Relative Slip in Damage Zones: Insights from Bishop Tuff Normal Faults

Abstract:

Faults are embedded in damage zones where rocks are intensely fractured. Greater compliance in these zones impacts earthquake behavior on master faults. To understand these impacts, the extent and mechanical properties of damage zones need to be known. To approach this question, we target active normal faults in Bishop Tuff, California, and examine whether their damage zones obey some scaling relations in their architecture and relative slip. Normal faults are ideal targets as they form high and clear escarpments that ease their identification and the measure of their total slip. Using satellite (Pléiades), airborne (LiDAR) and drone images (resolution 70, 50, and 1 cm, respectively), we built georeferenced high-resolution ortho-images and DEMs of the Bishop site. In these data, we identified a set of primary faults (lengths 0.1-10 km) and delineated their associated zones of secondary faulting off their trace. We then measured the vertical cumulative slips across the faults with different methods. In particular, we developed a Matlab code, Auto_throw, that does not need any prior knowledge of the faults (but mean strike of master fault). Using piecewise linear fitting of elevation profiles across the faults, the code defines "flat" sections offset by steeper, "scarp" sections. Based on a Monte-Carlo method, the vertical offset between two neighboring flat sections is calculated. The hundreds of solutions per profile are then analyzed statistically, so that eventually, only actual fault scarps are revealed with their vertical throw measured. Preliminary results show that: 1) secondary faults are detected away from the master fault trace, up to distances equal to ~10% of the master fault length; 2) damage is greater close to the master fault, in an “inner zone” of width <~5% of the master fault length, which accommodates ~35% of the total master fault slip; 3) off this zone, secondary splays form an outer damage zone, which accommodates ~25% of the total master fault slip.