Off-fault damage and acoustic emission distributions during the evolution of structurally-complex faults over series of stick-slip events

Thomas H. Goebel, Thorsten W. Becker, Charles G. Sammis, Georg Dresen, & Danijel Schorlemmer

Published June 2014, SCEC Contribution #1902

Key words:
fault structure, fractal roughness, acoustic emissions, seismic cycle, core deformation zone
Tectonic faults contain structural complexity at many different scales. Variations in fault structure, e.g. fault roughness or fault width, influence the location and dynamics of large earthquakes as well as the distribution of small seismic events. In nature, roughness and seismicity distributions can rarely be studied simultaneously. Consequently, little is known about their evolution with successive seismic cycles. Here, we investigate the connection between fault structure and near-fault distributions of acoustic emission events over series of stick-slip cycles in the laboratory. We conducted a set of experiments on rough fault zones that developed from initial, fracture surfaces while monitoring stress and seismic activity. We determined acoustic emission density distributions as function of fault normal distance based on high-accuracy hypocenter locations during subsequent interslip periods. The characteristics of these distributions were closely connected to different structural units of the faults, i.e. the fault core, off-fault and background damage zone. The core deformation zone was characterized by consistently high seismic activity, whereas the off-fault damage zone displayed a power-law decay of seismic activity with increasing distance from the fault core. The exponents of the power-law distributed off-fault activity increased with successive stick-slip events so that later interslip periods exhibited a more rapid spatial decay of seismic activity as function of fault-normal distance. This increase was strongest during the first one to three interslip periods and reached almost constant values thereafter. The relatively faster spatial decay of acoustic emission events for later interslip periods is likely an expression of decreasing fault zone complexity and roughness. Our results indicate a close relationship between fault structure, stress and seismic off-fault activity. A more extensive mapping of seismic off-fault activity-decay has the potential to significantly advance the understanding of fault zone properties including variations in fault roughness and stress.

Goebel, T. H., Becker, T. W., Sammis, C. G., Dresen, G., & Schorlemmer, D. (2014). Off-fault damage and acoustic emission distributions during the evolution of structurally-complex faults over series of stick-slip events. Geophysical Journal International, 197(3), 1705-1718. doi: 10.1093/gji/ggu074.