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191.
南沙群岛永暑礁岩体稳定性评价 总被引:2,自引:0,他引:2
永暑礁表层为松散的生物砂砾堆积,表层以下为礁灰岩,夹有2个沉积间断面。礁体呈岩盆结构,从外向内胶结程度依次降低。在综合研究了永暑礁自然环境特征、地层岩性以及岩体结构特征的基础上,认为永暑礁整体稳定性良好,不会发生整体倾覆的现象,浅层土体也不会发生剪切、滑移等工程地质问题。并利用非线性弹性本构模型,对永暑礁礁体地基与边坡在不同应力作用下发生变形与破坏的过程及机理进行了探讨,研究结果认为,在外部应力作用下,珊瑚礁内的应力状态将发生变化,局部出现应力集中,边坡出现拉应力。200kPa的荷载使礁体边坡产生的水平位移小于1cm,岩体边坡仍是稳定的,但最大垂直位移达到1.8cm,有可能出现珊瑚砂的整体剪切破坏。 相似文献
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Abstract. Living, symbiont-bearing, larger foraminiferids restricted to the photic zone incorporate mechanisms blocking solar radiation and water current transportation. They can be divided in two groups according to their wall structure: 1, Families with opaque test walls, reducing light penetration: Peneroplids inhabit shallow-water environments from the intertidal ( Peneroplis in large numbers) down to 40 m ( Dendritina , preferring sandy substrates). Soritids are restricted to subtidal environments with less water turbulence and range from the reef moat down to 60 m. Amphisorus and Marginopora are common down to 30 m, whereas Parasorites is restricted to deeper parts of the reef slope. Alveolinella can be found in the upper 40 m of the reef slope. 2. Families with hyaline test walls, adjusted for light penetration: amphisteginids inhabit the whole photic zone, demonstrating a correlation of test flattening with increasing depth. Calcarinids cling with their spines to firm substrates such as algal thalli or macroids. Baculogypsina is restricted to high energetic regions on the reef flat. Calcarina inhabits similar environments, extending its distribution range within fore reefs down to 80 m. Baculogypsinoides is a dominant calcarinid genus from 30 to 70 m depth. Within nummulitids, Heterostegina prefers hard substrates on the reef slope, while Assilina (= Operculina ) is frequent in deeper parts and is independent of substrates. Sandy bottoms from 30 to 60 m are inhabited by the last living representative of the genus Nummulites, N. venosus . The giant foraminifer Cycloclypeus , with test sizes up to 7 cm, is restricted to fore reef areas below 50 m down to the base of the photic zone. 相似文献
194.
Abstract. The sponge fauna of three neighbouring submarine caves of the Island of Majorca (Balcaric Islands) is studied along a light gradient; 45 species have been recorded and systematic data are given for those showing morphological, pigmentary, or spicule differences. Adaptation to darkness and precarious trophic conditions is also discussed. 相似文献
195.
L. I. Gonzalez de Vallejo R. Capote L. Cabrera J. M. Insua J. Acosta 《Marine Geophysical Researches》2003,24(1-2):149-160
A series of clastic dikes and tubular vents were identified in southern Tenerife (Canary Islands). These features are the result of seismic liquefaction of a Holocene sand deposit, as the consequence of a high intensity paleoearthquake. The peak ground acceleration (pga) and magnitude of the paleoearthquake generating these liquefaction features were estimated by back calculation analysis. A representative value of 0.30 ± 0.05 g was obtained for the pga. From this, an earthquake intensity of IX was estimated for the liquefaction site. Magnitude bound methods and energy based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M = 6.8. The zone in which the liquefaction structures are found has undergone tectonic uplift and is affected by two faults. One of these faults was responsible for displacing Holocene materials. Dating of the uplifted sand formation indicates an age of 10,081 ± 933 years, the liquefaction features ranging from this age to 3490 ± 473 years BP. This paleoearthquake was of much greater magnitude than those known historically. Faults with neotectonic activity are significant features that should be borne in mind when assessing the seismic hazards of the Canary Islands, presently considered as low and mainly of volcanic origin. 相似文献
196.
根据1990年春季至1991年冬季采自南日岛海域浮游动物样本,初步鉴定出98种浮游动物和12类浮游幼虫。桡足类种类和数量均占优势。精致真刺水蚤、中华哲水蚤、肥胖箭虫、中华假磷虾和短尾类氵蚤状幼虫为优势种。本文报道了浮游动物总生物量,总个体数和种类数的季节分布及平面分布,讨论其分布与环境的关系。 相似文献
197.
198.
J. Acosta E. Uchupi D. Smith A. Muñoz P. Herranz C. Palomo P. Llanes M. Ballesteros 《Marine Geophysical Researches》2003,24(1-2):59-90
The meso-scale (km) morphology of the well-studied volcanic rift zones on the Island of Hawaii is compared to the morphology of the lesser known rift zones of La Palma and El Hierro, Canary Islands. We find that there are both differences and similarities in their morphologic characteristics. In general, the rift zones on La Palma and El Hierro are shorter (a few tens of km in length) than those on Hawaii (ranging up to >100 km in length), perhaps reflecting both magma supply and composition. Many of the rift zones on Hawaii have well defined axial zones, both on-and offshore. In contrast, the rift zones on La Palma and El Hierro display various geometries ranging from linear ridges having smooth to irregular crests to structures with a broad fan-like morphology in plan view. The pronounced fanning may be a reflection of: 1) the stress field within the rift being insufficient to trap dikes within a narrow region, 2) dike injection and volcanism shifting laterally through time, 3) volcanoes building nearly one atop of another in the Canary Islands, superimposing the stress field of one structure on the other and thus yielding a more complex distribution of gravitational stresses, and 4) low rate of magma supply producing low magma pressures and thus randomly oriented dike injections. Irregularities and curvature along the axes of the rifts on La Palma and El Hierro may be a reflection of differences in the rate of magma production. Unlike the volcanoes on the Island of Hawaii there may be insufficient volumes of lavas erupted on La Palma and El Hierro to smooth out irregularities. The superposition of rifts from different volcanoes may also add to topographic irregularities in the Canary Islands, especially if eruption rates are low. 相似文献
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Theresearchontheremotesensing'sinformationtreemodeloftheNanshacoralisletsandreef'sspatialstructure¥LiuBaoyin;WangYanfengandHa... 相似文献