Publication Date:
2011-06-01
Description:
Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811-1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismic-hazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5g, values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m/s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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