Squishy Faults: A Scaled Laboratory System to Study Slow Slip
In addition to traditional earthquakes that release energy over time scales of seconds to minutes, there are a wide range of slow slip behaviors that slip over time scales of hours to years, and are a critical part of the seismic cycle. I have developed a new type of scaled-down experimental system to better understand slow-slip events, and specifically, how stress drop scales for slow slip events.
This system allows active control of fault heterogeneity through both the fault roughness and the normal stress distribution, while simultaneously offering direct imaging of interfacial strains at high spatiotemporal resolution. This is also a truly three-dimensional fault with fully confined ruptures. The resultant fault has events with maximum slip velocities around 0.1 mm/s, and earthquakes with magnitudes between -12.7 and -8.7. Despite their small size, these earthquakes follow constant stress drop scaling across a wide range of conditions. In addition, this system also allows the exploration of nucleation behaviors, and offers enormous potential for controllably building in complexity to a model laboratory fault.
Measurements of Fault Roughness Across a Wide Range of Spatial Scales
The roughness of fault is a key factor in dictating its frictional properties, and thus, determining when and how much it will slip. Since active faults cannot be accessed directly, researchers instead focus on measuring faults that have been geologically exhumed and exposed. Past measurements of these natural faults have revealed surprisingly consistent roughness characteristics (with faults generally being understood to be self-affine) from nanometers up to kilometers. However, the techniques used to make these measurements, and the resulting analyses are limited in the scales they are sensitive to, leading to gaps at certain critical scales. I have developed two new techniques for measuring different scales of fault roughness, using laser profilometry and drone-based Structure from Motion (SfM). In addition, I am developing new techniques to analyze these rough surfaces and improve our understanding of the connection between fault roughness and earthquakes.
Three-Dimensional Dynamics of Fracture Roughness Generation at High Spatiotemporal Resolution
I have developed a new system that combines laser sheet microscopy and fast camera photography to measure growing fractures at ~15 x 15 x 15 micron resolution at 1000 volumes per second. These movies show how step-like instabilities on the fracture front grow and interact to generate roughness. The complex three-dimensional dynamics of these interactions lead to mechanical and topological constraints on three-dimensional fracture mechanics.
Wear Mechanisms at High Strains
Elastomers are commonly used in a number of applications that involve high frictional stresses. This often leads to wear. Different types of elastomers exhibit markedly different frictional behaviors. Using a total internal reflection contact imaging system, combined with high speed photography, to observe hemispheres of different materials as they skid, we observe radically different dynamics depending on the material. Dissipating the friction energy in these systems involves highly coupled thermal, elastic, plastic, and fracture-like responses, and as a result is, surprisingly, an excellent model system for understanding wear behaviors during faulting.
How Material Heterogeneity Makes Cracks Rougher
Fracture surface roughness is an important parameter that can dictate the fluid transport or friction/seismic properties of a material or fault. In addition, a crack surface preserves a time history of a fracture's motion through a material, and thus the roughness also reflects the fracture dynamics. One source of fracture roughness is the interaction between a crack and material heterogeneity, but this relationship is poorly understood since heterogeneity is a difficult parameter to control experimentally. I have developed two methods to control heterogeneity in brittle hydrogels and have used them to explore how the size and amount of heterogeneity affects fracture roughness, and have shown that the presence of heterogeneity at very high densities can generate crack surfaces that resemble those seen in complex natural materials like rocks.
The Rules of Fracture Roughness
Fracture roughness is not uniformly distributed across a surface, and instead is usually localized. For brittle fractures, this localized roughness comes in the form of step-like instabilities that drift laterally along the growing front and leave in their wake a linear scar of relief known as a step line. I have shown that these steps have a characteristic asymmetric shape that determines their motion and interactions and that there are only three unique types of interactions that steps can have. These interactions follow straightforward rules that, taken together, can be utilized to explain and predict large-scale complexity in fractures.