Although the southern embayment is characterized by only moderate historic seismicity (< M4.5), mounting paleoseismological evidence documents strong Quaternary earthquakes predating the activity of the NMSZ in this region. In particular three liquefaction fields in the area of Marianna, Arkansas and in Ashley County and Desha County stand out for the size of the structures (tens to hundreds of meters long), suggesting that they were triggered by large magnitude events about 5,000-7,000 years ago. While the timing and location of these liquefaction fields are studied with increasing detail, very little is known about the faults responsible for the earthquakes that triggered sand liquefaction. The southern profile crosses several mapped and suspected faults that are inferred to be responsible for the newly discovered liquefaction features. In many instances the presence of faults or fault zones is inferred from potential field data (e.g. East Reelfoot Rift Margin and White River fault zone) and corroborated by geomorphic structures, such as alignment of terraces (Big Creek fault zone), drainage pattern of the Mississippi River tributaries (e.g. Arkansas, Saline and White River) and changes in the Mississippi River longitudinal profile and sinuosity. Two of the suspected faults (i.e. Saline River and Arkansas River fault zones) follow the deformation corresponding to the northern front of the Paleozoic Ouachita orogen, buried by the Mesozoic and Cenozoic embayment sediments. This segment of the orogen underwent strong right-lateral transpression during late Paleozoic time, and it controls the structural frame of the southern embayment. A diffuse seismicity trend follows the front, and activity along the Saline River fault zone and the Arkansas River fault zone is inferred to result from Quaternary reactivation of the Paleozoic grain. However the geometry, location and Quaternary history of deformation of these structures are still unknown.

The southern profile will retrace a portion (~60 km from Tunica, MS to Helena, MS) of the survey carried out during the initial equipment/navigation tests of the pilot study. The quality of the data collected along this portion of the profile suffered from the continuous change in direction and speed as we perfected the acquisition procedure.