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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.
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
3.
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
4.
5.
The intersection of the Juan de Fuca ridge and Blanco fracture zone is characterized by unusually high amplitude magnetic anomalies (over 1500 nT) which appear to be associated with a body roughly 50 km in length and 20 km in width aligned along the fracture zone. Simple three-dimensional magnetic models indicate that this anomaly is probably caused by a highly magnetized block of material situated in the western end of the Blanco fracture zone near its intersection with the Juan de Fuca ridge. Rock magnetization studies of tholeiitic basalts dredged from this area confirm the presence of highly magnetized basalts near the ridge crest/transform fault intersection. These tholeiitic basalts are enriched in iron and titanium relative to “normal” oceanic tholeiites, apparently the result of extensive shallow fractionation involving olivine, plagioclase, and clinopyroxene. Magnetic model studies indicate that an average thickness of no more than 500 m of these iron-rich basalts is necessary to produce the observed anomaly pattern. Comparison of these basalts with samples previously dredged from the Juan de Fuca ridge crest suggests that these Fe-rich, highly magnetized basalts probably “leaked” out of the southernmost portion of the Juan de Fuca ridge. 相似文献
6.
V. Ramaswamy F.W. Jones H.W. Dosso W. Nienaber 《Physics of the Earth and Planetary Interiors》1980,22(1):60-67
Vertical magnetic fields for a three-dimensional numerical model, for a laboratory-analogue model, and from field stations for the Vancouver Island region of British Columbia, Canada, are compared. The numerical results are obtained using a three-dimensional finite-difference numerical technique employing a 25 × 25 × 25 mesh of grid points for a simplified mathematical model of the Vancouver Island region. The calculations are carried out for a source frequency of 0.004 Hz. The analogue model results for four traverses over the Island model and the field station values (obtained from transfer function analysis) for ten locations are those discussed previously by Nienaber et al. (1979a, b). General agreement exists between the numerical, analogue, and field station data, and comparison of results between these methods is important in three-dimensional electromagnetic induction studies of complex geomagnetic induction problems. 相似文献
7.
本研究做为电磁感应国际合作研究项目—EMSLAB计划的一部分,主要目的在于确定东太平洋年轻的海洋板块——Juan de Fuca板块及其周围地区岩石圈和软流圈的电性结构。 在对海底观测的大地电磁资料定性和定量分析的基础上,进行了二维数学模拟。在二维模型中,海洋地形和已知的表层构造被确定。通过改变模型的重要参数,评价响应函数和观测资料之间偏差的增加,检验模型的可靠性。结果表明,Juan de Fuca板块深部存在两个导电层,一个在40km,另一个在200km左右。这里岩石圈比其它较老板块的岩石圈薄 相似文献
8.
Stephen R. Hammond 《Bulletin of Volcanology》1997,58(8):617-627
Forty-three volcanoes located along the flanks of the Juan de Fuca Ridge were selected to study relationships between their
morphologies and off-axis magmatic processes. The volcanoes occur both in chains consisting of up to seven distinct cones
and isolated edifices. Nearly all of the volcanoes are circular, truncated cones with steep flanks and large, relatively flat
summit plateaus. In addition, most of these volcanoes also have prominent and distinctly offset calderas or craters. The most
striking characteristic of the volcanoes' morphology is that nearly all of their collapse structures are located on the sides
of the volcanoes which face the Juan de Fuca Ridge and many are breached with openings toward the ridge. A simple model based
on these observations accounts for these ridge-facing features. As plate motion transports a volcano away from its magma source
beneath the lithosphere, the volcano's magma supply conduits tend to lag behind. Eventually these conduits are abandoned and
ridgeward collapse structures are formed. It can be inferred from the model that, on average, individual volcanoes were active
for approximately 50 000 years and that most eruptions took place early in this interval. If most of the cone-building eruptions
occurred during the first thousand years or so, associated hydrothermal activity may have temporarily rivaled the present-day
yearly time-averaged hydrothermal output along the entire Juan de Fuca ridge axis.
Received: 1 September 1996 / Accepted: 13 January 1997 相似文献
9.
A deposit of Fe-rich, Al-poor, hydrothermal nontronite was recovered from the Juan de Fuca Ridge. Analyses show the deposit to be mineralogically and chemically similar to nontronite described at other oceanic localities. The deposit is located near the tip of a propagating segment of the Juan de Fuca Ridge. Rare earth elements and Sr isotopes indicate that the nontronite precipitated from seawater. A formation temperature of 57°C is suggested by oxygen isotopic composition. The low-temperature nontronite deposits apparently form from newly established hydrothermal systems associated with the propagating rift segment. More mature hydrothermal systems that deposit sulfide on the seafloor may develop from these low-temperature systems. 相似文献
10.
Numerical simulations with the Regional Ocean Modeling System (ROMS) are used to study the initial spin-up and the evolution of a mesoscale, topographically linked eddy under steady and variable wind conditions. The development of a pool of dense water on the southern Vancouver Island shelf allows cyclonic eddies formed by coastal upwelling off Cape Flattery to spread westward, ultimately contributing to the shelf-wide circulation known as the Juan de Fuca Eddy. This dense water arises through upwelling of water present in the underlying canyon system and tidal mixing over several shallow banks to the north. Tidal mixing is critical to the separation of the eddy from the coast. Although steady upwelling winds with a seasonal mean magnitude (combined with estuarine flow and tides) produce an eddy, only fluctuating winds with timescales and magnitudes typical of the region result in an eddy with a westward extent similar to seasonal observations. With each period of upwelling-favorable winds, newly upwelled water from the coast is entrained into the eddy which grows in size and moves westward. Wind events also significantly affect the baroclinic structure of the eddy. Specifically, during typical summer wind reversals, model surface drifters continue to move cyclonically within the eddy for several days after each downwelling wind event. Under upwelling-favorable wind conditions, model drifters exit the eddy to the southeast as the eddy and coastal upwelling fronts merge into a continuous southeastward shelf break jet. 相似文献
11.
Multi-scale modeling of Puget Sound using an unstructured-grid coastal ocean model: from tide flats to estuaries and coastal waters 总被引：1，自引：1，他引：0
Water circulation in Puget Sound, a large complex estuary system in the Pacific Northwest coastal ocean of the United States,
is governed by multiple spatially and temporally varying forcings from tides, atmosphere (wind, heating/cooling, precipitation/evaporation,
pressure), and river inflows. In addition, the hydrodynamic response is affected strongly by geomorphic features, such as
fjord-like bathymetry and complex shoreline features, resulting in many distinguishing characteristics in its main and sub-basins.
To better understand the details of circulation features in Puget Sound and to assist with proposed nearshore restoration
actions for improving water quality and the ecological health of Puget Sound, a high-resolution (around 50 m in estuaries
and tide flats) hydrodynamic model for the entire Puget Sound was needed. Here, a three-dimensional circulation model of Puget
Sound using an unstructured-grid finite volume coastal ocean model is presented. The model was constructed with sufficient
resolution in the nearshore region to address the complex coastline, multi-tidal channels, and tide flats. Model open boundaries
were extended to the entrance of the Strait of Juan de Fuca and the northern end of the Strait of Georgia to account for the
influences of ocean water intrusion from the Strait of Juan de Fuca and the Fraser River plume from the Strait of Georgia,
respectively. Comparisons of model results, observed data, and associated error statistics for tidal elevation, velocity,
temperature, and salinity indicate that the model is capable of simulating the general circulation patterns on the scale of
a large estuarine system as well as detailed hydrodynamics in the nearshore tide flats. Tidal characteristics, temperature/salinity
stratification, mean circulation, and river plumes in estuaries with tide flats are discussed. 相似文献
12.
A seafloor lava field was mapped within the summit caldera of Axial Volcano, Juan de Fuca Ridge, using SeaMARC I sidescan sonor and submersible observations. By analogy with similar subaerial features, we infer that several volcanic seafloor features here formed by the process of lava flow inflation. Flow inflation occurs within tube-fed lava flows when lava continues to be supplied to the interior of a flow that has ceased advancing, thus uplifting the flow's rigid surface and creating a suite of characteristic surface structures. Inflated lavas require a feeder lava tube or tube system connected to a remote lava source, and therefore we infer that inflated submarine lava flows contain lava tubes. Inflated flow features identified from sidescan sonar images elsewhere on Axial Volcano and within the axial valley of the southern Juan de Fuca ridge suggest that flow inflation is a widespread submarine volcanic process. 相似文献
13.
Stanley E. Church 《Earth and Planetary Science Letters》1976,29(1):175-188
Lead isotopic analyses have been repeated using silica gel for several samples from the Cascade Mountains which were previously analyzed by lead sulfide. The improved precision indicates that some of the scatter in the original data was due to thermal fractionation; however, the bulk of the data have not changed significantly. Two-point mixing lines are demonstrated for main cone-satellitic cone pairs from Glacier Peak, Mt. Baker and Mt. Shasta. Comparison with data on oceanic basalts from the Juan de Fuca and Gorda Ridge area indicates that hypothesis of mixing of mid-ocean ridge (MOR) basalt lead and “alkali basalt-like” lead from the oceanic crust is not tenable. Lead isotope analyses of pre-Astoria Fan sediments from DSDP Leg 18 sites and from the Eocene Tyee Formation indicate that the sedimentary continental detritus from the North American continent has the correct lead isotopic composition to be the continental component necessary to account for the Cascade Mountains lead isotopic array by mixing with Juan de Fuca-Gorda Ridge MOR basalts. However, from recent work on the structure of oceanic trenches by Karig and Sharman (1975), it does not appear that subduction of sediments is the rule. A model of crustal contamination and/or assimilation at the crust/mantle interface is the preferred explanation for the lead isotopic data from the Cascade Mountains. 相似文献
14.
The behaviour of the magnetic field variations over the North China-Korea coastal region is studied with the aid of a scaled laboratory analogue model. The model source frequencies simulate naturally occurring geomagnetic variations of 3–60 min periods. In-phase and quadrature magnetic Bx, By and Bz field measurements for the modelled region are presented for E- and B-polarizations. Large anomalous in-phase and quadrature model magnetic fields are observed over the Korea-Japan strait for E-polarization and over the Bohai strait for B-polarization due to current channelling through the straits. Large responses of the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period. In general, the effects of peninsulas, straits, bays and irregularities in the coastlines play an important role in the magnetic field responses both on-shore and off-shore for this complex North China-Korea coastal region. Model and field site induction arrows are compared for three sites west of Bohai Bay. 相似文献
15.
Alexander B. Rabinovich Richard E. Thomson Isaac V. Fine 《Pure and Applied Geophysics》2013,170(9-10):1529-1565
The major (M w = 8.8) Chilean earthquake of 27 February 2010 generated a trans-oceanic tsunami that was observed throughout the Pacific Ocean. Waves associated with this event had features similar to those of the 1960 tsunami generated in the same region by the Great (M w = 9.5) 1960 Chilean Earthquake. Both tsunamis were clearly observed on the coast of British Columbia. The 1960 tsunami was measured by 17 analog pen-and-paper tide gauges, while the 2010 tsunami was measured by 11 modern digital coastal tide gauges, four NEPTUNE-Canada bottom pressure recorders located offshore from southern Vancouver Island, and two nearby open-ocean DART stations. The 2010 records were augmented by data from seven NOAA tide gauges on the coast of Washington State. This study examines the principal characteristics of the waves from the 2010 event (height, period, duration, and arrival and travel times) and compares these properties for the west coast of Canada with corresponding properties of the 1960 tsunami. Results show that the 2010 waves were approximately 3.5 times smaller than the 1960 waves and reached the British Columbia coast 1 h earlier. The maximum 2010 wave heights were observed at Port Alberni (98.4 cm) and Winter Harbour (68.3 cm); the observed periods ranged from 12 min at Port Hardy to 110–120 min at Prince Rupert and Port Alberni and 150 min at Bamfield. The open-ocean records had maximum wave heights of 6–11 cm and typical periods of 7 and 15 min. Coastal and open-ocean tsunami records revealed persistent oscillations that “rang” for 3–4 days. Tsunami energy occupied a broad band of periods from 3 to 300 min. Estimation of the inverse celerity vectors from cross-correlation analysis of the deep-sea tsunami records shows that the tsunami waves underwent refraction as they approached the coast of Vancouver Island with the direction of the incoming waves changing from an initial direction of 340° True to a direction of 15° True for the second train of waves that arrived 7 h later after possible reflection from the Marquesas and Hawaiian islands. 相似文献
16.
We have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution. Using this technique, we have deciphered the evolutionary history of the Juan de Fuca area in the northeast Pacific, and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates. 相似文献
17.
Weiming Zhou Rob Van der Voo Donald R. Peacor 《Earth and Planetary Science Letters》1997,150(3-4):353-362
High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts. 相似文献
18.
利用收集到的各种来源共计7 959组的地震各向异性观测数据和21 750组应力场数据,结合板块绝对运动模型计算给出的各板块的运动规律,分别统计分析了板块运动与地震各向异性及应力场的相关性,并对板块运动对地震各向异性及应力场特征产生的影响进行了分析． 统计结果表明,阿拉伯、 加勒比、 胡安德富卡、 北美、 纳兹卡、 太平洋和南美板块上地震各向异性与板块运动均具有较好的相关性,而非洲、 南极洲、 澳大利亚、 欧亚、 印度和菲律宾板块上二者的相关性则相对较差. 讨论分析发现,板块运动拖动软流圈流动、 橄榄岩晶格优选方位、 化石各向异性和地幔流动或岩石圈流动等因素均在一定程度上控制并影响着地震各向异性与板块运动的一致性． 而板块基底拖曳力、 洋脊推力、 浮力作用和碰撞及俯冲作用等多种因素共同制约了板块运动与应力场的相关性,使得非洲、 可可斯、 欧亚、 胡安德富卡、 北美、 纳兹卡、 菲律宾和南美板块上二者的相关性较好,其它板块上其相关性则较差． 对于俯冲带地区,由于俯冲机制的复杂性和软流圈、 岩石圈地幔流动方向的不确定性,其板块运动与地震各向异性及应力场的相关性图像表现复杂,需要结合具体的俯冲带构造进行近一步研究． 相似文献
19.
The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate. 相似文献
20.
In 1996, long-term sealed-hole hydrological observatories with subseafloor temperature and pressure sensors were installed in four cased holes drilled by the Ocean Drilling Program into sedimented young oceanic crust east of the Juan de Fuca Ridge. Data recovered over a year later showed that all four holes displayed temperature profiles indicative of vertical fluid flow immediately prior to their being sealed. Warm water was being produced from basement in two cases, and cool ocean bottom water was being drawn into basement at the others. Linear flow rates of 60–200 m/h were estimated from the perturbation of the temperature profiles relative to undisturbed geothermal gradients at the sites. The pressure differentials driving the flow were also measured at the time of the observatory installations, allowing estimates of permeabilities of the upper crustal sections penetrated by the holes. Estimated permeabilities vary systematically with age, ranging from about 10−10 m2 in the youngest site (0.9 Ma) to 10−12 m2 in the oldest site (3.6 Ma), confirming an apparent reduction of permeability with age determined with packer experiments at three of the same sites. Combined with other estimates of permeabilities in the same holes using methods with different scales of investigation, the new permeability estimates also provide evidence for a significant scale dependence of permeability in the upper oceanic crust. 相似文献