Earthquake Rupture Speed Dependence on Normal Stress in Laboratory Experiments

Rupture speed plays a critical role in earthquake dynamics, seismic energy release, and ground shaking characteristics. While variations in rupture speed of earthquake fault slip from fast to slow are well-documented in nature and in the lab, the responsible mechanisms are not fully understood. Here we address the physical mechanisms for variations in rupture speed using an array of strain gage rosettes in direct shear experiments to estimate the rupture speed of stick-slip instabilities. The experiments were conducted with applied normal stresses spanning one order of magnitude, ranging from 2 to 20 MPa. High-speed records of shear strains at 13 equidistant locations along 15-cm-long granite faults were analyzed to understand the effects of normal stress on rupture dynamics. Our data follow the expectation that higher normal stress generally promotes faster rupture speeds, consistent with observations from natural fault systems.

Our analysis reveals the interplay between stress conditions, stored elastic energy, and fault behavior. The experiments provide insights into how changes in normal stress affect the propagation of frictional rupture along a simulated fault surface with a thin layer of moisturized quartz gouge (Min-U-Sil, 40). A concise relation between normal stress and rupture speed based on linear elastic fracture mechanics is derived to explain our observations.

Fracture energy scales linearly with normal stress, which tends to reduce rupture speed as normal stress increases. However, the greater difference between peak and residual strength at higher normal stresses allows for more energy to be released during fault slip. Thus, as normal stress increases, the energy release rate, which scales quadratically with normal stress, outpaces the linear increase in fracture energy, leading to higher rupture speeds.

Our results provide important information for seismic hazard assessment and the development of more accurate rupture models for earthquake forecasting. By clarifying the role of normal stress in modulating rupture speed, our work illuminates the complex interactions between stress conditions and earthquake rupture dynamics. Overall, our data underscore the significance of considering normal stress variations in seismological methods to improve earthquake estimations and hazard assessments.