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771.
Margarita Caballero Gabriela Vázquez Socorro Lozano-García Alejandro Rodríguez Susana Sosa-Nájera Ana Carolina Ruiz-Fernández Beatriz Ortega 《Journal of Paleolimnology》2006,35(1):83-97
The Sierra de Los Tuxtlas is a recently active volcanic field, with eruptions in 1664 and 1792. It holds one of the reserves
of tropical evergreen forest in Mexico, as well as several maar lakes. One of them, Lago Verde, was chosen for a three-fold
study (1) on its present limnological conditions, (2) on the algal community living in the water column and preserved in the
surface sediments; and (3) on its recent history (ca. 340 yr). The palaeolimnological study was based on multiproxy analyses
on core material dated by 210Pb, 137Cs and 14C. Lago Verde is a small, shallow lake with dilute, slightly alkaline water (CO32− + HCO3− > Cl− > SO42−, Na+ + K+ > Ca2+ > Mg2+). It is turbid, eutrophic, with high phosphorus levels. It is a warm polymictic lake, with thermal and oxygen stratification
establishing by midday during the warm months. The lake does not stratify in winter. Diatoms dominate the phytoplankton community
in the cold ‘nortes’ season, Cyanobacteria in summer, and Chlorophyta in autumn. Cyanobacteria (Chroococcales) are not well
preserved in the surface sediments while Chlorophyta are better preserved. Sedimentary diatoms are well preserved, dominated
by the three most abundant species in the water column: Achnanthidium minutissimum, Fragilaria capucina and Aulacoseira granulata. The base of the studied sequences is constrained by the historic eruption of 1664. The period from 1664 to 1963 is characterised
by a meso-eutrophic lake. Tropical Forest vegetation reaches maximum values between ca. 1800 and 1963. Between ca. 1785 and
1885 the lake was slightly shallower than in the rest of the 1664–1963 period, probably recording climatic variability. An
early pulse of anthropogenic disturbance was recorded by ca. 1921 and after ca. 1963 intense forest clearance and high erosion
rates led to a more turbid, more productive, nutrient-rich lake. The highest anthropogenic impact was reached by ca. 1988;
afterwards the lake and its basin reached a new balance, with the establishment of the present modern conditions. 相似文献
772.
Late orogenic intramontane basin development: the Granada basin, Betics (southern Spain) 总被引:2,自引:0,他引:2
The quantitative study of subsidence in the Granada basin, using decompaction and backstripping techniques, and contemporaneous relief development in the surrounding areas, especially in the Sierra Nevada, provides a good case example of the development of an intramontane basin. In the Granada basin, according to the interpretation of the seismic profiles and results of the backstripping analysis, subsidence and sedimentation rates were at a maximum in the late Tortonian and decreased progressively; meanwhile, the neighbouring areas were uplifted forming important relief. Chronostratigraphical revisions of the marine sediments show that the marine incursion that deposited sediments in the Granada basin lasted only 1.3 Ma, between 8.5 and 7.2 Ma. The gradual retreat of the sea in the Granada basin is not attributable to global eustatic fluctuations, but rather to uplift in the Sierra Nevada and its adjacent areas. From latest Tortonian to early Messinian times, the region became continental and the Granada basin acquired its present physiography and was differentiated as such. From the late Tortonian onwards, NNW–SSE compression combined with ENE–WSW extension affected the cordillera. In the Granada basin, extension controlled fault movements. There are two well-defined fault sets: the first trends 70°N–90°E, with low angle faults (less than 30°) dipping towards the north and south, defining the subsiding areas which have approximately E–W direction; whereas the second set has a NW–SE direction, and cuts and displaces the previous ones, defining the main subsiding areas in the eastern part of the basin. The reinterpretation of seismic profiles reveals that the subsiding axes within the Granada basin persisted from the Tortonian to the present because of continued displacements of the main faults. 相似文献
773.
Francisco J. Chávez-García Francisco Luzón Dimitris Raptakis José Fernández 《Pure and Applied Geophysics》2007,164(4):697-720
Deformation analysis and simulation of volcanic edifices require the construction of models of elastic properties of those
structures. In this paper we present an analysis of microtremor measurements recorded during the performance tests of two
temporary seismic arrays installed in the eastern portion of the Teide caldera in 1994. We take advantage of recent developments
of the SPAC method and use spatial cross-correlation computations to estimate phase velocity dispersion of Rayleigh waves
at the location of the arrays. We show that the extension of the standard SPAC method is valid in the case of our data, justifying
its use and supporting the generalization of the SPAC method to single station pairs. The phase velocity dispersion curve
obtained was inverted to recover the shear-wave profile at the site of the arrays. Our results indicate that the subsoil structure
of the caldera is laterally homogeneous at the scale of a few km about the location of the arrays. We obtained about 315 m
of volcanic sediments overlying rocks with a shear-wave velocity of 2 km/s. These results are robust and are a starting point
to further modelling of deformation, permanent or transient, at this volcanic edifice, which can be useful in the interpretation
of different observed fields. In fact, the computation of deformations and gravity changes due to possible volcanic intrusions
in two models; one considering the volcanic sediments and the other without considering them, provided different results in
the near field. 相似文献
774.
Three-dimensional displacements and gravity changes were measured at 20 points between 2000 and 2002 at Merapi volcano, Java,
Indonesia. Former models focused on the modeling of a single magmatic source located in the summit region of the volcano.
Such models do not fit to our measurements between 2000 and 2002. A new hybrid model approach is developed consisting of an
elastic-gravitational source described by a mass and energy term as well as a combined dip-slip/strike-slip fault zone in
the summit region. Both nonlinear optimization problems, given by the common inversion of three-dimensional displacements
and gravity changes, are solved by applying a genetic algorithm. The hybrid model fits the measurements accurately, tested
by Fisher test statistics. Furthermore, our model for Merapi volcano confirms previous structural models for this region so
that the new model is statistically proven as well as physically reliable. 相似文献
775.
L. Valenzuela-Vásquez J. Ramírez-Hernández J. Reyes-López A. Sol-Uribe O. Lázaro-Mancilla 《Environmental Geology》2006,51(1):17-27
Anomalous high fluoride concentration up to 7.59 mg/dm3 is found in groundwater from “La Victoria” area. This water is used to supply drinking water to Hermosillo City, Sonora. Geochemistry of groundwater, relationship between physicochemical parameters, hydrogeology and geologic setting were correlated to define the origin and the geochemical mechanisms of groundwater fluorine enrichment. High fluoride concentration is associated with high bicarbonates, pH and temperature, and it decreases toward the west and south of the area. Fluoride is in negative correlation to calcium concentration. Sodium sulphate facies of regional deep water flow are related to high fluoride concentration. High electric resistivity rocks associated with granites from the Sierra Bachoco basement might be the deep source of fluoride. Outcropping of Sierra Bachoco in the west causes upward regional flow. Groundwater of longer residence time can be pumped there. The anomalous area is restricted to “La Victoria” because calcareous paleozoic rocks outcrop to the south. 相似文献
776.
C. González-Nicieza M. I. Álvarez-Fernández A. Menéndez-Díaz A. E. Álvarez-Vigil 《Rock Mechanics and Rock Engineering》2006,39(5):421-444
Summary. The methods for designing pillars in underground mines are fundamentally based on empirical formulae that do not take into
account the quality of the rock mass as an input parameter. This makes them difficult to apply in other types of ground that
are different to those used to establish each empirical formula.
To avoid this inconvenience, the present paper examines existing empirical formulae to then propose a modification of these
formulae adjusting the resistance of the pillars on the basis of the RMR (Bieniawski’s Rock Mass Rating). The compression
safety factor of the pillars is analyzed for each modified formula and a study is carried out of shear failure if planes of
weakness exist in the pillars. Finally, the safety factors of the pillars in a marble mine situated in Alicante (Southern
Spain) were calculated in order to validate the new formulae.
From the results obtained, it is concluded that this new formulation determines the safety factor of pillars of the mine with
greater reliability, provided that the pillars are isolated. At the same time, the introduction of the RMR in the formulae
results in a better fit of the strength of each pillar to the characteristics of the rock mass. 相似文献
777.
Biomass equations for shrub species of Tamaulipan thornscrub of North-eastern Mexico 总被引:1,自引:0,他引:1
J. Nvar E. Mndez A. Njera J. Graciano V. Dale B. Parresol 《Journal of Arid Environments》2004,59(4):269
Nine additive allometric equations for computing above-ground, standing biomass were developed for the plant community and for each of 18 single species typical of the Tamaulipan thornscrub of north-eastern Mexico. Equations developed using additive procedures in seemingly unrelated linear regression provided statistical efficiency in total biomass estimates at the scales of both individual species and at the plant community. A single equation for each species improves efficiency in biomass estimates by 12.5% in contrast to using a single equation for the plant community. Therefore, additive equations developed in seemingly unrelated linear regression of parameter estimation are recommended to compute biomass components and total biomass for the species described. 相似文献
778.
Geomechanical characterisation of rock masses in Alpine regions: the Basque Arc (Basque-Cantabrian basin, Northern Spain) 总被引:1,自引:0,他引:1
Toms Morales Gorka Uribe-Etxebarria Jesús A. Uriarte Iigo Fernndez de Valderrama 《Engineering Geology》2004,71(3-4):343-362
The geomechanical characterisation of rock masses in Alpine regions is one of the main challenges currently facing rock mechanics. These rock masses, made up of very different materials and frequently tectonically disturbed, are affected by engineering works, giving rise to potentially important problems. Thus, in the Basque Country, these materials are at the cause of important fatalities occurring in recent years. In this context, we aim at characterising these materials with the ultimate purpose of predicting their behaviour. The characterisation is initially approached based on the GSI, mi and σci parameters, following the Hoek-Brown failure criterion. This information is supplemented with field and laboratory measurements, and by obtaining the RQD and RMR indices. Using the information obtained from 99 metering stations, we develop a graphic classification procedure, which allows us to distinguish materials based on their characteristics, behaviour, type and degree of associated problems, and type and effectiveness of solutions provided. Eight classes of material are established. 相似文献
779.
Cesar Aybar Carlos Fernández Adrian Huerta Waldo Lavado Fiorella Vega Oscar Felipe-Obando 《水文科学杂志》2020,65(5):770-785
ABSTRACTA new gridded rainfall dataset available for Peru is introduced, called PISCOp V2.1 (Peruvian Interpolated data of SENAMHI’s Climatological and Hydrological Observations). PISCOp has been developed for the period 1981 to the present, with an average latency of eight weeks at 0.1° spatial resolution. The merging algorithm is based on geostatistical and deterministic interpolation methods including three different rainfall sources: (i) the national quality-controlled and infilled raingauge dataset, (ii) radar-gauge merged precipitation climatologies and (iii) the Climate Hazards Group Infrared Precipitation (CHIRP) estimates. The validation results suggest that precipitation estimates are acceptable showing the highest performance for the Pacific coast and the western flank of the Andes. Furthermore, a meticulous quality-control and gap-infilling procedure allowed us to reduce the formation of inhomogeneities (non-climatic breaks). The dataset is publicly available at https://piscoprec.github.io/ and is intended to support hydrological studies and water management practices. 相似文献
780.
E. Paris D. B. Dingwell F. A. Seifert A. Mottana C. Romano 《Physics and Chemistry of Minerals》1994,21(8):510-515
The effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS). The XANES spectra collected at the Ti K-edge clearly show a variation with pressure that is related to changes in the geometrical environment around the Ti atoms. By comparison with spectra of standard materials, the XANES spectra of the glasses suggest a relatively low average coordination number (near 5) in samples quenched at low pressure and a higher coordination number (near 6) in samples quenched from the highest pressure. The combination of XANES data with density and compressibility measurements supports the idea that a mixture of 6- and lower coordinated (4- and/ or 5-coordinated) Ti geometries are present in the 1 bar glass, and an increasing proportion of 6-coordinated Ti occurs in the glasses synthesized at progressively higher pressures. 相似文献