Fault Frequency, Strain, and Subseismic Fault Prediction

Faults are among the most numerous deformation features on Earth. Faults of all scales are important for reservoir and aquifer characterization due in part to their influence on fluid transmissivity. Small faults outnumber large faults and the distribution is well-described by some form of power law. However, this relationship alone is not sufficient to estimate fault populations below some threshold of detection (e.g., subseismic faults). We have measured exposed normal faults in outcrops of Cretaceous carbonate rocks in Texas and determined bulk fault-accommodated strain directly from observed faults. From these data, we demonstrate that fault frequency correlates with extensional strain for extensions from 0 to 15%. A relationship between fault frequency and strain provides a means by which subseismic fault populations may be estimated from strain information together with the displacement distribution of the detectable fault population. Strain estimates derived from geometric-kinematic models or geomechanical modeling alone, or these estimates coupled with the seismically imaged portion of a fault network, can be used to predict subseismic faults. This approach is particularly powerful in that it incorporates the heterogeneity in fault frequency and displacement distribution as a function of structural position.