SCEC Award Number 16164 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Role of fault geometry on the spatial distribution of the slip budget
Investigator(s)
Name Organization
Phillip Resor Wesleyan University Michele Cooke University of Massachusetts Amherst Scott Marshall Appalachian State University
Other Participants John Hossain (MA student at Wesleyan University)
SCEC Priorities 4b, 1a, 4a SCEC Groups Geodesy, Geology, SDOT
Report Due Date 03/15/2017 Date Report Submitted 04/24/2017
Project Abstract
 A fundamental problem in earthquake physics is how stress is transferred from plate motion to faults. Kinematic models assume that long-term fault slip rates will sum to the plate velocity; however, in a number of locations in southern California slip rates determined from modeling geodetic data differ significantly from geologic estimates. In this study we use mechanical models to investigate how releasing steps may affect estimates of fault slip over geologic time. A suite of 2D models reveals how fault length, friction and step geometry affect kinematic efficiency and the distribution of slip rate for idealized fault systems. We find that although systems with longer segments are more efficient, accommodating ~55-86% of plate displacement, geologic studies are unlikely (p<50%) to yield representative slip rate estimates. Systems with short segments are less efficient, but more likely to yield representative rates, particularly when summing slip on overlapping segments. A 3D model of the San Jacinto fault illustrates how fault system geometry may impact slip rate estimates along a real fault system. Modeled slip rates are faster in the middle of fault segments and slower within releasing steps, consistent with published geologic estimates. The mean slip rate along the modeled fault trace, however, is significantly slower than the average of the geological rates. Our results suggest that the location of geologic slip rate studies may govern their suitability for hazard estimates and that models can be used to put point measurements of slip into the context of slip distribution throughout a fault system.
Intellectual Merit This project contributes toward a better understanding of how stress is transferred from plate motion to crustal faults where it is released during fault slip (priority for SCEC 4). Specifically, the project explores how along strike variation in fault complexity may lead to spatially variable slip rate (SCEC Science Priority 4b) with implications for our ability to extract representative slip rates from geologic and geodetic observations (SCEC Science Priority 1a). Although several previous studies have proposed methods for better estimating uncertainty of geologic slip rates, little attention has been given to characterizing how representative geologic slip rates may be.

Specific contributions include: 1) a 2D parametric modeling study of releasing steps that highlights which fault geometries are most, and least, likely to yield representative slip rate estimates from geologic and geodetic studies based on measures of their slip rate distribution and kinematic efficiency. 2) A quantification of the effects of summing slip across overlapping segments in stepovers. 3) A 3D modeling study of the San Jacinto fault and vicinity that illustrates how segmentation along this fault system may lead to spatially variable slip rates that are locally consistent with geological estimates (SCEC Science Priority 4a). Our results suggest that the location of geologic slip rate studies may govern their suitability for hazard estimates and that models can be used to put point measurements of slip into the context of slip distribution throughout a fault system.
Broader Impacts This project has continued to foster collaborations between researchers at Wesleyan University, the University of Massachusetts, and Appalachian State University. The project provided support for a graduate student (John Hossain) at Wesleyan University to attend the SCEC annual meeting. The results of this project will benefit society by helping geoscientists to better characterize slip rates and seismic hazard of active strike slip faults.
Exemplary Figure Figure 3. Results from 3D mechanical model of the San Jacinto Fault and vicinity. A. Surface traces of faults from 3D model. B. Slip distribution at the Earth’s surface along faults of the San Jacinto Valley, Anza and Coyote Creek sections segments of the San Jacinto fault from a 3D mechanical model of the region. Blue shaded region corresponds to uncertainties in tectonic loading and white line shows aver-age slip distribution. The black dashed line shows the averaged model surface dextral slip summed across the overlapping faults. The grey band shows the geodetic range from Lindsey and Fialko (2013). The red line shows the 40 km wide average of slip rates at all depths along the modeled fault. Bars show geologic estimates of slip rates from 1) Prentice et al. (1986), 2) Rockwell (2008), 3) Sharp (1981), 4) Janecke et al. (2011), 5) Blisniuk et al. (2010) and 6) Blisniuk et al. (2013).
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