Fault Angle Control on Potential Seismic Slip in the Illinois Basin Region

The possibility of reactivation of faults due to enhancement of pore pressure in rocks surrounding faults and fractures in the deep subsurface is a challenge associated with injection practices. Reactivation can result in induced seismicity, some of which may be significant enough to be felt or even damaging. Of the key factors that influence the propensity for fault slippage—pore pressure, orientation of the stress tensors, and frictional coefficient orientation of the fault plane—only the last factor can be assessed for its contribution to the possibility of reactivation. This investigation assesses a simplified version of fault orientation relative to the stress tensors and ranks the propensity of movement as a three-level risk (high, medium, and low). Fault segments in the fault systems located within the Illinois basin and surrounding portion of the eastern Midcontinent are assigned a risk based on their relative orientation to the principal horizontal stress. Horizontal stress tensors are arrayed relative to the fault segments in two different manners: a generalized single value for the average stress orientation (N60°E for the entire domain), and a locally specific orientation of the stress tensors based on an inversion of earthquake fault-plane solutions and stress indicators (30 × 30 km cells across the domain). Comparison of the results of these two methods of portraying the angle of the maximum horizontal stress tensor relative to the fault segment orientation reveals several areas of divergence in assigned fault-slip risk. These changes are especially apparent within portions of the Wabash Valley of southwestern Indiana and the Shawneetown-Rough Creek fault system of western Kentucky and southern Illinois. The assessment of fault-slip risk potential based on fault orientation relative to the orientation of the principal horizontal stress is improved by incorporating local stress tensor orientations over a single regional value.