 1 km from the active fault; (3) the greatest number of landslides
(1,147 counts), landslide density (3.2 counts/km2), and landslide area ratio (2.3 ha/km2) was found within Miocene sandstone and siltstone, Precambrian schist and quartzite, and Eocene and Paleocene limestone and
shale, respectively; (4) there was a slight trend that large landslides occurred on vertically convex slopes rather than on
concave slopes; furthermore, large landslides occurred on steeper (30° and more) slopes than on gentler slopes; (5) many large
landslides occurred on slopes facing S and SW directions, which is consistent with SAR-detected horizontal dominant direction
of crustal deformation on the hanging wall. 相似文献
875.
876.
Masahiro?ChigiraEmail author Fengjun?Duan Hiroshi?Yagi Takahiko?Furuya 《Landslides》2004,1(3):203-209
An airborne laser scanner can identify shallow landslides even when they are only several meters in diameter and are hidden by vegetation, if the vegetation is coniferous or deciduous trees in a season with fewer leaves. We used an airborne laser scanner to survey an area of the 1998 Fukushima disaster, during which more than 1,000 shallow landslides occurred on slopes of vapor-phase crystallized ignimbrite overlain by permeable pyroclastics. We identified landslides that have occurred at the 1998 event and also previous landslides that were hidden by vegetation. The landslide density of slopes steeper than 20° was 117 landslides/km2 before the 1998 disaster. This event increased the density by 233 landslides/km2 indicating that this area is highly susceptible to shallow landsliding. 相似文献
877.
The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ18O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ18O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ18O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ18OH2O and δDH2O values range from 8 to 12%. and ?3 to +42%., respectively.δ18OH2O values calculated from δ18O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ18O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are 18O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ18OH2O and δDH2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. 18O-enrichment in andesites may be produced by isotopic exchange with high 18O waters at elevated temperatures and sufficiently high water/rock ratios. 相似文献
878.
Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the 81Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the 81Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture 36Ar, 80Kr, 82Kr and/or 128Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of 128Xe from neutron capture on 127I vary among four measured specimens of Cumberland Falls (0.5-76 × 10−14 cm3STP/g), but are correlated with those of radiogenic 129Xe, implying that the concentrations of (128Xe)n and (129Xe)rad reflect variable abundances of iodine among specimens. The ratios of (128Xe)n/(129Xe)rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic 129Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10−7, 9.4 × 10−10 and 2.8 × 10−10 cm3STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely 244Pu-derived, and about 10-20% seems to be 238U-derived at 136Xe. The observed (136Xe)Pu corresponds to 0.019-0.16 ppb of 244Pu, from which the 244Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ17O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites. 相似文献
879.
Yoshiro Yamashita Hiromitsu Haba Hiroshi Shimizu 《Geochimica et cosmochimica acta》2007,71(14):3458-3475
In order to understand the fractionation of Re and Os in marine environments, their removal from artificial seawater to Tokyo Bay sediments is studied using a multitracer technique. The chemical processes of the removal of Re and Os are also estimated based on their speciation analyses by X-ray absorption fine structure (XAFS) spectroscopy. The partitioning experiments, which use the multitracer technique, provide information on Re and Os regarding (i) their distributions between artificial seawater-sediment systems, (ii) their complexation with humic acid, and (iii) their carriers in sediments. In addition, XAFS spectroscopy provides direct information on the chemical states of Re and Os in the sediments.In an artificial seawater-sediment system containing a multitracer, Re is removed from the artificial seawater only under a reducing environment. The speciation of Re by X-ray absorption near-edge structure (XANES) suggests that the majority of Re remains as in the artificial seawater even under highly reducing conditions, during laboratory time scale (about 2 weeks). Moreover, XANES simulation shows that some Re exists at a lower oxidation state, such as ReO2, in the reducing sediment. These results can be explained by the slow kinetics of the reaction which is similar to those suggested by previous geochemical studies.In contrast, Os is readily removed from the artificial seawater into sediments under various redox conditions. Even under oxic conditions, a large fraction of Os is removed from the artificial seawater to sediments without organic matter. Based on the Os XANES study, it is confirmed that the oxidation states of Os incorporated in the reducing sediment and oxic sediment are trivalent and tetravalent, respectively. Sequential extraction suggests that the main carrier of Os in the organic-rich sediment is either ferromanganese oxides or organic matter, and that the Os in these two fractions may correspond to hydrolyzed insoluble Os species and Os species interacting with organic matter, at lower valence, respectively. The results of distribution study of Os in the absence and presence of humic acid (HA) also imply that Os assumes more than one chemical species, and a small fraction of Os may interact with HA in the experimental system. Meanwhile, extended X-ray absorption fine structure (EXAFS) confirms that the first neighboring atom of Os in the reducing sediment is oxygen. If Os(VIII) is the main dissolved species in seawater, as is expected thermodynamically, reductive removal may control the enrichment of Os in the sediment. Osmium, which is removed as Os(IV), is reduced further to Os(III) by a diagenetic process and may be complexed with organic matter in the reducing sediment.The results of the removal behaviors of Re and Os obtained in the current study show that Re can be removed from the artificial seawater only under highly reducing conditions within 2 weeks, but Os removal from the artificial seawater can be found under various redox conditions. Thus, a high 187Re/188Os ratio can occur only in reducing sediments, such as black shales. The high187Re/188Os ratio, in turn, makes black shales suitable for Re-Os dating. In contrast, authigenic sediments (and minerals) under oxic environments can enrich Os, but since Re is not distributed to the sediments under oxic conditions, this will cause a much lower 187Re/188Os ratio than that of seawater. The Os isotope system of these materials can be used as a paleo-marine environmental tracer since the 187Os/188Os ratio cannot grow significantly due to its extremely low 187Re/188Os ratio. 相似文献
880.
Devolatilization equilibria in graphite-pyrite-pyrrhotite bearing pelites with application to magma-pelite interaction 总被引:1,自引:0,他引:1
The proportions of species in a C-O-H-S fluid in equilibrium with graphite, pyrite and pyrrhotite were calculated for a range of pressure, temperature and
conditions, using the equilibrium constants and mass balance method, for ideal and non-ideal mixing in the fluid. Under typical metamorphic conditions, H2O, CO2, CH4 and H2S are the principal fluid species with H2S favored by higher temperatures, lower pressures and lower
conditions. The dominance of H2S in the fluid at high temperatures leads to values of
becoming significantly less than 1, and causes hydrous minerals to dehydrate at lower temperatures than the case when
. The production of H2S-bearing fluids provides a mechanism for the selective transfer of sulfur from a graphite-pyrite-pyrrhotite bearing pelite into a pluton via a fluid phase, without requiring wholesale melting and assimilation of rocks. Such a process is feasible if a magma is intruded by stoping, which allows a significant volume of pelite country rock to be raised rapidly to temperatures approaching that of the magma. H2S-bearing fluids produced from graphite-pyrite-pyrrhotite pelites (due either to magmatic intrusion or regional metamorphism) may also mobilize ore-forming metals as sulfide complexes. 相似文献