Detection of Lowermost Mantle Heterogeneity Using Seismic Migration of Diffracted S-Waves

The bottom of Earth's mantle hosts strong seismic wave speed heterogeneities. These are commonly detected via forward modeling of seismic waveforms, which can include time-consuming waveform synthesis and visual inspection. Furthermore, such imaging has been most commonly carried out with waves that have limited global coverage. In this work, we investigate the efficacy of the diffracted S ($S_{\text{diff}}$) wavefield, which has global coverage to map core-mantle boundary heterogeneity. We implement a Kirchhoff migration algorithm to objectively investigate the presence or absence of postcursors to $S_{\text{diff}}$, caused by ultralow velocity zones (ULVZs) and other sharp velocity contrasts. Our approach makes use of the expected moveout of ULVZ-generated $S_{\text{diff}}$ postcursors as a function of distance from great-circle path at the base of the mantle. We investigate epicentral distances $>95°$, where $S_{\text{diff}}$ includes asymptotic S/ScS up to diffraction. We test the algorithm using synthetic waveforms calculated for models that include lowermost mantle wavespeed heterogeneity via a recently proposed hybrid simulation approach. Our results demonstrate that the migration approach, when applied to a single event, can well resolve the location of heterogeneity structures in the azimuthal direction, but is less accurate at constraining the along-great circle path location. To locate ULVZ structure accurately, heterogeneity maps from different earthquakes with crisscrossing raypaths are combined. Lastly, we provide real-data proof-of-concept examples which detect ULVZs with different sizes that have been proposed in past work. These include the Hawaiian ULVZ, which is roughly 1,000 km across and a ULVZ beneath the Himalayas with a lateral extent of only 200 km.