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Connecting Acoustic Emission Event Locations, Aftershock Density and b-values before and after Stick-Slips to Changes in Topography of Rough Fracture Surfaces during Frictional Sliding Experiments

GOEBEL, T. H. W., USC, Los Angeles, CA, tgoebel@usc.edu; BECKER, T., USC, Los Angeles, CA, twb@usc.edu; SCHORLEMMER, D., SCEC, Los Angeles, CA, ds@usc.edu; STANCHITS, S., Schlumberger Terra Tek, Salt Lake City, UT, stanch@gfz-potsdam.de; RYBACKI, E., GFZ-German Research Center for Geosciences, Potsdam, Germany, erik.rybacki@gfz-potsdam.de; DRESEN, G., GFZ-German Research Center for Geosciences, Potsdam, Germany, dre@gfz-potsdam.de

Frictional properties of fault surfaces fundamentally influence the local strength of the seismogenic crust. We investigate the relationship between fault roughness and the creation of micro-cracks and damage by analyzing acoustic emission (AE) events emitted during sliding of naturally developed rough surfaces in the laboratory. We developed a three stage procedure, where initially rock samples are fractured under triaxial loading, followed by fault locking due to confining pressure increase and a final stage of fault reactivation. We observe three different types of behaviors: (1) frictional sliding with decreasing differential stress (quasi creeping fault) (2) linear and non linear stress increase, slow as well as abrupt stress drops (3) approximately linear stress increase followed by abrupt stress drop events between 10 to 200 MPa (stick slips with saw tooth pattern). We performed a detailed spatial analysis of event clusters before and after stick slips, primarily focusing on their b-values. Stress drop events appear to nucleate at the periphery of low b-value regions while “aftershocks” occur within in most cases. In a further analysis step we investigate the temporal variations of b-values for AE events within low b-value regions. We observe a characteristic drop in b-values before followed by high b-values after stress drops events. In case of frictional sliding experiments, the fault topography from X-ray scans and visual inspection indicate that low b-value regions coincide with areas of positive topography. The fault structure consists of a gouge layer, a damage zone with varying crack density and the host rock. Natural rough faults strongly favor the creation of spatially and temporal distinct AE clusters in laboratory experiments which have similar characteristics to seismicity observed on crustal scales.

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Last Modified: 2011 Aug 10

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