Laboratory Observations of Linkage of Preslip Zones Prior to Stick-Slip Instability

Field and experimental observations showed that preslip undergoes a transition from multiple to single preslip zones, which implies the existence of linkage of preslip zones before the fault instability. However, the observations of the linkage process, which is significant for understanding the mechanism of earthquake preparation, remains to be implemented due to the limitations of observation methods in previous studies. Detailed spatiotemporal evolutions of preslip were observed via a high-speed camera and a digital image correlation method in our experiments. The normalized length of preslip zones shows an increase trend while the normalized number of preslip zones (N_N) shows an increase followed by a decrease trend, which indicate that the expansion of the preslip undergoes a transition from increase to linkage of the isolated preslip zones. The peak N_N indicates the initiation of the linkage of preslip zones. Both the linkage of the preslip zones and the decrease in the normalized information entropy of fault displacement direction indicate the reduction of spatial complexity of preslip as the instability approaches. Furthermore, the influences of dynamic adjustment of stress along the fault and the interactions between the asperities and preslip on the spatial complexity of preslip were also observed and analyzed.