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.