Characterizing potential offshore seismic sources using high‐resolution geophysical and seafloor sampling programs: An example from Cape Egmont fault zone, Taranaki shelf, New Zealand

Abstract

Standard marine geological techniques (high‐resolution seismic reflection methods, piston cores) are used to investigate the neotectonics and paleoseismology of an active part of an offshore fault system, the Cape Egmont fault zone, Taranaki continental shelf, New Zealand. The main active structure within the zone, the Cape Egmont fault, is a reactivated Pliocene to Recent normal to oblique‐slip fault that is associated with shallow crustal seismicity and exhibits a 53‐km long, 1‐to 5‐m high, low‐angle drape fold or “scarp” at the seafloor. Four, possibly five, fault segments varying in length from 7–51 km are delineated using geometric, structural, and stratigraphic information. Empirical estimates of maximum credible earthquake magnitudes, based upon seismic moment calculations, surface rupture lengths, and vertical surface offsets, suggest that the prehistoric earthquake(s) that formed the present seafloor surface trace probably had a magnitude of at least MW 6.4(MW, moment magnitude) and could have been as large as MS 7.3–7.4(MS, surface wave magnitude). Tentative maximum recurrence intervals for such events, inferred for each segment, range from 8000–13,000 years, with lower magnitude events having correspondingly shorter recurrence intervals of 500–900 years. Without careful study of offshore late Quaternary faults and fault zones, particularly where they are contiguous with areas of active onshore deformation, existing regional seismic hazard predictions may be inaccurate.