## 2 Materials and Methods  
### Sample and Loading Conditions  
Most of the devastating earthquakes are located in the upper crust, which has a granodioritic bulk composition [31]. Therefore, the use of the granodiorite as a sample to simulate the earthquake rupture process is representative. A granodiorite sample with size of 300 \(\times\) 300 \(\times\) 50 mm was cut through to form a bent fault. The fault surface was ground with a diamond wheel with a particle size of 150\(\#\). The roughness of the fault surface was \(\sim\)100 \(\mu\)m before loading. The elastic modulus, Poisson's ratio, and shear modulus of the sample were 60 GPa, 0.27, and 24 GPa, respectively, which were tested via a uniaxial press machine. As shown in Figure 1, the bend point divided the fault into two segments of equal length. The segment with a small angle (42.5\({}^{\circ}\)) from the direction of \(\sigma_{1}\) was referred to as segment \(S_{I}\), while the other segment with a larger angle (47.5\({}^{\circ}\)) from the direction of \(\sigma_{1}\) was referred to as segment \(S_{II}\). The angle between the two segments was 5\({}^{\circ}\). The bend point was located on one diagonal of the sample and was offset 6.549 mm from the geometric center of the sample. Axis \(D\) coincides with the fault trace. The common origins of axes \(D\), \(X\), and \(Y\) were located at the bend point of the fault. The coordinates of segments \(S_{I}\) and \(S_{II}\) on axis \(D\) were negative and positive, respectively. During the experiment, the sample was placed in a horizontal biaxial hydraulic servo control loading apparatus for loading. The maximum load in each axis of the loading apparatus was 1000 kN. The range of the displacement rate of the piston in each axis of the loading apparatus was from 0.01 to 100 \(\mu\)m/s. In order to ensure that the loading system was stable and the loading process was not interrupted even in the case of stick-slip motion of the sample, and to produce the suitable stick-slip cycle durations for observation, we used the following loading mode. The loads along the \(X\)-direction and \(Y\)-direction were synchronously increased from 0 to 4.63 MPa, then the load along the \(X\)-direction was held constant at 4.63 MPa, while the \(Y\)-direction was transferred to a displacement rate control of 0.5, 0.1, and 0.05 \(\mu\)m/s successively to make the sample generate dextral stick-slip motions. The sample used in this experiment was a repetition of previous studies [23; 24] and the loading procedure was also similar to previous studies [23; 24]. The variations of the differential stress (\(\sigma_{1}\)-\(\sigma_{2}\)) applied to the end of the sample by the apparatus and the displacement of the piston of the loading end along the \(Y\)-direction (dy) with time in the experiment are shown in Figure 2. Each stress drop in Figure 2 corresponds to a stick-slip instability event of the fault. See our previous paper [25] for details on the loading system. Figure 1: Experimental design. The field of view of the high-speed camera covers the entire sample surface. The \(D\)-axis coincides with the fault trace. The common origins of the \(D\)-, \(X\)-, and \(Y\)-axis coincide with the bend point of the fault. The fault is divided into segments \(S_{I}\) and \(S_{II}\). The red lines on both sides of the fault are each composed of 1700 measuring points (pixels), which are symmetric with the fault and offset 5.839 mm from the fault. T1–T14 are the numbers of 14 strain gage groups mounted along the fault on the bottom sample surface. The inset pointing to the T5 strain gage group shows details of the  
arrangement of the three strain gages forming the strain gage group on the sample surface. The illustration on the right is an enlarged view of the fault slip gages in the dashed circle around the fault bend. The red solid circles are the measuring points (pixels) with a 5.839 mm offset from the fault.