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

从实验和理论上都知道，在动态破裂扩展之前，存在一个缓慢的前兆滑动阶段。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报非常有帮助。这项研究的目的是找到一种方法，通过主动监测穿过模拟断层的传输波来检测成核过程，其中断层之间有一层断层。在此基础上，利用离散元方法对透射波的变化机理进行了数值模拟。通过一系列的室内实验，我们发现粘滑总是伴随着较大的前兆滑动和较大的泥层剪胀而最终发生的。研究还发现，在前兆滑动和泥层膨胀过程中，透射波的幅度明显减小，并用DEM模拟方法对其机理进行了探讨。DEM模拟很好地再现了实验的全过程，即在最终粘滑之前发生了前兆滑移和剪胀。通过模拟，我们可以观察到应力累积过程中发生的情况。这一点概括如下。1.在施加剪应力之前，应力链在断层泥中均匀分布。2.随着剪应力的增加，有选择地产生少量的强柱，以支撑总力。这可以被视为一种自组织临界性(SOC)。3.随着剪力的进一步增大，柱发生转动，同时产生前兆滑移和剪胀。泥层内部结构的这种变化使波的透射率极低。4.当柱状结构破裂时，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.