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Task 3 -Evaluation of Fault Geometry, Rupture Segmentation, and Maximum Magnitude.
Critical to understanding earthquake size potential in Las Vegas Valley is the determination of
the dimensions of a potential earthquake source, establishing the connectivity and/or
segmentation of earthquake faults, and establishing a methodology for incorporating these data
into a maximum magnitude estimate. The total lengths of faults will be determined using the new
geologic map and will include estimates of potential uncertainties (e.g., a possible extension to
an isolated fault trace). The subsurface geometry of faults will be estimated using gravity
models, existing seismic reflection data, and a small number of deep boreholes. These data will
help determine whether faults project to the base of the seismogenic zone or are truncated by a
master fault at shallower levels, such as the westward-dipping Frenchman Mountain fault that
lies along the western base of Frenchman Mountain.
Many of the known faults in Las Vegas Valley intersect each other at the surface. Better
documentation and understanding of this connectivity is needed, including whether it means that
simultaneous failure along multiple faults during a single earthquake is either more or less likely.
If connectivity is small, individual earthquakes may only rupture a single fault trace. This
assessment will be made using available paleoseismic information, structural and kinematic
relations between faults, and displacement profiles along faults. Displacement profiles along the
faults will be made using pre-developmental elevation data, existing topographic maps, and field
measurements of present-day topography.
Earthquake rupture models for the faults will be developed based on structural, geometric, and
available paleoseismic information. For example, a fault may fail as a single entity during an
earthquake or it may fail along with an adjacent fault; these would be two different earthquake
rupture models. In some cases, faults may have substantial discontinuities along them that would
be considered as possible rupture ends. In this case, an earthquake model that is smaller than the
total fault length would be considered. Parameters associated with the earthquake rupture models
will be used in the earthquake hazard analysis.
A contemporary strategy will be developed for the scaling of earthquake size that uses multiple
fault parameters, such as fault length and co-seismic displacement. The strategy will incorporate
direct magnitude calculations by using earthquake moment estimates based on rupture model
dimensions and co-seismic offset. It will also incorporate the estimation of magnitude values
using fault parameters and regressions developed from documented historical earthquakes.
Regressions used in national analyses, such as by Wells and Coppersmith (1994), will be
considered, as well as research from the University of Nevada, Reno and the University of
Nevada, Las Vegas, such as Wesnousky (2008). Using the developed strategy, potential
earthquake magnitudes will be estimated. for the faults in the valley.
Task 3 contains six subtasks:
Task 3.1 -Determination of Fault Parameters,
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