Recent Deformation in the Mississippi Embayment from High-Resolution Reflection Data

The New Madrid seismic zone (NMSZ), located in the Mississippi embayment (Central United States), is known for the highest rate of seismic activity east of the Rocky Mountains as well as the destructive sequence of earthquakes (> 7.5M) that occurred in New Madrid, Missouri during a three-month period in 1811-1812. Here we present preliminary results of a 300 km-long high-resolution marine seismic reflection survey conducted in June 2008 along the Mississippi river from Caruthersville, Missouri, to Helena, Arkansas. Paleoseismological observations in this area, just south of the NMSZ, indicate that seismic activity has occurred here that is not correlated with the present NMSZ active fault system. Our acquisition program was designed to image the sedimentary sequences from the southern part of the NMSZ through this area with the goal to identify and characterize concealed faults inferred from the paleoseismic evidence. The reflection data were acquired using a dual chamber 245/245 cm3 (15/15 in3) mini-GI airgun firing at 13.8 MPa (2000 psi) every 10-12 seconds and a 24 channel 75 m-long active streamer. The 4 m minimum offset, 3.125 m group interval and 1.6 m CMP spacing enabled us to successfully image and identify recent deformation in the sedimentary layers (< 1 km depth) with unprecedented resolution. An ecosounder (or CHIRP) was used in conjunction with the aforementioned seismic array to map near surface structure (< 15 m). Observed structures from the CHIRP data suggest small-scale faulting and geomorphologic forms. Reflections up to an approximate depth of 1 km allow us to map the Paleozoic, Cretaceous, Tertiary and Quaternary sequences throughout much of the profile, revealing that the deformation is accommodated within discrete zones where the unconsolidated sediments are folded and faulted. In particular, the data show pronounced folding of the sedimentary layers from the Paleozoic to the Eocene/Quaternary unconformity. Some of the most significant deformation is imaged near the presently seismically active southern arm of the NMSZ. Furthermore, Quaternary deformation is also visible approximately 30 km south of the Blytheville arch in an area where no seismicity is observed today.