An investigation of the seismic hazards of the El Paso-Juarez region: The nature and extent of the southern east Franklin Mountains fault zone

Abstract

The El Paso-Ciudad Juarez Region is situated at the southern end of the active Rio Grande Rift within the Mesilla and Hueco basins. The Hueco basin is separated from the eastern Franklin Mountains by a prominent fault, the Eastern Boundary Fault. Studies of this fault have revealed fault scarps that displace Quaternary deposits (Keaton et al., 1995; Raney and Collins, 1990; Lovejoy, 1976). This fault is cataloged as an active fault in the National Seismic Hazard Map with an estimated slip rate of 0.1 mm/yr. A fault with similar characteristics as the Eastern Boundary Fault extends further to the south into Mexico. This continuation of the Eastern Boundary Fault is comprised of two segments that are hypothesized to lie beneath the urbanized portion of El Paso and Ciudad Juarez. The location of these segments is inferred by an earthquake hazards study of the El Paso region (Keaton, 1993). The objective of this research was to locate and attempt to determine how the fault segments in downtown El Paso-Ciudad Juarez interconnect using precise gravity readings. For this I used two gravimeters; for the El Paso area a Lacoste Romberg was used and then a CG5 gravimeter was used in Ciudad Juarez. The tilt derivative filter applied to the gravity data taken reveals the fault segments correspond well to those inferred by previous researchers who did not have any geophysical data for use in their interpretation. The results help to assess seismic hazards within the El Paso-Ciudad Juarez region. The data also reveal several new intriguing anomalies related to basement structure and faults that need to be more closely investigated in planned geophysical surveys.