HR: 16:45h
AN: NS24A-04    [Abstracts]
TI: Geophysical Characterization of Yucca Fault in Yucca Flat, Nevada Test Site, Nevada
AU: * Burton, B L
EM: blburton@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, CO 80225, United States
AU: Haines, S S
EM: shaines@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, CO 80225, United States
AU: Sweetkind, D S
EM: dsweetkind@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 973, Denver, CO 80225, United States
AU: Asch, T H
EM: tasch@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, CO 80225, United States
AB: Yucca Fault is a north-striking, east-dipping normal fault located in the eastern half of Yucca Flat, an elongate basin on the Nevada Test Site in the Basin and Range province. The fault offsets alluvial material in the shallow subsurface and has a prominent surface scarp; abundant drill-hole data record offsets of 200 to 400 m on underlying Miocene volcanic rocks and Paleozoic carbonate bedrock. Understanding the geometry and physical properties of the Yucca Fault and other fault zones in Yucca Flat is an important component in assessing the role of faults in the groundwater flow and transport at the Nevada Test Site. From 2005 to 2007, we conducted near- surface geophysical studies across the surface trace of the Yucca Fault to determine the feasibility of the chosen methods in helping to evaluate the extent of footwall damage and to investigate possible hanging wall splay faults. In particular, we selected an area of the fault that exhibited marked sinuosity and multiple splays that result from the interaction of en echelon linked segments. We acquired direct current (dc) resistivity, compressional (P) and shear wave seismic, ground magnetic, and audio magnetotelluric (AMT) data. The dc resistivity data, including both two-dimensional and three-dimensional data sets, show a 200 ohm-m higher resistivity on the hanging wall than on the footwall. These data sets provide the best high-resolution detail of the subsurface geometry of the fault zone for the top 30 to 35 m. These data correlate well with detailed surface fracture mapping, where subtle changes in the shallow resistivity structure are interpreted to be a result of structural disruption of caliche cementation within the alluvium. The P-wave seismic data show a lower velocity zone that corresponds with the surface expression of the fault. The shear wave data, however, appear to suffer from converted (P-wave) arrivals produced by the shallow caliche layer and therefore do not provide useful information. The total-field magnetic data exhibit a relatively sharp 180 nT transition across the fault, which we interpret to be due to the fault offset of alluvial fill and underlying Miocene welded tuff. Two AMT profiles were collected coincident with two of the dc resistivity profiles, and they exhibit lateral variations in resistivity that correlate with the mapped surface expression of Yucca Fault. At shallow depths, the AMT data show the same resistivity trend across the fault as do the dc resistivity data, but the AMT data show the opposite transition at greater depths. The results of these geophysical methods encompass varying scales of resolution and depths of investigation and aid in the characterization of fault effects in the alluvium and underlying volcanic rocks.
DE: 0600 ELECTROMAGNETICS
DE: 0925 Magnetic and electrical methods (5109)
DE: 0935 Seismic methods (3025, 7294)
DE: 1875 Vadose zone
DE: 8010 Fractures and faults
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting