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1.
A preliminary survey of seven trace metals in three species of oyster from the Knysna estuary, South Africa, has been carried out. The results which were obtained indicate that this area is as yet unpolluted and provide base-line levels against which future trace-metal pollution can be measured. Two species of oyster are being grown at a number of sites as part of a long-term programme to monitor trace-metal levels in the estuary.  相似文献   

2.
Pressure-vacuum lysimeters are an inexpensive means of collecting numerous water samples from the same location in the unsaturated zone over a period of time. However, prior studies have indicated that the chemistry of water samples may be altered by the collection technique, creating concern about the representativeness of the pore water samples obtained. A study conducted using soil water pressure-vacuum lysimeters in outwash sand and glacial till deposits demonstrates that for non-dilute-solution samples the effect on pH of sampling with lysimeters is minimal, and that measured major cation and anion concentrations are representative of the natural pore water; trace-metal concentrations can be significantly altered by collection procedures at low concentrations.  相似文献   

3.
We study density-driven flow in a fractured porous medium in which the fractures are represented as manifolds of reduced dimensionality. Fractures are assumed to be thin regions of space filled with a porous material whose properties differ from those of the porous medium enclosing them. The interfaces separating the fractures from the embedding medium are assumed to be ideal. We consider two approaches: (i) the fractures have the same dimension, d, as the embedding medium and are said to be d-dimensional; (ii) the fractures are considered as (d − 1)-dimensional manifolds, and the equations of density-driven flow are found by averaging the d-dimensional laws over the fracture width. We show that the second approach is a valid alternative to the first one. For this purpose, we perform numerical experiments using finite-volume discretization for both approaches. The results obtained by the two methods are in good agreement with each other.  相似文献   

4.
Aftershock sequences of the magnitude M W =6.4 Bingöl earthquake of 1 May, 2003 (Turkey) are studied to analyze the spatial and temporal variability of seismicity parameters of the b value of the frequency-magnitude distribution and the p value describing the temporal decay rate of aftershocks. The catalog taken from the KOERI contains 516 events and one month’s time interval. The b value is found as 1.49 ± 0.07 with Mc =3.2. Considering the error limits, b value is very close to the maximum b value stated in the literature. This larger value may be caused by the paucity of the larger aftershocks with magnitude M D ≥ 5.0. Also, the aftershock area is divided into four parts in order to detect the differences in b value and the changes illustrate the heterogeneity of the aftershock region. The p value is calculated as 0.86 ± 0.11, relatively small. This small p value may be a result of the slow decay rate of the aftershock activity and the small number of aftershocks. For the fitting of a suitable model and estimation of correct values of decay parameters, the sequence is also modeled as a background seismicty rate model. Constant background activity does not appear to be important during the first month of the Bingöl aftershock sequences and this result is coherent with an average estimation of pre-existing seismicity. The results show that usage of simple modified Omori law is reasonable for the analysis. The spatial variability in b value is between 1.2 and 1.8 and p value varies from 0.6 to 1.2. Although the physical interpretation of the spatial variability of these seismicity parameters is not straightforward, the variation of b and p values can be related to the stress and slip distribution after the mainshock, respectively. The lower b values are observed in the high stress regions and to a certain extent, the largest b values are related to Holocene alluvium. The larger p values are found in some part of the aftershock area although no slip occurred after the main shock and it is interpreted that this situation may be caused by the alluvium structure of the region. These results indicate that the spatial distribution in b and p values are generally related to the rupture mechanism and material properties of an aftershock area.  相似文献   

5.
Achache et al.'s averages over 10 west European observatories of the annual mean magnetic components X, Y, Z are studied from 1953 to 1979. A 1970 jerk is fitted to the data by least squares, and the fit is quantitatively compared with that of a polynomial having the same number of free parameters as the jerk (a quintic). A crude correction for the 11-y sunspot cycle is also attempted. The jerk and quintic are equally good fits to X, the jerk is clearly better for Y, and the quintic is modestly better for Z. The jerk amplitudes of X and Z but not Y depend heavily on whether a sunspot correction is made, and such a correction may be required to estimate the energy of the internal part of the jerk in the manner of Malin and Hodder. The sunspot correction to Y probably cannot be brought above the noise because it is small and highly correlated with both the jerk and the quintic. The sunspot corrections to X and Z are statistically significant but depend on whether the core signal is taken as a jerk or a quintic. Courtillot and Le Mouël's physical model for the jerk predicts that the jerk amplitudes of X, Y, Z will be proportional to the longitudinal derivatives of X, Y and Z. This prediction is roughly verified for the magnitudes but not the signs; it is just possible that the signs in X and Z are not statistically significant.  相似文献   

6.
Microearthquake spectra from the Shillong region are analyzed to observe the effect of attenuation and site on these spectra. The spectral ratio method is utilized to estimate the Q values for both P- and S-waves in the subsurface layer, wherein the ratio of spectral amplitudes at lower and higher frequencies are taken into consideration for three stations at varying epicentral distances. Average estimates of Q P and Q S are 178 and 195. The ratio of Q S to Q P is estimated to be greater than 1 in major parts of the Shillong area, which can be related to the dry crust prevailing in the Shillong region. Typically, the variation in corner frequencies for these spectra is inferred to be characteristic of the site. Simultaneously, observations from spectral content of local earthquakes recorded at two different stations with respect to the reference site yield greater amplification of incoming seismic signals in the frequency range of 2–5 Hz, which is found to be well supported by the existing local lithology pertinent to that region.  相似文献   

7.
Free oscillation and body wave data are used to construct average Q models for the earth. The data set includes fundamental and overtone observations of the radial, spheroidal and toroidal modes, ScS observations and amplitudes of body waves as a function of distance. The preferred model includes a low-Q zone at both the top and the bottom of the mantle. In these regions the seismic velocities are likely to be frequency dependent in the “seismic” band. Absorption in the mantle is predominantly due to losses in shear. Compressional absorption may be important in the inner core.A grain-boundary relaxation model is proposed that explains the dominance of shear over compressional dissipation, the roughly frequency independent average values for Q and the variation of Q with depth. In the high-Q regions, the lithosphere and the midmantle (200–2000 km), Q is predicted to be frequency dependent. However, the low-Q regions of the earth, where Q is roughly frequency independent, dominate the observations of attenuation.  相似文献   

8.
Tropical harmful algal blooms (HABs) are increasing in frequency and intensity and are substantially affecting marine communities. In October/November 2008 a large-scale HAB event (> 500 km2, dinoflagellate Cochlodinium polykrikoides) in the Gulf of Oman caused the complete loss of the branching corals, Pocillopora and Acropora spp., and substantial reductions in the abundance, richness and trophic diversity of the associated coral reef fish communities. Although the causative agents of this C. polykrikoides bloom are unknown, increased coastal enrichment, natural oceanographic mechanisms, and the recent expansion of this species within ballast water discharge are expected to be the main agents. With rapid changes in oceanic climate, enhanced coastal eutrophication and increased global distribution of HAB species within ballast water, large-scale HAB events are predicted to increase dramatically in both intensity and distribution and can be expected to have increasingly negative effects on coral reef communities globally.  相似文献   

9.
The depth changes in the b-value and density of the number of earthquakes in different magnitude bins (M ≥ 1.8, M ≥ 3.0, M ≥ 3.5) are analyzed using highly accurate seismological observations carried out in 1955–1991 at the Garm prognostic area in Tadjikistan. It is found that the observed b-values are controlled by the variations in the proportion between weak and strong earthquakes. Two horizons with different patterns of the b-value are identified in the Earth’s crust above and below a depth of 15–16 km. The b-value in the upper and lower horizons is close to 0.8 and 1.2, respectively. The lower horizon is marked by almost complete absence of relatively strong earthquakes with M ≥ 3.0. The observed changes in the b-value with increasing depth could probably be due to the increase in the strength of crustal material caused by the growth in temperature and confining pressure in the depth interval from 0 to 15 km. The transitional interval between the upper and lower crustal horizons (～13–18 km), which is characterized by a sharp drop in seismic activity, can probably be associated with the zone of the phase transition of crustal material from an elastic brittle state to a plastic state, as suggested by some authors. Typically, the top of this zone hosts the hypocenters of the strongest earthquakes in a given territory. The correlation is established between the crustal areas with low b-values and the locations of the strongest earthquakes in the region. It is suggested that the three-dimensional mapping of the b-value can be helpful for estimating the location, depth, and maximal magnitude of the probable strong earthquakes in seismically active regions and can be used to assess seismic risks.  相似文献   

10.
Strong motion records taken during earthquakes in Turkey are used to calculate Newmark displacements in slopes. These displacements are then utilized in developing a novel displacement-based methodology to select the seismic coefficient which is used to calculate pseudostatic safety factor. In the first step of the study, calculated Newmark displacements are evaluated in three different categories which are as follows: using all data, using data for different earthquake magnitude (M) ranges with and without distance constraint and using data for different peak acceleration (amax) ranges. For all categories, different equations are obtained to assign slope displacements as a function of the ratio of yield acceleration to peak acceleration. The results show that categorization of data is an important issue, because the displacements are earthquake magnitude and peak acceleration dependent. In the second step, equations obtained for different peak acceleration ranges are used to propose charts linking upper bound slope displacements (D), seismic coefficients (kh) and pseudostatic safety factors (PSF), which are three important parameters of a pseudostatic approach. This enables the kh values be chosen based on the allowable displacements, instead of the current applications based on judgement and expertise. The results show that kh values for any allowable displacement should be based on anticipated amax values, while use of high PSF values results in lower displacements. Extensive comparison with solutions from the literature is also made. The methodology is best suited for earthquake triggered shallow landslides in natural slopes, consisting of materials which do not lose strength during dynamic loading.  相似文献   

11.
We examine the influence of attenuation and site on the spectra of microearthquakes having origin within the Shillong region. The ratios of spectral amplitudes at lower and higher frequencies are measured for three different stations at varying epicentral distances to estimate Q value for both P- and S-wave in near and sub-surface layer. The average estimates of Q P and Q S are found to be 178 and 195. The ratio of Q S to Q P emerges to be greater than unity in major parts of the Shillong area, suggesting dominance of dry crust prevailing in Shillong region. The variation in corner frequencies for these spectra is inferred to be characteristics of the site. Besides, the disparity in spectral content with reference to hard rock site yields the inference that the incoming seismic signals get amplified considerably while traversing from southern part to northeastern part of Shillong, best outlined at 2 to 5 Hz, which is well corroborated by the existing lithology.  相似文献   

12.
Earthquakes began to occur in Koyna region (India) soon after the filling of Koyna Dam in 1962. In the present study, three datasets 1964–1993, 1993–1995, and 1996–1997 are analyzed to study the b-value and fractal dimension. The b-value is calculated using the Gutenberg–Richter relationship and fractal dimension D corr. using correlation integral method. The estimated b-value and D corr. of this region before 1993 are found to be in good agreement with previously reported studies. In the subsequent years after 1995, the b-value shows an increase. The estimated b-values of this region are found within the limits of global average. Also, the pattern of spatial clustering of earthquakes show increase in clustering and migration along the three zones called North-East Zone, South-East Zone (SEZ), and Warna Seismic Zone. The earthquake events having depth ≤5 km are largely confined to SEZ. After 1993, the D corr. shows decrease, implying that earthquake activity gets clustered. This seismic clustering could be helpful for earthquake forecasting.  相似文献   

13.
Coral trout (Plectropomus spp.) are the main target species for commercial fishers in the eastern Torres Strait Reef Line Fishery (ETS RLF). The four species of coral trout known to occur in Torres Strait: Plectropomus leopardus, Plectropomus maculatus, Plectropomus areolatus and Plectropomus laevis are currently managed as a single species in Torres Strait, as there is no species-specific biological information available for the region which could be used to assess whether species differ in their response to fishing pressure. The aim of our study was to determine whether it is appropriate (biologically) to manage coral trout in the ETS RLF as a single species group or whether different management arrangements are required for some species. We used catch data and biological data from samples collected by commercial fishers to examine the distribution within Torres Strait and estimate a range of biological parameters for P. leopardus, P. maculatus and P. areolatus. Insufficient P. laevis samples were collected to reliably examine this species. Results indicated that the population biology, particularly the reproductive biology, of P. areolatus was substantially different to both P. leopardus and P. maculatus. Although it is difficult to predict the response to fishing, P. areolatus may be more vulnerable to fishing than P. leopardus and P. maculatus, due to the larger size at sex change observed for this species and the very low proportion of males protected by the current minimum size limit. Therefore, while the common management arrangements for P. leopardus and P. maculatus appear to be adequate for these species, separate management arrangements are needed for the sustainable harvest of P. areolatus populations in the ETS. Specifically, we recommend the introduction of a maximum size limit for P. areolatus, in addition to the current minimum size limit, which may allow a proportion of males some protection from fishing.  相似文献   

14.
Several seismological projects focused on the deep structure of the Scandinavian Mountains, in Norway and neighbouring Sweden. We use these recordings to study seismic anisotropy by analysing the birefringence of SKS and SKKS phases. These phases, which should be polarised radially, are split into an additional transverse component if they propagate through an anisotropic medium. Our results are directions Φ of the apparent fast shear wave polarisation and delay times δt between the split phases. For station KONO in Southern Norway, we find frequency-dependent Φ and δt values, indicating a depth-dependent anisotropy. Additionally, Φ and δt values vary with epicentre backazimuths in Norway, indicating a complex anisotropic structure in the crust and upper mantle. Stacking of the SKS/SKKS waveforms improves the signal-to-noise ratio along one station line and allows us to better determine the splitting parameters. A unique and complete model of the complex anisotropy cannot be obtained due to the limited observed backazimuth range. Near-surface tectonic structures correlate with the splitting pattern and thus the crust is one anisotropic layer in the region. Partly preferred orientations in the rock fabric at the surface can be correlated with Φ. Below one or more anisotropic layers must exist to explain the backazimuth- and frequency-dependent observations, as well as the long δt values (>2 s) which cannot be explained with crustal anisotropy alone. The spatial distribution of the splitting results indicates that different tectonics units, e.g. the Sveconorwegian, the Central and Northern Svecofennian and the Caledonian nappes, are each characterised by specific anisotropic signatures.  相似文献   

15.
A lattice Boltzmann (LB) model is proposed for simulating fluid flow in porous media by allowing the aggregates of finer-scale pores and solids to be treated as ‘equivalent media’. This model employs a partially bouncing-back scheme to mimic the resistance of each aggregate, represented as a gray node in the model, to the fluid flow. Like several other lattice Boltzmann models that take the same approach, which are collectively referred to as gray lattice Boltzmann (GLB) models in this paper, it introduces an extra model parameter, ns, which represents a volume fraction of fluid particles to be bounced back by the solid phase rather than the volume fraction of the solid phase at each gray node. The proposed model is shown to conserve the mass even for heterogeneous media, while this model and that model of Walsh et al. (2009) [1], referred to the WBS model thereafter, are shown analytically to recover Darcy–Brinkman’s equations for homogenous and isotropic porous media where the effective viscosity and the permeability are related to ns and the relaxation parameter of LB model. The key differences between these two models along with others are analyzed while their implications are highlighted. An attempt is made to rectify the misconception about the model parameter ns being the volume fraction of the solid phase. Both models are then numerically verified against the analytical solutions for a set of homogenous porous models and compared each other for another two sets of heterogeneous porous models of practical importance. It is shown that the proposed model allows true no-slip boundary conditions to be incorporated with a significant effect on reducing errors that would otherwise heavily skew flow fields near solid walls. The proposed model is shown to be numerically more stable than the WBS model at solid walls and interfaces between two porous media. The causes to the instability in the latter case are examined. The link between these two GLB models and a generalized Navier–Stokes model [2] for heterogeneous but isotropic porous media are explored qualitatively. A procedure for estimating model parameter ns is proposed.  相似文献   

16.
The specific quality factor Q of the crust was derived from explosion seismic waves by measuring the change of spectrum ratio with distance from the source. The spectrum for two wave groups, which are supposed to be transmitted entirely in the upper crust and transmitted primarily in the lower crust, respectively, were used. The source function could be represented by a simple function with one parameter. The value of Q and the parameter were determined simultaneously by the least-squares method. The same Q value was obtained for the upper and the lower crust and was estimated to be in the range Q = 1000–2000.  相似文献   

17.
The temporal variations in the chemical and isotopic compositions of urban stormwater under different land uses, and their dependence on physical parameters such as precipitation intensity, stormwater discharge, cumulative stormwater volumes and the size of the drainage area, were investigated in the coastal city of Ashdod, Israel. During 2000/2001 and 2001/2002, 46 stormwater events were intensively monitored for precipitation distribution and intensity at three stations across the city, and for stormwater discharge at seven stations draining 85% of the city area. Sixty-eight and 202 precipitation samples were collected and analyzed for chemical and isotopic compositions, respectively, as were 186 stormwater samples, collected from the drains during 15 of the 46 events. Land use had only a minor effect on the concentrations of major ions and trace elements. Conversely, the concentrations and variety of volatile and semi-volatile organic compounds were significantly higher in stormwater generated in the industrial area than in that draining from residential areas. Ion and trace-metal concentrations were very low (below drinking-water standards) in 97% of the stormwater samples collected from all drains. Stormwater concentrations were higher at stations draining a larger area, thereby linking concentrations to the length of the stormwater flow paths. A first-flush effect was documented on both a seasonal and event basis for both ions and trace elements. The high concentrations of fecal coliform bacteria exceeded the drinking-water standards and displayed a random pattern. The isotopic ratios of oxygen and hydrogen in the stormwater suggest very little exposure to the atmosphere, resulting in very limited fractionation. The presence of fecal coliforms, ammonium in some samples, and specific ratios of oxygen and nitrogen isotopes, suggest that although the sewer and stormwater-collection systems are separated, wastewater, possibly from overflowing sewers, contributed to the drained stormwater.  相似文献   

18.
—?The number and geometric distribution of putative mantle up-welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 Euler poles of the observational NUVEL model. For an assumed distribution of up-welling centers (called “cell-cores”) the corresponding cell boundaries are constructed by a Voronoi division of the spherical surface; the resulting polygons are called “Bénard cells.” By assuming the flow-kinematics within a cell, the viscous coupling between the flow and the plates is estimated, and the Euler poles for the plates are computed under the assumption of zero-net-torque. The positions of the cell-cores are optimized for the HS2-NUVEL1 Euler poles by a method of successive approximation (“subplex”); convergence to one of many local minima occurred typically after ～20,000 iterations. Cell-cores associated with the fourteen HS2-NUVEL1 Euler poles converge to a relatively small number of locations (8 to 10, depending on interpretation), irrespective of the number of convection cells submitted for optimized distribution (from 6 to 50). These locations are correlated with low seismic propagation velocities in tomography, uniformly occur within hotspot provinces, and may specifically be associated with the Hawaiian, Iceland, Reunion/Kerguelen (Indian Ocean), Easter Island, Melanesia/Society Islands (South Pacific), Azores/Cape Verde/Canary Islands, Tristan da Cunha (South Atlantic), Balleny Islands, and possibly Yellowstone hotspots. It is shown that arbitrary Euler poles cannot occur in association with mantle Bénard convection, irrespective of the number and the distribution of convection cells. Nevertheless, eight of the observational Euler poles – including the five that are accurately determined in HS2-NUVEL1 (Australia, Cocos, Juan de Fuca, Pacific, and Philippine) – are “Bénard-valid” (i.e., can be explained by our Bénard model). Five of the remaining six observational poles must be relocated within their error-ellipses to become Bénard-valid; the Eurasia pole alone appears to be in error by ～115°, and may actually lie near 40°N, 154°E. The collective results strongly suggest Bénard-like mantle convection cells, and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the Euler poles. The magnitudes of the computed Euler vectors show, however, that basal shear cannot be the exclusive driving force of plate tectonics, and suggest force contributions (of comparable magnitude for perhaps half of the plates) from the lithosphere itself, specifically subducting slab-pull and (continental) collision drag, which are provisionally evaluated. The relationship of the putative mantle Bénard polygons to dynamic chaos and turbulent flow is discussed.  相似文献   

19.
Ballast water exchange was measured for the first time in Bilbao Harbour, one of the most active of northern Spain. Between 1997 and 2006, 41,900,980.34 ballast water tn were loaded and 13,272,709 tn were discharged. Bilbao Harbour appears to be mainly a source of ballast water, 90% of which would be discharged in European harbours. We estimated that vessels carrying liquid and solid bulk have higher probabilities of exporting ballast water, whereas those with liquid bulk and containers are more likely to introduce it. From 30 potentially harmful phytoplankton species identified to date near harbour facilities, there would be a high risk of exporting at least Alexandrium minutum, Dinophysis sp., Heterosigma akashiwo, Karlodinium sp., Ostreopsis cf. siamensis, Pfiesteria-like and Prorocentrum minimum. Invasion risk by ballast water was tested by analyzing the response of six strains of H. akashiwo from different geographic areas to varying salinity. Results show that successful growth of foreign strains would be possible.  相似文献   

20.
—Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.¶Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.¶The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed. This ambiguity can be substantially reduced when shear waves (S-waves) are considered. The directional distribution of S-wave velocities and of the S-wave splitting exhibits characteristic patterns for the different olivine texture types. This could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle. A fundamental requirement for a more comprehensive interpretation is the availability of detailed S-wave observations. The maximum S-wave splitting in the peridotites investigated coincides with the maximum of the faster (leading) S-wave. This may be of importance to detect S-wave splitting in future seismic studies.  相似文献   

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