Abstract

In January 2011, a sequence of earthquakes occurred in close proximity to a well, which was being hydraulically fractured in south‐central Oklahoma. The hydraulic fracturing of the Picket Unit B Well 4–18 occurred from 16 January 2011 18:43 through 22 January 16:54 UTC. This vertical well penetrated into the mature Eola‐Robberson oil field. Earthquakes were identified by cross correlating template waveforms from manually identified earthquakes and cross correlating these templates through the entire operation period of the Earthscope USArray Transportable Array (TA) station X34A. This produced a series of 116 earthquakes, which occurred from 17 January 2011 19:06 through 23 January 3:13 UTC with no other similar earthquakes identified at other times prior to or post‐hydraulic fracturing. The identified earthquakes range in local magnitude (ML) from 0.6 to 2.9, with 16 earthquakes ML 2 or greater and a b‐value of 0.98. There is a strong temporal correlation between hydraulic fracturing and earthquakes. This correlation is strengthened because hydraulic fracturing operations ceased for ∼2 days due to bad weather, and earthquakes can be observed to cease during this period and resume after hydraulic fracturing had resumed. Earthquakes were relocated using cross‐correlated phase arrivals and bootstrap iterations of hypoDD. Locations were well constrained for 86 earthquakes. These earthquake locations clearly delineate a fault which strikes ∼166°, subparallel to the mapped minor fault sets in the area, and dips steeply to the west. The earthquakes appear to have occurred at shallow depths from ∼2 to 3 km and within ∼2.5  km horizontally of the well. The first earthquake occurred ∼24 hrs after hydraulic fracturing began at the well. This delay is consistent with the diffusion of pore pressure in the subsurface over a distance of ∼2  km.

Online Material: Results from bootstrap hypoDD relocations using cross‐correlation phase arrivals.

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