The evolution of frictional strength, stability, and fracture permeability is intimately linked both to the seismic cycle and to the impact of hydraulic stimulation for fractured reservoirs. However, despite this importance, the poromechanical relationships between fault permeability and strength remain unclear. The present study explores this relationship via laboratory experiments for concurrent shear‐flow on smooth fractures of Westerly granite. The novelty of these experiments is that the shear velocity is precisely controlled during the measurement of fracture permeability. Results indicate permeability enhancement during velocity‐weakening (potentially unstable) frictional slip. To decipher key processes contributing to this response, we evaluate the state of contacting asperities and of fracture surface asperities via digital rock fracture modeling of statistically equivalent surfaces. We propose two plausible mechanisms constraining the relationship between friction and permeability evolution—one based on changes in asperity contact distribution and one on shear‐induced dilation triggered by changes in fault slip velocity. These mechanisms should be taken into account in interpreting field observation such as the abrupt permeability increase of natural faults at the onset of seismic slip.

We have developed a new approach for reflection seismic imaging using microseismic waveform data. We took the specific character of microseismic downhole data into account and were able to image regions in the direct vicinity or inside of a seismically active zone. We used a directional migration method that reduced imaging artifacts caused by the low aperture of the data, but required estimation of the wavefield polarization. We also used hodogram analysis to identify parts of data that provided reliable polarization estimates. Only these parts were then used for imaging, whereas the rest of the data are excluded. The approach was successfully applied to microseismic waveforms recorded during and after the stimulation of the enhanced geothermal system in Basel, Switzerland. This data set included more than 2000 events recorded at five sparsely distributed borehole sensors. The reflection images revealed a distinct network of reflectors in the vicinity of the open hole section within the injection well. The results were compared with the injection borehole ultrasonic image revealing a quite good agreement. Combined analysis of imaged reflectors, event locations, and event focal mechanisms indicated that the approach resolved mostly aseismic highly permeable fluid pathways, whereas most of the seismicity occurred in the second-order fractures.