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ABSTRACT: Geologic Hazards Assessment at the Idaho National Engineering Laboratory, Southeastern Idaho

Richard P. Smith, William R. Hackett, Ivan G. Wong, Suzette M. Jackson

The Idaho National Engineering Laboratory (INEL), an 890 mi2 facility on the Eastern Snake River Plain (ESRP) in southeastern Idaho, is operated by the U.S. Department of Energy for energy research and development. The ESRP, a Neogene and Quaternary volcanic province produced by passage of the Yellowstone mantle plume, has intimate spatial and genetic relationships to the northern part of the Basin-and-Range province.

Work is in progress in two major aspects of seismic hazards assessment: identification and characterization of seismic sources, and prediction of potential strong ground motions at specific sites. Investigations of mechanics of ESRP volcanic rift zone formation have furnished information important to both seismic and volcanic hazards assessment.

Results to date include the following. Although some ESRP volcanic rift zones are colinear with basin-and-range faults, their surface deformation features are related to dike emplacement rather than to projection of basin-and-range tectonism onto the Plain. Trenching of a NE-trending scarplet along the NW edge of the ESRP shows that these features are not associated with faulting. The most serious seismic threat is associated with the southern segment of the Lemhi fault, a basin-and-range fault north of the ESRP. Based on the Band Limited White Noise methodology, strong ground motions resulting from possible earthquakes on this fault depend upon the configuration of low velocity materials interbedded in the sequence of basaltic lava flows comprising the upper kilometer of crust in the ESRP. These materials consist mostly of alluvial and lacustrine sediments and a thick layer of unwelded rhyolitic tuff. Their seismic velocities (Vp) range from 1 to 2 km/sec, whereas that for the basaltic lava flows varies from 3 to 5 km/sec. The location, thickness, and number of layers of these materials can affect modeled peak ground accelerations by as much as 25%.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990