Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu
Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu
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DOI：
10.1029/98jb00752
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发表时间：
1998-07
影响因子：
--
通讯作者：
F. Tajima;S. Grand
中科院分区：
文献类型：
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作者：
F. Tajima;S. Grand
通过区域距离地震波形与合成地震记录的对比，研究了西北太平洋过渡带的地震结构。通过对波形进行建模，我们将不包含在大多数P波层析成像实验中使用的走时数据中的次要波的走时纳入其中。这些二次波对660公里不连续面以上的速度高度敏感，因此增加了与许多层析成像实验中使用的地震结构无关的过渡带地震结构的信息。在波形比较中，我们只使用了两个一维模型。Iasp91(在浅层稍加修改为aiasp)被用作全球平均结构，M3.11模型被用来代表深俯冲板块附近的过渡带结构。M3.11型在525到660公里的深度上比iasp91快，但也有30公里深的“660”公里的不连续。我们的结果表明，西北太平洋之下的过渡带结构发生了强烈的变化。日本西北部俯冲带和南千岛群岛下的样本数据与M3.11模型吻合得很好。位于朝鲜半岛以西的渤海深部构造似乎也在过渡带显示出与板块相关的速度异常。相比之下，使用全球平均模式IASP91对黄海至东中国海的地震数据采样进行了更好的模拟。在菲律宾海下采集的数据显示，过渡带的速度异常高，但那里660公里的不连续面的深度几乎没有明显变化。总体结果与过去的工作一致，表明俯冲板块在过渡带内处于平坦或堆积的相当宽广的区域，尽管我们发现发生这种情况的区域以及上地幔内的板块总体积比过去的研究中看到的要少得多。菲律宾板块下方不寻常的构造可能是由于板条在该地区下方相对较新的下降所致。板块本身是冷的，因此速度异常快，但更深的地幔可能还没有冷却到足以加深660公里的不连续。
The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one-dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper “660” km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab-related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.