An experimental trial to detect nucleation processes of faults with a gouge layer by transmission waves

在实验和理论上，众所周知，在动态破裂传播之前存在一个缓慢的前兆滑移阶段。如果我们能够以某种方式探测到前兆滑动，这将对即将到来的地震预测非常有帮助。本研究旨在通过主动监测断层间断层泥层的透射波，找到一种检测成核过程的方法。在此基础上，我们利用离散元法（DEM）进行数值模拟，试图阐明透射波变化的机理。通过一系列的室内实验，我们发现一个粘滑总是在一个大的前兆滑动和断层层的大膨胀之后最终发生的。在断层泥层的前兆滑动和扩张过程中，透射波的振幅明显减小。通过DEM模拟研究了这一观测的机理。DEM模拟较好地再现了整个实验过程，即在最终粘滑之前发生了前兆滑动和扩容。模拟使我们能够观察到应力积累过程中发生了什么。总结如下。1.剪切应力作用前，断层泥层内应力链分布均匀。2.随着剪应力的增加，有选择地形成少量的强柱，以维持总力。这可以看作是一种自组织临界性（SOC）。3.随着剪力的进一步增大，柱体发生旋转，同时引起前期滑移和剪胀。断层泥层内部结构的这种变化使得波的透射率极低。4.当柱状结构被破坏时，会发生较大的动力滑移。

It has been, experimentally and theoretically, well known that there is a slow stage of precursory slip prior to a dynamic rupture propagation. If we were able to somehow detect the precursory slip, it would be very helpful for forthcoming earthquake prediction. This research aims to find a way to detect nucleation processes by actively monitoring transmission waves across a simulated fault with a gouge layer in between. Further we try to elucidate the mechanism of change in transmission waves by a numerical simulation using DEM (Discrete Element Method).By a series of laboratory experiments, we found that a stick-slip always finally occurred following a large precursory slip and a large dilatancy of the gouge layer. It was also found that the amplitude of the transmission waves was remarkably decreased during the process of precursory slip and dilation of the gouge layer.The mechanism of this observation was investigated by means of DEM simulation. The DEM simulation well reproduced the whole process of the experiment, that is, a precursory slip and a dilatancy occurred prior to a final stick-slip. The simulation enabled us to observe what was going on during the stress accumulation process. This is summarized as follows. 1.The stress chain is homogeneously distributed in the gouge layer before shear stress application. 2.As the shear stress is increased, a small number of strong columns are selectively created, which sustains the total force. This may be regarded as a kind of self-organized criticality (SOC). 3.As the shear force is further increased, the columns are rotated, which simultaneously causes the precursory slip and the dilatancy. This change in the inner structure of the gouge layer makes the wave-transmissivity extremely low. 4.When the columnar structures are broken, a large dynamic slip occurs.