--> --> Abstract: The Importance of Secondary Faulting, Active Strike-Slip Fault Zones of Southern California, by J. Johnson, J. Slosson, and C. Grey; #90992 (1993).

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JOHNSON, JEFFREY, Jeffrey A. Johnson, Inc., Del Mar, CA, and JAMES SLOSSON, and CLIFF GREY, Slosson and Associates, Van Nuys, CA

ABSTRACT: The Importance of Secondary Faulting, Active Strike-Slip Fault Zones of Southern California

Currently, greater attention is being given to the importance of secondary and/or conjugate faulting on the triggering of tectonic earthquakes, the identification of fault zones bounded by active faults, the identification and origin of fault-related geomorphic features within strike-slip fault zones, and the potential for coseismic rupture of secondary faults. Recent observations by the authors following the Landers earthquake of June 28, 1992, clearly suggest that coseismic, conjugate Riedel shears or faults can have an adverse effect on engineered structures and develop significant geomorphic features suitable for the identification of strike-slip fault zones and the hazards associated with zone-bounding faults and secondary shearing. It has long been known the R1 and R2 conjugate hears are common features along strike-slip fault zones. However, repeated differential and possibly sequential movement of R1 and R2 faults can result in the development of "R1" and "R2" ridges. "R2" ridges form at a relatively high angle to the fault zone. Relative slip along R1 faults is greater than R2 faults resulting in "R1" ridges that offset "R2" ridges or appear to truncate the linear channels between parallel "R2" ridges that form due to differential erosion and tectonic uplift. The geomorphic pattern of "R1" and "R2" ridges is a potential indicator than an area is within a significant shear or fault zone. Additionally, through-going faults should bound the zone on both sides, the potential for coseismic secondary faulting within the zone is significant, and tensional fractures can also develop within the zone. These geomorphic features were recently used by the authors in the analysis of potential seismic hazards affecting water storage facilities in southern California.

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