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131.
通过对多分量地震资料的分析,我们发现随着频率的增加横波分裂时差减小.对于深部接收的VSP数据来说快横波的偏振方向保持不变,而对于浅层接收的VSP数据来说偏振方向却存在一个最大可以达到20°的旋转.尽管多尺度随机分布微裂隙岩石物理模型已经成功地模拟并解释了横波分裂时差随频率变化的现象,却不能解释与频率相关的横波分裂.据推测,如果微裂隙的排列方向和大裂隙的排列方向不同,利用低频信息获得的偏振方向将指示裂隙主方向,而利用高频信息获得的偏振方向则指示微裂隙方向.在背景多孔隙介质中存在多组裂隙的情况下,推导出垂直入射条件下横波偏振方向的解析式,给出了系统研究横波在介质中传播的方法.研究结果表明,横波偏振方向会随着频率的变化而变化,并且在入射方位、角度一定的条件下,是裂隙方位和密度的函数,这些认识可能有助于揭示观测到的、依赖频率变化的横波偏振现象. 相似文献
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实验室可以产生两种振动模式的横波,一种是剪切振动,另一种是扭转振动,在各向同性介质中两模式的横波速度是相同的,但它们的振动特性不一样,前者表现出很强的偏振特性,后者为无偏振特性.实验测试表明无偏振特性的扭转波在各向异性介质中传播时也会出现两种速度不同的扭转波,速度值与剪切横波的快慢横波速度值一致.用扭转波换能器接收时,这快慢扭转波的波形振幅不受各向异性方位影响.通过两块均匀的各向异性样品,用实验观测揭示了各向异性介质中扭转波的一些传播特征. 相似文献
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Preliminary seismic anisotropy in the upper crust of the south segment of Xiaojiang faults and its tectonic implications 下载免费PDF全文
The Xiaojiang faults,striking north-to-south(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults. 相似文献
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Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian Møre coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2–7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget. 相似文献
139.
Peter Strogen 《Geological Journal》1988,23(2):121-137
The Carboniferous succession in southeast County Limerick, on the southeastern margin of the Shannon Trough, is Courceyan to mid-Namurian in age and over 1900 m thick. The lithostratigraphy is described in detail. Its most important aspect is the presence of two thick volcanic sequences, a Chadian one of the alkali basalt to trachyte suite and one of Asbian age dominated by limburgites and ankaramites. The associated Dinantian carbonates are of shelf or ramp facies throughout, and no fundamental division into shelf and basin facies occurs as in the Dublin and Craven Basins in early Viséan times. Rapid differential subsidence between this area and the Shannon Estuary began during deposition of the late Courceyan to early Chadian Waulsortian facies but was less marked in the remaining Viséan when much of the volcanic topography was preserved by rapid basinal subsidence. There was basinal inversion in the late Dinantian to lower Namurian, followed by renewed subsidence in mid-Namurian times. This contrasts with the continuous rapid subsidence of the area further west on the Shannon Estuary. This behaviour, together with a comparison of that of nearby Carboniferous basins such as the Dublin, South Munster, and Craven Basins, which lack substantial volcanic sequences, suggests an origin in a transtensional regime rather than one of simple crustal stretching. 相似文献
140.
通过对洱源-江川宽角反射地震资料的叠前偏移处理成像,得到了一个类似于近垂直入射多道地震数据的记录剖面.反射剖面图像与地壳速度结构共同揭示出地壳厚度由剖面西北端(约45 km)向东南端(约40 km)减薄.在洱源-楚雄西北附近深度约10 km处存在一组向东南倾斜的强反射震相,其东南约50 km处存在向西北倾斜的强射震相.易门断裂两侧地壳反射属性具有明显的差异,易门断裂之西北方向深度25~40 km处,中下地壳内存在两组强振幅,向东南方向上倾的地震反射同相轴,并被楚雄-建水断裂后期所错断,易门断裂之东南方向上,地壳内反射较为均一,未见特别明显的强间断面反射信息,这个反射结构被解释为印度板块东向俯冲与藏东缘地壳物质东向逃逸综合作用导致下地壳增厚和厚地壳变形的结果. 相似文献