InSAR data reveal that the largest hydraulic fracturing-induced earthquake in Canada, to date, is a slow-slip event.

For tectonic earthquakes, slip rate spans a continuum from creep to supershear earthquakes, where slow slip events (SSEs) are important in releasing stress without radiating damaging seismic energy. Industrial-scale subsurface fluid injection has caused induced earthquakes, but the role of SSEs in fault activation is currently unclear. Ground-deformation observations, measured by satellite radar, show that SSEs up to magnitude 5.

The role of aseismic slip in hydraulic fracturing-induced seismicity.

Models for hydraulic fracturing-induced earthquakes in shales typically ascribe fault activation to elevated pore pressure or increased shear stress; however, these mechanisms are incompatible with experiments and rate-state frictional models, which predict stable sliding (aseismic slip) on faults that penetrate rocks with high clay or total organic carbon. Recent studies further indicate that the earthquakes tend to nucleate over relatively short injection time scales and sufficiently far from the injection zone that triggering by either poroelastic stress changes or pore pressure diffusion is unlikely. Here, we invoke an alternative model based on recent laboratory and in situ experiments, wherein distal, unstable regions of a fault are progressively loaded by aseismic slip on proximal, stable regions stimulated by hydraulic fracturing.