LOCATING THE NORTHERN CALAVERAS FAULT INTERSECTION FOR THE ALAMEDA SIPHON NO. 4

Engineering design of the SFPUC Alameda Siphon No. 4 requires identification of the fault crossing location and development of fault-rupture displacement estimates and facility response. To support these calculations, the initial phase involved a site-specific fault characterization. The existing Alameda Siphons are components of the Hetch Hetchy water system, which transmits water stored at Hetch Hetchy Reservoir to customers in the SF Bay area. The three existing Alameda Siphons cross the Northern Calaveras fault within the Sunol Valley and are vulnerable to damage in a major seismic event as a result of fault displacement. The characteristic earthquake for the 45-km Northern Calaveras fault is estimated to be M6.8 by the most recent USGS Working Group of California Earthquake Probabilities report (WGCEP, 2008).

The principal objectives of the fault-rupture characterization described herein were to assess location and width of active strand(s) and characterize the amount and distribution of fault-rupture displacement in support of pipeline design. Site-specific geologic, seismologic, and geotechnical information were collected and integrated with pre-existing datasets, including creep indicators in existing pipelines, to determine fault location and geometry, style of faulting and relative degree of activity.

The primary strand of the Northern Calaveras fault at the latitude of the Alameda Siphons is located through the zones of creep-related deformation in Siphons No. 1 and 3, and is interpreted to lie within a 120-foot-wide zone that intersects Siphon No. 2 orthogonally. At the latitude of Siphon No. 4, the fault lies within a 100-foot-wide zone, which intersects the proposed corridor at an angle of about 71 degrees and will result in extensional pipeline deformation during right-lateral fault movement. Based on the results summarized here, the design fault crossing infers deformation will occur within a 30 foot primary zone of faulting anywhere within the 100 to 120 ft fault zone. The design approach used to mitigate the pipeline’s fault rupture hazard, proven successful during construction, is a result of a working relationship from start to finish between the geologic and engineering design team.