Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan
Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan
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DOI：
10.1029/2004jb003486
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发表时间：
2005-10-12
影响因子：
3.9
通讯作者：
Kennett, BLN
中科院分区：
文献类型：
--
作者：
Furumura, T;Kennett, BLN
俯冲的太平洋板块充当高频信号的有效波导，在深地震期间，经常在日本北部东海岸产生异常大的强度。高强度区域的波形记录显示，P波和S波都以低频（f < 0.25 Hz）开始，随后是大的高频（f bb - 20 Hz），尾波较长。这种行为不能用一个由低衰减的高速板块组成的简单俯冲带模型来解释。通过对太平洋俯冲带观测到的宽带波形分析和地震波传播的数值模拟，证明了在俯冲板块中传播的高频导波是由板块结构非均质性对地震波的散射引起的。我们首选的非均质性模型是由von Karmann函数描述的平行于板块边缘的细长散射体，其下倾相关长度约为10 km，相关长度更短，厚度约为0.5 km。波速波动与平均背景模型的标准差约为2%。这种新的非均质板模型在厚度上对波长小于相关距离的地震波产生明显的散射，但波长较长的低频波很容易穿透这种层状结构。其结果是具有频率选择性的传播特性，具有更快的低频相位，随后是具有很长尾的大高频信号。俯冲板块顶部原洋壳的低波速通道效应对于解释观测到的分散信号和超长高频尾波是没有必要的。利用地球模拟器超级计算机对太平洋俯冲带的高频地震波传播进行了三维模拟，并考虑了板块非均质性，清晰地展示了高频地震波在板块中传播的散射波导效应。非均质模式的大强度区域从震源向日本北部迁移，沿太平洋海岸呈细长带，几乎与太平洋板块深部事件的观测结果相当。
The subducting Pacific plate acts an efficient waveguide for high-frequency signals and often produces anomalously large intensity on the eastern seaboard of northern Japan during deep earthquakes. The waveform records in the region of high intensity show a low-frequency (f < 0.25 Hz) onset for both P and S waves, followed by large, high-frequency (f > 2 Hz) later arrivals with a long coda. This behavior is not explained by a simple subduction zone model comprising a high-velocity plate with low attenuation. From the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the Pacific subduction zone we demonstrate that the high-frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure. Our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von Karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0.5 km in thickness. The standard deviation of wave speed fluctuations from the averaged background model is about 2%. This new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness, but low-frequency waves, with long wavelengths, can easy tunnel through such lamina structure. The result is frequency-selective propagation characteristics with a faster low-frequency phase followed by large and high-frequency signals with very long coda. A low-wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high-frequency coda. Three-dimensional simulations, using the Earth simulator supercomputer for modeling of high-frequency seismic wave propagation in the Pacific subduction zone including plate heterogeneity, clearly demonstrate the scattering waveguide effects for high-frequency seismic waves traveling in the plate. The region of large intensity for the heterogeneous model migrates away from the hypocenter into northern Japan with an elongated zone along the Pacific coast, almost comparable to the observations from deep events in the Pacific plate.