Were the 2011 Prague, Oklahoma Earthquakes Natural, Induced, or Something in Between?

Two hypotheses have been proposed for the origin of the Prague, Oklahoma earthquake sequence, which is under debate. I offer a third explanation, whereby this sequence was triggered by approximately 12 million cubic m of wastewater injected into the epicentral zone, from five high-volume injection wells. This sequence included three earthquakes of M5 or greater: a M5 foreshock, the M5.7 main shock, and a M5 aftershock (Keranen et al., Geology, 2013). The main shock, which caused substantial damage, is of exceptional importance in the central and eastern United States because it is the second largest earthquake that has been recorded in this vast region. Because of its importance in assessing seismic hazard, especially in central Oklahoma, there is an urgent need to understand its origin. The first hypothesis argues that the Prague earthquakes are of natural origin (Keller and Holland, OGS website, 2013). As shown by Ellsworth et al. (abstract, SSA Meeting, 2012,), the seismicity in central Oklahoma has increased dramatically starting in 2009. This increase is inconsistent with natural processes that are likely to occur in this geologically stable area. Instead, it seems more likely that the increase in seismicity is related to the increase of fluid injection activities related to expanding oil and gas operations. Even so, the Prague earthquakes themselves could be a natural sequence. The second hypothesis argues that the M5 foreshock was triggered by pore pressure increase due to wastewater injected from two wells near the northeast end of the epicentral zone. This M5 event, in turn, increased the Coulomb stress on a pre-existing fault triggering the M5.7 mainshock and its numerous aftershocks (Keranen, et al., 2013). This hypothesis focuses on two nearly co-located disposal wells, Stasta 1 and 2, that together injected nearly 100,000 cubic m of wastewater into the basal aquifer during the years leading up to the Prague earthquakes. There are, however, five much higher-volume disposal wells in the zone of epicenters, each of which injected more than 1 million cubic m into the same aquifer. Indeed, the highest volume well, Wilzetta 1, which injected nearly 4 million cubic m starting in 1999, is also at the northeast end of the zone of epicenters, close to the two Stasta wells. Accordingly, the third hypothesis, favored here, is based on a relationship developed by McGarr (2013, manuscript submitted for publication) that gives the maximum seismic moment as less than or equal to the product of the total volume of injected fluid and the modulus of rigidity. This upper bound is consistent with observations of seismic moments and corresponding injected volumes from numerous case histories during the past 50 years including the Prague sequence for which a total injected volume of 12 million cubic m resulted in a maximum seismic moment of about 3.9e17 Nm.