Complex Scattering at the Core-Mantle Boundary Observed in Short-Period Diffracted P-Waves

We identified diffracted P-waves, Pdiff, in 15 short-period records from Global Digital Seismic Network data, to infer heterogeneities near the core-mantle boundary (CMB). These records are classified into three types by their waveforms: 1) a clear phase observed at the predicted arrival time of Pdiff with late arrivals of minor energy; 2) continuous high-energy arrivals, called tail, lasting for 30-50 s after an initial phase; and 3) an initial phase observed unclear and a long tail with large amplitude of more than 50 s. From particle motions in the tail and comparisons between Pdiff and direct P-waves propagating through the lower mantle, the characteristics of each type are caused by scattering due to heterogeneities deep in the Earth, probably near the CMB. Assuming that the corresponding heterogeneities are only the result of irregular core-mantle topography, we estimate the CMB configuration from the above observations and some simple numerical simulations including the multiplescattering effect. The first type of Pdiff corresponds to a core-mantle topography with a small number of bumps, while many bumps of various sizes exist for the other two types. We cannot explain the long duration and high-energy level in the tail of the third type only by the scattering from CMB irregularity. The Iong tail of the third type suggests the existence of some structures that trap scattered waves effectively, such as a layer where the P-wave velocity gradient is negative or a low-velocity zone just above the CMB.