Abstract

Allan E. Seward Engineering Geology, Inc. was subcontracted in 1992 and 1993 to perform a geologic investigation of the San Gabriel fault zone (SGFZ) for a proposed civic center site for the City of Santa Clarita near Bouquet Junction, southern California. The segment of the SGFZ traversing the site is considered active in the last 11,000 years and was placed within an Alquist-Priolo earthquake fault zone in 1988 based on studies conducted approximately 3 km northwest of the site. Ninety (90) trenches and one dozer cut, totaling approximately 11,380 lineal feet, and the existing 550 foot long railroad cut at Bouquet Junction were logged in detail to evaluate the lateral extent, geometry and activity of the SGFZ on the site. The main trace of the fault was found to extend southeastward from the railroad cut at Bouquet Junction across the northern margin of the site, in close proximity to the trace shown on the Alquist-Priolo Map. The main trace was observed to consist of several strands which cut steeply dipping to overturned, south-facing bedding of the Saugus Formation. Subsurface investigations indicate that an undated stream terrace deposit has been offset between 10 and 13 feet and the overlying soil has been offset 3 feet up to the north. This offset soil and apparent offset of the adjacent ridgeline, which constricts the drainage to the west, supports Holocene, dip-slip movement on the SGFZ. Most of the strands observed along the main trace dip steeply to the south but are interpreted to dip steeply to the north at depth based on oil well data. A north-dipping thrust was observed south of the main trace on the western portion of the site which shows approximately 25 to 35 feet of dip-slip offset of the Quaternary Pacoima Formation. On the eastern portion of the site, the north-dipping thrust appears to die out or bends eastward and intersects the main trace. South of the main trace and east of the north-dipping thrust, a complex zone of cross faults and south-dipping faults which cut the Pacoima Formation were encountered. One of these faults correlates with a south-dipping bedding plane fault documented in the MWD Saugus tunnel located just west of the site. Other south-dipping faults previously mapped on the western margin of the site and just east of the site apparently die out laterally and/or are overlain by the upper portion of the Pacoima Formation. Based on the evidence of Holocene and late Quaternary activity, a building setback was designated on the site extending at least 50 feet on either side of the main zone, the north-dipping thrust, the south-dipping faults which cut the upper Pacoima Formation and the intermediate splay faults.