Evolution of Faulting Induced by Deep Fluid Injection, Paradox Valley, Colorado

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

High pressure fluid injection into a sub-horizontal confined aquifer at 4.3 km to 4.6 km depth induced > 7000 recorded earthquakes between 1991 and 2012 within once seismically-quiescent Paradox Valley in Colorado, with magnitudes ranging up to Mw 3.9. Alignment of earthquake epicenters define trends of nine vertical fault zones near the well that offset left-stepping normal faults of the Wray-Mesa fault system within the Paradox Valley structural basin. Over time earthquake hypocenters expand laterally away from the well and vertically above and below the pressurized aquifer with continued fluid injection. Hypocenters, rakes, and strikes of 2061 well constrained focal mechanisms show that most induced earthquakes occur on steeply-dipping dextral strike-slip faults that begin at acute angles to the strikes of the fault zones, then evolve into faults nearly aligned with the fault zones as deformation continues. A laboratory experiment with similar boundary conditions indicates that these faults are Riedel shears formed to accommodate fault slip. A fully coupled poroelastic analysis of stress from fluid injection shows that Riedel shears first occur when the effective stresses exceeding Coulomb failure conditions are between 0.35 MPa and 0.4 MPa. As they form, the Riedel shear faults affect the local stress state to produce anastomosing fault zone structures that coalesce over time to form horizontally and vertically expanding vertical fault zones. The three largest induced earthquake ruptures occurred along three of the nine vertical fault zones. Among all developing fault zones, seismicity rates were highest in the first decade of injection as fault zones developed, and then substantially declined with time in the second decade of injection.

Presenting Author: Roger P. Denlinger

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