NEAR-SURFACE CHARACTERIZATION OF THE COMMERCE GEOPHYSICAL LINEAMENT USING S-WAVE SEISMIC METHODS

Shallow seismic reflection imaging was used to characterize the significance of near-surface tectonic deformation associated with the northeast-trending Commerce Geophysical Lineament (CGL), a 600-km-long gravity and magnetic anomaly that extends from Arkansas to Indiana. Three sites along the central CGL were investigated, at Idalia and Qulin in southeastern Missouri, and near Tamms in southern Illinois. The CGL in these areas is expressed by subtle topographic escarpments, photolineaments, and subsurface faulting in Paleozoic through Quaternary deposits.

The upward continuation of basement faults into near-surface sediments in the central United States (CUS) is often difficult to image with conventional compressional wave (P-wave) seismic methods because the soft sediments distribute deformation in complex zones of folding and small-scale faulting, and rarely propagate large, continuous fractures. However, shear-wave (S-wave) seismic reflection profiling has proven to be extremely effective for imaging near-surface deformation, primarily because increased seismic resolution is often gained in the water-saturated, unconsolidated sediments of the CUS.

The reflection data were collected in SH mode (sensitive to horizontally polarized shear waves) on 12- and 24-channel engineering seismographs. A variety of spread configurations, geophone frequencies, and source-receiver intervals were tested. Seismic energy was generated by horizontal impacts of a sledgehammer on a steel I-beam oriented perpendicular to the spread.

The processed seismic profiles exhibit coherent seismic reflection energy to depths of around 70 m. Reflections from the Quaternary/Tertiary boundary, Tertiary/Cretaceous boundary, and top of Paleozoic have been interpreted. Offsets in reflections and changes in reflection amplitude and coherency suggest the presence of a high-angle fault zones at all three sites. Disruptions of shallow reflections suggest that some of the faults extend upward to within 15 m of the surface (Holocene?).

The interpreted seismic profiles suggest deformation with similar structural styles over large sections of the CGL. Correlation with existing paleoseismic and paleoliquefaction data will further improve our understanding of the CGL as a potential seismogenic source in the CUS.