Self-limiting earthquake dynamics and spatio-temporal clustering of seismicity enabled by off-fault plasticity

Earthquakes are among nature’s deadliest and costliest hazards. Understanding mechanisms for earthquake nucleation, propagation, and arrest is key for developing reliable operational forecasts and next generation seismic hazard models. While significant progress has been made in understanding source processes in linear elastic domains, the response of the rocks near the fault is complex and likely to be inelastic due to the extreme stresses and deformations associated with fault slip. The effect of this more realistic fault zone response on seismic and aseismic fault slip is poorly understood. Here, we simulate sequence of earthquake and aseismic slip of a fault embedded in an elastic-viscoplastic bulk subject to slow tectonic loading. We show that off-fault plasticity significantly influences the source characteristics. Specifically, off-fault plasticity may lead to partial ruptures and emergence of spatial segmentation as well as hierarchical temporal seismic clustering. Furthermore, co-evolution of fault slip and off-fault bulk plasticity may lead to heterogeneous rupture propagation and results in pockets of slip deficit. While the energy dissipated through plastic deformation remains a small fraction of the total energy budget, its impact on the source characteristics is disproportionally large through the redistribution of stresses and viscous relaxation. Our results suggest a new mechanism of dynamic heterogeneity in earthquake physics that can be active for both small and large earthquakes and may have important implications on earthquake size distribution and energy budget. Furthermore, this plasticity-induced self-limiting crack dynamics may be relevant for other dynamic fracture applications and design of dynamically tough materials.

Ultrafast phased-array imaging with pump excitation for closed-crack imaging

Nondestructive evaluation of closed cracks is one of the most challenging problems in ultrasonic testing. Here, we propose a novel closed-crack imaging technique combining ultrafast phased-array with pump excitation. The pump excitation with kHz frequency can effectively induce the crack opening/closing behaviors since it can generate a large displacement of 1000 nm order. At the same time, ultrafast phased-array imaging, i.e. plane wave imaging, with MHz frequency stroboscopically captures the high-speed crack dynamics induced by pump excitation. We successfully proved the concept in a closed fatigue crack specimen.

Fracture Nucleation Phenomena and Thermally Activated Crack Dynamics in Monocrystals

We explore irreversible thermally activated growth of cracks which are shorter than the Griffith length. Such a growth was anticipated in several studies [Golubović, L. & Feng, S., (1991). Rate of microcrack nucleation, Physical Review A 43, 5223. Golubović, L. & Peredera, A., (1995).  Mechanism of time-delayed fractures, Physical Review E 51, 2799]. We explore this thermally activated growth by means of atomistic Monte-Carlo dynamics simulations of stressed monocrystals. This crack growth is stepwise. Each step is marked by nucleation of a microcavity close to the crack tip, and by creation of a passage connecting the microcavity and the crack. If the external tensile stress is weak, many such nucleation events occur before the crack length reaches the Griffith size. In addition to the simulations, we also present an analytic theory of the stepwise thermally activated crack growth. The theory explains surprising observation form our simulations that the thermally activated crack growth remains fairly well directed in spite of the stochastic nature of the crack growth process.

Assessing Soil Crack Dynamics and Water Evaporation during Dryings of Agricultural Soil from Reduced Tillage and Conventional Tillage Fields

Crack formation and development have been a general concern in agricultural science. Cracks contribute to soil aeration, aggregate formation, and easy root penetration. However, cracks accelerate soil desiccation, allow deep infiltration of pesticides/pollutants through preferential flow, and pollute the shallow water table in Belgium. Cracks have mostly been studied in pure clay or in high-clay-content soil (Vertisol). Yet, in Wallonia, cracks were also present on silt–loam soil (Luvisol). This study tried to cover this gap by analysing crack dynamics and evaporation process, during drying kinetics of the Luvisol. Soils were collected directly from the agricultural field and processed on a small drying chamber in which an evaporation test took place. A ceramic IR emitter heated the chamber while sensors (DHT22) measured the temperature and relative humidity. A digital camera took photos of the soil surface at 30-min intervals. A balance and tensiometer were linked to a datalogger (CR800), and recorded the soil hydraulic properties (evaporation rate, etc.). Cracks were assessed from small samples (~5 cm × 1cm thick) and big samples (~20 cm size × 1.6 cm thick). Three treatments were considered, including disturbed soil, conventional tillage and reduced tillage. For big samples, results showed higher crack formation on disturbed soil > reduced tillage > conventional tillage, due to loose soil cohesion, soil organic content, soil aggregation, biological activities, and soil porosity. The soil evaporation rate was also greater in disturbed soil > reduced tillage > conventional tillage. Cracks opening exposed a large quantity of soil water to the atmosphere without it passing through the soil matrix. For small samples, the repetitive drying experiments increased cracks’ length and width, especially for the dense samples. The results indicated the presence of pre-existing (or micro-) cracks in the soil samples. Future study is needed to assess the presence of pre- (micro-) cracks in soil using X-ray microtomography.

Fluid-driven cracks in an elastic matrix in the toughness-dominated limit

The dynamics of fluid-driven cracks in an elastic matrix is studied experimentally. We report the crack radius R ( t ) as a function of time, as well as the crack shapes w ( r , t ) as a function of space and time. A dimensionless parameter, the pressure ratio Δ p f /Δ p v , is identified to gauge the relative importance between the toughness (Δ p f ) and viscous (Δ p v ) effects. In our previous paper (Lai et al. 2015 Proc. R. Soc. A 471 , 20150255. ( doi:10.1098/rspa.2015.0255 )), we investigated the viscous limit experimentally when the toughness-related stresses are negligible for the crack propagation. In this paper, the experimental parameters, i.e. Young’s modulus E of the gelatin, viscosity μ of the fracturing liquid and the injection flow rate Q , were chosen so that the viscous effects in the flow are negligible compared with the toughness effects, i.e. Δ p f /Δ p v ≫1. In this limit, the crack dynamics can be described by the toughness-dominated scaling laws, which give the crack radius R ( t )∝ t 2/5 and the half maximum crack thickness W ( t )∝ t 1/5 . The experimental results are in good agreement with the predictions of the toughness scaling laws: the experimental data for crack radius R ( t ) for a wide range of parameters ( E , μ , Q ) collapse after being rescaled by the toughness scaling laws, and the rescaled crack shapes w ( r , t ) also collapse to a dimensionless shape, which demonstrates the self-similarity of the crack shape. The appropriate choice of the viscous or toughness scaling laws is important to accurately describe the crack dynamics. This article is part of the themed issue ‘Energy and the subsurface’.