On the absence of an ultralow-velocity zone in the North Pacific
On the absence of an ultralow-velocity zone in the North Pacific
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北太平洋不存在超低速区
DOI：
10.1029/2009jb006420
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发表时间：
2010
期刊：
影响因子：
3.4
通讯作者：
Rost S
中科院分区：
文献类型：
--
作者：
Rost S
我们利用夏威夷大岛附近发生的一场异常大的地震，研究了夏威夷和北美之间东北太平洋下方的核心地幔边界（CMB）。 CMB 的密集采样是使用北美大量高质量台站（包括加利福尼亚州、俄勒冈州、华盛顿州和阿拉斯加州的网络以及 EarthScope 的 USArray 台站）记录的核心反射相位 PcP 实现的。我们应用自适应叠加技术在特定相位（即P和PcP）上实现最佳记录对齐，然后叠加地震图以获得具有相对较高信噪比的PcP和P的求和道。人们注意到，CMB 处的异常超低速区 (ULVZ) 分层存在于通过断层扫描成像的最下地幔大低剪切速度区下方的太平洋各个部分。 ULVZ 结构以 PcP 前体的形式产生异常的 PcP 波形变化。然而，这些PcP数据缺乏前兆能量，表明（1）缺乏ULVZ分层或（2）存在ULVZ并且比我们的检测阈值薄，即厚度小于几公里。我们使用合成波形模型来建立研究 PcP 之前的时间窗口以获取精细 CMB 分层生成的前体的敏感性和实用性。这些结果与太平洋其他地区 ULVZ 结构的证据相结合，表明 ULVZ 是间歇性的，并且可能只能在地幔流收集 ULVZ 物质的区域中检测到，无论是否部分熔融，可能位于上升流或羽流下方（或附近）。
Using an unusually large earthquake near the big island of Hawaii, we study the core mantle boundary (CMB) beneath the north‐northeastern Pacific between Hawaii and North America. A dense sampling of the CMB is achieved using the core‐reflected phasePcPrecorded at a large number of high‐quality stations in North America, including networks in California, Oregon, Washington, and Alaska, as well as at EarthScope's USArray stations. We apply an adaptive stacking technique for optimal record alignment on specific phases (namelyPandPcP) and subsequently stack seismograms to obtain summation traces possessing relatively high signal‐to‐noise ratios ofPcPandP. Anomalous ultralow‐velocity zone (ULVZ) layering at the CMB has been noted to exist in various parts of the Pacific beneath the lowermost mantle large low shear velocity province imaged by tomography. ULVZ structure produces anomalousPcPwaveform variations in the form of precursors toPcP. ThesePcPdata, however, lack precursory energy, indicating either that (1) ULVZ layering is lacking or (2) that a ULVZ is present and thinner than our detection threshold, i.e., less than a few kilometers thick. We use synthetic waveform modeling to establish the sensitivity and utility of investigating the time window ahead ofPcPfor precursors generated from fine‐scale CMB layering. These results, combined with evidence for ULVZ structure in other parts of the Pacific, suggest that ULVZs are intermittent and possibly only detectable in regions where mantle currents collect ULVZ material, whether or not partially molten, presumably beneath (or near) upwellings or plumes.