Summary. Linear-programming methods are powerful and efficient tools for objectively analysing seismic focal mechanisms and are applicable to a wide range of problems, including tsunami warning and nuclear explosion identification. The source mechanism is represented as a point in the six-dimensional space of moment-tensor components. Each observed polarity provides an inequality constraint, linear with respect to the moment tensor components, that restricts the solution to a half-space bounded by a hyperplane passing through the origin. The intersection of these half-spaces is the convex set of all acceptable solutions. Using linear programming, a solution consistent with the polarity constraints can be obtained that maximizes or minimizes any desired linear function of the moment tensor components; the dilatation, the thrust-like nature, and the strike-slip-like nature of an event are examples of such functions. The present method can easily be extended to fit observed seismic-wave amplitudes (either signed or absolute) subject to polarity constraints, and to assess the range of mechanisms consistent with a set of measured amplitudes. 相似文献
The expert system approach enables geochemical evidence to be integrated with geological, petrological and mineralogical evidence in identifying the eruptive setting of ancient volcanic rocks. This paper explains the development of ESCORT, an Expert System for Characterization of Rock Types. ESCORT uses as its knowledge base a set of dispersion matrices derived from a geochemical data bank of some 8000 immobile element analyses, together with tables of magma-type membership probabilities based on a-priori knowledge, the dominant flow type, the facies of intercalated sediment, the basement to the volcanic sequence, the presence or absence of an iron-enrichment trend and the phenocryst phase assemblage. Inferencing is based on the Bayes Decision Rule, adapted to take into account possible uncertainties in the geological evidence. Upper and lower probability thresholds are used to decide whether an interpretation is likely or unlikely. Evaluation of the system shows that it is able to overcome many of the well-publicized ambiquities attached to geochemical discrimination diagrams, as well as providing a unified method of decision making and a measure of the correctness of the interpretation. 相似文献
Oxygen isotope fractionations between zoisite and water have been studied at 400–700°C, PH2O = 13.4 kbar, using the three-isotope method described by Matsuhisaet al. (1978) and Matthewset al. (1983a). The zoisite-waier exchange reaction takes place extremely slowly and consequently direct-exchange calibration of equilibrium fractionation factors was possible only at 600 and 700°C. Fractionation factors at 400–600°C were determined from samples hydrothermally crystallized from a glass of the anhydrous zoisite composition. At 600°C, both exchange procedures gave identical fractionations within experimental error. Scanning electron microscope studies showed that the zoisite-water exchange reaction occurs largely by solution-precipitation mass-transfer mechanisms. The slow kinetics of zoisite-water exchange may be typical of hydrous silicates, since additional experiments on tremolite-water and chlorite-water exchange also showed very low rates. When the zoisite-water fractionation factors determined in this study are combined with the quartz and albite-water data of Matsuhisaet al. (1979) and the calcite-water data of O'Nellet al. (1969), mineral-pair fractionations are obtained for which the coefficients “A” in the equation 1000 In α = A × 106T?2 are:
Many geophysical problems involve solving for the time derivative of the Laplacian of a streamfunction, particularly three-dimensional ocean circulation studies. This paper demonstrates the existence of two instabilities when relaxation methods are used to solve the resulting Poisson equation. One occurs as a result of over-relaxation. The other occurs even if under-relaxation is used, provided that there are variations in Coriolis parameter and that extrapolation provides an initial guess for the relaxation process. Both instabilities equilibrate with (erroneous) amplitudes determined by the degree of dissipation present. In the case of the latter instability, the amplitude can be six orders of magnitude larger than the error criterion for the relaxation; its structure resembles basic modes, which persist indefinitely. Some ways of circumventing the problem are discussed. 相似文献
Within a period of a few weeks toward the end of the Allerød Interstadial, the major Plinian eruption of the Laacher See volcano produced some 20 km3 of eruptiva, covering and preserving the late-glacial landscape in the German Central Rhineland over an area of more than 1000 km2. Correlation of terrestrial archives with the Greenland ice-core records and improved calibration of the radiocarbon timescale permit a precise, accurate age determination of the Laacher See event some 200 yr before the onset of the Younger Dryas cold episode. Carbonized trees and botanical macrofossils preserved by Laacher See Tephra permit detailed regional paleoenvironmental reconstruction and show that open woodland were typical for the cool and humid hemiboreal climatic conditions during the late Allerød. This woodland provided the habitat for a large variety of animal species, documented at both paleontological and Final Paleolithic archeological sites preserved below Laacher See deposits. Of special interest are numerous animal tracks intercalated in Middle Laacher See deposits at the south of the Neuwied Basin. This knowledge may help to evaluate possible supraregional impacts of this volcanic event on northern hemispheric environment and climate during the late Allerød. 相似文献
A mathematical model has been produced to examine the impact of sewage sludge and fertilizer application to arable land and the effect of different crop regimes on the amount of nitrate leached to chalk groundwater. Previous work on nitrate leaching has concentrated on either a soil science or a hydrogeological approach with little overlap between the two. This study considered both fields to obtain an overall picture of the nitrate leaching process. IMPACT is a layered deterministic N-leaching model which predicts the nitrogen loads entering groundwater daily from arable land, and can be used as a management tool in development of sludge application and agricultural policy. The model relates nitrogen species movement resulting from the application of sewage sludge and fertilizer to differing vegetation-soil-hydrogeological conditions. Field data collected at three sites on the unconfined chalk aquifer of East Anglia, England over a two and a half year period was used to produce an initial conceptual model and to constrain the mathematical model during development. IMPACT simulates nitrogen and transport processes in the soil and unsaturated zone of the chalk. The nitrogen processes include: mineralisation of soil organic-N and sewage sludge organic-N, nitrification; crop uptake; volatilization; denitrification; and N inputs from fertilizers and precipitation. A mixing cell method is used to model solute transport in both the soil and chalk. Matrix flow and combined fissure-matrix flow are considered for the chalk. The model enables examination of the relationship between the arable/hydrogeological systems and the environmental implications of sludge application and of different arable regimes. Results are of use in developing strategies for arable farming and sludge application in areas sensitive to nitrate leaching. This Part 1 paper describes the model development approach. Results of associated modelling scenarios are presented separately in the associated Part 2 paper. 相似文献
A layered deterministic N-leaching model, IMPACT, has been calibrated using data from two study sites on the unconfined Chalk aquilfer of East Anglia, UK. The model predicts nitrogen species movement resulting from the application of sewage sludges and fertilizers to arable land for different vegetation-soil-hydrogeological conditions. One site received sludge in the form of digested sewage cake (DSC) for the first time during the study period, whilst the other site had over 15 years history of liquid undigested sludge (LUS) applications at 3 year intervals. Site data included: 3-monthly concentration profiles at 0.3 m intervals to depths of up to 6 m for N-species and chloride; unsaturated potential measurements; water level and saturated groundwater solute concentrations, fertilizer and sludge input; daily recharge, and soil/chalk type and moisture content. The observed average movement rate for nitrate peaks in the Lower Chalk, measured at one site, was 0.2 m year−2. Leachate peaks were not observed annually but approximately every third year, being associated with large sludge applications and ploughing of grass crops. Significant correlation between observed and modelled nitrate profiles in soil and chalk were obtained which demonstrated applications. The relationship between crop demand, application times of fertilizers and sludge, nitrate availability and recharge was shown strongly to control the shape of nitrate profiles in the soil and chalk and the quantity of nitrate leached tochalk. The change in hydrogeological conditions at the soil-chalk contact and associated potential for denitrification was also shown to exert a significant control on the shape of the nitrate profile. Following calibration, different arable crop and sludge application regimes were examined for a 6 year period and ranked according to their nitrate leaching risk. Of the modelled cereal farming scenarios, the crop/sludge regime giving the least nitrate leaching was a late autumn surface spread application of DSC followed by winter cereals, while highest nitrate leaching was generated by an autumn injection of LUS followed by spring cereals. Field and modelled results may be used in the development of sludge disposal policies to arable land particularly with regard to sludge types, application times, and following crop types and fertilizer requirements. Overall, observed and model data demonstrate the importance of examining nitrate leaching as a continuum from the soil through the chalk to the water table. 相似文献
The physical and chemical characteristics of Lake Bonney, a permanently ice-covered closed basin lake in Taylor Valley, Antarctica are influenced significantly by local climate. The rising lake-levels of the past thirty years indicate a recent change in the local climate. We explored the significance of twentieth century changes in lake-level as a climate-change indicator by using a hydrologic model for the basin and a Monte Carlo simulation based on the variability in the available 30 yrs of hydrologic record. We compared the lake-level in the retrospective simulations with a measurement surveyed by Robert Scott's party in 1903. All the retrospective simulations based on the observed inflows yielded estimates that the lake was dry in 1903. It was necessary to remove 6 yrs from the observed 21-yr record for the retrospective simulation to match the measured 1903 lake-level for 50% of the simulations. From these analyses, we conclude that the period from 1969 to the present has had greater availability of solar radiation for meltwater generation, possibly brought about by changing cloud-cover patterns and coupled with a gradual warming trend. A third simulation indicated that an annual increase in inflow of about 3% between 1903 and 1973 would be required to match the 1903 measurement. 相似文献
The terranes which outcrop to the north and south of the westward extension of the Jebha Fault show considerable differences in their nature and structural organisation. The pre-Oligocene basement is composed of synorogenic synclines filled with detrital deposits. In the North, the synclinal troughs are oriented North-South and filled with siliciclastic material. The synclinal structures on top of the western extension of the Jebha Fault are orientated N 140° and are also filled with the same material. South of this westward extension of the Jebha Fault, the orientation of the synclinal structures is north-south; they are filled with bioclastic calcareous material.The synorogenic synclines, situated on top of the Jebha-Arbaoua limit show characteristics of the detached sedimentary cover by their position and orientation. This cover formed the Arbaoua-Jebha lateral ramp during its movement towards the west. Developement of this ramp is explained by the crustal discontinuity inherited from the period of rifting.The resulting palaeomorphological relief allows the difference between the zones situated at each part of the limit to be understood, in particular, it permits the understanding of the differences in the detrital material, in its role as a barrier opposing the arrival of bioclastic calcareous material from the African margin (situated southwards of the limit).The Arbaoua-Jebha lateral ramp appears as a major and original feature of the Rift chain. 相似文献
