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In this paper we consider a spatially flat Friedmann-Robertson-Walker (FRW) cosmological model with cosmological constant,
containing a stiff fluid and a classical Dirac field. The proposed cosmological scenario describes the evolution of effective
dark matter and dark energy components reproducing, with the help of that effective multifluid configuration, the quintessential
behavior. We find the value of the scale factor where the effective dark energy component crosses the phantom divide. The
model we introduce, which can be considered as a modified ΛCDM one, is characterized by a set of parameters which may be constrained
by the astrophysical observations available up to date. 相似文献
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Source characterisation of low frequency events at Stromboli and Vulcano Islands (Isole Eolie Italy)
The discussion about the source of low frequency events generally wrecorded on active volcanoes is still open and needs deeper understanding of the phenomena involved in their generation. Most of the models view such a phenomenon as a source effect (oscillation of volcanic fluids in conduits or cracks), although a different explanation as a path or site effect exists. In the present paper we analysed 26 seismic signals recorded at Vulcano and 60 at Stromboli in order to put some constraints on the functional shape of the recorded signals. They evidence all the characteristics of the low frequency events. The spectral analysis reveals sharp peaks in the range 0.5–4 Hz, while the cepstra suggest that the signals are composed by a two-sine kind function. This suggestion is confirmed by the two-dimensional projections of the reconstructed phase space. The correlation dimension of the attractor is very close to 3 for both the volcanoes confirming the existence of a toroidal structure in the phase space. This implies that we can suggest as a model for the low frequency events source a physical mechanism very similar to a Duffing oscillator or any other quasiperiodic one. In particular in the cases of the analysed volcanoes we recognise two independent oscillations. 相似文献
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Dieter Heymann Franco Cataldo Mark h. Thiemens Roelof Fokkens Nico m. m. Nibbering Ronald d. Vis 《Meteoritics & planetary science》2000,35(2):355-361
Abstract— Toluene extracts from two (CSn)x photopolymers were examined with high‐performance liquid chromatography (HPLC), matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry, particle‐induced x‐ray emission (PIXE), and 12C(d,p)13C nuclear microprobing. the extracts contained elemental s and at least from 17 to 20 distinct cmsn compounds with m/z less than ～500 amu. Whereas H2S is the dominant S‐bearing compound of the normal jovian atmosphere, elemental S, CS, and CS2 were observed at Shoemaker‐Levy 9 cometary impact sites and at altitudes of the transiently disturbed jovian atmosphere where photodissociation and photopolymerization occur. It is uncertain whether the CS2 molecular densities were sufficiently large for both to occur, but photopolymerization could have occurred during larger impacts of Jupiter's history. Because the known stable CmSn compounds are yellow, orange and deep red, they could contribute significantly to the colors of the jovian clouds. 相似文献
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In this article we investigate and develop specific aspects of Friedmann-Robertson-Walker (FRW) scalar field cosmologies related to the interpretation that canonical and phantom scalar field sources may be interpreted as cosmological configurations with a mixture of two interacting barotropic perfect fluids: a matter component ρ 1(t) with a stiff equation of state (p 1=ρ 1), and an “effective vacuum energy” ρ 2(t) with a cosmological constant equation of state (p 2=?ρ 2). An important characteristic of this alternative equivalent formulation in the framework of interacting cosmologies is that it gives, by choosing a suitable form of the interacting term Q, an approach for obtaining exact and numerical solutions. The choice of Q merely determines a specific scalar field with its potential, thus allowing to generate closed, open and flat FRW scalar field cosmologies. 相似文献
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Mario Castellano Fabrizio Ferrucci Cataldo Godano Sebastiano Imposa Girolamo Milano 《Bulletin of Volcanology》1993,55(5):357-361
Data from a portable array of three-component digital stations, run at Mt Etna from 1988 to early 1990, highlight the seismic behaviour of the volcano before the 1989 eruption, one of the most significant in terms of energy of the last two decades. After a two-year period of weak and discotinuous seismicity, the depth of the seismically active volumes was observed to become shallower a few months before the volcanic event. The overall migration of the events, inferred by hypocentral locations and decreases of S-P time differences at two stations, agrees with other geophysical forerunners and allows further insights into the changes in the stress field leading to the eruption. 相似文献
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Following the increase in seismic activity which occurred near Isernia (Molise, Central Italy) in January 1986, a digital seismic network of four stations with three-component, short-period seismometers, was installed in the area by the Osservatorio Vesuviano. The temporary network had an average station spacing of about 8–10 km and, in combination with permanent local seismic stations, allowed the accurate determination of earthquake locations during an operating period of about one month. Among the 1517 detected earthquakes, 170 events were selected with standard errors on epicentre and depth not greater than respectively 0.5 and 1.5 km. The most frequent focal depths ranged between 4 and 8 km, while the epicentres distribution covered a small area NE of Isernia of about 10 km2. A main rupture zone could not be clearly identified from the spatial distribution of the earthquakes, suggesting a rupture mechanism involving heterogeneous materials. The activity was characterized by low energy levels, the largest earthquake, on January 18, 1986, havingM
D
=4.0. The time sequence of events and pattern of seismic energy release revealed a strong temporal clustering of events, similar to the behaviour commonly associated with seismic swarms. 相似文献
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We study the spatio–temporal scaling properties of the NCSN California catalogue for the period 1975–1995 (about 137 000 events) using a multifractal approach. The study is based on a new method of declustering earthquake catalogues based on looking for independent events. This technique makes use of the properties of the multifractal distribution of the inter-event times. A comparison test between our method and the Gardner and Knopoff (GK hereafter) approach reveals that our method produces a catalogue of independent events (monofractal Poissonian catalogue), while the GK declustered catalogue remains multifractal. The main difference lies in the assumption that the events can be considered to be correlated at large distances. The spatial properties of the declustered catalogue reveal a distribution of epicentres that is more complex than the distribution for the complete catalogue. We suggest that such results are very similar to the scaling properties of fully developed turbulence. 相似文献
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Landslides control the spatial and temporal variation of channel width in southern Taiwan: Implications for landscape evolution and cascading hazards in steep,tectonically active landscapes 
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下载免费PDF全文 Brian J. Yanites Nate A. Mitchell Joshua C. Bregy Grace A. Carlson Kirstyn Cataldo Margaret Holahan Graham H. Johnston Amelia Nelson Jeffery Valenza Matthew Wanker 《地球表面变化过程与地形》2018,43(9):1782-1797
Intense precipitation or seismic events can generate clustered mass movement processes across a landscape. These rare events have significant impacts on the landscape, however, the rarity of such events leads to uncertainty in how they impact the entire geomorphic system over a range of timescales. Taiwan is steep, tectonically active, and prone to landslide and debris flows, especially when exposed to heavy rainfall events. Typhoon Morakot made landfall in Taiwan in August of 2009, causing widespread landslides in southern Taiwan. The south to north trend in valley relief in southern Taiwan leads to spatial variability in landslide susceptibility providing an opportunity to infer the long‐term impact of such landslide events on channel morphology. We use pre‐ and post‐typhoon imagery to quantify the propagating impact of this event on channel width as the debris is routed through the landscape. The results show the importance of cascading hazards from landslides on landscape evolution based on patterns of channel width (both pre‐ and post‐typhoon) and hillslope gradients in 20 basins along strike in southern Taiwan. Prior to Typhoon Morakot, the river channels in the central part of the study area were about 3–10 times wider than the channels in the south. Following the typhoon, aggradation and widening was also a maximum in these central to northern basins where hillslope gradients and channel steepness is high, accentuating the pre‐typhoon pattern. The results further show that the narrowest channels are located where channel steepness is the lowest, an observation inconsistent with a detachment‐limited model for river evolution. We infer this pattern is indicative of a strong role of sediment supply, and associated landslide events, on long‐term channel evolution. These findings have implications across a range of spatial and temporal scales including understanding the cascade of hazards in steep landscapes and geomorphic interpretation of channel morphology. Copyright © 2018 John Wiley & Sons, Ltd. 相似文献