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Updated Mapping Of Surface Traces Of The San Cayetano Fault Zone Between Fillmore And Piru, Ventura Basin, Southern California


The San Cayetano Fault Zone (SCFZ) is a major east-west-trending north-dipping thrust fault, along the northern edge of the Santa Clara River valley between Upper Ojai Valley and Piru Creek in Ventura County. It is divided into two sections based on a prominent right-step in the fault trace, which forms a lateral ramp along Sespe Creek. At two places in the eastern section, the SCFZ bifurcates into two strands with one located varying distances from the mountain front and another trending higher up through the foothills. One of these splits is found in the vicinity of Fillmore and another near Piru. In each of these cases, it appears the lowermost trace is the active strand, while the upper strand appears inactive. Where the fault exists as a single fault trace near the base of the mountains, it is also active. The California Geological Survey (CGS) recently evaluated the SCFZ in the Piru quadrangle for the Alquist-Priolo Earthquake Fault Zoning program. Here, the “Main strand” continues northeast in the foothills and the “Piru strand” trends more easterly in the alluvium, south of the mountain front. Both strands lose stratigraphic separation and die out east of Piru Creek. Previous geologic mapping in the 1970s through 1990s located the Main strand in the foothills; however, the Piru strand and portions of the Main strand in the alluvium at the base of the mountains are mapped in varying locations, largely inferred from aerial photos and groundwater data. Recently flown Light Detection and Ranging (LiDAR) data and paleoseismic studies allowed for better interpretation of the geomorphic features associated with the SCFZ. These data show the active portions of the SCFZ have discrete geomorphic expression, including prominent south-facing scarps in the alluvium up to 8 m high. In the eastern Fillmore quadrangle, the SCFZ trends nearly north-south as part of the lateral ramp connecting the two primary sections of the fault zone. Here the same pattern is seen with the inactive “Goodenough strand” located in the foothills and the “Main strand” in the alluvium near the mountain front. Recent paleoseismic studies by others, performed east of Fillmore, confirm the lower strand is active. An inactive “upper strand” was mapped during that study, but not as high in the hills as the other two inactive strands to the north and east. In the Fillmore area scarps are not well preserved due to erosion from Sespe and Pole Creeks, which flow near the base of the mountains.