Evaluating Potential Linkages of the Pitas Point and Ventura Fault Systems, California: Implications for Seismic Hazards

Abstract

The offshore Pitas Point and onshore Ventura faults are aligned along strike, which has led some in the geological community to propose their connection at depth. However, this connection requires that onshore and offshore fault segments have similar subsurface geometries. This project presents an analysis of the subsurface geometry and kinematics of the Pitas Point fault to assess this potential linkage with the Ventura fault via the Sisar decollement. Our analysis specifically focuses on the region beneath the Dos Cuadras anticline in the eastern Santa Barbara Channel, where we constrain and render the geometry of the Pitas Point fault using 3-D seismic reflection data and well control. We determine a permissible range of fault geometries through seismic interpretation, cross section construction, and balancing. The Pitas Point fault is comprised of several splays. Our analysis suggests that the main splay (referred to as the Upper Pitas Point fault (UPP)), has a listric geometry -shallowing its dip with depth. The UPP fault soles to the regional detachment at about 8.1 km. A similar mid-crustal detachment has been proposed for the onshore Ventura fault (Sisar decollement), and mapped to the south of our study area as the Lower Pitas Point fault (LPP). This common detachment provides a direct structural linkage between the Upper Pitas Point and Ventura faults, supporting the proposal that these structures are part of a combined fault system. We combine this new fault interpretation with elements of the SCEC Community Fault Model to generate a complete representation of the Ventura, Pitas Point, and Southern San Cayetano fault (V-PP-SSC) system, which has a combined surface area of about 2610 km2. Utilizing the slip rate range of 6.6-10.5 mm/yr as presented by Hubbard et al. (2014) and corroborated by Marshall et al. (2017), we calculate potential maximum moment magnitude (7.4) and a recurrence interval (125-290 years) for earthquakes on this system. Due to the nature of the Santa Barbara Channel and Ventura basins, and high slip rates for the V-PP-SSC system, the combined fault system described in this study poses great risk to the urban populations of southern California.