Total station geologic mapping: an innovative approach to analyzing surface-faulting hazards

We have developed an innovative application of high-precision geologic mapping with an electronic total station to assess the potential for seismic surface rupture in areas of Los Alamos National Laboratory (LANL). Our method of total station mapping enables recognition of secondary faults, with as little as 30 cm of vertical displacement that are not exposed at the surface, have no topographic expression, and would otherwise likely go unnoticed. It has been applied to preclude the presence of faulting in large areas (several km2) of proposed and existing critical facilities at LANL. The method involves surveying of points on geologic features, and detailed computer-aided and field analyses of anomalies in the elevations of surveyed points. We examine vertical anomalies in elevations that are the result of dominantly normal and reverse faulting; however, the method could also be applied to strike-slip faulting. Surveying of geologic contacts allows for easy integration of geologic data into a Geographical Information System (GIS) and detailed 3D analysis of small-scale structures. Field data are analyzed in profiles, 3D surface diagrams, and maps that are constructed with a variety of commercially available software packages. We apply the method to delineate volcanic map-unit boundaries in the 1.2-million-year-old Tshirege Member of the Bandelier Tuff to characterize portions of the Pajarito fault system. The ability of this method to identify faults with very small displacements that otherwise might be unrecognizable allows for discrimination of varying styles of deformation, decreases in displacement along strike through splaying into many smaller faults, monoclinal flexures, and cross structures between faults.