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Previous seismological studies have focused on the large-scale heterogeneity of the D" layer beneath East Asia. Here, we use high-frequency PcP precursors recorded at dense networks in China and Japan to constrain the small-scale heterogeneity in this region. By forward modeling of the stacks of recorded PcP precursory signal, we find that the small-scale heterogeneity in the D ''" layer beneath East Asia is characterized by a P wave velocity perturbation of 0.1-0.2% and a dominant length scale of 4-20 km. Although the resolved length scale is similar to the dimension of the subducted oceanic crust, the weak velocity perturbation is difficult to explain by basaltic rock in the lowermost mantle. Postperovskite transition of the supposed slab might also not be viable for generating weak heterogeneity; thus, new explanations are needed from geodynamics and mineral physics to account for the different strength of the large- and small-scale heterogeneity.

Plain Language Summary Most global tomography models show high-velocity anomalies near the core-mantle boundary beneath East Asia, which is part of the circum-Pacific fast anomalies in the lowermost mantle. Previous seismological, mineral physical, and geodynamical studies have proposed two hypotheses to interpret the origin of high-velocity anomalies: the accumulation of old slabs and the perovskite to postperovskite phase transition. In this work, by modeling PcP precursors in recordings, we observe weak small-scale heterogeneity, which cannot be explained by these two hypotheses; thus, new mechanisms are needed to account for the different strength of heterogeneity between large scale and small scale.