The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.

Transpressive plate motion in the coastal region between Monterey Bay and Los Angeles is distributed over a complex system of active strike-slip faults, subparallel reverse and reverse-oblique faults, and related folds. Seismotectonic responses to interplate stresses vary markedly along this portion of the plate margin. Coastal central California is divided into structurally and physiographically distinct seismotectonic domains separated by major, predominantly Quaternary, boundary faults. Internally, seismotectonic domains are marked by distinctive styles and orientations of Quaternary faulting and folding, historical seismicity patterns, geomorphic expression, and basement rock characteristics. Five principal seismotectonic domains are recognized in this study: Transverse Ranges domain, Santa Maria Basin-San Luis Range domain, coastal Franciscan domain, Salinian domain, and western San Joaquin Valley domain. Major domain boundaries include the San Andreas, Nacimiento-Rinconada, San Gregorio-Hosgri, Big Pine, and Santa Monica-Raymond-Sierra Madre-Cucamonga faults. The Transverse Ranges domain is characterized by pronounced north-northeast-oriented maximum horizontal compressive stress and associated Quaternary crustal shortening, west-trending reverse and left-lateral reverse-oblique faults and earthquake focal mechanisms, and a frequent occurrence of damaging earthquakes. The Santa Maria Basin-San Luis Range domain has low to moderate rates of Quaternary tectonism, active west- to northwest-striking reverse faults, and low to moderate seismicity with mainly reverse and left-lateral reverse-oblique focal mechanisms. The coastal Franciscan domain includes numerous northwest-striking, mainly northeast-dipping, faults with uncertain earthquake potentials. Moderate seismicity and reverse and right-lateral reverse-oblique earthquake focal mechanisms indicate significant northeast-directed convergence and broad internal deformation of weak Franciscan Complex basement. The Salinian domain includes a moderate- to high-relief western region marked by abundant northwest-striking faults with uncertain Quaternary histories, and an eastern region with generally low relief and few recognized surface faults. Seismicity within the domain is sparse, typically with right-lateral strike-slip focal mechanisms. The western San Joaquin Valley domain is marked by young folds associated with active thrust and reverse faults in its central and southern portions and both shear and contractional deformation in the north. Seismicity occurs at a low to moderate rate, with mainly reverse and thrust fault focal mechanisms.