IDENTIFICATION OF FAULT PATTERNS FROM INDUCED EARTHQUAKES AT PARADOX VALLEY, COLORADO: INFERENCES ABOUT TRIGGERING AND MAXIMUM MAGNITUDE (Invited Presentation)

Precise relative hypocenters and focal mechanisms of earthquakes induced by long-term fluid injection at the Paradox Valley Unit (PVU) brine disposal well provide detailed information about previously unidentified subsurface faults. Using distinct epicenter lineations and focal mechanism trends, we identify a prevailing set of near-vertical conjugate fault orientations in most clusters of induced seismicity. The observed strong preferential shear slip along the faults with one of these orientations over a wide area and at all azimuthal directions from the well is consistent with event triggering on pre-existing faults due to an increase in pore pressure. This assumes a simple Mohr-Coulomb stress model and a uniform regional maximum horizontal stress direction consistent with observations from the World Stress Map Project. The earthquake epicenters and focal mechanisms also indicate a third fault orientation, observed only in a concentrated area near the injection well. The shear slip observed along this third fault orientation is inconsistent with the simple pore-pressure triggering model and suggests that additional factors are present, such as a local rotation of the horizontal stresses, a different prevailing orientation of pre-existing faults, or event triggering due to stress transfer from injection-related subsurface deformation.

Individual fault segments along which induced seismicity occurs can be seismically active for several years prior to the occurrence of a larger event that ruptures the entire fault segment. In the case of the M_W 4.0 induced earthquake that occurred in January, 2013, most of the fault segment on which it occurred experienced dozens of smaller events during the preceding 13 years. Hence, previously or currently active fault segments identified by epicenter lineations and focal mechanisms can provide estimates of the locations and maximum magnitudes of potential future induced earthquakes. The longest potentially contiguous, seismically active fault segment identified at PVU lies within a 3.3-km radius of the injection well and has a length of ~4.0 km, yielding maximum magnitude estimates of M_W 5.0 to M_W 5.7 for a circular crack model and assumed stress drops of 2 to 20 MPa.