Considerations on the seismotectonics of the Northern Apennines

The Northern Apennines have been subdivided into homogeneous zones, on the basis of recent structural evolution and crustal structure, in which the earthquake distribution can find a coherent framework. These zones, whose physiography is in strict connection with their structure, are: the Internal Peri-Tyrrhenian Belt; the External or Main Belt; the Buried Belt; and the Pede-Alpine Homocline. Earthquake activity has a tendency to cluster along well-defined bands, particularly in the easternmost border of the Peri-Tyrrhenian Belt, as well as along the zone between the External Belt and the Buried Belt, i.e. along the Padanian margin of the Northern Apennines.A minimum of seismic activity seems to be correlated with some zones of the External Belt, as well as with the Late Tertiary and Quaternary magmatic province of Tyrrhenian Southern Tuscany.The fault-plane solutions are coherent with the structural picture.A tentative seismotectonic model of the Northern Apennines is discussed.     

Formation and evolution of inclined accretion disks in intermediate polars

The results of 3D modeling of the formation of the accretion disks of intermediate polars are presented. A model with misaligned rotation axes of accretor and the orbit is onsidered, in which it is assumed that the white dwarf has a dipolar magnetic field with its symmetry axis inclined to the whitedwarf rotation and orbital axes. The computations show that, in the early stages of formation of the disk, the action of magnetic field is able to create the initial (seed) inclination of the disk. This inclination is then supported mainly by the dynamical pressure of the flow from the inner Lagrangian point L₁. As themass of the disk increases, the inclination disappears. Under certain conditions, the disk inclination does not arise in systems with misaligned white-dwarf rotation and orbital axes. The influence of the magnetic field and asynchronous rotation of the accretor may result in the formation of spiral waves in the disk with amplitudes sufficient to be detected observationally.     

Effect of Aspect Ratio on the Monotonic Shear Behaviour: Micromechanical Interpretations

The effect of aspect ratio on the mechanical behaviour and micromechanics of two different assemblies during drained triaxial shearing are reported in this paper. Discrete element simulations are done on two different sets of assemblies—first assembly consists of particles with aspect ratio 1.0 and second assembly consists of particles with aspect ratio 1.5. A log normal distribution of particle size is adopted for both the samples. The constitutive behaviour of the assemblies and the evolution of the microstructure of the samples under shearing are closely examined and is related to the aspect ratio of the particles constituting the assembly. The spherical harmonic distributions of contact forces and contact normals along with 3-D histograms are plotted to give quantitative information of the variation of these parameters as the loading progresses. The results indicate that as the aspect ratio increases, there is an increase in the maximum deviatoric stress at the macroscopic level. At the microscopic level, the values of the anisotropic coefficients which are representative of the microparameters also show an increase in the magnitude for the assembly with higher aspect ratio particles.     

A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue

We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the ( M _V ,( B − V ) _T ) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Strömgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV ( RI )_C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of R _C I _C photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars.
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.     

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.     

Resolving the Azimuthal Ambiguity in Vector Magnetogram Data with the Divergence-Free Condition: Theoretical Examination

We demonstrate that the azimuthal ambiguity that is present in solar vector magnetogram data can be resolved with line-of-sight and horizontal heliographic derivative information by using the divergence-free property of magnetic fields without additional assumptions. We discuss the specific derivative information that is sufficient to resolve the ambiguity away from disk centre, with particular emphasis on the line-of-sight derivative of the various components of the magnetic field. Conversely, we also show cases where ambiguity resolution fails because sufficient line-of-sight derivative information is not available. For example, knowledge of only the line-of-sight derivative of the line-of-sight component of the field is not sufficient to resolve the ambiguity away from disk centre.     

Incoherent radiation from relativistic electrons with power energetic spectrum

It is shown that an incoherent high-frequency radiation from an ensemble of relativistic particles with the power energy distribution is described by a certain general expression which covers practically all the cases of particle radiation in random electromagnetic fields of cosmic radiation sources.     

用天然地震探测青藏高原中部地壳、上地幔结构

从西藏南部的定日、嘎拉至青海铜铁山的天然地震探测剖面,实际路线长约２０００ｋｍ,布设了约１１０台便携地震仪,记录了数百次远震和近震事件,采用多种方法进行了资料处理与解释．依据ＳＫＳ,ＰＫＳ,ｐｓ等横波分裂特征计算的青藏高原中部上地幔的地震各向异性表明：研究区各构造单元内的地震各向异性有明显变化,发现上地幔各向异性快速波的偏振方向与造山带的走向不完全一致．在雅江缝合线、崩错－嘉黎、唐古拉山口、昆仑山口几条断裂带处南、北各向异性出现显著的差异,而金沙江缝合线和班公－怒江缝合线的南、北则没有明显的各向异性变化．由Ｐ波走时残差,利用层析技术反演了４００ｋｍ深度内的速度图像,可以看出近地表１００ｋｍ范围内速度的不均匀变化与地表划分的构造单元很吻合,进一步佐证了青藏高原是由不同时期的微板块拼合而成的认识．在青藏高原中部１５０ｋｍ深度以下发现了多处低速区．在金沙江缝合带下方约２００ｋｍ深度处有一长２５０ｋｍ以上、延伸１５０ｋｍ的低速体,推测可能是一地幔柱．利用ＰＳ转换波划分的界面,显示出青藏高原北部具有低速层和高速层交替出现的地壳结构．