A microslip model for the seismic response analysis of drained cohesionless soil deposits

Constitutive equations for drained cohesionless soils under triaxial stress states, based on a microslip model with internal friction, are proposed and applied to the analysis of the seismic response of soil deposits. The microslips, assumed localized in the discontinuity surfaces passing through the particle-to-particle contacts, describe the effects of the sliding and rolling between grains of sand. Once the response of the model under cyclic shearing stresses and strains superimposed on the geostatic stress state has been analysed, the constitutive equations are applied to a one-dimensional shear model for the dynamic analysis of the layered deposits. An application to an instrumented soil deposit is then developed, the experimental results of which refer to both strong and weak motion earthquakes; the comparisons between the experimental and theoretical results point out the peculiarities and validity limits of the model. The model here developed may be applied also to the analysis of two- and three-dimensional sites.     

Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896

DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa_17.5±2.1, [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (δ17O = +2.55, δ18O = +3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (∼10 vol%), typically in the form of corroded lithic fragments <100 μm in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of ∼80-100 GPa. The relatively young 3.704 ± 0.035 Ga 40Ar-39Ar crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history.Based on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of ∼1630°C under relatively slow cooling rates (∼10°C h⁻¹) to ∼1300°C, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes’ Law indicates a settling velocity >1 m h ⁻¹ during the first few hours of crystallization on asteroidal bodies of >25 km radius.The ∼3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid.The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in IIE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Δ17O = 0.77 ± 0.04) should be extended to IIEs to verify this possible petrogenetic link.     

Tectonic Control of the Triassic Santa Maria Supersequence of the Paraná Basin, Southernmost Brazil, and its Correlation to the Waterberg Basin, Namibia

The Middle and Late Triassic Santa Maria Basin, exposed in southernmost Brazil, and Waterberg Basin, in Namibia, are herein interpreted as part of en échelon small basins in southern West Gondwana. The main structures are the Waterberg-Omaruru Fault which can be connected to a NW-strike anastomosed fault zone in Brazil. Based on field structural and stratigraphic analysis two populations of NW-strike fractures, named A-type and B-type, are recognized. A-type fractures (Az = 280°-290°) occur in the Sanga do Cabral Supersequence, underlying units of Santa Maria Basin, as extension of the Waterberg-Omaruru Fault during the Early Triassic. B-type fractures (Az = 295°-345°) are observed in all studied stratigraphic units, from the Triassic Sanga do Cabral Supersequence to the Early Cretaceous Botucatu/Serra Geral formations. Based on the structural analysis we propose that NNE-SSW extension reactivated structures of the Damara Belt, Namibia, with a propagation towards Rio Grande do Sul State forming an anastomosing normal fault system and related-rift basin by Early-Middle Triassic time. The A-type fractures were preferentially active by this phase and the B-type ones are interpreted as secondary link segments within the anastomosing system. During the Gondwana break-up and South Atlantic opening (rift to proto-oceanic phase, Aptian) the B-type fractures were reactivated as normal faults by N50°E-S50°W extension.     

Soil erosion prediction at the basin scale using the revised universal soil loss equation (RUSLE) in a catchment of Sicily (southern Italy)

Soil erosion by water is a serious problem in southern Italy, particularly in Sicily which is one of the Italian administrative regions prone to desertification. Soil erosion not only affects soil quality, in terms of agricultural productivity, but also reduces the availability of water in reservoirs. This study was conducted in the Comunelli catchment in south-central Sicily, to predict potential annual soil loss using the revised universal soil loss equation (RUSLE) and to test the reliability of this methodology to predict reservoirs siltation. The RUSLE factors were calculated for the catchment using survey data and rain gauge measurement data. The R-factor was calculated from daily, monthly and annual precipitation data. The K-factor was calculated from soil samples collected in May and November 2004. The LS topographic factor was calculated from a 20 m digital elevation model. The C- and P-factors, in absence of detailed data, were set to 1. The results were compared with those obtained from another soil loss estimation method based on ¹³⁷Cs and with the soil loss estimated from the sediment volume stored in the Comunelli reservoir between 1968 and 2004.     

Simulation of the summer circulation over South America by two regional climate models. Part I: Mean climatology

Summary This study investigates the capabilities of two regional models (the ICTP RegCM3 and the climate version of the CPTEC Eta model – EtaClim) in simulating the mean climatological features of the summer quasi-stationary circulations over South America. Comparing the results with the NCEP/DOE reanalysis II data it is seen that the RegCM3 simulates a weaker and southward shifted Bolivian high (BH). But, the Nordeste low (NL) is located close to its climatological position. In the EtaClim the position of the BH is reproduced well, but the NL is shifted towards the interior of the continent. To the east of Andes, the RegCM3 simulates a weaker low level jet and a weaker basic flow from the tropical Atlantic to Amazonia while they are stronger in the EtaClim. In general, the RegCM3 and EtaClim show, respectively a negative and positive bias in the surface temperature in almost all regions of South America. For both models, the correlation coefficients between the simulated precipitation and the GPCP data are high over most of South America. Although the RegCM3 and EtaClim overestimate the precipitation in the Andes region they show a negative bias in general over the entire South America. The simulations of upper and lower level circulations and precipitation fields in EtaClim were better than that of the RegCM3. In central Amazonia both models were unable to simulate the precipitation correctly. The results showed that although the RegCM3 and EtaClim are capable of simulating the main climatological features of the summer climate over South America, there are areas which need improvement. This indicates that the models must be more adequately tuned in order to give reliable predictions in the different regions of South America.     

Effects of Amazonian deforestation on climate: a numerical experiment with a coupled biosphere-atmosphere model with soil hydrology

Summary A coupled biosphere-atmosphere statistical-dynamical model (SDM) is used to study the climatic effects of Amazonian deforestation. A soil moisture model based on BATS has been incorporated into the SDM in order to study the biogeophysical feedback of change in surface characteristics to regional climate due to the deforestation. In the control experiment, the mean annual and mean seasonal climate is well simulated by the model when compared with NCEP/NCAR reanalysis data. In the deforestation experiment, the evergreen broadleaf trees in the Amazonian region are substituted by short grass. The effects of Amazonian deforestation on regional climate are analysed taking into account the model simulations for the land portion of the latitude belts comprising the tropical region. Amazonian deforestation results in regional climate changes such as a decrease in evaporation, precipitation, available surface net radiation and soil moisture content, and an increase in temperatures and sensible heat flux. The reduction in transpiration was responsible for the most part of the decrease in total evapotranspiration. The reduction in precipitation was larger than the decrease in evapotranspiration so that runoff was reduced. The simulation of the diurnal cycle of the surface temperature shows an increase in temperature during the day and a decrease at night, which is in agreement with observations, whereas earlier GCM experiments showed an increase both during the day and night. In general, the changes in temperature and energy fluxes are in good agreement with GCM experiments, showing that the SDM is able to simulate the characteristics of the tropical climate that are associated with the substitution of forest by pasture areas.     

Macroseismic and mechanical models for the vulnerability and damage assessment of current buildings

The European Commission funded the RISK-UE project in 1999 with the aim of providing an advanced approach to earthquake risk scenarios for European towns and regions. In the framework of Risk-UE project, two methods were proposed, originally derived and calibrated by the authors, for the vulnerability assessment of current buildings and for the evaluation of earthquake risk scenarios: a macroseismic model, to be used with macroseismic intensity hazard maps, and a mechanical based model, to be applied when the hazard is provided in terms of peak ground accelerations and spectral values. The vulnerability of the buildings is defined by vulnerability curves, within the macroseismic method, and in terms of capacity curves, within the mechanical method. In this paper, the development of both vulnerability and capacity curves is presented with reference to an assumed typological classification system; moreover, their cross-validation is presented. The parameters of the two methods and the steps for their operative implementation are provided in the paper.     

Anomalous soil CO2 degassing in relation to faults and eruptive fissures on Mount Etna (Sicily, Italy)

 The relationships between soil gas emissions and both tectonic and volcano-tectonic structures on Mt. Etna have been studied. The investigation consisted of soil CO₂ flux measurements along traverses orthogonal to the main faults and eruptive fissures of the volcano. Anomalous levels of soil degassing were found mainly in coincidence with faults, whereas only 49% of the eruptive fissures were found to produce elevated CO₂ soil fluxes. This result suggests that only zones of strain are able to channel deep gases to the surface. According to this hypothesis, several previously unknown structures are suggested. Based on our geochemical data, new structural maps of different areas of Etna are proposed. The soil CO₂ fluxes observed in this study are higher than those measured in a 1987 study, and they are consistent with the higher level of volcanic unrest during the current study. Received: 20 March 1998 / Accepted: 17 June 1998     

Microdeformation of lacustrine laminite sequences from Late Miocene formations of SE Spain: an interpretation of loop bedding

Lacustrine laminated sediments (laminites) present in Late Miocene formations of the Híjar Basin, SE Spain, display well developed loop bedding, a structure consisting of bundles of laminae that are sharply constricted at intervals, giving a morphology of loops or links of a chain. The laminite sequences, which are interbedded with turbidite marlstones, were accumulated on the bottom of a permanently stratified lake developed in a rapidly subsiding basin limited by 010° and 105° normal faults. As deduced from both macro- and microdeformational analyses, the basin evolved under an extensional stress field throughout the Late Miocene. Four main types of loops, simple and complex loops with subcategories, have been recognized within the laminite sequence. Simple loops of type 1 show the best definite pattern, quite similar to ' pinch and swell structures ', a type of boudinage typical of stretching of alternating beds where the competence contrast is not strongly marked. The remaining loop types display contortion and occasional breakage of laminae (microfaulted edges) indicative of microdeformation near the boundary between the ductile-brittle deformational fields. The distribution of the various loop types across the laminite sequence reflects an interplay between progressive lithification of the laminites as sedimentation progressed and tectonic stresses which affected the sediment sequence. Accordingly, a mechanism of deformation under an extensional stress field, ultimately related to the creep movement of the main basin faults which resulted in successive seismic shocks of low magnitude, is proposed to explain the formation of loop bedding in the laminites.     

The Stellar Content of Active Galaxies

We present the results of a long-slit spectroscopic study of 38 active and four normal galaxies. Stellar absorption features, continuum colours and their radial variations are analysed in an effort to characterize the stellar population in these galaxies and detect the presence of a featureless continuum underlying the starlight spectral component. Spatial variations of the equivalent widths of conspicuous absorption lines and continuum colours are detected in most galaxies. Star-forming rings, in particular, leave clear fingerprints in the equivalent widths and colour profiles. We find that the stellar populations in the inner regions of active galaxies present a variety of characteristics, and cannot be represented by a single starlight template. Dilution of the stellar lines by an underlying featureless continuum is detected in most broad-lined objects, but little or no dilution is found for most of the 20 type 2 Seyferts in the sample. Colour gradients are also ubiquitous. In particular, all but one of the observed Seyfert 2s are redder at the nucleus than in its immediate vicinity. Possible consequences of these findings are briefly outlined.