Shear velocity variation within the D″ region beneath the central Pacific

Small-scale variability of shear velocity (V_s) structure in the D″ region beneath the central Pacific is imaged using 442 broadband tangential component S waveforms recorded in western North America for 37 intermediate and deep focus Tonga-Fiji earthquakes. Double-array stacking of spatially binned subsets of data reveals lateral variations in the relative timing and amplitude of deep mantle discontinuity reflections on scale lengths of about 130 km across the ∼6° × 8° region of D″ sampled. Waveform modeling using localized one-dimensional structures indicates variations of the V_s increase at the D″ discontinuity ranging from 0.5% to 2.3%, and discontinuity depths ranging from 2490 to 2735 km, deepening and weakening from southwest-to-northeast across the study area. Two abrupt V_s reductions are also detected within the D″ layer. We thus present a laterally variable three-layer model of the D″ region beneath the central Pacific. The complex structure may be associated with lateral thermal and chemical gradients that produce a lens of postperovskite material above an ultralow-velocity zone on the margin of a large low shear velocity province.