Earthquakes that are triggered or caused by human activities are called induced seismicity. The human activities that may cause induced seismicity include quarrying, mining, dam reservoir impoundment, extraction of ground water, production of oil or gas, injection of wastewater or fluids underground, the injection of fluids to enhance oil or gas recovery, geothermal stimulation, or hydraulic fracturing.
Alberta has a long history of induced earthquakes, dating back to the 1970s. These older cases were related to conventional oil and gas technologies. Since 2010, there has been a sudden increase in the rate of earthquakes. This newest increase is dominantly induced by unconventional technologies: in particular, hydraulic fracturing of the Duvernay Formation near Fox Creek. However, there are also other noteworthy cases.
Today, the majority of earthquakes recorded in Alberta are induced. Despite this, only a small fraction of hydraulic fracturing wells actually cause earthquakes. Many of these findings are highlighted in a previous study (Atkinson et al., 2016). Click any of the links below to learn more.
Causing induced earthquakes requires a perfect storm of conditions. First, a well that is injecting fluids underground (such as by hydraulic fracturing) must have a pre-existing fault nearby. Second, that fault must be critically stressed, which means that it is already nearly ready to slip naturally. Third, there must be a means for the inducing well to change the stresses/forces to cause the slip on the fault. It is thought that encountering all of these conditions at the same place and same time is quite rare. There is also scientific evidence to support this thought (Schultz et al., 2016; Pawley et al., 2018; Galloway et al., 2018).
Two main processes contribute to changing the stress/forces on the fault to cause induced earthquakes. In the first case (left hand side in the figure below), fluid injected into the ground is able to move along fault and fracture pathways. Increasing fluid pressure from the well increases pressure along the critically stressed fault. The increase of fluid pressure on the fault hydraulically ‘props’ it open, reducing the forces clamping the fault closed, allowing it to slip. In the other case (right hand side in the figure below), fluid injected into the ground changes the mass and volume in the subsurface. This change of stresses/forces is transmitted through the rock and may change the stresses/forces on a nearby fault, allowing it to slip.
Schematic diagram explaining how hydraulic fracturing can induce an earthquake. Adapted from Schultz et al., 2017.
Seismologists examine several characteristics of an earthquake sequence to determine whether or not it was induced (Davis & Frohlich, 1993). Briefly, the criteria for determination are:
• Are these earthquakes in an area that has not commonly had earthquakes?
• Is there an increase in the rate of earthquakes in this area?
• Are the earthquakes occurring at the same time as the suspected human activity?
• Are the earthquakes within a reasonable distance from the suspected human activity?
• Are the human-caused changes in stress/forces on a fault large enough to explain the seismicity?
For example, the AGS studied recent earthquakes west of Fox Creek using the above line of reasoning to determine their cause. In this study, they were able to show hydraulic fracturing operations in the Fox Creek area induced the nearby earthquakes (Schultz et al., 2017).
West of Fox Creek, low magnitude earthquakes first started in December of 2013. These events occurred sporadically, timed with nearby hydraulic fracturing operations in the Duvernay Formation (Schultz et al., 2015). These events went unnoticed, until the event that was felt in Fox Creek on January 23, 2015 (4.4 ML). This resulted in the creation of a traffic light protocol to manage these events (Alberta Energy Regulator, 2015).
Locations of earthquakes (red circles) recorded near Fox Creek compared to nearby hydraulic fracturing operations (colored areas). Adapted from Schultz et al., 2018.
Since then, the earthquakes near Fox Creek have been well studied. Earthquakes are triggered on basement-rooted faults that intersect the Duvernay Formation (Schultz et al., 2017). These faults move in a strike-slip manner (Wang et al., 2018). It also appears that the correct geological conditions are required to induce earthquakes; that not every hydraulic fracturing operation will cause seismicity (Schultz et al., 2016; Pawley et al., 2018). For wells that are in a susceptible region, the more fluid is pumped into a fault the more induced earthquakes that are induced (Schultz et al., 2018); when the pumping stops, the earthquake rate begins to decrease. This area continues to be studied, to better understand the nature of induced earthquakes.
Earthquakes in the Rocky Mountain House Seismogenic Zone occur ~30 km southwest of Rocky Mountain House. This was Alberta’s first case of induced seismicity. Earthquakes here first started in the 1970s and peaked in 1988. A study by the Geological Survey of Canada (Wetmiller, 1986) linked these earthquakes to secondary recovery in the Strachan D3-A sour gas reservoir in the Leduc Formation. In fact, it took nearly five years of secondary gas production from this formation before earthquakes began. To date, the largest event induced there was ~4.0 ML.
Graph of earthquakes recorded near Rocky Mountain House (bars) compared to gas production (line) from the Strachan D3-A sour gas field. Adapted from Stern et al., 2013.
The mechanism was thought to be related to the removal of large quantities of gas causing compaction of the pore spaces in the rock formation (Baranova et al., 1999). The loss of volume and mass in the rock formation lowered the stress acting vertically relative to the horizontal stresses. This model is consistent with the types of earthquake slip that was observed in the vicinity of the Strachan gas field.
Earthquakes in the Brazeau area occur 30 km north of Nordegg, Alberta. Earthquakes here first started in the 1990s and have been ongoing. A study linked these earthquakes to wastewater disposal in the Rundle Group (Wetmiller, 1986). In fact, it took three years of waste water disposal before earthquakes began. To date, the largest event induced there was ~4.0 ML.
Graph of earthquakes in the Brazeau River Cluster (bars) compared to monthly injection rates of fluid disposal in the Cordel Field (lines). Adapted from Schultz et al., 2014.
This study concluded that it took nearly three years before the increases in underground pressures were able to diffuse outwards to a fault that was already close to slipping. Overall, the increased seismicity in this region was consistent with wastewater disposal induced events.
More than 60 small earthquakes (up to 3.0 ML) were detected from December 2011 to March 2012 north of Cardston, Alberta. This area had no previously known seismic activity. The timing of these earthquakes was simultaneous to a single well hydraulically fracturing the Exshaw Formation. It was found that earthquakes occurred directly below this well, in the crystalline basement. It was concluded that these earthquakes were caused by the nearby hydraulic fracturing well (Schultz et al., 2015).
Location of Cardston earthquake swarm. Adapted from Schultz et al., 2015.
Graph of earthquakes near Cardston compared to a nearby hydraulic fracturing well. Adapted from Schultz et al., 2015.
Further study into this area demonstrated the presence of a basement-rooted fault that intersected the sedimentary Exshaw Formation. Geological inference of this fault’s properties confirmed a permeable conduit that would allow pore pressure from the operation in the Exshaw to diffuse down into the crystalline basement, where the earthquakes occurred (Galloway et al., 2018).
Schultz, R., Atkinson, G., Eaton, D. W., Gu, Y. J., & Kao, H. (2018). Hydraulic fracturing volume is associated with induced earthquake productivity in the Duvernay play. Science, 359(6373), 304-308, doi: 10.1126/science.aao0159 Open Access link to Science article.
Kavanaugh, J., Schultz, R., Andriashek, L. D., van der Baan, M., Ghofrani, H., Atkinson, G., & Utting, D. J. (2018). A New Year’s Day Icebreaker: Icequakes on Lakes in Alberta, Canada. Canadian Journal of Earth Sciences, doi: 10.1139/cjes-2018-0196.
Galloway, E., Hauck, T., Corlett, H., Pană, D., & Schultz, R. (2018). Faults and associated karst collapse suggest conduits for fluid flow that influence hydraulic fracturing-induced seismicity. Proceedings of the National Academy of Sciences, 115(43), E10003-E10012, doi: 10.1073/pnas.1807549115.
Pawley, S., Schultz, R., Playter, T., Corlett, H., Shipman, T., Lyster, S., & Hauck, T. (2018). The geological susceptibility of induced earthquakes in the Duvernay play. Geophysical Research Letters, 45(4), 1786-1793, doi: 10.1002/2017GL076100.
Wang, R., Gu, Y. J., Schultz, R., & Chen, Y. (2018). Faults and Non‐Double‐Couple Components for Induced Earthquakes. Geophysical Research Letters, 45(17), 8966-8975, doi: 10.1029/2018GL079027.
Schultz, R., Wang, R., Gu, Y. J., Haug, K., & Atkinson, G. (2017). A seismological overview of the induced earthquakes in the Duvernay play near Fox Creek, Alberta. Journal of Geophysical Research: Solid Earth, 122(1), 492-505, doi: 10.1002/2016JB013570.
Atkinson, G. M., Eaton, D. W., Ghofrani, H., Walker, D., Cheadle, B., Schultz, R., ... & Liu, Y. (2016). Hydraulic fracturing and seismicity in the Western Canada Sedimentary Basin. Seismological Research Letters, 87(3), 631-647, doi: 10.1785/0220150263.
Schultz, R., Mei, S., Pană, D., Stern, V., Gu, Y. J., Kim, A., & Eaton, D. (2015). The Cardston earthquake swarm and hydraulic fracturing of the Exshaw Formation (Alberta Bakken play). Bulletin of the Seismological Society of America, 105(6), 2871-2884, doi: 10.1785/0120150131.
Schultz, R., Stern, V., & Gu, Y. J. (2014). An investigation of seismicity clustered near the Cordel Field, west central Alberta, and its relation to a nearby disposal well. Journal of Geophysical Research: Solid Earth, 119(4), 3410-3423, doi: 10.1002/2013JB010836.
Stern, V. H., Schultz, R. J., Shen, L., Gu, Y. J., & Eaton, D. W. (2013). Alberta earthquake catalogue, version 1.0: September 2006 through December 2010. Alberta Geological Survey Open-File Rept, 15, 36, URL: https://ags.aer.ca/publications/OFR_2013_15.html.