Using SP precursor waves to detect upper-mantle discontinuities
Using SP precursor waves to detect upper-mantle discontinuities
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DOI：
10.1093/gji/ggy369
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发表时间：
2018-09
影响因子：
2.8
通讯作者：
A. Reiss;C. Thomas;T. Lecocq
中科院分区：
文献类型：
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作者：
A. Reiss;C. Thomas;T. Lecocq
我们调查的可用性转换phaseSP和它的前兆，这反映了下的上地幔地震不连续面。与PP和SS波相反，SP相位并不反映源和接收器之间的中间位置，而是反映大圆路径上的大约四分之三。这导致数据覆盖范围扩大，特别是在通常部署很少接收器的海洋地区。由于传播时间和入射角相似，很难区分SP和PS相位。使分离这两种波成为可能的一个特征是它们的偏振。因此，我们开发了一种偏振滤波器，它允许检测的前驱信号的SP在vespagrams。在这项可行性研究中，我们分析了在美国移动台阵西部记录的所有方位角方向的地震，Mw ≥ 5.8，震中距离在80°和140°之间。尽管这种方法有几个限制，如有限的距离和深度范围的前兆信号是明确可识别的，这项研究导致52个事件显示的信号反射了410和/或660公里的不连续的底面。我们的平均结果表明，加深410公里的不连续性下的阿拉斯加湾，阿拉斯加中部和加拿大西部和浅410公里的不连续性下的北方东太平洋和墨西哥海岸。对于660公里的不连续性，我们发现较少的反射。在阿拉斯加中部和墨西哥的太平洋海岸下面，这种不连续性似乎略有升高。阿拉斯加南部海岸和加拿大部分地区显示出加深了660公里的间断。这些观测结果与前人对PP-前兆、SS-前兆和接收函数的研究结果一致。我们表明，SP前兆是一种成功的新方法来映射上地幔地震不连续。
We investigate the usability of the converted phaseSPand its precursors, which reflect off the underside of upper-mantle seismic discontinuities. In contrast toPPandSSwaves, theSPphases do not reflect midway between source and receiver but about three quarters on the great circle path. This leads to extended data coverage, especially in oceanic regions, where usually few receivers are deployed. Due to similar traveltimes and incidence angles,SPis difficult to distinguish from thePSphase. One feature that makes it possible to separate these two waves is their polarization. Therefore, we developed a polarization filter, which allows detecting precursor signals ofSPin vespagrams. For this feasibility study, we analysed events from all azimuthal directions withMw≥ 5.8 and ranging between 80° and 140° epicentral distance recorded at the western part of the Transportable Array in the United States. Even though this method has several restrictions like limited distance and depth ranges for which the precursor signals are clearly identifiable, this study resulted in 52 events showing signals reflected off the underside of the 410 and/or 660 km discontinuity. Our averaged results show a deepened 410 km discontinuity beneath the Gulf of Alaska, central Alaska and western Canada and a shallower 410 km discontinuity beneath the northern east Pacific Ocean and the coast of Mexico. For the 660 km discontinuity we find fewer reflections. This discontinuity seems to be slightly elevated in central Alaska and beneath the Pacific shore of Mexico. The southern coast of Alaska and parts of Canada show a deepened 660 km discontinuity. These observations agree with previous results ofPP-precursors,SS-precursors and receiver function studies. We show thatSPprecursors are a successful new approach to map upper-mantle seismic discontinuities.