We apply the Backus‐Gilbert approach to normal mode center frequency data, to constrain jumps in P, S, bulk‐sound speed and density at the “660” discontinuity in the earth’s mantle (∼650–670 km depth). Different 1‐D models are considered to compute sensitivity kernels. When using model PREM (Dziewonski & Anderson, 1981, Physics of the Earth and Planetary Interiors, 25, 297–356. doi:10.1016/0031‐9201(81)90046‐7) as reference, with a “660” at 670 km depth, the best‐fitting jumps in density, P‐ and S‐wave speeds range from (5.1–8.2)%, (5.3–8.0)%, (5.0–7.0)%, respectively, so the PREM values lie outside the ranges of acceptable density and P wave speed jumps. When shifting the depth of “660” to 660 km, the density and S wave speed jumps increase, while the P‐wave speed jump decreases. Normal mode data do not support a global transition at 650 km depth. The density jumps are closer to those of pyrolite than PREM, while our bulk‐sound wave speed jumps suggest a larger garnet proportion at “660.”

中文翻译：

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使用常规模式数据通过Backus-Gilbert方法在660 km间断处约束密度和弹性特性的跳跃

我们将Backus-Gilbert方法应用于正常模式的中心频率数据，以限制地幔“ 660”不连续（深度约650-670 km）处的P，S，体音速度和密度的跳跃。考虑使用不同的一维模型来计算灵敏度内核。当使用PREM模型（Dziewonski＆Anderson，1981，地球与行星内部物理，25，297–356。doi：10.1016 / 0031-9201（81）90046-7）作为参考时，在670 km处为“ 660”深度，最合适的密度跃变，P波和S波速度分别在（5.1–8.2）％，（5.3–8.0）％，（5.0–7.0）％的范围内，因此PREM值不在此范围内可接受的密度和P波速度跳跃。当将“ 660”的深度更改为660 km时，密度和S波速度跳跃增加，而P波速度跳跃减少。普通模式数据不支持650 km深度的全局过渡。密度跃变比PREM更接近于黄铁矿，而我们的声波波速跃变表明，石榴石的比例更大，为“ 660”。