Testing Structural Model Predictions Against Geodetic Data in the Western Transverse Ranges, Southern California

Yuval Levy, Scott T. Marshall, Thomas K. Rockwell, & John H. Shaw

Published August 14, 2019, SCEC Contribution #9590, 2019 SCEC Annual Meeting Poster #211

In the Western Transverse Ranges of southern California, paleoseismic evidence has documented large uplift events along the Ventura-Pitas Point fault system. Given the number of competing models in the region we conducted a comprehensive structural analysis in order to advance and refine our understanding of the subsurface architecture. Our structural model was developed from a series of cross sections that span 140 km of the range in order to capture the lateral variability. This model incorporates existing structural models, surface data from maps, well data, seismic surveys, the observed geological vertical rates and the southward migration of folding in time. This comprehensive model details an imbricate thrust system that evolved since the Pliocene, with the dominant faults dipping to the north as they shoal from ~16-30˚ degrees at depth to ~45-60˚ near the surface. While much of the shallow fault complexity derived from the surface geology and subsurface well data has no impact on the interseismic deformation patterns, GPS data can be used to constrain the deep fault geometry. The dip of the lower ramp in the model is constrained by the observed geologic vertical and horizontal rates, yielding a range of plausible dips of 16-30˚. To better constrain the lower ramp dip, we compare the regional vertical motions measured by GPS to a series of kinematic models with a range of locking depths and lower ramp dips. In the western sections of the range, the GPS data is spatially sparse and exhibits spatially incoherent uplift patterns, so we focus our efforts on matching the deformation in the eastern sections of the system. Initial results suggest that the geodetic data in the eastern portion of the Ventura-Pitas Point fault is best fit by models with dips of 25-30˚, locking depths of 10-15 km, and reverse slip rates of 4-5 mm/yr. While a 15 km locking depth is shallower than seismologic observations suggest, models using deeper locking depths produce uplift too far north compared to the GPS observations. The 4-5 mm/yr modeled slip rate is consistent with the lower range of estimates from geologic studies in the region. This work demonstrates that by adding the constraints provided by GPS data to a wide range of geological and geophysical data, we can better constrain fault geometry at depth, and produce an overall better constrained fault model.

Key Words
Western Transverse Ranges, Geodetic, Structure, Fault model

Citation
Levy, Y., Marshall, S. T., Rockwell, T. K., & Shaw, J. H. (2019, 08). Testing Structural Model Predictions Against Geodetic Data in the Western Transverse Ranges, Southern California. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy