This article by Sumy et al (2014) is particularly notable because it reports on the largest event to date related to gas extracted from the bedrock with a magnitude of M5.7. The team of authors looks at the M5.7 event in the context of a thousand other events registered during the injection phase (1993 - 2011). Of these, 110 were large enough to be evaluated in terms of hypocenter parameters and focal mechanism solutions.
The authors looked at the (temporally) neighboring events relative to the event just examined depending on when they had occurred (foreshocks or aftershocks). Within the 2011 Oklahoma sequence, the M5.7 event in November 2011 registered a foreshock of a size of M0.5 and an aftershock of a size of M5.0 in addition to many smaller events. The M5.0 foreshock, which (like all others) was located along the (200 km long) Wilzetta Fault, had previously been classified as being connected to several disposal wells operated since 1993.
The authors asked the question whether the M5.7 event could be a direct result of the M5.0 foreshock and whether this means that the classification as “induced” can also be transferred to the M5.7 event, which would make this event the largest known fluid-induced event to date. In this context, the authors examined the effect of the examined series’ earthquakes on the local stress field, which changes after each new event (coseismic stress changes). In order to be able to find out whether these stress changes could trigger a shear event, it was important to determine whether crack criteria (e.g. Coulomb criterion) were at least temporarily exceeded for a specific fault with a specified orientation. Here, the authors applied a value of 0.01 Mpa (0.1 bar) from literature sources as the sufficient additional stress value, a conservative assumption considering that the fault is critically stressed and, so to speak, “ready to go”. Relevant friction values in this context were varied in a larger range.
A very careful analysis of all events of the series that were suitable for evaluation leads to the conclusion that “M5.0 foreshocks may have triggered the cascading failure and thus the subsequent earthquakes along the Wilzetta Fault”.
