The evolution of fault strength and behavior during the initial stages of slip plays an important role in driving the onset of instability and fault weakening. Using small-displacement triaxial experiments on quartz sandstone, this study highlights the rapid onset of microstructural change on fault interfaces and identifies new evidence for an evolution in physical processes with increasing slip and velocity. Pre-ground fault surfaces have been slipped over a range of velocities (0.36 µm s–1 to 18 cm s–1) and at normal stresses comparable to upper- to mid-crustal conditions (92–287 MPa). Microstructural analysis of the fault interfaces reveals the formation of amorphous material at displacements <170 µm and slip durations < 1 ms. Mechanical and microstructural observations have been combined with numerical modeling to present the first documented examples of a transition from mechanical amorphization to flash heating, then frictional melting, with changes in slip conditions. The sequence of processes activated during the initial stages of fault movement may provide new insights into factors that influence the onset of slip in the seismogenic crust.

Low displacement fracture damage plays an important role in influencing the behavior and mechanical evolution of faults. Fracture damage zones influence slip behavior through changing near‐field stress orientations, altering fluid pathways and modifying fault structure. Here we use small displacement triaxial experiments to explore the development of fault zone damage, frictional lock‐up, and the generation of new faults using samples with preground faults, oriented in 5° increments between 25° and 65° relative to the shortening direction. With increasing reactivation angle, faults support higher peak normal stresses (104–845 MPa) and behavior transitions from stable sliding to stick slip. Frictional melting occurs on surfaces where stick slip is initiated, forming micron‐thick layers that locally weld asperity contacts. The extent of melt welding is correlated with normal stress and melt‐welded zones increase fault cohesion. Distribution of fracture damage adjacent to the fault is spatially correlated with melt‐welded zones and the corresponding concentrations of stress and elastic strain. In a process referred to as “adhesive wear,” fractures bypassing welded zones transfer melt‐adhered material from one side of the fault to the other, forming new geometric asperities. On faults with high reactivation angles (55°–60°) the increase in cohesive strength resulting from melt‐welded contacts drives fault lock‐up after an initial slip event; subsequent slip localizes on new, favorably oriented faults. Given their size, melt‐welded zones are likely to be short‐lived in nature but may play a significant and previously unrecognized role in the development of fault‐related damage.

Evolution of fault strength during the initial stages of seismic slip plays an important role in the onset of velocity‐induced weakening, which in turn, leads to larger earthquake events. A key dynamic weakening mechanism during the early stages of slip is flash heating, where stress concentrations at contacts on the interface lead to the rapid generation of heat. Although potential weakening from flash heating has been extensively modeled, there is little recorded microstructural evidence of its physical manifestations. We present results of a series of triaxial experiments on synthetic faults in quartz sandstone. Samples were subjected to a variety of normal stresses and ambient temperatures, to induce a range of slip event sizes and sliding velocities. We show the microstructural evolution of asperity interactions from the onset of flash heating through to the formation of grain‐scale areas of sheared melt. Using microstructural observations and mechanical data from the experiments, we model temperature and the viscoelastic behavior of the glass. Results suggest that, in the earliest stages of slip asperity contacts melt, but temperatures remain too low for viscous shear to occur within the melt layer. Instead melted asperities behave as glassy solids, facilitating continued frictional heating. With further slip, increased asperity temperatures allow the transition to viscous shear within the melt layer, facilitating weakening. These results highlight the dynamic evolution of the viscoelastic properties of the melt and resulting effects on asperity strength. Such complexity has, to‐date, not been fully addressed in modeling of flash heating.

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We present a free-space optical displacement sensor for measuring geological slip event displacements within a laboratory setting. This sensor utilizes a fiberized Mach-Zehnder based optical heterodyne system coupled with a digital phase lock loop, providing a large dynamic range (multiple centimeters), high displacement resolution (with an amplitude spectral density of [Formula: see text] m/[Formula: see text] for frequencies above 100 Hz), and high velocity tracking capabilities (up to 4.96 m/s). This displacement sensor is used to increase the displacement and the time sensitivity for measuring laboratory-scale earthquakes induced in geological samples by using a triaxial compression apparatus. The sensor architecture provides an improved displacement and time resolution for the millisecond-duration slip events, at high containment and loading pressure and high temperatures. Alternatively, the sensor implementation can be used for other non-contact displacement readouts that required high velocity tracking with low noise and large dynamic range sensing. We use 13 high-velocity slip events in Fontainebleau sandstone to show the large dynamic range displacement tracking ability and displacement amplitude spectral densities to demonstrate the optical readout’s unique sensing capabilities.