J. M. Bull

The use of high-resolution seismic reflection profiles for fault analysis in the near-shore environment, Weymouth Bay, Dorset, England, United Kingdom

High?resolution seismic reflection profiles using a Chirp source image a north?south extensional fault set, which cuts rocks of Upper Jurassic age, cropping out on the seafloor of Weymouth Bay, Dorset, England, United Kingdom. The same fault set cuts rocks of similar age along the adjacent coast, and field mapping can be compared directly with the Chirp profiles. Survey lines were shot perpendicular to the fault strike to produce dip sections from which displacements could be measured. One hundred and fifty?three faults were picked on a 15 km line, yielding a fault density of ?10 km?1, similar to that measured in the coastal section. Chirp?resolved fault displacements as small as 0.5 m and a maximum fault displacement of 221 m could be inferred from the data. Distribution analysis of offshore fault data indicated that fault displacement is power law with a well constrained exponent of ? 0.9. This value is consistent with the power law exponent estimate for fault displacement, over the scale range 2–8 m, onshore. Thus Chirp near?shore seismic reflection profiles can infill a data gap for fault size?frequency relationships that commonly occurs when combining data from outcrops/cores and conventional seismic reflection profiles.