Multiscale Transects Illuminate the Shallow-to-Deep Fault Architecture of the Western Transverse Ranges, Incorporating Constraints from Tomography

Yu-Huan Hsieh, & John Suppe

Submitted September 7, 2025, SCEC Contribution #14861, 2025 SCEC Annual Meeting Poster #TBD

The Transverse Ranges, straddling the Big Bend of the San Andreas fault, are both data-rich and complex. The current transpressive tectonics since ~6Ma is characterized by M6-7+ earthquakes on thrust faults that build the topographic mountain belt in addition to major San Andreas and other strike-slip events that make little topographic contribution. The Transverse Ranges also inherit a deep fault architecture that includes ~90° clockwise transtensional rotation and earlier subduction channel underplating. The network of active thrust and strike-slip faults in the uppermost crust is known from decades of earthquake hazards research (SCEC CFM7), but how they extend downward through the crust is still elusive. Here we show how full-waveform and ambient-noise anisotropic Vs and Vp tomographies image velocity boundaries and velocity inversions in 3D that, when combined with seismicity and upper crustal geologic data, illuminate major deeper structure of the western Transverse Ranges.

We are constructing new multiscale transects of the western Transverse Ranges by integrating data from the surface to the Moho. In the upper crust, we use the exceptional Dibblee maps plus petroleum well data. Rick Allmendinger's GMDE software is used for projecting surface geologic data in 3D to produce much more data-rich representations of the upper crust. Here we focus on the downward deep crustal extent of two well-documented active faults that bound the deep deforming Ventura Basin [1] the San Cayetano thrust at the northern edge, which has experienced M7+ earthquakes and shows ~10mm/yr shortening and [2] the Oakridge fault at the southern edge.

We observe a regionally extensive ~20° north-dipping low-velocity channel under the highest topography of the western Transverse Ranges, extending ~60km northward from the Ventura Basin to the base of the crust north of the San Andreas fault, with Vs and Vp velocities ranging from sedimentary to metasedimentary. The roof thrust of the ~15-km-thick channel, below overlying basement velocities, projects upward to the San Cayetano thrust and downward to intersect and perhaps truncate the San Andreas fault, where abundant thrust mechanisms are observed. The floor thrust is seismically illuminated, including the Fillmore swarm. Below the floor thrust is a steep 60-70° N-dipping velocity boundary extending to the Moho, which is apparently the deep Oakridge fault bounding the Ventura rift basin.

Key Words
Active Faults, Tomography, Low-Velocity Channel, Multi-Scale, Fault Model, San Cayetano Thrust, Western Transverse Ranges

Citation
Hsieh, Y., & Suppe, J. (2025, 09). Multiscale Transects Illuminate the Shallow-to-Deep Fault Architecture of the Western Transverse Ranges, Incorporating Constraints from Tomography. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)