Experimental study of frictional shear resistance under rapid variations of normal stress and near-field ground motion of thrust earthquakes

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

Submitted August 16, 2021, SCEC Contribution #11630, 2021 SCEC Annual Meeting Poster #173

A proper formulation for the evolution of shear resistance during rapid changes in normal stress is essential for many earthquake source problems. 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 normal stress. Here, we generate dynamic ruptures in an experimental setup mimicking natural thrust earthquakes and characterize their dynamics near the free surface to explore this effect. The experimental measurements show significant normal stress reduction as the ruptures interact with the free surface, with shear frictional resistance exhibiting a significant lag in response to the variations in normal stress. The observed response can be fit using rate-and-state friction with enhanced dynamic weakening and Prakash-Clifton law, featuring weakening parameters that depend on normal stress. Our results indicate that the delay is associated with an evolution distance that is 2–3 orders of magnitude larger than that of rate-and-state friction. Further, in order to consider an interface more directly relevant to natural faults, we conduct experiments of dynamic ruptures along interfaces enriched with rock gouge. In these initial tests, gouge is placed away from the free surface and displays a rich slip behavior consistent with significant initial strengthening followed by pronounced weakening, as can be described by rate and state friction enhanced with flash heating. Next, we will add rock gouge close to the free surface to study the effects of rapid normal stress changes on the rock gouge behavior. Using this experimental configuration reproducing thrust earthquakes, we study the effects of asymmetric geometry and heterogeneous frictional resistance on the ground motion. Our full-field measurements confirm the asymmetry between the surface motions of the hanging and footwalls, with larger velocity magnitudes at the hanging wall. At the same time, we find that the horizontal surface velocity components are larger and the attenuation in surface velocity with distance from the fault trace is generally slower at the footwall wall than at the hanging wall. Measurements of the rotations in surface motions during the interaction of the ruptures with the free surface confirm experimentally the existence of a torqueing mechanism that leads to reduction in normal stress near the free surface for thrust earthquakes

Key Words
dynamic friction, thrust earthquakes, rock gouge, ground motion

Citation
Rubino, V., Tal, Y., Rosakis, A. J., & Lapusta, N. (2021, 08). Experimental study of frictional shear resistance under rapid variations of normal stress and near-field ground motion of thrust earthquakes. Poster Presentation at 2021 SCEC Annual Meeting.


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