High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations
High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations
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南加州圣哈辛托断层带记录的高频钻孔地震图。
DOI：
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发表时间：
1991
期刊：
影响因子：
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通讯作者：
P. Shearer
中科院分区：
文献类型：
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作者：
R. Aster;P. Shearer
美国地质调查局在圣哈辛托断层带的南加州基岩地区建立了两个钻孔地震仪阵列（KNW-BH 和 PFO-BH）。这些站点距离 Anza 网络地面站不到 0.4 公里，并在 300 m 深度、150 m 深度和地面部署了三分量地震仪。使用区域和近区域初始纵波质点运动，井下水平地震仪的定向精度可达约 5°。
在 KNW-BH 井下记录的剪切波表明，先前在（地表）KNW Anza 站点（KNW-AZ）观察到的初始 S 波粒子运动的强排列并未在近地表风化层中产生。 KNW-BH 地面仪器位于高度风化区顶部，显示出与井下和用螺栓固定在露头上的 KNW-AZ 观测到的初始 S 波偏振方向显着不同（约 20°）的初始 S 波偏振方向。尽管井下初始剪切波质点运动方向与剪切波分裂假设一致，但正交偏振慢剪切波的观测结果通常难以捉摸，即使是在 300 m 处记录的地震图中也是如此。 Sfast 和 Sslow 水平偏振 S 波表观相对速度的互相关测量表明，浅剪切波各向异性与观测到的初始 S 波质点运动方向一致，在 300 至 150 m 之间为 2.3 ± 1.7%，在 150 至 0 m 之间为 7.5 ± 3.5%。
Two borehole seismometer arrays (KNW-BH and PFO-BH) have been established in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey. The sites are within 0.4 km of Anza network surface stations and have three-component seismometers deployed at 300 m depth, at 150 m depth, and at the surface. Downhole horizontal seismometers can be oriented to an accuracy of about 5° using regional and near-regional initial P-wave particle motions.
Shear waves recorded downhole at the KNW-BH indicate that the strong alignment of initial S-wave particle motions previously observed at the (surface) KNW Anza site (KNW-AZ) is not generated in the near-surface weathered layer. The KNW-BH surface instrument, which sits atop a highly weathered zone, displays a significantly different (≈ 20°) initial S-wave polarization direction from that observed downhole and at KNW-AZ, which is bolted to an outcrop. Although downhole initial shear-wave particle motion directions are consistent with a shear-wave splitting hypothesis, observations of orthogonally polarized slow shear waves are generally elusive, even in seismograms recorded at 300 m. A cross-correlation measure of the apparent relative velocities of Sfast and Sslow horizontally polarized S waves suggests shallow shear-wave anisotropy, consistent with the observed initial S-wave particle motion direction, of 2.3 ± 1.7 per cent between 300 and 150 m and 7.5 ± 3.5 per cent between 150 and 0 m.