ABSTRACT: Faulting and Tectonic Model of the Las Posas Anticline, Western Transverse Ranges, and Implications for Seismic Risk in the Ventura Basin Region, California
GATH, ELDON M., and ROBERT A. WHITNEY, Leighton and Associates, Inc., Diamond Bar, CA
The western Transverse Ranges in southern California are undergoing north-south shortening at a rate of about 13-20 mm/yr. Much of the deformation is accommodated on the northerly dipping San Cayetano and southerly dipping Oak Ridge thrust faults. The Las Posas anticline and associated east-west-trending Springville thrust are modeled as a back-thrust to the Oak Ridge fault.
Trenching of the Springville fault and the Las Posas anticline show that five types of faulting affect the hanging wall structure. In the axial area are east-west-trending normal (keystone) bending moment faults and north-south-trending tear faults. In the western and central sections of the hanging wall are east-west-trending intercalation faults (back thrusts), and in the eastern section are east-west-trending strain-partitioning strike-slip faults. Along the southern base of the anticline is the east-west-trending, northerly dipping Springville thrust.
Trenching studies indicate the Springville fault has only recently broken to the surface, consequently inactivating bending moment and tear faults in the crestal areas of the anticline, as well as the back-thrusting intercalation faults. Recently activated strain-partitioning faults occur in the eastern section of the anticline where the Springville fault zone trends about 20 degrees from east-west.
The results of this investigation are significant for understanding secondary, coseismic, rupture hazards in youthful fold structures. Seismotectonic modeling of the structures within the anticline was successfully applied to characterize the various faulting mechanisms and to evaluate hazard to a proposed development.