Modern microbialites and their environmental significance, Meiji reef atoll, Nansha (Spratly) Islands, South China Sea

Meiji (Mischief) coral atoll, in Nansha (Spratly) Islands, South China Sea, consists of an annular reef rim surrounding a central lagoon. On the atoll rim there are either protuberant 'motu' (small coral patch reefs on the rim of atoll) islets or lower sandy cays that contain modern microbialite deposits on the corals in pinnacles and surrounding bottoms of the atoll. Microbialites, including villiform, hairy, and thin spine growth forms, as well as gelatinous masses, mats and encrustation, developed on coral colonies and atoll rim sediments between 0 and 15 m deep-water settings. The microbialites were produced by natural populations of filamentous cyanobacteria and grew on (1) bulbous corals together with Acropora sp., (2) on massive colonies of Galaxea fascicularis, (3) on dead Montipora digitata, and (4) on dead Acropora teres, some hairy microbialite growing around broken coral branches. This study demonstrates that microbial carbonates are developed in coral reefs of South China Sea and indicates that microbial processes may be important in the construction of modern reef systems. The results have significance in the determination of nature and composition in microorganisms implied in the formation ancient microbialites, and permit evaluation of the importance of microbial deposits in mo-dern coral reefs and of 'microbialites' in biogeochemical cycles of modern coral reef systems. The re-sults also provide evidence of modern analogues for ancient microbialites in shallow-water settings, and combine with sedimentological studies of ancient microbialites to understand their controls.     

Numerical simulation of strong ground motion for the <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>8.0 Wenchuan earthquake of 12 May 2008

The Wenchuan earthquake of 12 May 2008 is the most destructive earthquake in China in the past 30 years in terms of property damage and human losses. In order to understand the earthquake process and the geo-morphological factors affecting the seismic hazard, we simulated the strong ground motion caused by the earthquake, incorporating three-dimensional (3D) earth structure, finite-fault rupture, and realistic surface topography. The simulated ground motions reveal that the fault rupture and basin structure control the overall pattern of the peak ground shaking. Large peak ground velocity (PGV) is distributed in two narrow areas: one with the largest PGV values is above the hanging wall of the fault and attributed to the locations of fault asperities and rupture directivity; the other is along the northwestern margin of the Sichuan Basin and caused by both the directivity of fault rupture and the amplification in the thick sediment basin. Rough topography above the rupture fault causes wave scattering, resulting in significantly larger peak ground motion on the apex of topographic relief than in the valley. Topography and scattering also reduce the wave energy in the forward direction of fault rupture but increase the PGV in other parts of the basin. These results suggest the need for a localized hazard assessment in places of rough topography that takes the topographic effects into account. Finally, had the earthquake started at the northeast end of the fault zone and ruptured to the southwest, Chengdu would have suffered a much stronger shaking than it experienced on 12 May, 2008. Supported by the U.S. National Science Foundation (Grant Nos. EAR 0738779 and OCE 0727919), the National Basic Research Program of China (Grant No. 2004CB418404), and partially by the National Nature Science Foundation of China (Grant No. 40521002)     

Observation and study on the whole process of cloud-to-ground lightning using narrowband radio interferometer

A narrowband radio interferometer has been developed and used to locate the entire sources of VHF radiations from a negative cloud-to-ground (CG) lightning discharge which contains 19 strokes. This system uses five antennas to form an array consisting of short- and long-baselines along two or- thogonal directions. The system error which comes from frequency conversion is reduced by phase detection through direct high frequency amplifying. An interactive graphic analysis procedure is used to remove the fringe ambiguities which exist inherently in interferometry and to determine the direction of lightning radiation sources in two dimensions (azimuth and elevation) as a function of time at a time resolution of microsecond orders. With the developed system, the whole progression process in time and space of a lightning flash can be reconstructed. In this paper, combining the synchronous data of electric filed change and VHF radiation, the whole processes of an example negative CG flash have been studied in detail. It is found that the preliminary breakdown event of the CG flash started from negative charge region and exhibited firstly a downward pregression and then an upward propagation. There were very intense and continuous radiations during stepped leaders which became much stronger when the first return stroke began. In contrast, there were less and only discrete radiations during dart leaders. Stepped leader and dart leader may transform to each other depending on the state of the ionization of the path. The progression speed of initial stepped leaders was about 105 ms?1, while that was about 4.1×106 and 6.0×106 ms?1 for dart leaders and dart-stepped leaders, respectively. M events produced hook-shaped field changes accompanied by active burst of radiations at their begin- nings. Followed these active radiation processes, M events appeared to contact finally into conducting main discharge channels. The mean progression speed of M events was about 7×107 ms?1, greater than that of the dart leaders and dart-step leaders. K events and attempted leaders were essentially the same as dart leaders except that they could not reach the ground and initiate return strokes.     

中国东部史前地震事件的的识别问题

中国东部地区气候湿润,物理化学风化作用强烈,加上地震周期长,人类活动频繁,地质事件遗迹难寻;另一方面,东部地区经济蓬勃发展,建(构)筑物和公众地震安全要求攀升.本文为解决这一日益尖锐矛盾作了相应综述和分析,并提出了一些建议:从经验预报统计样本局限性分析入手,综述史前地震识别标志和事件时序的分析研究现状,在此基础上提出了中国东部地区史前地震事件强化研究思路,即充分利用大规模考古发掘和工程所揭露的机遇,抢救性编录、分析.同时与考古、地质等相关部门联手,反演并重构全新世以来天然变形事件时序,用于弥补经验性预报的样本局限,从而科学预测地震趋势.作者的初步实践表明,这一思路是可行的,通过持之以恒的努力,有望达到预期目标.     

Ionospheric electromagnetic perturbations observed on DEMETER satellite before Chile M7.9 earthquake

Based on the ionospheric electromagnetic data observed on DEMETER satellite of France, the ionospheric electromagnetic signals were analyzed within 10 days before Chile M7.9 earthquake on November 14, 2007. It is found that, low frequency electromagnetic disturbances began to increase in a large scale of latitude, and reached to a maximum one week prior to the earthquake, and at about three days before the quake, the peak values shifted to lower latitude. Taking three days as a group, spatial images of a fe...     

Features and mechanism of neotectonic deformation of the Xishan fault system west of Urumqi

We have made careful field investigation and trench exploration to the Xishan fault system in west of Urumqi,and an integrated analysis in conjunction with data of deep seismic sounding.Our result suggests that under the SN-oriented compressive stress,the Xishan block moves steadily toward the Tianshan Mountains in south,resulting in southward thrust-slip and crustal shortening,particularly the southward thrust of the Xishan fault which serves as the main boundary in south.North of the Xishan fault are the ...     

Nonlinear seismic soil–pile–structure interactions: Shaking table tests and FEM analyses

In this paper, a soil–pile–structure model is tested on a shaking table subject to both a sinusoidal wave and the acceleration time history of the scaled 1940 El Centro earthquake. A medium-size river sand is compacted into a 1.7-m-high laminar rectangular tank to form a loose fill with a relative density of 15%. A single-storey steel structure of 2.54 ton is placed on a concrete pile cap, which is connected to the four end-bearing piles. A very distinct pounding phenomenon between soil and pile is observed; and, the acceleration response of the pile cap can be three times larger than that of the structural response. The pounding is due to the development of a gap separation between soil and pile, and the extraordinary large inertia force suffered at the top of the pile also induces cracking in the pile. To explain this observed phenomenon, nonlinear finite element method (FEM) analyses with a nonlinear gap element have been carried out. The spikes in the acceleration response of the pile cap caused by pounding can be modeled adequately by the FEM analyses. The present results suggest that one of the probable causes of pile damages is due to seismic pounding between the laterally compressed soil and the pile near the pile cap level.     

佘山地震台防雷技术研究与应用

在详细分析雷电灾害特征的基础上,阐述了防雷工作的主要技术手段。并针对地震台站的构成特点,研究了台站综合防雷系统的设计方法和技术措施。同时,以余山地震台为例,具体说明了防雷措施的应用方法及应用效果。     

欧拉梁动力反应初边值问题的微分求积解

动荷载作用下欧拉梁动响应的计算是一个初边值问题,通常很难得到解析解,传统数值方法一般是把空间和时间分别离散进行求解,计算相对复杂,效率也不高.针对分布动荷载作用下欧拉梁的振动偏微分方程,采用传统微分求积法,在空间和时间上同时进行离散;对于所有非0阶的初/边值条件,采用嵌入法在权系数计算中予以考虑.算例的数值结果与精确解的对比证明采用传统微分求积法处理此问题是可行的,而且是高效的.对于实际工程中的其他类似问题,该方法同样适用.     

基于组合权重的模糊物元模型在砂土液化势评价中的应用

目前,砂土液化问题仍是岩土工程界重要的研究方向.鉴于砂土液化评价的复杂性及其影响因素的不确定性和模糊性,将熵值理论与模糊集和贴近度相结合,建立了基于熵权与专家打分法所获权重的组合权重的模糊物元模型,有效地避免了权重分配困难的问题.该模型以一种新的方法进行砂土液化势的评价,并且结合实例应用,通过与现场标贯试验对比,进一步验证了该模型的合理、简便和实用.