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.

This work includes new interpretations of shallow offshore geologic structure between Point Arguello and the Santa Maria River within California’s 3-mi coastal limit. These interpretations are based on multi-sensor high-resolution seismic reflection data collected during January and February 1986. Water depths within the survey area range from 16 m (50 ft) nearshore to 70 m (230 ft) 3 mi west of Point Arguello. The sea floor slopes between 0.3 and 0.5° south-westward. The thickness of unconsolidated Quaternary sediment in the survey area ranges from 0 (bedrock outcrop) to almost 50 m (165 ft) off Point Arguello. The survey area crosses the boundary between the northwest-trending Coast Ranges and the east-trending Transverse Ranges. The onshore faults and folds can be traced offshore in the seismic sections. From north to south, these faults include: (1) Pezzoni-Casmalia-Orcutt frontal fault, (2) Lions Head fault, (3) Santa Ynez River fault system, (4) Lompoc-Solvang fault, (5) Cañada-Honda fault, and (6) several unnamed faults offshore Point Arguello. These faults are tentatively classified as potentially active because they do not offset a Pleistocene erosion surface and the Holocene unconsolidated sediments overlying that surface do not show offset in the seismic records. Although the faults are tentatively classified as potentially active, they may be seismically active as suggested by the limited earthquake data in the area. The seismic data show that the north-northwest-striking Hosgri fault zone decreases in both vertical and right-slip displacement toward the south. In the northern and central parts of the survey area, the fault zone consists of two subparallel branches. In the south, near Purisima Point and near the boundary between the Transverse and Coast Ranges, the north-northwest strike of the fault zone changes toward the east and the fault zone shows splays. Our interpretation of the data is that this area of splays may be the terminus of the Hosgri. If that is correct, then the amount of surface rupture due to earthquakes along this segment of the Hosgri is likely to be small, if surface ruptures occur at all.

ABSTRACT The Half Moon Bay oil field was first developed in the 1880’s and further drilling has been done in each “oil boom” of the past 100 years. The field has produced an estimated 58,000 barrels of oil from about 19 wells within a maximum area of 155 acres. Recent peak field production was 11 B0PD in 1985. Efforts to develop new production have been severely limited by Coastal Zone restrictions. The main reservoir in the Half Moon Bay oil field consists of very thin sandstone layers within lower Pliocene (and upper Miocene?) mudstones of the Purisima Formation, at depths of 240 to 3,085 ft (73 to 940 m). Oil is high gravity: 43°-55° API. Petroleum is trapped on two separate structural features. Recent drilling has been concentrated in the Verde area on the northwest-trending Purisima anticline, where the Purisima Formation overlies a thick upper Miocene sequence. The potential for Monterey Shale production on that structure has not been adequately evaluated. Most of the earlier wells and production were located to the northeast in the Purisima Creek area in a fault block on which the Purisima Formation lies unconformably on lower Miocene and Eocene beds. The Eocene Butano Sandstone in that block has produced a very minor amount of oil. The northwest-striking faults which bound that block had significant pre-Pliocene offset. They appear to extend north to join the San Gregorio fault, and south to merge with a previously mapped major fault in the La Honda area. Similarities between the stratigraphic sections in the Half Moon Bay and Point Reyes areas support earlier estimates of about 44 mi (70 km) of right slip along the San Gregorio fault since the end of Miocene time. [Note: this paper is partially excerpted from A. J. Horn ( 1983 ), “The Resurrection of the Half Moon Bay Oil Field, San Mateo County, California”. Because that article emphasized the Purisima Creek area of the oil field, additional sources have been utilized to extend and update its coverage.]