锚杆对节理围岩稳定性影响的离散元研究

本文针对巷道围岩多被节理切割的特点,利用离散元研究了全长锚杆对节理围岩稳定性的影响。结果表明,尽早锚固,有利于围岩的稳定;同时,巷道各部位围岩的变形与破坏具有明显协同性或关联性,应预以系统考虑对围岩的加固。     

Arch Filament Systems Associated with X-Ray Loops

Using multi-wavelength observations obtained with the Tenerife telescopes (VTT and GCT) and with the Yohkoh satellite, we observed new emerging flux with an associated arch filament system (AFS) in the chromosphere and bright X-ray loops in the corona. We observed the change of connectivity of the X-ray loop footpoints which may be at the origin of the occurrence of a subflare. Densities, gas and magnetic pressures of cold AFS and hot loops were derived and discussed. The extrapolation of the photospheric magnetic field observed with the GCT in a linear force-free field assumption (constant ) shows that this region, in spite of having roughly a global potential configuration, consists of two systems of arch filaments. We found these two systems best fitted with two sheared magnetic topologies of opposite values of ± 0.1 Mm^-1     

中国岩溶地区石漠化贫困问题研究进展

岩溶地区是实施乡村振兴战略的坚中之坚,是极易出现返贫的地区之一,有必要梳理总结岩溶地区消除石漠化贫困的历程、成果和存在的问题,巩固脱贫攻坚成果,夯实乡村振兴的理论基础。本研究以岩溶地区实施重大生态工程的时间节点为主线,参考各阶段前人取得的阶段性和标志性成果,系统梳理了石漠化贫困的相关概念和研究历程;然后对各阶段的研究热点进行整理,归纳总结了石漠化与农村贫困的关系、石漠化致贫机制、消除石漠化贫困的策略、防治工程减贫效益评价等4个方面的前人研究成果。研究表明:石漠化贫困与石漠化区贫困的界限不清,是造成石漠化防治工程和精准扶贫衔接脱节的原因之一,应突出石漠化区生态脆弱的特性,将生态建设有机融入巩固脱贫攻坚成果同乡村振兴有效衔接之中,实现生态、资源、人口和经济各要素的耦合协调发展;工作超前,理论基础研究落后的现实依然存在,应加强石漠化区乡村人地关系及相对贫困的研究,探究石漠化阻碍乡村振兴的制约机制;此外,在岩溶地区生态质量不断提升的背景下,应优化石漠化防治工程后评价体系,凸显社会效益和经济效益评价的重要性,总结经验、汲取教训,巩固岩溶地区脱贫攻坚成果,实现脱贫攻坚与乡村振兴有效衔接。     

Seismic constraints on magma chambers at Hekla and Torfajökull volcanoes,Iceland

Hekla and Torfajökull are active volcanoes at a rift–transform junction in south Iceland. Despite their location next to each other they are physically and geologically very different. Hekla is an elongate stratovolcano, built mainly of basaltic andesite. Torfajökull is a prominent rhyolitic centre with a 12-km-diameter caldera and extensive geothermal activity. The scope of this study is to examine the propagation of body waves of local earthquakes across the Hekla–Torfajökull area and look for volumes of anomalous S-wave attenuation, which can be evidence of magma chambers. So far the magma chamber under Hekla has been modelled with various geophysical means, and its depth has been estimated to be 5–9 km. A data set of 118 local earthquakes, providing 663 seismic rays scanning Hekla and Torfajökull, was used in this study. The major part, 650 seismograms, did not show evidence for S-wave attenuation under these volcanoes. Only six seismograms had clear signs of S-wave attenuation and seven seismograms were uncertain cases. The data set samples Hekla well at depths of 8–14 km, and south part of it also at 4–8 km and 14–16 km. Western Torfajökull is sampled well at depths of 4–14 km, eastern and southern Torfajökull at 6–12 km. Conclusions cannot be drawn regarding the existence of magma beyond these depth ranges. Also, magma volumes of smaller dimensions than about 800 m cannot be detected with this method. If a considerable molten volume exists under Hekla, it must be located either above 4 km or below 14 km. The former possibility seems unlikely, because Hekla lacks geothermal activity and persistent seismicity, usually taken as expressions of a shallow magma chamber. An aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of Torfajökull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber. Our results indicate that this volume cannot be molten to a great extent because S-waves travelling through it are not attenuated. Intense geothermal activity and low-frequency earthquakes are possibly signs of magma in the south part of Torfajökull, but a magma chamber was not detected there in the areas sampled by this study.Editorial responsibility: T. Druitt     

Volcanic tremor related to the 1991 eruption of the Hekla volcano,Iceland

Volcanic tremor at the Hekla volcano is directly related to eruptive activity. It starts simultaneously with the eruptions and dies down at the end of them. No tremor at Hekla has been observed during non-eruptive times. The 1991 Hekla eruption began on 17 January, after a short warning time. Local seismograph stations recorded small premonitory earthquakes from 16:30 GMT on. At 17:02 GMT, low-frequency volcanic tremor became visible on the seismograph records, marking the onset of the eruption. The initial plinian phase of the eruption was short-lived. During the first day several fissures were active but, by the second day, the activity was already limited to a segment of one principal fissure. The eruption lasted almost 53 days. At the end of it, during the early hours of 11 March, volcanic tremor disappeared under the detection threshold and was followed by a swarm of small earthquakes. At the start of the eruption, the tremor amplitude rose rapidly and reached a maximum in only 10 min. The tremor was most vigorous during the first hour and started to decline sharply during the next hour, and later on more gently. During the eruption as a whole, the tremor had a continuous declining trend, with occasional increases lasting up to about 2 days. Spectral analysis of the tremor during the first 7 h of the eruption shows that it settled quickly, within a couple of minutes, to its characteristic frequency band, 0.5–1.5 Hz. The spectrum had typically one dominant peak at 0.7–0.9 Hz, and a few subdominant peaks. Hekla tremor likely has a shallow source. Particle motion plots suggest that it contains a significant component of surface waves. The tremor started first when the connection of the magma conduit with the atmosphere was reached, suggesting that degassing may contribute to its generation.     

Estimating the Impact of Global Change on Flood and Drought Risks in Europe: A Continental, Integrated Analysis

Most studies on the impact of climate change on regional water resources focus on long-term average flows or mean water availability, and they rarely take the effects of altered human water use into account. When analyzing extreme events such as floods and droughts, the assessments are typically confined to smaller areas and case studies. At the same time it is acknowledged that climate change may severely alter the risk of hydrological extremes over large regional scales, and that human water use will put additional pressure on future water resources. In an attempt to bridge these various aspects, this paper presents a first-time continental, integrated analysis of possible impacts of global change (here defined as climate and water use change) on future flood and drought frequencies for the selected study area of Europe. The global integrated water model WaterGAP is evaluated regarding its capability to simulate high and low-flow regimes and is then applied to calculate relative changes in flood and drought frequencies. The results indicate large ‘critical regions’ for which significant changes in flood or drought risks are expected under the proposed global change scenarios. The regions most prone to a rise in flood frequencies are northern to northeastern Europe, while southern and southeastern Europe show significant increases in drought frequencies. In the critical regions, events with an intensity of today's 100-year floods and droughts may recur every 10–50 years by the 2070s. Though interim and preliminary, and despite the inherent uncertainties in the presented approach, the results underpin the importance of developing mitigation and adaptation strategies for global change impacts on a continental scale.     

Spectral investigation of chromospheric fine structure

H spectra and effectively simultaneous filtergrams were taken at the Fraunhofer Observatory on Capri with the 35 cm domeless Coudé. The spatial resolution of the 19 best spectra selected for analysis was estimated to be 1–2 arc sec. The comparison of several hundred H line profiles emitted by typical chromospheric structure elements with theoretical prediction yielded strong evidence to suggest that the chromosphere consists of two parts: A lower, rather uniform layer at rest superposed by clouds (condensations of great spatial variability) which constitute the well-known structure pattern of H filtergrams. For most image points the line-of-sight velocity, optical thickness, source function and Doppler broadening of these clouds could be determined. While the values of the latter three quantities were found to be similar to what Beckers (1968) has found for limb spicules the velocity of the bright and of the dark mottles is considerably smaller than to be expected if these features were the spicules as seen on the disk. However, our results do not rule out the possibility that the spicules rise at the centers of rosettes where they are difficult to detect.Mitteilung aus dem Fraunhofer Institut No. 105.