Seismic study of geochmical heterogeneity in the mid lower mantle.

来自美国西部短周期地震台网的数据显示了异常的后期阶段，我们将其解释为来自马里亚纳俯冲带NW中下地幔非均质性的s - p转换波。物体的几何形状是通过两步方法绘制的：(1)通过阵列技术相对于直接P波测量的到达时间、慢度和后期到达的方位角，计算S-to-P散射的可能性；(2)通过使用相似度直接使用波形。下地幔研究区至少探测到3个亚平行天体，倾角为30°~ 40°，侧向范围大于500 km × 300 km。考虑到它们转换短周期波的高效率，与非均质物体相关的弹性性质在几公里内似乎发生了变化。以前的地震层析成像研究没有预测到非均质层周围明显的速度异常。这可能是因为……更常见的是，它们低于现有方法的分辨率限制。这三个主要非均质在垂直方向上相距近300公里，并且每个物体之间似乎没有明显的异常。然而，最清晰的中间物体的更细网格尺寸（0.1°× 0.1°× 10km）的相似横截面表明，该物体不是单一界面，而是由相距近50 km的双界面组成。纯千米尺度的热异常必须是地幔的瞬态特征，因此极不可能是非均质性的原因。它们更应该代表下地幔矿物各向异性晶格优选方向的变化或急剧变化。然而，由于集中的放射性核或由变形引起的粘性剪切加热产生的热量，热异常可能叠加在这种变化上。对这些倾斜物体的一种合理的构造解释是，它们代表了在太平洋板块当前俯冲之前俯冲的岩石圈板块的残余物。虽然我们不能仅仅从这些观察结果中得出产生这种非均质性的机制的结论性论点，但我们认为它们可能代表了混沌地幔流中拉伸、变薄和双重的岩石。少

Data from western United States short period seismic networks show anomalous later phases which we interpret as S-to-P converted waves from heterogeneities in the mid-lower mantle NW of the Mariana subduction zone. The geometry of the objects is mapped by a two-step approach : (1) computing the S-to-P scattering likelihood with the arrival time, slowness, and azimuth of the later arrivals measured by array techniques relative to direct P waves, and (2) by direct use of the waveforms using semblance. At least three subparallel objects are detected in the lower mantle study region, with the dips of 30 deg to 40 deg southward and with a lateral extent larger than 500 km by 300 km. The elastic properties associated with the heterogeneous objects appear to change within several kilometers, considering their high efficiency at converting short period waves. Previous seismic tomography studies have not predicted significant velocity anomalies around the heterogeneities. This is probably becau … More se they are below the resolution limit of present methods. The three major heterogeneities are separated from each other nearly 300 km vertically, and significant anomalies seem absent between each object. Nevertheless, semblance cross sections with a finer grid size 0.1 deg by 0.1 deg by 10km of the clearest intermediate object show a hint that the object is not a single interface but is actually composed of double interfaces nearly 50 km apart. Purely thermal anomalies of kilometers scale must be transient features in the mantle, and thus are highly unlikely to be the cause of the heterogeneities. They should rather represent either or sharp changes in preferred orientations of anisotropic lattice of the lower mantle minerals. Thermal anomalies, however, may be superimposed on such variations because of heat generation either by concentrated radio-active nuclei or by viscous shear heating due to deformations. A plausible tectonic interpretation of these dipping objects is that they represent remnants of subducted lithospheric plates which preceded the current subduction of the Pacific plate. Although we cannot make conclusive arguments on what the mechanism of creating such heterogeneities from these observations alone, we suggest that they may represent stretched, thinned, and duplexed rocks within chaotic mantle flows. Less