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G. S. Heinson A. White L. K. Law Y. Hamano H. Utada T. Yukutake J. Segawa H. Toh 《Marine Geophysical Researches》1993,15(2):77-100
From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained. 相似文献
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2008年5月12日汶川MS8级地震的发生不是局部地区孤立的构造事件,研究汶川地震的孕震机制,应该把局部分析和区域分析相结合,关注地壳上地幔直至地幔过渡带的深部结构.基于近年来在东北、华北和汶川地震附近地区进行的深部结构电磁探测结果,结合地震学等其他资料,从太平洋板块的俯冲、印度板块的碰撞和松潘甘孜地块的推挤三个“层次”探讨分析汶川特大地震的成因.太平洋板块向亚洲大陆的俯冲作用,导致中国大陆东部地幔过渡带深度较普遍地存在着停滞的板片,它对汶川地震的影响不可忽视.印度板块与青藏高原的碰撞,使组成高原的各地块发生向北和向东的运动,各地块向东的运动作用于南北地震带中南段,影响到该区域的地震活动.松潘甘孜地块向四川地块的推挤,使松潘甘孜地块运动方向和龙门山断裂带形成“丁”字形结构,龙门山断裂带显示为较陡直的电性边界,加剧了汶川地震前的应力积累,可能是汶川地震发生的最直接的诱因. 相似文献
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With the prospect of studying the relevance of the topographic core-mantle coupling to the variations of the Earth’s rotation and also its applicability to constraining the core surface flow, we investigate the variability of the topographic torque estimated by using core surface flow models accompanied by (a) uncertainty due to the non-uniqueness problem in the flow inversion, and (b) variance originating in that of geomagnetic secular variation models employed in the inversion. Various flow models and their variances are estimated by inverting prescribed geomagnetic models at the epoch 1980. The subsequent topographic torque is then calculated by using a core-mantle boundary topography model obtained by seismic tomography. The calculated axial and equatorial torques are found subject to the variability of order 1019 and 1020 Nm, respectively, on which (b) is more effective than (a). The variability of the torque is attributed even to (a) and (b) of the large-scale flows (degrees 2 and 3). Yet, it still seems unlikely for the decadal polar motion with the observed amplitude to be excited exclusively by the equatorial topographic torque associated with any of reasonable core surface flow models. It is also confirmed that, with the topography model adopted here, the axial topographic torque on a rigid annulus in the core (coaxial with the Earth’s rotation axis) associated with any of reasonable flow models is larger by two orders of magnitude than the plausible inertial torque on such cylinders. This implies that any core surface flow model consistent with the topographic coupling does not exist, unless the topography model is appropriately modified. Nevertheless, the topographic coupling might provide not only a weak constraint for explaining the decadal LOD variations, but also the possibility to probe the core surface flow and the core dynamics. 相似文献
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2008年5月12日汶川Ms8级地震的发生不是局部地区孤立的构造事件,研究汶川地震的孕震机制,应该把局部分析和区域分析相结合,关注地壳上地幔直至地幔过渡带的深部结构.基于近年来在东北、华北和汶川地震附近地区进行的深部结构电磁探测结果,结合地震学等其他资料,从太平洋板块的俯冲、印度板块的碰撞和松潘甘孜地块的推挤三个"层次"探讨分析汶川特大地震的成因.太平洋板块向亚洲大陆的俯冲作用,导致中国大陆东部地幔过渡带深度较普遍地存在着停滞的板片,它对汶川地震的影响不可忽视.印度板块与青藏高原的碰撞,使组成高原的各地块发生向北和向东的运动,各地块向东的运动作用于南北地震带中南段,影响到该区域的地震活动.松潘甘孜地块向四川地块的推挤.使松潘甘孜地块运动方向和龙门山断裂带形成"丁"字形结构,龙门山断裂带显示为较陡直的电性边界,加剧了汶川地震前的应力积累,可能是汶川地震发生的最直接的诱因. 相似文献
15.
Joseph E. John Toh H. Fujimoto H. Iyengar R.V. Singh B.P. Utada H. Segawa J. 《Marine Geophysical Researches》2000,21(1-2):1-21
Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85°E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85°E Ridge along 14°N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across 85°E Ridge along 12°N latitude. Here we used the data from Annamalainagar Magnetic Obervatory (ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9°N latitude with land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding (VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 m and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 m) followed by a very low resistive (0.1–50 m) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features such as deep-sea fans of Bay of Bengal, the resistive 85°E Ridge and the sea water column above the seafloor stations. VGS and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal. 相似文献
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网式大地电磁（Network-MT,N-MT）法采用长数公里至数十公里的电话线为电极线测量电场,很难形成两条笔直且相互垂直的电极线,因此阻抗张量的计算不如大地电磁法中直接.本文依据阻抗张量的旋转规则提出了一种计算N-MT阻抗张量的简便算法.依据该算法计算了中国东北地区5个N-MT测站的阻抗张量,获得了基于阻抗张量的视电阻率、相位曲线和最佳主轴方位角分布图像,为我国东北N-MT资料的进一步处理和解释提供了基础性数据.此外,本文对比分析了朝阳测站中6条N-MT观测电极线上阻抗张量旋转值与观测值之间的差异,重点讨论了产生这种系统性偏差的各种因素,提出其主要因素可能来自“电场等效各向异性”效应,即测站附近的地壳内部存在与观测电极线尺度相比拟的横向非均匀构造,而测站各电极线沿不同方向跨越不均匀构造,此时各电极线上的电场分量不遵循同一电场矢量的分解准则,导致地表观测三角形内阻抗分量不满足统一的阻抗张量旋转规则. 相似文献