Seismicity Induced by Wastewater Injection in Washington County, Ohio: Influence of Preexisting Structure, Regional Stress Regime, and Well Operations

Abstract

Recent seismicity in Washington County, Ohio, has been suggested to be induced by wastewater disposal operations despite injection ~2 km above the Precambrian basement. We investigated the relationships between disposal well locations and operational histories, spatiotemporal patterns of seismicity enhanced by waveform correlation, and mapped subsurface structures. We also analyzed proxies for in situ stress conditions to evaluate existing principal stress orientations and magnitudes. Double-difference relocations of small (ML ≤ 2.1) seismicity revealed linear clusters defining a NE-SW orientation that parallels the trend of small-amplitude folds in Upper Paleozoic rocks within 1–2 km of the injection well. Hypocenter depths of ~3.1 to 4.3 km indicate seismicity occurred along faults below the Silurian injection interval in lower Paleozoic strata and Precambrian crystalline basement. Similar folds/faults imaged in nearby seismic reflection lines were associated with basement-involved fault systems that cut the injection interval targeted by the injection well, providing permeability pathways for fluid pressure increases that initiated slip. Interpreted faults were also optimally oriented in the regional stress field for reactivation due to injection-related decreases in effective stress. Well completion records suggest that during the recorded seismicity, the Long Run-1 well was injecting disposal fluids close to or above the fracture pressure of the targeted Silurian injection interval. A comparison of observed seismicity rates and monthly injection records from the disposal well also indicate a close correspondence between earthquake frequency and injection rate. Taken together, these results provide a physical framework for understanding why wastewater disposal operations induced the Washington County seismic sequence.