A Ground-Penetrating Radar Study Of Quaternary Faulting In The Benton Hills Area, Southeast Missouri

Two grids of ground-penetrating radar profiles were acquired in the Benton Hills area, southeast<br>Missouri. The first grid, consisting of six north-northeast trending GPR profiles, was acquired adjacent and<br>parallel to the Upper Rainbow Trench (MDNR-USGS trench), near the southern edge of the Benton Hills<br>Escarpment. The Upper Rainbow Trench crosses a section of the Commerce Geophysical Lineament, and<br>exposes several near-vertical deep-seated faults that displace juxtaposed Quaternary and Tertiary strata and<br>extend to the surface. These exposed faults and the shallow strata were imaged on the GPR data and correlated<br>across the grid of profiles. The interpretation of the GPR data confirms that the recent faults trend<br>predominantly west-southwest, essentially parallel to the Commerce Fault Lineament.<br>The second grid, consisting of nine north-northeast trending GPR profiles, was acquired to the westsouthwest<br>of the Benton Hills across the projected extension of the Commerce Geophysical Lineament. The<br>GPR grid was centered adjacent to a reflection seismic anomaly (interpreted deep-seated fault). The<br>interpretation of the GPR data correlates with the seismic data and suggests that the anomalous seismic feature<br>is indeed a shallow fault (that extends to the base of disturbed soil). These data support the thesis that the<br>Commerce Geophysical Linearnent extends to the west-southwest of the Benton Hills and into the lowlands of<br>the Mississippi Embayment.<br>The interpretations of the GPR data are significant for two reasons. First, these data indicate that GPR<br>can be used to detect shallow faults in southeast Missouri, even where such faults are overlain by a thin veneer<br>,of surficial soil, or by disturbed soil (agricultural areas). Second, the GPR data supports the thesis that the<br>Commerce Fault extends into the Mississippi Embayment. This indicates that significant tectonic activity has<br>occurred in southeast Missouri outside of the New Madrid area of active micro-seismicity, implying that the<br>earthquake risk in southeast Missouri is greater than generally thought. It also supports the thesis that tectonics<br>played an important role in the development in the Mississippi Embayment.