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.
