Constraints on the Small Scale Heterogeneity in D" from Precursors to Short Distance PcP Wave and Implication for Roughness of Core-Mantle Boundary

Volumetric heterogeneity in D" layer and topography variation of the Core Mantle Boundary (CMB) are well established on large scales (thousand kms) to intermediate scales with seismological approaches. However, there are controversies regarding the level of heterogeneity in D" layer at small scales ( a few km - 10 km), with lower bound estimate of 0.1% to a few percent. And there are very limited reports of small scale topography of CMB. We take advantage of the small amplitude PcP waves at near podal distances (0-10 degree), and use the ratio of short period (1 Hz) PcP and its precursors to constrain level of small scale heterogeneity in the D" layer. We computed short period synthetic seismograms with 2D finite code for a series of volumetric heterogeneity models in the crust and in D", and find that PcP is not observable if the heterogeneity in D" is above 2%. We will present evidences of clearly observed PcP at short distances and argue for weak small scale heterogeneity in D". Assuming topography of CMB is related to isostasy, the volumetric heterogeneity in D" can be used to estimate CMB roughness.