Stratigraphy and Geologic Structure at the SCC and NISC Building Sites, Technical Area 3, Los Alamos National Laboratory, New Mexico

Ten closely spaced, shallow (<100 ft) drill cores were obtained from the 1.22-Ma-old Bandelier Tuff at a 4-acre site for proposed construction at Los Alamos National Laboratory, New Mexico. The goal of the investigation was to identify faults that may have potential for earthquake-induced surface ruptures at the site. Careful mapping of contact surfaces within the Bandelier Tuff was supplemented with results of geochemical analyses to establish unit boundaries with a high degree of accuracy. Analysis shows that the upper contact surface of Unit 3 of the Bandelier Tuff provides no evidence of faults beneath the building site, and that the subsurface structure is consistent with a shallowly dipping (< 2degree), unbroken block. Because no significant or cumulative faulting events have disturbed the site in the last 1.22 million years, it is unlikely that surface rupture will occur at the site in future large earthquakes. Uncertainty analysis suggests that this method would detect faults with ge2 ft of cumulative stratigraphic separation