The Pen Branch Fault is a reactivated, high angle, reverse basin border fault that dips to southeast from the basement (Triassic-Paleozoic-Precambrian in age) to near vertical through the Atlantic Coastal Plain sediments (Late Cretaceous to Recent in age) of the Savannah River site in South Carolina. The fault movement has occurred through Late Cretaceous to Tertiary. Faulting might penetrate as shallow as the Dry Branch and the Tobacco Road Sand Formations (Late Eocene).

An investigation with high resolution reflection seismic data is undertaken along the Pen Branch Fault. Five of the seismic lines are reprocessed to help in determining the upward depth of penetration of this fault. This is done by utilizing refracted arrivals from the multifold reflection data.

The shallowest refractors (4 - 18 m) imaged have an average velocity of 1700 m/s. All of the lines exhibit events that are not flat lying across the data where the fault is believed to be. The lines possess deformation such as offsets, upwarping and channels. Deformation can be related to the Pen Branch Fault as shallow as 4 m from the surface. Displacements along the lines vary from 1 m up to 5 m. Reverse sense of motion is mainly exhibited along the fault zone that is covered by this study. The events resolved portray the Pen Branch Fault in a fault zone of subparallel faults and splays.