With the development of both conventional and unconventional resources in Canada, induced seismicity caused by anthropological activities has been observed, documented, and studied. Induced events are generally smaller in magnitude than the most 'important' earthquakes documented by Natural Resources Canada. The largest natural earthquakes are generally located in the coastal regions of the country. The majority of large, natural seismic events in Western Canada are located near the Cascadia and Juan de Fuca Subduction Zones. The majority of large, natural seismic events in Eastern Canada are localized to distinct seismic zones like the Charlevoix-Kamouraska region. The only 'important' interior earthquakes, as classified by Natural Resources Canada, are the two strike-slip fault failures of local magnitude (ML) 6.6 and 6.9 observed in the Nahanni region of the Northwest Territories. Induced earthquakes, however, tend to occur in a 150-km-wide band east of the Canadian Rocky Mountains where the tectonic strain rate is relatively high. Induced seismicity in Canada is mainly related to hydraulic fracturing and wastewater disposal. Within the central Western Canadian Sedimentary Basin (WCSB), evidence shows that geological factors likely influence the nature of induced seismicity related to hydraulic fracturing operations, as they share two characterizations in terms of spatial distributions: 1. Pre-existing basement-controlled faults are more likely to be triggered based on the focal depth analysis of earthquake clusters; 2. The lateral distributions of earthquake clusters are significantly correlated with the margins of the fossil reef structures in the Swan Hills Formation. These phenomena can be explained by the regional- and local-scale geological evolution of this area. First, basement tectonics play a role in reef growth which were nucleated on elevated structures during the Devonian period. Second, the generation of dolomitized strata requires deep-seated faults to transport Mg-enriched fluid, which can later provide transport conduits for injected fluids, thereby creating hydraulic connections with the reservoir. Dolomitization of Devonian strata increased the formation permeability and generated greater fluid diffusivity, thus more induced seismicity occurred. A total of 216 induced earthquakes occurred between 2009 and 2011 at the Etsho and Kiwigana fields in Horn River, Canada. Of those, 19 were between magnitudes (ML) 2 and 3, and the largest felt event reached ML 3.8. Seismicity was temporally correlated to pumping fluids during hydraulic fracture treatments, with earthquakes starting several hours after the onset of pumping. Since at least 2009, the Horn River Basin has been the site of induced seismicity associated with oil and gas activities. The BC Oil and Gas Commission states that over 8,000 hydraulic fracturing completions have had no associated anomalous seismicity in this region between 2009 and 2011. Studies on induced seismicity have been ongoing since the 1970s. Focus on the cause of induced seismicity has shifted from activities related to conventional resources like mining to unconventional resource exploration and production. Barriers to understanding induced seismicity processes include lack of access to subsurface hydrogeological and geomechanical data, insufficient stress state data, and limited records of seismicity at the nucleation process scale.
Mechanism
The mechanism of induced seismicity can be categorized based on different causes. It is widely accepted that impoundment of reservoirs, mining, oil and gas exploration and production, including injecting fluids to the subsurface and extracting oil and gas from the underground, are related to induced seismic events. Industrial operations that create tremor movement of the ground, such as mining and seismic data collecting, are often a cause of induced seismicity. In the vicinity of deep mining activities, seismicities are often related to rockbursts - the violent failure of rock due to excavation. The magnitudes of these seismic events, however, are dependent on the local geological settings, such as the rock properties, faulting system, and regional stresses. For geophysical exploration activities, usually seismic waves are generated by man-made explosions to help the geophysicists understand the underground formations and structures. These explosions are far away from the population-dense areas. Induced seismicity related to fluid injection is generally triggered by two basic mechanisms (Fig. 2): pore pressure perturbation via direct hydrologic connections and/or a change in total stress on pre-existing fault through poroelastic transmissions. A fault will be activated once the shear stress on the fault plane τ {\displaystyle \tau } reaches a critical value τ c {\displaystyle \tau _{c}} :
τ c = c + ( σ n − P p ) tan ϕ {\displaystyle \tau _{c}=c+(\sigma _{n}-P_{p})\tan \phi }
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