Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone
Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone
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DOI：
10.1029/2021jb021653
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发表时间：
2021-01
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影响因子：
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通讯作者：
Roque A. Soto Castaneda;G. Abers;Z. Eilon;D. Christensen
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作者：
Roque A. Soto Castaneda;G. Abers;Z. Eilon;D. Christensen
阿拉斯加俯冲带的地震部署提供了密集的地震波场采样，从而限制了热结构和俯冲几何形状。我们测量了206个站点的远震体波的振幅和相位谱比对P和S的衰减，这些站点来自区域和短期阵列。平行远震走时测量提供了相同尺度的地震速度信息。这些数据表明，在俯冲系统的弧前持续低衰减，在弧后持续高衰减，类似于局部地震衰减研究，但频率低10倍。这种模式既可以在库克湾的正常太平洋俯冲区域看到，也可以在有俯冲争论的弗兰格尔火山场看到。这些观测结果证实了后者下方以俯冲为主的热状态。旅行时间显示了俯冲岩石圈比地震活动性更深的证据，而衰减测量似乎主要反映了小于150公里深的地幔温度，这是地幔最接近其固体的深度，并且可能发生与俯冲相关的融化。在厚厚的沉积盆地上，旅行时间显示出强烈的延迟。衰减信号没有显示出盆地吸收的证据，尽管一些盆地显示出异常丰富的高频能量信号，随之而来的是负的表观衰减。在盆地外，这些数据与220 km以上的地幔衰减一致，在数量上与地表波和局地震体波的观测结果相似。P和S衰减之间的差异主要表明剪切模量松弛。总体而言，衰减测量显示出一致的、相干的俯冲相关结构，与行程时间互补。
Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short‐term arrays. Parallel teleseismic travel‐time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local‐earthquake attenuation studies but at 10× lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction‐dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction‐related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high‐frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local‐earthquake body waves. Differences between P and S attenuation suggest primarily shear‐modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction‐related structure, complementary to travel times.