Near-seafloor Environment and Potential Fault around the Gas Hydrate Drill Site of the Ulleung Basin, East Sea

To perform environmental monitoring of deep-sea drill sites (e.g., gas leakage and seafloor deformation) and for optimal placement of monitoring equipment on the seafloor, the exact fault trace and any possible fault offsets need to known. We mapped the structures around a growth fault near the gas hydrate drill site. Then, based on the contour lines, we derived individual two-dimensional (2D) chirp data and merged 3D volumes. The chirp data revealed a clear growth fault slightly south (about 210 m) of drill site. The fault was mapped across all inlines of the 3D volumes. Fault offsets were determined for five individual layers and a clear change in offset with depth was observed. A reduced offset was noted below layer L2 (∼1 m) but it was to 2.5 m for both underlying layers L3 and L4. Based on five piston cores across the survey area between North Zone and South Zone, the linear sedimentation rate for the Holocene showed almost no difference (16.5–16.7 cm/kyr). The rate below the Holocene showed a difference between the North Zone (lower; 12.6–12.8 cm/kyr) and the South Zone (higher; 14.8–16.1 cm/kyr). This change is explained by a sudden reduction of fault activity during the Holocene.