Pore fluids are ubiquitous throughout the lithosphere and are commonly cited as the cause of slow-slip and complex modes of tectonic faulting. We investigate the role of fluids for slow-slip and the frictional stability transition and find that the mode of fault slip is mainly unaffected by pore pressures. We shear samples at effective normal stress (σ’n) of 20 MPa and pore pressures Pp from 1 to 4 MPa. The lab fault zones are 3 mm thick and composed of quartz powder with median grain size of 10 µm. Fault permeability evolves from 10-17 to 10-19 m2 over shear strains up to 26. Under these conditions, dilatancy strengthening is minimal. Slow slip may arise from dilatancy strengthening at higher fluid pressures but for the conditions of our experiments slip rate-dependent changes in the critical rate of frictional weakening are sufficient to explain slow-slip and the stability transition to dynamic rupture.

We exploit nonlinear elastodynamic properties of fractured rock to probe the micro-scale mechanics of fractures and understand the relation between fluid transport and fracture aperture and area, stiffness proxy, under dynamic stressing. Experiments are conducted on rough, tensile-fractured Westerly granite specimen subject to triaxial stresses. Fracture permeability is measured from steady-state fluid flow with deionized water. Pore pressure oscillations are applied at amplitudes ranging from 0.2 to 1~MPa at 1~Hz frequency. During dynamic stressing we transmit acoustic signals through the fracture using an array of piezoelectric transducers (PZTs) to monitor the evolution of fracture interface properties. We examine the influence of fracture aperture and contact area by conducting measurements at effective normal stresses of 10, 12.5, 15, 17.5, and 20~MPa. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film. These experiments are conducted separately with the same fracture and they map contact area at stresses from 9 to 21~MPa. The resulting ‘true’ area of contact measurements made for the entire fracture surface and within the calculated PZT sensor footprints, numerical modeling of Fresnel zone. We compare the elastodynamic response of the the fracture using the stress-induced changes ultrasonic wave velocities for a range of transmitter-receiver pairs to image spatial variations in contact properties, which is informed by fracture contact area measurements. These measurements of the nonlinear elasticity are related to the fluid-flow, permeability, in response to dynamic stressing and similar comparisons are made for the slow-dynamics, recovery, of the fracture interface following the stress perturbations.

We perform a suite of laboratory friction experiments on saw-cut Westerly Granite surfaces and probe frictional state evolution in response to step changes in normal stress. The experiments are conducted with the objective of illuminating the origin of friction memory effects and the fundamental processes that yield friction rate and state dependence. In contrast to previous works, we measure directly the fault slip rate and account for changes in slip rate caused by normal stress perturbations. Further, we complement mechanical data acquisition by continuously probing the faults with ultrasonic pulses. We conduct the experiments at room temperature and humidity conditions in a servo controlled biaxial testing apparatus in the double direct shear configuration. The normal stress perturbations are carried out during steady shearing over a range of shear velocities, from 0.02 - 100 μm/s. We report observations of a transient shear stress and friction evolution with step increases and decreases in normal stress. Specifically, we show that shear stress evolves in a two-stage fashion – first linear-elastically, then inelastically in response to the normal stress step. We find that the excursions in slip rate resulting from the changes in normal stress must be accounted for in order to accurately predict fault strength evolution. The effects of induced changes in fault slip rate are also apparent in elastic wave properties. Ultrasonic wave amplitudes increase instantly in response to normal stress steps and then gradually decrease to a new steady state value, in part due to changes in fault slip rate. This decrease is strongly related to accelerated creep at the fault interface. We also demonstrate that steady state amplitudes are a reliable proxy for real contact area (RCA) at the fault interface. Previous descriptions of frictional state evolution during normal stress perturbations have not adequately accounted for large slip velocity excursions. Here, we do so by using the measured ultrasonic amplitudes as a proxy for frictional state during transient shear stress evolution. Our work improves understanding of induced seismicity and triggered earthquakes with particular focus on simulating static triggering and stress transfer phenomena using rate-and-state frictional formulations in earthquake rupture models.

Tectonic faults fail in a continuum of modes from slow earthquakes to elastodynamic rupture. Precursory variations in elastic wavespeed and amplitude, interpreted as indicators of imminent failure, have been observed in limited experimental and natural settings for this spectrum of slip modes. Such variations are thought to arise from microcracking within and around the fault zone. However, the physical mechanisms and connections to fault creep are not well understood. Here, we vary loading stiffness to generate a range of slip modes and measure fault zone properties using elastic waves transmitted through the fault. We find that elastic wave amplitudes show clear changes before failure. The temporal onset of amplitude reduction scales with lab earthquake magnitude and the magnitude of this reduction varies with fault slip. Our data suggest that continuous seismic monitoring in proximity to natural faults could be useful for assessing fault state and seismic hazard potential.

Machine learning (ML) techniques have become increasingly important in seismology and earthquake science. Lab-based studies have used acoustic emission data to predict time-to-failure and stress state, and in a few cases the same approach has been used for field data. However, the underlying physical mechanisms that allow lab earthquake prediction and seismic forecasting remain poorly resolved. Here, we address this knowledge gap by coupling active-source seismic data, which probe asperity-scale processes, with ML methods. We show that elastic waves passing through the lab fault zone contain information that can predict the full spectrum of labquakes from slow slip instabilities to highly aperiodic events. The ML methods utilize systematic changes in p-wave amplitude and velocity to accurately predict the timing and shear stress during labquakes. The ML predictions improve in accuracy closer to fault failure, demonstrating that the predictive power of the ultrasonic signals improves as the fault approaches failure. Our results demonstrate that the relationship between the ultrasonic parameters and fault slip rate, and in turn, the systematically evolving real area of contact and asperity stiffness allow the gradient boosting algorithm to ‘learn’ about the state of the fault and its proximity to failure. Broadly, our results demonstrate the utility of physics-informed machine learning in forecasting the imminence of fault slip at the laboratory scale, which may have important implications for earthquake mechanics in nature.

Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide-hold-slide experiments over a range of fault slip rates, from plate rate (5 cm/yr) to ~1 mm/s and quantified the frictional velocity dependence and healing rates for a range of lithologies at different stresses. The friction velocity dependence (a-b) and critical slip distance Dc increase with fault slip rate in our experiments. We observe a transition from velocity weakening to strengthening at slip rates of ~0.3 µm/s. This velocity dependence of Dc could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay-rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1-10 years are consistent with the geodetically-inferred low stress drops and healing rates characteristic of the Hikurangi SSEs.