Seismic study of geochmical heterogeneity in the mid lower mantle.

美国西部短周期地震台网资料显示异常的后期震相，我们将其解释为马里亚纳俯冲带中下地幔西北向非均质性的S-P转换波。通过两步方法绘制目标的几何图形：(1)利用阵列技术测量的相对于直接P波的到达时间、慢度和方位来计算S对P的散射似然率；(2)直接利用相似波的波形。在下地幔研究区至少发现了三个近平行的物体，它们向南倾斜30度到40度，横向范围大于500公里×300公里。考虑到它们在转换短周期波方面的高效率，与非均匀物体相关的弹性性质似乎在几公里内发生变化。以前的地震层析成像研究没有预测到非均质层周围的显著速度异常。这可能是因为…更重要的是，它们低于现有方法的分辨率极限。这三个主要的非均质性在垂直方向上相距近300公里，每个物体之间似乎没有显著的异常。然而，最清晰的中间物体的网格尺寸为0.1度×0.1度×10公里的相似横截面表明，该物体不是单一界面，而实际上是由相距近50公里的双界面组成的。千米尺度的纯热异常必然是地幔的瞬变特征，因此不太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择优取向的变化。然而，热异常可能叠加在这种变化上，因为集中的射电活动核产生热量，或者由于形变产生粘性剪切加热。对这些俯冲物体的一种可信的构造解释是，它们代表了太平洋板块当前俯冲之前的俯冲岩石圈板块的残留物。虽然我们不能仅从这些观测中得出结论，说明造成这种不均质性的机制是什么，但我们认为，它们可能代表了混乱的地幔流动中的拉伸、变薄和双重岩石。较少

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