DETAILED HVSR ANALYSIS OF GLACIAL BURIED VALLEY AND UPLAND AREAS, BUTLER COUNTY, OHIO

This study presents the results of a detailed Horizontal-Vertical Spectral Ratio (HVSR) passive-seismic recording program from glacial buried-valley and upland settings in southwestern Ohio. Study goals were to evaluate the sensitivity of the HVSR method and best practices regarding field sampling techniques and data analysis. The study was conducted within and near the valley of Four Mile Creek near Oxford in Butler County, Ohio and includes >100 HVSR records taken at stations on an ~100-m grid spacing. The area investigated covers >1 km² of the modern floodplain, part of a tributary drainage, and adjacent upland regions. Quaternary deposits, consisting of Holocene fluvial channel sand/gravel and overbank mud and Pleistocene glacial till, outwash sand/gravel, and lacustrine clay, are >70 m thick in the valley but thin rapidly towards both valley margins where Ordovician bedrock is exposed. Upland areas are generally covered by <10 m of till. Numerous drilling logs from water and geothermal wells and a rotosonic core of the entire valley-fill assemblage permits subsurface calibration of HVSR frequency peaks. Study results indicate a systematic decrease in peak HVSR frequency from >30 Hz near valley-margin bedrock exposures to ~2 Hz in the deepest part of the valley fill. The close spacing of the recording stations permits identification of key components of the HVSR spectral signature that can be used to distinguish the fundamental frequency of the valley fill deposits. Overall findings highlight both the utility and limitations of the HVSR method in determining Quaternary thickness, the vertical and lateral variability of heterogenous buried valley deposits, and shear wave velocity variations between the valley fill and upland regions.