TELESEISMIC RESIDUAL ANALYSIS OF THE NORTH AMERICAN MIDCONTINENT CRATONIC PLATFORM USING DATA FROM AN EARTHSCOPE FLEXARRAY EXPERIMENT

We completed an analysis of 301 teleseismic earthquakes recorded by the EarthScope Ozark-Illinois-INdiana-Kentucky (OIINK) FlexArray Experiment to better understand the structures underlying the Illinois Basin and Ozark Dome. We deployed 70 broadband seismometers at a 25-km grid throughout southern Missouri, Illinois, and Indiana embedded within the EarthScope Transportable Array. We measured teleseismic P-wave arrival times for an 8-month period (May to Dec. 2012) using a multichannel array cross-correlation method. Travel-time residual can be reliably determined for most teleseismic events with magnitudes greater than 4.7 at distances from 25 to 90 degrees. We analyzed the spatial distribution of relative residual times for individual events by plotting residuals as a function of station position. The most commonly observed pattern, independent of source location or depth, was for negative residuals (up to -0.7 sec) for sites in the Ozarks and westernmost Illinois Basin, and more positive residuals (up to +0.5 sec) in the central and eastern Illinois Basin. We interpret the observation of negative residuals in the west and positive in the east as largely the result of the relative high velocity of the igneous and metamorphic rocks of the Ozark Dome relative to those of the sedimentary rocks that comprise the Illinois Basin. These differences may also result from systematic differences in crustal thickness and mantle velocities between the two areas. Initial estimates of crustal thicknesses and P-wave velocities through the strata of the Illinois Basin indicate that a combination of the two is responsible for the observed patterns. More recently the time period of analysis was extended to 15 months (Jan. 2012 to Mar. 2013). The new arrivals were cross-correlated, and then inverted for P-wave velocity. Initial results of this method determined that the upper 200 km of mantle is separated into two blocks of differing velocities with a transition zone following the Ohio River boundary between Illinois and Kentucky. This result can be partially due by the lower velocities of the rocks that fill the Illinois Basin. Further investigation into the crustal thickness variations and the effect of the basin fill across the study area must be completed before an accurate interpretation can be made.