SHEAR WAVE CHARACTERIZATION OF QUATERNARY SEDIMENTS FOR GEOLOGIC MAPPING AND GEOHAZARDS ASSESSMENT

The Kentucky Geological Survey (KGS) and University of Kentucky Department of Earth and Environmental Sciences (UK-DEES) have collaborated to collect seismic shear-wave profiles in Quaternary sediments along the Ohio River valley near Henderson and Owensboro, Kentucky. New shear wave profiles were collected in 2005 and 2008 as part of a geologic field mapping project and supporting a regional seismic hazard assessment. Data from a MS thesis by Higgins (1996) was also included in the study. Support for the data collection was provided by NEHRP, STATEMAP, the KGS and the UK-DEES. Each profile was carefully reviewed, and velocity layers identified on the profiles were associated with mapped sediment deposits. Three products have been derived from detailed examination of the resulting data set.

First, these data provide an improved characterization of three-dimensional geometry of the unconsolidated sediments in the map area. Despite well-documented potential errors with deriving absolute thickness of sediments from interpreted seismic profiles, the thickness data agree well with nearby well logs where available.

Second, the compiled data on the shear-wave characteristics of general geologic map units in the project area illustrate distinct patterns of variability in shear-wave velocity for different sediment packages. The mean shear-wave velocity of the different map units was relatively similar, but the variability within each map unit (e.g. standard deviation) was much more distinct.

Third, Vs₃₀ and associated NEHRP soil classifications derived for each profile show consistent patterns within each map unit, with variability in classification attributable primarily to variation in bedrock topography. When combined with the recently completed detailed surficial geologic mapping in the area, this allows for improved detailed mapping of and development of detailed HAZUS input layers that will be useful for local hazard assessment and modelling.