## 4 Discussion  
### Reliability Test of the DIC Method via Strain Gage Measurement  
The above fault slip velocity measured by the DIC method spanned a wide range of at least 3 orders of magnitude, as shown in Figure 4, Figure 5, and Figure 6. Although the high slip velocity was easily observed by the DIC method with a high signal-noise ratio, the reliability of the observed low slip velocity needs to be tested, especially in the first cycle of the alternative propagation stage where the slip velocity can be as low as below 1 \(\upmu\)m/s, as shown in Figure 4a, Figure 5a, and Figure 6a. Such a low slip velocity leads to a problem of whether the observed velocity was a fault slip or a strain buildup. To make it clear, a high precision measurement of the shear strain along the fault is necessary because local shear strain will drop as local instability occurs in the fault [35; 36]. Therefore, we used the evolution of shear strain at the 14 locations along the fault to test whether the observed slip velocity in the first cycle of the alternative propagation stage, as shown in Figure 4a, Figure 5a, and Figure 6a, was a fault slip or a strain buildup. Since the sampling rate of the strain acquisition system was only 100 Hz, the shear strain measurement was unable to test the rupture in other cycles of the alternative propagation stage.  
Figure 7 shows the evolution of the shear strain in a duration of 7 s covering the first cycle of the alternative propagation stage of the three stick-slip events. The subgraphs (a) and (b), (c) and (d), (e) and (f) in Figure 7 correspond to events E1, E2, and E3, respectively. The changes of the average shear strain in the whole fault, the segment \(S_{I}\), and the segment \(S_{II}\) with time are shown on the left side of Figure 7. The changes of the shear strain of each strain gage group with time are shown on the right side of Figure 7. The red triangles in Figure 7a,c,e mark the turning points in the average shear strain of the fault segments. Figure 7a shows that the average shear strain in segment \(S_{I}\) increased and decreased just before and after the red triangle (\(t\sim-2.8\) s), respectively, while the average shear strain continuously increased in segment \(S_{II}\) at the red triangle. These indicate that segment \(S_{I}\) began to slip at \(t\sim-2.8\) s. Figure 7b shows that the rupture initiated near group T1, of which the shear strain dropped first. These are consistent with the results observed in Figure 4a.  
The two red triangles in Figure 7c show that the average shear strain in segment \(S_{II}\) changed from ascending to descending at \(t\)\(\sim\)\(-2.8\) s, while the average shear strain in segment \(S_{I}\) changed from decreasing to rising at \(t\)\(\sim\)\(-2.4\) s. This indicates that the rupture gradually shifted from segment \(S_{I}\) to segment \(S_{II}\) between \(t\)\(\sim\)\(-2.8\) s and \(t\)\(\sim\)\(-2.4\) s, which is consistent with the evolution of the rupture observed in Figure 5a.  
The red triangle in Figure 7e shows that the average shear strain began to rise slowly in segment \(S_{I}\) but decreased in segment \(S_{II}\) after \(t\)\(\sim\)\(-3.1\)s, indicating that the rupture initiated in segment \(S_{II}\) before it propagated to segment \(S_{I}\) during this period. Figure 7f shows that the shear strain of group T11 dropped first, which indicates that the rupture initiated near group T11. These are consistent with the observations in Figure 6a.  
The consistency of the observations between the strain gages and the DIC method indicates that the rupture evolution shown in Figure 4a, Figure 5a, and Figure 6a are reliable.