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181.
P-SH conversion is commonly observed in teleseismic P waves, and is often attributed to dipping interfaces beneath the receiver. Our modelling suggests an alternative explanation in terms of flat-layered anisotropy. We use reflectivity techniques to compute three-component synthetic seismograms in a 1-D anisotropic layered medium. For each layer of the medium, we prescribe values of seismic velocities and hexagonally symmetric anisotropy about a common symmetry axis of arbitrary orientation. A compressional wave in an anisotropic velocity structure suffers conversion to both SV -and SH -polarized shear waves, unless the axis of symmetry is everywhere vertical or the wave travels parallel to all symmetry axes. The P-SV conversion forms the basis of the widely used 'receiver function' technique. The P-SH conversion occurs at interfaces where one or both layers are anisotropic. A tilted axis of symmetry and a dipping interface in isotropic media produce similar amplitudes of both direct ( P ) and converted ( Ps ) phases, leaving the backazimuth variation of the P-Ps delay as the main discriminant. Seismic anisotropy with a tilted symmetry axis leads to complex synthetic seismograms in velocity models composed of just a few flat homogeneous layers. It is possible therefore to model observations of P coda with prominent transverse components with relatively simple 1-D velocity structures. Successful retrieval of salient model characteristics appears possible using multiple realizations of a genetic-algorithm (GA) inversion of P coda from several backazimuths. Using GA inversion, we determine that six P coda recorded at station ARU in central Russia are consistent with models that possess strong (> 10 per cent) anisotropy in the top 5 km and between 30 and 43 km depth. The symmetry axes are tilted, and appear aligned with the seismic anisotropy orientation in the mantle under ARU suggested by SKS splitting. 相似文献
182.
183.
C. Peirce R. B. Whitmarsh R. A. Scrutton B. Pontoise F. Sage J. Mascle 《Geophysical Journal International》1996,125(3):781-795
During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge—the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running ∼ 150 km, ENE-WSW between 3°55'N, 3°21'W and 4°23'N, 2°4'W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ivoire Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin.
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data. 相似文献
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data. 相似文献
184.
185.
以单位为基础的中国城市内部生活空间结构─兰州市的实证研究 总被引:47,自引:8,他引:47
本文着眼于中国城市居民生活的最基本组织─单位,通过考察其形成与作用、日常生活类型及空间分布特征等,得出中国城市的内部生活空间结构由三层次构成:(1)由在世界各种城市地域组织中独具一格的单位构成的基础生活圈;(2)同质单位为主形成的低级生活圈;(3)以区为基础的高级生活圈。这种独特的生活空间结构是在社会主义计划城市下行政管理和生活居住规划的双重影响下形成的。 相似文献
186.
北京地区遥感环形构造信息提取与金矿预测 总被引:7,自引:2,他引:7
解译了北京地区航片上的环形构造,对环形构造解译图进行了数字化处理,运用数理统计方法分析了环形构造的空间分布特征及其与金矿化作用的关系,最后得到了环形构造密度异常与金矿分布之间具有正相关关系等新认识。 相似文献
187.
华北板块南缘栾川群研究 总被引:7,自引:0,他引:7
栾川群出露于东秦岭北部。在栾川群分布区域,古华北板块不同时代的地层往南逆冲,构成北秦岭的太华推覆体。栾川群存在于该推覆体内部的特定逆冲岩席之中,沿这一逆冲岩席追索,确定古华北板块南缘的栾川群是连续沉积的,而受后期推覆构造的改造,栾川群在推覆体前缘的分布是断续相连的。根据地层之间的接触关系、岩浆活动、沉积作用的研究,对栾川群的划分提出了新的看法,认为栾川群由三川组、南泥湖组、煤窑沟组及大红口组构成。栾川群形成于震旦纪晚期,由湖坪及潮下斜坡等滨浅海沉积物构成。栾川群的岩浆岩为具双众数特征的岩套,具典型的大陆裂谷岩浆岩特征。并认为中元古代以来,古华北板块南缘处于裂陷拉张环境。早期的裂陷形成了熊耳群、汝阳群、官道口群及竞坪群,晚期裂陷的过程形成了栾川群、陶湾群,其进一步发育导致以二郎坪群为代表的洋壳生成。 相似文献
188.
本文从区域构造背景出发,根据波拉湾煤勘探区地表及钻孔资料综合分析,建立了逆冲断层F1、F2及其配套的f1、f2……构造,理顺了错综复杂的构造格局,将推覆构造理论成功地用于援外生产实践。 相似文献
189.
天山地震活动区的深部构造特征 总被引:1,自引:0,他引:1
以地壳介质的P波速度分布和导电性为基础,描述了天山地区几个主要地震活动区域深部构造特征,地震大都发生在地壳内不同速度块体之间的过渡带附近;沿着天山的南缘和毗邻塔里木盆地的西昆仑、阿尔金等山前地带,壳内高导层由北向南、由东向西逐渐加深并增厚,它们与所在地区震源深度的分相相吻合。这种不稳定的地壳结构有长期受力情况下非常容易发生形变甚至破裂而引发地震。 相似文献
190.
首次发现报道了曼信一带海相火山岩微枕状构造的地质特征。主要根据野外第一手资料,分析探讨了曼信—老厂一带火山岩的时代,主要认识是:“依柳组”火山岩的主要形成于晚二叠世;老厂地区泥盆纪—中晚石炭世有少量基性—中性火山活动及三叠纪火山活动。 相似文献