NEW REGULATORY FAULT RUPTURE HAZARD ZONES BY STATE OF CALIFORNIA

The California Geological Survey (CGS) has released new and revised Alquist-Priolo Earthquake Fault Zone (APEFZ) maps in Alameda and Ventura Counties. APEFZ maps are regulatory documents defining zones in which detailed fault investigations are required for property development. The zones are intended to preclude construction across Holocene-active faults. Completed Fault Evaluation Reports (FER) evaluated geomorphology and available subsurface data, and faults are zoned based the criteria ‘sufficiently active’ and ‘well-defined’, as per CGS SP 42. As part of these new map releases, CGS now combines APEFZ maps with Seismic Hazard Zone (SHZ) maps in one product, when both are available.

In Alameda County, the existing APEFZ in the Hayward Quadrangle is revised to include two branches of the Hayward Fault (HF). The Ashland Fault (AF) branches northwest from the HF and extends approximately 2 km through an area of extensive residential development. It is interpreted to be an oblique reverse fault that merges with the Hayward at relatively shallow depths. The AF was previously encountered during local construction and subsequently zoned by the County. Evidence for Holocene activity on the AF includes a curvilinear scarp in Holocene alluvium, and bedrock juxtaposed against Holocene soils. Scarp heights range from 3 m to over 8 m. The HF APEFZ is also revised to include a smaller branch to the northeast, which occupies an area of mostly public land with the exception of an abandoned quarry where a multi-unit development was previously proposed.

In Ventura County, the existing Piru Quadrangle SHZ map now incorporates a new APEFZ for the San Cayetano Fault (SCF). The SCF is a major east-west-trending north-dipping reverse fault, extending from Upper Ojai Valley eastward to Fillmore, then along the base of the Topatopa Mountains where it terminates approximately 5 km east of Piru. Within the quadrangle, the SCF bifurcates into the Main and Piru strands. South-facing scarps up to 8 m high within alluvium were interpreted as evidence that portions of the Main and Piru strands were Holocene-active. Two paleoseismic trenches previously excavated across a prominent scarp along the Piru strand indicate 4.3 to 5 m of apparent offset related to the most recent event.