SCEC Award Number 15028 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Role of fault geometry on the spatial distributions of the slip budget
Investigator(s)
Name Organization
Scott Marshall Appalachian State University Michele Cooke University of Massachusetts Amherst Elizabeth Madden University of Massachusetts Amherst Phillip Resor Wesleyan University
Other Participants
SCEC Priorities 4b, 1a, 4a SCEC Groups SDOT, Geology, Geodesy
Report Due Date 03/15/2016 Date Report Submitted 10/13/2015
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 in this manner 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 scales. A suite of 2D models of idealized fault systems reveals how fault length, friction and step geometry affect kinematic efficiency of the system and the distribution of slip rate along the faults. We find that although systems with longer fault segments are more efficient, accommodating ~56-86% of plate displacement, point sampling of these systems during geologic studies is actually less likely to yield representative slip rate estimates. 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 simple 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 Pri-ority 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 3D mod-eling 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 fostered collaborations between researchers at Wesleyan University, the University of Massachusetts, and Appalachian State University. The project introduced PI Resor to the SCEC community. The project provided training for PI Madden, a postdoctoral researcher at the University of Massachusetts (now at Munich). 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. Portions of faults with representative slip rates (mean +/- 1 mm/yr) highlighted in red. B. Slip distribution along the San Jacinto Valley, Anza and Coyote Creek segments. Shaded region corresponds to uncertainties in tectonic loading and white line shows average slip distribution. Red line and pink band illustrate mean slip rate with uncertainty bounds (+/- 1 mm/yr.) for entire fault. Bars show geologic estimates of slip rates from 1) Prentice et al (1986), 2) Rockwell et al (2008), 3) Sharp (1981), 4) Janecke et al. (2010), 5) Blisniuk et al (2010) and 6) Blisniuk et al (2013). C. Probability density distribution of slip from model. Red and green bars are mean and median values. Slip is normalized to maximum slip.