A PASSIVE SEISMIC INVESTIGATION OF THE CRUSTAL STUCTURE UNDER OHIO

Although gravity and magnetic field data have been used to infer variations in the crustal structure of Ohio, seismic evidence regarding the depth to the Mohoroviĉic discontinuity is sparse. First arrivals from 13 regional earthquakes recorded by the Ohio Seismic Network between 2001 and 2006 were examined to investigate variations in crustal composition and thickness in Ohio and attempt to calculate the depth to the mantle. The average crustal structure consists of two layers: Paleozoic sedimentary rock over crystalline crust (phase Pg) overlying the (phase Pn). The average apparent P-wave velocities for these crustal structures are 4.8 km/s and 5.5 km/s respectively. Reduced travel time curves of first arrivals show no direct evidence of a higher (~ 6.8 km/s) velocity lower crust (crustal phase Pb). Paleozoic sedimentary rock thickness, determined from well data, ranges from 700 m in western Ohio to 4 km in southeastern Ohio. Calculated thicknesses of the Paleozoic sedimentary rock (-5.8 ± 8.8 km in western Ohio, 26.9 ± 25.8 km in southeastern Ohio) and granitic crystalline crust (20.1 ± 10.1 km in western Ohio, 57.6 ± 29.6 km in southeastern Ohio) beneath each station, determined from regional earthquake residuals (1.8 ±1.8 seconds early in western Ohio, 5.0 ± 5.4 seconds late in southeastern Ohio), have a mean value higher than thicknesses derived from well data and gravity and magnetic field interpretations. However, our results involve large standard deviations that span crustal models previously proposed. Comparing these thicknesses determined from regional earthquakes with thicknesses determined from teleseismic earthquakes and EARS seismic data show small variations beneath most stations. Those stations with large variations between regionally, teleseismically, and EARS determined thicknesses also have a small number of regional earthquake observations.