Application of receiver function method for characterizing the mantle transition zone structures in central and southern Alaska
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摘要:
太平洋板块向北美板块俯冲，在北美洲阿拉斯加大陆边缘形成了碰撞造山带和俯冲构造带，前人对该区域阿拉斯加俯冲带地幔转换带的研究存在分歧，一种观点认为太平洋板块俯冲至阿拉斯加中南部下方的地幔转换带内部，引起地幔转换带增厚，另一种观点则认为地幔转换带增厚是由于库拉板块残留造成的。因此，研究阿拉斯加俯冲带区域地幔转换带的精确成像对于厘清太平洋板块俯冲机制和俯冲过程具有重要意义。本文利用美国地震联合学会（IRIS）在阿拉斯加区域布设的USArray部分固定台站接收到远震事件的波形资料，采用天然地震P波接收函数与共转换点道集叠加的方法，反演得到了阿拉斯加山脉和育空高原410 km、660 km间断面和地幔转换带的厚度结构。结果表明：由于太平洋板块俯冲至地幔转换带内部，使得410 km间断面在阿拉斯加山脉东部和育空高原与阿拉斯加山脉交界处的弧形区域出现了抬升现象，抬升幅度为0～20 km，进而导致在该区域出现不同程度的地幔转换带增厚的现象，验证了第一种观点。另外，该地区地幔转换带结构显示，660 km间断面在阿拉斯加山脉东部以及阿拉斯加山脉与育空高原交界的东部出现下沉现象，故此推测阿拉斯加中南部部分区域太平洋板块已经俯冲至地幔转换带底部。
Abstract:The subduction of the Pacific Plate towards the North American Plate has formed a collisional mountain belt and subduction zone at the edge of the continent in Alaska. Previous studies on the mantle transition zone beneath the Alaska subduction were in debate. It is believed that the thickening of the transition zone is due to the subduction of the Pacific Plate, or due to the thickening is caused by the residual Kula Plate. Therefore, accurate imaging of mantle transition zones in the Alaskan subduction zone is of great significance for understanding the subduction mechanism and courses of the Pacific Plate. This study provided waveform data from teleseismic events received in several stations of the US Array deployed by Incorporated Research Institutions for Seismology (IRIS) in the Alaska region, and adopts the natural earthquake P-wave receiver function and common conversion point stacking method to obtain 410 km and 660 km discontinuity sections and mantle transition zone thickness structure of the Alaska Range and Yukon Plateau. The new results show that due to the subduction of the Pacific Plate into the mantle transition zone, a 410 km long discontinuity is uplifted in the arc-shaped area of the eastern Alaska Mountains and the junction of the Yukon Plateau and the Alaska Mountains, with an uplift of 0～20 km, which in turn led to different degrees of thickening of the mantle transition zone in this area, which verifies the first view. In addition, the structure of the mantle transition zone in this area shows that a 660 km long discontinuity has subsided in the eastern part of the Alaska Mountain Range and the eastern part of the border between the Alaska Mountains and the Yukon Plateau. Therefore, it is assumed that the Pacific Plate in parts of south-central Alaska has subducted to the bottom of the mantle transition zone.
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Keywords:
- receiver function /
- mantle transition zone /
- discontinuity /
- Alaska
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图 3 本文涉及到的大于6.5级地震事件的分布及与研究区域的位置关系图
图中黄色三角为研究区域，红色五角星为地震发生位置，小的红色圆圈为震中距25°的范围，大的红色圆圈为震中距100°的范围。
Figure 3. The distribution of earthquakes with magnitude greater than 6.5 and the position of the study area
Yellow triangle: the study area; red stars: earthquake epicenter; small red circles: the range of 25° from the epicenter, and large red circles: the range of 100° from the epicenter.
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图 4 P波通过莫霍面产生转换震相和多次波示意图
Figure 4. Illustration of wave conversion and multiple reflections at the Moho
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图 6 进行去仪器响应、去趋势性和去均值之后的地震记录
Figure 6. Seismic records after instrument response removal, detrending, and mean removal
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图 7 ENZ坐标系与TRZ坐标的关系示意图
Figure 7. Illustration of the relationship between ENZ and TRZ coordinate systems
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图 8 截取P波到达时前10 s至P波到达后120 s的地震记录
Figure 8. Seismic records extracted from 10 seconds before P-wave arrival to 120 seconds after P-wave arrival
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图 9 部分地震台站选取的接收函数显示
A为BPAW台站下方按照后方位角排列的接收函数，B为HAD台站下方按照后方位角排列的接收函数。
Figure 9. Display of selected receiver functions in some seismic stations
A represents the reception pattern beneath the BPAW station arranged according to the rear azimuth, and B represents the reception pattern beneath the HAD station arranged according to the rear azimuth.
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图 10 阿拉斯加山脉及育空高原地区下方的410 km、660 km间断面起伏与地幔转换带厚度
Figure 10. Topography of the 410 km and 660 km discontinuities and thickness of the mantle transition zone
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图 11 5条测线位置及沿测线的共转换点叠加剖面图
Figure 11. The distribution of 5 profile lines and stacked profile of common conversion point along the test line
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