THE MAJOR MISSISSIPPIAN SEISMIC EVENT IN SOUTHERN OHIO AND ITS PROBABLE ORIGIN

Ohio is relatively aseismic compared to other areas of the United States, but geologic evidence of paleoseismicity in bedrock outcrops shows that the state has experienced strong earthquakes during the Silurian (Onasch, C. M., and Kahle, C. F., 2002), Devonian (Baird, G. C., et al., 2013 and Blood, D. R., et al., 2019), and Mississippian (Fakhari, M. D., 2018) Periods. The Mississippian large-scale seismites (1.8–4.6 m thick) in siltstone and mudstone layers of the Cowbell Member of the Borden Formation are exposed in the roadcut along SR-823 north of Portsmouth in southern Ohio. Three major exposures of the seismites are between coordinates N 38° 53’ 52.13”, W 82° 58’ 12.33” and N 38° 52’ 8.71”, W 82° 55’ 51.94”. This seismite horizon is observable in limited nearby roadcuts in radius of 30 kilometer to the north and east in Ohio and to the south in Kentucky.

Clear evidence of liquefaction and hydroplastic deformation of the unconsolidated and semilithified sediments are displayed as large-scale intralayer north–south oriented folds, faults, convolute lamination, sediment intrusion, flame structures, and bedding detachments in the roadcuts. The seismite horizon is sandwiched between undeformed horizontal parallel layers of the unit. These large-scale seismites in the Cowbell Member show that the area experienced a major earthquake during the sedimentation of the unit. Analysis of the geometry of the convoluted seismite beds and the north–south concentration of the strike of the deformed beds suggests that the epicenter of the triggering earthquake was at the east or west of the affected area. It is most likely a high magnitude earthquake generated in the subduction zone of the Alleghenian mountains to the east during the early Alleghenian orogeny was the cause of the soft sediment deformation and forming these seismites.