Earthquake Hazard Class Mapping by Parcel in Las Vegas Valley

Earthquake Hazard Class Mapping by Parcel in Las Vegas Valley Preliminary hazard map of a portion of Las Vegas Valley, Nevada, prepared from measurements of 1265 sites as of March 2008. (Map book 163 – green numbers – was mapped later that year.) A sharp NEHRP B-C boundary (green to orange) is visible between map books 176 and 177; the NEHRP C-D boundary (orange to red in map book 162) is much more complicated, requiring dense measurements to locate accurately.
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Clark County, Nevada is completing the Nation's very first effort to map earthquake hazard class systematically through an entire urban area. The resulting geotechnical shear-velocity map is a layer added to the County's existing soil-hazards map. The map already includes layers such as fault proximity, ground subsidence, and potential for swelling clay soils. The new earthquake-hazard map layer will be available on the County's web GIS interface. Geotechnical shear velocity is a basic building block in foundation design, and in predictions of potential for ground liquefaction, landslides, and seismic shaking, as a few examples. The map will be used in development and disaster response planning, in addition to its direct use for building code implementation and enforcement. Clark County contracted with the Nevada System of Higher Education to apply the refraction micro-tremor geophysical surveying technique, a State of Nevada-owned technology, to the hazard classification of about 500 square miles Las Vegas Valley.
</p><p>The parcel map includes almost 9000 measurements that classify parcels on the NEHRP hazard scale, from NEHRP A at least hazardous to NEHRP E at the most. No site in Nevada has yet been measured to be less hazardous than NEHRP B or more hazardous than NEHRP D; and this represents the range of classifications found in unincorporated Clark County. The measured parcel map shows a clearly definable NEHRP B-C boundary on the west side of Las Vegas Valley, rarely more than 300 m wide. The NEHRP C-D boundary is on the other hand much more complex. Using the parcel map in computing shaking in the Valley for scenario earthquakes is crucial for obtaining realistic predictions of ground motions. For Las Vegas the principal earthquake hazard is from the Furnace Creek fault system, capable of M7.5 events. Animations of shaking show the expected strong trapping and long shaking durations within basins, as well as diffusion and scattering of energy between the many basins in the region. Despite affecting only the very shallowest zone of models (<30 m), the Vs30 geotechnical shear-velocity from the parcel map shows clear correlation to 0.3-Hz PGV predictions in basins. Increasing basin thicknesses from 0 to 1.3 km correlate with increased PGV, but the basin effect at 0.3 Hz saturates for basin thicknesses greater than 1.3 km; deeper parts of the basin show variance and uncertainty of a factor of two in predicted PGV.
</p><p>Acknowledgements: Research supported primarily under contract to the Clark County Building Department; also partially by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 08HQGR0015; and by a 2006 Fulbright Senior Scholar award to Louie for work in New Zealand. 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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