DEFINING THE 3D GEOMETRY OF THE VENTURA-PITAS POINT FAULT SYSTEM IN SOUTHERN CALIFORNIA USING COMPLEX FOLD GEOMETRIES: IMPLICATIONS FOR LARGE MULTI-SEGMENT EARTHQUAKES

The Ventura-Pitas Point fault system, located in the western Transverse Ranges, is one of the largest earthquake sources in southern California. Holocene marine terraces suggest that deformation occurs in discrete 7-9 meter uplift events. This system is unique in that the onshore Ventura fault has a non-planar ramp-flat-ramp geometry that links at depth with several of the largest, fastest slipping faults in the area. This geometry has the potential to cause large magnitude events that rupture multiple faults and pose significant hazards.

We aim to characterize the geometry of the Pitas Point fault system, the direct offshore extension of the Ventura thrust, in the Santa Barbara Channel, using the complex fold geometries imaged in both 2D and 3D industry seismic reflection data. We use the fold geometries to define underlying fault geometries and supplement our interpretations with extensive well control, including dipmeter logs and horizon tops.

Through a series of balanced cross sections, created using fault-related folding theory, we build a 3D model of the Pitas Point fault system, which defines the nature of its linkage with the onshore Ventura fault and other large regional faults at depth. Based on the geometries of the Dos Cuadras and Pitas Point anticlines, we show that the fault has a ramp-flat-ramp geometry similar to that of the onshore Ventura fault. Both structures share a mid-crustal detachment defining their linkage. However, while all of the slip on the blind Ventura fault is consumed by folding on the south limb of the Ventura Avenue anticline, some of the slip on the Pitas Point fault extends farther south into the channel into the Pitas Point and Mid-Channel anticlines. We focus particularly on these structures to fully characterize the distribution of slip along strike. The improved 3D fault geometries further support the prospect for large, multi-segment ruptures and have implications for the associated tsunami and ground shaking hazards as well as geodetic strain and loading patterns.