Implications of principal stress axes and eigenvalue ratios on critical orientation of fractures. Application to two tectonic regions in Alaska

The principal stress axes and eigenvalue ratios of the stress tensors from two active seismic regions in Alaska (eastern Aleutian Arc and eastern Gulf of Alaska) are computed. These results are obtained using focal mechanisms of earthquakes with magnitudes ranging from 5.5 to 8.2 and show for both regions slightly different compressional states with an orientation of the pressure axis of N26°W for the eastern Gulf of Alaska and N27°W for the east Aleutian Arc. The eigenvalue ratios for the eastern Gulf of Alaska and east Aleutian Arc are 0.84 and 0.99, respectively. The algorithm used in computations allows us to detect some focal mechanisms which are incoherent with respect to the obtained stress tensors. These incoherences are explained in terms of some tectonic features of the region. The orientations of possible pre-existing fractures which would need an unrealistically high maximum shear stress to start a slip on their fault planes are also investigated. These critical orientations depend on a constitutive frictional law as well as on eigenvalue ratios and eigenvectors of the stress tensors. The orientation, which is given in our case by the angle between the normal vector to a fracture and the regional tensional axis, is approximately 59° for the eastern Aleutian Arc. This angle ranges from 49° to 59° for the eastern Gulf of Alaska. It must be pointed out that fractures which need unrealistically high shear stress to start slip on their fault planes are defined by a very narrow band of possible angles between tensional direction and the normal vector to fault plane.     

Elastic-anelastic Regional Structures for the Iberian Peninsula Obtained from a Rayleigh Wave Tomography and a Causal Uncoupled Inversion

The elastic and anelastic structure of the lithosphere and asthenosphere of the Iberian Peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of Rayleigh wave fundamental mode. The database consists of surface wavetrains recorded at the broadband stations located in the Iberian Peninsula on the occasion of the ILIHA project. Path-averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and, subsequently, local dispersion curves were computed according to the Yanovskaya-Ditmar formulation. First, a principal component analysis (PCA) and the average linkage (AL) clustering algorithm are applied to these local values in order to classify the Iberian Peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves, without previous seismotectonic constraints. Second, averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere. This purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients. The main features of the homogeneous regions are discussed by taking as reference the Hercynic, Alpine and Neogene domains of the Iberian Peninsula, and two questions affecting the reliability of the elastic-anelastic models are revised. First, the coherence of the shear-velocity and Q_β⁻¹ models obtained by causal uncoupled inversion for each region is analysed. Second, the influence of the causal phase and group velocities on the shear-velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non-causal inversion.     

Are changes in the structure of nematode assemblages reliable indicators of moderate petroleum contamination?

This study assesses through a multiple before-after-control-impact (MBACI) design the effects of diesel oil on the structure of nematode assemblages in unvegetated tidal flats of a subtropical estuary. Oil-exposed treatments were contrasted with controls for a duration of four successive days before and after an experimental spill in three distinct areas of the Paranaguá Estuarine Complex (Southern Brazil). No significant differences were observed in nematode total density, number of taxa and the overall assemblage structure between the control and impact treatments from before to after the experimental spill. This reinforces the idea that, despite being good indicators of environmental stress, free-living marine nematodes are able to tolerate low concentrations of hydrocarbons and to survive in moderately contaminated areas. We also show that robust experimental designs are useful to avoid confounding expected natural variability with the effects of a mild impact.     

Lateral variations of attenuation coefficients, group and phase velocities of rayleigh waves in Europe

Rayleigh wave group and phase velocities and attenuation coefficients are investigated in the 15–60-s period range for earthquakes occurring in the European area and registered at European stations. Observed group and phase velocities show clear differences in the period range 20–40 s, whereas attenuation coefficients are differentiated from 45 s to larger periods. From the shape of dispersion and attenuation curves, two broad zones have been isolated and designated as Western Europe and Southeastern Europe. Although some scatter is present, specially for attenuation, the data suggest that in general the shear velocity and Q_β values are higher for Southeastern Europe than for Western Europe. For Southeastern Europe there is a channel of low velocity and low Q_β, apparently correlated, located at the lower crust: in both regions there is a channel of low velocity and low Q_β values located at the uppermost mantle. In each case there also exists a channel of high Q_β values located at the lower crust but at slightly different depths.     

A Methodology for the Definition of Geotechnical Mine Sectors Based on Multivariate Cluster Analysis

This paper offers a new method for the definition of geotechnical sectors in open pit mines based on multivariate cluster analysis. A geological-geotechnical data set of a manganese open pit mine was used to demonstrate the methodology. The data set consists of a survey of geological and geotechnical parameters of the rock mass, measured directly in several points of the mine, structured initially in twenty-eight variables. After the preprocessing of the data set, the clustering technique was applied using the k-Prototype algorithm. The squared Euclidean distance was used to quantify the proximity between numerical variables, and the Jaccard's coefficient of similarity was used to quantify the proximity between the nominal variables. The different cluster results obtained were validated by the multivariate analysis of variance. The identification of cluster structures was achieved by plotting them on the mine map for spatial visualization and definition of geotechnical sectors. These sectors are spatially contiguous and relatively homogeneous regarding their geological–geotechnical properties, indicated by a high density of points of the same group. It was possible to observe a great adherence of the proposed sectors to the mine geology, demonstrating the practical representativeness of the clustering results and the proposed sectors.

     

Some Statistical Characteristics of Monthly and Annual Pluviometric Irregularity for the Spanish Mediterranean Coast

Summary  Seven series of monthly pluviometric amounts, sometimes exceeding recording periods of 100 years and compiled by the Instituto Nacional de Meteorología (Spain), are used to study the irregularity of the pluviometric regime along the Spanish Mediterranean coast and nearby Atlantic coast. First of all, three statistical functions (gamma, log-normal and a combination of Poisson and gamma distributions) and moment-ratio diagrams are used to model the monthly and annual empirical distributions of precipitation amounts, each distribution being tested by means of the Kolmogorov-Smirnov test. It is noteworthy that, whereas most of the monthly cases require the gamma distribution, the pluviometric behaviour of the summer months is well described by the Poisson-gamma distribution. Moreover, both the log-normal and the gamma distributions satisfactorily model empirical annual amounts. Consequently, rainfall amounts are not identically distributed along a year for each gauge tested. Second, temporal trends deduced for annual and seasonal amounts are computed and their statistical significance evaluated. The most notable fact is that, although some linear trends are close to 1 mm/year, their significance levels exceed the assumed threshold value and, excepting the winter season for Barcelona, they are considered non-significant from a statistical point of view. Finally, by again using monthly and annual amounts, three temporal irregularity indexes are computed for each pluviometric series, the temporal disparity of the rainfall patterns of the Mediterranean region being enhanced as a result. It addition to the temporal irregularity, a change with latitude is observed both in the parameters of the statistical distributions and the temporal irregularity indexes for the rain gauges analysed. The two most southerly rain gauges constitute a special case in comparison with the remaining stations, because they also receive the Atlantic influences due to their proximity to this ocean. Received February 25, 1999/Revised August 2, 1999     

Tourmaline composition and boron isotopes record lateritic weathering during the Great Oxidation Event

Identifying evidence of oxidative weathering in the geological record is essential to trace the evolution of Earth's atmosphere oxygenation. Metamorphosed residues of lateritic weathering have been identified as two rock types in the 2.1‐Ga‐old Cercadinho Formation, Piracicaba Group, Quadrilátero Ferrífero of Minas Gerais. One is tourmaline–hematite–sillimanite–kyanite quartzite; the other is rutile–tourmaline–hematite–muscovite phyllite. Both rocks have abundant tourmaline with δ¹¹B values between about ?17‰ and ?13‰. The Cercadinho tourmaline is roughly parallel to the povondraite–“oxy‐dravite” join of meta‐evaporitic tourmaline, in its more aluminous segment, offset to higher contents of iron. These compositional and isotopic characteristics of the Cercadinho tourmaline indicate that continental evaporitic brines interacted with aluminium‐ and iron‐rich residues of lateritic weathering. The abundance of disseminated tourmaline, a mineral poorly reported from palaeosols worldwide, implies a boron‐rich brine overprint on the lateritic profile before the onset of metamorphism, reflecting a climatic change from humid to arid conditions in a continental setting. The recognition of lateritic weathering in the Cercadinho Formation contributes to the amount of evidence for increased levels of atmospheric oxygen between 2.22 and 2.06 Ga ago.     

Winter and autumn daily precipitation patterns in Catalonia,Spain

Summary A principal component analysis (PCA), based on a network consisting of 60 pluviometric gauges and their daily precipitation data, is attempted in order to describe the main winter and autumn patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation correlation matrix and rotated component loadings are then deduced and extensively interpreted. The PCA results are then used in a clustering process (Average Linkage), leading to two rainfall divisions, one for each season, which are then compared.With 5 Figures     

Seasonal patterns of suspended sediment transport in an abandoned farmland catchment in the Central Spanish Pyrenees

The suspended sediment response of a small catchment subjected to farmland abandonment and subsequent plant recolonization was studied in relation to its hydrological functioning. The analysis of data over a seven‐year period demonstrated that suspended sediment yield was greatly influenced by the occurrence of intense, low‐frequency events. Greater amounts of suspended sediment were exported during spring, when the catchment was hydrologically more active. Rainfall intensity and baseflow at the start of a flood event had a strong influence on the sediment response, suggesting that several hydrological processes were active within the catchment. SSC (suspended sediment concentration)‐Q hysteretic loop analysis at the event scale aided understanding of the sedimentological and hydrological behaviour of the catchment. During the study period the SSC‐Q loops showed a high degree of seasonality and two main patterns strongly related to catchment wetness were distinguished. When the catchment was dry (mainly during summer and the beginning of autumn) the predominant process was infiltration excess runoff over sparsely vegetated areas close to the main channel. Under these conditions, floods exhibited a counter‐clockwise hysteretic loop and were characterized by a small streamflow response, short duration and high SSC. Under wet conditions (mainly during winter and spring), saturation excess runoff was increasingly dominant over vegetated areas. Under these conditions, floods exhibited a clockwise hysteretic loop, and were characterized by a larger streamflow response, longer duration and higher suspended sediment yield. The lower SSC during the falling stage of the hydrograph is likely to be due to dilution effects related to the contribution of clean water resulting from enlargement of the saturated areas, together with an increase in the baseflow discharge. Copyright © 2009 John Wiley & Sons, Ltd.