A comparison of seismicity rates and fluid-injection operations in Oklahoma and California: Implications for crustal stresses

Fluid injection into deep wellbores can increase pore pressure, reduce effective stress, and trigger earthquakes. The extent of the seismogenic response to injection provides insight into how close faults are to failure in the injection-affected area. The seismogenic response to injection operations in hydrocarbon basins is examined in California and Oklahoma. Changes in spatial and temporal seismicity rates are tested for significant variations, and timing and location of such variations are determined based on nonparametric modeling of background seismicity rates. Oklahoma has experienced a recent surge of seismic events, which exceeded the 95% confidence limit of Poissonian background rates in c. 2010. Annual injection volumes in Oklahoma increased systematically between 1998 and 2013 and have been connected to several earthquake sequences. In California, injection volumes increased monotonically between 2001 and 2009; however, the seismogenic response was limited and was devoid of large-scale background rate increase. A detailed comparison of injection parameters in Oklahoma and California included well density, wellhead pressures, peak and cumulative rates, and injection depths. No detectable difference was found that could readily explain the observed changes in seismicity rate in Oklahoma and the lack thereof in California. A strongly different seismogenic response to similar pressure perturbations indicates that the injection parameters considered are only of secondary influence on the resulting earthquake activity. The primary controls on injection-induced earthquakes might be the specific geologic setting and the stress state on nearby faults. Introduction — Injection-induced earthquakes in California and the central and eastern United States The injection of waste fluids into deep disposal wells and its environmental consequences are a growing concern in the central and eastern United States. Such injection activities can increase pore pressures and poroelastic stresses, which might trigger earthquakes on faults close to failure (e.g., Ellsworth, 2013; Kim, 2013). Several regions in the central and eastern United States exhibited a pronounced increase in seismic activity coincident with injection operations in nearby wastewater-disposal wells. The corresponding seismicity sequences include the 2011 Mw 4.7 sequence in Guy, Arkansas (Horton, 2012); the Mw 3.9 in Youngstown, Ohio (Kim, 2013); and the Mw 5.7 Prague, Oklahoma, earthquake sequence (Keranen et al., 2013). Many of those sequences were associated with nearby wastewater injection into high-permeability aquifers overlying igneous basement. The basement layers, which are connected hydraulically to the reservoirs above, host the majority of the induced earthquakes, including the largest-magnitude events. Previous studies examined isolated cases of likely injectioninduced seismicity. However, a synoptic identification of induced Thomas Göbel seismicity and its underlying causes is still missing in Oklahoma and the central United States. In central California, a comprehensive regional study revealed that induced seismicity is rare considering the extensive injection activity that has occurred in close proximity to active faults (Aminzadeh and Göbel, 2013). The authors identified three induced-seismicity sequences with magnitudes as high as Mw 4.7, based on a rigorous statistical assessment of correlations between injection and seismic activity. This study focuses on a large-scale assessment of differences in injection operations and possibly induced seismicity in California and Oklahoma. Those two states exhibit strong differences in tectonic deformation and seismic activity. Although Oklahoma experienced generally low seismicity rates until 2009 (Ellsworth, 2013), seismicity rates in California are high as a result of pervasive tectonic deformation along many active faults. To isolate the possible influence of injection activity from tectonic earthquakes in California, I limit the analysis to seismicity that occurred in major hydrocarbon basins. The article is structured as follows. First, I determine the cumulative distribution of earthquake rates and annual injection rates in California and Oklahoma between 1980 and 2014. I then test for statistically significant increases in background seismicity rates and determine when and where they occur. Finally, I examine whether differences in seismicity rate can be traced to differences in injection parameters between California and Oklahoma. Seismicity and injection data in California and Oklahoma This work concentrates on the most widely available and homogeneous seismicity and injection data sets, including earthquake catalogs, fluid-injection volumes, wellhead pressures, and injection depths. Injection data have been archived by the Oklahoma Corporation Commission (OCC) since c. 1975 and by the California Department of Conservation, Division of Oil, Gas and Geothermal Resources (DOGGR), since c. 1977. The seismicity record is archived by the U. S. Geological Survey Advanced National Seismic System in California and is available from Oklahoma Geological Survey. Much of the seismicity in California is tectonically driven and localized along major fault traces. I exclude earthquakes along major faults and solely select seismicity within large hydrocarbon basins. For this purpose, I compute the largest convex hull of well-location vertices, using a Delaunay triangulation algorithm. The subset of seismicity within the hull corresponds to 12% of the total seismicity in California (Figure 1). For Oklahoma, essentially the entire earthquake record (99%) is used. I evaluate changes in network recording quality as a function of time based on variations in the magnitude of completeness (Mc). The latter is computed by minimizing the misfit between the observed frequency-magnitude-distribution and the modeled 1California Institute of Technology. http://dx.doi.org/10.1190/tle34060640.1. D ow nl oa de d 06 /0 2/ 15 to 2 07 .1 51 .3 5. 79 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg /