Faults--Offshore of Pacifica map area, California

This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore of Pacifica map area, California. The vector data file is included in "Faults_OffshorePacifica.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshorePacifica/data_catalog_OffshorePacifica.html.

The Offshore of Pacifica 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 (sheets 8, 9; 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 (sheet 8). 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 Pacifica 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 (sheet 9; 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 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的本部分提供了加利福尼亚州帕西菲卡近海（Offshore of Pacifica）区域地质地貌图相关的断层数据。矢量数据文件收录于"Faults_OffshorePacifica.zip"，可通过http://pubs.usgs.gov/ds/781/OffshorePacifica/data_catalog_OffshorePacifica.html访问获取。

帕西菲卡近海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（right-lateral transform boundary），被多条活动断层（active faults）切割，这些断层共同构成了弥散剪切带（distributed shear zone），包括圣安德烈亚斯断层（San Andreas Fault）、圣格雷戈里奥断层（San Gregorio Fault）东支、金门断层（Golden Gate Fault）以及土豆补丁断层（Potato Patch Fault）（图幅8、9；Bruns等，2002；Ryan等，2008）。这些断层被全新世沉积物（Holocene sediments，主要为Qms、Qmsb、Qmst地层单元）覆盖，未显现海底地表出露特征，通过地震反射数据（seismic-reflection data）完成填图（图幅8）。圣安德烈亚斯断层是主要的板块边界构造，呈北西向延伸贯穿研究区；其与海岸线的交点分别位于研究区以北10公里处的帕西菲卡潟湖，以及研究区以南3公里处的贻贝岩（Mussel Rock）。该段圣安德烈亚斯断层的估算滑动速率（slip rate）为17至24毫米/年（美国地质调查局，2010），破坏性极强的1906年加州大地震（Great 1906 California earthquake，震级M 7.8）被认为是在旧金山近海数公里范围内的研究区内的圣安德烈亚斯断层上发震的（图幅9；Bolt，1968；Lomax，2005）。

圣安德烈亚斯断层构成了两个截然不同的基底地体（basement terranes）的边界：东侧为上侏罗统至下白垩统（Upper Jurassic to Lower Cretaceous）的弗朗西斯科混杂岩（Franciscan Complex），西侧为晚白垩世花岗岩及更古老的变质岩组成的萨利尼亚地块（Salinian block）。弗朗西斯科混杂岩（未细分地层单元KJf）在洛博斯角（Point Lobos）及其以北区域形成海底露头（seafloor outcrops），与陆上露头（onland exposures）相邻。本次地质图的陆上部分基于岩性（lithologies）与时代差异，将弗朗西斯科混杂岩划分为13个不同地层单元，但由于缺乏直接观测和（或）采样数据，近海区域无法进行同等精度的地层划分。

本次断层填图主要通过解译地震反射剖面数据（seismic reflection profile data）完成（详见S-15-10-NC与F-2-07-NC）。相关地震反射剖面的数据采集时间为2007年至2010年之间。

参考文献
Bolt, B.A., 1968, 1906年加州地震的震源：《美国地震学会通报》，第58卷，第457–471页。
Bruns, T.R., Cooper, A.K., Carlson, P.R. 及 McCulloch, D.S., 2002, 根据高分辨率地震反射数据推断的法拉隆湾水下圣安德烈亚斯与圣格雷戈里奥断层带结构，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年加州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. 及 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变形：《构造物理学》，429卷（1-2期），第209–224页。
美国地质调查局与加州地质调查局, 2010, 美国第四纪断层与褶皱数据库，2012年4月5日访问，源自USGS官网：http://earthquake.usgs.gov/hazards/qfaults/.