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231.
For 5 months before the 2001 Mt. Etna eruption, a progressive gravity decrease was measured along a profile of stations on
the southern slope of the volcano. Between January and July 2001, the amplitude of the change reached 80 μGal, while the wavelength
of the anomaly was of the order of 15 km. Elevation changes observed through GPS measurements during a period encompassing
the 5-month gravity decrease, remained within 4–6 cm over the entire volcano and within 2–4 cm in the zone covered by the
microgravity profile. We review both gravity and elevation changes by a model assuming the formation of new cracks, uniformly
distributed in a rectangular prism. The inversion problem was formulated following a global optimization approach based on
the use of Genetic Algorithms. Although it is possible to explain the observed gravity changes by means of the proposed analytical
formulation, the results show that calculated elevation changes are significantly higher than those observed. Two alternative
hypotheses are proposed to account for this apparent discrepancy: (1) that the assumptions behind the analytical formulation,
used to invert the data, are fallacious at Etna, and thus, numerical models should be utilized; (2) that a second process,
enabling a considerable mass decrease to occur without deformation, acted together with the formation of new cracks in the
source volume. 相似文献
232.
233.
Paola Morasca Luca Malagnini Aybige Akinci Daniele Spallarossa R. B. Herrmann 《Journal of Seismology》2006,10(3):315-333
In order to empirically obtain the scaling relationships for the high-frequency ground motion in the Western Alps (NW Italy), regressions are carried out on more than 7500 seismograms from 957 regional earthquakes. The waveforms were selected from the database of 6 three-component stations of the RSNI (Regional Seismic network of Northwestern Italy). The events, M
W ranging between 1.2 and 4.8, were recorded within a hypocentral distance of 200 km during the time period: 1996–2001. The peak ground velocities are measured in selected narrow-frequency bands, between 0.5 and 14 Hz. Results are presented in terms of a regional attenuation function for the vertical ground motion, a set of vertical excitation terms at the reference station STV2 (hard-rock), and a set of site terms (vertical and horizontal), all relative to the vertical component of station STV2.The regional propagation of the ground motion is modeled after quantifying the expected duration of the seismic motion as a function of frequency and hypocentral distance. A simple functional form is used to take into account both the geometrical and the anelastic attenuation: a multi-variable grid search yielded a quality factor Q(f) = 310f
0.20, together with a quadri-linear geometrical spreading at low frequency. A simpler, bi-linear geometrical spreading seems to be more appropriate at higher frequencies (f > 1.0 Hz). Excitation terms are matched by using a Brune spectral model with variable, magnitude-dependent stress drop: at M
w 4.8, we used Δσ = 50 MPa. A regional distance-independent attenuation parameter is obtained (κ0 = 0.012 s) by modelling the average spectral decay at high frequency of small earthquakes.In order to predict the absolute levels of ground shaking in the region, the excitation/attenuation model is used through the Random Vibration Theory (RVT) with a stochastic point-source model. The expected peak-ground accelerations (PGA) are compared with the ones derived by Ambraseys et al. (1996) for the Mediterranean region and by Sabetta and Pugliese (1996) for the Italian territory. 相似文献
234.
M. P. Rao Paolo Castracane Stefano Casadio Daniele Fuá Giorgio Fiocco 《Boundary-Layer Meteorology》2004,111(1):85-108
The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime. 相似文献
235.
Relative motion of the Adriatic with respect to the confining plates: seismological and geodetic constraints 总被引:3,自引:0,他引:3
236.
Daniele Nizzoli Giuseppe Castaldelli Marco Bartoli David Thomas Welsh Patricia Arriaga Gomez Anna Elisa Fano Pierluigi Viaroli 《Marine Ecology》2002,23(S1):297-306
Abstract. Dark respiration rates, dissolved inorganic nitrogen (DIN) fluxes and nitrification rates were measured at two sites in the microtidal Sacca di Goro lagoon in September 2000. DIN fluxes correlated with the biomass of the dominant macrofauna species (the amphipod Corophium spp. at station Giralda and the polychaete Neanthes spp. at station Faro). Respiration (> 6 mmol O2 m−2 h−1 ) and ammonium fluxes (> 80μmol N m−2 h−1 ) were higher at station Giralda despite the lower organic matter content (4.5 %) and lower macrofauna biomass (4 g AFDW m−2 ). At both sites ammonium fluxes were significantly correlated with the biomass of the benthic infauna, but Corophium stimulated ammonium NH4 + fluxes 3-fold compared to Neanthes. The amphipod also enhanced nitrification rates (> 300 μmol N m−2 h−1 ) due to the high density of its burrows, the higher NH4 + regeneration rates and the enhanced oxygen supply to the bacteria. 相似文献
237.
This paper presents a non-iterative approach to solve Kepler’s Equation, M = E − e sin E, based on non-rational cubic and rational quadratic Bézier curves. Optimal control point coordinates are first shown to be
linear with respect to orbit eccentricity for any eccentric anomaly range. This property yields the development of a piecewise
(e.g., 3, 4) solving technique providing accuracies better than 10−13 degree for orbit eccentricity e ≤ 0.99. The proposed method does not require large pre-computed discretization data, but instead solves a cubic/quadratic
algebraic equation and uses a single final Halley iteration in only a few lines of code. The method still provides accuracies
better than 10−5 degree for the near parabolic worst case (e = 0.9999) with very small mean anomalies (M < 0.0517 deg). The complexity of the proposed algorithm is constant, independent of the parameters e and M. This makes the method suitable for extensive orbit propagations.
Presented at the 7th Dynamics and Control of Systems and Structures in Space Conference, July 18–22, 2006, Greenwich, England. 相似文献
238.
Sixto R. Fernández-López Giulio Pavia Elisabetta Erba Myett Guiomar Maria H. Henriques Roberto Lanza Charles Mangold Davide Olivero Daniele Tiraboschi 《Swiss Journal of Geoscience》2009,102(2):271-295
The Bathonian Global Stratotype Section and Point (GSSP) is proposed at the base of limestone bed RB071 (bed 23 in Sturani
1967) in the Ravin du Bès Section (43° 57′ 38′′ N, 6° 18′ 55′′ E), Bas-Auran area, “Alpes de Haute Provence” French department.
The Ravin du Bès Section, as formal candidate GSSP for the base of the Bathonian Stage, satisfies most of the requirements
recommended by the International Commission on Stratigraphy: 1) The exposure extends over 13 m in thickness. At the Bajocian-Bathonian
transition, no vertical (bio-, ichno- or tapho-) facies changes, condensation, stratigraphic gaps or hiatuses have been recorded.
Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant
constraints. 2) There is a well-preserved, abundant and diverse fossil record across the boundary interval, with key markers
(ammonites and nannofossils) for worldwide correlation. The base of the Bathonian Stage and Zigzag Zone in Bas-Auran corresponds
to the first occurrence level of Gonolkites convergens Buckman, which coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils, as secondary global marker, are present in all beds and allow characterizing the Bajocian-Bathonian
transition. 3) Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray
data corroborate an Early Bathonian deepening half-cycle of second order. 4) The criteria of accessibility, conservation and
protection are assured by the “Réserve Naturelle Géologique de Haute Provence”. The Cabo Mondego Section (Portugal) is suggested
as the Bathonian auxiliary section and point (ASSP) within this GSSP proposal. 相似文献
239.
The study illustrates the critical role of accurate geological structural mapping to delineate crystalline aquifer zones more prone to high health risk due to elevated dissolved As in drinking wells.The analysis revisits the results from more than 1200 groundwater samples collected over ten years from domestic wells across the Tampere region(Finland).It is demonstrated that the highest dissolved As concentrations in the region(up to 2230 μg/L)are exclusively found near major faults and deformation zones(FDZs)detected via geophysical and geological surveys,and that a clear correlation exists between dissolved concentrations and the distance from the FDZs(r).Almost all values exceeding the drinking water limit(10 μg/L)occur at r8 km,while concentrations above 100 μg/L occur at r4 km.Solidphase As concentrations in bedrock show less dependency on FDZ than aqueous concentrations.This behavior is explained considering different mechanisms,which include enhanced sulfide oxidation and fracture connectivity,promoting preferential transport of dissolved As to FDZs and mixing of waters from different redox zones,mobilizing preferentially As(Ⅲ)or As(V).Fe hydro-oxides may also precipitate/dissolve preferentially because of FDZs,while residence time may influence the contact time between water and As-bearing minerals.It is concluded that the accurate mapping of FDZs,and in general of structural geology,provides an important preliminary information to identify where localized,sitespecific characterization of hydrogeology and geochemistry is more urgent to reduce As-related health risk from groundwater intake. 相似文献
240.
Diatom response to mid-Holocene climate change in Lago di Massaciuccoli (Tuscany,Italy) 总被引:2,自引:0,他引:2
A sedimentary sequence from the Mediterranean coastal basin of Lago di Massaciuccoli (Tuscany, Italy) was analyzed for diatoms,
covering two periods over the past ca. 7,000 years. The site was selected because it is situated in a sensitive position at
the limit between Mediterranean and Central European climates and biomes. Our focus is on the impact of accelerated human
activity during the recent past (water uptake in the catchment, sand extraction, wastewater discharge) and on a phase of evident
change between 6,600 and 5,400 cal. BP. The diatom record suggests fresh-water conditions and rather high lake levels until
ca. 6,000 years ago. The subsequent shift towards brackish conditions peaked at around 5,500 cal. BP. We relate this shift
to a pervasive change towards a drier climate that has been observed elsewhere in the Mediterranean and Northern African regions,
and stands in contrast to the shift towards a cooler and more humid climate in the nearby Alps (200–350 km distant) and in
central Europe. Pollen and charcoal records from a previous study on the same sedimentary sequence were used to gain additional
insights about the causes of the changes in the diatom assemblages and apply numerical methods to search for common trends
and correlations. 相似文献