Mantle Discontinuity Structure From 3D ScS Reverberation Mapping

地幔是地球表面薄地壳和深铁芯之间的部分，是驱动地表板块运动的活跃“引擎”。由于化学变化和温度波动，它在所有长度尺度上都是不均匀的，并且不断演变。地震学提供了一种探索地幔的有力手段，而层析成像地图，类似于医学上的猫扫描，是我们最好的工具。但是断层扫描忽略了地幔结构的重要方面，最明显的是局部反射。该项目对经典的地幔勘探工具进行了改造，利用极低频横波在地幔中“混响”，作为石油工业风格的“散射波”成像技术。研究结果将利用新的地震记录数据集和俯冲带的大量地震，绘制出地幔中清晰的边界——“反射器”。地幔中的地震反射体是地幔矿物晶体结构变化、成分突变、地幔熔融区域和各向异性织物快速转变（材料性质的方向性变化，与灯芯绒织物的颗粒相似）的结果。每一个都是对地幔组成、状态（固体、液体或两者的混合）和动力学（它在哪里运动，有多快）的重要诊断，它们的地图为断层摄影提供了关键的补充信息。我们推荐的新方法很好地利用了这些波携带的信息，并承认地幔在所有三个维度上都是复杂和可变的。它将应用于两个独立的数据集，一个是由日本“Hi-Net”阵列的近500台仪器、日本及其周边地区的60多个F-net地震仪站以及中国东部、朝鲜和俄罗斯东南部的其他地震仪站产生的。这里的目标包括地幔过渡带中停滞的太平洋“板块”（深度在400至700公里之间的层），过渡带顶部存在熔融层的证据，以及可能与过去俯冲有关的中地幔反射物。第二个研究的地理区域是西南太平洋和珊瑚海。虽然那里的台站覆盖范围要有限得多，但中深地震活动的丰富和多个中心足以弥补这一点。这里的目标与日本相似。在这两种情况下，结合改进的成像工具和密集的数据，可以发现文献中没有类似物的反射器。该提案的更广泛的影响是支持和培训了两名研究生，以及这项工作对理解地幔流动和地幔中水的分布的效用。

Earth's mantle, the portion of the planet between the thin surface crust and the deep iron core, is an active "engine" driving movement of the surface plates. Because of chemical variability and temperature fluctuations, it is heterogeneous at all length scales and constantly evolving. Seismology provides a powerful means of exploring the mantle, and tomographic maps, akin to medical cat-scans, are our best tool. But tomography misses important aspects of mantle structure, most notably localized reflectors. This project offers reformulation of a classic mantle exploration tool, using very low frequency shear waves "reverberating" through the mantle, as an oil-industry style "scattered-wave" imaging technique. The result will make use of new seismogram datasets and abundant earthquakes in subduction zones to map out sharp boundaries--"reflectors"--in the mantle. Seismic reflectors in the mantle are the result of changes in crystal structure of mantle minerals, abrupt changes in composition, regions of mantle melting, and rapid transitions in anisotropic fabric (directional variation in material properties, not unlike the grain of corduroy fabric). Each is an important diagnostic of mantle composition, state (solid, liquid or a mix of both) and dynamics (where is it moving and how fast), and maps of them provide critical complimentary information to tomography. The new method we recommend makes good use of the information these waves carry and acknowledges that the mantle is complex and variable in all three dimensions. It will be applied to two separate data sets, one produced by the nearly 500 instruments of the "Hi-Net" array in Japan, the 60-plus seismograph stations of the F-net in and around Japan, and other seismograph stations in eastern China, the Koreas, and southeastern Russia. Targets here include the stagnant Pacific Ocean "slab" in the transition zone of the mantle (the layer between 400 and 700 km depth), evidence of a melt layer atop the transition zone, and mid-mantle reflectors possibly related to past subduction. The second geographic region of study is the SW Pacific and Coral Sea. While station coverage is much more limited there, the abundance and multiple centers of intermediate and deep seismicity adequately compensates. Targets here are similar to Japan. In both cases, the combination of an improved imaging tool and dense data offers the possibility of discovering reflectors without analogs elsewhere in the literature. Broader impacts of the proposal are the support and training of two graduate students, and the utility of this work for understanding mantle flow and perhaps the distribution of water in the mantle.