Experimental study of the rupture nucleation on periodically heterogeneous faults

Abstract:

Seismologic and geodetic observations, such as observation of repeaters, foreshocks migration or pre-seismic slow deformation, suggest that slow preparatory processes may occur before some large earthquakes. In experimental studies, a slow and localized nucleation phase is consistently observed, growing slowly and continuously, and accelerating toward the fast radiating phase of the rupture. However, it is difficult to link these experimental observations to the observations made on natural faults, because these experimental studies have been conducted on unrealistic smooth homogeneous faults.

We present experimental observations that explore the effects of fault heterogeneity on the rupture process, and especially on the nucleation phase. We conducted friction experiments between two polycarbonate plates, with a periodically heterogeneous friction interface. We image and monitor the rupture propagation with an ultrafast video camera, acoustic transducers, and a strain gauge. We show that in this heterogeneous case, the nucleation process no longer consists in a monotonic growth of the rupture velocity. Instead, the propagation of the rupture nucleation tip alternates between slow and fast episodes until a critical nucleation length is reached. This preparatory process is thus partly radiating waves, and is not completely silent as in the homogeneous case. We show that the global length and duration of this nucleation process are anti-correlated to the stress drop. Finally, we also observe a large variability in the rupture process due to the balance between the friction heterogeneity, which is controlled, and the stress heterogeneity, which arises from the loading process and the successive slip events.