Imaging of the seismic discontinuities that bound the mantle transition zone (at 410- and 660-km depths) is critical for understanding mantle dynamics and composition. A type of seismic data that is highly sensitive to the detailed structure of these discontinuities is “triplication” data, i.e., three seismic phases that sample above, at, and below the discontinuity. However, utilizing the triplication data remains challenging, as it is difficult to separate the three individual phases that arrive close in time. In this talk, I introduce a novel technique that untangles the three arrivals and brings out the high-frequency information that is not easily accessible in the original form of the data, based on the theory that the three phases arrive at distinct horizontal speeds. Taking advantage of a dense array of seismic stations in Japan, I apply this method to the 410- and 660-km discontinuities around the Kuril subduction zone, a region northeast of Japan. My analyses reveal a diffuse and, at some locations, a "missing" 410-km discontinuity that cannot be explained solely by the presence of water, which suggests the significance of other effects such as high iron content in olivine. Moreover, a “double” 660-km discontinuity separated by only 10 to 20 km is observed. The additional discontinuity is compatible with a garnet phase transformation that can obstruct mantle flow in cold environments, with the implication that variable garnet concentrations can explain different subducting behaviors of slabs. Going beyond the seismic structure, I also present a promising approach for understanding the rheological structure of the mantle based on geodetic observations of deep earthquakes.