SCEC Award Number 18131 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Evolution of frictional shear resistance in response to rapid variations of normal stress
Investigator(s)
Name Organization
Ares Rosakis California Institute of Technology
Other Participants (1) Senior Personnel: Rubino, Vito
SCEC Priorities 4a, 1d, 3g SCEC Groups FARM, GM, Seismology
Report Due Date 03/15/2019 Date Report Submitted 05/01/2019
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 experiments allow for full-field measurements of displacements, strains, and stresses by combining digital image correlation (DIC) technique with ultra high-speed photography. As part of this project we have designed a fast post-processing algorithm that supplements the local DIC solution and enforces the continuity of tractions across the interfaces of shear ruptures. This procedure has allowed us to obtain more physically meaningful stress fields near the interface, which is important when DIC is applied to study the dynamics of laboratory frictional ruptures. Our experiments show that the method is capable of providing very coherent full-field measurements of fault-parallel velocity, shear stress, and fault-normal stress close to the free surface. These measurements provide an experimental evidence for significant reductions in normal stress as the rupture breaks the free surface; they also demonstrate a delayed response of shear stress resistance to the normal stress variations.
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 4. Experimental evidence for delayed shear stress resistance evolution after the rapid normal stress change. (a) Slip rate vs. time at two points on the interface located 1.1 mm and 11.5 mm from the free surface (the locations are shown in Figure 2). (b) Shear and normal stress vs. time at the point located 1.1 mm from the free surface (c) Resolved frictional resistance 1.1 mm from the free surface. The black circles represent the ratio between observed shear and normal tractions. When the incoming rupture is reflected at the free surface (at a time t = 70 us and slip of 25 um), σ rapidly decreases and because of a delayed response of the frictional shear resistance, the ratio τ/σ increases. The blue curve represents the frictional resistance calculated using an enhanced-weakening RS law developed for the experimental interface by Rubino et al. (2017) and applied to the experimentally measured slip rate, assuming an immediate response of frictional resistance to variations in normal stress. Clearly, this formulation, which does not account for the delay, cannot match the observed response (Tal et al., in preparation). The apparent strengthening in the dynamic friction coefficient with respect to the value of ~0.3 is indicative of the delayed decrease in the shear stress with respect to the decrease in the normal stress, which causes the ratio to go up.