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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Large angle reflection from a dipping structure recorded as a PKIKP precursor: Evidence for a low velocity zone at the core-mantle boundary beneath the Gulf of Mexico
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Large angle reflection from a dipping structure recorded as a PKIKP precursor: Evidence for a low velocity zone at the core-mantle boundary beneath the Gulf of Mexico

机译：记录为PKIKP前兆的浸入结构的大角度反射：墨西哥湾以下岩心幔边界处低速带的证据

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摘要

We observed a clear phase-like arrival prior to the PKIKP wave at a broadband seismic array in eastern Tibet from an intermediate depth earthquake occurring in Guatemala. The measured incident angle and back azimuth of this phase indicate that it originates from scattering near the core-mantle boundary (CMB) of the source side. This phase, however, was not observed from another earthquake that is only 60 km away, suggesting that scattering depends strongly on the angles of the incident waves. Ray tracing and diffraction migration indicates that the precursor is a large-angle reflection from a dipping structure in the lowermost mantle east of Mexico. The seismic reflector dips northward by similar to 52(sic) and is centered at -93.31(sic)W and 23.89(sic)N with an east-west extension of similar to 100 km. A decrease of P-wave velocity by a few to 10% is required to explain the amplitude and polarity of the phase. It is unlikely to explain the large P-wave velocity contrast and the large dipping feature with the post-perovskite phase transition. The reflector is located in a region where the lowermost mantle is marked by a high velocity anomaly related to the subducted Farallon slab. Previous numerical modeling suggested that a substantial amount of hot mantle could be trapped beneath a slab over long periods of time, leading to the formation of a mega-plume. Thus, the observed sharp dipping boundary here might correspond to the edge of a low velocity zone below the subducted Farallon slab. Mixed results on the presence of an ultra low velocity zone in the region have been reported by previous seismic studies. We speculated that the low velocity zone below the cold slab was developed as a consequence of long-term heating from core.
机译：我们从危地马拉发生的中深度地震中观察到了PKIKP波之前，在藏东东部的宽带地震阵列中出现了清晰的类似相位的到达。测得的该相位的入射角和后方位角表明它是源于源侧核幔边界（CMB）附近的散射。但是，只有在相距仅60 km的另一次地震中没有观察到这一阶段，这表明散射在很大程度上取决于入射波的角度。射线追踪和衍射迁移表明，该前驱体是来自墨西哥东部最低地幔中一个浸没结构的大角度反射。地震反射器向北倾斜约52（sic），中心为-93.31（sic）W和23.89（sic）N，东西向延伸约100 km。为了说明相位的幅度和极性，需要将P波速度降低几到10％。钙钛矿后的相变不可能解释大的P波速度反差和大的倾角特征。反射器位于最下部的地幔以与俯冲的法拉隆板有关的高速异常为特征的区域。先前的数值模型表明，很长一段时间内，大量的热幔可能会被困在板下面，导致形成巨型泡沫。因此，这里观察到的尖锐的浸入边界可能对应于俯冲的法拉隆板下方的低速带的边缘。先前的地震研究已经报道了该地区存在超低速带的混合结果。我们推测，冷板下方的低速区是由于岩心长期加热而形成的。

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