Earthquakes Induced by Hydraulic Fracturing in Poland Township, Ohio

Abstract:

Felt seismicity induced by hydraulic fracturing is very rare with only a handful of reported cases worldwide. Using an optimized multi-station cross-correlation template matching routine, 77 earthquakes were identified in Poland Township, Mahoning County, Ohio that were closely related spatially and temporally to active hydraulic fracturing operations. We identified earthquakes as small as M ~1 up to M 3, one of the largest earthquakes induced by hydraulic fracturing in the United States. These events all occurred 4-12 March 2014 and the rate decayed once the Ohio Department of Natural Resources issued a shutdown of hydraulic fracturing at a nearby well on 10 March. Using a locally derived velocity model and double difference relocation, the earthquake epicenters occurred during six stimulation stages along two horizontal well legs that were located ~0.8 km away. Nearly 100 stages in nearby wells at greater distances from the earthquake source region did not coincide with detected seismicity. During the sequence, hypocenters migrated ~600 m along an azimuth of 083 degrees defining a vertically oriented plane of seismicity close to the top of the Precambrian basement. The focal mechanism determined for the M 3 event had a vertically oriented left-lateral fault plane consistent with the earthquake distribution and the regional stress field. The focal mechanism, orientation, and depth of hypocenters were similar to that of the 2011 Youngstown earthquake sequence that occurred ~20 km away, but was correlated with wastewater injection instead of hydraulic fracturing. Considering the relatively large magnitude of these events and the b-value of 0.85, it appears the hydraulic fracturing induced slip along a pre-existing fault/fracture zone optimally oriented in the regional stress field.