DETERMINING BASIN STRUCTURE OF THE HUECO AND SOUTHERN MESILLA BOLSONS, WEST TEXAS, SOUTHERN NEW MEXICO AND NORTHERN CHIHUAHUA: GEOPHYSICAL CHALLENGES IN AN URBAN SETTING

Over the past five years we have been using a conjunction of microgravity, magnetic, water well log, electrical resistivity, and MASW studies to image the complex structure of the Hueco and Mesilla Bolsons in the urbanized El Paso/Ciudad Juarez metropolitan area, home to over 2 million residents. Our research builds on extensive regional geophysical work conducted by G.R. Keller and his students in the 1980’s to study the structure of the southern Rio Grande rift. The bolsons are the primary groundwater source for much of the region. Faults within the basins show evidence for recent (< 64,000 years) tectonic movement and are associated with low level (M<4) seismicity. Our results indicate the main boundary fault on the west side of the Hueco 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 El Paso can be traced south into Ciudad Juarez. They appear to serve as a barrier between fresh and saline water with the northern Hueco Bolson aquifer, while minor (< 5 km long) intrabasin faults may serve as conduits for vertical and lateral flow of brackish water in the northwestern aquifer. At least two additional concealed intrabasin faults are located beneath the central metropolitan area where sediment thickness exceeds 1800 m and surficial soils have site classes of D to E. Thick sediments and soft soils could lead to significant earthquake strong ground motion in these urbanized regions. In contrast, gravity data suggest an extensive subsurface andesite body (10 x 20 km) is associated with the Cristo Rey laccolith in the southern Mesilla Bolson leading to thinner soil cover and stiffer soils.