Subduction Dynamics at the Northwestern Pacific Slab Edge: Constraints of Tomography in Kamchatka
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摘要:
堪察加半岛位于太平洋板块的西北边缘处。太平洋板块沿堪察加海沟俯冲进入地幔，而在板块边缘处，其俯冲特征是否有不同？本研究从IRIS网站下载76个固定台站记录到的来自2239个近震事件和75个远震事件的77141条P波到时数据，利用近震-远震联合层析成像方法（TOMOG3D）获得堪察加地区壳幔内的三维P波速度结构。成像结果显示，研究区域下方上地幔内存在非常明显的高速异常块体，且与深源地震的空间分布高度一致。分析认为，该高速异常体为俯冲的西太平洋板块，俯冲角度和深度沿堪察加海沟由北向南均逐渐增加。地幔过渡带和下地幔顶部存在明显的高速异常块体，可能是因为堪察加半岛下方的太平洋俯冲板块在边缘或深部发生岩石圈熔融或拆沉现象，该高速异常块体即为拆沉的岩石圈。本文的成像结果中还可清晰地观察到2个板块窗口。堪察加地区浅部火山前线下方出现大范围的低速异常，可能是由于俯冲板块脱水或流经板块窗口的地幔流热物质导致。
Abstract:Kamchatka is located at the northwestern edge of the Pacific plate. The Pacific slab is subducting into the mantle along the Kamchatka trench. Are there any differences in subduction features near the edge? To answer this question, seismic tomography was applied to 77,141 P-wave arrival times of 2239 local earthquakes and 75 teleseismic events recorded at 76 permanent stations to study the three-dimensional velocity structure to a depth of 700 km below Kamchatka. A clear high-velocity anomaly was evident beneath the study region, which is consistent with the distribution of intermediate-depth and deep-focus earthquakes. This high-velocity anomaly was interpreted as the subducting Pacific slab with its subduction angle and depth gradually increasing from the north to south along the Kamchatka trench. Another high-velocity anomaly appeared in the mantle transition zone and the uppermost lower mantle, which may reflect a piece of detached oceanic lithosphere due to melting of the subducting slab near the slab edge. Two slab windows were also found, through which hot mantle materials flowed from the subslab to the upper-mantle wedge. Large-scale low-velocity anomalies exist under the volcanic front, which reflect hot and wet upwelling flow in the mantle wedge due to the slab dehydration.
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图 1 堪察加半岛地形与地震台站分布
蓝色方块为台站，白色和红色三角形分别代表死火山和活火山；字母S和K分别代表舍维留奇火山和克柳切夫火山。
Figure 1. Topography of the Kamchatka Peninsula and distribution of seismic stations (blue squares)
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图 3 远震事件分布
图中的黑色三角形代表研究区域的中心位置，黑色正方形代表远震事件，同心圆圈层旁边的数字代表圆圈上的点到中心点的距离。
Figure 3. Distribution of teleseismic events used in the current study
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图 4 横向网格间隔为1°×1°的检测板测试结果
Figure 4. Results of a checkerboard test with a lateral grid interval of 1°×1°
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图 6 阻尼系数折中曲线
最佳阻尼系数为30。
Figure 6. Trade-off curve for selecting the optimal damping parameter
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图 7 堪察加地区壳幔速度异常水平剖面图
色棒表示速度异常（％），红色代表低速异常，蓝色则代表高速异常。
Figure 7. Map views of P-wave velocity anomalies at different depths below Kamchatka
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图 9 堪察加壳幔速度异常纵向剖面图（剖线AA′–HH′，位置见图8）
剖面图中白色圆圈表示剖线附近的地震；红色和蓝色分别表示低速和高速异常（见色棒）；图中3条虚线从上至下分别代表33 km处的莫霍面，410 km和660 km深度处速度不连续面。
Figure 9. Vertical cross-sections of velocity anomalies along the profiles (AA′–HH′) shown in Fig.8
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图 10 堪察加壳幔速度异常纵向剖面图（剖线II′-NN′，位置见图8）
剖面图上白色圆圈用来表示剖线附近相关的地震事件；红色区域和蓝色区域分别表示剖面图上的低速和高速异常区域（见色棒）；图上3条虚线从上至下分别代表33 km处的莫霍面，410 km和660 km深度处速度不连续面。
Figure 10. Vertical cross-sections of velocity anomalies along the profiles (II′–NN′) shown in Fig.8
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图 11 两种不同速度模型的可恢复测试结果
（a）和（b）为两种不同的输入模型，（c）和（d）为对应的输出模型。4个剖面结果均沿剖线AA′（见图8）。
Figure 11. Results of two restoring resolution tests
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