A PASSIVE SEISMIC INVESTIGATION OF THE CRUSTAL STUCTURE UNDER OHIO

Although gravity and magnetic field data have been used to infer variations in crustal structure in Ohio, seismic evidence regarding the depth to the Mohoroviĉiæ discontinuity is sparse. First arrivals from 17 regional earthquakes recorded by the Ohio Seismic Network between 2000 and 2006 were examined to investigate variations in crustal composition and thickness in Ohio and attempt to calculate the depth to the mantle. The Ohio Seismic Network consists of 25 stations well distributed across Ohio that can be accessed via the Web. 14 stations recorded multiple earthquakes used in this study. A composite time-distance curve compiled from these 17 earthquakes was used to calculate an average crustal structure for Ohio. The average crustal structure was determined to have two layers: Paleozoic sedimentary rock over the crystalline crust, (phase Pg), overlying a horizontal mantle (phase Pn). The average apparent P-wave velocities for crustal structures are 4.8 km/s and 5.6 km/s respectively, overlying the mantle There was 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 700m in western Ohio to 4 km in southeastern Ohio. Preliminary average seismic thicknesses for the Paleozoic sedimentary rock and crystalline crust, computed from the composite time-distance curve, are 4 km and 59 km, respectively. This calculated crustal thickness, too thick for the craton, suggests increasing velocity with depth within the crust must occur although first arrival data fail to document its presence. Reduced traveltime plots for each earthquake used mean apparent velocity across the array for each event to determine time residuals for each station. Residuals range from 0.6 early to 1.8 seconds late. Average station residuals are used to infer variations in Paleozoic sediment and crystalline crustal thicknesses under each station. These thicknesses will then be compared with crustal thicknesses from surrounding regions and published thickness variations derived from interpreting potential field data.