Experimental Study on Near-Fault High-Frequency Strong Motion

对预先存在人工断层的岩石试件进行了粘滑实验，观测了粘滑运动产生的近断层高频弹性波。主要结果如下：1）观测到的近断层高频强震持续时间与局部破裂时间近似成正比。2)观测到的最高峰值加速度为几十km/s^2，考虑到地震的大小和频率差异，这相当于天然地震的1000 gal。此外，我们模拟粘滑运动的动态行为，在我们的实验室实验中观察到：一些圆形的凸体分布在一个给定的断层面上。断层上的每一个微凸体在宏观破裂前缘到达后破裂。然而，我们认为，在破裂前沿到达的时间和破裂开始的粗糙体之间存在着一定的时滞。假设单个微凸体的时滞服从一定的概率密度函数。合成的强震动很好地再现了观测到的强震动特征，基于该模型的上述结果表明，既存断层不稳定滑动过程中观测到的高频强震动主要是由断层面凹凸体的脆性破裂产生的。由此可见，即使在预先存在的断层上宏观稳定滑动的情况下，也应该观察到由于粗糙体的脆性断裂而产生的声发射（AE）。为了验证这一点，进行了稳定滑动的实验上的岩石标本与预先存在的故障。在宏观稳定滑动期间观察到许多AE，以证实我们的理论预期。进一步发现，随着宏观滑动速度的增加，单位滑动距离的声发射数量和声发射的平均幅度减少。

A stick-slip experiment is performed on a rock specimen with a pre-existing artificial fault to observe near-fault high-frequency elastic waves generated by stick-slip motion. The main results are as follows : 1) The observed near-fault high-frequency strong-motion duration is approximately proportional to the local breakdown time. 2) The highest peak acceleration observed amounts to a few tens of km/s^2, which is considered to correspond to about 1000gal in the case of natural earthquakes, when the differences is size and frequency are taken into account. Further we model the dynamic behavior of stick-slip motion observed in our laboratory experiment : A number of asperities circular in shape are assumed to distribute on a given fault surface. Each asperity on the fault is to be ruptured after the arrival of the macroscopic rupture front. We consider, however, that there exists some time lag between the arrival time of rupture front and the rupture initiation of the asperity. It is assumed that the time lags for individual asperities obey a certain probability density function. The synthesized strong motions reproduce well the characteristics of the observed strong motions.The above result based on the model implies that high-frequency strong motions observed during unstable sliding on a pre-existing fault are generated mainly by brittle fracture of asperities on the fault surfaces. It follows from this that acoustic emission (AE) due to brittle fracture of asperities should be observed even in the case of macroscopically stable sliding on a pre-existing fault. To examine this, an experiment of stable sliding is performed on a rock specimen with a pre-existing fault. A number of AEs are observed during macroscopically stable sliding to confirm our theoretical expectation. It is further found that the number of AEs per unit slip distance and the average magnitude of AEs are decreased with an increase in macroscopic sliding-speed.