The source of the 1722 Algarve earthquake: evidence from MCS and Tsunami data

The 27 December 1722 Algarve earthquake destroyed a large area in southern Portugal generating a local tsunami that inundated the shallow areas of Tavira. It is unclear whether its source was located onshore or offshore and, in any case, what was the tectonic source responsible for the event. We analyze available historical information concerning macroseismicity and the tsunami to discuss the most probable location of the source. We also review available seismotectonic knowledge of the offshore region close to the probable epicenter, selecting a set of four candidate sources. We simulate tsunamis produced by these candidate sources assuming that the sea bottom displacement is caused by a compressive dislocation over a rectangular fault, as given by the half-space homogeneous elastic approach, and we use numerical modeling to study wave propagation and run-up. We conclude that the 27 December 1722 Tavira earthquake and tsunami was probably generated offshore, close to 37°01′N, 7°49′W.     

Hydrothermal alteration and fluid inclusion study of the Lower Cretaceous porphyry Cu–Au deposit of Tiámaro, Michoacán, Mexico

The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.     

Deglaciation sequences in the Permo-Carboniferous Itararé Group, Paraná Basin, southern Brazil

The Late Carboniferous–Early Permian Itararé Group is a thick glacial unit of the Paraná Basin. Five unconformity-bounded sequences have been defined in the eastern outcrop belt and recognized in well logs along 400 km across the central portion of the basin. Deglaciation sequences are present in the whole succession and represent the bulk of the stratigraphic record. The fining-upward vertical facies succession is characteristic of a retrogradational stacking pattern and corresponds to the stratigraphic record of major ice-retreat phases. Laterally discontinuous subglacial tillites and boulder beds occur at the base of the sequences. When these subglacial facies are absent, deglaciation sequences lie directly on the basal disconformities. Commonly present in the lowermost portions of the deglaciation sequences, polymictic conglomerates and cross-bedded sandstones are generated in subaqueous proximal outwash fans in front of retreating glaciers. The overlying assemblage of diamictites, parallel-bedded and rippled sandstones, and Bouma-like facies sequences are interpreted as deposits of distal outwash fan lobes. The tops of the deglaciation sequences are positioned in clay-rich marine horizons that show little (fine-laminated facies with dropstones) or no evidence of glacial influence on the deposition and likely represent periods of maximum ice retreat.     

Hydrogeological investigation in Santiago Island (Cabo Verde) using magnetotellurics and VLF methods

A geoelectromagnetic research was carried out in the Santa Cruz region (Santiago Island, Cabo Verde) during June 2004. The survey comprised MT soundings and VLF profiles. The main purpose of the MT profile, carried out across three important valleys associated with freshwater aquifers, was to study the tectonic structures correlated to seawater infiltration. The VLF method was used inside of the valleys for investigating shallow structures related to the aquifer contamination by seawater.Numerical modelling shows that the ocean effect is not important for MT data collected at periods shorter than 1 s. The MT data were inverted using a two-dimensional approach, to obtain the sub-superficial electrical conductivity distribution. The VLF data were processed applying the Karous–Hjelt filters to obtain the equivalent current distribution and inverted using 2-D approach. The results obtained in one of the most important valleys show anomalous current concentration/low resistivity (<20 Ω m) areas at depths greater than 40 m that may correspond to an increase in seawater content.The MT data modelling show that the deep zones beneath the valley are strongly fractured representing good pathways for seawater circulation. The depth of the conductive zones increases from south to north, suggesting a northward decreasing of the seawater infiltration effect. This observation correlates very well with in situ geochemical observations.     

Assessment of failure susceptibility of soil slopes using fuzzy logic

Generally, the process of land occupation in urban areas involves spaces that are not suitable for construction. In most cases these areas are subject to landslides. Therefore it is mister the development of models to evaluate the susceptibility of occurrence of landslides in these areas. For this, Fuzzy Logic is used herein for modeling such areas where landslides are susceptible to occur and, therefore, a direct evaluation is important. The possibility of capturing the judgment and the modeling of linguistic variables are the main advantages of using Fuzzy Logic. These models are capable to capture the factors directly affecting the slope stability and also the inter-relationship amongst them. These factors were chosen by experts to whom a questionnaire was sent. Fuzzy Logic was then used to transform the linguistic variables into fuzzy number, allowing thus, the calculation of failure potential index (FPI). Herein the MAX-MIN Mamdani strategy for the inference of the rule base was used. This methodology has been applied to identify the susceptibility of landslides in a chaotic occupied urban area of Itaperuna City in northeastern of Rio de Janeiro, Brazil, where some occurrences have been reported.     

Un exemple de volcanisme calco-alcalin de type orogénique mis en place en contexte de rifting (Cambrien de l'oued Rhebar,Meseta occidentale,Maroc)

The Middle Cambrian calc-alkaline Oued Rhebar volcanic complex (western Meseta, Morocco) compares with rocks originated in orogenic contexts. The La/Nb ratios are relatively high (5.2), suggesting a lithospheric mantle origin. The La/Ta ratios, higher than 26, and the negative Nb anomaly indicate a lithospheric source contaminated by the continental crust. These rocks were generated in the Mesetian Mid-Cambrian rift and would have inherited their orogenic signature from the partial melting of a previously metasomatized mantle. To cite this article: H. El Hadi et al., C. R. Geoscience 338 (2006).     

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u _* when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u _*). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h /|L_MO| over the time scale t_MO~|L _MO|/(10u_*) (where L _MO and t _MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h /|L _MO| (>>1), over the time scale t _MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.     

The role of recharge and evapotranspiration as hydraulic drivers of ion concentrations in shallow groundwater on Everglades tree islands,Florida (USA)

Recently, evapotranspiration has been hypothesized to promote the secondary formation of calcium carbonate year‐round on tree islands in the Everglades by influencing groundwater ions concentrations. However, the role of recharge and evapotranspiration as drivers of shallow groundwater ion accumulation has not been investigated. The goal of this study is to develop a hydrologic model that predicts the chloride concentrations of shallow tree island groundwater and to determine the influence of overlying biomass and underlying geologic material on these concentrations. Groundwater and surface water levels and chloride concentrations were monitored on eight constructed tree islands at the Loxahatchee Impoundment Landscape Assessment (LILA) from 2007 to 2010. The tree islands at LILA were constructed predominately of peat, or of peat and limestone, and were planted with saplings of native tree species in 2006 and 2007. The model predicted low shallow groundwater chloride concentrations when inputs of regional groundwater and evapotranspiration‐to‐recharge rates were elevated, while low evapotranspiration‐to‐recharge rates resulted in a substantial increase of the chloride concentrations of the shallow groundwater. Modeling results indicated that evapotranspiration typically exceeded recharge on the older tree islands and those with a limestone lithology, which resulted in greater inputs of regional groundwater. A sensitivity analysis indicated the shallow groundwater chloride concentrations were most sensitive to alterations in specific yield during the wet season and hydraulic conductivity in the dry season. In conclusion, the inputs of rainfall, underlying hydrologic properties of tree islands sediments and forest structure may explain the variation in ion concentration seen across Everglades tree islands. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluating the performance of CMIP3 and CMIP5 global climate models over the north-east Atlantic region

One of the main sources of uncertainty in estimating climate projections affected by global warming is the choice of the global climate model (GCM). The aim of this study is to evaluate the skill of GCMs from CMIP3 and CMIP5 databases in the north-east Atlantic Ocean region. It is well known that the seasonal and interannual variability of surface inland variables (e.g. precipitation and snow) and ocean variables (e.g. wave height and storm surge) are linked to the atmospheric circulation patterns. Thus, an automatic synoptic classification, based on weather types, has been used to assess whether GCMs are able to reproduce spatial patterns and climate variability. Three important factors have been analyzed: the skill of GCMs to reproduce the synoptic situations, the skill of GCMs to reproduce the historical inter-annual variability and the consistency of GCMs experiments during twenty-first century projections. The results of this analysis indicate that the most skilled GCMs in the study region are UKMO-HadGEM2, ECHAM5/MPI-OM and MIROC3.2(hires) for CMIP3 scenarios and ACCESS1.0, EC-EARTH, HadGEM2-CC, HadGEM2-ES and CMCC-CM for CMIP5 scenarios. These models are therefore recommended for the estimation of future regional multi-model projections of surface variables driven by the atmospheric circulation in the north-east Atlantic Ocean region.