Influence of geological setting on stress released by hydraulic fracture-induced earthquakes

Recent studies of injection-induced seismicity suggest that induced events have lower stress drops than tectonic earthquakes, suggesting stress drop can potentially be used as a discriminant parameter between natural and anthropogenic earthquakes. The main physical mechanisms responsible for low stress drop earthquakes, that is earthquakes that release less stress during failure than the average natural earthquake in a similar tectonic setting, are related to the increase in pore pressure and fault lubrication that result from the presence of fluids on the fault surface. Fluids reduce the effective normal stress applied on a fault allowing it to slip under lower stress conditions. Hough (2014) found that induced earthquakes have a factor of 2 to 10 lower stress drops relative to natural earthquakes, for 11 injection-induced moderate earthquakes in central and eastern United States based on their intensity reports. Sumy et al. (2016) also found a factor of 10 difference between the stress drops of natural and induced earthquakes when studying 87 Mw1.8 to Mw3.5 induced earthquakes in Oklahoma. A few other studies found opposing results. Huang et al. (2016) found the stress drops of 25 small earthquakes in central Arkansas to be comparable to Californian tectonic earthquake stress drops, and Viegas and Abercrombie (2011) also found the stress drops of a sequence of moderate earthquakes in Colorado to be comparable to the stress drops of Californian earthquakes. However, Viegas and Abercrombie (2011) noted that Central and North America intraplate earthquakes are expected to have higher stress drops than plate boundary California earthquakes (Viegas et al., 2010) making the comparison a false equivalency. Small earthquakes recorded during two hydraulic fracturing treatment programmes targeting stratigraphically equivalent formations in two regions of the Western Canadian Sedimentary Basin (Muskwa in Horn River Basin and Duvernay in Fox Creek), show very different stress drop estimates. Earthquakes in the Horn River Basin show lower stress drops by a factor of 10 to 20 than earthquakes in Fox Creek (Duvernay Basin). While the seismic events from both sites are generated during similar injection programsme, we propose that the differences in the observed stress drops are caused by different regional stress characteristics, as reflected by the differences in geology. The Horn River Basin stratigraphy is dominantly horizontal, whereas the Fox Creek region is characterized by the presence of the Leduc Formation reefs. These dolomitized reef complexes cross-cut the Duvernay shale formation, where both the Duvernay and overlying Ireton formations drape over the reef facies (Stoakes, 1980). We propose that earthquakes occurring in higher stressed regions have higher stress drops, as these regions are subjected to higher strain rates and store larger amounts of elastic strain.