Seismic fault zone trapped noise
Seismic fault zone trapped noise
复制标题
地震断层带截留噪声
DOI：
10.1002/2014jb011217
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
Philippe Roux
中科院分区：
文献类型：
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作者：
G. Hillers;Michel Campillo;Y. Ben‐Zion;Philippe Roux
断层带之间和内部的系统速度对比可以导致头波和陷波，这为地震和断层力学的许多方面提供了重要的结构单元的直接信息。在这里，我们从断层阵列记录的散射地震波场中构造捕获波。利用噪声相关场的特性，研究了环境波场与断裂带环境之间的频率相关相互作用。临界频率fc≈0.5 Hz定义了一个阈值，在此阈值以上，断层内散射波场与环境噪声相比具有更高的各向同性和相干性。断层内传播方向的随机性增加会产生一个波场，该波场被困在与低速损伤区相关的波导/腔状结构中。密集的空间采样允许近场焦点点的分辨率，它出现在坍缩的叠加中，时间反转波前。焦斑的形状取决于局部介质性质，基于焦斑的断层波速正态分布表明，与远场走时反演的估计一致，速度降低了~ 50%。合成相关场的到达时间模式可以调整为与观测模式的属性相匹配，从而提供了一种基于噪声的成像工具，可以补充对捕获弹道波的分析。这些结果对于研究断层损伤带的内部性质具有广泛的适用性，因为与捕获的弹道波相比，控制捕获噪声出现的机制限制较少。
Systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics. Here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array. The frequency‐dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field. A critical frequency fc ≈ 0.5 Hz defines a threshold above which the in‐fault scattered wavefield has increased isotropy and coherency compared to the ambient noise. The increased randomization of in‐fault propagation directions produces a wavefield that is trapped in a waveguide/cavity‐like structure associated with the low‐velocity damage zone. Dense spatial sampling allows the resolution of a near‐field focal spot, which emerges from the superposition of a collapsing, time reversed wavefront. The shape of the focal spot depends on local medium properties, and a focal spot‐based fault normal distribution of wave speeds indicates a ∼50% velocity reduction consistent with estimates from a far‐field travel time inversion. The arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern, providing a noise‐based imaging tool that can complement analyses of trapped ballistic waves. The results can have wide applicability for investigating the internal properties of fault damage zones, because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves.