INITIAL MASW ANALYSIS OF THE CINCINNATI METRO AREA AND IMPLICATIONS FOR SEISMIC HAZARD IN HAMILTON COUNTY, OHIO

The Cincinnati metro area, with ~1.76 million inhabitants, has a deceptively complex subsurface geology which may result widely variable earthquake shaking hazard across the metro area. Though the area does not have significant nearby faults, large historic earthquakes (including the New Madrid earthquake sequence), have caused damage in the Cincinnati area. Bedrock exposures throughout the Cincinnati area are dominantly Ordovician limestones and shales. However, Cincinnati sits just south of the southern margin of Wisconsinan glaciation and just north of the southern margin of Illinoian glaciation. Glaciation and glacial outwash generated deeply-carved fluvial systems that then partially filled with unconsolidated drift sediment. These areas represent the paths of major fluvial systems in the Cincinnati region, including the Ohio River, Great Miami River, Little Miami River, and the Mill Creek. Additionally, some areas with the thickest glacial drift no longer have major fluvial systems, including a buried valley between Fairfax, Norwood, and St. Bernard. These buried linear to sinuous valleys are 1-3 km wide, filled with up to 110 m of unconsolidated glacial till. This variable subsurface geology likely results in a wide variety of shear wave velocities throughout the Cincinnati metro area. Therefore, the purpose of this proposed study is to collect initial Vs30 (shear wave velocity averaged over the upper 30 m) estimates in the Cincinnati area, focused on the buried valley systems.

For the initial Vs30 analysis we plan to collect multichannel analysis of surface wave (MASW) data using a Geometrics ES-3000 24-channel seismometer. MASW surveys were done at two locations (Greater Miami River at Miamitown and Bechtold Park in Blue Ash), and surveys will be completed at several other locations across Hamilton County in Spring 2023. Vs30 will be calculated using Geometrics SeisImager/SW software. Ultimately Vs30 data will be used to identify areas of increased shaking hazard within Cincinnati metro and assess the affect of buried valleys on shear wave velocity.