SCEC Award Number 13099 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Nucleation of dynamic rupture on faults with heterogeneous rate- and state-dependent friction (aging law)
Investigator(s)
Name Organization
Robert Viesca Tufts University
Other Participants
SCEC Priorities 4, 3, 5 SCEC Groups FARM, DRCV, CS
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 We report on results of study into the development of (dynamic rupture-nucleating) slip instabilities on surfaces whose frictional strength is determined by a slip rate- and state-dependent friction law. Specifically, we examine conditions in which frictional parameters vary spatially, as in current models of the seismic cycle. We remarkably find (1) that spatial variations of friction parameters create preferential sites for slip-instability development (and ultimately, earthquake nucleation in such models), and (2) that the manner of that development can be determined a priori if the distribution of the frictional parameters are known. We also highlight (3) results contrasting the influence of heterogeneity in determining the expected nucleation patch size, relative to that expected under homogeneous conditions.
Intellectual Merit Understanding of nucleation of dynamic rupture on faults with slip rate- and state-dependent friction is often built on full numerical simulations of the process [e.g., Dieterich, 1992; Kaneko and Lapusta, 2008]. Complementary analytical work [e.g., Rice and Ruina, 1983; Rubin and Ampuero, 2005] has provided means to anticipate some results and quickly assess the characteristic magnitude of potentially observable quantities (e.g., the acceleration of moment release and the length scale over which it takes place). In our work, we have found that there remain large classes of problems for which such solutions may be found. This includes problems with homogeneous fault properties as studied in past analytical work, but of particular interest are more representative cases with heterogeneous properties as commonly proposed for models of the seismic cycle and aseismic transients. The useful information and the ease with which analytical solutions may be found (relative to full numerical simulations) warrant obtaining them when possible. In addition to studying the nucleation process and informing model systems, the results help to answer the question, what information do we risk losing by homogenizing a potentially very heterogeneous fault friction? Looking towards seismic observations (e.g., Nadeau and Johnson, 1998; Rubin et al., 1999; Waldhauser et al., 1999) to identify variations at a finer scale, we examine sample case studies.
Broader Impacts This project has laid the groundwork for subsequent projects for an early-stage Ph.D. student (Sohom Ray). It has also provided initial results for an NSF proposal to study nucleation under rate and state friction on a broader scope. That proposal was ultimately funded and supports both the PI and the graduate student (Ray).
Exemplary Figure Figure 3. Demonstration that nucleation sites may be determined solely by frictional parameter distributions. (left) Distribution of frictional properties, here given by depth-dependent a(x) and uniform b. (right) Snapshots of normalized slip rate profiles with depth (greyscale; time increasing with progressive darkening) showing that slip instability is attracted to occur with a predictable location and manner (red-dashed) in spite of an initial forcing centered at a different rate-weakening location (open circle).
Figure credit: Robert C. Viesca, Tufts University