This line shapefile contains fault lines in the offshore area of Half Moon Bay, California. The map area lies about 12 km southwest of the San Andreas Fault, the dominant structure in the distributed, right-lateral, transform boundary between the North American and Pacific plates. The map area straddles the right-lateral San Gregorio Fault, the most important structure west of the San Andreas Fault in this broad zone. The San Gregorio is part of fault system that occurs predominantly in the offshore, extending about 400 km from Point Conception on the south to Bolinas and Point Reyes on the north (Dickinson and others, 2005), intersecting land at a few coastal promontories. In the Offshore of Half Moon Bay map area, the San Gregorio Fault forms a distributed shear zone about 2 to 4.5 km wide that includes two primary diverging fault strands. The eastern strand (also known as the Seal Cove Fault or Coastways Fault) roughly parallels the shoreline, lies onshore for about 3 km at Pillar Point, and locally forms the boundary between outcrops of Cretaceous grantic rocks to the east and Purisima Formation to the west. The western strand (also known as the Frijoles Fault) lies entirely offshore and forms a boundary between the Purisima Formation on the east and undifferentiated Cretaceous and (or) Tertiary rocks (Pigeon Point Formation?) of the Pigeon Point structural block (McCulloch, 1987) on the west. The Pigeon Point block forms a northwest-trending bedrock ridge that extends offshore for about 30 km from Pescadero Point and forms the northwest boundary of the outer Santa Cruz Basin (McCulloch, 1987). Cumulative lateral slip on the San Gregorio Fault zone is thought to range from 4 to 10 mm/yr in this region (U.S. Geological Survey, 2010). Bathymetric (Bathymetry--Offshore Half Moon Bay, California, DS 781) and seismic-reflection data (see field activity S-15-10-NC) reveal that the offshore outcrops of the Purisima Formation between the eastern and western strands of the San Gregorio Fault Zone are spectacularly folded, faulted and rotated by the strike-slip motion and drag along the faults. The entire map area lies along strike with the young, high topography of the Santa Cruz Mountains and Coast Ranges. This regional uplift has been linked to a northwest transpressive bend in the San Andreas Fault (for example, Zoback and others, 1999). Uplift of nearby marine terraces at rates up to 0.44 mm/yr confirms that this uplift includes the coastal zone (Weber and others, 1995). Faults were primarily mapped by interpretation of seismic reflection profile data (see field activity S-15-10-NC). The seismic reflection profiles were collected between 2007 and 2010. Contains representation class for line symbols from Section 2 of FGDC Digital Cartographic Standard for Geologic Map Symbolization (FGDC Document Number FGDC-STD-013-2006). The map that show these data are published in Open-File Report 2014–1214, "California State Waters Map Series--Offshore of Half Moon Bay, California." This layer is part of USGS Data Series 781.In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP) to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats and geology within the 3-nautical-mile limit of California's State Waters. CSMP has divided coastal California into 110 map blocks, each to be published individually as United States Geological Survey Open-File Reports (OFRs) or Scientific Investigations Maps (SIMs) at a scale of 1:24,000. Maps display seafloor morphology and character, identify potential marine benthic habitats and illustrate both the seafloor geology and shallow (to about 100 m) subsurface geology. Data layers for bathymetry, bathymetric contours, acoustic backscatter, seafloor character, potential benthic habitat and offshore geology were created for each map block, as well as regional-scale data layers for sediment thickness, depth to transition, transgressive contours, isopachs, predicted distributions of benthic macro-invertebrates and visual observations of benthic habitat from video cruises over the entire state. These data are intended for science researchers, students, policy makers, and the general public. This information is not intended for navigational purposes.The data can be used with geographic information systems (GIS) software to display geologic and oceanographic information. Additionally, this coverage can provide a geologic map for the public and geoscience community to aid in assessments and mitigation of geologic hazards in the coastal region and sufficient geologic information for land-use and land-management decisions both onshore and offshore.