In March 2020, the Province of British Columbia introduced measures aimed at slowing the spread of COVID-19, including the closure of schools and childcare facilities on 17 March and the closure of many businesses (in particular those that included daily human interaction) on 21 March. Although these closures did not include a government enforced closure of hydraulic fracturing activities, the stalling economy led to stock prices of operators within the KSMMA plummeting to record lows and as such the suspension of most activities (Stu Venables, British Columbia Oil and Gas Commission, personal communication, January 2021). Up until this point in 2020, similar patterns of seismicity to other years were observed in the KSMMA (Fig. ). A total of 4268 events were detected from the onset of data collection (22 January 2020) from the updated EO array (yellow triangles; Fig. ) to 1 April. Following the initial closure of businesses on 21 March, there is evidence of ongoing hydraulic fracture operations for ∼10 d, with associated heightened seismicity (Fig. b). This reflects the continuation of planned operations by companies within the KSMMA, following which no new operations were initiated due to the diminishing economy. Magnitudes of recorded seismicity prior to lockdown at the end of March range from ML -0.7 to ML 2.9.

At the beginning of April, a period of relative seismic quiescence began in the KSMMA (Fig. b; grey background). The COVID-19 pandemic not only limited the movement of people and shut businesses, but also caused an economic downturn in the energy sector, leading to the cancellation of many operations. Seismicity since the resumption of activities in the later summer months, ∼4 months after the lockdown began, is once again temporally clustered, with a total of 3176 events being recorded from 6 August 2020 to 1 January 2021. The largest magnitude event of 2020 occurred on 11 September at 22:37 UTC with an estimated ML of 3.1 (using ; previously estimated as ML 3.4 using ), after which proximal operations were suspended in line with the traffic light protocol introduced for the KSMMA . Seventy-three precursory events occurred over approximately 4 h prior to the ML 3.1 event, with events located within a small spatial extent (∼300 m × 150 m), probably directly related to ongoing operations in the area due to the correlation in space and time of events and fluid injection. Events within this precursory sequence had magnitudes between ML 0.2 and ML 2.6 and were all located at depths of approximately 2.05 km. Moment tensor results for the ML 3.1 event suggest a focal mechanism dominated by strike-slip .

A clear reduction in the number of seismic events was observed during the lockdown period from April to August 2020 in the KSMMA (Fig. b; grey background). Over the ∼4 months of relative quiescence, only 389 events were detected using the EO network and available public stations in the area. For comparison, 344 events were detected on this network over a single week from 8 to 15 February when operations were fully underway. On average during this period, the magnitude of events were smaller than during time periods when activity was driven by ongoing operations.

A reduction in seismic noise and therefore ground motion is also evident in the KSMMA following the introduction of government restrictions and the subsequent economic downturn in March 2020 (Fig. ). Unfortunately, the most central seismic stations in the EO array were not installed until immediately before and immediately after the end of lockdown (March and May, respectively) and therefore could not be used to analyse the long term changes in seismic noise. We chose station KSM08, located in the east of the KSMMA, approximately 14 km due north of the settlement of Rolla, due to the long, uninterrupted seismic data recorded at this station, as well as its proximity to recent dense clusters of ongoing seismicity (Fig. ). The recent seismicity close to KSM08 suggests that a number of wells in the vicinity were active prior to the economic downturn related to the COVID-19 pandemic and government-imposed lockdowns in March 2020. Heightened seismic ground motion is evident at KSM08 through January to March, as operations are ongoing (Fig. ). A significant decrease in average seismic ground motion is observed following the government restrictions in late March 2020, with the average displacement sitting well below the daytime mean calculated prior to lockdown. As restrictions ease, we see a large increase in ground motion following the reopening of businesses in May 2020, although this once again tails off through June and July. The decrease in average ground motion in the late summer is observed at all of the stations in the network, to varying degrees, and is believed to be related to the downturn in the industry (as a result of the COVID-19 pandemic) during which operations were suspended by companies in response to the COVID-19 pandemic (Stu Venables, British Columbia Oil and Gas Commission, personal communication, January 2021). The re-introduction of operations in August is clear from an increase in ground displacement and seismic noise, which has remained elevated ever since (although not as high as pre-lockdown levels).

Seismicity occurring during the period of quiescence from April to August 2020 within the KSMMA exhibits a number of characteristics indicative that it is a (latent) consequence of previous operations in the area. Figure  shows the temporal and spatial evolution of seismicity during this period. Firstly, perhaps unsurprisingly, seismicity does not occur in a distinct temporal pattern that exhibits clustering (Fig. a). A small number of events (about five) occur each day throughout the 4 month period. Event magnitudes also reveal no discerning patterns with time, with all events registering ML -0.7 to ML 1.2. Furthermore, the frequency index (FI) suggests no temporal patterns during the period of relative quiescence. The FI is a proxy for the spectral content of each waveform based upon the ratio of energy in low- and high-frequency windows , calculated at a single station. We show our analysis for station KSM06 (Fig. ) due to its proximity to the majority of the ongoing seismicity during this period of relative quiescence, although all stations within the EO network suggested no temporal patterns in the FI. A negative FI means the waveform is dominated by low-frequency energy (in this case 1–40 Hz); a positive FI demonstrates a majority of energy in the high-frequency band (40.1–80 Hz). Overall, the seismicity detected during the period of relative quiescence within the KSMMA shows no discerning temporal characteristics.

Spatially, seismicity detected during the COVID-19 lockdown period exhibits characteristics that are similar to the previously detected seismicity in the KSMMA (Fig. b). Most events occur in a corridor orientated NW–SE, similar to the spatial distribution of seismicity prior to lockdown. Some spatial clustering is evident (e.g. in May in the south; shown in yellow), but given the limited number of events this is difficult to determine with certainty. Most events during the quiescence period occur at focal depths of ∼0–4 km, which is similar to events prior to lockdown within the KSMMA, if potentially slightly shallower. Target formations for hydraulic fracturing within the KSMMA (Upper and Lower Montney) typically sit between 2 and 2.5 km (total vertical depth), with saltwater disposal (SWD) injecting at shallower depths (Michelle Gaucher, British Columbia Oil and Gas Commission, personal communication, November 2020). This suggests that events detected during the quiescence were generated in formations similar to those that occur when active hydraulic fracturing and SWD was ongoing.

Seismicity generated during this period of quiescence appears to share many characteristics with seismicity generated during hydraulic fracturing operations within the KSMMA. Although low in number, the event rate per day remains fairly constant throughout the ∼4 month period of limited hydraulic fracturing operations, with no apparent temporal decay (Fig. a). This contrasts the usual pattern of seismicity during active hydraulic fracturing operations, which is highly temporally (and spatially) clustered around the wells operating (Fig. ) e.g.. Figure b also suggests no change in the recorded magnitudes of events pre- and during lockdown. Furthermore, no discernible changes in the FI are observed during the lockdown period (Fig. a). In volcanic environments (for which the FI was developed), a low FI is thought to be indicative of the presence of fluid moving through the system directly influencing the generation of seismicity e.g.. However, in volcanic environments, low FI values typically contain energy of ≤5 Hz, with high FI values focused up to 15 Hz. The waveforms observed here contain much higher frequency energy than this, which is a common trait of seismicity generated in hydraulic fracturing environments e.g.. have suggested that events dominated by low-frequency energy in hydraulic fracturing environments may be indicative of slow-slip or aseismic deformation. Therefore, the low-frequency nature of the events identified here may be indicative of either aseismic deformation or the presence of fluids within the system, however discerning between the two processes and their relative importance is still an area of active research.

The estimated magnitude of completeness (Mc) during the lockdown period is ∼0.4 (Fig. ). We calculate Mc using the maximum-curvature method of . Given the reduction in noise during the period of quiescence (Fig. ), it is perhaps no surprise that a low Mc is identified. However, the Mc for 2020 (Mc∼0.4; Fig. b) cannot directly be compared to the Mc for 2018 (Mc∼1.0; Fig. a) since different methods were used in their magnitude calculations ( vs. ). The lowering of Mc between these years is fundamentally related to an increase in the monitoring capacity and detectability of small seismic events within the KSMMA . The estimated b value of 1.96 (Fig. ; calculated using a least-squares linear regression for events with magnitudes ≥ Mc) is similar to b values estimated from seismicity associated with hydraulic fracturing experiments in western Canada, suggesting an abundance of lower magnitude events . The fact that no large magnitude events were detected during the period of quiescence (no events ≥ ML 1.5) directly influences the estimated b value in this case. Interestingly, higher b values (as in this case) have typically been attributed to seismicity generated in normal faulting regimes . The KSMMA is strongly influenced by the Fort St. John Graben complex, an asymmetrical half graben that has also undergone significant strike-slip and rotational movement upon reactivation of the basement faults in the area , which may also be directly influencing the estimated b value. Furthermore, in hydraulic fracturing environments, b values of >2 have been associated with the stimulation of natural fractures at depth, with smaller b values associating with large-scale tectonic faults . In our case, this would suggest that the seismicity being generated may be related to both complex natural fracture systems and large-scale faults in the area, since the b value lies close to 2.

Seismicity during the quiescence appears to be spatially concurrent with previous seismicity in the area (Figs. b and ). However, there appears to be very little correlation between the spatial extent of seismicity and the most recent hydraulic fracturing activity in the area (active in March 2020 prior to lockdown; green squares in Fig. b). Seismicity during the quiescence appears in two planar elongated features extending in a NW–SE direction with lengths of up to 30 km (eastern segment), if assumed to be one feature. These features are not coincident with any known large-scale faults in the area e.g.. Seismicity recorded during this period of quiescence is generally located at a similar depth to the target formations of the Montney (∼2 km), as well as in the formations above. This suggests hydraulically connected pathways above the injection zone, perhaps within mechanically stronger lithologies, as has been previously suggested by in the Fox Creek region of Alberta (another area undergoing intensive hydraulic fracturing operations).

The generation of induced seismicity has often been successfully correlated to a number of injection parameters, including the injected volume of fluid e.g. and/or the pumping rate e.g.. Temporally, data are too sparse to draw conclusions as to whether any of these parameters directly influence the generation of latent seismicity within the KSMMA, although given that hydraulic fracturing operations during our period of interest were extremely limited, we know that almost all of this seismicity cannot be a direct response to this type of fluid injection. Only one well pad was operated during this period of quiescence, from 19 to 24 July (Stu Venables, British Columbia Oil and Gas Commission, personal communication, January 2021), and only four events have both temporal correlation (i.e. occurred during this time window) and spatial correlation (i.e. occurred within 5 km of this well pad). However, there is evidence that a small number of seismic events identified from April to August 2020 may be associated with SWD. Within the KSMMA, only eight SWD wells were active in 2020, compared to hundreds of hydraulic fracturing wells. Of these, only one well was active during our period of investigation (Fig. b). We infer that the seismicity occurring on 13 April 2020 (Fig. a; upper panel), where over 20 events were registered on the same day (significantly above the background rate of seismicity during this quiescence), may be due to SWD, accounting for ∼6 % of the detected seismicity during this period of quiescence. In this case, ongoing sustained SWD occurred ∼2 km away from these events. This offset is not unusual for SWD and associated seismicity; found an offset of ∼3.5 km between SWD and associated seismicity in Alberta.

PPSD is one of the most common methods used to characterize ambient seismic noise. However, the level of smoothing, the size of the data window used in analysis and the methodology itself may all influence the PPSD calculation and distort features of interest . Smoothing is primarily undertaken in order to reduce the uncertainty associated with the PPSD estimates and means that short spikes in noise (e.g. due to wind gusts or seismic activity) do not dominate the spectrum. In our case, the reduction in ground motion is much easier to determine from the average of the PPSD rather than individual estimates (Figs.  and ; green vs. grey lines). Although we use a period smoothing of 0.025 octaves, this is likely to provide adequate spectral resolution of spectral peaks, as shown by and therefore impacts our results minimally. We also use a window of 30 min (overlapping by 50 %) to try to reduce spectral leakage and variance when calculating the PPSD.

Earthquakes, and other transient signals, are likely to impact the estimation of ambient noise by generating large spikes in the data. However, the removal of seismicity from datasets is generally accepted as not necessary since they are low-probability occurrences within generally high-probability ambient seismic noise . Only teleseismic earthquakes appear to have any real affect upon PPSD calculations . A number of teleseismic events were detected during the period of quiescence analysis (e.g. a Mw 7.8 event on 20 July 2020, 99 km off the coast of Alaska) that may influence our calculation of PPSD. However, since we see no peak in the average ground motion at these times (e.g. no substantial peak in July 2020; Fig. ), we suggest that teleseismic events are not majorly influencing our results.

One signal that does clearly influence our PPSD results in Figs.  and  is wind and other meteorological phenomena. Poor weather conditions (with wind gusts exceeding 80 km h-1, ) were reported in the KSMMA at the end of April 2020 during an otherwise quiet time period when there were limited hydraulic fracturing operations and limited movement of people due to lockdown measures. Since the noise generated from wind gusts penetrates a wide frequency band, we are unable to completely filter it out. Using a filter between 4 and 14 Hz tries to eliminate some of these transient signals mostly associated with meteorological and oceanic conditions.

The suspension of almost all operations within the KSMMA in the summer of 2020 gives us unique insight into seismicity that cannot be directly correlated with injection, which has always been the inferred triggering mechanism for most (if not all) of the seismicity within the KSMMA. The characteristics of the seismicity generated during this period suggest no fundamental differences in terms of temporal or spatial patterns or magnitudes to previous seismicity within the KSMMA that can be correlated with injection. In fact, many of the characteristics appear to be equivalent to events detected prior to lockdown. Prior to the development of the Montney play, low magnitude natural seismicity within the KSMMA was undetectable, given the limitations of the available seismic networks in place. The Canadian National Seismograph Network (CNSN) recorded 20 earthquakes (ML 2.5–ML 4.3) from 1984 to 2008 within the KSMMA, which are assumed to be a mixture of natural events and those associated with early anthropogenic activities in the area. Three clusters of events in November to December 1984 (ML 2.2–ML 2.8), January to February 1992 (ML 2.5–ML 3.5) and December 1992 to January 1993 (ML 2.5–ML 4.1) have been associated with oil extraction and fluid injection (water) in the Eagle West and Eagle fields, just north of Fort St. John . One additional natural event occurred with a significantly larger magnitude in March 1986 (Mw 5.4) north-east of Prince George, British Columbia , still a significant distance away from the study area.

In order to investigate the likelihood that this detected seismicity during the period of quiescence is natural seismicity, we calculate the expected recurrence rates of seismicity within the KSMMA greater than ML 2.5 from historical data, which is the magnitude of completeness used for the determination of seismic hazard maps in Canada due to detection thresholds from the Canadian public seismic network. The total number of earthquakes detected by the national network from 1984 to 2008 within the KSMMA was 20 , suggesting a recurrence interval of 0.83 events per year. It is not unsurprising that during the period of quiescence, no events greater than ML 2.5 were detected, given this calculated recurrence interval. Following the Gutenberg–Richter formula , it stands that there should be a 100-fold increase in the event rate to estimate the number of events > ML 0.5 (assuming a b value of 1.0), suggesting an event count of 83. We use a b value of 1.0, rather than our calculated b value of 1.96 (Fig. ), since this is the expected b value for an area dominated by natural seismicity e.g.. Therefore, a maximum of 21 % of events detected during the period of relative quiescence might be attributable to natural seismicity, in addition to the 6 % attributable to SWD and one fluid injection well. However, this leaves ∼65 % (a further 8 % may be attributable to dynamic triggering effects; see below) of seismicity generated during this period of relative quiescence that is difficult to explain by either of these mechanisms and we suggest is more likely produced as a remnant to previous operations and therefore directly related to previous states of stress. With events being generated over 4 months since the almost complete cessation of operations, the state of stress at depth must be near-critical for an extended period of time in order to generate this latent seismicity.

It is widely reported that earthquakes can be generated by transient stress changes related to the passage of seismic waves (i.e. dynamic triggering) e.g.. In some cases, this dynamic triggering can also be delayed by days or weeks following a teleseismic event, potentially related to the re-distribution of pore fluid from the passing seismic waves or through initial aseismic slip on faults triggering seismicity . During the period of quiescence (28 March to 6 August 2020), 43 earthquakes of >M6 were reported by the that may have the potential to cause dynamic triggering. We follow the methodology set out by , whereby we first select only the teleseismic events that generated an estimated peak ground velocity of greater than 0.2 cm s-1 at any station within the KSMMA, as defined by , whereby 1log⁡A20=M-1.66log⁡10δ-2, and 2PGV≈2πA20T, where A20 is the peak waveform amplitude when filtered at 20 s, M is the magnitude, δ is the epicentre–station distance (in degrees) and T is the surface wave period (assumed to be 20 s). This method identified 40 events from the original list of teleseismic events. We then calculated the β statistic by 3βN1,N2,t1,t2=N2-EN2varN2, which is a quantitative measure of the level of dynamic triggering, representing the standard deviation in the background seismicity rate after a remote event. N1 and N2 are the number of earthquakes detected before (t1) and after (t2) the remote event, respectively. Here, we take t1 and t2 to be 12 h. E(N2)=N1×t2/t1 is the expected number of earthquakes after the main shock based on the background seismicity rate. If no earthquakes occur in t1 (i.e. before the main shock), N1 is set to 0.25 based on the equivalent range of the probability density function . When β≥2, there is sufficient statistical evidence (at a 95 % confidence level) that there is a significant increase in the seismic event rate following the remote event .

We identify seven remote earthquakes that generate a β value ≥ 2 (Fig. ), including the largest event to have occurred in 2020 located 99 km SSE of Perryville, Alaska, on 22 July at 06:12 UTC with Mw 7.8 . The increase in event count in the KSMMA following this remote event is difficult to determine unless the above statistical analysis is performed, as it is difficult to see an increase in seismicity when daily event counts are used. In some cases, however, such as following the Mw 6.1 event on 31 May 2020, 43 km W of Lampa, Peru, a significant increase in the number of events detected in the KSMMA is clear. Our analysis therefore suggests that a maximum of 8 % of the seismicity detected during this period of relative quiescence may be attributed to dynamic triggering, in particular the events on 31 May 2020. This rudimentary calculation does not take into account the potential spatial migration of events in response to the teleseismic event, only that if a significant increase in event count following the teleseismic event in the next 12 h is observed, all events within this 12 h window are determined to be related to dynamic triggering. However, ∼65 % of the detected seismicity cannot be attributed to primary activation mechanisms such as this and, therefore, in our opinion are the result of latent ongoing processes.

Understanding how the stress field has evolved in time and space within the KSMMA, in particular with respect to the development of unconventional oil and gas operations in the area, is an extensive ongoing research topic. Very limited data regarding the state of stress prior to unconventional resource development in the area are available, since natural seismicity was almost non-existent e.g. and details of such seismicity are often incomplete with large associated errors e.g.. Furthermore, the sparse nature of regional recording systems prior to unconventional development means that detailed understanding of the source characteristics of identified seismic events, which may allow insight into the state of stress, is not possible. investigated the changes in background regional seismicity within the Horn River Basin, British Columbia, before and after hydraulic fracturing operations became prominent in the area. The Horn River Basin is similar to the KSMMA in the fact that it is dominated by unconventional resource operations. Their study suggests that background seismicity dramatically increases following the introduction of hydraulic fracturing, correlating in time and space with ongoing operations, in a similar manner to what we have observed in the KSMMA. However, their study fails to detail how the seismicity responds when hydraulic fracturing operations stop completely and whether there is a long-term change in the overall background rate of seismicity. If found to be true, this would indicate that the state of stress has changed at depth to allow for such an increase.

Further evidence for a true change in stress at depth may be observed through changes in the principal stress axes. The World Stress Map suggests an average SHmax orientation of ∼40∘ in the area close to Fort St. John, taken from borehole breakouts . calculated SHmax from focal mechanism analysis of 66 events with magnitudes between 1.5 and 4.6 within the KSMMA associated with hydraulic fracturing and found the value to be ∼22 to ∼33∘, significantly rotated relative to the estimate from the World Stress Map. This would suggest that there is complexity in the stress distribution within the KSMMA, particularly spatially, possibly related to the structural complexity in the area as a result of the Fort St. John Graben complex . The dominance of this tectonic feature throughout the KSMMA influences the identified source mechanisms of seismicity, which are extremely varied within a small spatial extent, suggesting a complex and heterogeneous stress regime . As a consequence, determining a permanent change in the principal stress axes before hydraulic fracturing began, during the dominance of hydraulic fracturing operations and after the investigated period of quiescence requires further investigation.

The analysis presented here provides a rare opportunity to study seismicity in a period of relative quiescence during a time when the cessation of hydraulic fracturing operations within the KSMMA is not temporally or spatially limited (for example, as is the case when a seismic event ≥ M3 occurs) nor is it due to the depletion of the entire reservoir when operations cease indefinitely. Instead, operations were suspended for ∼4 months across a wide area due to an economic downturn as a result of the COVID-19 pandemic which has once again seen an uptake in activity since August 2020 when the market recovered (Fig. b). We are interpreting this identified seismicity as latent (i.e. long-lived with no obvious direct primary activation source) and suggest that it may be a response to previous fluid injection in this area, despite its long-lived nature. This may well represent a new normal background rate of seismicity in this area, since pore pressures at depth are presumably higher (in particular within less permeable formations) due to previous injected fluid, thus reducing the effective normal stress at depth, or may be the result of fault weakening with time due to previous operations. However, until operations within the KSMMA cease permanently we will be unable to accurately measure this new background rate of seismicity or determine whether the rates of seismicity observed here continue for sustained time periods (i.e. years).

The generation of seismicity in response to hydraulic fracturing is typically attributed to either fluid migration models, poroelastic phenomena or potentially aseismic slip e.g.. In the fluid migration model, pore fluid pressures are significantly increased upon fluid injection reducing the effective normal stress within a fault zone, which is sufficient to trigger seismicity e.g.. Given the temporal and spatial correlation between seismicity and hydraulic fracturing operations within the KSMMA, this appears to be a likely cause of some seismicity. Under this model, the seismicity rate is usually observed to be proportional to the pore pressure and is assumed to track the injection rate . Consequently, a slow and steady decrease in the rate of seismicity over time would be expected to occur, as fluid pressure leaks into the surrounding formations before seismicity returns to the background (i.e. natural) rate. Since seismicity during the period of quiescence is long-lived, shows no decay and cannot be attributed to increased fluid injection, another process must be involved in its generation. Furthermore, if pore fluid pressure and relaxation as a direct consequence of immediate injected fluid was the trigger of the seismicity during this period of quiescence, we would expect the seismicity to spatially migrate directly outwards from the most recently injected wells. We see no evidence of this (Fig. b), suggesting direct pore fluid migration cannot be held responsible for the triggering of this sequence.

Seismicity triggered by pore pressure diffusion can be estimated by determining the propagating pore pressure fluid front (rt) related to the hydraulic diffusivity in a homogeneous isotropic saturated poroelastic medium by 4rt=4πDt, where D is the hydraulic diffusivity and t is time since injection. If the triggering front (rt) closely follows the maximum distance of seismicity through time, then pore pressure diffusion is thought to play a central role in the triggering of this seismicity e.g.. Diffusivity (D) is generally assumed to range in the Earth's crust between 0.1 and 10 m2 s-1 , although in areas affected by hydraulic fracturing it is thought to generally be in the range of 0.1 to 2 m2 s-1 e.g.. suggested similar diffusivity values determined from seismicity related to hydraulic fracturing in the Montney formation to the north-west of the KSMMA, although others have speculated that much smaller diffusion values would be expected in shale formations . Higher values of diffusivity in hydraulic fracturing scenarios are believed to be the result of faults and fractures at depth acting as fluid corridors e.g.. However, the seismicity generated in the KSMMA during the period of quiescence shows no coherence with a triggering front from the most recently active injection wells (Fig. ), suggesting that pore pressure diffusion is not the dominant mechanism responsible for triggering these earthquakes.

Other models proposed for the generation of seismicity in response to hydraulic fracturing suggest that both pore pressure and poroelastic effects are jointly responsible e.g.. In these instances, the increased pore pressure due to injection is thought to load the surrounding rock matrix, altering the stress field, often at great distances from the original injection site if the region is well hydraulically connected. Again, however, these models suggest that seismicity is generated as a response to injecting fluid into the Earth, which was occurring only on a very minor scale at the time of this seismicity. Given that the stress field is unlikely to be sustained at critical following the cessation of fluid injection, we would also expect a decay in seismicity with time e.g.. We do not observe this. Alternatively, the trapping of fluids within a fault zone with only minor fluid migration along the fault, could result in slow changes to the effective stress due to changes in pore pressure . In this scenario, seismicity should migrate spatially outwards from this fault zone as the effective stress migrates. We also see no evidence of this spatial migration (Fig. b).

Recently, it has been suggested that aseismic slip may play an important role in the generation of seismicity associated with hydraulic fracturing at distances extending beyond the fluid-pressurized zones through the transmission of an elastic stress perturbation e.g.. For hydraulic fracturing regimes in Canada, suggested that distal unstable regions of a fault may be loaded by aseismic slip that initiated due to an increase in pore pressure within a stable zone, leading to the generation of seismicity. Once slip is initiated, far-field intraplate stresses may repeatedly reload unstable regions of the fault, leading to relatively steady seismicity rates, which may continue for long periods of time. They suggest the driving stresses of such behaviour are most likely to be elevated pore pressures (as a result of ongoing hydraulic fracturing in the area) becoming trapped within fault zones due to low permeabilities within many formations, sustained by tectonic forces. suggested that far-field aseismic deformation may also be sustained by fault weakening, which may accelerate with time. Fault weakening may be further enhanced due to degradation of slip surfaces from chemical and hydrothermal fluid-rock interactions, as suggested in geothermal reservoirs by .

Given that in the absence of the cessation of operations the detection of latent seismicity is extremely difficult, there are few examples of long-lived seismicity associated with hydraulic fracturing operations that may offer insight into the generation of such activity. One recent example comes from a long-lived seismic swarm in Alberta, where seismicity was observed over 10 months after injection ceased and was interpreted as being driven primarily by aseismic slip . The authors suggest that the steady seismicity rate over a number of months (swarm-like behaviour of seismicity rather than a typical mainshock–aftershock sequence) and a lack of hypocentre migration cannot be easily explained by a fluid-migration model (which is often the most favoured model in hydraulic fracturing environments) and is instead better explained by an aseismic slip model .

We favour this interpretation of aseismic slip playing an important role in the initiation of this seismicity since ongoing hydraulic fracturing operations are not required to generate ongoing seismicity (as is the case in the fluid-migration model); instead, the trapping of fluids that have been previously injected within fault zones may be enough to sustain the generation of seismicity. The latent seismicity identified here is persistent over ∼4 months, lacks evidence of hypocentre migration and shows no evidence of a mainshock–aftershock type sequence as the magnitude distribution remains constant (i.e. is swarm-like in its behaviour). In addition we see no evidence of a propagating pore pressure fluid front and detected events are dominated by lower frequencies, all of which appear to be characteristic of the aseismic slip model . Geodetic methods (e.g. GPS, InSAR) may be able to measure such aseismic deformation e.g., however, at this time no data of this nature are available for the KSMMA.