Evolution of frictional shear resistance in response to rapid variations of normal stress

Yuval Tal, Vito Rubino, Ares J. Rosakis, & Nadia Lapusta

Submitted August 14, 2019, SCEC Contribution #9684, 2019 SCEC Annual Meeting Poster #162

A proper formulation of the shear-resistance evolution during rupture is essential for many earthquake source problems, including simulations of the ground motion. While the shear resistance is typically assumed to be proportional to normal stress, several studies have shown a gradual, slip-dependent, response of the frictional shear resistance to rapid variations in the normal stress. Here we use laboratory experiments to quantify normal stress variations and the corresponding evolution of the frictional shear resistance produced by the interaction of dynamic ruptures with the free surface, similarly to what occurs in natural thrust events. The experiments also enable studying the potential fault opening and ground motion of the simulated thrust faults. In order to image and quantify the full-field behavior of the ruptures near the free surface, we use an experimental technique (Rubino et al., 2017) that combines ultra-high speed photography and digital image correlation to produce maps of dynamic displacements, velocities, strains, and stresses at intervals of less than a microsecond. It allows us to image the evolution of fault-normal stress and fault-parallel shear stress, and thus to study how friction evolves under the conditions of rapid normal stress variations. Our experiments provide very coherent full-field measurements of fault-parallel velocity, shear stress, and fault-normal stress close to the free surface. The ruptures arrive to the free surface at a supershear speed, resulting in small initial increase in normal stress followed by a significant reduction. Additional reduction is observed at the arrival of a trailing Rayleigh disturbance, with a temporal complete release (possible opening) in experiments that were conducted under small initial compressive load. The experiments show a significant delay in the response of shear resistance to the variations in normal stress. The data is fitted well with a frictional resistance formulation that (1) accounts for normal stress variations via a Prakash-Clifton formulation with a large evolution distance, and (2) uses a combined rate and state and flash heating friction for the evolution of the friction coefficient.

Tal, Y., Rubino, V., Rosakis, A. J., & Lapusta, N. (2019, 08). Evolution of frictional shear resistance in response to rapid variations of normal stress. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)