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1.
Electromagnetic induction in the Vancouver Island region for a uniform inducing source field for 300 s period is investigated with the aid of three-dimensional (3-D) numerical and analogue model results and field site measurements. The thin sheet numerical model, based on the subducting Juan de Fuca plate analogue model ofDosso
et al., consists of a 5km thick non-uniform thin sheet (comprising the lateral conductivity contrasts arising from the land, the varying depth ocean, and the sediment) underlain by a four-layer conductive structure. The four-layer conductive structure beneath the non-uniform thin sheet simulates the effect of the Juan de Fuca plate subducting Vancouver Island. To examine the effects of the ocean channel depth between Vancouver Island and the British Columbia (Canada) mainland, numerical results were obtained for two channel depths (0 and 600 m). The results indicate that the channel plays an important role in the geomagnetic response in the central and inner coastal regions of Vancouver Island. The general agreement of the 3-D numerical model induction arrows with the analogue model and field site induction arrows for 300 s supports the premise of a layered conductive substructure dipping at a small angle, at most, beneath Vancouver Island.Lithoprobe Publication No. 311. 相似文献
2.
Measurements of geomagnetic induction responses for a laboratory analogue model, that includes a simulation of the subducting Juan de Fuca Plate, are compared with those obtained at three sites on Vancouver Island. Good agreement between model and field responses at the central Vancouver Island site is observed over the period range 3–60 min, while at the east and west coastal sites, good agreement is achieved only for periods greater than 20 min. At shorter periods, departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation, and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model. Nevertheless, the analogue model results are consistent with the premise of a Juan de Fuca plate, underlain by the conductive asthenosphere, subducting at a shallow depth beneath Vancouver Island. 相似文献
3.
The response of the electric and magnetic field variations over the San Juan Bay region of Vancouver Island is studied using a scaled laboratory analogue model. The laboratory frequencies simulate periods of 10 and 100 s in the geophysical problem. The model results indicate that, for both E- and H-polarization of the source field, induced current in the ocean is deflected around Cape Flattery and channelled into the Juan de Fuca Strait. With increasing period, the proportion of current channelled into the Strait decreases sharply. Induced current in the strait is also funnelled into San Juan Bay, a finger-shaped bay ca. 4 km wide and 7.5 km long, for both polarizations of the source field. The effect of the Bay on the field response is confined to the local region, within approximately 6 km of the centre of the Bay. Good agreement between field station and analogue model Hz results was obtained for the San Juan Bay. The behaviour of the Parkinson arrow for these two stations is examined with the aid of the analogue model results. 相似文献
4.
在地球物理问题中,有两种方法通常用于解释地磁和地电异常,这就是数字模型技术和实验室模拟模型法。实验室模拟模型对于分析不易求得数学解的问题非常有用,并且已经广泛用于研究复杂的地球物理问题。本文评述了国外特别是加拿大开展模拟模型研究的情况;描述了平面波、线电流、磁偶极等各种类型的场源模型;给出了该法在研究海岸效应、岛陆通道电流、海浪电磁效应和各向异性导体等方面的应用。模拟模型测量和数字计算的比较结果显示了非常好的一致性,这就进一步证实了模拟模型法对于研究复杂二维和三维感应问题的可靠性。该项研究工作对于了解天然电磁场源的性质、进行地球物理勘探以及研究地球地壳和地幔的电性结构都十分有益。 相似文献
5.
W. Nienaber H.W. Dosso V.R.S. Hutton 《Physics of the Earth and Planetary Interiors》1981,27(2):122-132
The results of a laboratory electromagnetic analogue model study, which employs a horizontal inducing field over a simple model of the British Isles region, delineate the location and frequency dependence of the major coast effect induction anomalies of the Scotland region. Contours of amplitudes, amplitude ratios, and in-phase and quadrature parts of the model field measurements are presented. The model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared.While major discrepancies occur between model and field Hz amplitudes, the Hz gradients across Scotland, which can be attributed to the coast-effect, are comparable in value, although sometimes reversed in sign. Superimposed on this coast-effect, the field data indicate the existence of current concentrations associated with the Great Glen and the Southern Uplands faults and possibly also of currents within the Scottish mainland near the east and west coasts. 相似文献
6.
V. Ramaswamy W. Nienaber H.W. Dosso F.W. Jones L.K. Law 《Physics of the Earth and Planetary Interiors》1975,11(2):81-90
The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean is investigated using both a three-dimensional finite-difference numerical method and a scaled analogue model method. The effect of a coastline located at some distance from the island is included in the study. The numerical model results and the scale model results are compared for various traverses across the island. The results indicate a high degree of compatibility between the two methods for studying problems involving three-dimensional conductivity structures. 相似文献
7.
H.W. Dosso V. Ramaswamy F.W. Jones D.J. Thomson 《Physics of the Earth and Planetary Interiors》1974,9(2):108-110
This work compares experimental analogue model measurements and finite-difference numerical calculations of the electric and magnetic fields for the cases of induction in a highly conducting plate and wedge embedded in a poorly conducting host earth. The results indicate very good agreement between the experimental and theoretical results and confirm the validity of the analogue and numerical methods for studying complex induction problems. 相似文献
8.
本文分析了魏格纳等求得的重力位球谐系数,得出n≤11阶系数与大尺度地幔对流相关,而12≤n≤25阶系数反映小尺度地幔对流;计算了小尺度地幔对流引起的华北地区壳下应力场,该应力场具有挤压性质,挤压中心在山西临汾—洪洞一带(111.5°E;36°N),应力大小为0.2—0.75×10~8达因·厘米~(-2),整个应力场具有规则的中心对称特征;壳下应力场的几何特征与震源机制解的结果、大震裂缝走向与错动方式、地震的空间分布等均彼此具有良好的一致性,对唐山与海城地震,这种一致性尤为明显。看来,似乎小尺度地幔对流及其造成的壳下应力场对包括目前这次强震高潮在内的华北地区地震的孕震过程起着控制作用 相似文献
9.
Isaac V. Fine Josef Y. Cherniawsky Alexander B. Rabinovich Fred Stephenson 《Pure and Applied Geophysics》2008,165(11-12):2019-2044
Alberni Inlet is a long and narrow fjord adjacent to Barkley Sound on the Pacific Coast of Vancouver Island, Canada. Port Alberni, at the head of the inlet, was affected in 1964 by the largest Pacific tsunami waves in Canadian history. We use observations and results from two numerical models to investigate the resonant characteristics of the region and amplification of tsunami waves in Barkley Sound and Alberni Inlet. The first model (A) was forced at its open boundary with a stationary autoregressive signal, similar to the observed background noise. The second model (B) used an initial sea-level deformation from a potential earthquake off California in the southern segment of the Cascadia Subduction Zone, producing transient tsunami waves. Spectral, cross-spectral and frequency-time (f-t) analyses of the observations were used to examine the resonant properties and topographic response of the local area. The respective results show large admittance functions over a wide 0.5–0.9 cph frequency band, implying a low Q factor but high amplification of arriving waves. This unusual behavior is a result of two effects: A quarter-wave resonance of the system for its fundamental Helmholtz mode and amplification due to the narrowing of the channel cross section from Barkley Sound to Alberni Inlet. The model A numerical results agree favorably with the observations, indicating an energetic resonant mode at frequency of ~0.53 cph (112 min), with its nodal line located near the entrance to Barkley Sound and amplification factor value close to 12. The results from the tsunami propagation model (B) yield spectral characteristics similar to those from the model A and from the observations. The maximum tsunami current speed for this scenario is 2.4 ms?1 in Sproat Narrows, which divides Alberni Inlet into two parts, while the largest computed wave height is 1.6 m in the northern Alberni Inlet, in the area of Port Alberni. 相似文献
10.
The movement of sediment through mountain river networks remains difficult to predict, as processes beyond streamflow and particle size are responsible for the entrainment and transport of bedload sediment. In deglaciated catchments, additional complexity arises from glacial impacts on landscape organization. Research to date indicates that the quantity of sediment stored in the channel is an important component of sediment transport in systems which alternate between supply and transport limited states, but limited long-term field data exist which can capture storage-transfer dynamics over a timescale encompassing episodic supply typical of mountain streams. We use a 45-year dataset with annual and decadal-scale data on sediment storage, channel morphology, and wood loading to investigate the spatial and temporal organization of storage in Carnation Creek, a previously glaciated 11 km2 catchment on Vancouver Island, British Columbia. Sediment is supplied episodically to the channel, including additions from debris flows in the early 1980s just upstream of the studied channel region. Analyzing the spatial and temporal organization of sediment storage along 3.0 km of channel mainstem reveals a characteristic storage wavelength similar to the annual bedload particle travel distance. Over time, two scales of variation in storage are observed: small-scale fluctuation of 3–10 years corresponding to local erosional and depositional processes, and larger scale response over 25–35 years related to supply of sediment from hillslopes. Complex relationships between storage and sediment transfer (i.e., annual change in storage) are identified, with decadal-scale hysteresis present in storage-transfer relations in sites influenced by hillslope sediment and logjams. We propose a conceptual model linking landscape organization to temporal variability in storage and to storage–export cycles. Collectively, our results reaffirm the importance of storage to sediment transport and channel morphology, and highlight the complexity of storage–transport interactions. © 2019 John Wiley & Sons, Ltd. 相似文献
11.
2017年8月8日四川阿坝州九寨沟发生M_W6.6地震,震源机制解显示该地震为左旋走滑型地震。对震中周围的GPS连续站观测资料进行处理,获得高频GPS动态形变和静态同震水平位移。震中100km范围内四川松潘和甘肃武都站观测到1 Hz动态形变。距离震中约69km的松潘站观测的同震水平位移为7.4mm。根据少量的GPS静态同震位移反演的同震破裂模型显示本次地震的最大滑动量为376mm,地震矩为7.25×1018 N·m,等效矩震级为M_W6.6。正演计算的同震三维形变场显示本次地震的最大水平位移可达4~5cm,垂直位移呈四象限分布,最大可达1.5cm,区域内10个流动GPS站可观测到同震形变。 相似文献
12.
琼中地磁台以磁报震方法探索 总被引：1，自引：1，他引：0
通过对海南低纬度地区地磁场垂直分量的特殊变化与海南岛及其邻近地区地震活动的对比分析,并结合实用化攻关成果,总结出一套利用琼中地磁台垂直分量１１年变化周期、相关系数Ｒ值变化曲线和日变化“双低点”异常预报海南岛及其邻近地区地震活动的方法。 相似文献
13.
根据软弱场地土上地铁车站结构大型振动台模型试验结果,以软件ABAQU S为平台,采用记忆型嵌套面黏塑性动力本构模型和动塑性损伤模型,分别模拟土体和车站结构混凝土的动力特性,建立了土-地铁车站结构非线性动力相互作用二维和三维有限元分析模型,对各种试验工况下地基土-地铁车站结构体系的地震反应进行了数值模拟,并与试验结果进行了对比。结果表明：二维、三维数值模拟与振动台模型试验结果基本一致,三维模型可更好地模拟软弱场地与地铁车站结构的动力相互作用及模型结构的动力反应。数值模拟结果和振动台试验结果可相互验证其可靠性。 相似文献
14.
Kyoko Ohashi Jinyu Sheng Keith R. Thompson Charles G. Hannah Harold Ritchie 《Continental Shelf Research》2009
A numerical shelf circulation model was developed for the Scotian Shelf, using a nested-grid setup consisting of a three-dimensional baroclinic inner model embedded inside a two-dimensional barotropic outer model. The shelf circulation model is based on the Princeton Ocean Model and driven by three-hourly atmospheric forcing provided by a numerical weather forecast model and by tidal forcing specified at the inner model's open boundaries based on pre-calculated tidal harmonic constants. The outer model simulates the depth-mean circulation forced by wind and atmospheric pressure fields over the northwest Atlantic Ocean with a horizontal resolution of 1/12°. The inner model simulates the three-dimensional circulation over the Gulf of St. Lawrence, the Scotian Shelf, and the adjacent slope with a horizontal resolution of 1/16°. The performance of the shelf circulation model is assessed by comparing model results with oceanographic observations made along the Atlantic coast of Nova Scotia and in the vicinity of Sable Island (on the Scotian Shelf) during two periods: October 2000–March 2001 and April–June 2002. Analysis of model results on Sable Island Bank indicates that tidal currents account for as much as ∼80% of the total variance of near-bottom currents, and currents driven by local winds account for ∼30% of the variance of the non-tidal near-bottom currents. Shelf waves generated remotely by winds and propagating into the region also play an important role in the near-bottom circulation on the bank. 相似文献
15.
利用中国科学院国家空间科学中心廊坊站(40.0°N,116.3°E)钠荧光多普勒激光雷达2011年至2013年共约82 h的钠原子数密度和垂直风观测数据,分析了廊坊地区中间层顶区域大气重力波耗散引起的钠原子输送特征.分析得到,90~100km处重力波耗散引起的平均钠原子垂直通量整体为负,钠原子向下输送,在93 km处达到最大负值-1.47×10~8m~(-3)m·s~(-1),85~90km处平均钠原子垂直通量为正,钠原子向上输送,但通量值随高度递减.钠原子垂直通量方向在90km处发生转变,垂直通量随高度的变化造成钠原子汇聚,汇聚效应引起的平均钠原子产生率最大值在91km处达到了1.40×10~8m~(-3)/h,该值超过了相同高度上模式计算流星烧蚀注入引起的钠原子产生率峰值,说明重力波耗散对钠层结构的形成具有重要影响.与美国SOR和Maui观测结果相比,平均钠原子产生率峰值大小相近,但出现高度不同,说明大气重力波耗散引起的物质输送具有显著的地域变化特征.研究结果可为大气物质输送理论的完善以及大气金属层物理模式的改进提供观测事实参考. 相似文献
16.
Susan E. Allen 《Surveys in Geophysics》2004,25(3-4):221-247
The shelf-break acts as a separator between the coastal ocean and the open ocean. Circulation (particularly deep near-bottom flow) is restricted from crossing the bathymetry. Eddies become elongated in the region of the shelf-break restricting exchange. An estimate of the horizontal eddy diffusivity over the shelf-break of less than 10m2s-1 is found from a numerical model. Various mechanisms are responsible for the weak cross-isobath flow that does occur. One is the increase of the Rossby number over small-scale topography such as submarine canyons. Along-shore flow (in the direction opposite to Kelvin wave propagation) generates upwelling through submarine canyons. A review of upwelling through submarine canyons is given. The deep cross-shelf flow generated by the canyons is shown to be as significant as the wind-driven upwelling in some regions. Examples for the reduction of flow across the shelf-break and for upwelling through canyons are taken from the West Coast of Vancouver Island. 相似文献
17.
Numerical Simulations of Tsunami Waves and Currents for Southern Vancouver Island from a Cascadia Megathrust Earthquake 总被引：1，自引：0，他引：1
Josef Y. Cherniawsky Vasily V. Titov Kelin Wang Jing-Yang Li 《Pure and Applied Geophysics》2007,164(2-3):465-492
The 1700 great Cascadia earthquake (M = 9) generated widespread tsunami waves that affected the entire Pacific Ocean and caused
damage as distant as Japan. Similar catastrophic waves may be generated by a future Cascadia megathrust earthquake. We use
three rupture scenarios for this earthquake in numerical experiments to study propagation of tsunami waves off the west coast
of North America and to predict tsunami heights and currents in several bays and harbours on southern Vancouver Island, British
Columbia, including Ucluelet, located on the west coast of the island, and Victoria and Esquimalt harbours inside Juan de
Fuca Strait. The earthquake scenarios are: an 1100-km long rupture over the entire length of the subduction zone and separate
ruptures of its northern or southern segments. As expected, the southern earthquake scenario has a limited effect over most
of the Vancouver Island coast, with waves in the harbours not exceeding 1 m. The other two scenarios produce large tsunami
waves, higher than 16 m at one location near Ucluelet and over 4 m inside Esquimalt and Victoria harbours, and very strong
currents that reach 17 m/s in narrow channels and near headlands. Because the assumed rupture scenarios are based on a previous
earthquake, direct use of the model results to estimate the effect of a future earthquake requires appropriate qualification. 相似文献
18.
A. V. Gorbatikov F. G. Montesinos J. Arnoso M. Yu Stepanova M. Benavent A. A. Tsukanov 《Surveys in Geophysics》2013,34(4):463-489
To study the deep structure of El Hierro Island, Canarian Archipelago, we have used a microseismic sounding method (MSM) based on the fact that heterogeneities of the Earth’s crust disturb the spectrum of the low-frequency microseismic field in their vicinity. So, at the Earth’s surface, the spectral amplitudes of definite frequency f above the high-velocity heterogeneities are decreasing, and above the low-velocity ones they are increasing. Moreover, the frequency f is connected with the depth of a heterogeneity H and the velocity of the fundamental mode of Rayleigh waves V R(f) through the relation H ≈ 0.4V R(f)/f. From these relations, the MSM lets us model the subsurface structure in a 3D context by inverting the amplitude-frequency spatial distribution of the microseismic field of low frequency. The validity of the method is shown through of numerical simulations and previous applications with known or verified solutions. This MSM is now used to invert the microseismic data registered in El Hierro Island. The obtained subsurface model reveals two large intrusive bodies beneath the island. Joint interpretation of microseismic and gravimetric data and their comparison with the available geological studies relate the central-eastern intrusive body to the early stage of the island formation. With respect to the western intrusive body, at the depths of 15–25 km, an area with lowest seismic velocities is identified, where we suggest that a modern magmatic reservoir is located. This reservoir could be associated with the recent submarine eruption in October 2011 and the accompanying seismic swarm, which started in July 2011. Several correlations between the shallowest structures identified by the gravity and MSM approaches are also found. Besides the numerical simulation and previous studies of this method, the correlation between gravity results, the MSM model, the geological information and the possible explanation of the features of the seismic swarm through the model obtained offer us a valid proof about the plausibility of the subsurface structures identified from MSM. 相似文献
19.
Site response analysis with two‐dimensional numerical discontinuous deformation analysis method 
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下载免费PDF全文 The capability of the numerical discontinuous deformation analysis (DDA) method to perform site response analysis is tested. We begin with modeling one‐dimensional shear wave propagation through a stack of horizontal layers and compare the obtained resonance frequency and amplification with results obtained with SHAKE. We use the algorithmic damping in DDA to condition the damping ratio in DDA by changing the time step size and use the same damping ratio in SHAKE to enable meaningful comparisons. We obtain a good agreement between DDA and SHAKE, even though DDA is used with first order approximation and with simply deformable blocks, proving that the original DDA formulation is suitable for modeling one‐dimensional wave propagation problems. The ability of DDA to simulate wave propagation through structures is tested by comparing the resonance frequency obtained for a multidrum column when modeling it with DDA and testing it in the field using geophysical site response survey. When the numerical control parameters are properly selected, we obtain a reasonable agreement between DDA and the site response experiment in the field. We find that the choice of the contact spring stiffness, or the numerical penalty parameter, is directly related to the obtained resonance frequency in DDA. The best agreement with the field experiment is obtained with a relatively soft contact spring stiffness of k = (1/25)(E × L) where E and L are the Young's modulus and mean diameter of the drums in the tested column. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
20.
A numerical method is used to calculate the electromagnetic fields associated with a three-dimensional conductivity anomaly. The source field is due to horizontal magnetic dipoles placed at two different positions with respect to the conductivity anomaly. The transfer functions and related perturbation and induction arrows associated with the fields are calculated and compared with the arrows obtained from a uniform source calculation. The results show the source effect on the induction arrows and indicate that the perturbation arrows provide a method of outlining the spatial extent of the anomaly. The transfer function calculations are made for both exact and approximate normal fields. In the transfer function calculation the anomalous fields are correlated with a normal field as suggested by Schmucker (1970) and Cochrane and Hyndman (1970). 相似文献