Poroelastic Shear Stresses Below Injection Target Intervals Encourage Vertical Strike-Slip Earthquakes

Abstract:

Waste-fluid injection into sedimentary rock is believed to have induced strike-slip earthquakes on historically inactive near-vertical faults in underlying crystalline basement beneath Youngstown, Ohio in 2011 (Kim, 2013). Two other examples of vertical strike-slip basement seismicity - the 1986 M4.8 earthquake near Painesville, Ohio (Nicholson et al., 1988) and sequences near Ashtabula, Ohio between 1986 and 2003 (Seeber et al., 2004) - were likely induced, although the Painesville event occurred 12 km from the nearest active injection well, and the Ashtabula activity persisted for years after injection ceased. In each case, hypocenters were about 1 km deeper than injection zones, prompting the hypothesis that fluid flow along faults triggered earthquakes by raising hypocentral fluid pressure. While a M4-5 earthquake likely requires a pre-existing fault 0.5-1 km in extent, favorably oriented in the pre-existing stress field, high fault-zone hydraulic conductivity has not been demonstrated. Assuming faults affect neither fluid flow nor the stress field, a layered axisymmetric finite element model (implemented in COMSOL) can simulate coupled poroelastic stress and pore-pressure distributions in isotropic, homogeneous basement with higher stiffness and lower permeability than target injection zones. Simulated radial and tangential poroelastic stresses resolved onto vertical planes lead to Coulomb stress changes favoring strike slip on portions of such planes beneath the injection zone. More specifically, the maximum horizontal shear stress increment on a vertical fault occurs where a line to the well is at 45° to the fault. The maximum shear strain is of opposite sense in adjacent quadrants, possibly explaining activation of en echelon faults in Youngstown, Ohio. Similar configurations of the wells, faults, and earthquakes in these case histories suggests that poroelastic stresses can govern the locations of strike-slip earthquakes on basement faults.