SHALLOW SEISMIC REFLECTION IMAGING OF THE COMMERCE GEOPHYSICAL LINEAMENT NEAR QULIN, MISSOURI

Shallow seismic reflection imaging was used to characterize the significance of near-surface tectonic deformation associated with the northeast-trending Commerce Geophysical Lineament (CGL) near Qulin (southeastern), Missouri. The CGL in the area is expressed by subtle topographic escarpments, photolineaments, and subsurface faulting in Paleozoic through Quaternary deposits. An east-west oriented shear-wave seismic reflection profile (Qulin 1) was collected approximately 4 km east of Qulin, across a suspected fault zone, in order to identify the location and style of neotectonic deformation. The reflection data were collected in SH mode (sensitive to horizontally polarized shear waves) on a 24-channel engineering seismograph (Seistronix RAS-24). The active spread was a portable landstreamer with 12 receivers (14 Hz horizontal geophones oriented perpendicular to the seismic line) spaced at 4-m intervals. The seismic profile was shot off-end with 2-m source offsets (274 shotpoints were recorded on the Qulin 1 profile for a total length of 1,096 m). Seismic energy was generated by five horizontal impacts of a 1.8-kg sledgehammer on a 10-kg steel I-beam oriented perpendicular to the spread. Acquisition parameters on the seismograph included a 1.0 s record length and 0.5 ms sampling interval.

The processed seismic profile exhibits coherent seismic reflection energy to depths of about 70 m. Based on correlation with previously acquired compressional-wave seismic reflection data, a reflection from the Quaternary/Tertiary boundary is visible around 50 m depth. Offsets in reflections and changes in reflection amplitude and coherency suggest the presence of a fault zone (steep eastward dipping faults) on the western half of the profile. Disruptions of a shallow reflection within the Quaternary section show that some of the faults extend upward to within 15 m of the surface. The fault zone is approximately 400 m wide, shows down-to-the-east offsets, and correlates very well with a zone of faults imaged on a seismic reflection profile from a previous study in the area. This correlation suggests a northeast-striking fault zone with down-to-the-southeast offset and deformation extending upward into the shallow subsurface.