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11.
Motivated by the discovery of the millisecond pulsars, we consider the effect of rapid rotation on the envelope of a neutron star. Solving the equation of hydrostatic equilibrium we find expressions for the density and oblateness as functions of radius and polar angle. 相似文献
12.
Poisson-distributed patterns of explosive eruptive activity 总被引:1,自引:0,他引:1
Servando De la Cruz-Reyna 《Bulletin of Volcanology》1991,54(1):57-67
The study of patterns of eruption occurrence could lead to a better understanding of the physics behind the volcanic process. However, various attempts to find a single statistical distribution that describes the occurrences of volcanic eruptions have not been successful. Global data show that, if the energies of point events in time (eruptions) are properly accounted above a certain noise level, the stochastic process — whose realization consists of explosive volcanic events — can be well represened by a Poisson point process, though not necessarily stationary. Many previous attempts to describe patterns of eruption occurrences were hampered by counting events with all levels of explosivity in the same category. When eruptions are separated by their sizes, the occurrence patterns of the higher magnitude eruptions become clearly Poissonian. In this study eruptions are classified by size using the Volcanic Explosivity Index (Newhall and Self 1982). Further analysis of the magnitude-characterized eruption data shows direct relations among the energy of eruptions, mean rate of occurrences and distribution of repose intervals between eruptions. An important result from the analysis of energy and mean rate of occurrence data is that, for global data, the product of those parameters is a constant. Simple load-and-discharge models provide an explanation of the random features of the volcanic processes. These considerations lead to the definition of a constinuous magnitude scale for volcanic eruptions which can consistently measure the energy and the rate-of-occurrence of eruptions over a wide range of values. 相似文献
13.
Jorge Luis De Souza 《Pure and Applied Geophysics》1991,136(2-3):245-264
The Rayleigh wave phase and group velocities in the period range of 24–39 sec, obtained from two earthquakes which occurred in northeastern brazil and which were recorded by the Brazilian seismological station RDJ (Rio de Janeiro), have been used to study crustal and upper mantle structures of the Brazilian coastal region. Three crustal and upper mantle models have been tried out to explain crustal and upper mantle structures of the region. The upper crust has not been resolved, due basically to the narrow period range of the phase and group velocities data. The phase velocity inversions have exhibited good resolutions for both lower crust and upper mantle, with shear wave velocities characteristic of these regions. The group velocity data inversions for these models have showed good results only for the lower crust. The shear wave velocities of the lower crust (3.86 and 3.89 km/sec), obtained with phase velocity inversions, are similar to that (=3.89 km/sec) found byHwang (1985) to the eastern South American region, while group velocity inversions have presented shear velocity (=3.75 km/sec) similar to that (=3.78 km/sec) found byLazcano (1972) to the Brazilian shield. It was not possible to define sharply the crust-mantle transition, but an analysis of the phase and group velocity inversions results has indicated that the total thickness of the crust should be between 30 and 39 km. The crustal and upper mantle model, obtained with phase velocity inversion, can be used as a preliminary model for the Brazilian coast. 相似文献
14.
The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of -Fe2O3 with selected Fe2+ or Fe3+ compounds, it is found that the calculated Fe3+
f values are somewhat overestimated with respect to those of Fe2+. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively. 相似文献
15.
Luis A. Spalletti Ignasi Queralt Sergio D. Matheos Ferrn Colombo Jorge Maggi 《Journal of South American Earth Sciences》2008,25(4):440-463
The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO2 contents, variable abundances of K2O and Na2O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones. 相似文献
16.
Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon,Northern Adriatic Sea,Italy)
Stefano Covelli Jadran Faganeli Cinzia De Vittor Sergio Predonzani Alessandro Acquavita Milena Horvat 《Applied Geochemistry》2008
The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g−1, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g−1) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L−1, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m−2 day−1) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m−2 day−1) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m−2 day−1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column. 相似文献
17.
Rain-induced debris and mudflow triggering factors assessment in the Santiago cordilleran foothills,Central Chile 总被引:1,自引:1,他引:0
Debris and mudflows are some of the main geological hazards in the mountain foothills of the Chilean capital city of Santiago.
There, the risk of flows triggered in the basins of ravines that drain the range into the city increases with time due to
the city growth. A multivariate statistical study based on the logistic regression method is presented. The model provides
equations that allow the computation of combined meteorological triggering factors associated with a probability of rain-induced
flow occurrence. Daily rainfall, accumulated rainfall and the snowfall level, traditionally considered as the relevant factors,
are analysed for a 25-year period. The results show a strong relevance of the rainfall on the day of the flow event over the
other factors. However, the relatively low probabilities returned for some real flow events suggest that the model does not
capture all the significant variables and the problem is more complex than as it has been traditionally assumed, and further
investigations are needed to develop predictive models of flow triggering. 相似文献
18.
19.
A. P. K. De Zoysa 《Ocean Engineering》1978,5(3):209-223
A numerical solution and computer program developed to analyse undersea flexible cable problems is presented. The cable is subject to hydrodynamic loads from a surrounding fluid medium. The object is to determine the cable profile and tensile forces developed. The cable equations and boundary conditions result in a classical two-point boundary value problem. An iteration ‘shooting method’ is used for solution. The program has been written to analyse a class of towing and anchoring problems. However the analysis is general and applicable to most steady-state cable problems. 相似文献
20.
南海尖峰海山多金属结壳地球化学 总被引:4,自引:5,他引:4
南海尖峰海山多金属结壳富含30多种元素,其锰铁矿物主要由钡镁锰矿,δ-MnO_2和FeOOH·xH_2O组成。与其它海区的结壳相比,尖峰海山结壳富含Cu、Ni、Ba、Zn、Pb等元素,而Co、Ti、稀土元素(REE)、Sr等元素相对较贫。研究表明,HREE亏损,具明显的Ce正异常,较明显的Tb正异常和Yb负异常。这是氧化弱碱性海洋环境所致。结壳是水成作用的产物,它的形成受南海独特的古海洋环境所控制,海底火山热液作用,可能也是影响因素之一。 相似文献