--> --> Quantifying Fault Stability in the Fort Worth Basin, Texas
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Quantifying Previous HitFaultNext Hit Stability in the Fort Worth Basin, Texas


From 1970 through 2005 there were 2 Mw ≥2.5 earthquakes in the Ft. Worth Basin (FWB) and from 2006 through mid-2018 there have been 125. Evidence from structural geological analysis, site-specific pore-pressure modeling, temporal and spatial correlation between seismicity and subsurface injection activities, and proximity of seismicity to known or suspected faults, leads to a general scientific consensus that this increase has been induced by increases in pore fluid pressure from waste water injection and from cross-Previous HitfaultNext Hit pore pressure imbalance due to oil and gas related injection and production. Shallow crustal earthquakes result from the reactivation of and slip on a pre-existing Previous HitfaultNext Hit or faults. The larger the Previous HitfaultNext Hit surface area that slips, and the stronger the rock involved, the larger the magnitude of the resulting seismic event. There are two important corollaries of this: (i) earthquake magnitude scales with Previous HitfaultNext Hit size and rock strength, and (ii) earthquakes can only occur if there is a pre-existing Previous HitfaultNext Hit appropriately oriented within the ambient stress state for reactivation if sufficient pore fluid pressure increase or imbalance should occur. Here we consider three factors that control the likelihood that a Previous HitfaultNext Hit will be reactivated by an increase in pore fluid pressure: ambient stress state, orientation and shape of Previous HitfaultNext Hit(s), and frictional strength of the Previous HitfaultNext Hit(s). We use newly developed 3D models of pore pressure evolution and Previous HitfaultNext Hit geometries, together with stress data from a variety of sources, including wells and seismicity, to characterize Previous HitfaultNext Hit stability within the FWB. Previous HitFaultNext Hit stability is assessed on the basis of Previous HitfaultNext Hit area combined with its proximity, in stress terms, to reactivation, here expressed as pore pressure perturbation required for slip (ΔPf crit ). In a fully 3-dimensional stress state, faults that have large areas with low values of ΔPf crit are less stable than faults that have small areas with low values of ΔPf crit . The stability of Previous HitfaultTop populations as well as individual faults can be evaluated, and the likely effects of different stress models can be compared.