论文部分内容阅读
假想的地幔热地幔柱预期地震波速低,因而可支持类似于断层带地震导波、光纤波和海洋声音固定和测距通道声波的弹性导波传播。地幔柱导波在波导中对几何复杂性并不敏感,其频散会使其在地震记录中与众不同并提供可以补充地震层析成像波导结构的有关信息。检测这类波将成为地幔柱存在新一类强有力的证据。嵌于无限介质中的圆柱形通道支持两类轴对称弹性波振型,即扭转振型和纵向—径向振型。扭转振型有与柱体表面横切的直线质点运动。纵向—径向振型在含有圆柱体轴的平面上有椭圆形质点运动,在轴附近有反向运动。椭圆形质点运动的方向随距离与轴反向:基本振型为一倍,第一谐波为两倍,依此类推。每种振型只有在超过其截止频率时才存在,相速度和群速度在无限介质中等于剪切波速。在高频率时两种速度在通道中接近剪切波速。所有振型在它们的群速度中均有最小值,在地震记录上产生艾里相位。对于剪切波速有百分之几的反差尽管对于地球的热地幔柱而言是现实的,但最大的信号是反向频散的,并有约0.1～1Hz和15～30s持续时间的优势频率。可观测到的地幔柱波至少有两个可能的来源:(1)深地幔中地幔柱与高振幅核震相焦散线的相交;(2)向下传播的导波在核幔边界ScS状的反射。最近广泛部署的宽带地震仪使搜索这些波成为可能。
Hypothetical mantle plumes are expected to have low seismic velocities and therefore support elastic guided-wave propagation similar to fault guided seismic, fiber-optic and oceanic sound fixtures and ranging channel acoustic waves. Mantle guided waves are not sensitive to geometrical complexity in waveguides and their dispersion will make them distinctive from seismic records and provide information that can complement seismic tomography waveguide structures. The detection of such waves will be a powerful new evidence for the existence of mantle plumes. Cylindrical channels embedded in an infinite medium support two types of axisymmetric elastic wave modes, torsional modes and longitudinal-radial modes. Torsional vibration mode has a straight line with the cylindrical surface of the particle movement. Longitudinal - radial modes have elliptical mass motion in the plane containing the cylinder axis and reverse motion in the vicinity of the shaft. The direction of elliptical particle motion reverses with distance and axis: the fundamental mode is doubled, the first harmonic is doubled, and so on. Each mode shape exists only when its cut-off frequency is exceeded, and the phase velocity and group velocity are equal to the shear wave velocity in an infinite medium. At high frequencies the two velocities approach the shear wave velocity in the channel. All modes have minimum values in their group velocities, producing an Airy phase on the seismic record. A few percent of the shear wave velocity contrast is realistic for the Earth’s thermal mantle plume but the maximum signal is backscattered and has dominant frequencies of about 0.1 to 1 Hz and 15 to 30 s duration . There are at least two possible sources of mantle plumes that can be observed: (1) the intersection of the mantle plume with the high-amplitude core-phase facula in the deep mantle; and (2) the downward-propagating guided wave at the core-mantle boundary Reflection. Broadly deployed broadband seismometers have made it possible to search for these waves.