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Constraints on rigid zones and other distinct layers at the top of the outer core using CMB underside reflected PKKP waves
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Clear PKKP, a P wave reflects off the core-mantle boundary on the core side, is recorded by the transcontinental USArray from two deep earthquakes occurred in South America and Tonga, and one intermediate-depth earthquake in the Hindu Kush region. We compare the PKKP waveforms with the direct P waves to investigate the fine structures near the core-mantle boundary, with a primary focus on the core side. We find no evidence for the existence of a sedimentary layer of lighter elements with a thickness above a few hundreds of meters beneath the reflection points of the two deep events, which are located at the Ninety-East Ridge and South Africa. On the other hand the PKKP wave duration of the Hindu Kush event is almost twice as long as that of the P wave, suggesting that multiple reflections may be occurring at the core-mantle boundary located beneath the Antarctic, which is located inside the so-called tangent cylinder of the outer core. The tangent cylinder is an imaginary cylindrical region suggested by geodynamics studies, which has different flow pattern and may have a higher concentration in lighter elements as compared to the rest of the outer core. One possible explanation of the elongated PKKP is a thin distinct layer with a thickness of a few kilometers at the top of the outer core, suggesting that precipitation of lighter elements may occur at the core-mantle boundary. Our data also indicate an extremely low QP of 312, approximately 40% of the PREM average (~780), within the large-scale low-velocity anomaly in the lowermost mantle beneath Pacific. 相似文献
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USArray及其近期科学发现 总被引:2,自引:0,他引:2
美国台阵(USArray)是EarthScope计划的主要内容之一。旨在探测北美大陆变形、结构及其演化。USArray主要由平移/滚动台阵和机动台阵两部分构成。滚动台阵由400个宽频带地震台站组成,以向前滚动的方式扫过美国本土和阿拉斯加。利用来自该台阵的数据,科学家能获得地球内部精细图像,藉此考察地震和火山的过程。作为滚动台阵的补充,机动台阵被用于对关键目标进行详细观测和短期测量。美国台阵计划启动5年来,采集的数据已经提供了足以令科学界考虑重新审视地震物理、火山过程、核幔结构、活动变形构造、大陆结构及其演化、地球动力学和地壳流体等基本科学问题的基础资料。 相似文献
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