Tuesday - October 8, 2024

 6:00 to 8:00 p.m. PDT

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"Seismic Profiling Across the Eglington and Frenchman Mountain Faults to Identify and Constrain Quaternary Fault Motion"

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By: Professor Lee Liberty

Boise State University, Idaho.   

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ABSTRACT:

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Active fault characterization in urban settings presents many challenges. The first challenge is to estimate the slip history of suspected active faults through subsurface imaging approaches. The second challenge is to operate in an urban setting where the seismic risk from damaging earthquakes is often greatest. In these urban settings, the ground surface is often modified through development, access across known or suspected faults can be restricted, and costs for geophysical surveys can be prohibitively expensive. In this presentation, I discuss my low-cost active source seismic imaging approach using near surface methods specifically designed for urban settings. I will show examples from both Las Vegas and other urban areas where I have identified and characterized the latest Quaternary fault displacements and where mechanical properties of the shallowest soils have been modified through active tectonic processes. I will then discuss the geometry and history of motion of two fault systems that extend through the Las Vegas metropolitan area.

The 11-km long intrabasin Eglington fault, part of the Las Vegas fault system, is expressed as a surface warp in Quaternary sediments. Hypotheses regarding the mechanisms responsible for Eglington warp formation include: coseismic warping, climatically modulated tectonic displacement, and differential sediment compaction. My results suggest that both tectonic and non-tectonic processes contribute to motion along the Eglington fault.

The 18-km long Frenchman Mountain fault is a west-dipping, range-bounding normal fault on the eastern side of the Las Vegas metropolitan area. The main Quaternary trace of the fault is expressed as a zone of scarps in alluvial fan surfaces, but subsurface displacement across this surface warp is relatively small, suggesting little long-term slip on this fault strand. This suggests that other presumably large-offset faults have accommodated the bulk of motion along the eastern Las Vegas margin; and these faults have yet to be fully characterized. While subsurface imaging of these faults is still limited, this study advances our understanding of the seismic hazard for the Las Vegas region.

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BIO:

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Lee Liberty received a BS Geology in 1987 from Syracuse University with a senior thesis focused on paleomagnetism. He then worked for the US Geological Survey in 1988 through 1989 where he focused on the integration of geological and geophysical datasets to construct three-dimensional earth models. In 1992, Lee received a MS Geophysics degree from the University of Wyoming where he focused on seismic reflection imaging using datasets from Nevada’s Basin and Range. He then moved to Boise State University as a Research Associate where he focused on addressing near surface hydrologic and geologic problems using a variety of geophysical methods. In 2002, Lee transitioned to the role of research faculty and was elevated to the position of a full Research Professor in 2008. Since 2008, Lee has broadened his research to develop new seismic methodologies towards an improved understanding of soil and rock mechanical properties in the near surface to address problems of societal interest.

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