Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California

We mapped an ∼94-km-long portion of the right-lateral Hosgri fault zone in offshore central California using a dense network of high-resolution seismic reflection profiles, marine magnetic data, and multibeam bathymetry. These data document the location, length, and continuity of multiple fault strands, highlight fault-zone heterogeneity, and demonstrate the importance of fault trend, fault bends, and fault convergence in the development of shallow structure and tectonic geomorphology along strike-slip faults.

Eight sections (A through H) of the Hosgri fault are mapped. The fault trends ∼335° to 341° in the southern ∼40 km of the study area (sections A through C) where shallow deformation is primarily dilational. The absence of tectonic uplift in this area has contributed to localization of the Santa Maria River and delta and, as a result, Holocene sediments cover the fault zone. The Hosgri fault generally trends 329° to 337° in the central ∼24 km of the study area (sections D through F), which coincides with oblique convergence of the Hosgri and the more northwest-trending Los Osos and Shoreline faults. This convergence has resulted in local restraining and releasing fault bends, transpressive uplifts, and extensional basins of varying size and morphology. Notably, development of a paired fault bend is linked to indenting and bulging of the Hosgri fault by a strong crustal block translated to the northwest along the Shoreline fault. Two diverging Hosgri fault strands bounding a central uplifted block characterize the northern ∼30 km of the Hosgri fault (sections G and H) in this area. The eastern Hosgri passes through significant releasing (329° to 335°) and restraining (335° to 328°) bends before passing onland at San Simeon; the releasing bend is the primary control on development of an elongate, asymmetric, 15-km-long × 300- to 2400-m-wide, “Lazy Z” sedimentary basin. The western strand of the Hosgri fault passes through a significant restraining bend (329° to 316°) and continues northward until slip is transferred to faults underlying the Piedras Blancas fold belt.

Earthquake hazard assessments should incorporate a minimum rupture length of 110 km based on continuity of the Hosgri fault zone through this area. Lateral slip rates may vary along the fault (both to the north and south) as different structures converge and diverge but are probably in the geodetically estimated range of 2–4 mm/yr.