SCEC Award Number 14040 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title Using Mechanical Models to Test Ventura Special Fault Study Area Alternative Fault Models
Name Organization
Scott Marshall Appalachian State University Gareth Funning University of California, Riverside Susan Owen National Aeronautics and Space Administration
Other Participants
SCEC Priorities 4a, 4b, 4c SCEC Groups USR, Geodesy, SDOT
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
Large coseismic offsets have been identified in the geologic record near the Ventura fault and the associated Ventura Avenue anticline, implying a local source for ~M8 earthquakes in the past [Hubbard et al., 2014]. Such large magnitude events are difficult to reconcile with the previous SCEC Community Fault Model (CFM) v4.0 discontinuous fault geometry. Recent work by Hubbard et al. [2014] provides evidence for a previously unrecognized ~80 km long and continuous fault surface extending from the San Cayetano fault through the Ventura fault and ~30 km offshore. Because of different subsurface interpretations of the fault geometry at depth [e.g. Hubbard et al., 2014; Kammerling et al., 2003], two potential Ventura fault geometries were tested. Both models share the same surface trace but differ in that the Hubbard et al. [2014] or “Ramp” model contains a nearly horizontal ramp section at depth. The Kammerling et al. [2003] representation (or “No Ramp” model) utilizes a constant dip angle and merges with the Red Mountain fault at a depth of 10 km. Creating two versions of the Ventura fault required modifications to ~10 faults since several faults are believed to be truncated by this or other structures. In the end, we find that the constant dip, or “No Ramp” model, fits the geologic slip rate data best. A comparison of interseismic deformation patterns may be able to help distinguish between the two Ventura fault geometries, although given the deep locking depth, the interseismic deformation patterns are likely to be rather diffuse.
Intellectual Merit This project contributes to the understanding of crustal deformation in southern California by using a novel three-dimensional mechanical modeling approach to simulate both interseismic and long-term deformation. A primary goal of the Ventura Special Fault Study Area (SFSA) is to determine the most likely fault structure for the region, and this work contributes to this effort by directly testing an updated fault system geometry for the greater Ventura region using a physics-based method. The final version of this work will directly test whether any geologic/geodetic rate discrepancies exist in the western Transverse Ranges, and if the geologic slip rates are more compatible with the Ramp or No Ramp fault representations. Our approach offers a quantitative assessment of the ability of the CFM to reproduce variations in slip and interseismic deformation in southern California. Furthermore, a product of this study will be a significantly updated fault model, which will be posted on PI Marshall’s website and provided for inclusion in a future release of the SCEC CFM.
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 Appalachian State University, PI Marshall has now begun training undergraduate students in GPS processing, dislocation modeling, and stress/strain theory. Marshall recently trained a Ph.D. student at the University of Massachusetts on GPS processing, and is currently working with two undergraduate geology students at Appalachian State University on fault modeling and geodesy. One student is writing a dislocation modeling code and modeling the CFM in the Los Angeles region, while the other is doing GPS time series processing to determine seasonal aquifer motions. 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 2: Mechanical model-predicted three dimensional slip distributions on the Ventura fault. A) The CFM v4.0 Ventura fault. B) The CFM v5.0 no ramp model. C) The CFM v5.0 ramp model. The view is oblique and to the southeast (refer to UTM axes). Note that the nearly horizontal ramp section causes slip rates to decrease dramatically locally. In the near surface, the fastest slip rates are near the Ventura Avenue anticline at the coastline near the city of Ventura. The apparent missing fault surface towards the left portion of the image in C) is the San Cayetano fault, which is included in the models, but not shown here.