SCEC Award Number 19093 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Evolution of frictional shear resistance in response to rapid variations of normal stress
Name Organization
Ares Rosakis California Institute of Technology
Other Participants Yuval Tal
Vito Rubino
SCEC Priorities 4a, 1d, 3g SCEC Groups FARM, Seismology, GM
Report Due Date 04/30/2020 Date Report Submitted 04/29/2020
Project Abstract
Friction formulations typically assume shear resistance to be proportional to normal stress. However, when normal stress changes rapidly enough, frictional shear resistance no longer obeys proportionality to the normal stress but rather evolves with slip gradually. In this project, we investigate the evolution of shear stress in response to rapid normal stress variations using laboratory experiments of spontaneously propagating dynamic ruptures. Our experiments produce variations in fault-normal stress due to the interaction of dynamic rupture with the free surface, similarly to what occurs in natural thrust events. Our experimental measurements clearly demonstrate the delay between normal stress changes and the corresponding changes in frictional resistance, with important implications for the dynamics of thrust earthquakes near the free surface. The experiments make use of full-field measurements of displacements, strains, and stresses by combining digital image correlation (DIC) technique with ultrahigh-speed photography, which thoroughly characterize rupture interaction with the free surface, including the large normal stress reductions. In particular, our results indicate that the delay in shear resistance response to variations in normal stress is associated with an evolution distance that is 2–3 orders of magnitude larger than that of rate-and-state friction.
Intellectual Merit Our study use laboratory experiments to investigate the evolution of shear stress in response to rapid normal stress variations as propagating dynamic ruptures interact with the free surface. That allows constraining and formulating constitutive laws that account for delayed response of the frictional shear resistance to variations in normal stress, which are critically important for investigations of several key earthquake source problems. Moreover, the experimental setup enables to study dynamics of thrust events as the ruptures break the free surface, to quantify the associated ground motions and normal stress reductions, and to estimate the potential for fault opening. Our project is also directly in line with the primary research objective of FARM working group to develop “physics-based fault models applicable to various spatial and temporal scales, such as nucleation, propagation and arrest of dynamic rupture”. It contributes to a number of research priorities of FARM, with the most relevant being: “Determine the mechanisms dominant in coseismic (dynamic) fault resistance, including the relative importance of various potential dynamic weakening mechanisms”.
Broader Impacts This project will contribute to our fundamental understanding of dynamic friction. Measuring local evolution of dynamic friction has important implications for understanding earthquake hazard since laws governing frictional resistance of faults are vital ingredients in physically based predictive models of the earthquake source. Moreover, thrust faults pose a major seismic risk for the LA area. In this project, we investigate experimentally and numerically the ground motions associated with the interaction of up-dip dynamic sub- Rayleigh and super-shear ruptures with the free surface, similarly to what occurs in natural thrust events. These measurements will enhance our understanding of the seismic risk associated with thrust events. A research scientist and a postdoctoral scholar have gained valuable research experience by participating in the project and interacting with the SCEC community.
Exemplary Figure Figure 3. Experimental evidence of pronounced delay in shear resistance response to rapid normal stress variations (Tal et al., 2020, PNAS, under review). Evolutions of experimental (black circles) and modeled (solid curves) effective friction with slip near the free surface for experiments conducted with two levels of normal stress: (A) Exp #1, at σ_0=11.6 MPa and (B) Exp #2, at σ_0=7.6 MPa. Two formulations are shown for each test: (1) Enhanced-weakening RS friction without accounting for a delayed response to variations in σ (blue); (2) Enhanced-weakening RS friction together with a Prakash-Clifton formulation (purple). The curves for both experiments were estimated using the model parameters constrained with the data of Exp #1. While the modeled curve works fairly well for Exp #1, there are some important discrepancies for Exp #2, conducted at a lower level of normal stress and we expect even larger discrepancies for tests to be conducted at lower normal stress levels. We postulate that accounting for the dependence of friction parameters fw and Vw upon normal stress may significantly reduce the discrepancies.