Using geophysical techniques to trace active faults in the urbanized northern Hueco Bolson, West Texas, USA, and northern Chihuahua, Mexico

The Hueco Bolson serves as one of the primary groundwater sources for the El Paso-Ciudad Juarez metropolitan region of over 2.5 million residents. The bolson lies at the point where the strike of the southern Rio Grande rift changes from north-south to northwest-southeast, likely due to its interaction with preexisting Mesozoic and Paleozoic structures. The region is tectonically active with recent (<750,000 years) movement along basin-bounding faults and low-level (M<4) seismicity. Over the past five years, we have used a combination of microgravity and water well logs to image the complex structure of this basin within an urbanized environment. Our results indicate the East Franklin Mountains fault, the main boundary fault on the west side of the bolson, extends as much as 30 km south of the end of its mapped surficial trace. Two intrabasin faults that have been mapped at the surface in the less developed portions of the city can be traced into central El Paso and Ciudad Juarez. These faults appear to control the boundary between fresh and saline water within the aquifer system beneath El Paso. Gravity modeling also suggests at least two additional concealed intrabasin faults are located beneath the metropolitan area.