IMAGING THE SOUTHERN EXTENSION OF THE BLACK HILLS FAULT SYSTEM, CLARK COUNTY, NEVADA

The Black Hills fault (BHF) is a SE-dipping normal fault forming the northwestern structural boundary of the Eldorado basin, between the growing communities of Henderson and Boulder City, Nevada. Fossett (2005) mapped 14 scarp-forming strands of the BHF in addition to 38 fault strands logged in a paleoseismic trench. The fault offsets Holocene strata and is thus considered to be active (Fossett, 2005). Earthquake magnitude estimates by Fossett (2005) based on surface rupture length (SRL) indicate an earthquake potential of Mw 5.7; however, estimates based on displacement values documented in the paleoseismic trench indicate a higher value of Mw 6.4-6.8. This implies that the subsurface rupture length is significantly greater than the length of the scarp and that the BHF poses a significant seismic hazard to the greater Las Vegas area.

To test this hypothesis, a three-component approach was utilized: Remote sensing, geologic mapping, and high-resolution seismic reflection surveying. Shuttle Radar Topography Mission (SRTM) and Landsat 7 ETM+ imagery were used for reconnaissance. Geologic mapping was performed to tie surface geology to the seismic cross-section. Additionally, high-resolution seismic data were acquired in SEG2 format, utilizing a vibroseis source, along two profiles. The first profile was acquired on a 60-channel system at 5 m station spacing with 30-160 Hz vibroseis sweeps of 15 s duration, using 40 Hz geophones that were co-located with the source locations. In an attempt to reduce coherent linear noise trains, the second profile was acquired on a 144-channel system at 3 m station spacing with 20-160 Hz vibroseis sweeps of 8 s duration, using 4.5 Hz geophones that were placed in between source locations.

The resulting seismic cross-sections were merged into an interpretive geologic cross-section showing several SE-dipping normal faults on strike with the BHF scarp. It is uncertain whether these faults are linked to the previously mapped strands of the BHF, or whether they represent unlinked segments of the Black Hills fault system. In either case, these newly identified strands suggest that the Black Hills fault system extends >6 km in length, and poses a greater seismic hazard than would be suggested by the 4.5 BHF scarp alone.