GPR SURVEYS IN SEARCH OF SAWMILL BRANCH, NEAR FORT DORCHESTER, SOUTH CAROLINA

DURA-GOMEZ, I., ADDISON, A., KNAPP, C.C., TALWANI, P., Department of Geological Sciences, University of South Carolina, and CHAPMAN, A., Colonial Dorchester State Historic Site, pradeep@sc.edu.

During the 1886 Charleston earthquake, two parallel tabby walls of Fort Dorchester (situated on the northern banks of the Ashley River) broke left-laterally. Offsets of 7 cm and 10 cm were measured in the northern and southern walls respectively and a strike of ~N25¼W was inferred for the causative Sawmill Branch fault. Considering that the 2.5 ft. thick walls of the fort broke during the earthquake, we made a plan to cut trenches across the strike to study any near surface evidence of the fault. To determine locations for trenching, we carried out GPR surveys. Eight profiles, varying in length from 10 m to 30 m, were run across the projected strike of the fault and one 50 m long profile parallel to it. Different antennas (50, 100 and 200 MHz) were used to capture the signature of the fault. Vertical displacements and other disruptions of distinctive markers were used to locate the fault(s). Preliminary results show that markers were disrupted at more than one place, suggesting the presence of a fault zone aligned approximately in a N30¼W direction. We plan to confirm these locations by cutting shallow trenches.