Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California
Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California
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DOI：
10.1785/bssa0850061805
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发表时间：
1995-12
影响因子：
3
通讯作者：
D. Jongmans;P. Malin
中科院分区：
文献类型：
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作者：
D. Jongmans;P. Malin
在加州州帕克菲尔德的瓦里安A-1井沿线，从0到1公里深的高增益三分量地震仪被用来记录附近微震的波形。尽管位于帕克菲尔德向斜的第三纪厚沉积物中，但该场地的S波放大率仅为3倍。沿井沿着的S波的谱含量和谱比表明，在该站点的顶部1 km的平均Qs为37，在不同的子间隔的Qs在8和65之间变化。根据初始S波偏振，瓦里安场地及其下方一定存在复杂的S波速度结构。这种结构似乎包括位置相关的各向异性以及陡峭的横向速度梯度。在1公里的深度，S波分裂平行和正常的圣安德烈亚斯断层带是一贯观察。这种分裂的尺度大约为0.01秒/千米。分裂S波之后，粒子运动似乎受事件震源机制控制。在1公里以上，上行S波衰减并改变偏振方向，新的分裂率为0.1秒/公里。独特的是，在圣安德烈亚斯断层上的一些事件，就在瓦里安现场下方，存在正常色散的大的后S波信号。我们建议，这些阶段是从圣安德烈亚斯故障引导到瓦里安网站沿着金山故障的断层带导波。
High-gain three-component seismometers from 0- to 1-km deep along the Varian A-1 well at Parkfield, California, were used to record the waveforms of nearby microearthquakes. Despite being in the thick Tertiary sediments of the Parkfield Syncline, the S-wave amplification at this site is only about a factor of 3. The spectral content and spectral ratios of S waves along the well show that the average Qs in the top 1 km at this site is 37, with the Qs in different subintervals varying between 8 and 65. Based on initial S-wave polarizations, a complex S-wave velocity structure must exist at and below the Varian site. This structure appears to include position-dependent anisotropy as well as steep lateral velocity gradients. At a depth of 1 km, S-wave splitting parallel and normal to the San Andreas fault zone is consistently observed. This splitting scales at roughly 0.01 sec/km. Subsequent to the split S waves, the particle motion seems to be controlled by event focal mechanism. Above 1 km, the upgoing S waves attenuate and change directions of polarization, with a new splitting rate of 0.1 sec/km. Uniquely, for some events on the San Andreas fault immediately below the Varian site, large, post-S-wave signals with normal dispersion are present. We propose that these phases are fault-zone guided waves channeled from the San Andreas fault to the Varian site along the Gold Hill fault.