Seismic event distributions and off-fault damage during frictional sliding of saw-cut surfaces with pre-defined roughness

The motion along upper crustal faults in response to tectonic loading is controlled by both loading stresses and surface properties, for example, roughness. Fault roughness influences earthquake slip distributions, stress-drops and possible transitions from stable to unstable sliding which is connected to the radiation of seismic energy. The relationship between fault roughness and seismic event distributions is insufficiently understood, in particular, the underlying mechanisms of off-fault seismicity creation in the proximity of rough faults are debated. Here, we investigate the connection between roughness and acoustic emission (AE) density with increasing fault-normal distance during loading of surfaces with pre-defined roughness. We test the influence of fault roughness and normal stress variations on the characteristics of AE off-fault distributions. To this end, two sets of experiments were conducted: one to investigate the influence of initial surface roughness at constant confining pressure, and the other to investigate the influence of fault-normal stresses at constant roughness. Our experiments reveal a power-law decay of AE density with distance from the slip surface. The power-law exponents are sensitive to both fault roughness and normal stress variations so that larger normal stresses and increased roughness lead to slower AE density decay with fault-normal distance. This emphasizes that both roughness and stress have to be considered when trying to understand microseismic event distributions in the proximity of fault zones. Our results are largely in agreement with theoretical studies and observations of across-fault seismicity distributions in California suggesting a connection between off-fault seismicity and fault roughness over a wide range of scales. Seismicity analysis including a possible mapping between off-fault activity exponents, fault stresses and roughness, can be an important tool in understanding the mechanics of faults and their seismic hazard potential. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society