共查询到20条相似文献，搜索用时 31 毫秒
1.
In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent. 相似文献
2.
Observations made with the co-located Rayleigh lidar and MST radar systems at Aberystwyth (52.4°N, 4.1°W) in Wales and radiosondes from Valentia (51.9°N, 10.2°W) in Eire are used to investigate the changes in the vertical propagation of gravity waves during periods of 4 days in June 1995 and February 1993. In each month, the lidar observations show that the wave activity in the upper stratosphere and lower mesosphere changes between two pairs of days. The radar and radiosonde measurements indicate that mountain waves make no contribution to the changes in intensity. Instead, the changes seem to arise largely from the presence or absence of long-period waves with vertical wavelengths near 8 and 10 km in June and February, respectively. The influence of such waves on the vertical wavenumber spectra is examined and related to the evidence for convective instabilities provided by the temperature profiles. 相似文献
3.
The propagation characteristics of blast-induced shock waves in a jointed rock mass have been monitored and studied. Accelerometers were set up on a rock surface along three lines, at 0°, 45° and 90° with respect to the orientation of the predominant joint strikes. Cylindrical charges were detonated in a charge hole, and ground accelerations in both vertical and radial directions at various points on the rock surface were recorded. Results show that rock joints have significant effects on the propagation characteristics of blast-induced shock waves. The amplitude and principal frequency of shock waves attenuate with the increase of distance from the charge centre, and the increase of incident angle between the joint strike and the wave propagation path. The measured data were compared with the empirical equations of shock wave attenuation proposed by other authors. The mechanism of rock joint effect, the attenuation of shock waves in relation to the propagation distance, the charge weight and the incident angle, are discussed in this paper. 相似文献
4.
A method for determination of characteristics of quasi-Rayleigh (qR) wave in a transversely isotropic homogeneous half-space with inclined axis of symmetry is outlined. The solution is obtained as a superposition of qP, qSV and qSH waves, and surface wave velocity is determined from the boundary conditions at the free surface and at infinity, as in case of Rayleigh wave in an isotropic half-space. Though the theory is simple enough, a numerical procedure for calculation of surface wave velocity presents some difficulties. The difficulty is caused by necessity to calculate complex roots of a non-linear equation, which in turn contains functions determined as roots of non-linear equations with complex coefficients. Numerical analysis shows that roots of the equation corresponding to the boundary conditions do not exist in the whole domain of azimuths and inclinations of the symmetry axis. The domain of existence of qR wave depends on the ratio of the elastic parameters: for some strongly anisotropic models the wave cannot exist at all. For some angles of inclination qR-wave velocities deviate from those calculated on the basis of the perturbation method valid for weak anisotropy, though they have the same tendency of variation with azimuth. The phase of qR wave varies with depth unlike Rayleigh wave in an isotropic half-space. Unlike Rayleigh wave in an isotropic half-space, qR wave has three components - vertical, radial and transverse. Particle motion in horizontal plane is elliptic. Direction of the major axis of the ellipsis coincides with the direction of propagation only in azimuths 0° (180°) and 90° (270°). 相似文献
5.
Dune bedforms and salt‐wedge intrusions are common features in many estuaries with sand beds, and yet little is known about the interactions between the two. Flow visualization with an echosounder and velocity measurements with an acoustic Doppler current profiler over areas of flat‐bed and sand dunes in the highly‐stratified Fraser River estuary, Canada, were used to examine the effect of dunes on interfacial mixing. As the salt‐wedge migrates upstream over the flat‐bed, mixing is restricted to the lower portion of the water column. However, as the salt‐wedge migrates into the dune field from the flat bed, there is a dramatic change in the flow, and large internal in‐phase waves develop over each of the larger dunes, with water from the salt‐wedge reaching the surface of the estuary. The friction Richardson number shows that bed friction is more important in interfacial mixing over the dunes than over the flat‐bed, and a plot of internal Froude Number versus obstacle (dune) height shows that the salt‐wedge flow over the dunes is mainly supercritical. Such bedforms can be expected to cause similar effects in interfacial mixing in other estuaries and sediment‐laden density currents, and may thus be influential in fluid mixing and sediment transport. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
6.
A new algorithm to correct the orientation error of the accelerometerat the Dahan Downhole Array, Hualien, Taiwan is developed. This algorithmconsists of three stages: (1) rotating two horizontal ground motions on thefree surface to the SH-SVdirection and SH axis offers a reference direction.(2) computing the synthetic downhole SH waves at a downhole station and (3)searching a rotation angle for downhole observation that yields a best waveformmatch between the synthetic and observed downhole seismograms. At this point, the rotated angle corresponding to the best waveform match can be considered as the orientation error. We selected five earthquakes with good data qualityfor analysis. Results show that this algorithm gives a more stable estimationthan a conventional method because it allows the selection of data from a wider time window for analyses. The estimated orientation error of the accelerometers at the Dahan Downhole Array after the 1999 reinstallation are40°, 114° and 285° at depths of 50, 100 and 200 m, respectively. 相似文献
7.
Mechanisms of earthquakes occurring on plate boundaries have been used to determine the relative motions of lithospheric plates in the east Pacific. By using both P-wave first motions and the radiation pattern of Rayleigh waves in the focal mechanisms determinations, the uncertainty in strike is held to about ± 10°. On the Chile fracture zone and the fracture zone at 4°S on the East Pacific Rise, the direction of motion is not parallel to the trend of epicenters. This supports the hypothesis that these fracture zones are composed of series of en-echelon transform faults. 相似文献
8.
针对结构隔震装置在较大的倾覆力作用下抗拉能力不足的问题,提出了一种带有楔形分离式轨道的新型滑移支座。新支座能够承受上部结构传递的荷载以及适应较大水平位移,同时利用楔形轨道与楔形滑块之间的相互作用提供抗拉能力。采用通用有限元程序ABAQUS建立了支座的三维实体有限元模型,对其抗压、抗拉、压剪和拉剪力学性能进行数值模拟研究,同时考虑了楔形角度的改变对支座力学性能的影响。通过分析表明：在竖向新支座具有较好的抗压和抗拉能力,抗拉性能受楔形角度的影响较大,角度大于60°时,支座的抗拉能力随着角度的减小不断提升;而当角度减小到60°时,楔形滑块与上部连接部分出现薄弱点,导致支座的抗拉能力开始降低。在压剪与拉剪的作用下,支座水平向的滞回曲线比较饱满,具有较好的水平向摩擦耗能能力且能够适应较大的水平位移。 相似文献
9.
The effect of topography and subsurface inhomogeneity on surface motion is investigated in the case of Rayleigh waves. In the previous paper, the same effect was investigated in the case of SV waves. Several types of topography, such as cliffs both with and without a soft layer at the foot of the slope, are considered. Computations are made using a new hybrid method combining a particle model with a finite element method. In cases of harmonic Rayleigh waves, surface motions with amplitudes as large as 1.5 to 5 times the horizontal surface displacement of the incident Rayleigh waves are produced near the slope and the sloping interface. When a Rayleigh wave propagating through a hard single-layered ground encounters a sloping interface where hard ground and soft ground make contact with each other, Rayleigh waves having two different, phase velocities are produced and they correspond to the fundamental mode, and the first mode determined by Haskell's method. In addition, the transient response when Rayleigh waves propagate through the cliff is also simulated. Assuming the vertical component of the Tokachi-oki Earthquake (1968) measured on the surface to be a Rayleigh wave, the incident Rayleigh wave can be obtained by a Fourier synthesis of eigenfunctions of Rayleigh waves. 相似文献
10.
K. Viswanathan 《Pure and Applied Geophysics》1972,101(1):67-73
Summary The aim of this work is to examine the critical loci which govern the limits to the depths to which a Rayleigh wave can penetrate into a Half-space. Both interior and surface sources are considered. The corresponding critical regions for the existence of these waves play a vital role in more complicated problems such as the calculation of the reflection and transmission coefficients of Rayleigh waves in a wedge.On leave from Defence Science Laboratory, Delhi, India. 相似文献
11.
P. Keckhut 《Annales Geophysicae》1995,13(6):641-647
Temperature and wind data obtained with Rayleigh lidar since 1979 and Russian rockets since 1964 are analyzed to deduce the summer response of the middle atmosphere to short-term solar UV changes. The equivalent width of the 1083 nm He I line is used as a proxy to monitor the short-term UV flux changes. Spectral analyses are performed on 108-day windows to extract the 27-day component from temperature, wind and solar data sets. Linear regressions between these spectral harmonics show some significant correlations around 45 km at mid-latitudes. For large 27-day solar cycles, amplitudes of 2 K and 6 m s−1 are calculated for temperature data series over the south of France (44°N), and on wind data series over Volgograd (49°N), respectively. Cross-spectrum analyses have indicated correlations between these atmospheric parameters and the solar proxy with a phase lag of less than 2 days. These statistically correlative results, which provide good qualitative agreement with numerical simulations, are both obtained at mid-latitude. However, the observed amplitudes are larger than expected, with numerical models suggesting that dynamical processes such as equatorial or gravity waves may be responsible. 相似文献
12.
An analytical model for describing the propagation and attenuation of Rayleigh waves along the free surface of an elastic porous medium containing two immiscible, viscous, compressible fluids is developed in the present study based on the poroelastic equations formulated by Lo et al. [Lo WC, Sposito G, Majer E. Wave propagation through elastic porous media containing two immiscible fluids. Water Resour Res 2005;41:W02025]. The dispersion equation obtained is complex-valued due to viscous dissipation resulting from the relative motion of the solid to the pore fluids. As an excitation frequency is stipulated, the dispersion equation that is a cubic polynomial is numerically solved to determine the phase speed and attenuation coefficient of Rayleigh waves in Columbia fine sandy loam permeated by an air–water mixture. Our numerical results show that, corresponding to three dilatational waves, there is also the existence of three different modes of Rayleigh wave in an unsaturated porous medium, which are designated as the R1, R2, and R3 waves in descending order of phase speed, respectively. The phase speed of the R1 wave is non-dispersive (frequency-independent) in the frequency range we examined (10 Hz–10 kHz) and decreases as water saturation increases, whose magnitude ranges from 20% to 49% of that of the first dilatational wave with respect to water content. However, it is revealed numerically that the R2 and R3 waves are functions of excitation frequency. Given the same water saturation and excitation frequency, the phase speeds of the R2 and R3 waves are found to be approximately 90% of those of the second and third dilatational waves, respectively. The R1 wave has the lowest attenuation coefficient whereas the R3 wave attenuates highest. 相似文献
13.
14.
Rayleigh wave dispersion can be induced in an anisotropic medium or a layered isotropic medium. For a layered azimuthally anisotropic structure, traditional wave equation of layered structure can be modified to describe the dispersion behavior of Rayleigh waves. Numerical stimulation results show that for layered azimuthal anisotropy both the dispersion velocities and anisotropic parameters depend principally on anisotropic S-wave velocities. The splitting S-wave velocities may produce dispersion splitting of Rayleigh waves. Such dispersion splitting appears noticeable at azimuthal angle 45°. This feature was confirmed by the measured results of a field test. The fundamental mode splits into two branches at azimuthal angle 45° to the symmetry axis for some frequencies, and along the same direction the difference of splitting-phase velocities of the fundamental model reaches the maximum. Dispersion splitting of Rayleigh waves was firstly displayed for anisotropy study in dispersion image by means of multichannel analysis of surface waves, the image of which provides a new window for studying the anisotropic property of media. 相似文献
15.
Observations of gravity waves in the upper and lower stratosphere by lidar and ozonesondes 总被引：1，自引：0，他引：1
Lidar observations of Rayleigh backscatter and temperature profiles measured by ozonesondes have been used to investigate gravity waves in the upper and lower stratosphere, respectively, over Aberystwyth (52.4°N, 4.1°W). Both data sets have been used to investigate the vertical wavenumber spectrum of the wave field at high wavenumbers. Similar analytic techniques applied to each data set enable direct comparison of the spectra. The possibility of laminar structures generated by differential advection contaminating gravity-wave fields deduced from temperature and/or density measurement is discussed and the behaviour of the wave-field mean potential energy reveals a seasonal cycle throughout the stratosphere with a late winter maximum and a summer minimum. 相似文献
16.
Heidi Houston Helen Anderson Susan L. Beck Jiajun Zhang Susan Schwartz 《Pure and Applied Geophysics》1993,140(2):331-364
To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (M
w
=7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (=275°, =61°, =156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×1020 N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes.We performed a simultaneous inversion ofP andSH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy. 相似文献
17.
针对隧道工程中TVSP（Tunnel Vertical Seismic Profiling）方法不适合于煤矿巷道超前探测的问题,介绍了煤矿巷道中的槽波TVSP超前探测方法原理和实现过程.通过理论和数值模拟证明：（1）当采用垂向集中力源（z方向）激发地震波时,地震波场z分量受到巷道影响较小,且主要为Rayleigh型槽波,较为适合槽波超前探测;（2）采用传播相对稳定且易于识别的槽波埃里相峰值时间代替传统TVSP方法中体波的初至走时时间,能够较准确地确定断层在掌子面前方位置和断层角度;（3）当断层与巷道掘进方向夹角较大时,断层交点位置（断层线与巷道掘进方向的交点）具有很好的定位精度,但角度误差则会较大;当实际断层交点位置距离迎头较远时,断层交点位置定位精度会下降,但角度误差则影响较小;（4）基于槽波的TVSP方法对观测系统要求不高,数据处理过程简单,计算量小. 相似文献
18.
Kehar Singh 《Pure and Applied Geophysics》1965,61(1):36-44
Summary The propagation of Rayleigh type waves in an axially symmetric inhomogeneous layer lying between two halfspaces is studied. The halfspaces are supposed to be identical in their elastic properties. The variation of the parameters in the layer is assumed to be of the form
where is a constant andz is the distance measured from one interface into the layer. With this assumption, the vector wave equation for the layer is separable. The solution is obtained in terms ofWhittaker's functions and the frequency equation of Rayleigh type waves is derived. 相似文献
19.
W. Baumjohann R. A. Treumann E. Georgescu G. Haerendel K.-H. Fornacon U. Auster 《Annales Geophysicae》1999,17(12):1528-1534
The Equator-S magnetometer is very sensitive and has a sampling rate of normally 128 Hz. The high sampling rate allows for the first time fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for a few wave cycles only, typically 0.25 s. They are right-hand circularly polarized waves with typical amplitudes of 0.5–1 nT at around one tenth of the electron gyrofrequency. The cone angle between wave vector and ambient field is usually smaller than 1.5°. 相似文献
20.
A theoretical investigation of the dynamic response of earth dams to the travelling seismic waves is presented. The earth dam is simplified as a truncated two-dimensional elastic wedge. The dam body consists of an isotropical linear viscoelastic material with homogeneous elastic modulus and density. The seismic waves travel along the longitudinal direction of the earth dam. The numerical calculations show the following. (i) For the longitudinal mode of vibration, the greater the ratio (H/L) of the height to the lenght of the complete wedge, the more the natural transverse period of vibrational of the two-dimensional wedge is less than that of the one-dimensional wedge. Especially for the first two natural transverse period, this influence is large. The decrease of the ratio of the natural transverse period for a two-dimensional wedge to that for a one-dimensional wedge with the ratio H/L is rapid for the higher than for the lower longitudinal modes. (ii) Comparing with the one-dimensional wedge, the natural transverse periods for the two-dimensional case in the complete wedge are lower, and they will increase as the coefficient of truncation, h/H increases. (iii) When the frequency of forced vibration is less than the natural transverse frequency for one-dimensional wedge, the amplification is less for a two-dimensional wedge than for a one-dimensional wedge. (iv) When the phase difference of ground motion at both ends of the dam equals π, the amplification for a two-dimensional wedge is less than that for a one-dimensional wedge, but when the phase difference equals nπ, (n > 1), the situation is reversed. (v) As the coefficient of truncation, h/H, increases, the displacement model partecipations decrease monotically. (vi) In general, the displacement caused by an earthquake is greater for a one-dimensional wedge than for a two-dimensional wedge when considering the seismic waves travelling, but the acceleration response of a two-dimensional wedge with long length of dam to travelling seismic waves with long dominant period is greater than that of a one-dimensional wedge. When the length of the dam is short enough, the response of a two-dimensional wedge without considering the influence of travelling seismic waves always gives the greatest value. 相似文献