Department of Geosciences, Princeton University, Princeton, NJ, USA.
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA.
Nat Commun. 2023 Mar 27;14(1):1714. doi: 10.1038/s41467-023-37067-x.
Seismic discontinuities in the mantle are indicators of its thermo-chemical state and offer clues to its dynamics. Ray-based seismic 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. Here, we show how to 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. We observe a thinned mantle transition zone southeast of Hawaii, and a reduction in impedance contrast around 410 km depth in the same area, suggesting a hotter-than-average mantle in the region. Here, we furthermore reveal a 4000-5000 km-wide reflector in new images of 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. Reverse-time migration full-waveform imaging is a powerful approach to imaging Earth's interior, capable of broadening our understanding of its structure and dynamics and shrinking modeling uncertainties.
地幔中的地震不连续面是其热化学状态的指标，并为其动力学提供线索。基于射线的地震方法虽然受到所做近似的限制，但已经详细绘制了地幔转换带不连续面，但尚未就中地幔不连续面的存在和性质得出明确结论。在这里，我们展示了如何使用基于波动方程的成像方法，即地震体波反射前的反向时间偏移，来揭示地幔转换带和中地幔不连续面，并解释其物理性质。我们观察到夏威夷东南部的地幔转换带变薄，同一地区的 410km 深度处的阻抗对比降低，表明该地区的地幔温度高于平均水平。此外，我们在中太平洋下方的中地幔新图像中揭示了一个 4000-5000km 宽的反射体，深度在 950-1050km 之间。这个深部不连续面表现出很强的地形，并且产生的反射与起源于 660km 不连续面的反射极性相反，这意味着在 1000km 附近存在阻抗反转。我们将这个中地幔不连续面与该地区上升的偏移地幔羽流的上游联系起来。反向时间偏移全波形成像方法是一种强大的地球内部成像方法，能够拓宽我们对地球结构和动力学的理解，并缩小建模不确定性。
