Best practices book - Flipbook - Page 321
Task I - Developing New Surficial Geologic and Fault Maps
A fundamental basis for understanding the surface expression, earthquake potential, and
earthquake occurrence along faults is a state-of-the-art, uniform surficial geologic map. This map
will be used to identify earthquake faults, measure the length of fault traces for magnitude
estimates, evaluate structural relationships between faults to understand if they fail together
during earthquakes, identify the age of offset geologic units along faults for estimating how often
earthquakes occur, and to identify areas with deposits that could be susceptible to liquefaction.
Developing a high-quality geologic map includes compiling all previous mapping; combining
and/or splitting out geologic units into a consistent mapping paradigm; conducting age dating of
key geologic units; new field mapping where needed; field checking of existing mapping; and
building a digital Geographic Information System (GIS) database of all map data. The map will
include the entire Las Vegas drainage basin (Fig. 4). This will be a surficial geologic map
focused on distinguishing variously aged Tertiary through Quaternary sediments, including basin
fill deposits, alluvial fans, and spring discharge deposits. The bedrock exposed in the study area
will not be analyzed in this study unless there is evidence of Quaternary faults cutting through
the bedrock.
A difficulty in assigning detailed geologic units in Las Vegas Valley, where several different
aged units have similar appearances, is a lack of dates from these deposits. This project will
develop over 100 dates from across the valley to fill this data gap. Optical luminescence and
accelerator radiocarbon dating will be conducted on the geologic units. The luminescence dates
will be principally collected by scientists from the U.S. Geological Survey and processed at their
laboratories. Radiocarbon dates are limited to the younger deposits, but can be important for the
most-recent paleoseismic history of faults and will be collected when opportunities for strategic
dates are discovered. The dates will be strategically collected to support the geologic mapping
and fault characterization investigations.
Task 1 has five subtasks:
Task 1.1 - Compiling Existing Mapping,
Task 1.2 -Age Determination Studies,
Task 1.3-New Geologic Mapping,
Task 1.4 - Compilation and Drafting of a New 1: 50,000 Geologic Map,
Task 1.5 - Final Map and Report.
The main effort for geologic mapping will be within the first two years, but follow up studies,
such as resolving dating of key units needed to constrain age and frequency of earthquakes, may
continue through the duration of the project. The final product will be a 1:50,000-scale surficial
geologic map, detailed descriptions of geologic units, and an overall discussion of the surficial
geology of the Las Vegas area. The map will be a peer-reviewed product with input from
geologists that are knowledgeable about the area. The final map will be available in digital and
hard copy formats.
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