A substructure method for earthquake analysis of structures including structure-soil interaction

A general substructure method for analysis of response of structures to earthquake ground motion, including the effects of structure-soil interaction, is presented. The method is applicable to complex structures idealized as finite element systems and the soil region treated as either a continuum, for example as a viscoelastic halfspace, or idealized as a finite element system. The halfspace idealization permits reliable analysis for sites where essentially similar soils extend to large depths and there is no rigid boundary such as soil-rock interface. For sites where layers of soft soil are underlain by rock at shallow depth, finite element idealization of the soil region is appropriate; in this case, the direct and substructure methods would lead to equivalent results but the latter provides the better alternative. Treating the free field motion directly as the earthquake input in the substructure method eliminates the deconvolution calculations and the related assumption—regarding type and direction of earthquake waves—required in the direct method. Spatial variations in the input motion along the structure-soil interface of embedded structures or along the base of long surface supported structures are included in the formulation. The substructure method is computationally efficient because the two substructures—the structure and the soil region—are analysed separately; and, more important, it permits taking advantage of the important feature that response to earthquake ground motion is essentially contained in the lower few natural modes of vibration of the structure on fixed base.     

On the origin of compressional intraplate stresses in South America

Intraplate stresses in middle South America are not negligible. We report thrust-faulting mechanisms for five intraplate earthquakes, which indicate a dominant horizontal deviatoric compressional stress oriented in a NW-SE direction. We conclude that this state of stress is due to forces connected with spreading on the Mid Atlantic Ridge and resistive forces exerted by the Caribbean plate to the north and the Nazca plate to the west. The existence and nature of the resistive forces is inferred from earthquake mechanisms and geological evidence presented in other studies. All the available intraplate stress data for Nazca and South America indicate that both plates are under deviatoric compression generated at spreading centers. The absence of tensional earthquake focal mechanisms, particularly in the Nazca plate near the trench, suggests that the forces associated with the gravitational sinking of subducted lithosphere are locally compensated. We present a simple numerical calculation of a non-subducting plate to show how the compressional deviatoric stresses in middle South America can be used to estimate an upper bound of about 10²¹ P for the viscosity of the mantle.     

Estimates of coda Q attenuation in the João Câmara area (Northeastern Brazil)

S coda wave of seventy-four local earthquakes recorded in a network of ten seismic stations were used to calculate coda Q attenuation (Q_c) in the João Câmara area (northeastern Brazil). The estimates show Q_c as a strong function of frequency in the range from 6.0 to 20.0 Hz. We found out that Q_c in João Câmara has a functional form given by Q_c= Q₀ f, where Q₀= 151 ± 99 and = 0.98 ± 0.05. If the standard deviations are taken into account,we conclude that there are no relevant changes in both Q₀ and values from one station to another. The estimated Q₀ values at the different stations suggest that the Samambaia fault is a boundary between two different seismic attenuation zones. In one side of the fault (left), where stations were installed in Pre-Cambrian terrain and thick sedimentary layer, the seismic attenuation is stronger than in the other side (stations installed in thin sedimentary layer and limestone outcrop).The anomalous Q₀ values in the left side of the Samambaia fault can be explained due to the presence of a shallow conductive layer in the upper crust( 10 km), such as proposed by Padilha et al. (1992). According to our results, if there is a conductive layer in the area, it probably spreads over João Câmara city and surrounding regions.However, more detailed investigation either with seismic methods (seismic attenuation,3D tomography with P and/or S wave velocities) or with other geophysical methods is needed to interpret the observed differences in Q₀ values between the two sides of the Samambaia fault.     

Fractal analysis of tidal channels in the Bahía Blanca Estuary (Argentina)

The fractal dimension (D) was estimated for nine tidal channels depicted in thematic mapper (TM) Landsat-5 imagery to derive information about the degree of geomorphological control on a tidal channel network characteristic of the Bahía Blanca Estuary (Argentina). Two methods, box counting and contiguity, were used to estimate fractal dimensions for each tidal channel. All channels produced D values close to 1, meaning that they are self-affine fractal features. However, these fractal dimensions do not represent the meandering pattern complexity characteristic of the tidal channels analysed. Although both methods allowed for estimation of D, the contiguity method showed that three of the channels actually are not fractal but have sinusoidal characteristics, a condition that was not detected by the former method.     

4: Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia

SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures.The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids (δ¹⁸O_fluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes.Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks (δ³⁴S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation.The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.     

Mineralogy,Mossbauer Characteristics,and K-Ar Isotopic Age of Glauconite from the Lower Cambrian Sediments,Western Lithuania

The paper presents results of lithological-mineralogical, Mossbauer, and isotope-geochronological study of globular phyllosilicates (glauconite and illite) from the Lower Cambrian Virbalis Formation in western Lithuania. K-Ar dates of these minerals (382 and 374 Ma) do not correspond to stratigraphic positions of the studied samples and indicate the loss of radiogenic ⁴⁰Ar, probably, in Middle Devonian. Simulation of cation distribution in the mineral structure and comparison of the obtained data with Mossbauer spectral characteristics suggest that loss of ⁴⁰Ar was caused by the structural rearrangement of globular phyllosilicates during the postdiagenetic substitution of Fe²⁺, Mg²⁺, and Fe³⁺ by Al³⁺, with the subsequent ordering of cation distribution in octahedral layers. These processes were presumably related to the same Caledonian tectonic events that were responsible for metasomatic dolomitization of Lower Paleozoic rocks in adjacent areas of the Baltic region at the terminal Silurian-Devonian. Diffraction analysis showed that globules consist of two micaceous phases with different Fe contents (illite and glauconite). Two alternative models are considered to explain the formation of two-phase globules: (1) disequilibrium conditions of diagenetic mineral formation; (2) transformation of glauconite into illite. Although the available data are insufficient to make an unambiguous choice between these models, the second mechanism seems to be more preferable.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 403–415.Original Russian Text Copyright © 2005 by Zaitseva, Gorokhov, Ivanovskaya, Zvyagina, Mel’nikov, Yakovleva.     

Effect of the cryolithic zone electrical properties on the medium-frequency surface wave at high latitudes

Result of the measurements of the cryolithic zone electrical properties by the radio impedance sounding method are presented. The surface impedance and geoelectric section of the Arctic permafrost have been determined. The experiments have been performed near Cherskii in the continental zone. The surface wave attenuation function is considered. The accuracy of the surface wave field calculations at high latitudes has been estimated based on the local electrical characteristics of the underlying cryogenic medium. The effect of the Arctic Ocean on the operating zone of navigation and communication facilities in the kilometer wave band has been experimentally estimated.     

Short-term changes in C and N distribution in soil particle size fractions induced by agricultural practices in a cultivated volcanic soil from Mexico

The purpose of this study was to assess the effect of agricultural practices on the characteristics of soil organic carbon (SOC). The study area is located in the Central Volcanic Belt (Michoacán) in Mexico. The soil is an Acrisol, acidic and rich in clays and sesquioxides. Experimental plots were treated with four different agronomic management systems between 2002 and 2003: traditional, improved traditional, organic and fallow. Each treatment was replicated twice. Samples were taken at depths of 0–10 and 10–20 cm and were subjected to a physical fractionation process by way of particle size. SOC was fractionated into fulvic acids, humic acids and humin. After two years of cultivation, the SOC content increased significantly with the organic management (2.2 mg g⁻¹ at 0–10 cm and 5.8 mg g⁻¹ at 10–20 cm). The C content of the soil fine particle fraction increased with the traditional and organic managements. The organic C and N contents of the silt + clay particle-size fraction were mainly concentrated as humin, indicating that this SOM should be stable and have a low risk of mineralization. The humin C content only decreased significantly under the traditional and fallow treatments. The N content of the humin fraction decreased significantly under the traditional management system (from 69% to 54%), indicating the low sustainability of this soil management. The C/N ratio of the soil increased significantly under all treatments, but mainly under the traditional and organic treatments (from 12.1 and 12.8 to 13.7 and 14.0, respectively). This indicates a decrease in humus quality. In addition, the C/N of the humin increased significantly (from 13.3 and 12.7 to 19.2 and 16.0, respectively).