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61.
Joo Carvalho Joo Cabral Rui Gonalves Luís Torres Luís Mendes-Victor 《Tectonophysics》2006,418(3-4):277-297
The study region is located in the Lower Tagus Valley, central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with a diffuse pattern, with historical earthquakes causing many casualties, serious damage and economic losses. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Cenozoic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Valley requires the use of indirect methodologies for their study. This paper focuses on the application of high-resolution seismic reflection method for the detection of near-surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the Tagus Valley, and that have been recognized on deep oil-industry seismic reflection profiles and/or inferred from the surface geology. These are a WNW–ESE-trending fault zone located within the Lower Tagus Cenozoic basin, across the Tagus River estuary (Porto Alto fault), and a NNE–SSW-trending reverse fault zone that borders the Cenozoic Basin at the W (Vila Franca de Xira–Lisbon fault). Vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out. According to the interpretation of the collected data, a complex fault pattern disrupts the near surface (first 400 m) at Porto Alto, affecting the Upper Neogene and (at least for one fault) the Quaternary, with a normal offset component. The consistency with the previous oil-industry profiles interpretation supports the location and geometry of this fault zone. Concerning the second structure, two major faults were detected north of Vila Franca de Xira, supporting the extension of the Vila Franca de Xira–Lisbon fault zone northwards. One of these faults presents a reverse geometry apparently displacing Holocene alluvium. Vertical offsets of the Holocene sediments detected in the studied geophysical data of Porto Alto and Vila Franca de Xira–Lisbon faults imply minimum slip rates of 0.15–0.30 mm/year, three times larger than previously inferred for active faults in the Lower Tagus Valley and maximum estimates of average return periods of 2000–5000 years for M 6.5–7 co-seismic ruptures. 相似文献
62.
P. González-Sampériz B.L. Valero-Garcés G. Jalut C. Martí-Bono A. Navas J.J. Dedoubat 《Quaternary Research》2006,66(1):38-52
Palynological, sedimentological and stable isotopic analyses of carbonates and organic matter performed on the El Portalet sequence (1802 m a.s.l., 42°48′00?N, 0°23′52?W) reflect the paleoclimatic evolution and vegetation history in the central-western Spanish Pyrenees over the last 30,000 yr, and provide a high-resolution record for the late glacial period. Our results confirm previous observations that deglaciation occurred earlier in the Pyrenees than in northern European and Alpine sites and point to a glacial readvance from 22,500 to 18,000 cal yr BP, coinciding with the global last glacial maximum. The patterns shown by the new, high-resolution pollen data from this continental sequence, chronologically constrained by 13 AMS 14C dates, seem to correlate with the rapid climate changes recorded in Greenland ice cores during the last glacial-interglacial transition. Abrupt events observed in northern latitudes (Heinrich events 3 to 1, Oldest and Older Dryas stades, Intra-Allerød Cold Period, and 8200 cal yr BP event) were also identified for the first time in a lacustrine sequence from the central-western Pyrenees as cold and arid periods. The coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the North Atlantic to mid latitudes. 相似文献
63.
A Practical Procedure for the Measurement of Fragmentation by Blasting by Image Analysis 总被引:1,自引:0,他引:1
Summary. The operation of a digital image analysis system in a limestone quarry is described. The calibration of the system, required
in order to obtain moderately reliable fragmentation values, is done from muckpile sieving data by tuning the image analysis
software settings so that the fragmentation curve obtained matches as close as possible the sieving. The sieving data have
also been used to extend the fragment size distribution curves measured to sizes below the system’s optical resolution and
to process the results in terms of fragmented rock, discounting the material coming from a loose overburden (natural fines)
that is cast together with the fragmented rock. Automatic and manual operation modes of the image analysis are compared. The
total fragmentation of a blast is obtained from the analysis of twenty photographs; a criterion for the elimination of outlier
photographs has been adopted using a robust statistic. The limitations of the measurement system due to sampling, image processing
and fines corrections are discussed and the errors estimated whenever possible. An analysis of consistency of the results
based on the known amount of natural fines is made. Blasts with large differences in the amount of fines require a differentiated
treatment, as the fine sizes tend to be the more underestimated in the image analysis as they become more abundant; this has
been accomplished by means of a variable fines adjustment factor. Despite of the unavoidable errors and the large dispersion
always associated with large-scale rock blasting data, the system is sensitive to relative changes in fragmentation. 相似文献
64.
Risk assessment of groundwater pollution using sensitivity analysis and a worst-case scenario analysis 总被引:5,自引:0,他引:5
This paper illustrates how sensitivity analysis and a worst-case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with several known groundwater pollution sources and nearby drinking water production wells. The main concern is whether the contamination sources threaten the drinking water wells of the area. A groundwater flow and transport model is set up to answer this question. Due to limited data availability, the results of this model are associated with large uncertainty. Sensitivity analysis and a worst-case scenario analysis are applied to estimate this uncertainty and build confidence in the model results. 相似文献
65.
C. Schnitchen D. J. Charman E. Magyari M. Braun I. Grigorszky B. Tóthmérész M. Molnár Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17
Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is
under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several
areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this
paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes
using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was
obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using
a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily
influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive
sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was
explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content,
confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture
and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania,
Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records
from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP,
3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow
which affected eastern Europe as well as western and central Europe. 相似文献
66.
Károly Posgay Tamás Bodoky Zoltán Hajnal Tivadar M. Tóth Tamás Fancsik Endre Hegeds Attila Cs. Kovács Ern Takács 《Geophysical Journal International》2006,167(1):187-203
The geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P -wave interval velocities to Meissner et al. 's activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes. 相似文献
67.
The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt 总被引:4,自引:0,他引:4
J. Sánchez España E. López Pamo E. Santofimia Pastor J. Reyes Andrés J. A. Martín Rubí 《Aquatic Geochemistry》2006,12(3):269-298
This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD)
waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters
were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO4=) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously
recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and
precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5
(stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates
of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially
sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO4=
anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous
free cations (Pb2+, Zn2+, Cu2+, Cd2+, Mn2+, Co2+, Ni2+) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely
scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10−1) of the SO 4= ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH
< 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co,
Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB. 相似文献
68.
Skier-triggered avalanches are the main cause of avalanche accidents in backcountry skiing. The risk of accidents during backcountry
skiing was analysed statistically and related to factors such as elevation level, aspect, stability rating and the time of
the year. The analysis is based on a database about terrain usage and avalanche accidents from a large heli-skiing operator
in Canada, which makes it possible to study the conditional probability of accidents given the recorded pattern of terrain
usage. This study shows that the historical risk of accidentally triggering an avalanche greater than size 1 depends highly
on the stability rating, with the highest risk occurring during “poor” stability. The risk is greater at high elevations,
and it is lower during the late season than earlier on. Skier risk does not depend as much on aspect as may be indicated from
avalanche data alone. However, it is relatively high in the N–NE–E sector. These factors are not independent of each other
and therefore analyses of combined factors were also performed. Questionnaires and interviews were used to gain knowledge
about the terrain selection of professional mountain guides. These results indicate that when selecting terrain, guides first
look at the overall shape and size of the terrain, but avalanche history of terrain and inclination are also important factors.
Finally, remarks in avalanche reports were analysed, and common human factors identified. 相似文献
69.
70.
José F. Noguera Lluís Rivero Xavier Font Andrés Navarro Francisco Martínez 《Environmental Geology》2002,41(8):898-905