New Mexico Geological Society Annual Spring Meeting — Abstracts

The Los Alamos National Laboratory (LANL) Seismic Hazards Program has done detailed geologic mapping of the Pajarito fault system for several years to better understand the location and nature of faulting, which is essential to assessing ground motion and seismic surface rupture hazards at the Laboratory. We have developed a mapping technique using a total station for precise mapping of flow and cooling unit contacts in the 1.2 m.y. old Tshirege Member of the Bandelier Tuff. The technique enables us to locate faults that would be overlooked by conventional mapping techniques, and have as little as 15 cm of vertical displacement. Total station mapping has allowed us to define broad zones of small faults and varying styles of deformation along strike of individual faults in the Pajarito fault system. Total station bedrock mapping has been accompanied by conventional geologic and geomorphic mapping, shallow drilling on urbanized mesa tops, and paleoseismic trenching. Within LANL, deformation in the fault system is distributed across a 1.8 to >3.5 km-wide, dominantly north-to northeasttrending, complex zone to the east of the main Pajarito fault escarpment. Along strike the Pajarito fault zone can be expressed at the surface as a monocline, a large normal fault, or a distributed zone of deformation with >135 m of down-east vertical offset in the Bandelier Tuff. The associated zone of deformation to the east includes normal and strike-slip faulting and monoclinal folding, with 0 to >30 m of distributed down-west vertical offset in the Bandelier Tuff.