A laboratory experiment to monitor the contact state of a fault by transmission waves
A laboratory experiment to monitor the contact state of a fault by transmission waves
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DOI：
10.1016/j.tecto.2005.10.035
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发表时间：
2006-02
期刊：
影响因子：
2.9
通讯作者：
N. Yoshioka;Koji Iwasa
中科院分区：
文献类型：
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作者：
N. Yoshioka;Koji Iwasa
我们进行了一系列实验室实验，其中弹性波穿过模拟断层传播。进行了两种类型的实验：（1）法向应力保持测试（NSHT）：法向应力保持恒定约3小时，无剪切应力，观察透射波。 (2)剪切应力增加试验(SSIT)：逐渐增加剪切应力直至发生粘滑事件。在应力积累的整个过程中连续观察传输波。我们重点关注施加剪切应力期间，尤其是前兆滑移期间传输波的变化。在NSHT中发现，传输波的振幅随着静止接触时间的对数线性增加。大约 3 小时后增加了几个百分点。粗糙接触处的蠕变是造成这种现象的原因。从理论上考虑，得出的结论是，实际接触面积随着静止接触时间的对数而增加。我们在 SSIT 中观察到，随着剪切应力的应用，波幅显着增加。这种现象不能归因于 NSHT 中观察到的时间效应。相反，它可以用 Tabor 提出的“结生长”机制来解释。结增长导致实际接触面积增加。发生结生长需要接触的材料在纯正常负载条件下已经是塑性的。计算机模拟证实我们的实验满足了这一要求。我们还发现，在发生粘滑事件之前，振幅增加的速率略有降低。还原井的开始时间与前兆滑移的开始时间一致。减少的原因是由于位移导致静止接触时间重置。 NSHT 的结果支持了这一解释。考虑到 SSIT 中的静止接触时间，我们预计波幅的变化最多只有百分之几。从这个意义上说，观察到的增长率略有下降是合理的。断层的静态刚度也随着前兆滑移而降低。研究还发现，低频波比高频波更能指示前兆滑移。这可能表明波长较长的低频波可以更好地指示故障的平均行为。然而，这个问题值得进一步调查。还发现相移可以很好地指示故障接触状态的变化。通过 Pyrak-Nolte 等人提出的传输系数理论，可以统一理解传输波的幅度和相位的变化。粗糙表面比光滑表面更容易产生更大的粘滑现象。粗糙表面的先兆滑移量比光滑表面的大。尽管通过计算机模拟证实粗糙表面比光滑表面具有更大的接触直径，但表面粗糙度（接触直径）与前驱滑移量之间的严格关系尚未建立。
We performed a series of laboratory experiments in which elastic waves were transmitted across a simulated fault. Two types of experiments were carried out: (1) Normal Stress Holding Test (NSHT): normal stress was kept constant for about 3 h without shear stress and transmission waves were observed. (2) Shear Stress Increasing Test (SSIT): shear stress was gradually increased until a stick-slip event occurred. Transmission waves were continuously observed throughout the process of stress accumulation. We focused on the change in transmission waves during the application of shear stress and especially during precursory slips. It was found in NSHT that the amplitude of transmission waves linearly increased with the logarithm of stationary contact time. The increase amounted to a few percent after about 3 h. Creep at asperity contacts is responsible for this phenomenon. From a theoretical consideration, it was concluded that the real contact area increased with the logarithm of stationary contact time. We observed in SSIT a significant increase in wave amplitude with shear stress application. This phenomenon cannot be attributed to the time effect observed in NSHT. Instead, it can be explained by the mechanism of “junction growth” proposed by Tabor. Junction growth yields an increase in real contact area. It is required for junction growth to occur that the material in contact is already plastic under a purely normal loading condition. A computer simulation confirmed that this requirement was satisfied in our experiments. We also found that the rate at which the amplitude increased was slightly reduced prior to a stick-slip event. The onset time of the reduction well coincides with the onset of precursory slip. The cause of the reduction is attributed to the reset of stationary contact time due to displacement. This interpretation is supported by the result of NSHT. Taking the time of stationary contact in SSIT into account, we may expect the change in wave amplitude to be, at most, only a few percent. The observed slight reduction in increasing rate is, in this sense, reasonable. The static stiffness of the fault also decreases with precursory slip. It was also found that low frequency waves are a better indicator of precursory slip than high frequency waves. This might suggest that low frequency waves with longer wavelength are a better indicator of average behavior of faults. The problem, however, merits a further investigation. The shifts in phase were also found to be a good indicator of the change in contact state of the fault. The changes in both amplitude and phase of transmission waves are unifyingly understood through the theory of transmission coefficient presented by Pyrak-Nolte et al. Rough surfaces have a tendency to give larger stick-slips than smooth surfaces. The amount of precursory slip is larger for rough surfaces than for smooth surfaces. Although it was confirmed by a computer simulation that rough surfaces have larger contact diameters than smooth surfaces, the rigorous relationship between the surface roughness (contact diameter) and the amount of precursory slips was not established.