Friction and faulting in heterogeneous systems

Doron Morad, University of California, Santa Cruz In natural fault surfaces, stresses are not evenly distributed due to variations in the contact population within the medium, causing frictional variations that are not easy to anticipate. These variations are crucial for understanding the kinematics and dynamics of frictional motion and can be attributed to both the intact material and granular media accommodating the principal slip zone. Here, I explore the effects of heterogeneous frictional environments using two different approaches: fracture dynamics on non-mobilized surfaces and granular systems on mobilized ones. First, I will present a quantitative analysis of laboratory earthquakes on heterogeneous surfaces, incorporating both laboratory-scale seismic measurements coupled with high-speed imaging of the controlled dynamic ruptures that generated them. We generated variations in the rupture properties by imposing sequences of controlled artificial barriers along the laboratory fault. We first demonstrate that direct measurements of imaged slip events correspond to established seismic analysis of acoustic signals; the seismograms correctly record the rupture moments and maximum moment rates. We then investigate the ruptures’ early growth by comparing their measured seismogram velocities to their final size. We investigate the laboratory conditions that allow final size predictability during the rupture early growth. Due to higher initial elastic energies imposed prior to nucleation, larger events accelerate more rapidly at the rupture onset for both heterogeneous and non-heterogeneous surfaces. Second, I present a new Couette-style deformation cell designed to study stress localization in two-dimensional granular media under different flow regimes. This apparatus enables arbitrarily large deformations and spans four orders of magnitude in driving velocity, from sub-millimeter to meters per second. Using photoelasticity, we measure force distribution and localization within the granular medium. High-speed imaging captures data from a representative patch, including both lower and upper boundaries, allowing us to characterize local variations in stress and velocity. For the first time, we present experimental results demonstrating predictive local granular behavior based on particle velocities, velocity fluctuations, and friction, as defined by [tau/sigma_n]. Our findings also reveal that stress patterns in the granular medium are velocity-dependent, with higher driving velocities leading to increased stress localization. These two end-member cases of frictional sliding, one dominated by gouge, and the second by intact surfaces, highlight two fundamental aspects of friction dynamics. The spatial distribution of heterogeneity directly influences stress distribution and, consequently, the stability of the medium. With these experimental methods, we can now measure and even control these effects.