Unprecedented quiescence in resource development area allows detection of long-lived latent seismicity

Abstract. Recent seismicity in Alberta and British Columbia has been attributed to ongoing oil and gas development in the area, due to its temporal and spatial correlation. Prior to such development, the area was seismically quiescent. Here, we show evidence that latent seismicity may occur in areas where previous operations may have occurred, even during a shutdown in operations. The global pandemic of COVID-19 furnished the unique opportunity to study seismicity during a period of anthropogenic quiescence. A total of 389 events were detected within the Kiskatinaw area of British Columbia from April to August 2020, which encompasses a period with no hydraulic fracturing operations during a government imposed lockdown. Apart from a reduction in seismicity rate, the general characteristics of the observed seismicity were similar to the preceding time period of active operations. During the shutdown, observed event magnitudes fell between M_L −1 and M_L 1.2, but lacked temporal clustering that is often characteristic of hydraulic-fracturing induced sequences. Hypocenters occurred in a corridor orientated NW-SE, just as seismicity had done in previous years in the area, and locate at depths associated with the target Montney formation or shallower (< 2.5 km). A maximum of 21 % of the detected events during lockdown may be attributable to natural seismicity, with a further 8 % being attributed to dynamic triggering of seismicity from teleseismic events. However this leaves over 70 % of the seismicity detected during lockdown being unattributable to primary activation mechanisms. Since we know this seismicity cannot be the result of direct pore-pressure increases (as no direct injection was occurring at the time) and we see no patterns of temporal or spatial migration in the seismicity, we suggest that this latent seismicity may be generated by aseismic slip as fluids (resulting from previous hydraulic fracturing experiments) become trapped within permeable formations at depth, keeping pore pressures in the area elevated, and consequently allowing the generation of seismicity. This is the first time that this latent seismicity has been observed in this area of British Columbia.