Michael Brudzinski , Maria Kozłowska

Eastern Ohio is an area of North America where a significant increase in seismicity rate was noted in the early 2010s. This increase has been associated with intensification of unconventional gas extraction performed in the Appalachian Basin and has been directly linked to two processes: hydraulic fracturing and disposal of the associated wastewater. In this paper we review the recent seismicity in the Appalachian Basin including various episodes of induced seismicity that were temporally and spatially linked to operational activity, but the activities have not been as pervasive as other areas of North America, such that the cases are typically isolated and provide opportunities to study the seismogenic process in detail. The observed seismicity is concentrated in a narrow corridor that extends north-south in eastern Ohio and into central West Virginia, perhaps due to differences in operational targets and geologic variations. Ohio appears to have a higher prevalence of seismicity induced by wastewater disposal than surrounding states, but this is based on limited number of cases. Ohio also has an order of magnitude higher prevalence of seismicity induced by hydraulic fracturing than surrounding states, and prior work has suggested this is due to the targeting of the deeper Utica-Point Pleasant formation in Ohio that is closer to basement rocks than the Marcellus formation in West Virginia or Pennsylvania. In areas where hydraulic fracturing has induced seismicity, the percentage of stimulated wells that produce detectable seismicity is approximately 10-33%. Detailed studies of induced seismicity via double difference relocation and focal mechanism analysis have revealed a series of linear fault segments, none of which correspond to previously mapped faults. Yet the remarkable coherence in their orientation suggests these were pre-existing, optimally-oriented, and critically-stressed. These fault orientations reveal a consistent regional stress field that only varies over a narrow azimuthal range from ~50°-74°. The strongest observed seismic events in Ohio appear to occur in the Precambrian basement and indicate that these rocks have the maturity needed to produce M > 2 earthquakes and hence the greater potential hazard.