Seismic Imaging of the Mt. Rose Fault, Reno, Nevada: A Landslide Block Cut by Faulting

Seismic Imaging of the Mt. Rose Fault, Reno, Nevada: A Landslide Block Cut by Faulting Interpreted depth section running approximately west (left) to east (right) across the Mt. Rose fault scarp at Sarmiento’s trench. No vertical exaggeration. Colors indicate velocities optimized from 1st-arrival picks, with unsaturated, fractured, slow Quaternary alluvium and Tertiary sands (blue at Vp=800-900 m/s) overlying Tertiary andesites (green and red). The interpreted Mt. Rose normal fault (yellow line) cuts an ancient landslide slip surface (red line) at low angle.
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The Reno-area basin, sited at the western limit of the Basin and Range province within the Walker Lane, borders the Carson Range of the Sierra Nevada Mountains on the east. Triangular facets observed along an abrupt range front of the Carson Range indicate active faulting, motivating research in this rapidly urbanized area. Within the Mt. Rose fan complex, we trace a continuous ~1.4 mile long, north-northeasterly striking scarp offsetting both pediment surfaces and young alluvium at fan heads. A paleoseismic trench excavated across the scarp north of Thomas Creek at ~39.4° N latitude by A. Sarmiento and S. Wesnousky reveals a sharp, planar, low angle contact presumed to be a low angle normal fault (LANF). The trench study alone does not allow for conclusive evidence for a LANF, suggesting the collection of a high-resolution shallow seismic reflection profile in November 2009. The application of advanced seismic processing revealed a low-angle structure as prominent as the alluvium- to-granite interface of the 1954 Dixie Valley rupture, a LANF [Abbott et al., 2001]. Commercial SeisOpt®@2DTM software and P-wave arrival times provided an optimized velocity model. The velocity section and survey shot gathers provided input for the Kirchhoff pre-stack depth migration (PSDM) in the figure, revealing the complex structure in full detail to 50 m depths. Layers of Tertiary sandstone are disrupted by an ancient landslide slip surface, which is cut more recently by offset of the Mt. Rose fault trace. Truncations of layers of underlying Tertiary andesite help locate the fault and constrain its dip to less than 30°. This advanced seismic exploration below a paleoseismic trench confirms fault structure and details the earthquake hazard within the populated urban basin.
</p><p>References
</p><p>Abbott, R. E., J. N. Louie, S. J. Caskey, and S. Pullammanappallil, 2001, Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada: J. Geophys. Res., 106, 4169-4181.
</p><p>Acknowledgements: Research partially supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number G09AP00051. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.</p>

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