Abstract

Induced seismicity due to fluid injection, including hydraulic fracturing, is an increasingly common phenomenon worldwide. Yet, the mechanisms by which hydraulic fracturing causes fault activation remain unclear. Here we show that pre-existing fracture networks are instrumental in transferring fluid pressures to larger faults on which dynamic rupture occurs. To date, studies of hydraulic fracturing-induced seismicity have used observations from regional seismograph networks at distances of 10's km, and as such lack the resolution to answer some of the key questions currently in the field. A high-quality dataset acquired at a hydraulic fracturing site in Alberta, Canada that experienced several events over MW 2.0 is presented for the purpose of analysing detailed mechanisms of fault activation. Both event hypocentres and measurements of seismic anisotropy reveal the presence of pre-existing fracture corridors that allowed communication of fluid-pressure perturbations to larger faults, over distances of up to a km or more. The presence of pre-existing permeable fracture networks can significantly increase the volume of rock affected by the pore pressure pulse, thereby increasing the probability of induced seismicity. This study demonstrates the importance of understanding the connectivity of pre-existing fracture networks as a tool for assessing potential seismic hazards associated with hydraulic fracturing of shale formations, and offers a conceptual understanding of induced seismicity due to hydraulic fracturing.