13 Structural Geology and Tectonics

Thursday, 19 May 2011
8:00 AM - 12:00 PM
Elm

INVESTIGATING THE SEISMIC HAZARDS OF THE PAHRUMP SEGMENT OF THE STATELINE FAULT SYSTEM

Paper number: 13-12
Program: Technical Sessions
Time: 11:20 AM – 11:40 AM

Authors

Abstract

The Stateline fault system (SFS) is a 200 km long zone of young dextral faults extending NW along the NV/CA state line from Primm, Nevada to Amargosa Valley. The SFS contains three segments: the Mesquite, Pahrump and Amargosa segments. This active system is a hazard to the 1.9 million people mostly 10 km away in Pahrump, Nevada and 40 km away in Las Vegas, Nevada. This study is to find magnitude, ages and frequency of paleoearthquakes so that future earthquakes are designed into zoning and development plans. This research used well logs, a trench survey, and 14C dates to establish (1) location of the SFS within Stewart Valley (SV), (2) ages of faulting events, (3) recurrence interval, (4) the along strike continuity as it extends from Pahrump Valley through SV, and (5) earthquake magnitude.

Well log lithology records, viewed in 3D; show a concealed dextral fault splay traversing the basin under the city of Pahrump. This splay had offset of at least 10 km since the valley playa deposition.

In SV, two 2.5 m deep and 250 m long trenches were excavated. Trench logs were created displaying disturbance of horizontal sediments as they cross or are cut by the fault. The trenches expose liquefaction/injection features, fault strands, and sediment layer brecciation with offset near fault strands. Sediment offset along with 14C dates imply three recent earthquakes: possibly as recent as 1956 AD +/- 1yr to 8595 +/- 45 yr BP. The SV portion of the SFS, initially thought to be purely strike-slip from the lack of obvious fault scarps, exhibited a flower/tulip structure, indicating normal and strike-slip motion. The SFS segment in SV, as approximated by earlier research, extends along the playa edge into the northern valley. Using surface rupture length, an estimate of maximum earthquake magnitude is M7.8, and is supported by presence of liquefaction and unit offset in the trenches. This presents a significant ground shaking hazard to Pahrump and Las Vegas, Nevada.