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711.
712.
Bing Shen Shuhai Xiao Alan J. Kaufman Huiming Bao Chuanming Zhou Haifeng Wang 《Earth and Planetary Science Letters》2008,265(1-2):209-228
To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (δ13C, δ18O, δ34S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ~ 10-m-thick Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Available chemostratigraphic data suggest that the Tereeken diamictite is probably equivalent to the Marinoan glaciation. Our new data indicate that organic and carbonate carbon isotopes of the Zhamoketi cap dolostone show little stratigraphic variations, averaging ? 28.2‰ and ? 4.6‰, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ34SCAS composition varies between + 9‰ and + 15‰ in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ34SCAS rises ~ 5‰ and varies more widely between + 10‰ and + 21‰. The range of δ34Spy of the cap dolostone overlaps with that of δ34SCAS, but direct comparison shows that δ34Spy is typically greater than δ34SCAS measured from the same samples. Hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone. In this model, CAS was derived from shallow, oxic surface waters with moderate sulfate concentration and depleted in 34S due to the post-glacial influx of sulfur from continental weathering. In contrast, pyrite was derived from anoxic bottom waters (or a sulfate minimum zone) with low sulfate concentration and 34S enrichment due to long-term syn-glacial sulfate reduction. The rapid shift in CAS abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation. Comparison with other post-Marinoan cap carbonates shows significant spatial heterogeneity in δ34SCAS, which together with strong temporal variation in δ34SCAS, points to generally low sulfate concentrations in post-Marinoan oceans. 相似文献
713.
Earth’s free wobble is often referred to as the Euler wobble (for the rigid case) or the Chandler wobble for the real case.
In this study, we investigate the theory of the free wobble of the triaxial Earth and demonstrate that: (1) the Euler period
should actually be expressed by the complete elliptic integral of first kind, and (2) the trace of the free polar motion is
elliptic, with the orientations of its semi-minor and major axes being approximately parallel to the Earth’s principal axes
A and B, respectively. Numerical calculations show that, due to the triaxiality of the Earth, the spin rate ω
3 fluctuates with the semi-Euler/Chandler period, although its amplitude (about 10−19 rad/s) is rather small and beyond the present measurement accuracy; the tilt of the instantaneous spin axis (or the amplitude
of the free wobble), θ, has a fluctuation whose amplitude is around 0.34 milli-arcsecond (mas), which could be detected by present observations.
Thus, we conclude that the Earth’s triaxial nature has little impact on ω
3, but has an influence on the polar motion which should not be ignored. On the other hand, our study shows that there is a
mechanism of frequency–amplitude modulation in the Chandler wobble which might be a candidate to explain the correlation between
the amplitude and period of the Chandler wobble. We compare the theoretical polar parameters (m
1, m
2) with the observed values for the Chandler components obtained from the data EOP (IERS) C 04, and find that they coincide
with each other quite well, especially for recent years. In addition, a polar wander towards 76.7°W, which is in agreement
with previous results given by other scientists, is also obtained. 相似文献
714.
湖南太阳山隆起区断层活动性研究 总被引:1,自引:0,他引:1
湖南省太阳山地区是洞庭湖西缘一个晚新生代时期隆起的低山丘陵区,山体走向为北北东方向,自南向北斜列展布了太阳山低山区和凤凰山低山区,二者之间为北北东向大龙站谷地.通过对该地区现有6条断层活动性的地质调查,获得了位于大龙站谷地东侧的肖伍铺断层断错中更新世晚期,距今12.3±1.O万年地层的地质证据,为中更新世晚期至晚更新世早期有活动的断层,表现为逆冲活动的性质.根据岗市-河袱断层带发育的断层泥及其测年结果可知,该断层可能在中更新世中期有过活动.其余4条断层没有第四纪以来活动的地质地貌表现,属于前第四纪断层.上述研究结果对认识1631年太阳山63/4级地震的发震构造是有意义的. 相似文献
715.
This paper presents an analytical investigation on the seismic design and response of coupled wall structures that use unbonded post‐tensioned steel coupling beams. Both monolithic cast‐in‐place reinforced concrete wall piers and precast concrete wall piers are considered. Steel top and seat angles are used at the coupling beam ends for energy dissipation. The seismic design of prototype structures to achieve target displacement‐based performance objectives is evaluated based on nonlinear static and dynamic time history analyses. Additional recommendations are provided on shear design. Comparisons with ‘conventional’ structures that use embedded steel coupling beams as well as isolated walls with no coupling are provided. The results indicate that while the peak lateral displacements of unbonded post‐tensioned coupled wall structures are larger than the peak displacements of structures with embedded beams, the residual displacements are significantly reduced as a result of the restoring effect of the post‐tensioning steel. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
716.
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. 相似文献
717.
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) 相似文献
718.
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. 相似文献
719.
Ionospheric electromagnetic perturbations observed on DEMETER satellite before Chile M7.9 earthquake 总被引:1,自引:0,他引:1
Xuemin Zhang Jiadong Qian Xinyan Ouyang Xuhui Shen Jin’an Cai Shufan Zhao Institute of Earthquake Science China Earthquake Administration Beijing China 《地震学报(英文版)》2009,(3):251-255
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... 相似文献
720.
Chuanyong Wu Jun Shen Jun Li Zhiyong Xiang Jun Hu Yili Ya'er Earthquake Administration of Xinjiang Uygur Autonomous Region Urumqi China 《地震学报(英文版)》2009,(1):69-74
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 ... 相似文献