The Boussinesq problem for soils with bounded non-homogeneity

The response of a compressible continuously non-homogeneous elastic soil to a static vertical point load on its surface is analytically investigated by using classical integral transform techniques and the extended power series method for obtaining the solution in the transform domain. The non-homogeneity is described by means of a depth-function which is non-zero at the surface and bounded at infinity and is capable in modelling both increasing and decreasing soil stiffness with depth. The influence of non-homogeneity on the displacements and stresses at the surface and in the interior is examined over a wide range on the governing parameters. © 1998 John Wiley & Sons, Ltd.     

Effect of torsional stiffness of prestressed concrete box girders and uplift of abutment bearings on seismic performance of bridges

Uplift of certain abutment bearings during earthquake may be utilized as a safety mechanism for the protection of the bridge against excessive stress. For this reason design codes such as Eurocode 8 permit the uplift of such bearings under certain conditions. Uplift of an abutment bearing occurs when the torque at the deck end exceeds a critical value. Therefore the torsional stiffness of the deck, before or after concrete cracking, is an important factor. In this work realistic values for the cracked torsional stiffness of the bridge deck are estimated from a parametric study of typical thin-walled prestressed box girders, based on non-linear analysis of softened space truss models proposed in the literature. Moreover the interaction between bearing uplift and pier flexural response is investigated and recommendations for the seismic design of bridges are proposed, taking into account the possibility of uplift of abutment bearings before or after yielding of the piers.     

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.     

Earthquake recurrence models and occurrence probabilities of strong earthquakes in the North Aegean Trough (Greece)

The determination of strong earthquakes’ recurrence time above a predefined magnitude, associated with specific fault segments, is an important component of seismic hazard assessment. The occurrence of these earthquakes is neither periodic nor completely random but often clustered in time. This fact in connection with their limited number, due to shortage of the available catalogs, inhibits a deterministic approach for recurrence time calculation, and for this reason, application of stochastic processes is required. In this study, recurrence time determination in the area of North Aegean Trough (NAT) is developed by the application of time-dependent stochastic models, introducing an elastic rebound motivated concept for individual fault segments located in the study area. For this purpose, all the available information on strong earthquakes (historical and instrumental) with M_w ≥ 6.5 is compiled and examined for magnitude completeness. Two possible starting dates of the catalog are assumed with the same magnitude threshold, M_w ≥ 6.5 and divided into five data sets, according to a new segmentation model for the study area. Three Brownian Passage Time (BPT) models with different levels of aperiodicity are applied and evaluated with the Anderson–Darling test for each segment in both catalog data where possible. The preferable models are then used in order to estimate the occurrence probabilities of M_w ≥ 6.5 shocks on each segment of NAT for the next 10, 20, and 30 years since 01/01/2016. Uncertainties in probability calculations are also estimated using a Monte Carlo procedure. It must be mentioned that the provided results should be treated carefully because of their dependence to the initial assumptions. Such assumptions exhibit large variability and alternative means of these may return different final results.     

Determining the causes for the dramatic recent fall of Lake Prespa (southwest Balkans)

Mediterranean lake–wetland systems are threatened by climate change and intensive human impacts. Individual lake responses to these threats are poorly known but urgently required to steer preservation strategies. The dramatic water-level fall (~8 m since 1987) of Lake Megali Prespa endangers this global biodiversity hotspot and the wider catchment’s water resources. Annual lake fluctuations are found to be strongly related to wet-season (Oct.–Apr.) precipitation variability, which is linked to the North Atlantic Oscillation. The lake primarily adjusts to sustained inflow changes through amending surface evaporation. Cumulative water abstraction since 1951 (~19 × 10⁶ m³/year: ~0.006% of lake volume) explains ~70% of the long-term decrease in surface evaporation; climate variability explains the remainder. Persistent low lake levels after 1995 are caused by water abstraction. Compared to 1952/53–1977/78, the period 1978/79–2003/04 experienced significant decreases in wet-season precipitation, snowfall and discharge; the number of very dry years increased.

EDITOR A. Castellarin; ASSOCIATE EDITOR D. Gerten     

Hydrocarbon potential of the middle Eocene-middle Miocene Mesohellenic piggy-back basin (central Greece): A case study

Two depocentres, >4200 m and >3200 m thick, have been recognized in the Mesohellenic piggy-back basin of middle Eocene to middle Miocene age, where submarine fans have accumulated unconformably over an ophiolite complex. The hydrocarbon potential is indicated by the presence of kerogen types II/III with minor amounts of type I; the evidence is mostly for wet gas and gas, with minor oil. Source rocks are the middle Eocene to lower Oligocene Krania and Eptachori formations, of up to 2000 m total thickness, reaching maturation during the early Miocene. The source rocks consist of outer fan and basin plain deposits. They are conformably overlain by the lower member (late Oligocene) of the up to 2600 m thick Pentalophos Formation, which consists mostly of thick submarine sandstone lobes. Possible stratigraphically trapped reservoirs include the lower member of the Pentalophos Formation, which overlies source rocks, as well as limestones tectonically intercalated within the ophiolite complex, underlying the source rocks. Traps may have formed also on the western side of an internal thrust (Theotokos Thrust), which influenced the evolution of the depocentres.     

Calcareous nannofossil and planktonic foraminiferal distributional patterns during deposition of sapropels S6, S5 and S1 in the Libyan Sea (Eastern Mediterranean)

In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.     

Geostationary secular dynamics revisited: application to high area-to-mass ratio objects

The long-term dynamics of the geostationary Earth orbits (GEO) is revisited through the application of canonical perturbation theory. We consider a Hamiltonian model accounting for all major perturbations: geopotential at order and degree two, lunisolar perturbations with a realistic model for the Sun and Moon orbits, and solar radiation pressure. The long-term dynamics of the GEO region has been studied both numerically and analytically, in view of the relevance of such studies to the issue of space debris or to the disposal of GEO satellites. Past studies focused on the orbital evolution of objects around a nominal solution, hereafter called the forced equilibrium solution, which shows a particularly strong dependence on the area-to-mass ratio. Here, we (i) give theoretical estimates for the long-term behavior of such orbits, and (ii) we examine the nature of the forced equilibrium itself. In the lowest approximation, the forced equilibrium implies motion with a constant non-zero average ‘forced eccentricity’, as well as a constant non-zero average inclination, otherwise known in satellite dynamics as the inclination of the invariant ‘Laplace plane’. Using a higher order normal form, we demonstrate that this equilibrium actually represents not a point in phase space, but a trajectory taking place on a lower-dimensional torus. We give analytical expressions for this special trajectory, and we compare our results to those found by numerical orbit propagation. We finally discuss the use of proper elements, i.e., approximate integrals of motion for the GEO orbits.     

A cervelleite-like mineral and other Ag-Cu-Te-S minerals [Ag2CuTeS and (Ag,Cu)2TeS] in gold-bearing veins in metamorphic rocks of the Cycladic Blueschist Unit, Kallianou, Evia Island, Greece

A cervelleite-like mineral, two unnamed silver sulfotellurides in the system Ag-Cu-Te-S [Ag₂CuTeS, (Ag,Cu)₂TeS], Te-rich polybasite and cadmian tetrahedrite occur in gold-bearing quartz veins in metapelites and faults within brecciated marbles of the Cycladic Blueschist Unit in the Kallianou area (southern Evia Island, Greece). The quartz veins and faults are discordant to syn-metamorphic structures and formed during ductile to brittle deformation in the final stages of exhumation of the Styra Nappe extrusion wedge (~21?Ma). Te-rich polybasite (up to 7.4 wt. % Te), cadmian tetrahedrite (up to 12.4 wt. % Cd), together with electrum (23?C54 wt. % Ag) and the sulfotellurides, are the main silver carriers in the mineralization. The two unnamed sulfotellurides, Ag₂CuTeS and (Ag,Cu)₂TeS are believed to be new quaternary minerals in the system Ag-Cu-Te-S. These minerals and the cervelleite-like phase could have exsolved from galena during cooling (below 200°C). Initial temperatures for the formation of the sulfotellurides, in the form of hessite-intermediate solid solution, at Kallianou may be up to 300°C under logfS₂ values between?~ ?11.5 to ?8.3, and logfTe₂ from?~ ?14.8 to ?7.8. The values of logfTe₂ and logfS₂ during re-equilibration (at ~200°C) were constrained to ?19.5 to ?15.2 and to ?15.8 to ?11.5 respectively.     

Macrobenthic community changes due to dystrophic events and freshwater inflow: Changes in space and time in a Mediterranean lagoon (Gialova lagoon, SW Greece)

This paper considers the hypothesis that changes in community structure through the control of the larvae maintenance and of the biological traits of the species mostly contribute to the spatio-temporal community pattern. This is supported by the results of the study, the changes to the macrobenthic community pattern deriving from two sampling periods in Gialova lagoon (SW Greece), carried out on a seasonal basis for two yearly periods: 1994-95 and 1998-99. Cognetti’s hypothesis that populations of the tolerant species occupying coastal marine habitats may belong to different species is another alternative hypothesis still to be tested. The importance of alternative management plans aiming at the amelioration of the hydrodynamic conditions of the lagoons and supported by continuous scientific monitoring is highlighted. Two canals bringing fresh water were opened at the beginning of the second sampling period, an intervention among others, suggested by the management plan proposed after the end of the first sampling period. The results show considerable variations in the values of the key environmental variables, for instance long periods with negative Redox potential values, decreased salinity and increased concentrations of the particulate organic matter and of the nutrients. The variables were correlated with the spatio-temporal community pattern, characterized during the second sampling period by: (i) greater dissimilarities among stations/seasons; (ii) disruption of the periodic trend observed on the seasonal scale; (iii) larger relative dissimilarities among the patterns stemming from the macrobenthos and the most abundant groups (polychaetes, molluscs, crustaceans); (iv) significant decrease in abundance or even the disappearance of several marine origin species, along with increased abundance in a few brackish-water species. All of the above changes in the community pattern are considered as early warning signals leading towards degradation, which has not yet been registered in the phylogenetic/taxonomic structure of the macrobenthic community.