The influence of a transitional stress regime on the source characteristics of induced seismicity and fault activation; evidence from the 30 November 2018 Fort St. John Ml 4.5 induced earthquake sequence

On 30 November 2018, a sequence of seismicity including a felt (ML approximately 4.5) induced earthquake occurred approximately 16 km southwest of Fort St. John, British Columbia. Using a local seismograph network around the epicentral region, we identified >560 seismic events over a two-week period, incorporating two mainshock events within a 45 min time interval, both with M (sub L) >4.3. This seismicity occurred close in location and depth to ongoing hydraulic fracturing operations. Using previously unpublished data, our analysis suggests that events, including the largest mainshock, occurred at the interval of fluid injection, which is shallower than previously reported. The events showed a mix of reverse, oblique normal, and strike-slip mechanisms within a well-defined structural corridor that forms the southern margin of the Fort St. John graben. The two mainshock events reveal opposing mechanisms: one as a reverse (re)activation of a normal fault (M (sub L) 4.5) and the other an oblique normal mechanism (M (sub L) 4.3). Stress inversion and bootstrap analysis of 72 well-constrained focal mechanisms indicate that the maximum principal stress direction is horizontal, oriented in a north-northeast direction (3 degrees -36 degrees ). However, the intermediate and minimum stress axes fluctuate between horizontal and vertical and are nearly equal in magnitude, indicating that both reverse and strike-slip regimes can occur in response to relatively small stress perturbations. Stress inversions using event subsets before and after the largest mainshock reveals an approximately 30 degrees counter-clockwise coseismic rotation of the principal stress axes in the hypocentral region. Furthermore, the observed seismicity suggests that the largest mainshock event exceeded the calculated M (sub max) using models based on injected volumes, suggesting that it may be an example of runaway rupture. This has important implications for risk analysis, because small changes in the stress field may be induced through ongoing operations in this area, destabilizing different faults within a complex structural environment.