Seismic evidence for a 1000-km mantle discontinuity under the Pacific

Seismic discontinuities in the mantle are indicators of its thermo-chemical state and offer clues to its dynamics. Ray-based imaging methods, though limited by the approximations made, have mapped mantle transition zone discontinuities in detail, but have yet to offer definitive conclusions on the presence and nature of mid-mantle discontinuities. We use a wave-equation-based imaging method, reverse-time migration of precursors to surface-reflected seismic body waves, to uncover both mantle transition-zone and mid-mantle discontinuities, and interpret their physical nature. Single-update full-waveform back-propagation-based imaging methods such as ours are well suited for large-scale geophysical problems conducted in realistic settings. In weak signal regimes, such as is the case for the relatively small precursors studied here, simple preconditioned adjoint solutions are often more robust than those obtained by the formal full inverse. With a contrast image of mantle discontinuities as the end goal, and de-emphasizing absolute amplitudes as unnecessary for model interpretation, given sufficiently dense data coverage, our procedure should become a valuable new approach for structural imaging of upper- and mid-mantle discontinuities. Here we observe a thinned mantle transition zone southeast of Hawaii, and a reduction in impedance contrast around 410 km depth in the same area. These observations coincide with anomalously low shear wavespeeds in the background tomographic model, suggesting a hotter-than-average mantle in the region. Our new images furthermore reveal a 4000—5000 km-wide reflector in the mid mantle below the central Pacific, at 950—1050 km depth. This deep discontinuity exhibits strong topography and generates reflections with polarity opposite to those originating at the 660 km discontinuity, implying an impedance reversal near 1000 km. We link this mid-mantle discontinuity to the upper reaches of deflected mantle plumes upwelling in the region. Some published seismic tomography models are in support of this interpretation, while others remain at odds---a discrepancy that our observations may help resolve.