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

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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.