A Century of Induced Earthquakes in Oklahoma

Abstract:

Seismicity rates have increased sharply since 2009 in the central and eastern United States, with especially high rates of activity in the state of Oklahoma. A growing body of evidence indicates that many of these events are induced, primarily by injection of wastewater in deep disposal wells. The upsurge in activity has raised the questions, what is the background rate of tectonic earthquakes in Oklahoma? And how much has the rate varied throughout historical and early instrumental times? We first review the historical catalog, including assessment of the completeness level of felt earthquakes, and show that seismicity rates since 2009 surpass previously observed rates throughout the 20^th century. Furthermore, several lines of evidence suggest that most of the significant (M_w > 3.5) earthquakes in Oklahoma during the 20^th century were likely induced by wastewater injection and/or enhanced oil recovery operations. We show that there is a statistically significant temporal and spatial correspondence between earthquakes and disposal wells permitted during the 1950s. The intensity distributions of the 1952 M_w5.7 El Reno earthquake and the 1956 M_w3.9 Tulsa county earthquake are similar to those from recent induced earthquakes, with significantly lower shaking than predicted given a regional intensity-prediction equation. The rate of tectonic earthquakes is thus inferred to be significantly lower than previously estimated throughout most of the state, but is difficult to estimate given scant incontrovertible evidence for significant tectonic earthquakes during the 20^th century. We do find evidence for a low level of tectonic seismicity in southeastern Oklahoma associated with the Ouachita structural belt, and conclude that the 22 October 1882 Choctaw Nation earthquake, for which we estimate M_w4.8, occurred in this zone.