昭通“7.12”暴雨滑坡泥石流灾害初步研究

昭通“7.12”暴雨过程诱发了区域性滑坡泥石流的强烈活动,造成严重的灾害。本文通过对灾害现场考察,剖析了暴雨成因、时空分布,并阐明了“7.12”暴雨滑坡泥石流区域分布和成灾特征,并以彝良县为例,探讨了暴雨与滑坡泥石流形成关系。根据昭通地区暴雨滑坡泥石流发生规律和发展趋势,提出了减灾对策。     

上海金山深水槽两侧第四纪地层错断现象的分析

浅层人工地震剖面记录的金山深水槽两侧第四纪上部地层的“错断”现象,仍是在深水槽发展、演化过程中,由于原有的冲刷面、冲刷陡坎被后期淤积而形成的在成分和结构上存在着物性差异的界面,也有一部分是深水槽演化过程中沿槽壁发生的水下滑坡。深水槽内部与两侧上部的反射界面不是同一时代地层的显示。因此,在金山深水槽两侧不存在第四纪活动断层     

用孔壁崩落法测定冀中平原地区地壳应力方向

本文叙述了孔壁崩落应力测量方法。分析和讨论了冀中平原地区的10口深孔地层倾角和四臂测井资料,得出该地区的孔壁崩落的方向是V12°W。由此确定的最大水平主应力方向为N78°E。此外,还与其它几种应力测量方法所取得的结果进行比较,表明具有良好的一致性关系。最后根据在冀中平原地区所取得的应力方向的结果,说明该地区应力方向是受着同一构适应力场的作用,其动力源是来自太平洋板块向西俯冲的结果。     

Hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material

A survey of flows was conducted at a river confluence with coarse bed material. Bridges were installed on both tributaries, at the confluence and farther downstream on the receiving stream. At these stations, flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage. Hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises. At bankfull discharge, average velocity is 1.6 times higher at the confluence than on either tributary. Flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence. The development of a zone of high shear stress is also associated with the cell of high flow velocity. Flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs. The acceleration is not related to the development of flow separation zones as observed by Best and Reid (1984) for wide junction angles, nor is it explained by the reduction of the friction exerted by the banks. Acceleration is associated with the plan geometry of the confluence, with the lateral slopes which permit water to converge, and with a reduction in grain roughness at the confluence. Owing to the curvature of the tributary and to the acute angle of entry, relative power losses through the confluence decrease with increasing stages.     

鄂西渝东区构造裂缝发育特征及力学机制

针对鄂西渝东区隔档式褶皱发育的构造特点,结合区域构造演化史,分别以隔档式褶皱和正弦曲线几何形态的褶皱建立模型,用有限元法模拟了褶皱不同构造部位的应力场分布特征。褶皱外侧为张应力集中区,内侧为压应力集中区,且应力矢量与地层产状平行,二者以中部既无明显挤压、亦无明显拉张的中和面为分界。通过野外露头和岩心观察,总结了鄂西渝东区构造裂缝发育的三种形式:具多层介质的非能干层中发育顺层裂缝;向斜内侧、背斜外侧的能干层中发育高角度剪切缝;背斜外侧、内侧具多层介质的非能干层中发育顺层裂缝与高角度缝共生的网状缝。这种特征与模拟结果呈现较高程度的一致性,可根据应力场模拟成果和裂缝发育地层的能干性进行合理解释。复向斜中的隆起区可作为页岩气勘探的重要目标。     

Using fault displacement and slip tendency to estimate stress states

We suggest that faults in high slip tendency orientations tend to develop larger displacements than other faults. Consequently, faults that accumulate larger displacements are more likely to be reliable indicators of the longer term stress field and should be weighted accordingly in paleostress estimation. Application of a stress inversion technique that uses slip tendency analyses and fault displacements to interpret populations of coherent normal faults within the Balcones Fault System of south-central Texas provides stress estimates that are consistent with established regional stress analyses. Although the method does not require measurement of slip directions, these data, where available, and sensitivity analyses of the angular mismatch between measured slip directions and those predicted by inverted stress states provide high confidence in the stress estimates generated using slip tendency analyses. Close inspection of the fault orientation and displacement data further indicates that subpopulations of faults with orientations different from the regional pattern have formed in response to stress perturbations generated by displacement gradients on an adjacent seismic scale fault.     

Present-day stress tensors along the southern Caribbean plate boundary zone from inversion of focal mechanism solutions: A successful trial

This paper presents a compilation of 16 present-day stress tensors along the southern Caribbean plate boundary zone (PBZ), and particularly in western and along northern Venezuela. As a trial, these new stress tensors along PBZ have been calculated from inversion of 125 focal mechanism solutions (FMS) by applying the Angelier & Mechler's dihedral method, which were originally gathered by the first author and published in 2005. These new tensors are compared to those 59 tensors inverted from fault-slip data measured only in Plio-Quaternary sedimentary rocks, compiled in Audemard et al. (2005), which were originally calculated by several researchers through the inversion methods developed by Angelier and Mechler or Etchecopar et al.The two sets of stress tensors, one derived from geological data and the other one from seismological data, compare very well throughout the PBZ in terms of both stress orientation and shape of the stress tensor. This region is characterized by a compressive strike-slip (transpressional senso lato), occasionally compressional, regime from the southern Mérida Andes on the southwest to the gulf of Paria in the east. Significant changes in direction of the maximum horizontal stress (σ_H = σ₁) can be established along it though. The σ₁ direction varies progressively from nearly east-west in the southern Andes (SW Venezuela) to between NW-SE and NNW-SSE in northwestern Venezuela; this direction remaining constant across northern Venezuela, from Colombia to Trinidad. In addition, the σ_V defined by inversion of focal mechanisms or by the shape of the stress ellipsoid derived from the Etchecopar et al.'s method better characterize whether the stress regime is transpressional or compressional, or even very rarely trantensional at local scale.The orientation and space variation of this regional stress field in western Venezuela results from the addition of the two major neighbouring interplate maximum horizontal stress orientations (σ_H): roughly east-west trending stress across the Nazca-South America type-B subduction along the pacific coast of Colombia and NNW-SSE oriented one across the southern Caribbean PBZ. Meanwhile, northern Venezuela, although dextral strike-slip (SS) is the dominant process, NW-SE to NNW-SSE compression is also taking place, which are both also supported by recent GPS results.     

2003年2月24日新疆巴楚—伽师M_S6.8地震的应力触发作用研究

自1997-1998年新疆伽师强震群后,伽师震区余震活动渐趋平静。2003年1月4日伽师发生了5.4级地震,其后50天在距其20km左右在巴楚-伽师地区发生了MS6.8地震,其后发生了7次MS5.0以上强余震,序列的余震丰富、强度大。通过计算几次强余震破裂面上产生的静态库仑破裂应力变化,结合余震活动的时空演化特征定量探讨了强余震活动的可能机理。结果表明,巴楚-伽师MS6.8地震不仅有利于分布与主破裂面方向较一致的余震发生,而且触发了与其共轭方向的地震活动。     

The Use of Fossil Caddisfly Assemblages in the Reconstruction of Flow Environments from Floodplain Paleochannels of the River Trent, England

This paper examines the use of fossil larval trichopteran communities to investigate and describe the flow environment of a paleochannel network in the middle reaches of the River Trent floodplain (UK). Previous research utilising fossil insect communities (principally O. Coleoptera, O. Chironomidae) has focussed upon climate reconstruction. However, larval trichopteran communities (O. Trichoptera) also offer an aquatic signal, giving information regarding channel habitat structure and flow environments. The taxonomic diversity of each fossil community and the categorisation of each taxon into a known flow group, using the “LIFE” methodology, facilitates the reconstruction of the river flow conditions at each site based upon known faunal associations with mean flow velocity. A total of 49 trichopteran taxa were recorded from 17 paleochannels (n = 170 samples). Detrended Correspondence Analysis (DCA) identified a environmental gradient on the first DCA axis that reflected variability in flow, from rapid/fast flow to slow flowing and standing water habitats. In addition, two distinct faunal groups were identified on the second DCA axis characteristic of (i) small nutrient rich lentic habitats; and (ii) larger water bodies displaying features of both lentic, lotic and ephemeral habitats where marginal vegetation is abundant on a mineral substratum. The results demonstrate that larval Trichoptera can be used in paleolimnological research to reconstruct a flow signal and provide additional information regarding the aquatic habitat structure. Their wider use in describing the aquatic environment, in association with other proxies (e.g., Chironomidae and Coleoptera), may provide a more holistic understanding of floodplain paleoenvironment succession.     

A grid-search inversion method looking for the best classification of polyphase fault/slip data

A novel, grid search-based stress inversion method is developed in this paper to find the global minima in the solution region for the classification of fault/slip data into many single-phase subsets. Exhaustively repetitious grid searches are taken to deal with possible local minima, in a departure from existing grid search-based inversion methods. Two stopping rules, to stop at the abrupt change of the objective function or at the least change of the classification, are adopted in the method to look for the best classification. Much calculation time is saved by using a modified version of conventional grid search. The feasibility of this proposed method is demonstrated by applying it to two artificial examples and two real examples. However, enormous time in calculation is still needed in the case of a data set either with a large number of data or for a large number of assigned subsets.