ASHBY, JAMES R., Mission Geoscience, Inc., Newport Beach, CA, and BROOKS R. SMITH, LSA Associates, Inc., Irvine, CA

ABSTRACT: The El Modeno Fault in Orange County, California: New Evidence for Holocene Activity

The El Modeno fault in north-central Orange County has long been suspected of being active in accordance with the guidelines of the Alquist-Priolo Special Studies Zones Act of 1972. The El Modeno fault was uncovered during a preliminary geotechnical investigation for a proposed shopping center development just south of the intersection of Chapman Avenue and Newport Boulevard in the city of Orange, California. Topsoil observed within the shear zone of the fault appeared to be visibly offset. As a result of this discovery, an Alquist-Priolo fault investigation was initiated, involving fault trenches excavated across the site. Close examinations of the fault trace exposed in the trenches conclusively showed that the surficial soil horizon was truncated. Previous geologic and geotechnical investigations in the vicinity of the site have failed to document undisputed evidence for Holocene activity (truncated geologic units proven to be less than 11,000 yr old) along the El Modeno fault. This lack of previous conclusive evidence is perhaps due in part to the general lack of datable organic material in Holocene soil horizons, and the distribution of displaced materials over a broad zone, a trait characteristic of reverse/right-lateral strike-slip displacement such as that along the El Modeno fault. Carbon-14 radiometric dating of the truncated soils revealed that surficial soils at this site are less than 2500 yr old. Thus, the El Modeno fault should be considered active in accordance with the present guidelines of the Alquist-Priolo Special Studies Act.

The potential zoning of this fault through urbanized areas of Orange County will undoubtedly create some zoning restrictions and may have implications for future planning and redevelopment activities in the area. Earthquakes, such as the recent one in Landers, California, have provided us with interesting insight into the behavior of fault ruptures through both urbanized and unimproved land. Additionally, examination of the behavior of the Landers rupture may have serious implications for our interpretation of expected fault kinematics along faults with similar types of displacements directly related to the El Modeno fault, such as the Peralta Hills thrust and the Whittier-Elsinore fault zone.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.