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Linkage of the Ventura-Pitas Point and Mid-Channel faults and its implications for large, multi-segment earthquakes

Jessica Don, Andreas Plesch, Mattie M. Newman, & John H. Shaw

Published August 13, 2019, SCEC Contribution #9479, 2019 SCEC Annual Meeting Poster #112

The Ventura-Pitas Point fault system, located in the western Transverse Ranges, is one of the largest earthquake sources in southern California. This fault system ruptures in large magnitude events as indicated by Holocene marine terraces which suggest discrete 7-9 meter uplift events (Rockwell et al., 1988; Hubbard et al., 2014; McAuliffe et al., 2015). Deformation of this scale requires the rupture of multiple fault segments and poses significant hazards (McAuliffe et al., 2015, Ryan et al., 2015; Marshall et al., 2017). Recent studies show that the Ventura fault has a unique non-planar ramp-flat-ramp geometry that links at depth with several of the largest, fastest slipping faults in the area (Hubbard et al., 2014; Marshall et al., 2017; Levy et al., 2017) suggesting the possibility for these large multi-segment ruptures.

We aim to characterize the geometry of the Pitas Point fault system, the direct offshore extension of the Ventura thrust, to further define the potential for large, multi-segment earthquakes. We use both 2D and 3D industry seismic reflection data to constrain the fault geometries. We supplement our interpretations with extensive well control, including dipmeter logs and horizon tops.

We build a 3D model of the Pitas Point fault system with a series of balanced cross sections created using fault-related folding theory and both inverse and forward modeling. We resolve a similar ramp-flat-ramp geometry in the Pita Point fault and the Ventura fault with a mid-crustal detachment defining their linkage. However, while all of the slip on the blind Ventura fault is consumed by folding in the Ventura Avenue anticline, our results suggest that a significant amount of the slip on the mid-crustal detachment does not transfer upward on the shallow ramp of the Pitas Point fault, but rather extends farther south beneath the Pitas Point and Mid-Channel anticlines on an active blind thrust system. We suggest that this reconciles the observation that the slip and slip rate on the Pitas Point fault appears less than that of the Ventura fault (Perea et al., 2017) despite their linkage. We focus on the Mid-Channel and Pitas Point anticlines to characterize the distribution of slip along strike in the Ventura-Pitas Point fault system. 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.

Key Words
Pitas Point, Ventura, Mid-Channel, fault system, 3D geometry, multi-segment ruptures

Don, J., Plesch, A., Newman, M. M., & Shaw, J. H. (2019, 08). Linkage of the Ventura-Pitas Point and Mid-Channel faults and its implications for large, multi-segment earthquakes . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology