SCEC Award Number 16171 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title High Resolution Geodetic Measurements of Deformation throughout the Ventura Special Fault Study Area Region
Investigator(s)
Name Organization
Scott Marshall Appalachian State University Gareth Funning University of California, Riverside Susan Owen National Aeronautics and Space Administration
Other Participants Hannah Krueger - Undergraduate student at Appalachian State University
SCEC Priorities 4a, 4b, 4c SCEC Groups SDOT, USR, Geodesy
Report Due Date 03/15/2017 Date Report Submitted 03/15/2017
Project Abstract
 Data from the SCEC Ventura Special Fault Study Area have provided new and significantly re-vised constraints on the subsurface structure of the Ventura-Pitas Point fault system in southern Cali-fornia; however, few data directly constrain fault surfaces below ~6 km depth. Here, we use geomet-rically complex three-dimensional mechanical models driven by current geodetic strain rates to test two proposed subsurface models of the fault system. We find that the model that incorporates a ramp geometry for the Ventura-Pitas Point fault better reproduces both the regional long term geologic slip rate data and interseismic GPS observations of uplift in the Santa Ynez Mountains. The model-calculated average reverse slip rate for the Ventura-Pitas Point fault is 3.5 ± 0.3 mm/yr, although slip rates are spatially variable on the fault surface with > 8 mm/yr predicted on portions of the lower ramp section at depth. This work effectively accomplishes two of the key goals of the Ventura SFSA: 1) to determine the most likely subsurface fault configuration, and 2) to identify and quantify the interseis-mic strain associated with the Ventura-Pitas Point fault.
Intellectual Merit This project contributes to the understanding of crustal deformation in southern California by us-ing a novel three-dimensional mechanical modeling approach to simulate both interseismic and long-term deformation. Two primary goals of the Ventura Special Fault Study Area (SFSA) are 1) to deter-mine the most likely fault structure for the region, and 2) to identify and quantify the interseismic de-ofmration associated with the Ventura-Pitas Point fault. This project has met both of these goals by creating and directly testing the two proposed subsurface fault system geometries for the greater Ventura region using a physics-based method. Our geodetic data processing and analysis has suc-cessfully identified the interseismic deformation associated with the Ventura-Pitas Point fault, and fur-thermore, we have shown that only the ramp model of Hubbard et al. [2014] fits the geodetic data. Our approach offers a quantitative assessment of the ability of the CFM to reproduce variations in slip and interseismic deformation in southern California and demonstrates that the ongoing efforts to revise and improve the SCEC CFM have been very worthwhile. Furthermore, the fault mesh produced in this study has been shared with numerous other SCEC researchers and is included as an electronic sup-plement in our recent GLR publication.

We hope that the success of this project will facilitate future studies of the slower slipping (but still hazardous) faults in southern California, including the Los Angeles and Ventura basin regions. There are still many facets of this fault system that are poorly studied, especially in the geodetic realm.
Broader Impacts This work has fostered collaborations between researchers at the Jet Propulsion Laboratory, the University of California Riverside, Harvard University, and Appalachian State University. At Appala-chian State University, PI Marshall now routinely trains undergraduate students in GPS/InSAR pro-cessing, dislocation modeling, and stress/strain theory. Marshall is currently working with two under-graduate geology students at Appalachian State University on fault modeling and geodesy. One stu-dent is modeling the faults of the Imperial Valley region using the CFM, while the other is processing GPS time series to determine seasonal aquifer motions. The student doing the GPS work was a co-author on our recent GRL publication. These efforts are aimed to produce future researchers that are better prepared for graduate school and the research community. Also, by training undergraduate students, interest and understanding of earthquake science is promoted. The results of this work will have an impact on society by more accurately characterizing the slip rates of faults, which in turn leads to improved seismic hazard estimates.
Exemplary Figure Figure 3b-e. b) N20W profile through GPS vertical velocities (gray triangles) in the western Trans-verse Ranges region. Blue curves show model predictions for the no ramp model. All velocities are relative to station CIRX. c) Cross-sections through the three dimensional model showing the fault ge-ometry at the profile location. Blue horizontal lines show the three locking depths plotted in part a). d-e) Same as b-c) but for the ramp model.