[editor’s note: the full report from the Oklahoma Geological Survey is available as a downloadable PDF at the link below]
On January 18, 2011, The Oklahoma Geological Survey (OGS) received a phone call from a resident living south of Elmore City, in Garvin County, Oklahoma, that reported feeling several earthquakes throughout the night. The reporting local resident had also offered that there was an active hydraulic fracturing project occurring nearby. Upon examination there were nearly 50 earthquakes, which occurred during that time. After analyzing the data there were 43 earthquakes large enough to be located, which from the character of the seismic recordings indicate that they are both shallow and unique. The earthquakes range in magnitude from 1.0 to 2.8 Md and the majority of earthquakes occurred within about 24 hours of the first earthquake. Careful attention and significant effort was put into obtaining the most accurate locations possible and gaining a reasonable estimate in the error in locations. The nearest seismic station is 35 km away from where the earthquakes occurred. Formal errors in location are on the order 100-500 m horizontally and about twice that for depth. Examination of different velocity models would suggest that the uncertainties in earthquake locations should be about twice the formal uncertainties. The majority of earthquakes appear to have occurred within about 3.5 km of the well located in the Eola Field of southern Garvin County. The Eola Field has many structures, which may provide conduits for fluid flow at depth. The well is Picket Unit B well 4-18, and about seven hours after the first and deepest hydraulic fracturing stage started the earthquakes began occurring. It was possible to model 95% of the earthquakes in this sequence using a simple pore pressure diffusion model with a permeability of about 250 mD (milliDarcies). While this permeability may be high it is less than those reported for highly fractured rock. The strong correlation in time and space as well as a reasonable fit to a physical model suggest that there is a possibility these earthquakes were induced by hydraulic-fracturing. However, the uncertainties in the data make it impossible to say with a high degree of certainty whether or not these earthquakes were triggered by natural means or by the nearby hydraulic-fracturing operation.