Structure of the Edge of the Pacific Low-Velocity Feature Investigated Using Philippine Source Array

The D- region of the lowermost mantle has been much studied in recent years, since understanding of its structure will shed light on mantle convection systems and core-mantle interactions. In particular, the role of the post-perovskite phase transition in creating the seismic discontinuities in D- has been the focus of much interest, along with the suggestion that the large scale seismic velocity reductions under Africa and the central Pacific may be due to piles of chemically and thermally distinct material. We investigate D- structure under the western Pacific, north-west of the Marshall Islands, by using earthquakes in the Philippine Sea recorded in Hawaii to construct source arrays. These events have core-mantle boundary bounce points which sample across the edge of the central Pacific low-velocity region found in seismic tomography of the lowermost mantle. Selection of groups of earthquakes with suitably similar source mechanisms and depths allows us to perform migrations of both P- and S-waves to show variations in the seismic velocity structure of D- across the edge of the low-velocity feature. This provides insight into the role of post-perovskite, thermal and chemical heterogeneity in the behaviour of the lower mantle.