We detect and measure diffusion along resonances in a quasi-integrable symplectic map for different values of the perturbation parameter. As in a previously studied Hamiltonian case (Lega et al., 2003) results agree with the prediction of the Nekhoroshev theorem. Moreover, for values of the perturbation parameter slightly below the critical value of the transition between Nekhoroshev and Chirikov regime we have also found a diffusion of some orbits along macroscopic portions of the phase space. Such a diffusion follows in a spectacular way the peculiar structure of resonant lines. 相似文献
The accelerated Kepler problem (AKP) is obtained by adding a constant acceleration to the classical two-body Kepler problem.
This setting models the dynamics of a jet-sustaining accretion disk and its content of forming planets as the disk loses linear
momentum through the asymmetric jet-counterjet system it powers. The dynamics of the accelerated Kepler problem is analyzed
using physical as well as parabolic coordinates. The latter naturally separate the problem’s Hamiltonian into two unidimensional
Hamiltonians. In particular, we identify the origin of the secular resonance in the AKP and determine analytically the radius
of stability boundary of initially circular orbits that are of particular interest to the problem of radial migration in binary
systems as well as to the truncation of accretion disks through stellar jet acceleration. 相似文献
Mt. Nyiragongo is one of the most dangerous volcanoes in the world for the risk associated with the propagation of lava flows.
In 2002 several vents opened along a huge system of fractures, pouring out lava which reached and destroyed a considerable
part of Goma, a town of about 500,000 inhabitants on the shore of Lake Kivu. In a companion paper (Favalli et al. in Bull
Volcanol, this issue, 2008) we employed numerical simulations of probable lava flow paths to evaluate the lava flow hazard on the flanks of the volcano,
including the neighbouring towns of Goma (DRC) and Gisenyi (Rwanda). In this paper we use numerical simulations to investigate
the possibility of significantly reducing the lava flow hazard in the city through the construction of protective barriers.
These barriers are added to the DEM of the area as additional morphological elements, and their effect is evaluated by repeating
numerical simulations with and without the presence of barriers. A parametric study on barrier location, size, shape and orientation
led to the identification of barriers which maximize protection while minimizing their impact. This study shows that the highest
hazard area corresponding to eastern Goma, which was largely destroyed by lava flows in 2002, cannot be effectively protected
from future lava flows towards Lake Kivu and should be abandoned. On the contrary, the rest of the town can be sheltered from
lava flows by means of two barriers that deviate or contain the lava within the East Goma sector. A proposal for the future
development of the town is formulated, whereby “new” Goma is completely safe from the arrival of lava flows originating from
vents outside its boundaries. The proposal minimizes the risk of further destruction in town due to future lava flows. 相似文献
Compositional control on the annealing kinetics of fission-tracks (FT) in apatite requires routine measurement of sample grain composition. However, for practical reasons the bulk composition of analysed grains is not routinely measured and instead grain chlorine content or etch-pit dimensions are used to characterise a samples annealing behaviour. A more desirable approach is to measure crystallographic parameters (i.e. unit cell dimension) of a grain as these represent the summed effect of all substitutions and crystal defects. We show how Raman microspectrometry can be used as a routine non-destructive tool to obtain rapid measurement of the crystallographic structure of apatite grains etched for FT analysis. Variations of unit cell parameter a are found to correspond to a systematic variation of Raman shift in the range of 452–440 cm− 1 for measurements made on c-parallel sections of apatite where the direction of the polarized incident beam is parallel to the c axis. 相似文献
We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.
We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.
The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.
ZS9 is made out of 36 zones where earthquakes with Mw > = 5 are expected. It also assumes that earthquakes with Mw up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.
Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates. 相似文献