Hydrogeologic implications of a fault scaling relationship in bedrock aquifers

Because faults in bedrock aquifers can act as conduits or barriers to groundwater flow, defining their length and geometry is important for hydrogeologic investigations. The purpose of this study was to determine fault lengths through the use of a length versus displacement fault scaling relationship. Analytical, statistical, and field methods were used to define this relationship and to identify and evaluate outliers, which were then examined for refinement of their length and displacement. This study resulted in redefining the length for 5 of the 80 normal faults evaluated within a structural domain in the Transition Zone physiographic province of Arizona. Three faults with partially concealed or queried surface traces had their lengths decreased to values closer to the expected value, based on the determined length versus displacement relationship, using analytical and statistical methods. One fault had its length extended, and one fault was divided into two segments, based on the length versus displacement relationship as determined through field investigation. Newly determined fault lengths were incorporated into a digital hydrogeologic framework model of the regional aquifer, which supplies the water for the headwaters of the Verde River in central Arizona. This fault scaling technique can be applied in numerous settings, requiring minimal time and expertise, to better constrain fault lengths and possibly fault geometries. The critical assumptions of the method are that there are numerous faults of known length and that the fault array is largely similar in the rocks the faults cut, so that rheological differences are minimized.