## 1 Introduction  
The experimental study of the evolution of earthquake ruptures is of great significance for understanding the underlying physical process of earthquake preparation and occurrence. Geological surveys and field observation data showed that fault geometry plays important roles in the initiation and propagation of earthquake ruptures [1; 2; 3; 4; 5; 6; 7; 8]. Numerical simulations analyzed the influences of the fault bend on the rupture process and the fault slip distribution, which revealed that the angle of the fault bend, the normal stress, and the loading mode play important roles in the initiation and propagation of the ruptures [9; 10; 11; 12; 13; 14]. Specifically, the rupture zone and overall slip distribution on the fault are controlled by the angle of the fault bend, while rupture velocity and detailed slip distribution around the bend are influenced by time-dependent normal stress changes caused by the rupture [12]. The fault bend will serve as an initiation and/or a termination point for the rupture via reducing normal stress on the dilatational segment and increasing normal stress on the compressive segment of the bent fault during dynamic ruptures [11]. The angle of the fault bend and the sliding direction of a dip-slipbent fault control the time and location of the rupture nucleation [13]. In addition, the rupture process of the Chi-Chi earthquake [10; 15], Landers earthquake [16], and Izmit earthquake [17] were reconstructed via numerical simulations to further reveal the mechanism of the influence of fault geometry on the earthquake rupture process using seismic and geodetic data combined with bent fault models. Physical experiments have been carried out to study the rupture process and the evolution of the relevant physical fields of bent faults. It was observed that the two segments on each side of the fault bend became active alternatively during sliding, which implied that the fault bend plays a role of a valve during sliding [18]; a two-step rupture propagation process was observed prior to the overall instability of the fault. Namely, the dynamic rupture started on a fault segment is stopped near the fault bend, which is restarted near the bend on the other fault segment after a certain delay time and leads to the overall slip of the entire fault without being arrested by the presence of the fault bend [19]. The influence of fault bends on the growth of sub-Rayleigh and intersonic dynamic shear ruptures was also studied in the laboratory [20]. In addition, the changes and characteristics of the deformation fields before and after the instability of the bent faults were also studied in the laboratory [21; 22; 23; 24]. From the above results based on experiments conducted in rock materials [18; 19; 21; 22; 23; 24], alternative activities were observed between the two fault segments on both sides of the bend before the overall instability of the fault. Since these experimental results were obtained via point contact type observation methods, such as strain gages, the spatial sampling was quite sparse. As a result, the detailed spatiotemporal evolution of ruptures cannot be observed. Accordingly, at least two problems remain unclear: (1) Is there only one alternative propagation of ruptures between the two fault segments on both sides of the bend prior to the overall instability of the fault as proposed in [19]? (2) What is the characteristic of the ruptures during their alternative propagation between the two fault segments? The solution to the two problems depends on the observation of the detailed spatiotemporal evolution of the ruptures. Therefore, it is necessary to further study the rupture evolution of the bent fault using high spatiotemporal sampling observation methods. An experimental study depends strongly on observations. Quantitative measurement of fault slip, slip velocity, and strain via intensive sampling in time and space is very important to reveal the detailed spatiotemporal evolution of fault ruptures.