Multicausal environmental severity: A flexible framework for microtidal sandy beaches and the role of polychaetes as an indicator taxon     

Nafsika Papageorgiou  Christos Arvanitidis  Anastasios Eleftheriou 《Estuarine, Coastal and Shelf Science》2006,70(4):643

The aim of this paper is to study the macrofaunal community dynamics and the biological–environmental interactions in the mid- and sublittoral ecosystems of the microtidal Mediterranean sandy shores. Four sandy beaches, three on the island of Crete and one on the northwest coast of Italy were selected to investigate the spatial and temporal changes in the community structure and the associated environmental variables. The littoral zone, which has not been adequately studied in the Eastern Mediterranean, presents special interest not only from the scientific point of view but also for practical reasons of ecological management. The multivariate techniques revealed that the community pattern of the sandy beach macrofauna is mainly spatial rather than temporal. There are pronounced differences in species composition and abundance of the macrofaunal assemblages of the mid- and sublittoral zone. The multicausal environmental severity hypothesis appears to be valid for the sandy beach macrofaunal communities of the Mediterranean. The abundance and composition of the macrofaunal assemblages are highly variable and are affected by the synergistic effects of many environmental variables. The polychaete taxonomic assemblage structure closely follows the macrofaunal community pattern. Differences between the two patterns may arise from the different responses that polychaetes may show to the environmental stress.  相似文献   

Determination of S-wave Site Response in Anchorage, Alaska in the 1–9 Hz Frequency Band     

S. K. Nath  N. N. Biswas  M. Dravinski  A. S. Papageorgiou 《Pure and Applied Geophysics》2002,159(11-12):2673-2698

—?Site response was estimated at 19 sites in the Anchorage basin in south-central Alaska, using 15 local earthquakes recorded with good signal-to-noise ratio by a temporary weak motion network. The receiver-function-type horizontal-to-vertical spectral ratios (HVSR) were computed at 1–9 Hz frequency band and the resulting HVSR contour maps at 1, 5 and 9 Hz are presented here. The spatial site response distribution shows considerable variation from the foothills of the Chugach Mountains in the east to the western part of Anchorage. The site response increases by a factor of 3 and 3.5 at 1 and 5 Hz, respectively, from the area of older glacial deposits in the eastern part of the city to the area occupied by the Bootlegger Cove formation, particularly in the section adjoining Knik Arm. At 9 Hz, the variation of HVSR from the east to the west is smaller, approximately by a factor of 2. Moreover, the trend of the HVSR variation at 1 and 5 Hz shows good correlation with that of the soil class obtained from surface measurements of S-wave velocity in the 0–30?m depth range and available results on ground failure susceptibility of Anchorage.  相似文献   

Contribution of electrical tomography methods in geotechnical investigations at Mavropigi lignite open pit mine,Northern Greece     

Hamdan Hamdan  Nikos Andronikidis  George Kritikakis  Nikos Economou  Zacharias Agioutantis  Paul Schilizzi  Chrysanthos Steiakakis  Christodoulos Papageorgiou  Panagiotis Tsourlos  George Vargemezis  Antonis Vafidis 《Environmental Earth Sciences》2014,72(5):1589-1598

In this paper, the application of 2D and 3D electrical resistivity methods in geotechnical investigations is explored through a case study in Northern Greece. These two methods were employed at a lignite surface mining operation where fracture zones and discontinuities have been recently observed close to the pit boundaries. The main aim of the geophysical survey was to estimate the inclination of the contact between the Neogene and Schist/Carbonate formations near the southern limits of the pit, as well as to estimate the thickness of the carbonate rocks on top of the Schist formations to evaluate the stability of the southern slopes. Synthetic data were initially generated to help plan an efficient electrical tomography survey, in a region with complex geology and irregular terrain. Three configurations (Wenner–Schlumberger and dipole–dipole or pole–dipole) proved essential in such conditions and helped improving the resolution of the resistivity section. The sections were then calibrated by boreholes. Finally, the geophysical survey provided invaluable data regarding the geometry of the bedrock and possible faults, which was essential for the slope stability calculations.  相似文献   

EChOSim: The Exoplanet Characterisation Observatory software simulator     

E. Pascale  I. P. Waldmann  C. J. MacTavish  A. Papageorgiou  A. Amaral-Rogers  R. Varley  V. Coudé du Foresto  M. J. Griffin  M. Ollivier  S. Sarkar  L. Spencer  B. M. Swinyard  M. Tessenyi  G. Tinetti 《Experimental Astronomy》2015,40(2-3):601-619

EChOSim is the end-to-end time-domain simulator of the Exoplanet Characterisation Observatory (EChO) space mission. EChOSim has been developed to assess the capability of the EChO mission concept to detect and characterise the atmospheres of transiting exoplanets. Here we discuss the details of the EChOSim implementation and describe the models used to represent the instrument and to simulate the detection. Software simulators have assumed a central role in the design of new instrumentation and in assessing the level of systematics affecting the measurements of existing experiments. Thanks to its high modularity, EChOSim can simulate basic aspects of several existing and proposed spectrometers including instruments on the Hubble Space Telescope and Spitzer, ground-based and balloon-borne experiments. A discussion of different uses of EChOSim is given, including examples of simulations performed to assess the EChO mission.  相似文献   

On the efficiency of Runge-Kutta-Nystrom methods with interpolants for solving equations of the form Y″ = F(T,Y, Y′) over short timespans     

Ch. Tsitouras  G. Papageorgiou  T. Kalvouridis 《Celestial Mechanics and Dynamical Astronomy》1992,53(4):329-346

Runge-Kutta-Nystrom (RKN) codes for the solution of the initial value problem for the general second order differential system have been developed recently, although the methodology on which they are based was known many years ago. In this paper we try to examine the efficiency of several known general Runge-Kutta-Nystrom (GRKN) methods by posing some criteria of cost and accuracy. These methods supplied with the corresponding interpolants, have been applied to some problems of Celestial Dynamics. The results obtained show that these codes have a good response in the approximation of the solution of these problems.Department of Mechanics  相似文献   

Variations of the specific barrier model—part II: effect of isochron distributions     

Benedikt Halldorsson  Apostolos S. Papageorgiou 《Bulletin of Earthquake Engineering》2012,10(4):1321-1337

The specific barrier model (SBM) is a particular case of a composite earthquake source model where the seismic moment is distributed in a deterministic manner on a rectangular fault plane on the basis of moment and area constraints. It is assumed that the fault surface is composed of an aggregate of subevents of equal diameter, the ‘barrier interval’. Furthermore, the subevents are assumed to rupture randomly and statistically independent of one another as the rupture front sweeps the fault plane. In the formulation of the far-field source spectrum of the SBM the ‘arrival time’ of the seismic radiation emitted by each subevent is specified via a probability density function (PDF). In the SBM the subevents are assumed to be of equal sizes (an assumption relaxed in a companion paper, referred to as Part I) and the PDF of ‘arrival times’ is assumed to be uniform. In this study we investigate the effects of different PDFs of ‘arrival times’ on the far-field source spectrum of the SBM. Different PDFs of ‘arrival times’ affect the source spectra primarily at the intermediate frequency range (between the first and second corner frequencies). Such effects become more pronounced as the earthquake magnitude increases. The far-field spectrum of seismic energy observed/recorded at a site depends on the location of the site relative to the causative fault plane, the location of rupture initiation (hypocenter) and the onset times of the rupturing subevents. All the above factors are effectively taken into account by the ‘isochrons’, which vary with source-site geometry. We investigate the selection of the appropriate PDF of seismic energy arrival times at a given site by computing isochrons for a grid of stations surrounding the earthquake fault, represented by the SBM. We show that only for stations located in a direction normal to the fault plane is the assumption of uniform PDF of ‘arrival times’ valid. At other sites non-uniform PDFs of ‘arrival times’ are observed. We identify and categorize the prevalent types of PDFs by directivity (forward vs. backward vs. neutral) and source-site distance (near-fault vs. far-field), show examples in which we group the stations accordingly. We investigate the effects of the different PDF-groups on the SBM source spectrum. Selection of the appropriate PDF for a given source-site configuration when simulating strong ground motions using the SBM in the context of the stochastic method is expected to yield more self-consistent, and physically realistic simulations.  相似文献   

Residual slip of sliding blocks induced by near‐fault ground motions          下载免费PDF全文

Kristel C. Meza Fajardo  Apostolos S. Papageorgiou 《地震工程与结构动力学》2017,46(7):1203-1220

The response of a rigid block supported on a horizontally moving foundation through a dry‐friction contact is investigated to near‐fault ground motions. Such motions can be thought of as consisting of a coherent component (‘pulse’) and an incoherent component, which can be described as a band‐limited ‘random noise’. The equation of motion of this strongly nonlinear system is reduced to a normalized form that reveals important parameters of the problem such as the critical acceleration ratio. The response of the sliding block to a set of uniformly processed near‐fault motions, covering a sufficiently wide range of magnitudes, is evaluated numerically for selected discrete values of the acceleration ratio. For each value of the critical acceleration ratio, the numerically computed residual slips are fitted with a Weibull (Gumbel type III) extreme value probability distribution. This allows the establishment of regression equations that describe accurately design sliding curves corresponding to various levels of non‐exceedance probability. The analysis reveals that the coherent component of motion contributes significantly to the response of the sliding block. Furthermore, the relevant acceleration in specifying the critical acceleration ratio is the (normalized) amplitude, α_{H_pulse}, of the pulse and not the (normalized) amplitude of the incoherent component α_H. Finally, the incoherent component is described quantitatively in terms of the root‐mean‐square acceleration a_RMS, and an attempt is made to understand its influence on the response of the sliding block. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

A Discrete Wavenumber Boundary Element Method for study of the 3-D response 2-D scatterers     

Apostolos S. Papageorgiou  Duoli Pei 《地震工程与结构动力学》1998,27(6):619-638

A mathematical formulation of the 2·5D elastodynamic scattering problem is presented and validated. The formulation is a straightforward extension of the Discrete Wave number Boundary Integral Equation Method (DWBIEM) originally proposed by Kawase¹ for 2D scattering problems and subsequently extended to the 3D problem by Kim and Papageorgiou.² It is demonstrated that the Green's function which is appropriate for a boundary formulation of the 2·5D elastodynamic scattering problem is the one corresponding to a unit force moving on a straight line with constant velocity. Such a Green's function is derived in the present study. The formulation may be used to study the wavefields in models of sedimentary deposits (e.g. valleys) or topography (e.g. canyons or ridges) with a 2D variation in structure but obliquely incident plane waves. The advantage of a 2·5D formulation is that it provides the means for calculations of 3D wavefields in scattering problems by requiring a storage comparable to that of the corresponding 2D calculations. © 1998 John Wiley & Sons, Ltd.  相似文献   

Application of high-order Runge-Kutta methods in the magnetic-binary problem     

Th. Simos  T. J. Kalvouridis  G. Papageorgiou 《Astrophysics and Space Science》1988,147(2):271-285

Our intention in this paper is to present the results obtained from a systematic comparative study of several high-order Runge-Kutta methods as applied in a composite problem of celestial dynamics, namely the magnetic-binary problem.  相似文献   

Monitoring the physical forcing of resuspension events in the Thermaikos Gulf—NW Aegean during 2001–2003     

E. Tragou  V. Zervakis  E. Papageorgiou  S. Stavrakakis  V.Lykousis 《Continental Shelf Research》2005,25(19-20):2315

This work constitutes an attempt to assess the relative importance of natural versus anthropogenic forcing for sediment resuspension on the shelf of the Thermaikos Gulf (NW Aegean) through a 2-year experiment in the framework of the E.U. project INTERPOL. Four periods of different hydrographic characteristics were identified, including two periods of stronger vertical homogenisation and two of stratified conditions. The former periods were characterized by stronger near-bed currents, while relatively weak internal wave motions dominated the periods of stratification. The near-bed currents showed strong coherence with the wind during the period of full homogenisation, whereas during stratification periods the wind provided indirect forcing evident mostly in the internal-wave bands. The site was too deep for the surface waves to cause any sediment resuspension. In conclusion, the observed near-bed currents at the site of interest did not appear to produce significant local resuspension of sediment; notably, the current-turbidity correlation suggested a shoreward transport of suspended material. The large-scale turbidity variability was rather related to the onset of the trawling period in mid-October 2001 and the increased riverine discharge in late summer/autumn 2002.  相似文献