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Task 2 - Earthquake Source Investigations
Earthquake source investigations are required to identify and characterize all potential
earthquake faults in the Las Vegas Valley. Although several major faults have been recognized
for decades, such as the "known" Quaternary faults in Figure 1, other faults have yet to be
considered as seismic sources. These other potential sources include uncharacterized lineaments,
suspected active faults, and buried or blind faults (blind faults usually do not break the surface
when they rupture but can cause strong ground shaking). A comprehensive study of the valley
using state-of-the-art techniques is needed to identify faults that have not been previously
recognized or that were not fully considered as potential earthquake sources. The proposed
investigation will conduct a comprehensive examination of available aerial photography and
other imagery (e.g., LiDAR), consider subsurface geology using the borehole data previously
compiled at UNLV, review gravity studies that define the shape of the bottom of the basin, and
possibly use new seismic reflection surveys to identify and determine the length of faults and
other parameters of potential earthquake sources.
Digital elevation models (DEMs) have many uses in earthquake characterization, including the
delineation of faults, the estimation of offsets along a fault, and delineation of the extent of
surficial geologic units. Along some faults, urban development obscures the natural topography.
In these areas historical photographs will be used to make pre-development DEMs using
"structure from motion" software, a program that uses photogrammetric triangulation from
overlapping photographs to produce a 3-D elevation model. A resultant DEM will be used to
map faults and make offset measurements.
The hydro-compaction hypothesis has been the most prevalent aseismic hypothesis cited for the
Las Vegas Valley faults. This hypothesis is predicated on the capacity for subsurface sediments
to compact with differential subsidence across lateral contrasts in material types, sediment
thickness, or variable groundwater conditions. In the hypothesis, this differential subsidence then
generates the observed faults. The capacity for differential hydro-compaction will be evaluated
using geologic drill hole logs that have been compiled at the University of Nevada, Las Vegas
and testing of drill hole samples stored at the Great Basin Science Sample and Records Library at
the Nevada Bureau of Mines and Geology. In our current understanding, subsurface information
indicates that the hydro- compaction process fails to account for displacement across several of
the major faults. However, if a significant differential hydro-compaction component is found to
be possible across a fault, the contribution of compaction will be incorporated into slip rate and
hazard estimates.
A regional fault study is proposed to develop and test tectonic models for faults in Las Vegas
Valley. A tectonic model incorporates other data into our understanding of deformation, or
strain, in the Las Vegas Valley, such as the long term geologic history of regional faults,
paleogeographic reconstructions, and geodetic information from precise OPS measurements.
Because a balance of deformation is sought, this model can indicate areas where unaccounted
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