Despite continuing studies in geodesy, seismology, and paleoseismology in the mid-continent/Mississippi Embayment area, there remains a paradox regarding the documented series of relatively large seismic events and the apparent low rates of deformation. As both modeling and observational studies improve, a variety of approaches are beginning to bear fruit. A promising hypothesis proposed to solve the apparent paradox is that deformation may be focused in different areas at different times, and that the present seismicity might not reflect the long-term behavior of the fault system in the Central US. Mounting evidence, mainly from paleoseismological studies, strongly suggests that the seismicity might have been migrating at least during the Holocene, although specific structures responsible for seismogenic faults other than the NMSZ system are not well documented.

In order to test the hypothesis of migrating faults, a pilot study conducted in the summer of 2008 collected 300 km of high-resolution seismic reflection data along the Mississippi river, from Caruthersville, MO, to Helena, AR. The pilot study was a collaborative effort between NEHRP and the NSF/Tectonics program. The data identified with unprecedented resolution the existence of areas of deformation and faulting involving Quaternary sediments, several of which outside of the NMSZ. Following up on these results, the proposed project focuses on the southernmost of these structures, imaged along the Mississippi river just 10 km northwest of Memphis, TN. The river profile crosses the structure in least at three locations that align along a ~60 km-long NE-SW striking trend. The structure is characterized by a large ~60 m-high (at the top of Cretaceous), asymmetric anticline bounded to the east by a fault that displaces the Paleozoic, Mesozoic and Cenozoic sediments, including the Holocene deposits. The imaged structure is interpreted as the southern continuation of the Meeman-Shelby Fault zone (MSF) (Williams et al., 2001). No seismicity is observed today along this fault, although the deformation of Quaternary deposits suggests that the fault has been active recently. The length and the Quaternary activity of this fault suggest that it might pose a more serious seismic hazard to Memphis than the NMSZ itself.

           The goal of this project is to image the onshore continuation of the recently imaged fault across the segment between two river crossings. The acquired high-resolution reflection seismic data will verify the location of the fault and help the characterization of the structure (style of faulting, variations in the third dimension). More importantly, identifying and imaging the fault on land will allow subsequent dating of key horizons through trenching, coring and geologic studies, providing the opportunity for bracketing the activity of the fault in time and to obtain age, amount of motions in the last events as well as recent slip rates. The project will open up several opportunities to investigate in detail the long-term behavior of this structure and contribute to the understanding of fault interactions in intraplate settings.