Large-scale failures on domes and stratocones situated on caldera ring faults: sand-box modeling of natural examples from Kamchatka, Russia

Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected caldera rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.Editorial responsibility: J. Stix     

Last interglacial and Holocene climatic development in the Norwegian Sea region: Ocean front movements and ice-core data

Planktonic foraminiferal evidence suggests that the ocean front systems between Polar and Atlantic surface waters in the Norwegian Sea generally were located closer to Greenland during Oxygen Isotope Substage 5e than in the Holocene. During both these periods oscillations have occurred in the position of the fronts. In the western Norwegian Sea region, the substage 5e influence of warm Atlantic waters was interrupted by a return to polar conditions. These findings support both ice-core data and evidence from Europe that the last interglacial was a period of rapid climatic shifts.     

Oscillatory propagation of an ionization-shock front

Based on numerical simulations, we show that the oscillatory propagation of a plane-parallel ionization-shock front is possible for a typical dependence of the interstellar-medium cooling function on density and temperature. In this case, the oscillation amplitude of the shock position in the presence of an ionization front can be several times larger than its value for a single shock wave. The variations in neutral and ionized gas velocities attributable to oscillations are comparable in order of magnitude and agree with the random velocities observed in H II regions.     

The shearing instability of ion flow at the high-latitude tail of magnetopause boundary layer

Shearing instability of ion flow in an inhomogeneous plasma background in the magnetopause boundary layer at the high-latitude magnetotail is studied in this paper. By considering tail-aligned currents, we find that the instability excitation strongly depends on the disturbed wavelength. A quasi-critical wave number for instability is obtained. For relatively long perturbations, the instability tends to be excited at the inner edge of the boundary layer. The stable surface waves at the magnetopause and the K-H instability at the inner edge of the boundary layer can exist at the same time. This may contribute to the continuous transfer of momentum toward the magnetosphere.     

球面正压大气的不稳定与南亚夏季风爆发动力机制的探讨

通过球坐标正压原始方程谱点的计算,发现南亚夏季风爆发前有正压不稳定发生,不稳定罗斯贝波其最大增幅位于南半球的西风急流处,可向低纬传播并能在低纬激发出旺盛的积云对流,该积云对流可造成已失稳的大气基本场发生重大调整,从而导致南亚夏季风的爆发.     

聚合物钻井液防塌机理的试验研究

在试验研究的基础上对比了无机聚合物硅酸盐和有机聚合物——聚乙烯醇（PVA）、部分水解聚丙烯酰胺（PHPA）、非水解聚丙烯酰胺（PAM）和聚丙烯酸钾防塌能力的大小；由试验得出吸附成膜速度的快慢和保护膜的致密程度是决定钻井液体系防塌能力的关键因素；钻井液体系的防塌能力是无机聚合物和有机物质协同作用的结果；部分水解聚丙烯酰胺钻井液体系的防塌能力较非水解聚丙烯酰胺钻井液体系的防塌能力强；Na-CMC和PHP的加入可提高吸附膜的致密性及体系的防塌能力。     

Implementation of a Flaw Model to the Fracturing Around a Vertical Shaft

Summary This paper investigates whether a micromechanical model developed to investigate the relation between the basic failure processes at grain-scale and the macroscopic failure pattern can be applied to model the fracturing around large-scale excavations. The simulated fracture pattern around a vertical shaft is compared to the fracturing around a shaft at a depth of 3400 m. The simulations suggest that wedge-shaped zones, called dog-ears, are formed by a progressive splitting-like failure of the rock. The fractures forming these slabs nucleate in the zones subjected to the highest compressive stresses. Both shear and tensile mechanisms are responsible for the fracturing. The dog-ears deepen and widen as spalling continues. Received June 5, 2001; accepted September 24, 2002; Published online January 21, 2003 Acknowledgements The simulations presented in this paper form part of the PhD study of B. Van de Steen at the KULeuven. The authors wish to thank the sponsors of the Deepmine project for the permission to use the information pertaining to the fracturing around the South shaft at Western Deep Levels. Authors' address: André Vervoort, Katholieke Universiteit Leuven, Faculty of Engineering, Department of Civil Engineering, Kasteelpark Arenberg 40, B-3001 Leuven, Belgium; e-mail: andre.vervoort@bwk.kuleuven.ac.be     

关于复阻尼理论的几点注记

给出了复阻尼本构方程在全频域的表达式，完成了向时域表达式的转换。中指出，基于Fourier频域的本构不能用于求解由初值确定的自由振动，只有延拓至复频域，才具备概括自由振动与强迫振动两种工况的能力。为了满足热力学定律的约束，对复频域本构必须附加限制条件。离散计算时，由于未实施这一限制条件，一定会导致失稳。中对解决失稳问题提出了若干建议。     

圆形涡旋中的惯性重力内波不稳定和对称不稳定

用Boussinesq近似下的轴对称径向二维柱坐标系中的线性扰动方程组，讨论了圆形大气涡旋系统中扰动的惯性重力不稳定和对称不稳定。在环境为正压情形时，惯性重力内波不稳定的条件为#A(μj/R0)2N2+n2F2<0；当环境为斜压时，具有平行型扰动特征的惯性重力波发展的条件为Ri＊<1-[(3/2)+m]2，此时表现为对称不稳定。可见，惯性重力内波不稳定和对称不稳定都可作为台风、气旋一类圆形涡旋中扰动形成和发展的机制。     

试论土的应力应变模式与滑坡发育过程的关系

滑坡按其发育过程的特点可区分为突然失稳型和渐进破坏型。本文分析了这两类滑坡在破坏机理和发生条件上的差异，阐明了土的应力应变模式与滑坡发育过程的特点有非常密切的关系。