REVISED ALQUIST-PRIOLO EARTHQUAKE FAULT ZONE MAP FOR THE WHITTIER FAULT, YORBA LINDA 7.5’ QUADRANGLE, ORANGE COUNTY, CALIFORNIA

The California Geological Survey (CGS) first evaluated the Whittier Fault for potential fault zoning in 1977. A revised evaluation was prepared in 1979, concluding segments of the Whittier Fault Zone were “sufficiently active and well defined” to be included within an Alquist-Priolo Earthquake Fault Zone (APEFZ). In 1980, an Earthquake Fault Zone map was issued for the Yorba Linda 7.5’ quadrangle. Since 1980, much development has taken place within the cities of Brea, Yorba Linda, and in the County of Orange accompanied by a number of geotechnical and fault studies within the mapped fault zone. Here we summarize the results of our current re-evaluation.

The Whittier Fault is a major structural element in the eastern Los Angeles basin. It is the longer of two northern extensions of the active Elsinore Fault Zone, the other being the Chino Fault. Primary sense of movement along this complex fault zone is right-lateral strike-slip, trending about N70^oW, with dips ranging from 65 to 80^oNE. Detailed paleoseismic studies have revealed complex fault characteristics at the Olinda Oil Field and Olinda Ranch sites. Gath et al. (1992) report the Whittier Fault consists of three active fault strands at Olinda Creek: the northern, central and southern strand. The central and southern strands form a positive flower structure: southern strand dipping northeast, central strand dipping southwest, with an elevated pressure ridge between the two faults. Work by Leighton and Associates reveals similar complex faulting relationships within Olinda Ranch. Patterson and Rockwell report Holocene sense of displacement is nearly pure strike-slip at Olinda Oil Field. Gath (1997) prepared a tectonic strip map along the Whittier Fault Zone, noting several geomorphic features indicative of active faulting, including an abundance of right-laterally deflected stream channels.

Detailed observations, made in these new studies and by CGS, provide a better understanding of the characteristics of active strands of this fault, which help with classification of geomorphic features recognized in other locations along the fault zone. CGS’ revision of this zone map is prompted by the abundance of new geologic data that identify locations of “sufficiently active and well defined” fault segments located outside previously designated zone boundaries.