Faults--Offshore of San Francisco Map Area, California

This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore San Francisco map area, California. The vector data file is included in "Faults_OffshoreSanFrancisco.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html. The Offshore of San Francisco map area straddles the right-lateral transform boundary between the North American and Pacific plates and is cut by several active faults that cumulatively form a distributed shear zone, including the San Andreas Fault, the eastern strand of the San Gregorio Fault, the Golden Gate Fault, and the Potato Patch Fault (Bruns and others, 2002; Ryan and others, 2008). These faults are covered by Holocene sediments (mostly units Qms, Qmsb, Qmst) with no seafloor expression, and are mapped using seismic-reflection data (see field activities S-15-10-NC and F-2-07-NC). The San Andreas Fault is the primary plate-boundary structure and extends northwest across the map area; it intersects the shoreline 10 km north of the map area at Bolinas Lagoon, and 3 km south of the map area at Mussel Rock. This section of the San Andreas Fault has an estimated slip rate of 17 to 24 mm/yr (U.S. Geological Survey, 2010), and the devastating Great 1906 California earthquake (M 7.8) is thought to have nucleated on the San Andreas a few kilometers offshore of San Francisco within the map area (Bolt, 1968; Lomax, 2005). The San Andreas Fault forms the boundary between two distinct basement terranes, Upper Jurassic to Lower Cretaceous rocks of the Franciscan Complex to the east, and Late Cretaceous granitic and older metamorphic rocks of the Salinian block to the west. Franciscan Complex rocks (unit KJf, undivided) form seafloor outcrops at and north of Point Lobos adjacent to onland exposures. The Franciscan is divided into 13 different units for the onshore portion of this geologic map based on different lithologies and ages, but the unit cannot be similarly divided in the offshore because of a lack of direct observation and (or) sampling. Faults were primarily mapped by interpretation of seismic reflection profile data (see field activities S-15-10-NC and F-2-07-NC). The seismic reflection profiles were collected between 2007 and 2010. References Cited Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society of America, v. 58, p. 457-471. Bruns, T.R., Cooper, A.K., Carlson, P.R., and McCulloch, D.S., 2002, Structure of the submerged San Andreas and San Gregorio fault zones in the Gulf of Farallones as inferred from high-resolution seismic-reflection data, in Parsons, T. (ed.), Crustal structure of the coastal and marine San Francisco Bay region, California: U.S. Geological Survey Professional Paper 1658, p. 77-117. Lomax, A., 2005, A reanalysis of the hypocentral location and related observations for the Great 1906 California earthquake: Bulletin of the Seismological Society of America, v. 95, p. 861-877. Ryan, H.F., Parsons, T., and Sliter, R.W., 2008. Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California. Tectonophysics, 429 (1-2), p. 209-224. U.S. Geological Survey and California Geological Survey, 2010, Quaternary fault and fold database for the United States, accessed April 5, 2012, from USGS website: http://earthquake.usgs.gov/hazards/qfaults/.本数据集（DS 781）的该部分提供了加利福尼亚州旧金山近海地图区域的地质与地貌图配套断层数据。矢量数据文件收录于"Faults_OffshoreSanFrancisco.zip"，可通过http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html获取。 旧金山近海地图区域横跨北美板块与太平洋板块之间的右旋走滑转换边界，被多条活动断层切割，这些断层共同构成分布式剪切带，包括圣安德烈亚斯断层（San Andreas Fault）、圣格雷戈里奥断层东支、金门断层以及马铃薯田断层（Bruns等，2002；Ryan等，2008）。上述断层被全新世沉积物（主要为Qms、Qmsb、Qmst地层单元）覆盖，无海底地表露头，通过地震反射数据完成填绘（参见野外作业S-15-10-NC与F-2-07-NC）。圣安德烈亚斯断层为核心板块边界构造，向西北方向延伸穿过本地图区域；其与海岸线的交点分别位于地图区域以北10公里的博利纳斯潟湖，以及地图区域以南3公里的贻贝岩。该段圣安德烈亚斯断层的估算滑动速率为17~24 mm/yr（美国地质调查局，2010），而破坏性巨大的1906年加利福尼亚大地震（震级M 7.8）被认为是在旧金山近海数公里的本地图区域内的圣安德烈亚斯断层上孕育发生的（Bolt，1968；Lomax，2005）。 圣安德烈亚斯断层构成两个截然不同的基底地体的边界：东侧为上侏罗统至下白垩统的弗朗西斯科杂岩（Franciscan Complex），西侧为晚白垩世花岗岩类及更古老变质岩组成的萨利尼亚地块（Salinian block）。弗朗西斯科杂岩（未细分地层单元KJf）在洛博斯角及其以北区域形成海底露头，与陆上出露区域相邻。本地质图的陆上部分基于岩性与年代将弗朗西斯科杂岩划分为13个不同地层单元，但由于缺乏直接观测与（或）采样手段，近海区域无法开展同等精度的地层划分。 断层主要通过地震反射剖面数据解译完成填绘（参见野外作业S-15-10-NC与F-2-07-NC）。本次研究的地震反射剖面采集于2007年至2010年间。 参考文献 Bolt, B.A., 1968, 1906年加利福尼亚大地震的震源：《美国地震学会通报》，第58卷，第457-471页。 Bruns, T.R., Cooper, A.K., Carlson, P.R., & McCulloch, D.S., 2002, 法拉隆湾水下圣安德烈亚斯与圣格雷戈里奥断层带结构——基于高分辨率地震反射数据的解译，收录于Parsons, T.主编：《加利福尼亚州旧金山湾沿岸及海域地壳结构》：美国地质调查局专业报告1658，第77-117页。 Lomax, A., 2005, 1906年加利福尼亚大地震震源位置及相关观测的重新分析：《美国地震学会通报》，第95卷，第861-877页。 Ryan, H.F., Parsons, T., & Sliter, R.W., 2008, 加利福尼亚州旧金山近海圣安德烈亚斯断层带相关的垂直构造形变：《构造物理学》，第429卷（1-2期），第209-224页。 美国地质调查局与加利福尼亚地质调查局, 2010, 美国第四纪断层与褶皱数据库, 2012年4月5日访问, 来自美国地质调查局官网: http://earthquake.usgs.gov/hazards/qfaults/.