An SH hybrid method and shear velocity structures in the lowermost mantle beneath the central Pacific and South Atlantic Oceans

Abstract

An SH hybrid method is developed for calculating synthetic seismograms involving two‐dimensional localized heterogeneous structures. The hybrid method is a combination of analytic and numerical methods, with the numerical method (finite difference) applied in the heterogeneous region only and analytical methods applied outside the region. Generalized ray theory solutions from a seismic source are used to initiate the finite difference calculation, and seismic responses at the Earth's surface are obtained from the finite difference output by applying the Kirchhoff theory. We apply the hybrid method and study SH wave propagation near the bottom of the mantle. The shear velocity structures and the interaction of SH waves with these velocity structures at the base of the mantle beneath the central Pacific and South Atlantic Oceans are of particular importance. The observed SH waves sampling these two regions of the core‐mantle boundary show very different characteristics across the epicentral distance range of 83°–108°. The SH waves sampling the core‐mantle boundary beneath the central Pacific show a linearly increasing delay of 4 s from 98° to 108° and discernible ScS phases up to an epicentral distance of 102°. The SH waves propagating through the base of the mantle beneath the South Atlantic Ocean, on the other hand, exhibit a linearly increasing delay of 10 s from 98° to 108°, discernible multiple ScS phases with a same slowness as the direct SH waves up to 108°, and rapid variations of waveforms across small epicentral distances. Synthetic tests indicate that while the observations sampling the central Pacific can be explained by a negative shear velocity gradient of 3% (relative to the preliminary reference Earth model) at the bottom 300 km of the mantle, those sampling the South Atlantic Ocean require a 300‐km‐thick bottom boundary layer with a larger negative velocity gradient (up to 10%) and steeply dipping edges. The negative velocity gradient at the base of mantle beneath the South Atlantic Ocean can be best explained by partial melt driven by a compositional change produced in the early Earth's history and a vertical thermal gradient within the layer, while that beneath the central Pacific may reasonably be attributed to pure thermal effects within a thermal boundary layer.