Song Teh-Ru Alex, Helmberger Don V, Grand Stephen P
Seismological Laboratory, Division of Geological and Planetary Science, California Institute of Technology, California 91125, USA.
Nature. 2004 Feb 5;427(6974):530-3. doi: 10.1038/nature02231.
The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.
地球地幔中410千米深处的地震不连续面通常被认为是（Mg,Fe）2SiO4（参考文献1、2）从橄榄石结构向瓦兹利石结构的相变所致。由于该不连续面深度对热扰动的响应，其深度变化常被用作地幔温度的代理指标。例如，冷异常会使410千米不连续面升高，因为其克拉珀龙斜率为正，而热异常则会使其降低。但是，地震波速非均匀性与不连续面地形之间的权衡常常阻碍对这些不连续面的详细分析，而且其结构往往显得非常复杂。在此，我们同时对美国西部410千米不连续面的地震折射波和散射波进行建模，以约束该区域的结构。我们发现，在美国西北部下方410千米不连续面之上存在一个低速带，其剪切波速度下降了5%，该低速带从俄勒冈州西南部延伸至盆地与山脉省北部。这个低速带厚度在20至90千米之间变化，且横向变化迅速。其空间范围与上覆火山作用的异常成分以及该区域上方观测到的地震“接收函数”观测结果均相符。我们将这个低速带解释为成分异常，可能是由于一个致密的部分熔融层所致，这可能与法拉隆板块先前的俯冲作用和弧后伸展有关。这样一个层的存在可能表明地球过渡带含水量较高。
