Characteristics and Alternative Mechanisms of Late Quaternary Faulting, Proposed LNG Terminal Site, Point Conception, California

Abstract

Late Quaternary faults at the proposed Point Conception LNG site consist of westerly-trending reverse faults that are parallel to bedding in the Pliocene bedrock beneath the terrace deposits, and southwesterly-trending cross-faults that offset displacements along the bedding-plane faults. These faults exhibit a number of unusual characteristics. The bedding-plane faults appear to be spaced at near-regular intervals of approximately 60–120 m, and the areas between the faults are relatively undisturbed. The zone of disruption along each fault is normally only a few centimeters, but a single-event displacement of 1.4 m can be demonstrated for the Arroyo fault and a 2.3 m displacement is suggested for the SF-1 fault. Such large displacements are normally associated with major fault zones and major earthquakes. Also, each fault appears to have experienced but one or two events, and “new faulting” along any of the numerous zones of weakness in the bedrock section beneath the site appears as likely as recurrent movement on a previously established fault. Flexural slip and backthrust models have been proposed as mechanisms for site faults. While the faults exhibit some characteristics expected from the flexural slip model, data on shoreline-angle elevations on both flanks of the Government Point syncline clearly prohibit flexure of this structure during the period of fault movements. In the absence of flexure, a flexural slip mechanism is untenable. A simple backthrust model does not explain the spacing of site faults, but an antithetic model taken from ramp regions of overthrusts could explain this characteristic. Whether such a model is appropriate is unknown, as the applicant9s consultants obtained no data that might be utilized in considering mechanisms other than flexural slip.