SCEC Award Number 22131 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
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 Dr Vito Rubino
SCEC Priorities 4a, 1d, 3g SCEC Groups FARM, Seismology, GM
Report Due Date 03/15/2023 Date Report Submitted 02/23/2023
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. Our new configuration features a rock gouge layer placed close to the free surface, which allows us to study the evolution of the shear resistance with rapid normal stress reduction along a gouge fault.
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 2. Snapshots of the fault-parallel (left column) and fault-normal (right) particle velocity at selected time frames (t=46 and 73 μs) alternated with the shear (left) and normal (right) stress maps for a test performed with P = 18 MPa and  = 29. The interface is colored in green and white to represent the portion along Homalite and gouge, respectively. A supershear rupture enters the field of view and is significantly attenuated when it transitions to the gouge layer, and leaves a trailing Rayleigh rupture in its wake. The first snapshots show a sub-Rayleigh rupture propagating along the gouge layer with nearly symmetric/anti-symmetric fault-normal/fault-parallel particle velocity fields and nearly constant normal stress along the interface. As the rupture impinges on the free surface the symmetry is broken and rapid and pronounced reductions in normal stress are observed.