ELECTRICAL RESISTIVITY SIGNATURES OF THE PAWNEE BILL MANSION CREEP, BLUE HAWK PEAK, OKLAHOMA

The 107-year old historical Pawnee-Bill Mansion sits at the edge of a 15° slope on Blue Hawk Peak, Pawnee County, Oklahoma, and has been observed to be creeping continuously downslope for the past 20 years. Structural damage of the mansion associated with the creep has been accelerated by the recent seismicity in parts of Central Oklahoma. As a result, the west-facing wall and raised patio of the mansion have accommodated ~8 cm of displacement over the years. Anecdotal reports suggest that the creep rate of the building has increased since the recent Mw5.8 Pawnee, Oklahoma earthquake of September 3, 2016. In order to determine the electrical properties of the subsurface that is indicative of the sliding surface inducing the creep, we conducted electrical resistivity surveys along multiple resistivity profiles across the mansion using an IRIS Syscal Pro 72 electrodes resistivity/induced polarization system with 1.5 m spacing. Our results show a general electrically-heterogeneous subsurface in which the house is directly underlain by a 2 m-thick, resistive substratum of 406-1407 Ωm, beneath which lies a thicker zone of 9.79-33.9 Ωm resistivity with a localized area of low resistivity (0.236-2.83 Ωm) at ~3.8-7.5m depth. This localized zone rests on top of a highly resistive area (117-406 Ωm) that is discontinuous throughout the profile. We interpret the zone of localized low resistivity (high conductivity) beneath the house as areas of high clay and water content, which could constitute a sliding surface beneath the house. Our study shows that electrical resistivity imaging is a useful geophysical method for investigating mass wasting processes.