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1.
Panayotis Lavvas 《Icarus》2009,201(2):626-633
By comparing observations from the Cassini imaging system, UV spectrometer, and Huygens atmospheric structure instrument, we determine an apparent radius of ∼40 nm, an imaginary index <0.3 at 187.5 nm and a number density of ∼30 particles cm−3 for the detached haze layer at 520 km in Titan's mesosphere. We point out that the detached haze layer is coincident with a local maximum in the measured temperature profile and show that the temperature maximum is caused by absorption of sunlight in the detached haze layer. This rules out condensation as the source of the layer. The derived particle size is in good agreement with that estimated for the size of the monomers in the aggregate particles that make up the main haze layer. Calculations of the sedimentation velocity of the haze particles coupled with the derived number density imply a mass flux , which is approximately equal to the mass flux required to explain the main haze layer. Because the aerosol size and mass flux derived for the detached layer agree with those determined for the main layer, we suggest that the main haze layer in Titan's stratosphere is formed primarily by sedimentation and coagulation of particles in the detached layer. This implies that high-energy radical and ion chemistry in the thermosphere is the main source of haze on Titan. 相似文献
2.
Two approaches for the modelling of turbulence in vegetated flows have been developed in the past. The “microscopic” approach
which is straightforward but limited to simple cases and the “macroscopic” approach which is based on Volume Average Theory
(VAT). In this study, aspects of Volume-Average (VA) analysis and modelling are investigated for turbulent vegetated flow
using computed three-dimensional results from the solution of the Reynolds-Averaged Navier-Stokes (RANS) equations around
a representative vegetal element. In particular (a) the VA transport equations for k and ε, based on VAT, are properly derived, (b) the Boussinesq hypothesis for the VA quantities, employed in 〈k〉-〈ε〉 turbulence models is tested, and (c) the values of the coefficients used in such turbulence models are assessed in comparison
with those used in the classical turbulence models. 相似文献
3.
Microseismicity and focal mechanisms at the western termination of the North Anatolian Fault and their implications for continental tectonics 总被引：4，自引：0，他引：4
Denis Hatzfeld Maria Ziazia Despina Kementzetzidou Panayotis Hatzidimitriou Dimitris Panagiotopoulos Kostas Makropoulos Panayotis Papadimitriou & Anne Deschamps 《Geophysical Journal International》1999,137(3):891-908
For seven weeks, a temporary network of 68 seismological stations was operated in Central Greece, in the region of Thessaly and Evia, located at the western termination of the North Anatolian Fault system. We recorded 510 earthquakes and computed 80 focal mechanisms. Seismic activity is associated with the NE–SW dextral North Aegean Fault, or with very young E–W-striking normal faults that are located around the Gulf of Volos and the Gulf of Lamia. The important NW–SE-striking faults bounding the Pilion, or the basins of Larissa and Karditsa, are not seismically active, suggesting that it is easier to break continental crust, creating new faults perpendicular to the principal stresses, than to reactivate faults that strike obliquely to the principal stress axes 相似文献
4.
Sofie Spatharis Sotiris Orfanidis Panayotis Panayotidis George Tsirtsis 《Estuarine, Coastal and Shelf Science》2011
The structure and diversity of sixteen macroalgal assemblages originating from two coastal locations in the Northern (Kavala gulf) and Central (Maliakos gulf) Aegean in Greece were explored by examining their relative abundance distributions (RADs) and fitting five stochastic niche-based models. A mechanistic interpretation of the underpinning assembly processes was attempted by relating the assumptions of the fitted models with available abiotic data corresponding to each assemblage. The random fraction niche-based model, assuming a random niche apportionment to species, was fitted to the majority of Maliakos assemblages characterized by more evenly distributed and speciose RADs, whereas the random assortment model, assuming no relationship among species abundance and niche size, was fitted to most of the Kavala assemblages characterized by steeper RADs with fewer species. Among the possible underlying mechanisms, wave exposure seems to play a key role in macroalgal assembly processes; however factors such as biogeography and hard substrate availability must be also taken into consideration. Short-term processes as changes in resource availability (nutrients and light), known as drivers of assembly rules in other primary producers (e.g. phytoplankton), do not considerably affect macroalgae in the oligotrophic Eastern Mediterranean, possibly due to their longer life spans. 相似文献
5.
Explicit finite‐element analysis for the in‐plane cyclic behavior of unreinforced masonry structures
A simple constitutive model is proposed for an in‐plane numerical analysis of unreinforced masonry structures, which are subject to cyclic loading, by using explicit dynamic procedures. The proposed model is implemented by using two‐dimensional plane‐stress finite elements. Three different constitutive relations that are based on the total strain in the global material system are used. Cracking and crushing are controlled through normal strains, whereas shear is controlled through shear strain. Separate hysteretic rules are adopted for each mode of damage. A numerical analysis of masonry walls that are subject to cyclic loading has demonstrated that the use of explicit procedures in conjunction with the proposed model results in an acceptable accuracy when compared with the experimental results. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
6.
Christos Makris Panagiota Galiatsatou Konstantia Tolika Christina Anagnostopoulou Katerina Kombiadou Panayotis Prinos Kondylia Velikou Zacharias Kapelonis Elina Tragou Yannis Androulidakis Gerasimos Athanassoulis Christos Vagenas Ioannis Tegoulias Vassilis Baltikas Yannis Krestenitis Theodoros Gerostathis Kostantinos Belibassakis Eugen Rusu 《Ocean Dynamics》2016,66(12):1603-1635
7.
Efthymios Lekkas Spyridon Mavroulis Panayotis Carydis Vassiliki Alexoudi 《Geotechnical and Geological Engineering》2018,36(4):2109-2142
On Tuesday, November 17, 2015 at 07:10:07 (UTC) a strong earthquake struck Lefkas Island (Ionian Sea, Western Greece) with magnitude Mw 6.4, depth of about 6 km and epicenter located 20 km southwest of Lefkas town. It was felt in Lefkas Island and the surrounding region and caused the death of two people, the injury of eight others, many earthquake environmental effects (EEE) and damage to buildings and infrastructures. Secondary EEE were observed in western Lefkas and classified as ground cracks, slope movements and liquefaction phenomena. Primary effects directly linked to surface expression of seismogenic source were not detected in the field. The maximum intensity VIIIESI 2007 was assigned to large-volume slope movements along western coastal Lefkas. Damage to buildings was mainly observed in villages located in Dragano-Athani graben arranged almost parallel to the northern segment of the Cephalonia Transform Fault Zone (Lefkas segment). Among structures constructed with no seismic provisions, the stone masonry buildings and monumental structures suffered most damage, while the traditional buildings of the area with dual structural system performed relatively well and suffered minor damage. Reinforced-concrete buildings were affected not so much by the earthquake itself but by the generation of secondary EEE. The maximum seismic intensities VIIIEMS-98 were assigned to villages located in Dragano-Athani graben due to very heavy structural damage observed on masonry buildings mainly attributed to the combination of the recorded high PGA values, the poor antiseismic design and construction of buildings and the geological and tectonic structure of the affected area. 相似文献
8.
George Kaviris Ioannis Fountoulakis Ioannis Spingos Christos Millas Panayotis Papadimitriou George Drakatos 《Acta Geophysica》2018,66(6):1341-1357
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. 相似文献
9.
Demetrios Cantzos Dimitrios Nikolopoulos Ermioni Petraki Panayotis Yannakopoulos Constantinos Nomicos 《Journal of Seismology》2018,22(6):1499-1513
The recent seismic activity in the eastern Aegean Sea, foregrounded by two major (ML?=?6.1 and ML?=?6.2) earthquakes occurring near Lesvos and Kos islands, respectively, is investigated in this work. Electromagnetic radiation measurements across different frequency bands and antenna orientations from a monitoring station in Agia Paraskevi, Lesvos Island, are analysed in order to reveal earthquake precursory signatures hidden in the electromagnetic data sequence. A straightforward, data-driven approach is employed in which day-to-day variations of the fractal characteristics of the measurements are adaptively monitored via a fractal spectral exponent similarity measure. The evolution of the fractal day-to-day variation in a 99-day period, which includes the two major earthquakes, shows a sustained, sudden increase lasting 1 to 4 days before every earthquake cluster of ML?=?4.0 and above. Most importantly, this day-to-day variation subsides shortly after and remains relatively low during the absence of earthquakes, thus alleviating the emergence of false alarms. 相似文献
10.
P. Galiatsatou P. Prinos 《Stochastic Environmental Research and Risk Assessment (SERRA)》2011,25(2):165-183
In the present paper a statistical model for extreme value analysis is developed, considering seasonality. The model is applied
to significant wave height data from the N. Aegean Sea. To build this model, a non-stationary point process is used, which
incorporates apart from a time varying threshold and harmonic functions with a period of one year, a component μ
w(t) estimated through the wavelet transform. The wavelet transform has a dual role in the present study. It detects the significant
“periodicities” of the signal by means of the wavelet global and scale-averaged power spectra and then is used to reconstruct
the part of the time series, μ
w(t), represented by these significant features. A number of candidate models, which incorporate μ
w(t) in their location and scale parameters are tried. To avoid overparameterisation, an automatic model selection procedure
based on the Akaike information criterion is carried out. The best obtained model is graphically evaluated by means of diagnostic
plots. Finally, “aggregated” return levels with return periods of 20, 50 and 100 years, as well as time-dependent quantiles
are estimated, combining the results of the wavelet analysis and the Poisson process model, identifying a significant reduction
in return level estimation uncertainty, compared to more simple non-stationary models. 相似文献