CAREER: Combining physical and numerical modes to characterize the contribution of semi-brittle rheology to deformation dynamics and strain transients.

Iowa State University, Ames IA

Investigators

Abstract

Tectonics faults are cracks in the Earth's crust accommodating the slow motions of tectonics plates. The stress accumulated around faults can be released slowly when faults creep over long periods of time. This viscous behavior is called aseismic because it does not produce earthquakes. Faults can also slip abruptly by brittle failure. Most tectonics faults exhibit both viscous (no earthquake) and brittle (earthquakes) behaviors. Understanding how fault behaviors interplay is critical to explain earthquakes and improve hazard forecasting. This task is, however, challenging because these behaviors occur on very different time scales. Here, the researchers use experiments and numerical modeling to explain fault dynamics. They use a unique combination of viscous and brittle materials which, mixed together, behave like faults. By deforming the mixture, they identify the factors producing slow deformation and abrupt slip. Using modeling, they apply their results to natural systems and explain the behaviors of fault. The project has strong implications for earthquake hazard forecasting. It provides support to an early-career female scientist, a postdoctoral associate and a graduate student. It offers training opportunities to undergraduate and high-school students. Furthermore, it fosters the development of teaching materials for a science communication class. The goal is to increase students' awareness of scientific methodology and strengthen their ability to communicate their results to a broad audience. This five-year award is co-funded by the Prediction of and Resilience against Extreme Events (PREEVENTS) program. Semi-brittle materials cover short time scales associated with rupture events (earthquakes) and much longer ones associated with viscous flow (ductile crust). Their range of behaviors can potentially explain the observed various fault dynamics. Yet, there is limited data available on the rheology of semi-brittle materials; and even less on materials involving grain comminution in the presence of fluids, as can occur in fault gouges. Here, the researchers use a mixture of viscous Carbopol and brittle hydro-gel spheres, which exhibits the semi-brittle behaviors underlying tectonic fault motions. Specimens are sheared in a new state-of-the-art apparatus which allows imaging fluid-flow, and particle motions and comminution. The team systematically investigates the effects on slip dynamics of grain comminution, distribution between brittle and viscous materials, normal force, total strain and viscosity. Experimental results are input into a numerical modeling which captures the physics of Earth materials on geological time scales. The model outcomes explore the parameter space and whether semi-brittle deformation can lead to the continuum between steady creep and stick-slip fault motions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.