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291.
Conductivity models were compiled in two geological provinces with thinsediments. The first province is the margin of the Precambrian East EuropeanPlatform. The second one is the Phanerozoic Rhenish Massif as a part of CentralGermany Hercinicum. In both provinces, a conducting layer was revealed at thebase of the upper crust by the magnetotelluric soundings. Its depth is around10 km in the East European Platform and around 15 km in the Rhenish Massif.The conductance of the layer reaches a few tens of Siemens in the first provinceand is almost an order of magnitude greater in the second one.A good correlation between the conductor and a seismic wave-guide (low-velocityzone) exists at the base of the upper crust. Simultaneous decrease of both electricalresistivity and seismic velocity, suggests an increase of porosity and permeability inpresence of saline water. The depth of rheological weak layer in the PhanerozoicRhenish Massif corresponds to the commonly accepted depth of the thermallyinduced brittle/ductile transition. Contrary similar layer in the Precambrian EasternEuropean Platform is much shallower than the thermally induced transition. Somenew mechanism should be considered. 相似文献
292.
Evaluation of density in layer compaction using SASW method 总被引:1,自引:0,他引:1
SASW test, which is non-intrusive and rapid in the field application, was used to evaluate the layer density in the roller compaction without performing the complicated inversion process. The concept of normalized shear wave velocity was introduced to minimize the effect of confinement in the density evaluation. SASW test was performed to determine the shear wave velocity of the layer, and the free–free resonant column (FF–RC) test was adopted to determine the correlation between the normalized shear wave velocity and density of the site, which is almost unique independent of confinement. Testing and data reduction procedures of both tests were briefly discussed and an evaluation procedure of the field density was proposed by effectively combining in-situ shear wave velocity determined by the SASW test with the correlation between the normalized shear wave velocity and the density determined by the FF–RC test. Finally, the feasibility of the proposed method was verified by performing a field case study at Hoengsung road construction site. Field densities determined by the proposed method matched well with those determined by sand cone tests, showing the potential of applying the proposed method in the field density evaluation. 相似文献
293.
A new formulation is proposed to model pounding between two adjacent structures, with natural periods T1 and T2 and damping ratios ζ1 and ζ2 under harmonic earthquake excitation, as non‐linear Hertzian impact between two single‐degree‐of‐freedom oscillators. For the case of rigid impacts, a special case of our analytical solution has been given by Davis (‘Pounding of buildings modelled by an impact oscillator’ Earthquake Engineering and Structural Dynamics, 1992; 21 :253–274) for an oscillator pounding on a stationary barrier. Our analytical predictions for rigid impacts agree qualitatively with our numerical simulations for non‐rigid impacts. When the difference in natural periods between the two oscillators increases, the impact velocity also increases drastically. The impact velocity spectrum is, however, relatively insensitive to the standoff distance. The maximum relative impact velocity of the coupled system can occur at an excitation period Tn* which is either between those of the two oscillators or less than both of them, depending on the ratios T1/T2 and ζ1/ζ2. Although the pounding force between two oscillators has been primarily modelled by the Hertz contact law, parametric studies show that the maximum relative impact velocity is not very sensitive to changes in the contact parameters. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
294.
Abstract: 3–D velocity images of the crust beneath the northern margin of the North China Plate have been constructed using P-wave travel time residuals of the latest earthquakes, with the data supplied by Chinese seismic networks.
The seismic image results indicate that there is a lateral heterogeneity in the crust beneath the northern part of the North China block. The velocity images of the upper crust show features closely related to the tectonic features on the surface. It can be seen from these velocity images of the vertical sections, and from the horizontal slice images at depths of 11 and 16 km that there exist East-West and North-East structures. The images indicate that the juncture zone of basin–and–range terrain is between the blue-colored high–velocity block corresponding to the Yanshan mountain range that developed during the Yanshan period in northwest Beijing and the green low-velocity area corresponding to the North China basin in southeast Beijing (Fig. 5). The juncture zone between high-velocity and low–velocity, and EW and NE fault zones have significant ore-control effects. From the chart of epicenters in the northern region of North China, we find that the epicenters of earthquakes are almost entirely distributed within the NE strip. Almost all major earthquakes took place in the transition strip between the high and low-velocity zones in the crust. The distribution of epicenters also reflects the strikes of known NE–faults. From the image sections along the latitude, we find that in the area between 114.0 E –118.0 E , there is a blue high-velocity block standing upright from the Moho to the upper crust (Fig. 6), from which can be deduced that some materials such as magma moved upward from the upper mantle during the history of its geological development. 相似文献
The seismic image results indicate that there is a lateral heterogeneity in the crust beneath the northern part of the North China block. The velocity images of the upper crust show features closely related to the tectonic features on the surface. It can be seen from these velocity images of the vertical sections, and from the horizontal slice images at depths of 11 and 16 km that there exist East-West and North-East structures. The images indicate that the juncture zone of basin–and–range terrain is between the blue-colored high–velocity block corresponding to the Yanshan mountain range that developed during the Yanshan period in northwest Beijing and the green low-velocity area corresponding to the North China basin in southeast Beijing (Fig. 5). The juncture zone between high-velocity and low–velocity, and EW and NE fault zones have significant ore-control effects. From the chart of epicenters in the northern region of North China, we find that the epicenters of earthquakes are almost entirely distributed within the NE strip. Almost all major earthquakes took place in the transition strip between the high and low-velocity zones in the crust. The distribution of epicenters also reflects the strikes of known NE–faults. From the image sections along the latitude, we find that in the area between 114.0 E –118.0 E , there is a blue high-velocity block standing upright from the Moho to the upper crust (Fig. 6), from which can be deduced that some materials such as magma moved upward from the upper mantle during the history of its geological development. 相似文献
295.
296.
地震前兆异常迁移速率的非线性变化特征及其力学解释 总被引:6,自引:0,他引:6
通过对长期,中短期以及短前兆异常迁移速率的研究,发现长期,中短期,短临前兆异常分别以每年10,100,1000km的量级向震中迁移,异常迁移速率不是线性增加,而为非线性递增,从理论上对前兆异常迁移性的存在与迁移速率的非线性变化进行了分析。这对不同时期的前兆异常判断以及地震的预测具有重要意义。 相似文献
297.
苍山5.2级地震前地震波速比的异常变化 总被引:1,自引:1,他引:1
本文对苍山5.2级地震前,鲁南地区地震波速比的时、空分布特征进行了分析,发现5.2级地震前,该区波速比的时、空分布都出现了较明显的低值异常。时间分布的异常形态为下降—低值—恢复—发震。空间分布则为近似圆形的低值异常区。异常区外围地区显示地震平静。苍山5.2级地震前,我们曾根据该区的波速比异常变化,对鲁南地区提出了预报意见。 相似文献
298.
A total of 11 earthquakes with 15 Rayleigh wave paths, recorded at 11 broadband digital PASSCAL seismometers installed in
the Tibet Plateau by the Sino-U.S. joint research group, were used to determine the phase velocity and attenuation coefficient
of surface waves in periods of 10–130 s. The average shear wave velocity and quality factor {ie271-1} structures in the crust
and upper mantle were obtained in this region. The result shows the average {ie271-2} is low and there exists a high attenuation
({ie271-3}=93–141) layer in the crust. The depth range of the low {ie271-4} value layer (16–42 km) is consistent with the
range of low velocity layer (21–51 km) in the crust. Below 63 km in the lower crust, {ie271-5} decreases with depth from 114
to 34 at depth of 180 km. The low shear wave velocity and low value of {ie271-6} at the same depth range in the crust indicate
that the rocks in the range is probably melted or partially melted. According to the shear wave velocity structure, the average
thickness of the crust is about 71 km and a clear velocity discontiniuty appears at the depth of 51 km. The low-velocity zone
(4. 26 km/s) at depth of 96–180 km may be corresponding to the asthenosphere.
Contribution No. 96A0047, Institute of Geophysics, SSB, China.
This study was supported by the National Natural Science Foundation of China. 相似文献
299.
使用一枚携带chaff的火箭观测到的垂直速度数据,研究中层大气垂直速度扰动细微结构。结果表明,垂直速度扰动有明显的层状结构,各层厚度约150-500m,层间垂直分离距离在几百米到1-3km间。在垂直速度方差和低Ri数峰值间以及动力不稳定和MST雷达回波区域间有好的对应关系。水平速度垂直波数谱与饱和谱在谱斜率和谱振幅上存在好的一致.这些观测结果表明,火箭测量到的细微结构可以用波场饱和解释。 相似文献
300.