Observations Relating to 3D Fault Geometry and Seismic Hazard in the Santa Barbara-Ventura Area: Findings from the SCEC Commemorative 1925 Santa Barbara Earthquake Workshop

Craig Nicholson, Chen Ji, Larry D. Gurrola, Marc J. Kamerling, Christopher C. Sorlien, & Kaj M. Johnson

Submitted September 7, 2025, SCEC Contribution #14516, 2025 SCEC Annual Meeting Poster #TBD

A field trip and workshop were held to commemorate the 1925 M6.5 Santa Barbara earthquake and to better understand the active fault systems in the Santa Barbara-Ventura area. Principal findings include: major onshore faults are steeply (>60°) S-dipping and oblique-strike-slip; these merge with N-dipping, offshore oblique-reverse faults (Red Mtn, Pitas Point) to form a master N-dipping oblique fault at depth. At Pitas Point, the 4 major (8-11 m) Holocene uplift events are anomalous and there are two active independent anticlinal folds, with Holocene displacement necessarily occurring on the S-dipping, listric Padre Juan fault. These observations contradict published models that rely exclusively on one anticlinal fold and only Holocene slip on the N-dipping Ventura fault. Based on extensive mapping, imaging, drilling, geodetic data, and seismicity, major faults (Red Mtn, San Cayetano, Santa Ynez, Oak Ridge) are oblique, exhibit steep dips (>50°) to depths of 10-12 km or more, and considerable deformation occurs in their footwall. Fault-related fold models used to model regional structure assume 2D deformation, faults are thrusts (pure reverse with dips <45°), a low-angle detachment at 7-8 km depth, and rigid footwalls. Major earthquakes (1925, 1978, 2013) are typically oblique and occur below the modeled detachment at 9-10 km depth. Extensive grids of 2D and 3D seismic reflection data allow mapping of the offshore segmented, oblique North Channel-Pitas Point-Red Mtn fault system for over 120 km, with components of strike-slip increasing to the west. Although uplift events at Pitas Point argue for the occurrence of multiple large earthquakes, the Pitas Point-Ventura fault is blind, and what correlations do exist farther along strike typically reflect smaller, fewer uplift or fold events since 7 ka. The lack of event correlation along strike and lack of surface or seafloor rupture suggests the earthquakes associated with Pitas Point uplift events are more like M7.0+, and not M7.7+ as previously inferred. Alternatively, one is left with the prospect of having multiple, shallow (3-10 km depth) M7.7+ earthquakes occurring on a blind fault and remaining otherwise hidden, which has not been seen anywhere else in the world. The other unresolved controversy is how to reconcile the assumed fold-and-thrust models used to model the regional fold structure (with detachments at 7-8 km depth) if the faults are planar, oblique, and defined by seismicity to 15-18 km depth.

Key Words
workshop findings, fault geometry, seismic hazard, model validation

Citation
Nicholson, C., Ji, C., Gurrola, L. D., Kamerling, M. J., Sorlien, C. C., & Johnson, K. M. (2025, 09). Observations Relating to 3D Fault Geometry and Seismic Hazard in the Santa Barbara-Ventura Area: Findings from the SCEC Commemorative 1925 Santa Barbara Earthquake Workshop. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)