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Solid Earth An interactive open-access journal of the European Geosciences Union
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Short summary
Earthquakes in Alberta and British Columbia are usually thought to be a response to human activities. The COVID-19 pandemic forced all such activities to stop for ~4 months. Yet earthquakes still occurred during this time, which was unexpected. Some of these earthquakes may be natural. However over 70 % of the earthquakes occurring during this time are thought to be the remnants of previous human activities in the area, which has altered the state of stress thus allowing earthquakes to occur.
Preprints
https://doi.org/10.5194/se-2020-203
https://doi.org/10.5194/se-2020-203

  08 Dec 2020

08 Dec 2020

Review status: this preprint is currently under review for the journal SE.

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

Rebecca O. Salvage and David W. Eaton Rebecca O. Salvage and David W. Eaton
  • Dept. of Geoscience, University of Calgary, Calgary, AB, T2N 1N4

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 ML −1 and ML 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.

Rebecca O. Salvage and David W. Eaton

 
Status: open (until 20 Jan 2021)
Status: open (until 20 Jan 2021)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Rebecca O. Salvage and David W. Eaton

Data sets

EO Seismic Network Data University of Calgary hosted by The IRIS Data Management Center (IRISDMC) https://doi.org/10.7914/SN/EO

Model code and software

Code for Noise Analysis Thomas Lecocq and Fred Massin https://doi.org/10.5281/zenodo.3820046

Rebecca O. Salvage and David W. Eaton

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Short summary
Earthquakes in Alberta and British Columbia are usually thought to be a response to human activities. The COVID-19 pandemic forced all such activities to stop for ~4 months. Yet earthquakes still occurred during this time, which was unexpected. Some of these earthquakes may be natural. However over 70 % of the earthquakes occurring during this time are thought to be the remnants of previous human activities in the area, which has altered the state of stress thus allowing earthquakes to occur.
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